US20050173031A1 - High strength steel sheet and method for manufacturing same - Google Patents

High strength steel sheet and method for manufacturing same Download PDF

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Publication number
US20050173031A1
US20050173031A1 US11/040,994 US4099405A US2005173031A1 US 20050173031 A1 US20050173031 A1 US 20050173031A1 US 4099405 A US4099405 A US 4099405A US 2005173031 A1 US2005173031 A1 US 2005173031A1
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Prior art keywords
steel sheet
less
cooling
high strength
temperature
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US11/040,994
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English (en)
Inventor
Kohei Hasegawa
Saiji Matsuoka
Yasuhide Ishiguro
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JFE Steel Corp
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JFE Steel Corp
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Assigned to JFE STEEL CORPORATION reassignment JFE STEEL CORPORATION ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: ISHIGURO, YASUHIDE, MATSUOKA, SAIJI, HASEGAWA, KOHEI
Publication of US20050173031A1 publication Critical patent/US20050173031A1/en
Priority to US12/386,518 priority Critical patent/US20090223607A1/en
Abandoned legal-status Critical Current

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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • 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
    • 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
    • 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
    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite
    • 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
    • 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

Definitions

  • the present invention relates to a high strength steel sheet most suitable for automobile body, reinforcements, wheels, and chassis parts and further for all kinds of machine structural parts, and to a method for manufacturing same.
  • a known response to the requirement is dual phase steel sheet structured by ferrite and martensite as the main phases, (the steel has several names of Dual Phase steel, DP steel, and composite structural steel). Owing to low yield ratio, (hereinafter referred to simply as YR), and high elongation, the dual phase steel sheet is superior in the press-formability such as draw-forming property and surface precision after press-forming (shape accuracy), thus the dual phase steel sheet drew attention as an automobile material, and the development thereof has been enhanced.
  • the dual phase micro-structure in a hot-rolled steel sheet is achieved during the cooling step after hot-rolling by transformation to polygonal ferrite much enough to enrich a solute element in the residual austenite, thus increasing in the quench-hardenability due to the transformation to martensite.
  • the technology emphasizes the control of precipitated amount of polygonal ferrite to form the micro-structure and to improve the mechanical characteristics. Accordingly, various development studies on the control of polygonal ferrite precipitation have been given.
  • Patent Documents 1 through 11 disclose methods combining with what is called the two-stage cooling process as a steel composition design.
  • the methods include the steps of: adding large amount of ferrite-stabilizing elements represented by Si, (and including P, Al, and the like); stopping cooling, in the cooling step after hot-rolling, at near A 1 temperature where the ferrite precipitation is accelerated; holding the temperature for about 10 seconds; and applying cooling again.
  • Patent Documents 12 through 15 disclose manufacturing methods to obtain desired steel sheet without adding the ferrite-stabilizing element. That is, the methods adopt a cooling-control pattern different from conventional method, for example, dividing the rapid cooling after finish-rolling into two stages.
  • Patent Documents 16 through 18 disclose methods to apply rapid cooling immediately after the hot-rolling.
  • Patent Document 16 adopts the above-method for a low Si steel.
  • Patent Documents 1 through 11 need to add excess Si, P, and Al, though they show favorable mechanical characteristics, thus they have problems of degradation in surface property caused by red-scale formation, degradation in coatability, and degradation in weldability. Consequently, their applications are limited.
  • the steel sheets manufactured by the methods according to Patent Documents 12 through 15 contain small amount of Si, P, and Al so that the cooling method in related art cannot fully progress the transformation from austenite to ferrite on the runout table after hot-rolling.
  • the volume percentage of polygonal ferrite becomes small, the volume percentage of martensite becomes large, and the polygonal ferrite grains become coarse by the same reason, which fails to attain adequate metallic micro-structure which is specified by the present invention.
  • the manufactured steel sheet shows higher than 0.6 of YR in the mechanical characteristics, which is an inferior characteristic.
  • YR is required to be 0.6 or less.
  • the method for manufacturing hot-rolled dual phase steel sheet according to the related art adopts either the addition of ferrite-stabilizing element (Si, P, Al, or the like) sacrificing the surface property and other features or the sacrification of mechanical characteristics.
