High strength steel plate and method for manufacturing high strength steel plate

Abstract

To provide a high strength steel plate having high strength and excellent hole expandability.SOLUTION: A high strength steel plate has a chemical composition containing, by mass%, 0.10-0.30% of C, 0.50-2.50% of Si, 0.50-3.50% of Mn, 0.100% or less of P, 0.020% or less of S, 0.010-0.100% of Al, 0.0100% or less of N, and the balance being Fe and inevitable impurities, and has a metal structure including 70 vol.% or more and less than 85 vol.% of the total of tempered martensite and bainite, and 5 vol.% or more of ferrite and 3 vol.% or more of retained austenite, wherein a region in the metal structure with a Mn concentration of 8.0 mass% or more is 0.05-1.5 area%.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a high-strength steel plate and a method for manufacturing the high-strength steel plate. [Background technology]

[0002] Steel sheets used for automobile parts and the like are required to have high strength as well as excellent formability.

[0003] WO 2018 / 030502 discloses a high-strength steel sheet having a tensile strength of 980 MPa or more and good formability. This steel sheet contains predetermined amounts of Cr and Mo, has an average grain size of 1.5 μm or less, an area ratio of the ferrite phase of 2% to 15%, an area ratio of the tempered martensite phase of 75% to 96%, and a total interface length per unit area between the untempered martensite phase and the ferrite phase and the untempered martensite phase and the tempered martensite phase of 6.3 × 10 8 μm / m 2 Over 5.0 x 10 11 μm / m 2 The following is the result.

[0004] JP 2016-194139 A discloses a high-strength cold-rolled steel sheet having a tensile strength of 980 MPa or more, good workability as evaluated by ductility and flangeability, and excellent crashworthiness. When the metallographic structure of this steel sheet is observed with a scanning electron microscope at a quarter position in the sheet thickness, the area ratio of ferrite is more than 10% and not more than 65% of the entire metallographic structure, and the remainder is a hard phase including quenched martensite and retained austenite, and at least one selected from the group consisting of bainitic ferrite, bainite, and tempered martensite.

[0005] Japanese Patent Application Laid-Open Publication No. 2015-193897 discloses a high-strength cold-rolled steel sheet with a tensile strength of 980 MPa or more and excellent ductility and bendability. When the structure of this steel sheet is observed with a scanning electron microscope at a position 1 / 4 of the steel sheet thickness, the area ratio of ferrite to the entire structure is 5% or more but less than 50%, with the remainder being a hard phase. Furthermore, when analyzed with an electron beam microprobe analyzer, it is found that there are 5% or more areas where the Mn concentration is 1.2 times or more the Mn concentration in the steel sheet, and when the fraction of the areas where the Mn concentration is 1.2 times or more the Mn concentration in the steel sheet is measured in 2 μm square sections, the standard deviation when measuring 100 sections is 4.0% or more.

[0006] WO 2008 / 042982 describes a method for treating an iron-based alloy, comprising the steps of bringing an iron-based alloy having a first microstructure transformable to a second microstructure to an austenite transformation temperature, rapidly heating the iron-based alloy to a temperature above the austenite transformation temperature, and immediately quenching at least a portion of the iron-based alloy with a quenching device adjacent to the heating device.

[0007] International Publication No. 2023 / 182279 discloses a cold-rolled steel sheet in which the metal structure contains a total of 85% by volume or more of tempered martensite and bainite, and structures other than tempered martensite, bainite, and ferrite account for 5% by volume or less, and the area in the metal structure where the Mn concentration is 8.0% by mass or more is 0.05 to 1.5% by area. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] International Publication No. 2018 / 030502 [Patent Document 2] Japanese Patent Application Laid-Open No. 2016-194139 [Patent Document 3] Japanese Patent Application Laid-Open No. 2015-193897 [Patent Document 4] International Publication No. 2008 / 042982 [Patent Document 5] International Publication No. 2023 / 182279 Summary of the Invention [Problem to be solved by the invention]

[0009] An object of the present invention is to provide a high-strength steel sheet having high strength and excellent formability, and more specifically, to provide a high-strength steel sheet having high strength and excellent hole expandability. [Means for solving the problem]

[0010] A high-strength steel sheet according to one embodiment of the present invention has a chemical composition, in mass%, of C: 0.10 to 0.30%, Si: 0.50 to 2.50%, Mn: 0.50 to 3.50%, P: 0.100% or less, S: 0.020% or less, Al: 0.010 to 0.100%, N: 0.0100% or less, and the balance: Fe and impurities. The high-strength steel sheet has a metallographic structure containing 70% by volume or more and less than 85% by volume of tempered martensite and bainite in total, 5% by volume or more of ferrite, and 3% by volume or more of retained austenite, and 0.05 to 1.5% by area of ​​regions in the metallographic structure where the Mn concentration is 8.0% by mass or more.

