Cold-rolled steel sheet and method for manufacturing the same

Abstract

The present invention provides a cold-rolled steel sheet which has high strength and excellent bore expanding properties. A cold-rolled steel sheet according to the present invention has a chemical composition that contains, in mass%, 0.10-0.30% of C, 2.50% or less 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, 0-0.100% of Ti, 0-0.100% of Nb, 0-0.50% of V, 0-0.50% of Mo, 0-0.50% of W, 0-0.0050% of B, 0-0.0100% of Ca, 0-0.0100% of Mg and 0-0.0100% of REM, with the balance being made up of Fe and impurities, while having a metal structure that contains a total of 85% by volume or more of tempered martensite and bainite and 5% by volume or less of structures other than tempered martensite, bainite and ferrite, with 0.05-1.5% by area of regions that have an Mn concentration of 8.0% by mass or more in the metal structure.

Description

[Technical Field]

[0001] The present invention relates to a cold-rolled steel sheet and a method for manufacturing the cold-rolled steel sheet. [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. [Prior art documents] [Patent documents]

[0007] [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 Summary of the Invention [Problem to be solved by the invention]

[0008] An object of the present invention is to provide a cold-rolled steel sheet having high strength and excellent formability, more specifically, to provide a cold-rolled steel sheet having high strength and excellent hole expandability. [Means for solving the problem]

[0009] The cold rolled steel sheet according to one embodiment of the present invention has a chemical composition, in mass%, of C: 0.10 to 0.30%, Si: 2.50% or less, 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 to 0.100%, Nb: 0 to 0.100%, V: 0 to 0.50%, Mo: 0 to 0.50%, W: 0 to 0.50%, B: 0 to 0. 0.0050%, Ca: 0-0.0100%, Mg: 0-0.0100%, REM: 0-0.0100%, balance: Fe and impurities, the metal structure contains a total of 85 volume % or more of tempered martensite and bainite, structures other than tempered martensite, bainite and ferrite are 5 volume % or less, and the area in the metal structure where the Mn concentration is 8.0 mass % or more is 0.05-1.5 area %.

[0010] A method for producing a cold-rolled steel sheet according to one embodiment of the present invention is a method for producing the above-mentioned cold-rolled steel sheet, and includes the steps of: heat-treating the hot-rolled steel sheet by holding it in a temperature range of 620 to 700°C for 10 minutes or more; cold-rolling the heat-treated steel sheet; holding the cold-rolled steel sheet in a temperature range of 880 to 1050°C for 10 seconds or more, and then cooling it to a temperature of 450°C or less at a cooling rate of 20°C / second or more; and, after cooling it to a temperature of 450°C or less in the cooling step, performing one of the following treatments (A) and (B): (A) A process in which the material is cooled to a temperature below 350°C and below the martensitic transformation start temperature (Ms point), then reheated and held in the temperature range of 300 to 450°C. (B) A process in which the temperature is maintained in the range of 300 to 450°C during cooling without reheating. [Effects of the Invention]

[0011] According to the present invention, a cold-rolled steel sheet having high strength and excellent hole expandability can be obtained. DETAILED DESCRIPTION OF THE INVENTION

[0012] The present inventors have conducted various studies to solve the above problems and have come to the following findings.

[0013] To increase the strength of cold-rolled steel sheets, appropriate heat treatment is required to create a metal structure primarily composed of tempered martensite and bainite. Furthermore, achieving a uniform distribution of Mn in the metal structure is effective for achieving excellent hole expandability. A 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.

[0014] Specifically, the metallographic structure contains a total of 85% by volume or more of tempered martensite and bainite, and further, the area ratio of regions in the metallographic structure where the Mn concentration is 8.0% by mass or more in the cross section of the steel sheet is 0.05 to 1.5% by area. A small area ratio of regions where the Mn concentration is 8.0% by mass or more (hereinafter referred to as "Mn-enriched regions") means that Mn is uniformly distributed. By reducing the area ratio of the Mn-enriched regions, it is possible to suppress the generation of MA and improve hole expandability. On the other hand, if the Mn-enriched regions are completely eliminated, excellent elongation cannot be obtained. By leaving only a small area ratio of the Mn-enriched regions (0.05% by area or more), work hardening during processing of the steel is promoted, improving the elongation of the steel.

