Classifications

• • 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
  • 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
• • 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
• • 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
  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur

Definitions

• the present invention relates to a high-strength steel sheet with a tensile strength (TS) of 980 MPa or more, suitable primarily for use in structural parts of automobile bodies and electrical parts, and to a method for manufacturing the same.
• TS tensile strength
• Patent Documents 1 and 2 disclose a technique in which, in the final annealing process, the sheet is cooled to a temperature below the Ms point, then reheated, and the martensite formed during cooling is tempered to achieve high stretch flangeability.
• Patent Document 3 discloses a method in which the steel sheet is tempered after being plated.
• Patent Document 4 discloses a method for producing steel sheets with good bending properties and bendability by plating the steel sheet, cooling it to a temperature below 200°C, and then further tempering it in the temperature range of 100°C to 600°C.
• Patent Document 7 discloses a technique for controlling the yield ratio by controlling the structure of quenched/tempered martensite, etc., and in its examples, it shows high yield ratios of over 85%.
• the area ratio of tempered martensite shown in this patent document is low, at around 40% at most, and the total elongation value is low at 9.9%, which is an issue.
• Patent Document 8 also shows a yield ratio of 85% in its examples, but has the problem of a low total elongation value of 9%.
• Patent No. 5463685 International Publication No. 2009/054539 Japanese Patent Application Publication No. 06-108152 Japanese Patent Application Laid-Open No. 2017-48412 Patent No. 6787525 Patent No. 6705561 Patent No. 6414246 Patent No. 6631760 Patent No. 6787526 International Publication No. 2017/179372
• Patent Document 4 discloses an invention that achieves good elongation properties by forming a surface decarburized layer, and explains that soft ferrite in the surface layer improves bending formability.
• the formation of soft ferrite in the surface layer not only reduces the strength level of the base material, but also promotes the formation of microcracks and voids during processing by increasing the number of microstructural interfaces between ferrite and martensite, which have different mechanical properties. Materials with tensile strengths exceeding 980 MPa are particularly susceptible to hydrogen embrittlement and fracture.
• the inventors have investigated a manufacturing method for improving the strength-ductility balance of high-strength steel plate that contains 70% or more primarily tempered martensite, with the remainder primarily austenite, and has a tensile strength (TS) of 980 MPa or more.
• TS tensile strength
• the present invention was made in light of the above circumstances. Its objective is to provide a high-strength steel plate and manufacturing method that achieves a TS of 980 MPa or more, a yield ratio (YR) of 70% or more, an El of 10,000/TS (MPa)% or more, i.e., El x TS ⁇ 10,000 MPa ⁇ %, while also providing hydrogen embrittlement resistance in the bent portion.
• the inventors conducted extensive research to resolve the above issues.
• the inventors focused on the upstream manufacturing processes, including the chemical composition of the steel, and primarily made improvements to the heat treatment process beyond simple adjustments of temperature and time.
• the gist of the present disclosure is as follows: [1] In mass%, C: 0.090% or more and 0.300% or less, Si: 0.50% or more and 2.50% or less, Mn: 1.8% or more and 4.0% or less, P: 0.100% or less, S: 0.0200% or less, Al: 0.200% or less, N: 0.0200% or less, and O: 0.0100% or less, Cu: 0.005% or more and 0.5% or less, Sn: 0.005% or more and 0.5% or less, and Sb: 0.001% or more and 0.07% or less and below, in a total amount such that 0.005 ⁇ 0.07 is satisfied, where ⁇ is a dimensionless quantity expressed as [Sb]+[Cu]/10+[Sn]/10, and [Sb], [Cu], and [Sn] represent the respective contents (mass%), and are zero when not contained, with the remainder consisting of Fe and unavoidable impurities.
• the steel structure at the 1/4 coil width position and the 1/4 plate thickness position satisfies the following: ferrite has an area fraction of 0% to 5%, retained austenite has a volume fraction of 3% to 20%, tempered martensite has an area fraction of 70% or more, and fresh martensite has an area fraction of 0% to 20%. Furthermore, the steel structure at the 1/4 coil width position and the outermost plate thickness layer satisfies the following: ferrite has an area fraction of 0% to 5%, retained austenite has a volume fraction of 3% to 20%, tempered martensite has an area fraction of 70% or more, and fresh martensite has an area fraction of 0% to 20%.
• a high-strength steel plate in which the proportion of retained austenite having a tensile strength ratio of 2.0 or more to the total retained austenite is 50% or less, a tensile strength TS and a yield strength YS are TS ⁇ 980 MPa and 100 ⁇ YS/TS ⁇ 70, and in the hardness distribution in the plate thickness direction, the relationship between the hardness value H025 at a position 1/4 of the plate thickness and the hardness value H005 at a position 1/20 of the plate thickness is 2.5 ⁇ 100 ⁇ (H025 ⁇ H005)/H025 ⁇ 20.0.