  • ferrite-stabilizing element Si, P, Al, or the like
  • Patent Documents 16 and 17 do not consider YR and the metallic micro-structure to attain the YR.
  • Patent Document 18 is a technology to manufacture a high Si steel, the surface property of the steel sheet becomes poor. To improve the surface property, Si may be decreased. If, however, the Si content is decreased, no adequate metallic micro-structure is obtained, and the YR characteristic becomes poor. Both the YR and the surface property cannot be satisfied at a time.
  • the inventors of the present invention found a phenomenon which significantly enhances the fine ferrite formation compared with conventional two-stage cooling process, even without adding excess ferrite-stabilization elements, by beginning the ultra-rapid cooling at 150° C./s or higher cooling rate within 2 seconds after the hot-rolling, followed by holding the temperatures between 750° C. and 600° C. for a specified period of time.
  • the inventors of the present invention applied the finding to the manufacture of dual phase hot-rolled high strength steel sheet, and have perfected the present invention.
  • the present invention provides a high strength steel sheet which consists essentially of 0.05 to 0.15% C, 0.5% or less Si, 1.00 to 2.00% Mn, 0.09% or less P, 0.01% or less S, 0.005% or less N, 0.01 to 0.1% Sol.Al, by mass, and the balance being Fe and inevitable impurities; and 60% or more of polygonal ferrite by volume, and 5 to 30% of martensite by volume.
  • the polygonal ferrite is preferably from 60 to 95% by volume.
  • the polygonal ferrite preferably has a mean grain size of 5 to 10 ⁇ m.
  • the martensite is preferably from 10 to 20% by volume.
  • the high strength steel sheet preferably contains 0.01 to 0.5% Si by mass.
  • the Si content is more preferably 0.25% or less by mass. Since Si has an effect to increase the strength, the Si content is preferably 0.01% or more.
  • the P content is preferably from 0.020 to 0.06% by mass.
  • the high strength steel sheet may further contain at least one element selected from the group consisting of 0.01 to 0.3% Mo, 0.001 to 0.05% Nb, 0.001 to 0.1% Ti, 0.0003 to 0.002% B, and 0.05 to 0.49% Cr, by mass.
  • the high strength steel sheet preferably has a yield ratio of 0.6 or lower. If the yield ratio exceeds 0.6, the shape accuracy deteriorates during press-forming.
  • the present invention also provides a method for manufacturing high strength steel sheet comprising the steps of: casting a slab consisting essentially of 0.05 to 0.15% C, 0.5% or less Si, 1.00 to 2.00% Mn, 0.09% or less P, 0.01% or less S, 0.005% or less N, 0.01 to 0.1% Sol.Al, by mass, and the balance being Fe and inevitable impurities; hot-rolling the cast slab, directly or heating thereof, at a temperature of Ar3 point or more to form a hot-rolled steel sheet; a primary cooling step of cooling the hot-rolled steel sheet beginning cooling thereof within 2 seconds after completing the hot-rolling to a temperature of from 750° C. to 600° C.
  • the slab may further contain at least one element selected from the group consisting of 0.01 to 0.3% Mo, 0.001 to 0.05% Nb, 0.001 to 0.1% Ti, 0.0003 to 0.002% B, and 0.05 to 0.49% Cr, by mass.
  • the cooling rate in the primary cooling step is preferably in a range from 150 to 1000° C./s, and more preferably from 200 to 700° C./s.
  • the cooling rate in the secondary cooling step is preferably in a range from 20 to 1000° C./s.
  • the coiling temperature is preferably in a range from 0° C. to 400° C.
  • the percentage for the ingredients of the steel, given in the description, is % by mass.
  • high strength steel sheet referred to herein signifies a steel sheet having more than 590 MPa of tensile strength (TS), which TS values are suitable for the machine structural parts.
  • TS tensile strength
  • the present invention provides high strength steel sheet having excellent formability and surface property.
  • the steel sheet manufactured by the present invention has low YR (0.6 or less) with high strength, high ductility, excellent press-formability, excellent surface property, and excellent spot weldability, thus the steel sheet can readily be applied to the automobile parts and machine structural parts. Since the high strength steel sheet can be manufactured by the conventional process for manufacturing mild steel sheet, and since the attained performance thereof is favorable without adding spec-ial elements, the manufacturing cost can be decreased. Accordingly, the high strength steel sheet according to the present invention is highly expected in practical uses in the future, and is expected to contribute to the conservation of global environment by the weight reduction of automobile and to the social development through the improvement of safety of automobile.
  • FIG. 1 is a graph showing the relation between the yield ratio (YR) and the primary cooling rate.
  • the high strength steel sheet according to the present invention specifies the composition as below, specifies the volume percentage of polygonal ferrite to 60% or more, specifies the volume percentage of martensite to a range from 5 to 30%, and specifies the mean grain size of polygonal ferrite to a range from 5 to 10 ⁇ m. These specifications are the most important conditions of the present invention. With the composition and micro-structure specified above, the high strength steel sheet having excellent formability and surface property can be obtained.
