Steel sheet and component including same

Abstract

The present invention provides: a steel sheet that has high strength, excellent ductility, and excellent resistance to internal cracking during bending; and a component that comprises the same. Provided are: a steel sheet characterized by having a predetermined chemical composition and including a metal structure that contains 70-95% bainite and 5-30% martensite by area percentage at a position of one-fourth of the plate thickness from the surface, the steel sheet also being characterized in that the number density of Si oxides having an equivalent circle diameter of 5 μm or more on the surface is less than 60 / mm2, and the difference between the maximum value and the minimum value of the surface roughness Ra is 1.5 μm or less; and a component that comprises the same.

Description

Steel plates and parts containing them

[0001] This invention relates to steel plates and parts containing them.

[0002] In recent years, the automotive industry has been demanding lighter vehicle bodies from the perspective of improving fuel efficiency. To achieve both vehicle weight reduction and collision safety, increasing the strength of the steel plates used is one effective method, and for this reason, the development of high-strength steel plates is progressing.

[0003] On the other hand, many automotive components are manufactured by press forming. It is generally known that the formability of steel sheets decreases as their strength increases.

[0004] In this regard, Patent Document 1 discloses a high-strength steel sheet having a predetermined chemical composition, wherein the microstructure includes, in the surface region from the steel sheet surface to the 1 / 10 thickness position, upper bainite accounting for 80% or more by area ratio and fresh martensite and / or retained austenite accounting for 2% or more by total area ratio, upper bainite accounting for 70% or more by area ratio and fresh martensite and / or retained austenite accounting for 3% or more by total area ratio in the internal region from the 1 / 10 thickness position to the 3 / 10 thickness position, the average grain size in the surface region from the steel sheet surface to the 1 / 10 thickness position is 6 μm or less, and the difference (HV2-HV1) between the hardness of the surface region from the steel sheet surface to the 1 / 10 thickness position and the hardness of the internal region from the 1 / 10 thickness position to the 3 / 10 thickness position is 5% to 15% of [0.3 × tensile strength (MPa)]. Patent Document 1 teaches that, according to the above configuration, a high-strength steel sheet can be obtained that possesses a tensile strength of 980 MPa or more, press formability, and bendability.

[0005] Patent Document 2 discloses a high-strength hot-dip galvanized steel sheet comprising a base steel sheet and a zinc plating layer formed on the surface of the base steel sheet, wherein the base steel sheet has a predetermined chemical composition and its microstructure comprises, by area fraction, 5 to 30% ferrite, 5 to 20% austenite, 50 to 80% bainite and martensite, and 2% or less (including 0%) precipitates, and the microstructure of the base steel sheet within 2 μm in the thickness direction from the surface comprises, by area fraction, 50% or more ferrite, 2% or less (including 0%) austenite, 50% or less (including 0%) bainite and martensite, and 2% or less (including 0%) precipitates. Patent Document 2 teaches that, according to the above configuration, a high-strength hot-dip galvanized steel sheet having strength and ductility at a level applicable to automobile body structural members can be provided.

[0006] Patent Document 3 describes a cold-rolled high-strength steel sheet having a predetermined chemical composition, wherein an internal oxide layer with a thickness of 1 to 5 μm exists on the surface of the cold-rolled high-strength steel sheet, the internal oxide layer has an iron matrix, the matrix contains oxide particles, the oxide particles are at least one of Si oxide or a composite oxide of Si and Mn, the surface of the steel sheet is not enriched with Si and Mn elements, the average diameter of the oxide particles is 50 to 200 nm, and the average spacing λ between oxide particles is 0.115 × (0.94 × [Si] + 0.68 × [Mn]). １／２ ×d≦λ≦1.382×(0.94×[Si]+0.68×[Mn]) １／２ A cold-rolled high-strength steel sheet is disclosed that satisfies the relationship ×d, where [Si] is the percentage of Si content in the steel, [Mn] is the percentage of Mn content in the steel, and d is the diameter of the oxide particles, with units of nm. Patent Document 3 teaches that according to the above configuration, it has excellent phosphate treatment and formability, a tensile strength of 980 MPa or more, an elongation of 20% or more, a room-temperature microstructure containing retained austenite, ferrite, martensite and / or bainite, and is suitable for the manufacture of structural and safety parts for automobiles with complex shapes and high requirements for formability and corrosion resistance.

[0007] Patent Document 4 describes a plated steel sheet having a hot-dip galvanized layer or an alloyed hot-dip galvanized layer on the surface of the steel sheet, wherein the steel sheet has a predetermined chemical composition, and sequentially from the interface between the steel sheet and the plating layer toward the steel sheet side, there is an internal oxide layer containing an oxide of Si and / or Mn, and a layer containing the internal oxide layer, and the Vickers hardness is t of the steel sheet. ０ / 4 copies (t ０ A high-strength plated steel sheet is disclosed, characterized in that it has a soft layer that satisfies 90% or less of the Vickers hardness in (where refers to the thickness of the steel sheet), and a hard layer composed mainly of martensite and bainite, wherein the average depth T of the soft layer is 20 μm or more, and the average depth t of the internal oxide layer is 4 μm or more and less than T, and the yield ratio YR (%), which is expressed as the ratio of tensile strength TS to yield stress YS (YS / TS), is 60% or more. Patent Document 4 teaches that with the above configuration, hydrogen embrittlement can be effectively suppressed, and a high-strength plated steel sheet with a tensile strength of 980 MPa or more can be obtained, which is excellent in all aspects of workability, including bendability, hole-expandability, and delayed fracture resistance.

[0008] Furthermore, steel sheets are required to have various properties in addition to high strength, and in relation to this, the following types of steel sheets are known that have controlled Si-containing oxides in the surface layer.

[0009] Patent Document 5 describes a steel sheet having a predetermined chemical composition, with a surface coverage rate of Si-based oxides of 1% or less, and a surface coverage rate of iron-based oxides of 40% or less, and Cu Ｓ / Cu Ｂ is 4.0 or less (Cu Ｓ This is the Cu concentration in the surface layer of the steel plate, Cu Ｂ A high-strength cold-rolled steel sheet is disclosed that satisfies the Cu concentration in the base material. Patent document 5 teaches that, according to the above configuration, it has high strength of 1180 MPa or more in tensile strength, while also having excellent delayed fracture resistance and chemical treatment properties.