• a method for producing a high-strength steel sheet comprising: removing scale from the steel sheet surface by pickling, and then heating a cold-rolled sheet obtained at a cold rolling reduction of 30% or more to a temperature of Ac3+10°C or higher and 950°C or lower; cooling the sheet at an average cooling rate of 10°C/s or higher and 100
• [7] The method for manufacturing a high-strength steel plate according to [6], wherein the steel is cooled from 400°C to a cooling stop temperature range of 50°C to 300°C at an average cooling rate of 5°C/s to 20°C/s, and then held in the cooling stop temperature range for 0.5 seconds or more.
• [9] The method for producing a high-strength steel sheet according to [6] or [7], further comprising hot-dip galvanizing.
• the present invention makes it possible to obtain a higher strength-ductility balance than ever before while maintaining a high yield strength ratio.
• the range of applications for automobile body parts is expanded, and it can also be used for parts that require complex press processing.
• it can make a significant contribution to reducing the weight of automobile bodies, and it can also improve the hydrogen embrittlement resistance of processed parts, particularly bent parts that are subject to particularly strict forming conditions, which is an issue specific to high-strength materials.
• C is one of the important basic components of steel, and in the present invention, it is an important element that affects the fractions of martensite, ferrite, and retained austenite. If the C content is less than 0.090%, the fraction of martensite decreases, making it difficult to achieve the desired tensile strength. On the other hand, if the C content exceeds 0.300%, the martensite becomes embrittled, making it difficult to achieve the desired elongation. Therefore, the C content is set to 0.090% or more and 0.300% or less.
• the preferred lower limit is 0.120% or more, and the more preferred lower limit is 0.150% or more.
• the preferred upper limit is 0.280% or less, and the even more preferred upper limit is 0.240% or less.
• Silicon (Si) is one of the important basic components of steel.
• silicon suppresses carbide formation during continuous annealing and promotes the formation of retained austenite, thereby affecting the hardness of martensite and the fraction of retained austenite. If the Si content is less than 0.50%, the fraction of retained austenite decreases, making it difficult to achieve the desired elongation. On the other hand, if the Si content exceeds 2.50%, the carbon concentration in the retained austenite increases excessively, resulting in reduced local ductility. For example, cracks are more likely to form on the surface layer during bending tests. Therefore, the Si content is set to 0.50% or more and 2.50% or less.
• the preferred lower limit is 0.60% or more, more preferably 0.80% or more.
• the preferred upper limit is 2.00% or less, more preferably 1.80% or less.
• Mn is one of the important basic components of steel, and in the present invention, it is an important element that affects the fraction of martensite. If the Mn content is less than 1.8%, the fraction of martensite decreases, making it difficult to achieve a TS of 980 MPa or more. On the other hand, if the Mn content exceeds 4.0%, the fraction of tempered martensite decreases, reducing local ductility. Bendability also decreases. Therefore, the Mn content is set to 1.8% or more and 4.0% or less.
• the preferred lower limit is 2.0% or more, more preferably 2.2% or more.
• the preferred upper limit is 3.8% or less, more preferably 3.6% or less.
• P 0.100% or less
• P segregates at prior austenite grain boundaries and embrittles the grain boundaries, reducing the local ductility of the steel sheet and decreasing elongation. It also reduces bendability. Therefore, the P content must be 0.100% or less. While there is no particular lower limit for the P content, it is preferable to set it to 0.001% or more because P is a solid-solution strengthening element and can increase the strength of the steel sheet. Therefore, the P content is set to 0.100% or less, preferably 0.070% or less.
• S 0.0200% or less
• S exists as sulfides and reduces the local ductility of the steel sheet, thereby reducing elongation. It also reduces bendability. Therefore, the S content must be 0.0200% or less. While there is no particular lower limit for the S content, it is preferably 0.0001% or more due to production technology constraints. Therefore, the S content is 0.0200% or less, preferably 0.0050% or less.
• Al 0.200% or less
• Al raises the A3 transformation point and causes a large amount of ferrite to be included in the microstructure, preventing high strength achieved by utilizing the martensite structure. Therefore, the Al content must be 0.200% or less.
• the Al content is preferably 0.001% or more because it suppresses the formation of carbides during continuous annealing and promotes the formation of retained austenite. A more preferred range is 0.150% or less.
• N 0.0200% or less
• N exists as a nitride and reduces the local ductility of the steel sheet, thereby reducing elongation. It also reduces bendability. Therefore, the N content must be 0.0200% or less. Although there is no particular lower limit for the N content, due to constraints on production technology, the N content is preferably 0.0001% or more. Therefore, the N content is 0.0200% or less, preferably 0.0050% or less.
• O exists as an oxide and reduces the local ductility of the steel sheet, thereby reducing elongation. It also reduces bendability. Therefore, the O content must be 0.0100% or less. While there is no particular lower limit for the O content, due to constraints on production technology, the O content is preferably 0.0001% or more. Therefore, the O content is 0.0100% or less, more preferably 0.0050% or less.
