Classifications

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  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
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  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0242—Flattening; Dressing; Flexing
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  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0263—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
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  • C22C—ALLOYS
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  • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
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  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
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  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of silicon
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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/52—Ferrous alloys, e.g. steel alloys containing chromium with nickel with cobalt
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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/18—Hardening; Quenching with or without subsequent tempering
• • C—CHEMISTRY; METALLURGY
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  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/62—Quenching devices
  • C21D1/673—Quenching devices for die quenching
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/74—Methods of treatment in inert gas, controlled atmosphere, vacuum or pulverulent material
  • C21D1/76—Adjusting the composition of the atmosphere
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  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/008—Martensite
• • C—CHEMISTRY; METALLURGY
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  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
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  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0236—Cold rolling
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  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0273—Final recrystallisation annealing

Definitions

• the present invention relates to a steel member and a steel sheet.
• a hot stamping technique has been employed as a technique for press-forming a material that is difficult to form, such as a high strength steel sheet.
• the hot stamping technique is a hot forming technique of heating a material to be subjected to forming and then forming the material.
• the material is formed after being heated. For this reason, the steel is soft at the time of forming and has good formability. Accordingly, even a high strength steel sheet can be accurately formed into a complex shape. Furthermore, since quenching is performed simultaneously with forming by a press die in the hot stamping technique, a steel member subjected to the forming has sufficient strength.
• Patent Document 1 discloses that the hot stamping technique makes it possible to impart a tensile strength of 1,400 MPa or more to a steel member subjected to forming.
• a vehicle is also required to have collision safety.
• the collision safety of a vehicle is evaluated on the basis of crushing strength and absorbed energy in a collision test for the entire vehicle body or some members.
• crushing strength largely depends on material strength, the demand for an ultrahigh strength steel member as a vehicle member dramatically increases.
• Patent Document 2 discloses a press-formed article that is excellent in toughness, has a tensile strength of 1.8 GPa or more, and is hot press-formed.
• Patent Document 3 discloses steel that has an extremely high tensile strength of 2.0 GPa or more and further has good toughness and good ductility.
• Patent Document 4 discloses steel that has an extremely high tensile strength of 1.8 GPa or more and further has good toughness.
• Patent Document 5 discloses steel that has an extremely high tensile strength of 2.0 GPa or more and further has good toughness.
• Patent Documents 1 to 5 do not contain any description regarding bendability, and higher demands may not be sufficiently met in a case where high strength steel having a tensile strength of more than 1.0 GPa is used for vehicle members.
• Patent Documents 1 to 5 there is room for further improvement in the techniques disclosed in Patent Documents 1 to 5 from the viewpoint of hydrogen embrittlement resistance.
• An object of the present invention is to provide a steel member that has high strength, excellent bendability, and excellent hydrogen embrittlement resistance (excellent hydrogen embrittlement resistance properties), and a steel sheet that is suitable as a material of the steel member.
• the present inventors have conducted studies to obtain excellent bendability and excellent hydrogen embrittlement resistance in a high strength steel member obtained from the hot-stamping of a steel sheet.
• the present invention has been made in consideration of the above-described problems.
• the gist of the present invention is as follows.
• a steel member (a steel member according to the present embodiment) and a steel sheet (a steel sheet according to the present embodiment) according to an embodiment of the present invention, and preferred manufacturing methods therefor will be described below.
• the steel member also includes a case where a steel member includes an alloy layer on a surface thereof and the steel sheet also includes a case where a steel sheet includes a plating layer (a case where the steel sheet is a plated steel sheet).
• the maximum B content in a range to a depth of 10 ⁇ m from the surface in the thickness direction is 5 times or more the B content at the 1/4-depth position.
• the C content is measured from the surface in the thickness direction using GDS and the distance from the surface to a position where the C content is equal to the C content at the 1/4-depth position for the first time is defined as a decarburization depth
• the decarburization depth is 20 ⁇ m or more
• the Vickers hardness at the 1/4-depth position is in a range of 310 to 890.
• the steel member may have a strength gradient.
• the steel member may include an alloy layer (coating) on the surface thereof.
• the surface of the steel member serving as a reference such as "a depth of 1/4 of a thickness from a surface in a thickness direction" or "a depth of 10 ⁇ m from the surface in the thickness direction", means a surface of a portion excluding the alloy layer (so-called base steel).
• the steel member according to the present invention has a chemical composition including, by mass%, C: 0.10% to 0.70%, Si: 3.00% or less, Mn: 0.01% to 3.00%, P: 0.100% or less, S: 0.0100% or less, N: 0.020% or less, O: 0.010% or less, B: 0.0002% to 0.0200%, Ti: 0% to 0.200%; Cr: 0% to 1.00%; Mo: 0% to 1.00%, Ni: 0% to 2.00%, Nb: 0% to 0.10%, Cu: 0% to 2.00%, V: 0% to 1.00%, Ca: 0% to 0.020%, Mg: 0% to 0.020%, Al: 0% to 1.00%, Sn: 0% to 1.00%, W: 0% to 2.00%, Sb: 0% to 1.00%, Zr: 0% to 1.00%, Se: 0% to 1.00%, Bi: 0% to 1.00%, As: 0% to 1.00%, Ta: 0% to 1.0
• the chemical composition of the steel member refers to a chemical composition of a portion excluding the vicinity of the surface (for example, a 1/4 position of the thickness from the surface of the steel member in the thickness direction: the 1/4-depth position).
• % regarding the content is mass% unless otherwise specified.
• the C is an element that increases the hardenability of steel and improves the strength of a steel member obtained from the hot stamping of the steel sheet.
• the C content is set to 0.10% or more. It is preferable that the C content is 0.15% or more.
• the C content is set to 0.70% or less. It is preferable that the C content is 0.60% or less.
• Si is an effective element for increasing the hardenability of steel and stably ensuring the strength of the steel member. For this reason, Si may be contained. In a case where the above-described effects are to be obtained, the Si content is preferably set to 0.05% or more and more preferably set to 0.10% or more.
• the Si content in the steel sheet is more than 3.00%, a heating temperature required for austenite transformation is significantly high in heat treatment. Accordingly, cost required for the heat treatment may be increased.
• the Si content is set to 3.00% or less.
• the Si content is preferably 2.00% or less and more preferably 1.80% or less.
• Mn is a very effective element for increasing the hardenability of steel and stably ensuring the strength of the steel member.
• Mn is an element that lowers Ac3 point and promotes the lowering of a quenching temperature. Since these effects are not sufficient in a case where the Mn content is less than 0.01%, the Mn content is set to 0.01% or more. It is preferable that the Mn content is 0.05% or more.
• the Mn content is set to 3.00% or less.
• the Mn content is preferably 2.80% or less and more preferably 2.50% or less.