[0010] Patent Document 6 discloses a method for producing a coated steel sheet coated with a zinc-based or aluminum-based coating, the method comprising: A) providing a steel sheet having a predetermined chemical composition, B) annealing the steel sheet comprising the following in this order, i) a preheating step of heating the steel sheet from room temperature to a temperature T1 between 550°C and Ac1+50°C, ii) H between 0.1 and 15% by volume ２ 2, with the balance consisting of inert gas, H ２ 2O, O ２ 2 and unavoidable impurities, having a dew point DP between -10°C and +30°C １ 1 in an atmosphere having the above, heating from temperature T １ 1 to a recrystallization temperature T between 720°C and 1000°C ２ 2, iii) H between 0.1 and 15% by volume ２ 2, with the balance consisting of inert gas, H ２ 2O, O ２ 2 and unavoidable impurities, having a dew point DP between -30°C and 0°C ２ 2, wherein the dew point DP １ 2 is higher than the dew point DP ２ 1, in an atmosphere having the above, maintaining the steel sheet at the recrystallization temperature T ２In the present invention, a soaking step is performed to hold the steel sheet, iv) a cooling step is performed, and c) the steel sheet is coated with a zinc-based or aluminum-based coating is disclosed. Furthermore, Patent Document 6 discloses a steel bulk manufactured by the above manufacturing method and having a predetermined chemical composition, a partially decarbonized layer on the steel bulk having a thickness of 20 to 40 μm, a carbon weight percentage of 5 to 20 percent of the carbon weight percentage of the bulk steel, and a microstructure containing at least 50% ferrite and at least one of the following components, namely bainite, martensite and / or retained austenite, and a layer on the partially decarbonized layer having a thickness of 5 to 40 μm and a carbon weight percentage of the bulk steel A decarbonized layer having a microstructure containing less than 5% carbon by weight and at least 90% ferrite is disclosed, the upper part of which the decarbonized layer comprises an internal oxide layer having a thickness of 2 to 12 μm and containing Mn, Si, Al, and Cr-based elemental oxides and mixed oxides of Mn, Si, Al, and Cr; a barrier layer having a thickness of 100 nm to 500 nm on the internal oxide layer; and a zinc-based coating layer having a thickness of 3 to 30 μm on the barrier layer. Patent Document 6 teaches that with the above configuration, both the target mechanical properties and excellent LME resistance can be satisfied.

[0011] International Publication No. 2022 / 209839, JP 2019-504196, JP 2019-531409, JP A 2018-193614, International Publication No. 2017 / 141953, JP 2023-525519

[0012] As mentioned above, it is generally known that the formability of steel sheets decreases as their strength increases. In addition, steel sheets sometimes contain a relatively large amount of Si to increase their strength and / or to create a desired hard structure. However, when steel sheets contain a relatively large amount of Si, a relatively large amount of Si oxide is generated during the manufacturing of the steel sheet, and this Si oxide may cause cracks to occur within the bend during bending deformation.

[0013] Therefore, the present invention aims to provide a steel sheet and a component containing the same that has high strength, excellent ductility, and excellent resistance to internal cracking during bending.

[0014] To achieve the above objective, the inventors focused on the microstructure and surface properties of steel sheets, particularly hot-rolled steel sheets, and conducted studies. As a result, the inventors discovered that by configuring the microstructure of a steel sheet having a predetermined chemical composition to contain bainite and martensite in specific proportions, the strength and ductility can be improved, and in addition, resistance to internal cracking during bending can be improved by controlling the variation in the number density of Si oxides and the surface roughness Ra on the surface of the steel sheet within a predetermined range. Thus, the inventors completed the present invention.

[0015] The present invention, which has achieved the above objectives, is as follows. (1) In mass%, C: 0.045-0.120%, Si: 0.30-3.00%, Mn: 1.20-3.00%, Al: 0.010-0.400%, P: 0.080% or less, S: 0.0100% or less, N: 0.0150% or less, O: 0.0100% or less, Ti: 0-0.180%, Nb: 0-0.100%, V: 0-1.000%, Cu: 0-1.000%, Cr: 0-2.000%, Mo: 0-3.000%, Ni: 0-1.000%, B: 0-0.0100%, Ca: 0-0.0500%, Mg: 0-0.050% The material has a chemical composition consisting of REM: 0-0.100%, Bi: 0-0.100%, Ta: 0-0.100%, Zr: 0-0.500%, Co: 0-3.000%, Zn: 0-0.200%, W: 0-0.200%, Sb: 0-0.500%, As: 0-0.100%, Sn: 0-1.000%, and the remainder being Fe and impurities. At a position 1 / 4 of the plate thickness from the surface, it has a metallic structure containing, in area percent, bainite: 70-95% and martensite: 5-30%. On the surface, the number density of Si oxide particles with an equivalent circle diameter of 5 μm or more is 60 particles / mm². ２A steel sheet characterized by having a surface roughness Ra of less than 1.5 μm, and the difference between the maximum and minimum values ​​of the surface roughness Ra being 1.5 μm or less. (2) The chemical composition is, in mass%, Ti: 0.001 to 0.180%, Nb: 0.001 to 0.100%, V: 0.001 to 1.000%, Cu: 0.001 to 1.000%, Cr: 0.001 to 2.000%, Mo: 0.001 to 3.000%, Ni: 0.001 to 1.000%, B: 0.0001 to 0.0100%, Ca: 0.0001 to 0.0500%, Mg: 0.0001 to 0.050%, REM: 0.0001 to 0.100%, Bi: 0.001 to 0.100%, Ta: 0.001 to 0.100%, (1) The steel sheet described above, characterized in that it contains at least one of the following: Zr: 0.001 to 0.500%, Co: 0.001 to 3.000%, Zn: 0.001 to 0.200%, W: 0.001 to 0.200%, Sb: 0.001 to 0.500%, As: 0.001 to 0.100%, and Sn: 0.001 to 1.000%. (3) The steel sheet described above, characterized in that it has a tensile strength of 980 MPa or more. (4) The steel sheet described above, characterized in that it has a plate thickness of 1.0 to 8.0 mm. (5) A part described above, characterized in that it contains the steel sheet described above, according to any one of the following: (1) to (4).

[0016] According to the present invention, it is possible to provide a steel sheet and a component containing the same that has high strength, excellent ductility, and excellent resistance to internal cracking during bending.

[0017] Figure 1 is a schematic diagram of a steel plate surface used to illustrate the measurement of the difference between the maximum and minimum values ​​of the surface roughness Ra.