• Cu and Sn contents are preferably 0.5% or less. While there is no particular lower limit for the contents, since these elements improve hardenability, the contents are more preferably 0.005% or more. A more preferable lower limit for Cu and Sn is 0.01% or more. An even more preferable upper limit for Cu and Sn is 0.30% or less.
• the Sb content be 0.07% or less. While there is no specific lower limit for the Sb content, it is more preferable that the content be 0.001% or more in order to obtain the effect of surface decarburization. More preferably, it should be 0.004% or more.
• ⁇ 0.005 or more and 0.07 or less
• Cu, Sn, and Sb are features of the present invention, and at least two of these elements must be contained, with ⁇ being 0.005 or greater.
• ⁇ is a dimensionless quantity expressed as [Sb] + [Cu]/10 + [Sn]/10, where [Sb], [Cu], and [Sn] represent their respective contents (mass%) and are zero when not contained.
• These elements are known as surface segregation elements, and adding a certain amount can suppress surface decarburization during the annealing process and ultimately suppress microstructural non-uniformity in the sheet thickness direction. More preferably, ⁇ is 0.010 or greater, and even more preferably, 0.015 or greater.
• the upper limit of ⁇ must be 0.07 or less. If it exceeds 0.07, bendability decreases.
• the upper limit of ⁇ is preferably 0.05 or less, and even more preferably, 0.03 or less.
• a high-strength steel sheet according to one embodiment of the present invention has a composition containing the above-mentioned components, with the balance including Fe and unavoidable impurities.
• a high-strength steel sheet according to one embodiment of the present invention has a composition containing the above-mentioned components, with the balance consisting of Fe and unavoidable impurities.
• unavoidable impurities include H, Zn, Pb, As, Se, Ge, Sr, and Cs. A total content of 0.100% or less of these impurities is acceptable.
• the high-strength steel plate of the present invention may further contain, by mass%, at least one element selected from Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Co: 0.010% or less, Ni: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, and Bi: 0.200% or less, either alone or in combination.
• V content be 0.200% or less. While there is no specific lower limit for the V content, it is more preferable that the V content be 0.001% or more, because V increases the strength of the steel sheet by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing. Therefore, if V is contained, its content should be 0.200% or less. A more preferable lower limit is 0.001% or more. A more preferable upper limit is 0.100% or less.
• Ta and W if each is 0.10% or less, do not form large amounts of coarse precipitates or inclusions, and do not reduce the local ductility of the steel sheet, resulting in no reduction in elongation. Furthermore, bendability is not reduced. Therefore, it is preferable that the Ta and W contents be 0.10% or less. While there are no specific lower limits for the Ta and W contents, it is more preferable that the Ta and W contents be 0.01% or more, since they increase the strength of the steel sheet by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing. Therefore, if Ta and W are contained, their contents should be 0.10% or less. More preferably, the Ta and W contents are 0.01% or more. An even more preferable Ta and W content is 0.08% or less.
• the Cr and Mo contents be 1.00% or less. While there are no specific lower limits for the Cr and Mo contents, it is more preferable that the Cr and Mo contents be 0.01% or more, as these elements improve hardenability. Therefore, if Cr and Mo are contained, their contents should each be 1.00% or less. More preferably, the Cr and Mo contents are 0.01% or more. An even more preferable Cr and Mo content is 0.80% or less.
• Co is 0.010% or less, coarse precipitates and inclusions do not increase, and the local ductility of the steel sheet is not reduced, so elongation does not decrease. Furthermore, bendability does not decrease. Therefore, it is preferable that the Co content be 0.010% or less. While there is no particular lower limit for the Co content, because Co is an element that improves hardenability, it is more preferable that the Co content be 0.001% or more. Therefore, if Co is contained, its content should be 0.010% or less. A more preferable Co content is 0.001% or more. An even more preferable Co content is 0.008% or less.
• Ni is 0.200% or less, cracks will not form inside the steel sheet during casting or hot rolling, and the local ductility of the steel sheet will not be reduced, so elongation will not be reduced. Furthermore, bendability will not be reduced. Therefore, it is preferable that the Ni content be 0.200% or less. While there is no specific lower limit for the Ni content, since Ni is an element that improves hardenability (generally an element that improves corrosion resistance), it is more preferable that the Ni content be 0.001% or more. Therefore, if Ni is contained, its content should be 0.200% or less, and more preferably 0.001% or more. An even more preferable Ni content is 0.150% or less.
• Ca, Mg, and REM if each is 0.0100% or less, do not increase coarse precipitates or inclusions, do not reduce the local ductility of the steel sheet, and therefore do not reduce elongation. Furthermore, bendability does not decrease. Therefore, it is preferable that the Ca, Mg, and REM contents be 0.0100% or less. While there are no specific lower limits for the Ca, Mg, and REM contents, since these elements spheroidize the shape of nitrides and sulfides and improve the local ductility of the steel sheet, it is more preferable that the Ca, Mg, and REM contents be 0.0005% or more. Therefore, if Ca, Mg, and REM are contained, their contents should each be 0.0100% or less.