[0011] A high-strength steel plate according to one embodiment of the present invention has a chemical composition, in mass %, of C: 0.10 to 0.30%, Si: 0.50 to 2.50%, Mn: 0.50 to 3.50%, P: 0.100% or less, S: 0.020% or less, Al: 0.010 to 0.100%, N: 0.0100% or less, Ti: 0.100% or less, Nb: 0.100% or less, V: 0.50% or less, Cr: 1.00% or less, Mo: 0.50% or less, W: 0.50% or less, B: 0.00 The alloy contains one or more elements selected from the group consisting of 50% or less of Mn, 0.0100% or less of Ca, 0.0100% or less of Mg, and 0.0100% or less of REM, with the balance being Fe and impurities. The alloy has a metal structure containing a total of 70% by volume or more and less than 85% by volume of tempered martensite and bainite, 5% by volume or more of ferrite, and 3% by volume or more of retained austenite, and the metal structure has a region where the Mn concentration is 8.0% by mass or more in 0.05 to 1.5% by area.

[0012] A method for producing a high-strength steel sheet according to one embodiment of the present invention is a method for producing the above-mentioned high-strength steel sheet, comprising the steps of: cold-rolling a hot-rolled steel sheet to a rolling reduction of 30% or more; heating the cold-rolled steel sheet to a temperature T1 of 860 to 1000°C; holding the cold-rolled steel sheet at the temperature T1 for less than 10 seconds; after holding at the temperature T1, holding at a temperature T2 of 800 to 900°C, which is lower than the temperature T1, for 10 seconds or more; and, after holding at the temperature T2, heating the steel sheet to 350°C. and a step of, after cooling to temperature T3, holding at temperature T4, which is 300 to 450°C and higher than temperature T3, for 50 seconds or more. In the step of heating to temperature T1, the average heating temperature from 750°C to temperature T1 is 100.0°C / second or more, and in the step of cooling to temperature T3, the average cooling rate from 600°C to 450°C is 10.0°C / second or more. [Effects of the Invention]

[0013] According to the present invention, a high-strength steel sheet having high strength and excellent hole expandability can be obtained. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is a diagram schematically showing a heat pattern of the heat treatment after cold rolling. DETAILED DESCRIPTION OF THE INVENTION

[0015] The present inventors have conducted various studies to solve the above problems and have come to the following findings.

[0016] To increase the strength of cold-rolled steel sheets, it is necessary to perform appropriate heat treatment to create a metal structure primarily composed of tempered martensite and bainite. Furthermore, to achieve excellent hole expandability, it is effective to create a metal structure containing an appropriate amount of ferrite and retained austenite, and furthermore, to uniformly distribute Mn within the metal structure. Uniform distribution of Mn can suppress the formation of martensite-austenite (MA) during heat treatment. In regions where Mn is concentrated, hardenability is enhanced, making MA more likely to form. MA is a very hard structure and is prone to becoming the starting point for cracks during hole expansion. Suppressing the formation of MA can improve hole expandability.

[0017] A decrease in MA also has the effect of increasing the amount of retained austenite. This is thought to be because, during cooling, unstable untransformed austenite transforms into MA or martensite, while stable untransformed austenite remains as retained austenite. An appropriate amount of retained austenite contributes to improving the workability (specifically, elongation and hole expandability) of cold-rolled steel sheets.

[0018] Specifically, the metallographic structure contains a total of 70% to less than 85% by volume of tempered martensite and bainite, 5% or more by volume of ferrite, and 3% or more by volume of retained austenite. Furthermore, the area ratio of the region in the cross section of the steel sheet where the Mn concentration is 8.0% by mass or more in the metallographic structure is 0.05 to 1.5% by area. A small area ratio of the region where the Mn concentration is 8.0% by mass or more (hereinafter referred to as "Mn-enriched region") means that Mn is uniformly distributed. Reducing the area ratio of the Mn-enriched region can suppress the generation of MA and improve hole expandability. On the other hand, completely eliminating the Mn-enriched region would prevent excellent elongation. Leaving only a small area ratio of the Mn-enriched region (0.05% or more by area) promotes work hardening during processing of the steel, improving the elongation of the steel.

[0019] To obtain such a metal structure, it is effective to carry out the following heat treatment. After cold rolling the hot-rolled steel sheet, it is heated to a temperature T1 of 860 to 1000°C. At this time, the average heating rate from 750°C to temperature T1 is 100.0°C / second or more. After holding at temperature T1 for less than 10 seconds, it is held at temperature T2 of 800 to 900°C, which is lower than temperature T1, for 10 seconds or more. This diffuses Mn in the metal structure, making the distribution of Mn uniform.

[0020] Next, the steel sheet is cooled to a temperature T3, which is 350°C or lower and the martensitic transformation start temperature or lower. At this time, the average cooling rate from 600°C to 450°C is 10.0°C / sec or higher. Thereafter, the steel sheet is held at a temperature T4, which is 300 to 450°C and higher than temperature T3, for 50 seconds or longer. This results in a metal structure mainly composed of tempered martensite and / or bainite, with an appropriate amount of ferrite and retained austenite.

[0021] The present invention has been completed based on the above findings. A high-strength steel plate according to one embodiment of the present invention will now be described.