[0015] To obtain this metal structure, the following heat treatment is effective. First, the hot-rolled steel sheet is subjected to heat treatment by holding it in the temperature range of 620 to 700°C for 10 minutes or more. This generates cementite in the steel, and Mn, generally 10 mass% or more, is concentrated therein. After cold-rolling this steel sheet, it is held at a temperature of 880 to 1050°C, which diffuses the Mn in the metal structure and homogenizes its distribution. Martensite and bainite are generated by cooling this steel sheet to a temperature of 450°C or less at a cooling rate of 20°C / sec or more.

[0016] In a preferred embodiment, the metal structure further contains 5% by volume or more of ferrite, and the average grain size of the ferrite is 10.0 μm or less. By containing 5% by volume or more of ferrite, the elongation can be further improved.

[0017] The present invention has been completed based on the above findings. A cold-rolled steel sheet according to one embodiment of the present invention will now be described.

[0018] [Cold rolled steel plate] [Chemical composition] A cold-rolled steel sheet 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 %.

[0019] 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%.

[0020] Si:2.50% or less Silicon (Si) is a solid solution strengthening element, but excessive Si content reduces the hot workability of steel. Therefore, the Si content is set to 2.50% or less. The upper limit of the Si content is preferably 2.00%, more preferably 1.50%. The lower limit of the Si content is preferably 0.10%, more preferably 0.30%.

[0021] 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%.

[0022] 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.

[0023] 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.

[0024] 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%.

[0025] 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%.

[0026] The chemical composition of the cold-rolled steel sheet according to this embodiment may contain one or more elements selected from the group consisting of Ti, Nb, V, Mo, W, B, Ca, Mg, and REM. All of these elements are optional elements. That is, the chemical composition of the cold-rolled steel sheet according to this embodiment may not contain some or all of Ti, Nb, V, Mo, W, B, Ca, Mg, and REM.

[0027] Ti: 0 to 0.100% Nb: 0 to 0.100% V: 0 to 0.50% Mo: 0 to 0.50% W: 0 to 0.50% Titanium (Ti), niobium (Nb), vanadium (V), 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, Mo, and W are contained. On the other hand, excessive contents of Ti, Nb, V, Mo, and W reduce the elongation of steel. Therefore, the contents of Ti and Nb are 0 to 0.100%, and the contents of V, Mo, and W are 0 to 0.50%. The lower limits of Ti and Nb are preferably 0.005%. The upper limits of Ti and Nb are preferably 0.060%, more preferably 0.040%. The lower limits of V, Mo, and W are preferably 0.01%. The upper limits of V, Mo, and W are preferably 0.40%, more preferably 0.30%.

[0028] B: 0 to 0.0050% Boron (B) improves the hardenability and strength of steel. This effect can be obtained 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 to 0.0050%. 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%.

[0029] Ca: 0 to 0.0100% Mg: 0 to 0.0100% REM: 0 to 0.0100% 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 set to 0 to 0.0100%. 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%, and more preferably 0.0040%. REM is a collective term for a total of 17 elements, including Sc, Y, and lanthanides, and the REM content refers to the total amount of the above elements.

[0030] The balance of the chemical composition of the cold-rolled steel sheet 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, Cr, and O.

[0031] [Metal structure] The metal structure of the cold-rolled steel sheet according to this embodiment contains tempered martensite and bainite in a total volume percentage of 85% or more. If the total volume percentage of tempered martensite and bainite is less than 85%, the required strength cannot be obtained. Note that the above-mentioned "bainite" includes tempered bainite. The bainite in the metal structure of the cold-rolled steel sheet 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 to be an independent structure, but is included in the total volume of the tempered martensite and bainite.

[0032] The average grain size of cementite contained in tempered martensite and bainite is more than 30 nm. The average grain size of cementite is measured by taking an extracted replica film from a cold-rolled steel sheet and observing it in three or more 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 preferably more than 50 nm.