• the Ca, Mg, and REM contents are 0.0005% or more.
• An even more preferable Ca, Mg, and REM content is 0.0050% or less.
• REM rare earth elements
• Sc, Y, and 15 elements ranging from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71, and the REM content here refers to the total content of these elements.
• Zr and Te if each is 0.100% or less, do not increase coarse precipitates or inclusions, and do not reduce the local ductility of the steel sheet, resulting in no reduction in elongation. Furthermore, bendability is not reduced. Therefore, it is preferable that the Zr and Te contents be 0.100% or less. While there are no specific lower limits for the Zr and Te contents, it is more preferable that the Zr and Te contents be 0.001% or more, as these elements spheroidize the shape of nitrides and sulfides and improve the local ductility of the steel sheet. Therefore, if Zr and Te are contained, their contents should be 0.100% or less. A more preferable Zr and Te content is 0.001% or more. An even more preferable Zr and Te content is 0.080% or less.
• Hf is 0.10% or less, the amount of coarse precipitates and inclusions will not increase, and the local ductility of the steel sheet will not decrease, so elongation will not decrease. Furthermore, bendability will not decrease. Therefore, it is preferable that the Hf content be 0.10% or less. While there is no specific lower limit for the Hf content, it is more preferable that the Hf content be 0.01% or more, as it is an element that spheroidizes the shape of nitrides and sulfides and improves the local ductility of the steel sheet. Therefore, if Hf is contained, its content should be 0.10% or less. A more preferable Hf content is 0.01% or more. An even more preferable Hf content is 0.08% or less.
• the Bi content be 0.200% or less. While there is no specific lower limit for the Bi content, because Bi is an element that reduces segregation, it is more preferable that the Bi content be 0.001% or more. Therefore, if Bi is contained, its content should be 0.200% or less. A more preferable Bi content is 0.001% or more. An even more preferable Bi content is 0.100% or less.
• Ti and Nb contents be 0.200% or less. While there are no specific lower limits for the Ti and Nb contents, it is more preferable that the Ti and Nb contents be 0.001% or more, since they increase the strength of the steel sheet by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing. Therefore, if Ti and Nb are contained, their contents should each be 0.200% or less. More preferably, the Ti and Nb contents are 0.001% or more. Even more preferably, the Ti and Nb contents are 0.100% or less.
• the B content is preferably 0.0100% or less. While there is no specific lower limit for the B content, because B is an element that segregates to austenite grain boundaries during annealing and improves hardenability, it is more preferable that the B content be 0.0003% or more. Therefore, if B is contained, its content should be 0.0100% or less. A more preferable B content is 0.0003% or more. An even more preferable B content is 0.0080% or less.
• the location that defines the structure is the 1/4 coil width and 1/4 plate thickness position. This is because the 1/4 coil width and 1/4 plate thickness positions are generally considered to be the locations that have the average properties and structure of the steel plate within the coil. Note that the coil width is the same as the plate width of the steel plate, and 1/4 coil width is equivalent to 1/4 plate width of the steel plate.
• Ferrite 0% or more and 5% or less. Electron microscope observation field area ratio
• Ferrite is a soft structure, and therefore is effective in improving workability.
• ferrite has a strong effect of reducing the strength of the steel plate.
• the ferrite referred to here may be polygonal ferrite, pseudo-polygonal ferrite, or granular bainitic ferrite.
• the ferrite content at the 1/4 position of the plate thickness is set to 5% or less. More preferably, it is set to 3% or less. Furthermore, an even more preferable condition is that the ferrite content is less than 1%. Note that ferrite does not necessarily need to be contained, so the lower limit is set to 0% or more.
• the ferrite area ratio is determined using the following method. First, a thickness cross section (L cross section) parallel to the rolling direction of the steel plate is polished and then etched with 3 vol. % nital. Ten fields of view are observed at 2000x magnification using a scanning electron microscope (SEM) at a position 1/4 of the plate thickness (a position corresponding to 1/4 of the plate thickness in the depth direction from the surface of the steel plate). Next, using the obtained structural images, the area ratio of each structure for the 10 fields of view is calculated using Media Cybernetics' Image-Pro. The average of the area ratios for these 10 fields of view is taken as the "ferrite area ratio.” In the above structural image, ferrite appears as a gray structure (base structure). Retained austenite and martensite appear as white structures.
• Retained austenite contributes to the ductility of the steel sheet through the TRIP effect and is an essential structure in the present invention.
• the lower limit is set to 3% or more.
• the upper limit is set to 20% or less.
• a more preferable lower limit is 6% or more.
• a more preferable upper limit is less than 16%. Note that this volume fraction of retained austenite can be converted into an area fraction.