[0022] [High strength steel plate] [Chemical composition] A high-strength steel plate according to an embodiment of the present invention has the chemical composition described below. In the following description, "%" for the content of an element means mass %.

[0023] C: 0.10 to 0.30% Carbon (C) improves the strength of steel. On the other hand, if the C content is too high, the elongation of the steel decreases. Therefore, the C content is 0.10 to 0.30%. The lower limit of the C content is preferably 0.12%, more preferably 0.15%. The upper limit of the C content is preferably 0.25%, more preferably 0.20%.

[0024] Si: 0.50 to 2.50% Silicon (Si) is a solid solution strengthening element, but if it is contained in excess, the hot workability of the steel will decrease. Therefore, the Si content is 0.50 to 2.50%. The upper limit of the Si content is preferably 2.00%, more preferably 1.50%. The lower limit of the Si content is preferably 0.60%, more preferably 0.70%.

[0025] Mn: 0.50 to 3.50% Manganese (Mn) improves the hardenability of steel and improves its strength. On the other hand, if the Mn content is too high, the elongation of steel decreases. Therefore, the Mn content is 0.50 to 3.50%. The lower limit of the Mn content is preferably 0.80%, more preferably 1.20%, even more preferably 1.55%, and even more preferably 1.60%. The upper limit of the Mn content is preferably 3.30%, more preferably 3.00%, and even more preferably 2.80%.

[0026] P:0.100% or less Phosphorus (P) is an impurity. P segregates at grain boundaries and reduces the elongation of steel. Therefore, the P content is 0.100% or less. The P content is preferably 0.050% or less, and more preferably 0.030% or less.

[0027] S: 0.020% or less Sulfur (S) is an impurity. S reduces the hot workability of steel. Therefore, the S content is 0.020% or less. The S content is preferably 0.015% or less, and more preferably 0.010% or less.

[0028] Al: 0.010 to 0.100% Aluminum (Al) is contained as a deoxidizer. On the other hand, if the Al content is too high, inclusions are formed and the elongation of the steel decreases. Therefore, the Al content is 0.010 to 0.100%. The lower limit of the Al content is preferably 0.015%, more preferably 0.020%. The upper limit of the Al content is preferably 0.080%, more preferably 0.060%.

[0029] N: 0.0100% or less Nitrogen (N) is an impurity. N reduces the elongation of steel. Therefore, the N content is 0.0100% or less. The upper limit of the N content is preferably 0.0060%, more preferably 0.0040%. On the other hand, excessive reduction of N increases manufacturing costs. The lower limit of the N content is preferably 0.0005%, more preferably 0.0010%.

[0030] The chemical composition of the high-strength steel sheet according to this embodiment may contain one or more elements selected from the group consisting of Ti, Nb, V, Cr, Mo, W, B, Ca, Mg, and REM. All of these elements are optional. That is, the chemical composition of the high-strength steel sheet according to this embodiment may not contain some or all of Ti, Nb, V, Cr, Mo, W, B, Ca, Mg, and REM.

[0031] Ti:0.100% or less Nb: 0.100% or less V:0.50% or less Cr:1.00% or less Mo: 0.50% or less W: 0.50% or less Titanium (Ti), niobium (Nb), vanadium (V), chromium (Cr), molybdenum (Mo), and tungsten (W) all have the effect of forming carbides and improving the strength of steel. This effect can be achieved even if even small amounts of Ti, Nb, V, Cr, Mo, and W are contained. On the other hand, excessive contents of Ti, Nb, V, Cr, Mo, and W reduce the elongation of steel. Therefore, the contents of Ti and Nb are 0.100% or less, the contents of V, Mo, and W are 0.50% or less, and the content of Cr is 1.00% or less. The lower limits of the contents of Ti and Nb are preferably 0.005%. The upper limits of the contents of Ti and Nb are preferably 0.060%, more preferably 0.040%. The lower limits of the contents of V, Mo, and W are preferably 0.01%. The upper limits of the contents of V, Mo, and W are preferably 0.40%, more preferably 0.30%. The lower limit of the Cr content is preferably 0.05%, and the upper limit of the Cr content is preferably 0.80%, and more preferably 0.50%.

[0032] B: 0.0050% or less Boron (B) improves the hardenability and strength of steel. This effect can be achieved even if even a small amount of B is contained. On the other hand, if the B content is excessive, the elongation of the steel decreases. Therefore, the B content is 0.0050% or less. The lower limit of the B content is preferably 0.0003%. The upper limit of the B content is preferably 0.0040%, and more preferably 0.0030%.

[0033] Ca:0.0100% or less Mg: 0.0100% or less REM: 0.0100% or less Calcium (Ca), magnesium (Mg), and rare earth elements (REM) all improve the hot workability of steel. Even small amounts of Ca, Mg, and REM can achieve this effect. On the other hand, excessive amounts of Ca, Mg, and REM result in the formation of inclusions, reducing the elongation of the steel. Therefore, the Ca, Mg, and REM contents are each 0.0100% or less. The lower limits of the Ca, Mg, and REM contents are preferably 0.0001%. The upper limits of the Ca, Mg, and REM contents are preferably 0.0060%, more preferably 0.0040%. REM is a collective term for 17 elements, including Sc, Y, and lanthanides, and the REM content refers to the total amount of these elements.