• the P is an element that causes the toughness and hydrogen embrittlement resistance of the steel member to deteriorate.
• the P content is more than 0.100%, the toughness and the hydrogen embrittlement resistance deteriorate significantly. Accordingly, the P content is limited to 0.100% or less. It is preferable that the P content is limited to 0.050% or less. Since it is preferable that the P content is small, the P content may be 0%. However, from the viewpoint of cost, the P content may be set to 0.001% or more.
• S is an element that causes the toughness, bendability, and hydrogen embrittlement resistance of the steel member to deteriorate.
• the S content is more than 0.0100%, the toughness, the bendability, and the hydrogen embrittlement resistance deteriorate significantly. Accordingly, the S content is limited to 0.0100% or less. It is preferable that the S content is limited to 0.0050% or less. Since it is preferable that the S content is small, the S content may be 0%. However, from the viewpoint of cost, the S content may be set to 0.0001% or more.
• N is an element that causes the toughness and hydrogen embrittlement resistance of the steel member to deteriorate.
• the N content is set to 0.020% or less.
• a lower limit of the N content does not need to be particularly limited and may be 0%.
• the N content may be set to 0.0002% or more or 0.0008% or more.
• O is an element that causes the toughness and hydrogen embrittlement resistance of the steel member to deteriorate.
• the O content is set to 0.010% or less.
• the lower limit of the O content does not need to be particularly limited and may be 0%.
• the O content may be set to 0.0002% or more, 0.0008% or more, or 0.001% or more.
• B is an element that has an action of dramatically increasing the hardenability of steel even with a very small amount. Further, B is an element that strengthens grain boundaries and increases toughness and hydrogen embrittlement resistance by being segregated at the grain boundaries, and is an element that suppresses the growth of austenite grains during the heating of the steel sheet. In addition, B is an effective element for obtaining a surface layer structure that includes tough grain boundaries and includes fine martensite as a main structure by being concentrated in the surface layer as described later. In a case where the B content is less than 0.0002%, these effects are not sufficient. For this reason, the B content is set to 0.0002% or more. The B content is preferably 0.0005% or more and more preferably 0.0010% or more.
• the B content is set to 0.0200% or less.
• the B content is preferably 0.0100% or less.
• the steel member according to the present embodiment may further contain one or more elements selected from Ti, Cr, Mo, Ni, Nb, Cu, V, Ca, Mg, Al, Sn, W, Sb, Zr, Se, Bi, As, Ta, Re, Os, Ir, Tc, Co, and REM in addition to the elements described above, in order to increase the strength, toughness, bendability, corrosion resistance, a deoxidation property, and hydrogen embrittlement resistance. Since these elements are optional elements and do not necessarily need to be contained, lower limits thereof are 0%. These elements may be contained as impurities as long as the amounts of the elements are equal to or less than upper limits to be described later.
• Ti is an element having an action of refining austenite grains by suppressing recrystallization and by forming fine carbide to suppress grain growth in a case where the steel sheet is to be heated to a temperature equal to or higher than Ac3 point and subjected to heat treatment. For this reason, an effect of significantly improving the toughness of the steel member is obtained in a case where Ti is contained.
• Ti is an element that is preferentially bonded to N contained in steel, suppresses the consumption of B caused by the precipitation of BN and promotes an effect of improving the hardenability caused by B to be described later. For this reason, Ti may be contained.
• the Ti content is set to 0.010% or more.
• the Ti content is more preferably 0.020% or more.
• the Ti content is set to 0.200% or less.
• the Ti content is preferably 0.100% or less.
• Cr is an effective element for increasing the hardenability of steel and stably ensuring the strength of the steel member. For this reason, Cr may be contained. In a case where the above-described effects are to be obtained, it is preferable that the Cr content is set to 0.01% or more.
• the Cr content is more preferably 0.05% or more and still more preferably 0.08% or more.
• the Cr content is more than 1.00%, the above-described effects are saturated and cost is increased. Further, Cr has an action of stabilizing iron carbide. Accordingly, in a case where the Cr content is more than 1.00%, coarse iron carbide is remains without melting during the heating of the steel sheet, so that the toughness of the steel member may deteriorate. Therefore, in a case where Cr is to be contained, the Cr content is set to 1.00% or less.
• the Cr content is preferably 0.80% or less.
• Mo is an effective element for increasing the hardenability of steel and stably ensuring the strength of the steel member. For this reason, Mo may be contained. In a case where the above-described effects are to be obtained, it is preferable that the Mo content is set to 0.01% or more. The Mo content is more preferably 0.05% or more.
• the Mo content is more than 1.00%, the above-described effects are saturated and cost is increased. Further, Mo has an action of stabilizing iron carbide. Accordingly, in a case where the Mo content is more than 1.00%, coarse iron carbide is remains without melting during the heating of the steel sheet, so that the toughness of the steel member may deteriorate. Therefore, in a case where Mo is to be contained, the Mo content is set to 1.00% or less.
• the Mo content is preferably 0.80% or less.
• Ni is an effective element for increasing the hardenability of steel and stably ensuring the strength of the steel member. For this reason, Ni may be contained. In a case where the above-described effects are to be obtained, it is preferable that the Ni content is set to 0.01% or more. The Ni content is more preferably 0.10% or more.
• the Ni content is set to 2.00% or less.
• the Ni content is preferably 1.00% or less.
• Nb is an element that has actions of forming fine carbide and increasing the toughness of steel with a grain refinement effect thereof. For this reason, Nb may be contained. In a case where the above-described effects are to be sufficiently obtained, it is preferable that the Nb content is set to 0.02% or more. The Nb content is more preferably 0.03% or more.
• the Nb content is set to 0.10% or less. It is preferable that the Nb content is 0.08% or less.
• Cu is an effective element for increasing the hardenability of steel and stably ensuring the strength of the steel member. For this reason, Cu may be contained. Further, Cu is an element that has an effect of improving the corrosion resistance of the steel member. In a case where the above-described effects are to be obtained, it is preferable that the Cu content is set to 0.01% or more. The Cu content is more preferably 0.05% or more.
• the Cu content is set to 2.00% or less.
• the Cu content is preferably 1.00% or less.
• V 0% to 1.00%
• V is an element that forms fine carbide and increases the toughness of steel with a grain refinement effect thereof. For this reason, V may be contained. In a case where the above-described effects are to be obtained, it is preferable that the V content is set to 0.01% or more. The V content is more preferably 0.10% or more.
• the V content is set to 1.00% or less.
• Ca is an element that has effects of refining inclusions contained in steel and improving the toughness of the steel member. For this reason, Ca may be contained. In a case where the above-described effects are to be obtained, it is preferable that the Ca content is set to 0.001% or more. The Ca content is more preferably 0.002% or more.
• the Ca content is set to 0.020% or less.
• the Ca content is preferably 0.010% or less and more preferably 0.005% or less.