• the method for measuring the volume fraction of retained austenite is as follows: The steel plate is mechanically ground in the thickness direction (depth direction) to 1/4 of the plate thickness, and then chemically polished with oxalic acid to create an observation surface. This observation surface is observed using X-ray diffraction. A Co K ⁇ radiation source is used as the incident X-ray, and the intensity of the diffraction peaks of the ⁇ 200 ⁇ , ⁇ 220 ⁇ , and ⁇ 311 ⁇ planes of fcc iron (austenite) is measured relative to the diffraction intensity of the ⁇ 200 ⁇ , ⁇ 211 ⁇ , and ⁇ 220 ⁇ planes of bcc iron.
• Tempered martensite 70% or more. Electron microscope observation field area ratio
• Tempered martensite is characterized by an aggregate of lath-shaped crystal grains containing iron-based carbides. It has higher ductility than fresh martensite and is also characterized by its tendency to achieve the high yield strength required to ensure the crashworthiness of automotive components.
• the area fraction of tempered martensite must be 70% or more. More preferably, the area fraction of tempered martensite is 75% or more, and even more preferably, 80% or more. While there is no particular upper limit to the area fraction of tempered martensite, in order to ensure the area fraction of retained austenite, the area fraction of tempered martensite is preferably 94.0% or less. The area fraction of tempered martensite can be measured by the method described in the Examples below.
• the effects of the present invention are not impaired even if bainite is contained in an area ratio of 20% or less.
• carbides such as pearlite (P) and cementite ( ⁇ ) and other known steel plate structures may also be included.
• the effects of the present invention are not impaired even if these are included, so long as their total area ratio is within a range of 10% or less.
• the other steel plate structures (remaining structures) can be confirmed and determined, for example, by SEM observation.
• the area ratios of tempered martensite, fresh martensite, pearlite, and bainite are measured as follows: A sample of annealed steel sheet is cut, and the cross section of the sheet thickness parallel to the rolling direction is polished and then etched with 3 vol. % nital. Three images are taken of each quarter-thickness position at 1500x magnification using a scanning electron microscope (SEM). Note that the quarter-thickness position refers to the position corresponding to 1/4 of the sheet thickness in the depth direction from the surface of the steel sheet. Observation at a higher magnification may be performed to confirm carbides in detail. Note that the steel sheet surface in the case of zinc plating refers to the interface between the zinc plating layer and the steel sheet.
• the area ratio of each structure is calculated using the obtained image data using Image-Pro manufactured by Media Cybernetics, and the average area ratio of each structure within the field of view is taken as the area ratio of each structure.
• fresh martensite is distinguished as a white or light gray region
• tempered martensite as a gray or dark gray region containing misoriented carbides
• pearlite as a black and white lamellar structure.
• the gray or dark gray region containing aligned carbides is bainite.
• this classification can be difficult during actual observation. For example, even tempered martensite may be observed to have aligned carbide orientation.
• the gray or dark gray region containing aligned carbides and a structure in which the structure interface extends linearly is considered bainite, while the gray or dark gray region containing other carbides is considered tempered martensite.
• fresh martensite is difficult to distinguish from retained austenite, which also appears in white or light gray regions. Therefore, after determining the area fraction of the white or light gray region, the volume fraction of retained austenite determined by the above method is considered to be the area fraction and subtracted to determine the area fraction.
• the structure of the outermost layer of the plate thickness includes the structure of the region approximately 20 ⁇ m from the outermost surface of the steel plate, extending into the plate thickness. This is because, when evaluating the structure fraction using SEM observation, it is necessary to observe the region several ⁇ m from the outermost layer into the plate thickness. Furthermore, the very outermost layer, within approximately 5 ⁇ m from the outermost surface of the steel plate, contains scale and is non-steady, so it may have a structure that is essentially different from this surface layer structure.
• the structure at the quarter coil width position and the outermost layer in the plate thickness has an area ratio of ferrite of 0% to 5%.
• the other structures are mainly composed of tempered martensite, and specifically, it is preferable that they are as follows.
• the volume fraction of retained austenite is 3% or more and 20% or less
• the area fraction of tempered martensite is 70% or more
• the area fraction of fresh martensite is 0% or more and 20% or less.
• the structure in the outermost layer of the plate thickness can be determined using a measurement method similar to the method for measuring the steel structure at the 1/4 position of the coil width and the 1/4 position of the plate thickness.
• microcracks are particularly likely to form when the aspect ratio ( ⁇ 1) is large, exceeding 2.0.
• ⁇ 1 refers to a value expressed as the major axis diameter/minor axis diameter of the crystal. While the detailed mechanism is unknown, it is thought that most of the microcracks formed after stress-induced transformation of prior austenite grains are linear along the prior austenite grain boundaries, which facilitates crack propagation.
• the proportion of retained austenite with an aspect ratio of 2.0 or more in the total retained austenite is low, and should be 50% or less.
• the proportion of retained austenite having an aspect ratio of 2.0 or more to the total retained austenite refers to the proportion of the area of retained austenite having an aspect ratio of 2.0 or more to the area of the total retained austenite.