[0018] <Steel Plate> The steel plate according to the embodiment of the present invention has the following composition in mass%, C: 0.045 to 0.120%, Si: 0.30 to 3.00%, Mn: 1.20 to 3.00%, Al: 0.010 to 0.400%, P: 0.080% or less, S: 0.0100% or less, N: 0.0150% or less, O: 0.0100% or less, Ti: 0 to 0.180%, Nb: 0 to 0.100%, V: 0 to 1.000%, Cu: 0 to 1.000%, Cr: 0 to 2.000%, Mo: 0 to 3.000%, Ni: 0 to 1.000%, B: 0 to 0.0100%, Ca: 0 to 0.0500%. The material has a chemical composition consisting of Mg: 0-0.050%, REM: 0-0.100%, Bi: 0-0.100%, Ta: 0-0.100%, Zr: 0-0.500%, Co: 0-3.000%, Zn: 0-0.200%, W: 0-0.200%, Sb: 0-0.500%, As: 0-0.100%, Sn: 0-1.000%, and the remainder being Fe and impurities. At a position 1 / 4 of the plate thickness from the surface, it has a metallic structure containing bainite: 70-95% and martensite: 5-30% by area percentage. On the surface, the number density of Si oxide particles with an equivalent circle diameter of 5 μm or more is 60 particles / mm². ２ It is characterized by being less than 1.5 μm, and the difference between the maximum and minimum values ​​of the surface roughness Ra being 1.5 μm or less.

[0019] As mentioned earlier, it is known that as the strength of steel sheets increases, their formability, such as ductility, decreases. Therefore, the inventors first investigated the metallographic structure of the steel sheet, in addition to optimizing its chemical composition. More specifically, the inventors found that by configuring the metallographic structure of a steel sheet having an optimized chemical composition, particularly one containing Si: 0.30 to 3.00% by mass, to include bainite and martensite in specific proportions, it is possible to achieve the desired high strength, more specifically a tensile strength of 980 MPa or more, while also achieving the desired ductility.

[0020] Si is effective in increasing the strength of steel sheets, and in addition, it is effective in forming hard structures such as martensite. Thus, Si is an important element in increasing the strength of steel sheets. For example, if Si is insufficient, from the perspective of the metallic structure, martensite may not be sufficiently formed, which may result in insufficient strength. On the other hand, in steel sheets containing a relatively large amount of Si, cracking during bending can become a significant problem. To explain in more detail, in steel sheets containing a relatively large amount of Si, more specifically in steel sheets containing 0.30% or more Si, Si oxide is formed at the interface between the Fe oxide (also called scale) that forms on the surface of the steel sheet and the steel. This Si oxide may remain on the surface, and may also remain as Si oxide on the surface after pickling, and this Si oxide becomes the initiation point of cracks during bending. In addition, because this Si oxide adheres firmly to the Fe oxide and the steel, the scale may not be sufficiently removed even by descaling using high-pressure water during hot rolling. In such cases, scale that was not sufficiently removed is pressed into the steel sheet surface by subsequent finish rolling, causing irregularities on the steel sheet surface and deteriorating the surface properties, which can lead to cracks in the bend originating from these surface irregularities. As a result, steel sheets containing a relatively high amount of Si are more prone to cracking in the bend. Therefore, in order to achieve the desired high strength and desired ductility, it is important to control the chemical composition of the steel sheet to contain Si: 0.30 to 3.00% by mass, as well as bainite and martensite in the specific proportions mentioned above. However, a relatively high Si content can exacerbate the problem of cracking in the bend during the bending process of the steel sheet.

[0021] Next, the present inventors conducted studies focusing on the surface properties of steel sheets in order to improve resistance to internal cracking during bending. As will be described in detail later in connection with the production method, by controlling the maximum temperature reached from after rough rolling to before descaling before finish rolling to 870 to 1130° C., excessive growth of Si oxide is suppressed, and it has been found that the level difference at the boundary between the descaled portion and the non-descaled portion during descaling can be reduced, whereby the difference between the maximum value and the minimum value of the surface roughness Ra can be controlled to 1.5 μm or less. As a result, unevenness on the surface of the steel sheet is reduced, the initiation point of internal cracking during bending forming is suppressed, and thereby resistance to internal cracking during bending can be improved.

[0022] However, it has been found that simply controlling the maximum temperature reached from after rough rolling to before descaling before finish rolling to 870 to 1130° C. increases the proportion of portions where scale is not removed by descaling before rolling, so that a relatively large amount of Si oxide remains on the surface, resulting in insufficient resistance to internal cracking during bending of the steel sheet. The present inventors have further proceeded with studies, and found that in addition to the above, by appropriately controlling the temperature after descaling following heating and extraction of a slab, penetration of Si oxide into grain boundaries is suppressed, Si oxide is easily removed, and coarse Si oxide remaining on the surface of the finally obtained steel sheet is reduced. More specifically, the number density of Si oxides having an equivalent circle diameter of 5 μm or more is 60 pieces / mm ２ It has been found that the number density can be controlled to less than the above. As a result, coarse Si oxides can be reduced, the initiation point of internal cracking during bending forming is suppressed, and thereby resistance to internal cracking during bending can be improved. In addition to controlling the difference between the maximum value and the minimum value of the surface roughness Ra to 1.5 μm or less, the number density of Si oxides having an equivalent circle diameter of 5 μm or more is 60 pieces / mm ２ It has been found that by controlling the number density to less than the above, the initiation points of internal cracking during bending can be reduced, whereby resistance to internal cracking during bending can be significantly improved.

[0023] From the foregoing, according to the steel sheet according to the embodiment of the present invention, despite having a high strength with a tensile strength of 980 MPa or more, excellent ductility can be achieved, and excellent resistance to internal cracking during bending can be achieved. Therefore, the steel sheet according to the embodiment of the present invention has high strength, excellent ductility, and excellent resistance to internal cracking during bending, and is particularly useful for use in the automotive field where these properties are required.

[0024] Hereinafter, the steel sheet according to the embodiment of the present invention will be described in more detail. In the following description, "%", which is the unit of content of each element, means "% by mass" unless otherwise specified. In addition, in the present specification, unless otherwise specified, the symbol "~" used to indicate a numerical range is used to mean that the numerical values described before and after it are included as the lower limit and the upper limit.