[0033] In the cold-rolled steel sheet according to this embodiment, the area ratio of the region 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 region where the Mn concentration is 8.0 mass % or more (hereinafter referred to as "Mn-enriched region") means that Mn is uniformly distributed. By reducing the area ratio of the Mn-enriched region, it is possible to suppress the generation of MA and improve hole expandability. On the other hand, if the Mn-enriched region is completely eliminated, excellent elongation cannot be obtained. By leaving the Mn-enriched region at 0.05 area % or more, work hardening during processing of the steel is promoted, and the elongation of the steel is improved.

[0034] 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 %.

[0035] The metal structure of the cold-rolled steel sheet according to this embodiment preferably further contains 5% by volume or more of ferrite, and the average grain size of the ferrite is 10.0 μm or less. By making the metal structure contain 5% by volume or more of ferrite, the elongation can be further improved. The lower limit of the volume fraction of ferrite is more preferably 10%. However, if the average grain size of ferrite is too large, the effect of improving elongation cannot be obtained. The upper limit of the average grain size of ferrite is more preferably 5.0 μm.

[0036] The remainder of the metal structure of the cold-rolled steel sheet according to this embodiment is, but is not limited to, pearlite, MA, retained austenite, etc. In the metal structure of the cold-rolled steel sheet according to this embodiment, the volume fraction of the remainder excluding martensite, tempered martensite, and ferrite is 5% or less. The upper limit of the volume fraction of the remainder is preferably 3%.

[0037] The cold-rolled 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, and even more preferably 1180 MPa. The upper limit of the tensile strength is not particularly limited, but is, for example, 1450 MPa.

[0038] The cold-rolled steel sheet according to this embodiment preferably has a breaking elongation of 8.0% or more, and the lower limit of the breaking elongation is more preferably 10.0%.

[0039] [Method of manufacturing cold-rolled steel sheets] Next, an example of a method for manufacturing the above-mentioned cold-rolled steel sheet will be described. The manufacturing method described below is merely an example and does not limit the manufacturing method of the cold-rolled steel sheet according to this embodiment.

[0040] 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.

[0041] The hot-rolled steel sheet is held in a temperature range of 620 to 700°C for 10 minutes or more. Hereinafter, this heat treatment is referred to as "post-hot rolling heat treatment." This post-hot rolling heat treatment forms cementite in the steel, and Mn is concentrated therein. If the holding temperature is too low or the holding time is too short, Mn is not sufficiently concentrated, and the area ratio of Mn-enriched regions in the final structure of the cold-rolled steel sheet may not be 0.05 area % or more. On the other hand, if the holding temperature is too high, austenite is formed in part of the structure, resulting in insufficient Mn concentration. The lower limit of the holding temperature for the post-hot rolling heat treatment is preferably 630°C. The upper limit of the holding temperature for the post-hot rolling heat treatment is preferably 680°C. The lower limit of the holding time for the post-hot rolling heat treatment is preferably 30 minutes, and more preferably 60 minutes. The upper limit of the holding time for the post-hot rolling heat treatment is not particularly limited, but may be, for example, 300 minutes. The cooling rate after holding may be any rate.

[0042] Furthermore, due to the thermodynamic properties of cementite, the concentration of Mn concentrated in cementite may become excessively high when heat treatment after hot rolling is performed at around 600° C. Therefore, if the temperature of heat treatment after hot rolling is less than 620° C., the local Mn concentration becomes excessively high, and Mn cannot be sufficiently diffused in heat treatment after cold rolling, which may make it impossible to reduce the area ratio of Mn-enriched regions to 1.5% or less.

[0043] The steel sheet that has been subjected to the heat treatment after hot rolling is then cold rolled, with the sheet thickness reduction rate in the cold rolling being, for example, 30 to 80%.

[0044] The cold-rolled steel sheet is held in a temperature range of 880 to 1050°C for 10 seconds or more, and then cooled to a temperature of 450°C or less at a cooling rate of 20°C / second or more. Hereinafter, this heat treatment is referred to as "post-cold rolling heat treatment."