• Proportion (%) of retained austenite having an aspect ratio of 2.0 or more to the total retained austenite 100 ⁇ (area ratio of retained austenite having an aspect ratio of 2.0 or more) / (area ratio of total retained austenite) Since retained austenite with a large aspect ratio is likely to form when a non-uniform carbon concentration distribution is formed by, for example, two-phase annealing, it is preferable to avoid two-phase annealing as much as possible.
• Retained austenite with an aspect ratio of 2.0 or more is evaluated using the SEM-EBSD method. For example, this can be obtained by acquiring and analyzing EBSD patterns from approximately three fields of view in a 20 ⁇ m ⁇ 50 ⁇ m region at the outermost surface layer in 0.05 ⁇ m increments. Other conditions can be determined by applying the method described in, for example, Patent Document 9.
• the tensile strength level of the steel sheet targeted by the present invention is 980 MPa or higher.
• the strength level is not limited to the 980 MPa class, and high-strength steel sheets up to approximately 2.0 GPa class, such as 1180 MPa class, 1300 MPa class, and 1470 MPa class, are also targeted.
• the manufacturing method described in the present invention can be applied to various materials regardless of the strength level of the steel sheet.
• TS and YS are values evaluated using a tensile test piece in the sheet width direction at a position 1/4 of the coil width. The tensile test is performed in accordance with the method described in JIS Z2241 using a JIS No. 5 test piece with the longitudinal direction perpendicular to the rolling direction.
• Yield ratio (YR) 100 x YS/TS is 70% or more]
• the steel sheet to which the present invention is directed has a high yield ratio of 70% or more, more preferably 75% or more, and even more preferably 80% or more.
• YS here refers to yield strength, and if an upper yield point clearly occurs, it is referred to as upper yield point strength, and if not, it is referred to as 0.2% proof stress strength.
• the soft layer on the surface side of the plate thickness a fundamental element of the present invention, is defined by data obtained from the micro-Vickers hardness test shown below. That is, the hardness distribution in the plate thickness direction must satisfy 2.5 ⁇ (H025 ⁇ H005)/H025 ⁇ 20.0.
• H025 is the hardness value at the 1/4 position of the plate thickness
• H005 is the hardness value at the 1/20 position of the plate thickness.
• the 1/20 position and the aforementioned outermost layer of the plate thickness do not necessarily overlap, but they may.
• Softening the surface layer without utilizing a ferrite structure can improve the ductility of the surface side, particularly the hydrogen embrittlement resistance of the bent portion.
• Hardness evaluation tests can be performed in accordance with the method described in JIS Z2244.
• the surface side of the plate thickness must be at least 2.5 times softer than the 1/4 position. More preferably, it is 3.0 or more.
• the upper limit of 100 ⁇ (H025 - H005) / H025 is set to 20.0 or less, and more preferably 15.0 or less.
• a steel material having the above-described composition is melted using a conventional refining process and then formed into a steel slab using a conventional ingot-blooming or continuous casting method.
• a thin steel slab with a thickness of 100 mm or less may be produced using a direct casting method.
• the steel slab is heated and held at a temperature of 1080°C to 1300°C, then subjected to hot rolling, followed by rough rolling and finish rolling to produce a hot-rolled sheet, which is then wound into a coil.
• the heating temperature is less than 1080°C, the alloy elements will be unevenly distributed in the steel, reducing the residual ⁇ fraction and impairing the properties of the final product sheet. Furthermore, heating at temperatures higher than 1300°C will excessively soften the steel billet, causing surface defects such as slab sagging and scabs, thereby impairing quality.
• the thickness of the hot-rolled sheet is preferably 0.8 mm to 4.0 mm.
• the rolling end temperature (finish hot rolling temperature or finish rolling end temperature) is set to 850°C or higher and 1000°C or lower. Hot rolling needs to be completed in the austenite single phase region in order to improve the strength and ductility balance after annealing by homogenizing the structure within the steel sheet and reducing the anisotropy of the material, so the finish rolling end temperature is set to 850°C or higher. On the other hand, if the finish rolling end temperature exceeds 1000°C, the hot rolled structure becomes coarse and the properties after annealing deteriorate. A more preferable range of the rolling end temperature is 875°C or higher and 950°C or lower.
• finish hot rolling temperature and coiling temperature are values measured at the center of the coil width.
• Skin pass rolling may be performed after the end of finish rolling and before annealing the hot-rolled sheet. Skin pass rolling can further flatten the shape of the steel sheet and also destroy scale formed on the hot-rolled sheet, thereby increasing the decarburization ability in the subsequent heat treatment step before cold rolling.
• the present invention performs the subsequent hot-rolled sheet annealing in a state in which the hot-rolled scale formed on the surface of the hot-rolled sheet is left without being removed by pickling.
• the decarburization of the steel sheet surface can be optimized.
• the first-stage annealing temperature should be above the Ac3 point.
