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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
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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
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  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/008—Martensite

Definitions

• Precipitation-hardening martensitic stainless steels such as SUS630 that are capable of achieving high strength by precipitation of an ⁇ -Cu phase through the addition of Cu are used for press plates used to manufacture multilayer printed wiring boards and other laminates by press forming.
• Patent Document 1 discloses a precipitation-hardening martensitic stainless steel that maintains high strength and toughness by controlling the ⁇ -Cu phase as well as an Ni 16 Ti 6 Si 7 -based intermetallic compound phase (G phase) through the addition of Ti and Si so as to distribute these phases in crystal grains.
• G phase that is, Ni 16 X 6 Si 7
• the component composition obtained by adjusting an amount of Nb, in particular, is clearly defined.
• Patent Document 2 discloses a manufacturing method of a press plate and a steel material thereof.
• the method includes subjecting an austenitic stainless steel having a high austenite ratio and containing 1.0 to 4.0 mass% of Si to aging treatment after necessary shape correction, and increasing a surface hardness to 430 HV or higher by strain aging.
• Precipitation-hardening martensitic stainless steels such as SUS630 are solution-treated at 1000°C or higher and then aging-treated at a temperature around 500°C to obtain a predetermined hardness (strength). That is, a precipitation-hardening martensitic stainless steel for a press plate requires a hardness that allows shape correction after solid solution treatment on the one hand, and the capability of being increased in strength to a sufficient hardness by subsequent aging treatment on the other hand.
• the present invention was made in view of circumstances such as described above, and an object thereof is to provide a precipitation-hardening martensitic stainless steel and a solid solution treatment material and an aging treatment material thereof capable of realizing a press plate having high hardness and excellent flatness.
• a precipitation-hardening martensitic stainless steel for a press plate in the present invention has a component composition including, by mass%, C: 0.01 to 0.07 %, Si: 0.05 to 2.25 %, Mn: 0.5 to 5.0 %, Ni: 4.5 to 10.0 %, Cr: 10.0 to 17.0 %, Mo: 0.1 to 1.50 %, Cu: 0.30 to 5.0 %, Al: 0.001 to 0.100 %, N: 0.001 to 0.020 %, Ti: 0.15 to 0.55 %, Nb: 0.15 to 0.45 %, Co: 0.03 to 0.80 %, P: 0.04% or less, and S: 0.0020% or less, with the remainder consisting of Fe and inevitable impurities.
• a solid solution treatment material of a precipitation-hardening martensitic stainless steel for a press plate has a two-phase structure of a martensite phase and a residual austenite phase, the residual austenite phase being in an amount of 0.2 to 8 vol%, a hardness of less than 380 HV, and a component composition including, by mass%, C: 0.01 to 0.07 %, Si: 0.05 to 2.25 %, Mn: 0.5 to 5.0 %, Ni: 4.5 to 10.0 %, Cr: 10.0 to 17.0 %, Mo: 0.1 to 1.50 %, Cu: 0.30 to 5.0 %, Al: 0.001 to 0.100 %, N: 0.001 to 0.020 %, Ti: 0.15 to 0.55 %, Nb: 0.15 to 0.45 %, Co: 0.03 to 0.80 %, P: 0.04% or less, and S: 0.0020% or less, with the remainder consisting of Fe and inevitable impurities.
• a precipitation-hardening martensitic stainless steel for a press plate requires a relatively low hardness after solid solution treatment on the one hand, and the capability of obtaining a sufficient hardness by aging treatment on the other hand.
• the inventors conducted extensive studies on the effect of the addition of Co along with Nb on martensitic stainless steels having such a hardness, and found that the desired hardness can be obtained both after solid solution treatment and after aging treatment by adjusting the components of the steel so that an austenite phase remains after the solid solution treatment.
• the component composition of such a steel may be as follows.
• the H value is in the range of 5.0 or greater described above, preferably within a range of 6.0 to 15.0, and more preferably within a range of 7.0 to 12.0. It should be noted that preferably an upper limit of the H value is set so as to prevent embrittlement of the steel material during manufacture.
• the strip steel material that is a solid solution treatment material is corrected in shape.
• a tension leveler can be suitably used.