• Mg is an element that has effects of refining inclusions contained in steel and improving the toughness of the steel member. For this reason, Mg may be contained. In a case where the above-described effects are to be obtained, it is preferable that the Mg content is set to 0.001% or more. The Mg content is more preferably 0.002% or more.
• the Mg content is set to 0.020% or less.
• the Mg content is preferably 0.010% or less and more preferably 0.005% or less.
• Al is an element that is generally used as a deoxidizing agent for steel. For this reason, Al may be contained. In a case where the above-described effect is to be obtained, it is preferable that the Al content is set to 0.01% or more.
• the Al content is set to 1.00% or less.
• the Al content mentioned here is the total Al content.
• Sn is an element that improves corrosion resistance in a corrosive environment. For this reason, Sn may be contained. In a case where the above-described effect is to be obtained, it is preferable that the Sn content is set to 0.01% or more.
• the Sn content is set to 1.00% or less.
• W is an element that increases the hardenability of steel and makes it possible to stably ensure the strength of the steel member. For this reason, W may be contained. Further, W is an element that improves corrosion resistance in a corrosive environment. In a case where the above-described effects are to be obtained, it is preferable that the W content is set to 0.01% or more.
• the W content is set to 2.00% or less.
• the W content is preferably 1.00% or less.
• Sb is an element that improves corrosion resistance in a corrosive environment. For this reason, Sb may be contained. In a case where the above-described effect is to be obtained, it is preferable that the Sb content is set to 0.01% or more.
• the Sb content is set to 1.00% or less.
• Zr is an element that improves corrosion resistance in a corrosive environment. For this reason, Zr may be contained. In order to obtain the above-described effect, it is preferable that the Zr content is set to 0.01% or more.
• the Zr content is set to 1.00% or less.
• Se is an element that improves hydrogen embrittlement resistance. For this reason, Se may be contained. In a case where the above-described effect is to be obtained, it is preferable that the Se content is set to 0.01% or more.
• the Se content is set to 1.00% or less.
• Bi is an element that improves hydrogen embrittlement resistance. For this reason, Bi may be contained. In a case where the above-described effect is to be obtained, it is preferable that the Bi content is set to 0.01% or more.
• the Bi content is set to 1.00% or less.
• the As content be set to 0.01% or more.
• the As content is set to 1.00% or less.
• Ta is an element that improves hydrogen embrittlement resistance. For this reason, Ta may be contained. In a case where the above-described effect is to be obtained, it is preferable that the Ta content be set to 0.01% or more.
• the Ta content is set to 1.00% or less.
• Re is an element that improves hydrogen embrittlement resistance. For this reason, Re may be contained. In a case where the above-described effect is to be obtained, it is preferable that the Re content be set to 0.01% or more.
• the Re content is set to 1.00% or less.
• Os is an element that improves hydrogen embrittlement resistance. For this reason, Os may be contained. In a case where the above-described effect is to be obtained, it is preferable that the Os content be set to 0.01% or more.
• the Os content is set to 1.00% or less.
• Ir is an element that improves hydrogen embrittlement resistance. For this reason, Ir may be contained. In a case where the above-described effect is to be obtained, it is preferable that the Ir content be set to 0.01% or more.
• the Ir content is set to 1.00% or less.
• Tc is an element that improves hydrogen embrittlement resistance. For this reason, Tc may be contained. In a case where the above-described effect is to be obtained, it is preferable that the Tc content be set to 0.01% or more.
• the Tc content is set to 1.00% or less.
• Co is an element that improves corrosion resistance in a corrosive environment. For this reason, Co may be contained. In a case where the above-described effect is to be obtained, it is preferable that the Co content be set to 0.01% or more.
• the Co content is set to 1.00% or less.
• REM is an element that has effects of refining inclusions contained in steel and improving the toughness of the steel member, as with Ca. For this reason, REM may be contained. In a case where the above-described effects are to be obtained, it is preferable that the REM content is set to 0.01% or more. The REM content is more preferably 0.02% or more.
• the REM content is set to 0.30% or less.
• the REM content is preferably 0.20% or less.
• REM refers to a total of 17 elements of Sc, Y, and lanthanoids, such as La and Nd, and the REM content means the total amount of these elements.
• REM is added to molten steel using, for example, an Fe-Si-REM alloy, and this alloy contains, for example, La, Nd, Ce, and Pr.
• an element other than the above-described elements, that is, the remainder includes Fe and impurities.
• the "impurities” are elements that are incorporated due to various factors including raw materials, such as ore and scrap, and a manufacturing step in a case where the steel sheet is industrially manufactured, and are acceptable in a range without adversely affecting the properties of the steel member according to the present embodiment.
• a method of industrially manufacturing the steel sheet is a blast furnace steelmaking method or an electric furnace steelmaking method, and also includes a level (impurity level) of impurities to be incorporated in a case where the steel sheet is manufactured by any of the methods.
• the chemical composition of the steel member can be obtained by the following method.
• the chemical composition of the steel member is obtained in a case where an analysis sample is cut out from the steel member and element analysis, such as inductively coupled plasma (ICP) atomic emission spectrometry, is performed.
• ICP inductively coupled plasma
• C and S may be measured using a combustion-infrared absorption method
• N may be measured using an inert gas fusion-thermal conductivity method
• O may be measured using an inert gas fusion-nondispersive infrared absorption method.
• the analysis sample is collected from a 1/4 position of the thickness from the surface of a base steel sheet in a thickness direction while avoiding a width-direction end portion of the base steel sheet so that the average chemical composition of the steel member over the entire thickness is obtained.
• the C content and the B content obtained here are the C content and the B content at the 1/4-depth position to be described later.
• the chemical composition is generally uniform in the steel member according to the present embodiment, it can be said that the C content and the B content obtained here are also the average chemical composition of the entire steel member except for a decarburized layer or the surface layer in which B is concentrated.
• a position corresponding to a depth of 1/4 of the thickness from the surface in the thickness direction will be described as a 1/4-depth position
• a position corresponding to a depth of 0.1 mm from the surface in the thickness direction will be described as a 0.1 mm-depth position.
• the distance from the surface to a position where the C content is equal to equal to the C content at the 1/4-depth position for the first time (a position closest to the surface among positions where the C content is equal to the C content at the 1/4-depth position) will be described as a decarburization depth.
• the maximum B content in a range to a depth of 10 ⁇ m from the surface is 5 times or more the B content at the 1/4-depth position and a decarburization depth is 20 ⁇ m or more.
• decarburizing and softening the surface layer are extremely effective to improve bendability. Since stress and strain to be generated are larger at an outer portion of the bent surface layer with regard to bending deformation, bendability can be improved in a case where the surface layer is softened to improve a fracture limit. In a case where the decarburization depth of the steel member is 20 ⁇ m or more, bendability is improved. For this reason, the decarburization depth is set to 20 ⁇ m or more.