• the first-stage annealing time is too short, solute carbon may not be able to diffuse sufficiently, so it is preferable to anneal for 5 seconds or more. There is no particular upper limit.
• the cooling rate it is preferable to cool to a temperature range of 400°C or higher.
• the cooling rate it is preferable that the cooling rate be 1°C/s or higher in the temperature range of 400°C or higher.
• the temperature is maintained or increased to a temperature range of 400°C or higher but not exceeding the Ac1 point, within the T temperature range. If the temperature is increased, it can be performed in a separate process (separate line) from the first annealing step without any problems.
• This second annealing step homogenizes the structure, thereby suppressing the formation of retained austenite with an aspect ratio of 2.0 or higher in the steel sheet surface. It also softens the hard structure formed by transformation from austenite, reducing the load of the subsequent cold rolling.
• the second annealing temperature exceeds the Ac1 point, an austenite phase is formed, which causes the structure to become non-uniform during subsequent cooling, resulting in a decrease in the proportion of retained ⁇ with the desired aspect ratio. Furthermore, if precipitation-strengthening elements such as Ti and Nb are added, the carbides and nitrides of these elements precipitate coarsely and do not contribute to a high yield ratio. On the other hand, if the second annealing temperature is below 400°C, the effects of surface decarburization and softening are not achieved. Additionally, the second annealing time is preferably 60 seconds or longer. If the second annealing time is less than 60 seconds, decarburization may not progress smoothly.
• the second annealing time there is no upper limit to the second annealing time, but from the perspective of productivity and cost, it is desirable to set it to 48 hours or shorter.
• the temperature must be between 400°C and the Ac1 point, and the temperature range T must also be satisfied. T is 5000 ⁇ - 600°C or greater.
• ⁇ [Sb] + [Cu]/10 + [Sn]/10, and this parameter corresponds to the amount of surface segregated elements Sb, Cu, and Sn. The larger the value, the greater the effect of suppressing decarburization during annealing.
• the annealing temperature T at which sufficient decarburization occurs can be determined according to the value of ⁇ . If the above temperature conditions are not met, decarburization of the surface layer will not occur sufficiently.
• the steel sheet is pickled and then cold-rolled to a desired thickness at a reduction of 30% or more.
• Cold rolling may be performed by tandem rolling (unidirectional rolling) or reverse rolling, or by using a known warm rolling technique. If the reduction is low, the driving force for recrystallization is low, which prevents sufficient recrystallization from occurring in the subsequent annealing process, leading to a decrease in the strength-ductility balance and a non-uniform structure, making it difficult to ensure the desired residual ⁇ fraction.
• the upper limit is not particularly specified, it is preferably 80% or less from the viewpoint of the rolling load.
• the resulting steel sheet is heat treated and, if necessary, plated. It is heated to a temperature between the Ac3 point + 10°C and 950°C, and then cooled at an average cooling rate of 10°C/s to 100°C/s to at least 400°C, and then cooled at an average cooling rate of 5°C/s to 20°C/s from 400°C to a cooling stop temperature range of 50°C to 300°C. It is preferably held in this cooling stop temperature range for 0.5 seconds or more. It is then heated to a temperature range of 250°C to 530°C, and then held in this temperature range for 10 seconds or more before cooling.
• the steel sheet is first heated to Ac3+10°C or higher and 950°C or lower.
• the heating rate is not particularly limited, but an excessively low rate may cause grain coarsening and impair toughness, so an average heating rate of 5°C/s or higher is preferred.
• the upper limit is not particularly specified, and there is no problem even if a heating rate exceeding 100°C/s is applied using an induction heating method or the like.
• the structure is austenitized, softening the strong carbon concentration distribution formed by surface decarburization during annealing before cold rolling.
• a key point of the present invention is not to achieve complete homogenization. If complete homogenization is achieved, the desired hardness distribution will not be formed. In the subsequent cooling process, hard martensite is formed. If the heating rate is lower than Ac3+10°C, ferrite remains during annealing, reducing the fraction of tempered martensite and significantly impairing strength. Furthermore, if the annealing temperature exceeds 950°C, austenite grains grow excessively, and the martensite structure after cooling also becomes coarse, resulting in a low YS. Furthermore, decarburization from the surface layer progresses during annealing, increasing the ferrite fraction in the surface layer of the steel sheet, resulting in a low YS. A more preferable range is Ac3+20°C or higher and 920°C or lower.
• Ac1 and Ac3 shown in this specification can be calculated from the following formulas (A) and (B) described in "Leslie Steel Science” (Maruzen Co., Ltd., translated and supervised by Shigeyasu Koda, published May 31, 1985, page 273).
• [ ] indicates the content (mass%) of each element, and the content of an element not contained in the steel sheet or an element below the lower detection limit of analysis may be calculated as 0 mass%.