[0025] [C: 0.045 to 0.120%] C is an element effective for increasing the strength of a steel sheet. In order to sufficiently obtain such an effect, the C content is set to 0.045% or more. The C content may be 0.050% or more, 0.055% or more, 0.060% or more, 0.065% or more, or 0.070% or more. On the other hand, when C is contained excessively, it may cause a decrease in ductility. Therefore, the C content is set to 0.120% or less. The C content may be 0.110% or less, 0.100% or less, 0.090% or less, or 0.080% or less.

[0026] [Si: 0.30 to 3.00%] Si is an element effective for increasing strength as a solid solution strengthening element. In addition, Si is an element effective for forming a hard structure such as martensite. In order to sufficiently obtain these effects, the Si content is set to 0.30% or more. The Si content may be 0.40% or more, 0.50% or more, 0.60% or more, or 0.70% or more. On the other hand, when Si is contained excessively, the cracking susceptibility of a slab increases, which may make handling of the slab difficult. Therefore, the Si content is set to 3.00% or less. The Si content may be 2.00% or less, 1.80% or less, 1.60% or less, 1.40% or less, 1.20% or less, or 1.00% or less.

[0027] [Mn: 1.20-3.00%] Mn is an element effective in increasing strength as a hardenability and solid solution strengthening element. To obtain these effects sufficiently, the Mn content should be 1.20% or more. The Mn content may be 1.40% or more, 1.60% or more, 1.80% or more, or 2.00% or more. On the other hand, excessive Mn content may lead to a decrease in ductility along with an increase in steel strength. For this reason, the Mn content should be 3.00% or less. The Mn content may be 2.80% or less, 2.60% or less, 2.40% or less, or 2.20% or less.

[0028] [Al: 0.010-0.400%] Al is an element that acts as a deoxidizing agent for molten steel. To obtain this effect sufficiently, the Al content should be 0.010% or more. The Al content may be 0.020% or more, or 0.030% or more. On the other hand, if Al is present in excess, coarse oxides may form, reducing toughness and ductility, which may lead to fracture during rolling. For this reason, the Al content should be 0.400% or less. The Al content may be 0.200% or less, 0.150% or less, 0.100% or less, 0.080% or less, or 0.060% or less.

[0029] [P: 0.080% or less] Excessive P content may negatively affect weldability and other properties. Therefore, the P content should be 0.080% or less. The P content may also be 0.050% or less, 0.030% or less, 0.020% or less, or 0.010% or less. The lower limit of the P content is not particularly limited and may be 0%, but excessive reduction will lead to increased costs. Therefore, the P content may be 0.001% or more, 0.003% or more, or 0.005% or more.

[0030] [S: 0.0100% or less] Excessive sulfur content can lead to the formation of large amounts of MnS, which can reduce toughness. Therefore, the sulfur content should be 0.0100% or less. The sulfur content may also be 0.0080% or less, 0.0060% or less, 0.0040% or less, or 0.0030% or less. The lower limit of the sulfur content is not particularly limited and may be 0%, but excessive reduction will lead to increased costs. Therefore, the sulfur content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more.

[0031] [N: 0.0150% or less] N is an element that forms coarse nitrides in steel sheets, reducing the workability of the steel sheet. A lower N content is preferable, so ideally it should be 0%. However, excessive reduction of the N content can lead to a significant increase in manufacturing costs. For this reason, the N content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, excessive N content can form coarse nitrides as described above, reducing the workability of the steel sheet. Therefore, the N content should be 0.0150% or less. The N content may also be 0.0100% or less, 0.0050% or less, 0.0040% or less, 0.0030% or less, or 0.0020% or less.

[0032] [O: 0.0100% or less] O is an element that is introduced during the manufacturing process. Excessive O content can lead to the formation of coarse inclusions, which can reduce the toughness of the steel sheet. Therefore, the O content should be 0.0100% or less. The O content may also be 0.0080% or less, 0.0060% or less, 0.0040% or less, or 0.0030% or less. The lower limit of the O content is not particularly limited and may be 0%, but reducing it to less than 0.0001% requires more time for refining, leading to a decrease in productivity. Therefore, the O content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more.

[0033] The basic chemical composition of the steel sheet according to the embodiment of the present invention is as described above. Furthermore, the steel sheet may optionally contain at least one of the following optional elements in place of a portion of the remaining Fe.

[0034] [Ti: 0-0.180%] Ti has the effect of improving the strength of steel sheets by forming carbonitrides in the steel and strengthening through precipitation. The Ti content may be 0%, but in order to obtain such an effect, the Ti content is preferably 0.001% or more. The Ti content may be 0.010% or more, 0.030% or more, 0.050% or more, 0.070% or more, or 0.090% or more. On the other hand, if the Ti content is excessive, the effect will saturate, and including more Ti in the steel than necessary will lead to an increase in manufacturing costs. Therefore, the Ti content should be 0.180% or less. The Ti content may be 0.160% or less, 0.150% or less, 0.140% or less, 0.130% or less, or 0.120% or less.

[0035] [Nb: 0-0.100%] Nb is an element that forms carbides, nitrides and / or carbonitrides in steel, contributing to the refinement of the microstructure and, consequently, the increased strength of the steel sheet through a pinning effect. The Nb content may be 0%, but to obtain such an effect, the Nb content is preferably 0.001% or more, and may be 0.002% or more, 0.005% or more, or 0.010% or more. On the other hand, if the Nb content is excessive, the effect will saturate, and including more Nb in the steel than necessary will lead to an increase in manufacturing costs. Therefore, the Nb content should be 0.100% or less. The Nb content may also be 0.080% or less, 0.060% or less, 0.040% or less, or 0.020% or less.

[0036] [V: 0-1.000%] V is an element that contributes to improving strength through precipitation strengthening, etc. The V content may be 0%, but in order to obtain such an effect, the V content is preferably 0.001% or more, and may be 0.002% or more, 0.005% or more, or 0.010% or more. On the other hand, if the V content is excessive, the effect will saturate, and including more V in the steel than necessary will lead to an increase in manufacturing costs. Therefore, the V content should be 1.000% or less. The V content may be 0.750% or less, 0.500% or less, 0.250% or less, 0.100% or less, or 0.050% or less.

[0037] [Cu: 0-1.000%] Cu is an element that contributes to improving strength through precipitation strengthening or solid solution strengthening. The Cu content may be 0%, but in order to obtain such an effect, the Cu content is preferably 0.001% or more, and may be 0.002% or more, 0.005% or more, or 0.010% or more. On the other hand, if the Cu content is excessive, the effect will saturate, and including more Cu in the steel than necessary will lead to an increase in manufacturing costs. Therefore, the Cu content should be 1.000% or less. The Cu content may be 0.750% or less, 0.500% or less, 0.250% or less, 0.100% or less, 0.050% or less, or 0.020% or less.