• decarburizing and softening the surface layer are effective with regard to the bendability of the steel member.
• carbon is an element that is segregated at grain boundaries and strengthens the grain boundaries, it is not preferable that the amount of carbon at the grain boundaries is reduced. Since carbon is an element that is segregated at grain boundaries and strengthens the grain boundaries, hydrogen embrittlement resistance and toughness may be reduced in a case where the amount of carbon at the grain boundaries is reduced and grain boundary strength is reduced.
• the surface has a ferrite-based structure.
• the reason for this is that, since ferrite is coarser and softer than the dense martensite-based structure of the base steel, there is a large mismatch between the ferrite-based structure of the surface layer and the martensite-based structure of the base steel, which causes deterioration in bendability, hydrogen embrittlement resistance, and toughness.
• the surface layer is decarburized and B is concentrated in the surface layer.
• B is an element that is segregated at grain boundaries and strengthens the grain boundaries as with carbon and improves hardenability. For this reason, in a case where B is concentrated in the surface layer, it is possible to obtain a martensite-based structure in which grain boundaries are tough and fine. Since having been decarburized, the martensite-based structure of the surface layer is softer than the martensite-based structure of the base steel and is obtained with a high level of bendability, hydrogen embrittlement resistance, and toughness.
• the steel member according to the present embodiment has a hardness (Vickers hardness (HV)) of 310 to 890 at the 1/4-depth position.
• the fact that the steel member has this Vickers hardness corresponds to the fact that the tensile strength of the steel member is in a range of 1.0 GPa to 3.1 GPa, and contributes to a reduction in the weight of a vehicle body and the improvement of collision safety thereof in a case where the steel member is applied to a vehicle component.
• a relationship between hardness and tensile strength is described in, for example, SAE J 417.
• the surface layer of the steel member according to the present embodiment is decarburized and softened and a strength gradient thereof is controlled from the surface layer toward the inside within the following range.
• bendability is further improved.
• the reason for this is considered as follows: The maximum strain and stress are applied to the outside bend surface as described above with regard to bending deformation and cracking is to progress from the surface to the inside. However, in a case where an appropriate strength gradient is present, there is an effect of suppressing the development of strain on the surface.
• the Vickers hardness at a position corresponding to a depth of 0.1 mm from the surface in the thickness direction (0.1 mm-depth position) is controlled within an appropriate range according to the Vickers hardness at the 1/4-depth position that has a high correlation with the tensile strength of the steel member.
• the Vickers hardness at the 0.1 mm-depth position is 0.95 times or less the Vickers hardness at the 1/4-depth position.
• the Vickers hardness at the 1/4-depth position is more than 700 and 890 or less (corresponding to a case where tensile strength is more than 2.4 GPa and 3.1 GPa or less), it is preferable that the Vickers hardness at the 0.1 mm-depth position is 0.86 times or less the Vickers hardness at the 1/4-depth position.
• the maximum principal plastic strain at the center of the outside bend surface layer is 0.260 or less.
• a bending angle is adjusted so that the same deformation as in a case where a steel member having a sheet thickness of 1.6 mm is deformed up to a bending angle of 50 degrees is obtained.
• a sheet thickness is 1.0 mm
• the maximum principal plastic strain at a bending outer center when a steel member is deformed up to a bending angle of 60 degrees is evaluated.
• a sheet thickness is 2.3 mm
• the sheet thickness is not limited, but it is preferable that the sheet thickness is in a range of 0.6 mm to 4.0 mm.
• the Vickers hardness at the 1/4-depth position is more than 530 and 700 or less (corresponding to a case where tensile strength is more than 1.8 GPa and 2.4 GPa or less), it is preferable that the Vickers hardness at the 0.1 mm-depth position is 0.88 times or less the Vickers hardness at the 1/4-depth position.
• the maximum principal plastic strain at the center of the outside bend surface layer is 0.280 or less.
• the Vickers hardness at the 1/4-depth position is more than 450 and 530 or less (corresponding to a case where tensile strength is more than 1.5 GPa and 1.8 GPa or less)
• the Vickers hardness at the 0.1 mm-depth position is 0.90 times or less the Vickers hardness at the 1/4-depth position.
• the maximum principal plastic strain at the center of the outside bend surface layer is 0.310 or less.
• the Vickers hardness at the 1/4-depth position is in a range of 310 to 450 (corresponding to a case where tensile strength is in a range of 1.0 GPa to 1.5 GPa), it is preferable that the Vickers hardness at the 0.1 mm-depth position is 0.95 times or less the Vickers hardness at the 1/4-depth position.
• the maximum principal plastic strain at the center of the outside bend surface layer is 0.340 or less.
• the Vickers hardness at the 0.1 mm-depth position is 0.70 times or more the Vickers hardness at the 1/4-depth position in any case.
• the degree of concentration of B in the surface layer (B concentration ratio: a ratio of the maximum B content in a range to a depth of 10 ⁇ m to the B content at the 1/4-depth position) and the decarburization depth can be obtained using GDS by the following methods.
• GDS low discharge spectrometry
• the outermost surface of the steel member the surface of a scale in a case where the steel member includes the scale on the surface thereof, and the surface of an alloy layer in a case where the steel member includes an alloy layer on the surface thereof
• a position where the Fe content becomes 95 mass% or more for the first time is defined as the surface.
• the maximum value among values of the B content in a range to a depth of 10 ⁇ m from the surface described above, which are obtained from the GDS analysis, is defined as the maximum B content.
• the maximum value among values of the B content in a range to a depth of 10 ⁇ m from a position where a total amount of Fe and Al becomes 70 mass% or more for the first time is defined as the maximum B content.
• the maximum value among values of the B content in a range to a depth of 10 ⁇ m from a position where a total amount of Fe and Zn becomes 70 mass% or more for the first time is defined as the maximum B content.
• a case where the steel member includes an Fe-Al-based alloy layer is a case where the Al content in an alloy layer region on the surface of the steel member, which is obtained from the GDS analysis, is 25 mass% or more.
• a case where the steel member includes an Fe-Zn-based alloy layer is a case where the Zn content in an alloy layer region on the surface of the steel member, which is obtained from the GDS analysis, is 25 mass% or more.
• the degree of concentration of B in the surface layer is calculated from the maximum B content and the B content at the 1/4-depth position that is obtained by the method described above.
• a position where the C content obtained from the same GDS analysis becomes equal to the C content at the 1/4-depth position described above for the first time is defined as an end of the decarburization depth, and the distance from the surface to this end is defined as the decarburization depth.
• the hardness at the 1/4-depth position and the hardness at the 0.1 mm-depth position are obtained by the following methods.