• the cooling rate after annealing is set to 10°C/s or more. This is because, particularly when the carbon concentration on the surface layer side has decreased due to annealing before cold rolling, an insufficient cooling rate will cause a high-temperature precipitation phase to form on the surface layer side, softening the steel sheet. More preferably, the cooling rate is set to 15°C/s or more, and even more preferably, 20°C/s or more. On the other hand, if the cooling rate is excessively high, uneven cooling will occur in the coil width direction, impairing the flatness of the sheet, so the cooling rate is set to 100°C/s or less.
• the average cooling rate is 5°C/s or higher and 20°C/s or lower
• the cooling is performed from 400°C to the cooling stop temperature at an average cooling rate of 5°C/s or more and 20°C/s or less.
• the cooling rate is set to 5°C/s or more.
• the upper limit is set to 20°C/s.
• the cooling stop temperature is set to 50°C or more and 300°C or less. If the cooling rate is lower than 50°C, the fraction of austenite, which is responsible for ductility, decreases excessively. On the other hand, if the cooling rate is higher than 300°C, the fraction of austenite becomes excessive, and in the subsequent cooling process, fresh martensite is generated while tempered martensite decreases, resulting in reduced ductility. A more preferable range is 80°C or more and 250°C or less.
• any known method may be used, such as gas cooling, oil cooling, mist cooling, and low-melting-point liquid metal cooling.
• the steel sheet After cooling to the above temperature, the steel sheet may be held at that temperature for 0.5 seconds or more without being immediately heated. According to experiments by the inventors, due to the impossibility of completely uniform cooling, a temperature difference of several tens of degrees may occur in the width direction of the steel sheet immediately after cooling is stopped. The temperature difference formed after cooling is stopped may affect the temperature difference in the width direction after the subsequent heating and cooling process. As a result, this may result in non-uniformity in the material properties in the coil width direction.
• a holding time of 0.5 seconds or more in the cooling stop temperature range is preferable because it equalizes the temperature difference in the coil width direction during holding, uniforms the material properties in the coil width direction after final cooling, and also enables uniform mechanical properties.
• a more preferable time is 1.0 seconds or more, and even more preferably 3 seconds or more.
• reheating Heating to a temperature range above the cooling stop temperature and above 250°C to 530°C, and then holding at that temperature range for 10 seconds or more]
• reheating is performed to temper the formed martensite and stabilize the retained austenite. Reheating may be performed immediately after the cooling stop, or after holding the temperature within a range in which carbides do not significantly precipitate. If the reheating temperature is excessively low, carbon diffusion to obtain the above-mentioned structure is not sufficiently promoted, and the desired strength-ductility balance cannot be ensured. Furthermore, if the reheating temperature is excessively high, coarse carbides precipitate, causing softening. A more preferred range is above the cooling stop temperature and between 280°C and 450°C.
• a more preferred holding time is 60 seconds or more, and even more preferably 180 seconds or more.
• hot-dip galvanizing may be performed within the above-mentioned range, i.e., within the temperature range of 250°C or higher and 530°C or lower, to produce a hot-dip galvanized steel sheet, or after hot-dip galvanizing, alloying treatment may be performed in the temperature range of 470°C or higher and 530°C or lower to produce an alloyed hot-dip galvanized steel sheet.
• the steel sheet of the present invention may be electroplated to produce an electroplated steel sheet.
• the steel sheet After the annealing step, the steel sheet must be cooled and then skin-pass rolled at an elongation rate of 0.02% or more to achieve flattening and increase the yield ratio.
• the upper limit of the elongation rate is preferably 2.0% or less, more preferably 0.5% or less.
• the lower limit of the elongation rate is preferably 0.05% or more.
• the above steel material with the chemical composition shown in Table 1 and the remainder consisting of Fe and unavoidable impurities, was melted and produced into thin steel sheets under the conditions shown in Table 2.
• the steel material was reheated, subjected to rough rolling and finish rolling, then cooled and coiled.
• the thickness of the hot-rolled sheet was adjusted to a range of 1.8 to 4.0 mm. It was then heat-treated with the hot-rolled scale remaining on the surface, and the scale was removed by pickling. It was then cold-rolled.
• the thickness of the cold-rolled sheet was adjusted to a range of 0.8 to 2.4 mm. It was then heat-treated (CR).
• Some of the sheets underwent hot-dip galvanizing (GI), galvannealed hot-dip galvanizing (GA), or electrogalvanizing (EG) during or after the heat treatment.
• GI hot-dip galvanizing
• GA galvannealed hot-dip galvanizing
• EG electrogalvanizing
• Test piece A JIS No. 5 test piece (gauge length 50 mm, parallel portion width 25 mm) was cut from the obtained steel plate at 1/4 of the plate width in the direction perpendicular to the rolling direction, and a tensile test was performed in accordance with JIS Z2241 (2022). The test pieces were butted together to measure El (total elongation), and a value of 10,000/TS (MPa)% or more was considered pass. In addition, the test piece was embedded in carbon resin so that the rolling direction and the plate thickness direction were the observation surfaces, and a hardness test was performed in accordance with JIS Z2244 to evaluate the structure.

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