[0038] [Cr: 0-2.000%] Cr is an element that enhances the hardenability of steel and contributes to improving its strength. The Cr content may be 0%, but to obtain such effects, it is preferable that the Cr content be 0.001% or more. The Cr content may be 0.010% or more, 0.020% or more, or 0.050% or more. On the other hand, if the Cr content is excessive, the effect will saturate, which may lead to an increase in manufacturing costs. Therefore, the Cr content should be 2.000% or less. The Cr content may be 1.500% or less, 1.000% or less, 0.750% or less, 0.500% or less, 0.300% or less, 0.200% or less, or 0.100% or less.

[0039] [Mo: 0-3.000%] Mo is an element that enhances the hardenability of steel and contributes to improving its strength. The Mo content may be 0%, but to obtain such effects, the Mo content is preferably 0.001% or more, and may be 0.010% or more, 0.020% or more, or 0.050% or more. On the other hand, if the Mo content is excessive, the effect will saturate, and including more Mo in the steel than necessary will lead to an increase in manufacturing costs. Therefore, the Mo content should be 3.000% or less. The Mo content may be 1.500% or less, 1.000% or less, 0.750% or less, 0.500% or less, 0.300% or less, or 0.100% or less.

[0040] [Ni: 0-1.000%] Ni is an element that contributes to improving strength through precipitation strengthening or solid solution strengthening. The Ni content may be 0%, but in order to obtain such an effect, the Ni content is preferably 0.001% or more, and may be 0.010% or more, 0.020% or more, or 0.050% or more. On the other hand, if Ni is included in excess, the effect will saturate, and including more Ni in the steel than necessary will lead to an increase in manufacturing costs. Therefore, the Ni content should be 1.000% or less. The Ni content may be 0.500% or less, 0.300% or less, 0.200% or less, 0.100% or less, or 0.080% or less.

[0041] [B: 0-0.0100%] B improves low-temperature toughness by segregating at grain boundaries and increasing grain boundary strength. The B content may be 0%, but to obtain this effect, it is preferable that the B content be 0.0001% or more. The B content may be 0.0002% or more, 0.0005% or more, or 0.0010% or more. On the other hand, if the B content is excessive, the effect will saturate, which may lead to an increase in manufacturing costs. Therefore, the B content should be 0.0100% or less. The B content may be 0.0050% or less, 0.0030% or less, 0.0020% or less, or 0.0015% or less.

[0042] [Ca: 0-0.0500%] Ca is an element that can control the morphology of nonmetallic inclusions. The Ca content may be 0%, but to obtain such an effect, it is preferable that the Ca content be 0.0001% or more, and may be 0.0005% or more, or 0.0010% or more. On the other hand, if Ca is included in excess, the effect will saturate, and including more Ca in the steel sheet than necessary will lead to an increase in manufacturing costs. Therefore, the Ca content should be 0.0500% or less. The Ca content may be 0.0200% or less, 0.0100% or less, 0.0080% or less, 0.0060% or less, 0.0040% or less, or 0.0020% or less.

[0043] [Mg: 0-0.050%] Mg is an element that can control the morphology of nonmetallic inclusions. The Mg content may be 0%, but to obtain such an effect, the Mg content is preferably 0.0001% or more, and may be 0.0005% or more, or 0.001% or more. On the other hand, if the Mg content is excessive, the effect will saturate, and including more Mg in the steel sheet than necessary will lead to an increase in manufacturing costs. Therefore, the Mg content should be 0.050% or less. The Mg content may also be 0.040% or less, 0.030% or less, 0.020% or less, or 0.010% or less.

[0044] [REM: 0-0.100%] REM is an element that can control the morphology of nonmetallic inclusions. The REM content may be 0%, but to obtain such an effect, the REM content is preferably 0.0001% or more, and may be 0.0005% or more, or 0.001% or more. On the other hand, if REM is included in excess, the effect will saturate, and including more REM in the steel sheet than necessary will lead to an increase in manufacturing costs. Therefore, the REM content should be 0.100% or less. The REM content may be 0.080% or less, 0.060% or less, 0.040% or less, or 0.020% or less. In this specification, REM is a collective term for 17 elements including scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanides from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71, and the REM content is the total content of these elements.

[0045] [Bi: 0-0.100%] Bi is an element effective in improving corrosion resistance. The Bi content may be 0%, but to obtain such an effect, it is preferable that the Bi content be 0.001% or more. The Bi content may also be 0.002% or more. On the other hand, if Bi is included in excess, the effect will saturate, and including more Bi in the steel plate than necessary will lead to an increase in manufacturing costs. Therefore, the Bi content should be 0.100% or less. The Bi content may also be 0.080% or less, 0.060% or less, 0.040% or less, or 0.020% or less.

[0046] [Ta: 0-0.100%] Ta is an element effective in controlling the morphology and increasing the strength of carbides. The Ta content may be 0%, but to obtain such effects, it is preferable that the Ta content be 0.001% or more. The Ta content may also be 0.002% or more. On the other hand, if the Ta content is excessive, the effect will saturate, and including more Ta than necessary in the steel sheet will lead to an increase in manufacturing costs. Therefore, the Ta content should be 0.100% or less. The Ta content may also be 0.080% or less, 0.060% or less, 0.040% or less, or 0.020% or less.

[0047] [Zr: 0-0.500%] Zr is an element that can control the morphology of nonmetallic inclusions. The Zr content may be 0%, but to obtain such an effect, it is preferable that the Zr content be 0.001% or more. On the other hand, if the Zr content is excessive, the effect will saturate, and including more Zr in the steel sheet than necessary will lead to an increase in manufacturing costs. Therefore, the Zr content should be 0.500% or less. The Zr content may also be 0.200% or less, 0.100% or less, 0.080% or less, 0.060% or less, 0.040% or less, or 0.020% or less.

[0048] [Co: 0-3.000%] Co is an element that contributes to improving hardenability and / or heat resistance. The Co content may be 0%, but to obtain these effects fully, it is preferable that the Co content be 0.001% or more. The Co content may also be 0.002% or more. On the other hand, if the Co content is excessive, the effect will saturate, and including more Co than necessary in the steel will lead to an increase in manufacturing costs. Therefore, the Co content should be 3.000% or less. The Co content may also be 1.500% or less, 1.000% or less, 0.500% or less, 0.200% or less, 0.100% or less, 0.050% or less, or 0.020% or less.