  • the high strength steel sheet can be manufactured by the sequential steps of: hot-rolling the steel at Ar3 pint or higher temperature; beginning cooling of the steel sheet within 2 seconds after the completion of hot-rolling; cooling the steel sheet to temperatures between 750° C. and 600° C.
  • Carbon is an important element to strengthen the martensitic phase.
  • the C content needs to be 0.05% or more. If, however, the C content exceeds 0.15%, austenite stabilizes, and the dual phase formation becomes difficult, which degrades the ductility. Accordingly, the C content is specified to a range from 0.05% to 0.15%. Regarding the spot weldability, the C content below 0.07% may degrade the tensile shear strength. If the C content exceeds 0.10%, the cross tension strength may decrease. Therefore, the C content is preferably in a range from 0.07 to 0.10%.
  • the P content exceeds 0.09%, the elongation is significantly degraded. Accordingly, the P content is specified to 0.09% or less. If the P content exceeds 0.06%, the toughness at welded section degrades to decrease the joint strength in some cases. Therefore, the P content is preferably 0.06% or less. Furthermore, the P content of 0.020% or more enhances the formation of polygonal ferrite to decrease YR. Thus the P content is preferably 0.020% or more.
  • Nitrogen is an impurity in the crude steel and degrades the formability of steel sheet as the base material. Accordingly, it is preferred to remove or reduce N in the steel making process as far as possible. Since, however, excess reduction of N increases the refining cost, the N content is specified to 0.005% or less, which level brings the N substantially harmless.
  • Aluminum is added for deoxidization and for precipitating N as AlN. If the Al content is less than 0.01%, the effect of deoxidization and denitrification becomes in sufficient. If the Al content exceeds 0.1%, the effect of Al addition saturates, which is uneconomical. Consequently, the Sol.Al content is specified to a range from 0.01 to 0.1%.
  • the volume percentage of polygonal ferrite is specified to 60% or more.
  • the volume percentage of polygonal ferrite is a critical condition to achieve the low YR characteristic which is a feature of the present invention. To attain 0.6 or lower YR, the volume percentage of polygonal ferrite is required to become 60% or more.
  • the polygonal ferrite is found in the ferritic phase, and is distinguished from the acicular ferrite in the morphology, and is limited to the one having 5 or lower ratio of maximum diameter to minimum diameter of the ferritic crystal grain.
  • the method for casting the slab is not specifically limited.
  • hot-rolling may be done directly or may be done after reheating after cooling.
  • the hot-rolling is conducted at Ar3 point or higher temperature. If the hot-rolling is done below the Ar3 point, the hot-rolling proceeds in the dual phase region of ferrite and austenite, which hinders the formation of polygonal ferrite, increases YR, and decreases the ductility.
  • the cooling begins within 2 seconds to cool the steel to a temperature range from 750° C. to 600° C., which is the holding temperature range, at cooling rates of 150° C./s or more.
  • the primary cooling which is given immediately after the hot-rolling is the most important condition to attain the effect of the present invention, (the effect of low YR attained by the enhancement of polygonal ferrite formation)
  • the holding step at temperatures of from 750° C. to 600° C. succeeding to the primary cooling allows the fine transformed polygonal ferrite to be drastically enhanced.
  • the steel After completing the primary cooling, the steel is held to a temperature range from 750° C. to 600° C. for 2 to 15 seconds. If the temperature range for holding the steel is above 750° C., the driving force of ferrite transformation becomes small, and no transformation enhancement effect is attained. If the temperature range therefor is below 600° C., the ferrite transformation which is controlled by the diffusion of Fe atoms delays, and satisfactory polygonal ferrite formation cannot be attained. If the holding time is less than 2 seconds, the ferrite transformation time is not sufficient, which fails to attain the low YR characteristic. If the holding time exceeds 15 seconds, the pearlite formation begins to degrade the mechanical characteristics.
  • the secondary cooling is conducted at cooling rates of 20° C./s or more, and the coiling of the steel sheet is done at temperatures of 400° C. or below.
  • the cooling rate in the secondary cooling is required to be 20° C./s or more to suppress the formation of pearlite and bainite during cooling. If the secondary cooling rate exceeds 1000° C./s, the metallic micro-structure becomes nonuniform within the sheet thickness range, and the mechanical characteristics may degrade. Therefore, the secondary cooling rate is preferably 1000° C./s or less.