• the hardness of the solid solution treatment material is less than 380 HV as described above, making it possible to make the burden on the manufacturing process for shape correction relatively small. For example, excellent flatness can be obtained by shape correction with a tension leveler alone.
• the flatnesses of the solid solution treatment materials after shape correction were all "acceptable” or better, and the hardnesses of the aging treatment materials were 450 HV or greater.
• the MA values described above were within the range of 1.0 to 4.5, the H values were in the range of 5.0 or greater, the residual y amounts of the solid solution treatment materials were within the range of 0.2 to 8.0 vol%, and the hardnesses were 380 HV or less, with the exception of Example 1.
• the hardness of the solid solution treatment material was relatively high at 386 HV, but the flatness was "acceptable.”
• Comparative Example 4 the amount of each element in the component composition was about the same as in the examples, but the H value was low. As a result, the hardness of the aging treatment material was low. Although the hardness of the solid solution treatment material was the same as in the examples, conceivably the precipitation hardenability in the aging treatment was low due to the low H value.
• Comparative Example 5 the content of Si was low. As a result, the hardness of the aging treatment material was low. Although the hardness was the same as in the examples at the time of the solid solution treatment material, conceivably sufficient hardness could not be obtained through the aging treatment due to the low Si content.
• Comparative Example 6 contained almost no Co, but the content and the MA value in other component compositions were substantially identical to those in Example 7. Comparative Example 6, despite having almost the same solid solution hardness as in Example 7 as well, had a lower value in aging hardness. That is, it was found that the Co content in Example 7 was an amount having almost no effect on the solid solution hardness, but significantly contributed to the aging hardness. Relationships between Comparative Example 7 and Example 11, and between Comparative Example 8 and Example 2 are also similar.
• C is an element that stabilizes the austenite phase while contributing to a rise in mechanical strength through solid solution treatment, suppresses the generation of ⁇ ferrite at high temperatures, and increases the residual austenite phase after solid solution treatment. Therefore, excessive content reduces the precipitation hardenability by the aging treatment. Considering these factors, C is within a range of 0.01 to 0.07 %, preferably within a range of 0.02 to 0.06 %, and more preferably within a range of 0.03 to 0.05 % by mass%.
• Si is added for deoxidization and is an important element for generating the G phase and obtaining mechanical strength through aging treatment.
• Si is a ferritizer, and excessive content leads to an increase in a ⁇ -ferrite phase, which deteriorates hot workability.
• Si is within a range of 0.05 to 2.25 %, preferably within a range of 1.05 to 2.05 %, and more preferably within a range of 1.35 to 1.85 % by mass%.
• Ni is an element that stabilizes the austenite phase, and is an important element that contributes to the generation of residual austenite while suppressing the generation of the ⁇ -ferrite phase, and further generates the G phase and contributes to a rise in mechanical strength through aging treatment. On the other hand, excessive content causes a significant amount of the austenite phase to remain, which reduces the mechanical strength after aging treatment. Considering these factors, Ni is within a range of 4.5 to 10.0 %, preferably within a range of 5.5 to 9.0 %, and more preferably within a range of 6.5 to 8.0 % by mass%.
• Cr is an element required to ensure corrosion resistance as a stainless steel and also contributes to the generation of residual austenite. On the other hand, excessive content causes generation of the ⁇ -ferrite phase, resulting in a decrease in hot workability. Considering these factors, Cr is within a range of 10.0 to 17.0 %, preferably within a range of 12.0 to 16.0 %, and more preferably within a range of 13.0 to 15.0 % by mass%.
• Mo is an element required to ensure corrosion resistance as a stainless steel and also contributes to the generation of residual austenite. On the other hand, excessive content causes generation of the ⁇ -ferrite phase, resulting in a decrease in hot workability. Considering these factors, Mo is within a range of 0.1 to 1.50 %, preferably within a range of 0.6 to 1.20 %, and more preferably within a range of 0.7 to 1.00 % by mass%.
• Cu is an element that stabilizes the austenite phase, and is an important element that contributes to the generation of residual austenite while suppressing the generation of the ⁇ -ferrite phase, and further generates the ⁇ -Cu phase and contributes to a rise in mechanical strength through aging treatment. On the other hand, excessive content causes a significant amount of the austenite phase to remain, which reduces the mechanical strength after aging treatment. Considering these factors, Cu is within a range of 0.30 to 5.0 %, preferably within a range of 0.40 to 3.0 %, and more preferably within a range of 0.50 to 1.5 % by mass%.