[0049] [Zn: 0-0.200%] Zn is an effective element for controlling the shape of inclusions. The Zn content may be 0%, but to obtain this effect sufficiently, it is preferable that the Zn content be 0.001% or more. The Zn content may be 0.002% or more, 0.005% or more, or 0.010% or more. On the other hand, if the Zn content is excessive, the number of inclusions will increase and may cause defects on the surface and inside of the steel sheet. Therefore, the Zn content should be 0.200% or less. The Zn content may be 0.150% or less, 0.100% or less, 0.080% or less, 0.060% or less, 0.040% or less, or 0.020% or less.

[0050] [W: 0-0.200%] W is an element that enhances the hardenability of steel and contributes to improving its strength. The W content may be 0%, but to obtain such an effect, the W content is preferably 0.001% or more, and may be 0.002% or more, 0.005% or more, or 0.010% or more. On the other hand, if W is included in excess, the effect will saturate, and including more W in the steel than necessary will lead to an increase in manufacturing costs. Therefore, the W content should be 0.200% or less. The W content may be 0.150% or less, 0.100% or less, 0.080% or less, 0.060% or less, 0.040% or less, or 0.020% or less.

[0051] [Sb: 0-0.500%] Sb is an element effective in improving corrosion resistance. The Sb content may be 0%, but to obtain such an effect, it is preferable that the Sb content be 0.001% or more, and may be 0.005% or more, 0.010% or more, 0.015% or more, or 0.020% or more. On the other hand, if the Sb content is excessive, the effect will saturate, and including more Sb in the steel sheet than necessary will lead to an increase in manufacturing costs. Therefore, the Sb content should be 0.500% or less. The Sb content may also be 0.200% or less, 0.100% or less, 0.050% or less, or 0.030% or less.

[0052] [As: 0-0.100%] As is an element effective in improving corrosion resistance. The As content may be 0%, but to obtain such an effect, it is preferable that the As content be 0.001% or more. On the other hand, if the As content is excessive, the effect will saturate, and including more As than necessary in the steel plate will lead to an increase in manufacturing costs. Therefore, the As content should be 0.100% or less. The As content may also be 0.050% or less, 0.040% or less, 0.030% or less, 0.020% or less, or 0.010% or less.

[0053] [Sn: 0-1.000%] Sn is an element effective in improving corrosion resistance. The Sn content may be 0%, but to obtain such an effect, it is preferable that the Sn content be 0.001% or more. The Sn content may also be 0.002% or more, or 0.005% or more. On the other hand, if the Sn content is excessive, the effect will saturate, and including more Sn in the steel than necessary will lead to an increase in manufacturing costs. Therefore, the Sn content should be 1.000% or less. The Sn content may also be 0.500% or less, 0.200% or less, 0.100% or less, 0.050% or less, 0.040% or less, 0.030% or less, 0.020% or less, or 0.010% or less.

[0054] In the steel sheet according to the embodiment of the present invention, the remainder other than the above-mentioned elements consists of Fe and impurities. Impurities are components that are mixed in during the industrial manufacture of steel sheets due to various factors in the manufacturing process, including raw materials such as ore and scrap, and components that are included in a range that does not affect the effects of the present invention.

[0055] The chemical composition of the steel sheet according to the embodiment of the present invention can be measured by general analytical methods. For example, the chemical composition of the steel sheet can be measured using inductively coupled plasma atomic emission spectroscopy (ICP-AES). C and S can be measured using the combustion-infrared absorption method, N can be measured using the inert gas fusion-thermal conductivity method, and O can be measured using the inert gas fusion-nondispersive infrared absorption method.

[0056] [Metal structure] [Bainite: 70-95%, and martensite: 5-30%] The metal structure of the steel sheet according to the embodiment of the present invention includes, by area percentage, bainite: 70-95% and martensite: 5-30%. By composing the metal structure of the steel sheet with these structures, it is possible to improve strength and ductility in a well-balanced manner. If the area percentage of bainite is high or the area percentage of martensite is low, the strength will decrease and the desired properties may not be obtained. Therefore, the area percentage of bainite should be 95% or less, for example, 93% or less, 90% or less, 87% or less, or 85% or less. Similarly, the area percentage of martensite should be 5% or more, for example, 7% or more, 10% or more, 13% or more, or 15% or more. On the other hand, if the area percentage of bainite is low or the area percentage of martensite is high, the ductility will decrease and the desired properties may not be obtained. Therefore, the area percentage of bainite should be 70% or more, for example, 72% or more, 75% or more, 78% or more, or 80% or more. Similarly, the area ratio of martensite may be 30% or less, but may also be 27% or less, 25% or less, 22% or less, or 20% or less.

[0057] [Remaining Structure] The remaining structure other than bainite and martensite may be 0% in area percentage, but if the remaining structure is present, it may be at least one of ferrite, pearlite, and retained austenite. If metallic structures other than bainite and martensite are included, for example, the area percentage of retained austenite may be 0 to less than 5%. The area percentage of retained austenite may be less than 5%, 3% or less, or 2% or less, and / or 0.01% or more, or 0.1% or more. The area percentage of the remaining structure is preferably small, for example, 0 to 5%. The area percentage of the remaining structure may also be, for example, 5% or less, 4% or less, or 3% or less, and / or 0.1% or more, 0.5% or more, 1% or more, or 2% or more.