• a sample for the observation of a cross section is collected from a 1/4 position of the width (short side) from the width-direction end portion of the steel member, and the Vickers hardness is measured at target depth positions (a 1/4-depth position and a 0.1 mm-depth position) according to JIS Z 2244-1 :2020.
• a test force is set to 100 gf. The measurement is performed five times at each depth, and the average value of measured values is used as hardness at that position.
• the maximum principal plastic strain at the center of the outside bend surface layer in a case where the steel member is deformed up to a predetermined bending angle is obtained by the following method.
• a steel member having a strength gradient on the surface thereof is modeled using a finite element method.
• the modeled steel member is set such that a range from the surface to a depth of 50 ⁇ m, a range having a depth of 50 ⁇ m to 100 ⁇ m, and a range having a depth of 100 ⁇ m to 250 ⁇ m respect to the entire sheet thickness have the same hardness as a steel member to be evaluated.
• a test jig used to perform a bending test of VDA238-100 is modeled.
• a punch diameter R is set to 0.4 mm
• a roller diameter is set to 30 mm ⁇
• the distance between the rollers is set to 2 ⁇ sheet thickness + 0.05.
• Pressing is performed up to a predetermined bending angle using the modeled steel member and the modeled test jig.
• the maximum principal plastic strain at the predetermined bending angle is output, and the maximum principal plastic strain at the center of the outside bend surface layer (a point where the maximum principal plastic strain on a bending ridge is maximum) is obtained.
• Software for analysis using a finite element method is not limited, and for example, Marc manufactured by MSC Software Corporation is used.
• the mesh size is set to 5 ⁇ m, and a friction coefficient ⁇ of a contact section between the steel member and the test jig is set to 0.05.
• the microstructure (internal structure) of the steel member according to the present embodiment at the 1/4-depth position is not limited, and it is preferable that the microstructure is a structure containing 80% or more of martensite by volume fraction.
• the Vickers hardness at the 1/4-depth position can be set to 310 or more (corresponding to a tensile strength of 1.0 GPa or more).
• martensite occupies 90% or more by volume fraction. More preferably, martensite occupies 95% or more. Martensite may occupy 100%. In a case where the volume fraction of martensite is small, it is difficult to obtain a tensile strength of 1.0 GPa or more.
• the microstructure of the steel member at the 1/4-depth position may contain residual austenite, bainite, ferrite, and/or pearlite as the remainder other than martensite.
• Martensite includes not only so-called fresh martensite but also tempered martensite and auto-tempered martensite.
• the auto-tempered martensite is tempered martensite generated during cooling at the time of quenching without heat treatment for tempering, and is generated by in-situ tempering of martensite generated due to self-heating associated with martensitic transformation.
• Microstructural fractions of the microstructure of the steel member at the 1/4-depth position can be measured by the following method.
• the area fraction of martensite (including fresh martensite, tempered martensite, and auto-tempered martensite) is measured by a transmission electron microscope (TEM) and an electron beam diffractometer attached to TEM.
• TEM transmission electron microscope
• a measurement sample including a 1/4 position of the width of the steel member from a width-direction end portion of the steel member (1/4-width position) and the 1/4-depth position of the steel member is cut out, and is used as a thin film sample for TEM observation.
• the range of this thin film sample which includes the 1/4-width position and the 1/4-depth position and has an area of 400 ⁇ m 2 or more, is observed with TEM.
• Martensite and bainite which have body-centered cubic lattices, and residual austenite, which has face-centered cubic lattices, are distinguished using an electron beam diffraction pattern of the thin film sample. Then, iron carbide (Fe 3 C) contained in martensite and bainite is found using the diffraction pattern, and a precipitation form thereof is observed to measure the microstructural fraction of each of martensite and bainite. Specifically, a structure is determined to be martensite (tempered martensite) in a case where the precipitation form is precipitation in three directions, and a structure is determined to be bainite in a case where the precipitation form is precipitation limited to one direction. Even in a case where precipitation of iron carbide is not observed, a structure is also determined to be martensite (fresh martensite).
• microstructural fractions of martensite and bainite to be measured by TEM are measured in area%.
• a value of an area fraction can be replaced with a volume fraction as it is.
• Carbide is observed to distinguish between martensite and bainite, but carbide is not included in the volume fraction of the structure in the present embodiment.
• the ferrite or the pearlite can be easily confirmed using an optical microscope or a scanning electron microscope. Specifically, a measurement sample including the 1/4-width position of the steel member and the 1/4-depth position of the steel member is cut out, and is used as a sample for observation. The cut sample is mechanically polished and subsequently mirror-finished. Next, etching is performed on the sample with a nital etchant to reveal ferrite and pearlite, and a range having an area of 40,000 ⁇ m 2 or more is observed using a scanning electron microscope to confirm the presence of ferrite or pearlite. A structure in which ferrite and cementite are alternately arranged in layers is determined to be pearlite, and a structure in which cementite is precipitated in a granular form is determined to be bainite.
• the steel member according to the present embodiment may include an alloy layer on at least a part of the surface (or the entire surface) thereof.
• the alloy layer may be an Fe-Al-based alloy layer or an Fe-Zn-based alloy layer.
• the Fe-Al-based alloy layer is an alloy layer containing 70 mass% or more of Fe and Al in total
• the Fe-Zn-based alloy layer is an alloy layer containing 70 mass% or more of Fe and Zn in total.
• the steel member has corrosion resistance in a case where the steel member includes the alloy layer, an effect of improving hydrogen embrittlement resistance can be obtained in a case where the steel member is used for a vehicle.
• the thickness of the alloy layer is in a range of 5 ⁇ m to 100 ⁇ m.
• a surface serving as a reference at each depth position is the surface of a portion (base steel) excluding the alloy layer.
• a cross section taken in the thickness direction is observed using a scanning electron microscope, so that the thickness of the alloy layer can be obtained.
• a measurement sample is cut out from a 1/2 portion of the steel member in a longitudinal direction (a position corresponding to 1/2 of a length from a longitudinal-direction end portion in the longitudinal direction) and a 1/4 portion of a width (a 1/4 position of the width from a width-direction end portion in the width direction) and is observed.
• An observation range of the microscope is set to, for example, a range having an area of 40,000 ⁇ m 2 or more at 400-fold magnification.
• the cut sample is mechanically polished and subsequently mirror-finished.
• the thicknesses of the alloy layer in ten arbitrary visual fields are measured, and an average value thereof is used as the thickness of the alloy layer.
• the measurement sample is observed using a BSE image (or a COMPO image)
• a clear contrast difference between the alloy layer and base steel (steel sheet substrate) is confirmed.
• it is possible to measure the thickness of the alloy layer by measuring a thickness from an outermost surface to a position where contrast is changed. The measurement is performed at 20 points arranged at regular intervals in an observation photograph, and the distance between the measurement points is set to 6.5 ⁇ m. Further, observation is performed in five visual fields in the above-described manner in the measurement, and the average value of measured values is used as the thickness of the alloy layer.