• Al is added for deoxidization and is an effective element for stably containing Nb and Ti, which are readily oxidized, resulting in a low adding yield in molten metal.
• Al increases the martensitic transformation initiation temperature (Ms point), and therefore excessive content inhibits the generation of residual austenite. Further, excessive content causes an increase in the ⁇ -ferrite phase, deteriorating hot workability.
• Al is within a range of 0.001 to 0.100 %, preferably within a range of 0.002 to 0.060 %, and more preferably within a range of 0.002 to 0.020 % by mass%.
• N is an element that stabilizes the austenite phase and contributes to the generation of residual austenite while suppressing the generation of the ⁇ -ferrite phase.
• excessive content causes a significant amount of the austenite phase to remain, which reduces the mechanical strength after aging treatment. Further, excessive content forms nitrides, mainly with Ti, generating a fracture origin of the press plate. Furthermore, excessive content may change the Ms point and cause slab cracking.
• N is within a range of 0.001 to 0.020 %, preferably within a range of 0.002 to 0.015 %, and more preferably within a range of 0.003 to 0.010 % by mass%.
• Ti is an important element that generates the G phase through aging treatment, and contributes to a rise in mechanical strength.
• excessive content causes generation of the ⁇ -ferrite phase, resulting in a decrease in hot workability. Further, excessive content may change the Ms point and cause slab cracking.
• Ti is within a range of 0.15 to 0.55 %, preferably within a range of 0.20 to 0.50 %, and more preferably within a range of 0.25 to 0.45 % by mass%.
• Nb is an important element that generates the G phase through aging treatment, and contributes to a rise in mechanical strength.
• excessive content causes generation of the ⁇ -ferrite phase, resulting in a decrease in hot workability. Further, excessive content may change the Ms point and cause slab cracking.
• Nb is within a range of 0.15 to 0.45 %, preferably within a range of 0.20 to 0.40 %, and more preferably within a range of 0.25 to 0.35 % by mass%.
• Co stabilizes the passivation film to increase corrosion resistance as a stainless steel and, as a stabilizing element of the austenite phase, lowers the Ms point and affects the generation of residual austenite. Further, Co increases the mechanical strength of the base material (matrix) by improving quenching performance, thereby ensuring the mechanical strength of the aging treatment material. On the other hand, excessive content causes excessive generation of the residual austenite in the solid solution treatment, making it no longer possible to obtain sufficient hardness even through the aging treatment, and reduces toughness. It should be noted that, after the aging treatment, in the plate materials fabricated from the steel in Examples 1 to 11 described above, it was observed that Co was included abundantly in the ⁇ -Cu phase and contained minimally in the G phase.
• Co affects the precipitation forms of the ⁇ -Cu phase and the G phase. Considering these factors, Co is within a range of 0.03 to 0.80 %, preferably within a range of 0.05 to 0.50 %, and more preferably within a range of 0.06 to 0.30 % by mass%.
• P is an element inevitably mixed in steel and is segregated at crystal grain boundaries and concentrated in the final solidification area during casting, which promotes solidification cracking and also results in a decrease in hot workability. Therefore, the content is desirably reduced to the extent possible, but excessive refining increases manufacturing costs. Considering these factors, P is 0.04% or less, preferably 0.030% or less, and more preferably 0.025% or less by mass%.
• S is an element inevitably mixed in steel and forms compounds with Mn, which become inclusions that reduce corrosion resistance. Furthermore, S segregates at grain boundaries and decreases hot workability. Therefore, the content is desirably reduced to the extent possible, but excessive refining increases manufacturing costs. Considering these factors, S is 0.0020% or less, preferably 0.0015% or less, and more preferably 0.0010% or less by mass%.
• an embodiment can be described as follows: To provide a precipitation-hardening martensitic stainless steel and a solid solution treatment material and an aging treatment material thereof capable of realizing a press plate having high hardness and excellent flatness.
• a solid solution treatment material has a two-phase structure of a martensite phase and a residual austenite phase, the residual austenite phase being in an amount of 0.2 to 8 vol%, and a hardness of less than 380 HV.
• the aging treatment material has a hardness of 450 HV or greater.

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