[0034] The balance of the chemical composition of the high-strength steel plate according to this embodiment is Fe and impurities. The impurities referred to here refer to elements that are mixed in from ores or scraps used as raw materials for steel, or elements that are mixed in from the environment during the manufacturing process. The impurities are not limited to these, but examples include Cu, Ni, and O.

[0035] [Metal structure] The metallographic structure of the high-strength steel plate according to this embodiment contains tempered martensite and bainite in a total volume ratio of 70% to less than 85%. If the total volume ratio of tempered martensite and bainite is less than 70%, the required strength cannot be obtained. On the other hand, if the total volume ratio of tempered martensite and bainite is 85% or more, it becomes difficult to obtain excellent workability. Note that the above-mentioned "bainite" includes tempered bainite. The bainite in the metallographic structure of the high-strength steel plate according to this embodiment may be partially tempered. Furthermore, although cementite is present in the tempered martensite and bainite, the cementite in the tempered martensite and bainite is not considered an independent structure, but is included in the total volume of the tempered martensite and bainite.

[0036] The average grain size of cementite contained in tempered martensite and bainite is preferably greater than 30 nm. The average grain size of cementite is measured by taking an extracted replica film from a high-strength steel sheet and observing it in at least three fields of view using a transmission electron microscope (accelerating voltage 200 kV) at a magnification of 10,000 to 100,000 times. The average grain size is the circle-equivalent diameter. When the metal structure is mainly composed of martensite that does not contain cementite (i.e., when the metal structure is mainly composed of as-quenched martensite), sufficient hole expandability cannot be obtained. The average grain size of cementite is more preferably greater than 50 nm.

[0037] The upper limit of the total volume fraction of tempered martensite and bainite is preferably 83%, more preferably 80%, and even more preferably 75%.

[0038] The metal structure of the high-strength steel plate according to this embodiment further contains 5% by volume or more of ferrite and 3% by volume or more of retained austenite. By making the metal structure further contain 5% by volume or more of ferrite and 3% by volume or more of retained austenite, it is possible to further improve elongation.

[0039] The lower limit of the ferrite volume fraction is preferably 8%, more preferably 10%. The upper limit of the ferrite volume fraction is preferably 30%, more preferably 25%, and even more preferably 20%. The lower limit of the retained austenite volume fraction is preferably 5%, more preferably 6%, more preferably 8%, more preferably 12%, and even more preferably 15%. The upper limit of the retained austenite volume fraction is preferably 20%, and even more preferably 18%.

[0040] If the average grain size of ferrite is too large, the effect of improving elongation may not be obtained.The average grain size of ferrite is preferably 10.0 μm or less, and more preferably 8.0 μm or less.

[0041] In the high-strength steel sheet according to this embodiment, the area in the metal structure where the Mn concentration is 8.0 mass % or more is 0.05 to 1.5 area %. A small area ratio of the area where the Mn concentration is 8.0 mass % or more (hereinafter referred to as "Mn-enriched area") means that Mn is uniformly distributed. By reducing the area ratio of the Mn-enriched area, it is possible to suppress the generation of MA and improve hole expandability. On the other hand, if the Mn-enriched area is completely eliminated, excellent elongation cannot be obtained. By leaving the Mn-enriched area at 0.05 area % or more, work hardening during processing of the steel is promoted, and the elongation of the steel is improved.

[0042] The lower limit of the Mn-enriched region is preferably 0.10 area %, more preferably 0.20 area %, and the upper limit of the Mn-enriched region is preferably 1.0 area %.

[0043] The remainder of the metal structure of the high-strength steel plate according to this embodiment is, but is not limited to, pearlite, MA, etc. The volume fraction of the remainder of the metal structure of the high-strength steel plate according to this embodiment, excluding tempered martensite, bainite, ferrite, and retained austenite, is preferably 5% or less. The upper limit of the volume fraction of the remainder is preferably 3%.

[0044] The high-strength steel sheet according to this embodiment preferably has a tensile strength of 980 MPa or more. The lower limit of the tensile strength is more preferably 1050 MPa, even more preferably 1180 MPa, even more preferably 1260 MPa, and even more preferably 1320 MPa. The upper limit of the tensile strength is not particularly limited, but is, for example, 1450 MPa.

[0045] The high-strength steel plate according to this embodiment preferably has a yield ratio (yield strength / tensile strength) of 0.80 or more, and the lower limit of the yield ratio is more preferably 0.85.

[0046] The high-strength steel sheet according to this embodiment preferably has a total elongation of 14.0% or more. The lower limit of the total elongation is more preferably 15.0%, even more preferably 16.0%, and still more preferably 17.0%.

[0047] [Manufacturing method for high-strength steel plates] Next, an example of a method for manufacturing the above-mentioned high-strength steel sheet will be described. The manufacturing method described below is merely an example and does not limit the manufacturing method of the high-strength steel sheet according to this embodiment.