• spot element analysis (a beam diameter of 1 ⁇ m or less) can be performed in the same observation range as described above using an electron probe microanalyzer (EPMA) to obtain the amounts of Fe, Al, Zn, and the like contained in the alloy layer.
• EPMA electron probe microanalyzer
• the alloy layer is analyzed at a total of 10 points in ten arbitrary visual fields, and the average value of obtained values is used as the amount of each of Fe, Al, and Zn contained in the alloy layer. Even in a case where an element other than Fe, Al, and Zn is contained, the amount of the element is obtained using the same method.
• the shape of the steel member according to the present embodiment is not particularly limited. That is, the steel member may be a flat sheet, or may be a formed body that is a steel sheet formed in a predetermined shape. A hot-formed steel member is often a formed body, and a case of a formed body and a case of a flat sheet are collectively referred to as a "steel member" in the present embodiment. Further, the steel member may be a tailored property material having different strengths depending on points. In this case, it is preferable that at least a part of the steel member has a tensile strength of 1.0 GPa (1000 MPa) or more.
• the tailored property material may be a material in which steel sheets having different chemical compositions, different strengths, or different sheet thicknesses are joined, or may be a material in which a part of a steel sheet is subjected to heat treatment.
• a steel sheet (which may be referred to as a steel sheet according to the present embodiment) that serves as a material of the steel member according to the present embodiment will be described. Heat treatment to be described later is performed on the steel sheet to be described below, so that the steel member can be obtained.
• the steel sheet according to the present embodiment includes a base steel sheet and an iron scale that is formed on a surface of the base steel sheet.
• the steel sheet according to the present embodiment may include a plating layer on the iron scale (a surface of the iron scale).
• a range of the chemical composition of a base steel sheet included in the steel sheet according to the present embodiment is the same as the range of the chemical composition of the steel member according to the present embodiment described above, and reasons for limiting the range are also the same as described above.
• a 1/4 position of a sheet thickness from the surface of the base steel sheet in a sheet thickness direction (1/4-depth position) is set as a representative position and element analysis is performed at this position with a general method, such as ICP, so that the chemical composition of the base steel sheet is obtained.
• the scale included in the steel sheet according to the present embodiment includes cracks at a density of 90 pieces/mm 2 or more.
• the base steel sheet of the steel sheet according to the present embodiment is decarburized using O that is contained in the scale formed on the surface of the base steel sheet by hot rolling or the like as described later.
• decarburization is performed by annealing in a coil shape (box annealing)
• the discharge of CO which is a decarbonizing reaction
• sheets more closely adhere to each other and are more sealed to each other at an inner portion of a coil For this reason, decarburization uniform in a longitudinal direction and a width direction of the coil is not obtained.
• discharge passages for CO are formed in a case where cracks are introduced into the scale, a decarbonizing reaction promptly and uniformly proceeds.
• the crack density of the scale is controlled in the steel sheet according to the present embodiment.
• the crack density of the scale can be obtained from the SEM observation of the surface.
• a sample having a size of about 15 mm is collected while avoiding an end portion (for example, within 50 mm from the end portion) of the steel sheet. At this time, attention is paid so that the scale is not peeled off, and a sample having a size of 15 mm 2 or more is cut out as necessary.
• the surface on which the scale is formed is observed with SEM, a BSE image is acquired, and the crack density is counted up using a cutting method.
• the sample is observed at 500-fold magnification so that an observed area has 50,000 ⁇ m 2 or more /per one visual field.
• the sample is observed using a BSE image (or a COMPO image)
• a clear contrast difference between the scale and cracks are confirmed.
• the obtained BSE image (or COMPO image) is cut into 10 vertical divisions and 10 horizontal divisions, the number of cracks overlapping with cutting-plane lines is counted and converted into a number density. Ten arbitrary visual fields are observed, and the average value of values of the crack density in the respective visual fields is used as the crack density of the scale.
• the steel sheet according to the present embodiment may include a plating layer (coating) on a part of the surface (or the entire surface) thereof.
• the plating layer may be an Al-based plating layer that mainly contains Al or a Zn-based plating layer that mainly contains Zn.
• the Al-based plating layer is a plating layer containing 70 mass% or more of Al
• the Zn-based plating layer is a plating layer containing 70 mass% or more of Zn.
• the shape of the steel sheet according to the present embodiment is not particularly limited. That is, the steel sheet may be a flat sheet, or may be a tailored property material in which steel sheets having different strengths or different sheet thicknesses are joined.
• a steel piece such as a slab, having the preferable chemical composition of the steel sheet according to the present embodiment described above is manufactured.
• Molten steel of which the chemical composition is adjusted to a predetermined chemical composition may be made into a steel piece by continuous casting or the like under publicly known conditions.
• the obtained steel piece is heated and hot-rolled to obtain a hot-rolled steel sheet.
• an iron scale (hot-roll scale) is formed on the surface of the steel sheet.
• Hot rolling conditions are not particularly limited, and may be appropriately set within a publicly known condition range according to characteristics of the steel sheet to be required.
• the hot-rolled steel sheet obtained in the hot rolling step is coiled in a coil shape.
• a condition such as a coiling temperature, is not particularly limited.
• the hot-rolled steel sheet on which the hot-roll scale is formed on the surface is subjected to uncoiling and light rolling reduction.
• a light rolling reduction having a rolling reduction of 0.2% or more scale cracks described above can be introduced.
• the upper limit of the rolling reduction is not particularly limited. However, since the scale may be peeled off in a case where the rolling reduction is 10.0% or more, it is not preferable that the rolling reduction is 10.0% or more.
• the rolling reduction is preferably less than 5.0%, less than 2.0%, or less than 1.0%.
• shot blasting may be performed on the hot-rolled steel sheet together with or instead of light rolling reduction.
• a steel sheet which includes a base steel sheet having a predetermined chemical composition and an iron scale formed on the surface of the base steel sheet and in which the iron scale includes cracks at a predetermined density, is obtained.
• a manufacturing method for the steel member according to the present embodiment is not limited, and the steel member can be manufactured by using, for example, a manufacturing method including the following steps for the above-described steel sheet.
• box annealing is performed on the steel sheet according to the present embodiment (the steel sheet in which the hot-roll scale including cracks is formed on the surface thereof) in a coiled state without the removal of a scale (in a so-called mill scale state).
• an annealing atmosphere is set to an inert gas atmosphere (an N 2 atmosphere, an H 2 atmosphere, or the like), and the steel sheet is annealed at a temperature of 650°C to 950°C for 4 to 30 hours.
• an annealing temperature is lower than 650°C or an annealing time is shorter than 4 hours, decarburization does not proceed sufficiently.