[0045] By holding the steel sheet in a temperature range of 880 to 1050°C for 10 seconds or more, Mn diffuses in the metal structure, resulting in a uniform Mn concentration distribution. If the holding temperature is too low or the holding time is too short, Mn does not diffuse sufficiently, and the area ratio of Mn-enriched regions in the final cold-rolled steel sheet structure may not be 1.5 area% or less. On the other hand, if the holding temperature is too high, the concentration of the Mn-enriched regions decreases due to elemental diffusion of Mn, and falls below the lower limit of Mn concentration. The lower limit of the holding temperature for the heat treatment after cold rolling is preferably 900°C, more preferably 920°C. The upper limit of the holding temperature for the heat treatment after cold rolling is preferably 1000°C. The lower limit of the holding time for the heat treatment after cold rolling is preferably 20 seconds, more preferably 30 seconds. The upper limit of the holding time for the heat treatment after cold rolling is not particularly limited, but is, for example, 300 seconds.

[0046] After the steel sheet is held in the temperature range of 880 to 1050°C, it is cooled to a temperature of 450°C or less at a cooling rate of 20°C / sec or more. At this time, a part of the structure is transformed into martensite. A metal structure containing 5% or more by volume of ferrite can be obtained by setting the cooling rate at this time to a relatively slow rate of 20 to 30°C / sec. If the cooling rate is less than 20°C / sec, excessive ferrite is formed, and it may not be possible to achieve a total volume fraction of tempered martensite and bainite of 85% or more in the final structure of the cold-rolled steel sheet. The upper limit of the cooling rate is not particularly limited, but is, for example, 200°C / sec.

[0047] After cooling to a temperature of 450°C or less, either (A) the material is once cooled to a temperature of 350°C or less and the martensitic transformation start temperature (Ms point) or less, and then reheated and held in the temperature range of 300 to 450°C for a predetermined time (preferably 100 seconds or more) (Quench & Partitioning treatment), or (B) the material is not reheated but is held in the temperature range of 300 to 450°C for a predetermined time (preferably 100 seconds or more) during cooling (Austempering treatment).

[0048] The cementite precipitated in the heat treatment after hot rolling is dissolved once in the heat treatment after cold rolling. The amount of dissolved cementite at this time is 90% by volume or more, preferably 95% by volume or more, of the cementite before the heat treatment after cold rolling. After the heat treatment after cold rolling and subsequent cooling, cementite is precipitated again in the metal structure by performing either the treatment (A) or (B) described above. This results in a metal structure containing 85% by volume or more of tempered martensite and bainite in total. Note that if the holding temperature is less than 300°C, cementite may not precipitate sufficiently (the average grain size of the cementite may not exceed 30 nm).

[0049] The cold-rolled steel sheet according to the present embodiment is manufactured by the above-described steps. According to the manufacturing method described above, a cold-rolled steel sheet can be obtained that has a metallographic structure containing 85% by volume or more of tempered martensite and bainite in total, with structures other than tempered martensite, bainite, and ferrite being 5% by volume or less, and in which the area of ​​the metallographic structure where the Mn concentration is 8.0% by mass or more is 0.05 to 1.5% by area.

[0050] The cold-rolled steel sheet and the manufacturing method thereof according to one embodiment of the present invention have been described above. According to this embodiment, a cold-rolled steel sheet having high strength and excellent hole expandability can be obtained. [Example]

[0051] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0052] 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.

[0053] [Table 1]

[0054] The hot-rolled steel sheets were heat-treated (heat treatment after hot rolling) at the temperatures and holding times shown in Table 2, and then cold-rolled at a thickness reduction rate (cold rolling rate) of 50% to obtain steel materials with a thickness of 1.0 mm.

[0055] [Table 2]

[0056] 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 "Heat treatment after cold rolling" column in Table 2.

[0057] Specifically, the test materials were heated to the temperature shown in Table 2 and soaked for the time shown in Table 2. After soaking, they were cooled to a temperature of 450°C or less at the cooling rate shown in Table 2. Test materials Nos. 1 to 7, 12 to 15, 17 to 21, 26 to 29, and 31 were subsequently cooled to a temperature of 350°C or less and the martensitic transformation start temperature (Ms point) or less, reheated, and then subjected to a quenching and partitioning treatment in which the material was held in the temperature range of 350 to 400°C for 300 seconds before being cooled to room temperature. Test materials Nos. 8 to 11, 16, 22, 24, 25, 30, and 32 were not reheated, but instead were subjected to an austempering treatment in which the material was held in the temperature range of 350 to 400°C for 300 seconds before being cooled to room temperature. Test material No. 23 was cooled to room temperature without any of the above treatments.