[0048] A hot-rolled steel sheet having the above-described chemical composition is prepared. The hot-rolled steel sheet can be produced, for example, by hot-rolling a slab having the above-described chemical composition by a conventional method and coiling it. The hot-rolling reduction ratio, the thickness after rolling, the cooling method to room temperature, the coiling conditions, etc. are not particularly limited.

[0049] The hot-rolled steel sheet is subjected to cold rolling with a reduction of 30% or more. If the reduction is too low, it becomes difficult to obtain the desired structure even if an appropriate heat treatment is subsequently performed. The lower limit of the reduction is preferably 50%. The upper limit of the reduction is preferably 80%, and more preferably 70%.

[0050] A predetermined heat treatment is applied to the cold-rolled steel sheet. Hereinafter, this heat treatment will be referred to as "post-cold rolling heat treatment." Figure 1 is a diagram that schematically shows the heat pattern of the post-cold rolling heat treatment. Figure 1 is a schematic diagram for explanatory purposes, and the length and inclination of the lines do not necessarily accurately reflect the actual temperature, holding time, heating rate, cooling rate, etc.

[0051] First, a cold-rolled steel sheet is heated to a temperature T1 of 860 to 1000° C. At this time, the average heating rate HR from 750° C. to the temperature T1 is set to 100° C. / second or more.

[0052] If the temperature T1 is too low, the structure does not transform to austenite, and the volume fraction of tempered martensite or bainite cannot be increased in the structure of the final high-strength steel sheet. On the other hand, if the temperature T1 is too high, the austenite grain size becomes coarse, and the final high-strength steel sheet cannot have a well-balanced structure of tempered martensite, bainite, ferrite, and retained austenite. The lower limit of the temperature T1 is preferably 880°C.

[0053] If the average heating rate HR is too low, ferrite recrystallization and grain growth will occur before the temperature T1 is reached. As ferrite recrystallization and grain growth progress, the density of grain boundaries, which serve as austenite precipitation sites, decreases, resulting in an increase in the size of individual austenite grains when a single-phase austenite structure is formed at temperature T1. If austenite grains become coarse, the grain size of tempered martensite and bainite in the final high-strength steel sheet structure will also increase, preventing excellent mechanical properties from being achieved. By setting the average heating rate HR to 100°C / s or higher, the structure is instantaneously exposed to high temperatures, reducing the size of reverse-transformed austenite, promoting Mn diffusion and eliminating Mn-enriched regions. In other words, Mn is more uniformly dispersed. The lower limit of the average heating rate HR is preferably 200°C / s, more preferably 300°C / s, even more preferably 400°C / s, and even more preferably 500°C. The upper limit of the average heating rate HR is not particularly limited, but is, for example, 1500°C / sec, preferably 1200°C / sec, 1000°C / sec, and preferably 800°C / sec. Heating at a heating rate of 100°C / sec or higher can be achieved, for example, but is not limited to, by high-frequency heating.

[0054] The step of heating to temperature T1 may involve, but is not limited to, switching the heating method at temperature Tc, which is a temperature of 600 to 750° C. For example, heating may be performed at a predetermined average heating rate from the start of heating to temperature Tc, and heating may be performed at an average heating rate HR from temperature Tc to temperature Tc.

[0055] 1 shows that heating is performed at a constant heating rate from the start of heating to 600°C and from 600°C to temperature T1, but this is merely an example, and the step of heating to temperature T1 only needs to have an average heating rate HR from 750°C to temperature T1 of 100°C / sec or more. Furthermore, the heating rate from the start of heating to 750°C may be any heating rate.

[0056] The temperature T1 is maintained for a time t1 of less than 10 seconds. If the time t1 is too long, the reverse-transformed austenite becomes coarse, and the final high-strength steel sheet does not have a well-balanced structure of tempered martensite, bainite, ferrite, and retained austenite. The upper limit of the time t1 is preferably 5 seconds, and more preferably 3 seconds. The lower limit of the time t1 is preferably 1 second.

[0057] After being held at temperature T1, the steel is held at temperature T2, which is 800 to 900°C and lower than temperature T1, for 10 seconds or more (t2). This allows Mn to diffuse in the metal structure, resulting in a uniform Mn concentration distribution. If temperature T2 is too low or time t1 is too short, Mn will not diffuse sufficiently, and the area ratio of Mn-enriched regions in the final high-strength steel sheet structure may not be 1.5 area % or less. On the other hand, if temperature T2 is too high, the concentration of the Mn-enriched regions will decrease due to elemental diffusion of Mn, falling below the lower limit of Mn concentration.

[0058] The lower limit of temperature T2 is preferably 810°C, more preferably 830°C. The upper limit of temperature T2 is preferably 870°C, more preferably 850°C. The lower limit of time t2 is preferably 20 seconds, more preferably 30 seconds, more preferably 40 seconds, and more preferably 60 seconds. The upper limit of time t2 is not particularly limited, but is, for example, 300 seconds.