• an annealing temperature is higher than 950°C or an annealing time is longer than 30 hours, the reduction reaction of the scale is completed and the supply of C from the inside of the steel sheet to the surface layer continues thereafter. Therefore, decarburization becomes shallow.
• a hot-rolled steel sheet to be supplied to the hot-rolled sheet annealing step has a sheet thickness of 9 mm or less, a sheet width of 2,100 mm or less, an outer diameter of 2,000 mm or less in the case of a coil, and a weight of 30 tons or less per coil.
• pickling is performed on the hot-rolled steel sheet subjected to the hot-rolled sheet annealing step.
• Publicly known hydrochloric acid or sulfuric acid may be used as pickling liquid.
• shot blasting may be performed before pickling to promote the mechanical peeling of the scale. For example, #60 may be applied as a shot size number.
• the pickling step may be omitted.
• cold rolling is performed on the steel sheet after the pickling step.
• a rolling reduction is not particularly limited. However, from the viewpoint of ensuring good flatness, it is preferable that a cumulative rolling reduction in the cold rolling is set to 30% or more. On the other hand, in order to avoid an excessive rolling force, it is preferable that a cumulative rolling reduction in the cold rolling is set to 80% or less.
• the cold rolling step may be omitted.
• annealing is performed on the cold-rolled sheet.
• the annealing is performed in a temperature range of 700°C to 950°C under a moist hydrogen atmosphere in which a dew point has become high due to humidification as an annealing atmosphere.
• the steel sheet in a two-stage heating furnace including a direct fire burner and a radiant tube, is heated to a temperature of 560°C to 650°C at an air-fuel ratio of 0.9 to 1.2 and is then heated to a temperature of 700°C to 950°C under an atmosphere in which an oxygen potential is -1.5 or more, a hydrogen concentration is in a range of 1 mass% to 20 mass%, and a dew point is in a range of - 10°C to +30°C. It is preferable that the steel sheet is retained at a temperature of 700°C or higher for 30 seconds or longer under an atmosphere in which hydrogen is contained and a dew point is high.
• the annealing step may be omitted or may be performed under conditions other than the above-described conditions.
• a coating is formed on the surface of the steel sheet to obtain a coated steel sheet.
• a method for forming the coating is not particularly limited, and a hot-dip plating method, an electro plating method, a vacuum vapor deposition method, a cladding method, a thermal spraying method, and the like can be used.
• the hot-dip plating method is the most popular in the industry.
• Examples of the coating include Al-based plating containing Al, Zn-based plating containing Zn, and the like.
• the plating step may be omitted.
• Fe is often contained in a plating bath as an impurity in addition to Al.
• Si, Ni, Mg, Ti, Zn, Sb, Sn, Cu, Co, In, Bi, Ca, mischmetal, and the like may be contained in the plating bath in addition to the above-described elements as long as 70 mass% or more of Al is contained.
• the steel sheet after the annealing step may be cooled to room temperature, then heated again, and subjected to plating, or may be cooled to a temperature of 650°C to 750°C near the temperature of the plating bath (that is, not cooled to room temperature) after being retained in the annealing step and then subjected to hot-dip plating.
• Pretreatment and post-treatment of the plating are not particularly limited, and precoating, solvent coating, alloying treatment, temper rolling, and the like can be performed.
• precoating, solvent coating, alloying treatment, temper rolling, and the like can be performed.
• retaining the steel sheet at 450°C to 800°C can also be applied as the alloying treatment.
• temper rolling is useful for shape adjustment and the like, and can achieve, for example, a rolling reduction of 0.1% to 0.5%.
• heat treatment is performed on the steel sheet, which has been subjected to the above-described steps and has a predetermined chemical composition, to obtain a steel member.
• the heat treatment is performed under conditions in which, for example, a steel sheet obtained by a method described above is heated to a temperature of Ac3 point to (Ac3 point + 300)°C at an average temperature rising rate of 1.0 to 1,000 °C/s and is cooled to Ms point or lower at an average cooling rate equal to or higher than an upper critical cooling rate.
• the productivity of the heat treatment deteriorates. For this reason, it is not preferable that the temperature rising rate is lower than 1.0 °C/s.
• the temperature rising rate is higher than 1,000 °C/s, a duplex grain structure is formed and bendability deteriorates. For this reason, it is not preferable that the temperature rising rate is higher than 1,000 °C/s.
• a heat treatment temperature is lower than Ac3 point (°C)
• ferrite remains after cooling and the strength is insufficient. For this reason, it is not preferable that a heat treatment temperature is lower than the Ac3 point (°C).
• a heat treatment temperature is higher than (Ac3 point + 300)°C
• a structure becomes coarse and hydrogen embrittlement resistance deteriorates. For this reason, it is not preferable that a heat treatment temperature is higher than (Ac3 point + 300)°C.
• the upper critical cooling rate is a minimum cooling rate at which austenite is supercooled to generate martensite without causing the precipitation of ferrite or pearlite in the structure. In a case where cooling is performed at a cooling rate lower than the upper critical cooling rate, ferrite or pearlite is generated and strength is insufficient.
• the steel sheet may be retained for 1 to 300 seconds within a range of the heating temperature ⁇ 10°C.
• tempering treatment may be performed in a temperature range of about 100°C to 600°C in order to adjust the strength of a steel member.
• Strip-shaped test pieces having a width of 30 mm and a length of 200 mm are cut out from the steel sheet according to the present embodiment, are heated to a temperature of 1,000°C at a temperature rising rate of 10 °C/s in a nitrogen atmosphere, are retained at that temperature for five minutes, and are then cooled to room temperature at various cooling rates.
• the cooling rates are set at intervals of 10 °C/s (here, 1 °C/s is followed by 10 °C/s) from 1 °C/s to 100 °C/s.
• the thermal expansion changes of the test piece, which is being heated and cooled are measured to measure Ac3 point and Ms point.
• the minimum cooling rate at which the precipitation of ferrite does not occur in the respective test pieces cooled at the above-described cooling rates is defined as the upper critical cooling rate.
• Ms point obtained from a thermal expansion change in a case where the test piece is cooled at a cooling rate equal to or higher than the upper critical cooling rate is used as Ms point of the steel member.
• hot forming such as hot stamping
• Ms point a temperature equal to or lower than Ms point
• a cooling rate equal to or higher than the upper critical cooling rate
• the hot forming include bending, drawing, stretching, hole expansion, flange forming, and the like.
• the present invention may be applied to a forming method other than press forming, for example, roll forming. As long as the thermal history is as to described above, hot forming may be repeatedly performed. Furthermore, the series of heat treatment may be repeated a plurality of times.
• the steel member according to the present embodiment includes both a formed body that is a steel sheet subjected to hot forming and a flat sheet that is subjected to only heat treatment.
• hot forming or heat treatment may be performed on a part of a steel sheet, which serves as a material, to obtain a steel member, which includes regions having different strengths, as the steel member according to the present embodiment.