[0058] The heat-treated test materials were subjected to microstructural observation, tensile testing, and hole expansion testing.

[0059] 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) and elongation at break (tEL / %) were calculated. The tensile strength and elongation at break were calculated as the average of the two tensile test pieces.

[0060] 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

[0061] The metal structure of the test material after the heat treatment was measured by the following method.

[0062] 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.

[0063] 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.

[0064] Furthermore, the regions identified as BCC phase were classified into ferrite and non-ferrite (tempered martensite or bainite) as follows.

[0065] 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.

[0066]

number

[0067] 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:

[0068]

number

[0069] R k (k=1 24) is the rotation matrix for transforming the cubic crystal's prescribed 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.

[0070] After this structural classification, the percentages of measurement points determined to be BCC phases other than ferrite (tempered martensite or bainite) and ferrite were calculated, and these were defined as the total volume fraction of tempered martensite and bainite, and the volume fraction of ferrite, respectively. The remaining regions included pearlite, MA, and retained austenite (regions identified as FCC phases based on EBSD), and were classified as "other regions."

[0071] The average grain size of ferrite was determined by calculating the average value of the circle equivalent diameter d in the EBSD measurement using the following formula. However, cases where the volume fraction of ferrite was less than 5% were not evaluated.

[0072]

number

[0073] 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

[0074] Mapping measurements of the Mn concentration in the tissue were performed using an EPMA (Electron Probe Micro Analyzer) on the same observation sample as above. Using a JEOL JXA-8530F, the electron beam acceleration voltage was set to 7 kV and the irradiation 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 Mn fluorescent X-rays (Kα rays) emitted from each point were measured using a wavelength-resolved detector.

[0075] From the obtained fluorescent 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

[0076] Here, I0 represents the number of fluorescent X-ray (Kα ray) counts 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 coefficient, and fluorescence correction coefficient, 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.

[0077] From the obtained Mn concentration distribution, the proportion of measurement points where the Mn concentration was 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 where the Mn concentration was 8.0 mass % or more.

[0078] The results are shown in Table 3. In Table 3, "TM+B" indicates the total volume fraction of tempered martensite and bainite, and "α" indicates the volume fraction of ferrite. * " indicates the area ratio of the region where the Mn concentration is 8.0 mass % or more.

[0079] [Table 3]

[0080] As shown in Tables 1 to 3, the test materials of test numbers 1 to 16 had a metallographic structure containing 85% or more by volume of tempered martensite and bainite in total, with structures other than tempered martensite, bainite, and ferrite being 5% or less by mass, and the area of ​​the metallographic structure containing regions with a Mn concentration of 8.0% or more by mass was 0.05 to 1.5% by area. The average grain size of cementite contained in the tempered martensite and bainite in all of these test materials exceeded 50 nm. These test materials had tensile strengths of 980 MPa or more, elongations of 8.0% or more, and hole expansion ratios of 40.0% or more. Among these, the test materials of test numbers 6, 7, 11, and 12 had a metallographic structure further containing 5% or more by volume of ferrite, with the average grain size of the ferrite being 10.0 μm or less. These test materials had particularly excellent elongation.

[0081] In contrast, the test materials of test numbers 17, 19, 21, 22, 24 to 26, 29, and 30 had a region with a Mn concentration of 8.0 mass% or more in the metallographic structure of more than 1.5 area%, and the hole expansion ratio was less than 40.0%. The large area ratio of the Mn-enriched region in the test materials of test numbers 17, 19, 24, 25, 27, 29, and 30 is thought to be due to the fact that the holding temperature of the heat treatment after cold rolling was too low. The large area ratio of the Mn-enriched region in the test materials of test numbers 21 and 26 is thought to be due to the fact that the holding time of the heat treatment after cold rolling was too short. The large area ratio of the Mn-enriched region in the test material of test number 22 is thought to be due to the fact that the temperature of the heat treatment after hot rolling was around 600°C.