[0059] After holding at temperature T2, the steel is cooled to temperature T3, which is 350°C or lower and the martensitic transformation start temperature or lower. At this time, the average cooling rate CR from 600°C to 450°C is set to 10.0 / s or higher. If temperature T3 is too high or the average cooling rate CR is too slow, it will be impossible to increase the volume fraction of tempered martensite or bainite in the final structure of the high-strength steel plate.

[0060] The upper limit of the temperature T3 is preferably 300°C, more preferably 200°C. The lower limit of the average cooling rate CR is preferably 20.0°C / sec, more preferably 30°C / sec, and even more preferably 40.0°C / sec. If the average cooling rate CR is too high, an appropriate amount of ferrite may not be obtained. The upper limit of the average cooling rate CR is preferably 60°C / sec.

[0061] After cooling to temperature T3, the material is held at temperature T4, which is 300 to 450°C and higher than temperature T3, for 50 seconds or longer for time t3. This causes cementite to precipitate in the metal structure. If temperature T4 is outside the above range or time t3 is too short, cementite may not precipitate sufficiently (the average grain size of cementite may not exceed 30 nm).

[0062] The lower limit of temperature T4 is preferably 320° C., and more preferably 340° C. The upper limit of temperature T4 is preferably 400° C., and more preferably 380° C. The lower limit of time t3 is preferably 120 seconds, and more preferably 240 seconds. The upper limit of time t3 is not particularly limited, but is, for example, 1000 seconds.

[0063] The above steps result in the production of a high-strength steel sheet. The above-described production method provides a high-strength steel sheet having a metallographic structure containing 70% by volume or more and less than 85% by volume of tempered martensite and bainite in total, 5% by volume or more of ferrite, and 3% by volume or more of retained austenite, and in which 0.05 to 1.5% by area of ​​the metallographic structure contains regions with a Mn concentration of 8.0% by mass or more.

[0064] The high-strength steel sheet according to one embodiment of the present invention and its manufacturing method have been described above. According to this embodiment, a high-strength steel sheet having high strength and excellent hole expandability can be obtained. [Example]

[0065] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0066] A 180 kg steel ingot having the chemical composition shown in Table 1 was melted in a high-frequency vacuum melting furnace and hot forged into a 30 mm thick slab. The resulting slab was hot rolled to a finishing temperature of 850 to 900°C using a hot rolling tester to produce a 2.0 mm thick hot-rolled steel sheet. After 3 to 10 seconds had elapsed since the completion of hot rolling, the steel sheet was cooled to a coiling temperature of 200 to 650°C, and then cooled to 200°C or below at a cooling rate of 20°C / s to simulate coiling of the steel sheet, to produce a hot-rolled steel sheet.

[0067] [Table 1]

[0068] Thereafter, cold rolling was carried out at the rolling reduction (cold rolling rate) shown in Table 2 to obtain steel materials.

[0069] [Table 2]

[0070] Test pieces measuring 160 mm in width and 230 mm in length were taken from the obtained steel material. Each of the test pieces was subjected to heat treatment (annealing) under the conditions shown in the "Manufacturing conditions" column of Table 2.

[0071] Specifically, the test material was heated to temperature T1 and held for time t1, and then held at temperature T2 for time t2. At this time, the heating method was switched at 600°C, so that the average heating rate from 400 to 600°C was HR0, and the average heating rate from 750°C to temperature T1 was HR. After holding at temperature T2, the material was cooled to temperature T3. At this time, the average cooling rate from 600°C to 450°C was CR. Then, heat treatment was performed by holding at temperature T4 for time t3.

[0072] The heat-treated test materials were subjected to microstructural observation, tensile testing, and hole expansion testing.

[0073] To evaluate the tensile properties, two JIS No. 5 test pieces were taken from each heat-treated test material, with the longitudinal direction parallel to the rolling direction (L direction), and tested at a tension speed of 10 mm / min. From the results, the tensile strength (TS / MPa), yield strength (YS / MPa), and total elongation (EL / %) were calculated. The tensile strength, yield strength, and total elongation were calculated as the average of the two tensile test pieces.

[0074] To measure the hole expansion ratio, two 90 mm × 90 mm test pieces were taken from the heat-treated test material, and a punched hole with an initial hole diameter d0 = 10 mm was drilled in the center of each piece. This hole was then expanded using a conical punch with a tip angle of 60°. When a crack appeared at the punched end and penetrated the plate thickness, the expansion was stopped and the load was removed. The hole diameter of the test piece was measured in two directions: parallel to the rolling direction, perpendicular to the rolling direction, and at a 45° angle. The average value was taken as the hole diameter d after hole expansion. The hole expansion ratio HER was evaluated using the following formula. The hole expansion ratio HER was calculated as the average of the two test pieces. HER(%)=(d-d0) / d0×100

[0075] The metal structure of the test material after the heat treatment was measured by the following method.