[0058] [Identification of Metallurgical Structure and Calculation of Area Ratio] The identification of the metallic structure and calculation of the area ratio in steel sheets are performed by optical microscopy observation and X-ray diffraction after etching with Nital reagent or Repera solution. Microscopy observation is performed on the thickness cross section parallel to the rolling direction and perpendicular to the sheet surface. While it is preferable that the thickness cross section be parallel to the rolling direction, it is not necessary to be parallel to the rolling direction if the rolling direction of the steel sheet cannot be determined. Specifically, first, a sample is taken from the steel sheet, and the observation surface of the sample is etched with Nital. Next, by performing image analysis on a microscopic image obtained with a field of view of 300 μm × 300 μm at a depth of 1 / 4 of the sheet thickness using an optical microscope, the area ratio of ferrite, the area ratio of pearlite, and the total area ratio of bainite and martensite are calculated. Crystals with equiaxed grains and no substructure can be identified as ferrite, while those with substructure can be identified as bainite and martensite. Next, using a sample whose observation surface has been reperforated, the total area ratio of retained austenite and martensite is calculated by performing image analysis on a micrograph obtained using an optical microscope in a 300 μm × 300 μm field of view at a depth of 1 / 4 of the plate thickness. The image analysis is performed using the "Analyze" function of the image analysis software "ImageJ," which allows for the calculation of the above-mentioned area ratios and the total area ratio. Here, "ImageJ" is open-source, public-domain image processing software that is widely used among those skilled in the art. Next, using a sample that has been surface-machined from the direction normal to the rolling surface to a depth of 1 / 4 of the plate thickness, the volume fraction of retained austenite is calculated by X-ray diffraction measurement. More specifically, first, the area from the surface of the steel plate to the 1 / 4 position of the plate thickness is removed by mechanical polishing and chemical polishing. Next, MoKα rays are used as characteristic X-rays on the surface of the polished sample to obtain diffraction peaks at (200) and (211) for the bcc phase, and at (200), (220), and (311) for the fcc phase. The structural fraction of retained austenite is calculated from the integral intensity ratio of these diffraction peaks and is taken as the volume fraction of retained austenite. Since the volume fraction of retained austenite is equivalent to the area fraction, this is taken as the area fraction of retained austenite.The area ratio of martensite is calculated by subtracting the obtained area ratio of retained austenite from the total area ratio of retained austenite and martensite calculated earlier. Finally, the area ratio of bainite is calculated by similarly subtracting the obtained area ratio of martensite from the total area ratio of bainite and martensite calculated earlier.

[0059] [Number density of Si oxide particles with an equivalent circular diameter of 5 μm or more: 60 particles / mm] ２ [Less than] On the surface of a steel plate according to an embodiment of the present invention, the number density of Si oxides having a circular equivalent diameter of 5 μm or more is 60 particles / mm². ２ It is controlled to be less than 60 particles / mm² of Si oxide with an equivalent circle diameter of 5 μm or more. ２ By controlling it to less than this, the number of crack initiations within the bend during bending deformation can be reduced, thereby improving resistance to internal bending cracking. From the viewpoint of improving resistance to internal bending cracking, a smaller number density of the Si oxide is preferable, for example, 55 particles / mm². ２ Below, 50 pieces / mm ２ The following or 45 pieces / mm ２ The following may also apply. On the other hand, the lower limit of the number density of the Si oxide is not particularly limited, but for example, Si oxide is 1 particle / mm ２ Above, 5 pieces / mm ２ Above, 10 pieces / mm ２ Above, 10 pieces / mm ２ Super, 11 pieces / mm ２ More than or 12 pieces / mm ２ That's fine too.

[0060] [Measurement of Si Oxide Number Density] The number density of Si oxides on the surface of a steel sheet is measured by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). First, if the steel sheet has scale on its surface, the sample is pickled before being subjected to the measurement of the number density. The pickling treatment is carried out by immersing the sample in hydrochloric acid with a concentration of 3 to 10 volume percent at a temperature of 85 to 98°C for 20 to 300 seconds. The pickling may be performed once or in multiple steps as necessary. The above pickling time (20 to 300 seconds) refers to the time of the single pickling if pickling is performed only once, and the total time of the multiple picklings if pickling is performed multiple times. By setting the pickling temperature to 85°C or higher, surface oxides can be sufficiently removed. Next, the surface of the steel plate is examined using a SEM with EDS at 200x magnification, with an observation area of ​​0.3 mm² per field of view. ２ The above steps are followed by observation of 120 fields of view. Next, Si oxides are identified by EDS analysis within the same field of view, and the number density of Si oxides with an equivalent circle diameter of 5 μm or more is determined. If peaks for Si and O are observed, the material is identified as a Si oxide. The average number density across the 120 fields of view is taken as the number density of Si oxides. In this specification, the equivalent circle diameter is the equivalent circle diameter based on the projected area of ​​the particle.

[0061] [Difference between maximum and minimum surface roughness Ra: 1.5 μm or less] In the steel sheet according to the embodiment of the present invention, the difference between the maximum and minimum surface roughness Ra is controlled to 1.5 μm or less. By controlling the difference between the maximum and minimum surface roughness Ra to 1.5 μm or less, the surface irregularities of the steel sheet are reduced, suppressing the occurrence of cracks in bending originating from these irregularities, thereby improving resistance to internal bending cracking. From the viewpoint of improving resistance to internal bending cracking, the smaller the difference between the maximum and minimum surface roughness Ra, the better. For example, it may be less than 1.5 μm, 1.2 μm or less, or 1.0 μm or less. The difference between the maximum and minimum surface roughness Ra is not particularly limited, but for example, it may be 0.1 μm or more, or 0.3 μm or more.

[0062] [Measurement of the difference between the maximum and minimum surface roughness Ra] The difference between the maximum and minimum surface roughness Ra of a steel sheet is measured as follows. More specifically, if the steel sheet has scale on its surface, the sample is first pickled before being subjected to roughness measurement. The pickling treatment is carried out by immersing the sample in hydrochloric acid with a concentration of 3 to 10 volume percent at a temperature of 85 to 98°C for 20 to 300 seconds. The pickling may be done once or in multiple steps as necessary. The above pickling time (20 to 300 seconds) refers to the time of the single pickling if pickling is performed only once, and the total time of the multiple picklings if pickling is performed multiple times. By setting the pickling temperature to 85°C or higher, surface oxides can be sufficiently removed. Next, the surface roughness of the steel sheet sample surface, or the steel sheet sample surface after pickling, is measured in one direction (0° direction). Here, the measurement length in a single measurement is set to 4 mm, and the measurement is taken in a direction perpendicular to the 0° direction (measurement direction). Next, 10 measurements are taken in the 0° direction at 10 mm intervals, i.e., a total of 40 mm (measurement length 4 mm × 10 measurements). However, if the measurable area is small and it is not possible to take 10 measurements parallel to the 0° direction at a position 10 mm away perpendicular to the measurement direction, for example, if the area of ​​the flat area is smaller than 100 mm, 10 measurements are taken parallel to the 0° direction at a position 1 mm away perpendicular to the measurement direction. With the center of the measurement area in the 0° direction as the center, measurement areas are rotated by 45°, 90°, and 135° (45° direction, 90° direction, and 135° direction), and the surface roughness of each measurement area is measured 10 times with a measurement length of 4 mm. Refer to Figure 1 for a more detailed explanation of the measurement of surface roughness in each direction. Figure 1 is a schematic diagram of the steel plate surface for measuring the difference between the maximum and minimum surface roughness Ra. In Figure 1, each straight line represents the area measured in a single measurement, i.e., an area with a measurement length of 4 mm. The dotted arrows indicate each direction. First, the surface roughness is measured over a measurement length of 4 mm in a direction perpendicular to the 0° direction. Next, measurements are taken 10 times at 10 mm intervals in the 0° direction, measuring a total measurement area of ​​40 mm (measurement length 4 mm × 10 times).The surface roughness is measured in the same manner for a total measurement area of ​​40 mm, centered on the center of the measurement area obtained by 10 measurements in the 0° direction, rotated by 45°, 90°, and 135° (in the 45°, 90°, and 135° directions). A roughness curve is obtained by sequentially applying contour curve filters with cutoff values ​​λc and λs to the measurement cross-sectional curve obtained from the measurement. Specifically, from the obtained measurement results, components with wavelengths λc of 0.8 mm or less and components with wavelengths λs of 2.5 μm or more are removed to obtain the roughness curve. Based on the obtained roughness curve, the arithmetic mean roughness is calculated in accordance with JIS B 0601:2013. The difference between the maximum and minimum values ​​of the surface roughness Ra is determined from the maximum and minimum values ​​of all obtained arithmetic mean roughness (40 points (10 points x 4 directions)).