  • the coiling temperature is required to be 400° C. or below to prevent the formation of pearlite and bainite after coiling, to form martensite, and to attain the target of 0.6 or lower YR. Furthermore, to prevent the fluctuations of strength within the coil, the coiling temperature is preferably 300° C. or below, and more preferably 200° C. or below. If the coiling temperature becomes below 0° C., the cooling by water becomes difficult so that the coiling temperature is preferably 0° C. or above.
  • a skin pass rolling may further be applied for shape-correction.
  • various surface treatments such as hot-dip galvanization and electro-galvanization may be applied to the high strength steel sheet according to the present invention as the base material.
  • the mechanical characteristics were determined by the test per JIS Z2241 with a JIS No. 5 Tensile Test sample (prepared by cutting the steel sheet lateral to the rolling direction).
  • the surface property was determined by visual observation in terms of presence/absence of red scale.
  • spot weldability spot-welding was given under a condition to form a nugget having the size of (5 x sheet thickness (mm)), and then the peal test using a chisel was applied to break the sheet to observe the fracture mode.
  • Steel No. 1 which is a comparative example had low C content, outside the range of the present invention, so that the hardness of martensite was unsatisfactory, which increased the YR value.
  • Steel Nos. 4 and 5 had the volume percentage of polygonal ferrite or the volume percentage of martensite outside the range of the present invention so that they failed to form favorable dual phase micro-structure and they gave high YR value.
  • Steel No. 9 had large C content outside the range of the present invention so that the ferrite formation delayed to fail in attaining favorable dual phase micro-structure, and resulted in high YR value, as well as degrading the spot weldability.
  • Steel No. 13 had large Si content outside the range of the present invention so that red scale was generated to give poor surface property.
  • Example A 75 10 8 341 620 30.6 0.55
  • Example B 75 10 9 342 610 31.1 0.56
  • Example C 80 10 10 354 610 31.1 0.58 Comparative D 50 10 13 447 630 30.2 0.71 example Comparative E 40 5 15 488 650 29.2 0.75 example
  • Example F 80 10 9 365 640 29.7 0.57
  • Example G 75 10 7 330 600 31.7 0.55
  • Example H 80 10 6 369 670 28.4 0.55 Comparative I 30 15 10 490 680 27.9 0.72 example
  • Symbol D which is a comparative example had a long period between the completion of rolling and the beginning of primary cooling, outside the range of the present invention, thus ferrite was formed irregularly before beginning the cooling, which resulted in unfavorable dual phase micro-structure and high YR value.
  • Symbol E had low primary cooling rate outside the range of the present invention so that ferrite was formed irregularly before beginning the cooling, which resulted in unfavorable dual phase micro-structure and high YR value.
  • Symbol I had high temperature of stopping the primary cooling outside the range of the present invention so that the formation of ferrite during the succeeding holding step became insufficient, which resulted in unfavorable dual phase micro-structure and high YR value.
  • Symbol M had low temperature of stopping the primary cooling outside the range of the present invention so that the formation of ferrite during the succeeding holding step became insufficient, which resulted in unfavorable dual phase micro-structure and high YR value.
  • Symbol N had insufficient holding time after the primary cooling outside the range of the present invention so that the formation of ferrite became insufficient, which resulted in unfavorable dual phase micro-structure and high YR value.
  • Symbol Q had long holding time after the primary cooling outside the range of the present invention so that pearlite was formed during holding step, which resulted in unfavorable dual phase micro-structure and high YR value.
  • FIG. 1 shows the relation between YR and the primary cooling rate for Steel No. 2.
  • the figure shows that favorable characteristics giving low YR value is attained at 150° C./s or higher primary cooling rate, which is the range of the present invention.
  • Symbol D failed to attain favorable result because the time before the primary cooling was 5 seconds, which is outside the range of the present invention.
  • the steel sheet according to the present invention has excellent press-formability and excellent surface property, the steel is also applicable to formed parts which emphasize the appearance.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Physics & Mathematics (AREA)
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  • Crystallography & Structural Chemistry (AREA)
  • Heat Treatment Of Sheet Steel (AREA)
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US11/040,994 2004-01-29 2005-01-20 High strength steel sheet and method for manufacturing same Abandoned US20050173031A1 (en)

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JP2004-020843 2004-01-29
JP2004020843 2004-01-29
JP2004326545A JP4470701B2 (ja) 2004-01-29 2004-11-10 加工性および表面性状に優れた高強度薄鋼板およびその製造方法
JP2004-326545 2004-11-10

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EP (1) EP1559797B1 (de)
JP (1) JP4470701B2 (de)
KR (1) KR100673424B1 (de)
CN (1) CN100439542C (de)
AU (1) AU2005200300C1 (de)
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