[0082] In the test materials of test numbers 18 and 27, the area where the Mn concentration in the metal structure was 8.0 mass% or more was less than 0.05 area%, and the hole expansion ratio was less than 40.0%. The reason why the area ratio of the Mn-enriched area in these test materials was small is thought to be that the temperature of the heat treatment after hot rolling was too low, so Mn did not concentrate in the cementite in the steel.

[0083] The test materials of test numbers 20, 28, 31, and 32 had a total volume fraction of tempered martensite and bainite of less than 85%, and a tensile strength of less than 980 MPa. The reason why the test materials of test numbers 20 and 28 had a total volume fraction of tempered martensite and bainite of less than 85% is thought to be because the cooling rate in the heat treatment after cold rolling was too slow. In addition, in these test materials, the hole expansion ratio was less than 40.0% due to the influence of excessive ferrite formation. The reason why the test materials of test numbers 31 and 32 had a total volume fraction of tempered martensite and bainite of less than 85% is thought to be because the Mn content or C content of the steel material was too low.

[0084] The test material of test number 23 had a total volume fraction of tempered martensite and bainite of less than 85%, and a hole expansion ratio of less than 40.0%. The reason why the test material of test number 23 had a total volume fraction of tempered martensite and bainite of less than 85% is thought to be that the test material was cooled to room temperature without undergoing either the Quench & Partitioning treatment or the Austempering treatment, and therefore most of the structure became martensite without precipitation of cementite.

[0085] 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: 2.50% or less, 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, Ti: 0 to 0.100%, Nb: 0 to 0.100%, V: 0 to 0.50%, Mo: 0 to 0.50%, W: 0 to 0.50%, B: 0 to 0.0050%, Ca: 0-0.0100%, Mg: 0 to 0.0100%, REM: 0-0.0100%, The balance is Fe and impurities. The metal structure contains tempered martensite and bainite in a total amount of 85% by volume or more, and structures other than tempered martensite, bainite, and ferrite are 5% by volume or less, The cold-rolled steel sheet 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 cold-rolled steel sheet according to claim 1, The metal structure further includes 5% by volume or more of ferrite, The cold-rolled steel sheet has an average grain size of ferrite of 10.0 μm or less.

3. The cold-rolled steel sheet according to claim 1 or 2, The chemical composition is, in mass %, Ti: 0.005-0.100%, Nb: 0.005-0.100%, V: 0.01-0.50%, Mo: 0.01 to 0.50%, and W: 0.01-0.50%, A cold-rolled steel sheet containing one or more selected from the group consisting of:

4. The cold-rolled steel sheet according to claim 1 or 2, The chemical composition is, in mass %, B: 0.0003 to 0.0050%, A cold-rolled steel sheet containing

5. The cold-rolled steel sheet according to claim 1 or 2, The chemical composition is, in mass %, Ca: 0.0001-0.0100%, Mg: 0.0001 to 0.0100%, and REM: 0.0001-0.0100%, A cold-rolled steel sheet containing one or more selected from the group consisting of:

6. The cold-rolled steel sheet according to claim 1 or 2, The tempered martensite and bainite in the metal structure contain cementite having an average grain size of more than 30 nm.

7. A method for producing the cold-rolled steel sheet according to claim 1 or 2, A step of heat treating the hot-rolled steel sheet by holding it in a temperature range of 620 to 700 ° C. for 10 minutes or more; cold rolling the heat-treated steel sheet; a step of holding the cold-rolled steel sheet in a temperature range of 880 to 1050°C for 10 seconds or more, and then cooling it to a temperature of 450°C or less at a cooling rate of 20°C / second or more; a step of cooling the steel sheet to a temperature of 450°C or less in the cooling step, and then performing one of the following treatments (A) and (B): (A) A process in which the steel is cooled to a temperature below 350°C and below the martensitic transformation start temperature (Ms point), then reheated and held in the temperature range of 300 to 450°C. (B) Treatment of holding the temperature in the 300 to 450°C range during cooling without reheating

Images