[0076] First, a specimen was taken from the heat-treated test material so that the cross section parallel to the rolling direction and thickness direction would be the observation surface. Electron backscatter diffraction (EBSD) analysis was performed on a 200 μm L × 100 μm T area of ​​the L cross section (a cross section parallel to the rolling direction and thickness direction) of the test material. The scan step was 0.1 μm, and the measurement points were arranged in a hexagonal grid.

[0077] When the EBSD data was analyzed and the crystalline phases were identified from the EBSD patterns at the measurement points, the regions identified as BCC phase were considered to be either tempered martensite, bainite, or ferrite, the regions identified as FCC phase were considered to be retained austenite, and the regions where the structure could not be identified (for example, when the Confidential Index value was less than 0.1 in the TSL EBSD system) were considered to be MA or cementite contained in pearlite.

[0078] Furthermore, the regions identified as BCC phase were classified into ferrite and non-ferrite (tempered martensite or bainite) as follows.

[0079] In the EBSD data, boundaries with a crystal orientation difference of 15° or more were defined as grain boundaries, and the area surrounded by the grain boundaries was considered as crystal grains. When the number of EBSD measurement points included within a BCC phase crystal grain is m, the rotation matrix P j For (j=1···m), the crystal orientation misorientation Δθ (unit: degree) between adjacent measurement points was calculated for all combinations using the following formula, and the average value M of these misorientations was obtained.

[0080]

number

[0081] P i or P j is a rotation matrix that can be calculated using the Euler angles of each measurement point obtained by EBSD measurement. Specifically, P i is obtained by substituting the Euler angles (φ1, Φ, φ2) of the ith measurement point (i = 1 m) obtained by EBSD measurement into the following equation:

[0082]

number

[0083] R k (k=1 24) is the rotation matrix for transforming the cubic crystal basis axes, and R k P i P jSelect k so that the rotation angle of is minimized. The i-th and j-th measurement points are selected to be adjacent. D[i, i] represents the value of the element in the i-th row and i-th column of the 3x3 matrix D. BCC grains with an average crystal orientation misorientation M of 0.5° or less were considered to be ferrite, and grains with an average misorientation M of more than 0.5° were considered to be tempered martensite or bainite.

[0084] After classifying the structure in this way, the percentages of measurement points determined to be BCC phases other than ferrite (tempered martensite or bainite) and measurement points determined to be ferrite were calculated, and these were taken as the total volume fraction of tempered martensite and bainite and the volume fraction of ferrite, respectively.

[0085] The average grain size of ferrite was determined by calculating the average value of the circle equivalent diameter d from the following equation in the EBSD measurement.

[0086]

number

[0087] where A i is the area of ​​the ith ferrite analyzed in the EBSD data, and d i is the circle equivalent diameter of the i-th ferrite. The circle equivalent diameter is the area of ​​the i-th ferrite (= A i ) means the diameter of a circle with an area equal to

[0088] Mn concentration mapping measurements were performed on the same observation sample as above using an EPMA (Electron Probe Micro Analyzer). Using a JEOL JXA-8530F, the electron beam acceleration voltage was set to 7 kV and the probe current to 20 nA. The electron beam scanned a 30 μm × 30 μm area of ​​the observation surface, dividing it into 500 × 500 points (arranged in a square grid), and the characteristic X-rays of Mn (Kα rays) emitted from each point were measured using a wavelength-resolving detector.

[0089] From the obtained characteristic X-ray count I, the Mn concentration X at each point is calculated.Mn was calculated using the following evaluation formula based on the ZAF correction: X Mn =I / I0×Z×A×F

[0090] Here, I0 represents the count number of characteristic X-rays (Kα rays) obtained when a standard pure Mn sample is subjected to EPMA measurement under the same electron beam conditions. Z, A, and F represent the atomic number correction, absorption correction factor, and correction factor, respectively. These values ​​were taken from Hiroyoshi Soejima, "Electron Beam Microanalysis," published by Nikkan Kogyo Shimbun, and known values ​​determined by the EPMA electron beam irradiation conditions and the composition values ​​of the steel material were used.

[0091] From the obtained Mn concentration distribution, the proportion of measurement points with a Mn concentration of 8.0 mass % or more to all measurement points (250,000 points) was calculated, and this was taken as the area ratio of the region with a Mn concentration of 8.0 mass % or more.

[0092] The results are shown in Table 3. In Table 3, "TM+B" indicates the total volume fraction of tempered martensite and bainite (except for test number 18, which is the volume fraction of as-quenched martensite), "α" indicates the volume fraction of ferrite, and "residual γ" indicates the volume fraction of retained austenite. "Grain size" indicates the average grain size of ferrite. * " indicates the area ratio of the region where the Mn concentration is 8.0 mass % or more.

[0093] [Table 3]

[0094] As shown in Tables 1 to 3, the test materials of test numbers 1 to 3, 5 to 7, 10, 19, and 20 contained a total of 70 to 85 volume % of tempered martensite and bainite, 5 volume % or more of ferrite, 3 volume % or more of retained austenite, and 0.05 to 1.5 area % of the region in the metal structure where the Mn concentration was 8.0 mass % or more. These test materials had a tensile strength of 980 MPa or more, an elongation of 14.0% or more, and a hole expansion ratio of 36.0% or more.