• the series of heat treatment can be performed by any method, and, for example, heating may be performed by high-frequency heating, energization heating, infrared heating, or furnace heating. Further, cooling may also be performed by water cooling, die cooling, or the like. City gas or nitrogen gas may be used for an atmosphere in a heating furnace in addition to the air. Furthermore, a dew point in the heating furnace may be controlled in order to suppress the generation of hydrogen during the heat treatment.
• hot-rolled steel sheets having a thickness of 3.2 mm and a sheet width of 1,000 mm
• the hot-rolled steel sheets were coiled at a temperature of 800°C or lower to obtain hot-rolled coils having an outer diameter of 1,700 mm and a weight of 14 tons.
• Steel sheets (blanks) having a predetermined size were cut out from the obtained coils having been subjected to light rolling reduction and were observed with SEM in the above-described manner, and the crack density of the scale of each steel sheet was evaluated.
• the measured chemical compositions were the same as the chemical compositions of the slabs.
• steel sheets each of which includes a base steel sheet having a predetermined chemical composition and a scale having a predetermined crack density, were obtained as steel sheets B1 to B21 satisfying the ranges of the present invention.
• results in which Comparative Examples b1 to b15 not satisfying the ranges of the present invention did not satisfy the chemical composition or the crack density of the scale were obtained.
• Hot-rolled sheet annealing was performed on the steel sheets shown in Table 2 under conditions shown in Table 3A.
• pickling and cold rolling were performed on some steel sheets (C2 to C30 and c2 to c16) among the steel sheets subjected to the hot-rolled sheet annealing to obtain cold-rolled steel sheets having a thickness of 1.6 mm.
• hot-dip A1 plating or hot-dip Zn plating was performed on some (C4 to C22, C24 to C30, and c5 to c16) of the cold-rolled steel sheets.
• Heat treatment was performed under the conditions shown in Table 3A on the hot-rolled steel sheets that were subjected to the hot-rolled sheet annealing, the cold-rolled steel sheets that were subjected to pickling, cold rolling, and annealing, or the cold-rolled steel sheets that were subjected to pickling, cold rolling, and annealing and subjected to plating during annealing (a cooling process after retaining). As a result, steel members were obtained.
• An Fe-Al-based alloy layer containing 70 mass% or more of Fe and Al in total or an Fe-Zn-based alloy layer containing 70 mass% or more of Fe and Zn in total were formed on the surfaces of the steel members, which were obtained in a case where the heat treatment was performed on the plated steel sheets, to have a thickness of 5 to 100 ⁇ m.
• a sample for the observation of a cross section was collected from a 1/4 position of the width (short side) from a width-direction end portion of the steel member, and the Vickers hardness was measured at each of a 1/4-depth position and a 0.1 mm-depth position according to JIS Z 2244-1 :2020.
• a test force was set to 100 gf. The measurement was performed five times at each depth, and an average value of measured values was used as hardness at that position.
• a tensile test was performed according to the regulation of ASTM Standard E8. After a soaked portion of the steel member is ground to a thickness of 1.2 mm, a half-sized sheet-shaped test piece (the length of a parallel portion: 32 mm, a sheet width of the parallel portion: 6.25 mm) of ASTM standard E8 was collected such that a test direction was parallel to a rolling direction.
• a strain gauge (gauge length: 5 mm, for example, FLAB-5 manufactured by Tokyo Measuring Instruments Laboratory Co., Ltd.) was attached to the center of the parallel portion of the test piece in a width direction and a length direction, and a room temperature-tensile test was performed at a strain rate of 3 mm/min to measure tensile strength (maximum strength).
• test piece was evaluated to have high strength in a case where the test piece had a tensile strength of 1,000 MPa or more.
• a bending test piece having a length of 60 mm in a direction parallel to the rolling direction and a length of 30 mm in a direction perpendicular to the rolling direction was collected from a soaked portion of the steel member, and a bending test was performed for this test piece according to the regulation of DA238-100.
• a bending angle has a correlation with strength. Accordingly, in the present example, the test piece was evaluated as more excellent in bendability than that in the related art in each tensile strength, specifically, in a case where the test piece had a bending angle of more than 70 degrees at a tensile strength of less than 1,500 MPa, in a case where the test piece had a bending angle of more than 55 degrees at a tensile strength of 1,500 MPa or more and less than 2,100 MPa, and in a case where the test piece had a bending angle of more than 45 degrees at a tensile strength of 2,100 MPa or more.
• a four-point bending test was performed for a test piece, and hydrogen embrittlement resistance was evaluated on the basis of the amount Hc of hydrogen that could be stored in the test piece until a limit where no cracking occurred. Specifically, a strip-shaped test piece having a width of 8 mm and a length of 68 mm was cut out as a test piece while avoiding an end portion of the steel member to be evaluated.
• the test piece was bent along the longitudinal direction of the test piece with a four-point support jig until strain corresponding to 3/5 of the tensile strength measured in the above-described tensile test for the steel member to be evaluated.
• the amounts of stored hydrogen were different
• the presence or absence of cracking was observed and the maximum (limit) amount Hc of hydrogen (mass ppm) at which no cracking occurred was obtained.
• the amount of hydrogen storage was changed by changing a dew point in the furnace in the heat treatment step and whether or not cracking occurred within 72 hours after a four-point bending test was observed.
• a plurality of test pieces were immersed in ammonium thiocyanate solutions having different concentrations after a four-point bending test to store hydrogen, and whether or not cracking occurred within 72 hours after the immersion was observed.
• the temperature of hydrogen stored in the steel member was raised at 100 °C/hr in thermal hydrogen analysis, and the amount of diffusible hydrogen released up to 250°C was defined as the amount of hydrogen contained in the steel member.
• the steel member was evaluated as excellent in hydrogen embrittlement resistance in a case where the amount Hc of hydrogen was 0.7 mass ppm or more in a steel member having a tensile strength equal to or higher than 1,500 and less than 2,000 MPa, in a case where the amount Hc of hydrogen was 0.5 mass ppm or more in a steel member having a tensile strength equal to or higher than 2,000 and less than 2,500 MPa, and in a case where the amount Hc of hydrogen was 0.3 mass ppm or more in a steel member having a tensile strength of 2,500 MPa or more.
• the invention examples C1 to C30 satisfying the ranges of the present invention had good results in both a structure and characteristics, and had high strength, excellent bendability, and excellent hydrogen embrittlement resistance.
• Comparative examples c1 to c16 not satisfying the ranges of the present invention are insufficient in terms of a chemical composition and the formation of a structure and were inferior in at least one of strength, bendability, or hydrogen embrittlement resistance, or any combination thereof were inferior.
• the steel member according to the present invention is particularly suitable to be used as a frame component of a vehicle.

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• 2023
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