[0095] In contrast, the test materials of test numbers 4, 8, 9, 11 to 18, and 21 to 24 had hole expansion ratios of less than 36.0%.

[0096] In the test material of test number 4, the total volume fraction of tempered martensite and bainite was less than 70%, which is thought to be because the reduction ratio in cold rolling was too small.

[0097] In the test material of test number 8, the total volume fraction of tempered martensite and bainite was less than 70%, which is thought to be because the average heating rate HR was too high.

[0098] In the test material of test number 9, the total volume fraction of tempered martensite and bainite was less than 70%, which is thought to be because the temperature T1 was too low.

[0099] In the test material of test number 11, the total volume fraction of tempered martensite and bainite was less than 70%, which is thought to be because the temperature T1 was too high.

[0100] In the test material of test number 12, the total volume fraction of tempered martensite and bainite was less than 70%, which is thought to be because the time t1 was too long.

[0101] In the test material of test number 13, the total volume fraction of tempered martensite and bainite was less than 70%, which is thought to be because the temperature T2 was too low.

[0102] In the test material of test number 14, the total volume fraction of tempered martensite and bainite was less than 70%, which is thought to be because the temperature T2 was too high.

[0103] In the test material of test number 15, the area in the metal structure where the Mn concentration was 8.0 mass % or more exceeded 1.5 area %, which is thought to be because the time t2 was too short.

[0104] In the test material of test number 16, the total volume fraction of tempered martensite and bainite was less than 70%, which is thought to be because the average cooling rate CR was too small.

[0105] In the test material of test number 17, the total volume fraction of tempered martensite and bainite was less than 70%, which is thought to be because the temperature T3 was too low.

[0106] The test material of test number 18 had a structure mainly composed of martensite as quenched. This is thought to be because the heat treatment of heating to temperature T4 after cooling to temperature T3 was not performed.

[0107] In the test materials of test numbers 21 to 23, the total volume fraction of tempered martensite and bainite was less than 70%, which is thought to be because the chemical composition was inappropriate.

[0108] The volume fraction of ferrite in the test material of test number 24 was less than 5%, which is thought to be due to an inappropriate chemical composition.

[0109] Although the embodiments of the present invention have been described above, the above-described embodiments are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiments, and the above-described embodiments can be appropriately modified and carried out without departing from the spirit of the present invention.

Claims

1. The chemical composition, in mass%, is C: 0.10-0.30%, Si: 0.50 to 2.50%, Mn: 0.50 to 3.50%, P: 0.100% or less, S: 0.020% or less, Al: 0.010-0.100%, N: 0.0100% or less, The balance is Fe and impurities. The steel has a metal structure containing tempered martensite and bainite in a total amount of 70% by volume or more and less than 85% by volume, ferrite in an amount of 5% by volume or more, and retained austenite in an amount of 3% by volume or more, The high-strength steel plate has a region in the metal structure where the Mn concentration is 8.0 mass% or more of 0.05 to 1.5 area%.

2. The chemical composition, in mass%, is C: 0.10-0.30%, Si: 0.50 to 2.50%, Mn: 0.50 to 3.50%, P: 0.100% or less, S: 0.020% or less, Al: 0.010-0.100%, N: 0.0100% or less, and further Contains one or more selected from the group consisting of Ti: 0.100% or less, Nb: 0.100% or less, V: 0.50% or less, Cr: 1.00% or less, Mo: 0.50% or less, W: 0.50% or less, B: 0.0050% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, and REM: 0.0100% or less, The balance is Fe and impurities. The steel has a metal structure containing tempered martensite and bainite in a total amount of 70% by volume or more and less than 85% by volume, ferrite in an amount of 5% by volume or more, and retained austenite in an amount of 3% by volume or more, The high-strength steel plate has a region in the metal structure where the Mn concentration is 8.0 mass% or more of 0.05 to 1.5 area%.

3. The high-strength steel plate according to claim 1 or 2, A high-strength steel plate, wherein the average grain size of the ferrite is 10.0 μm or less.

4. A method for producing the high-strength steel plate according to claim 1 or 2, A step of cold rolling the hot-rolled steel sheet at a rolling reduction rate of 30% or more; Heating the cold-rolled steel sheet to a temperature T1 of 860 to 1000°C; maintaining the temperature T1 for a time period of less than 10 seconds; After holding at the temperature T1, holding at a temperature T2 of 800 to 900° C., which is lower than the temperature T1, for 10 seconds or more; After holding at the temperature T2, a step of cooling to a temperature T3 that is 350°C or less and a martensitic transformation start temperature or less; After cooling to the temperature T3, the method comprises a step of holding the temperature T4, which is 300 to 450° C. and is higher than the temperature T3, for 50 seconds or more; In the step of heating to the temperature T1, the average heating temperature from 750°C to the temperature T1 is 100.0°C / sec or more, In the step of cooling to the temperature T3, the average cooling rate from 600°C to 450°C is 10.0°C / second or more.

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