[0063] If the steel sheet has a surface treatment film such as a plating layer or paint on its surface, the measurement is performed on the base metal surface obtained after removing the surface treatment film. The method for removing the surface treatment film can be appropriately selected according to the type of surface treatment film, within a range that does not affect the surface roughness of the base metal. For example, if the surface treatment film is a zinc plating layer such as electroplated zinc, electroplated Zn-Ni alloy, hot-dip galvanized zinc, alloyed hot-dip galvanized zinc, hot-dip Zn-Al alloy, hot-dip Zn-Al-Mg alloy, or hot-dip Zn-Al-Mg-Si alloy, the zinc plating layer can be dissolved using dilute hydrochloric acid with an inhibitor added. This allows only the zinc plating layer to be peeled off from the steel sheet. An inhibitor is an additive used to suppress changes in roughness due to the prevention of over-dissolution of the base metal. For example, hydrochloric acid diluted to 5 volume percent can be used with "Ibit No. 700BK," a corrosion inhibitor for hydrochloric acid pickling manufactured by Asahi Chemical Industry Co., Ltd., added to achieve a concentration of 0.6 g / L. Furthermore, if the surface treatment film is an aluminum plating layer such as molten aluminum plating, the Al plating is dissolved by sequentially immersing it in a dilute hydrochloric acid aqueous solution to which sodium hydroxide aqueous solution and hexamethylenetetramine have been added, in accordance with the description in JIS G 3314:2019, and continuing to immerse it until the foaming caused by the dissolution of the plating subsides. If the surface treatment film is electrodeposited coating, the electrodeposited coating is removed using a stripping agent (Neoriver SP-751: manufactured by Sansai Chemical Co., Ltd.).

[0064] [Plate Thickness] The steel plate according to the embodiment of the present invention is not particularly limited, but generally has a plate thickness of 1.0 to 8.0 mm. For example, the plate thickness may be 1.2 mm or more, 1.6 mm or more, or 2.0 mm or more, and / or 6.0 mm or less, or 4.0 mm or less.

[0065] The steel sheet according to the embodiment of the present invention may be a painted steel sheet having a paint layer on at least one surface. If the steel sheet has a plating layer and / or a chemical conversion coating, the paint layer may be formed on the plating layer and / or the chemical conversion coating. The paint layer is not particularly limited and may be any suitable paint layer known to those skilled in the art. The film thickness of the paint layer is also not particularly limited and may have any suitable film thickness. The paint layer generally includes an electrodeposited paint layer and may further include an intermediate paint layer, a base coat layer and a clear coat layer thereon.

[0066] As described above, the steel sheet according to the embodiment of the present invention can achieve excellent ductility and excellent resistance to internal cracking during bending, despite its high strength. Therefore, the steel sheet according to the embodiment of the present invention can reliably achieve both high strength and excellent workability, which are conflicting properties, and is particularly useful for use in parts in technical fields where these properties are required. In a preferred embodiment, an automotive part is provided that includes the steel sheet according to the embodiment of the present invention, particularly a part selected from the undercarriage, chassis, and bumper of an automobile. These automotive parts only need to include the steel sheet according to the embodiment of the present invention in at least a portion of them, and therefore at least a portion of these parts will satisfy the characteristics of chemical composition, metal structure, and surface properties described above. In parts of the steel sheet that do not come into direct contact with the mold during forming such as press forming, and where the degree of processing is relatively low, the characteristics of the metal structure and surface properties do not change particularly before and after forming.

[0067] [Mechanical Properties] [Tensile Strength (TS)] According to the steel sheet having the above chemical composition and metal structure, a high tensile strength, specifically a tensile strength of 980 MPa or more, can be achieved. The tensile strength is preferably 1000 MPa or more, 1040 MPa or more, or 1060 MPa or more. According to the steel sheet according to the embodiment of the present invention, despite having such a very high tensile strength, ductility and resistance to internal cracking during bending can be significantly improved by a specific combination of the chemical composition, metal structure, number density of Si oxide, and surface roughness Ra described above. There is no particular upper limit to the tensile strength, but for example, the tensile strength of the steel sheet may be 1470 MPa or less, 1250 MPa or less, or 1180 MPa or less. The tensile strength is measured by taking a JIS No. 5 test piece from the direction in which the longitudinal direction of the test piece is parallel to the direction perpendicular to the rolling direction of the steel sheet (direction C), and performing a tensile test in accordance with JIS Z 2241:2011. If the rolling direction of the steel plate cannot be determined, a JIS No. 5 test specimen may be taken from any direction within the surface of the steel plate. If it is difficult to take a JIS No. 5 test specimen, a JIS No. 13B test specimen may be used, or a small test specimen with a similar shape to a JIS No. 13B test specimen may be used.

[0068] [Total Elongation (EL)] According to the steel sheet having the above chemical composition and metal structure, in addition to high tensile strength, the total elongation can also be improved, and more specifically, a total elongation of 10.0% or more can be achieved. The total elongation is preferably 12.0% or more. There is no particular upper limit, but for example, the total elongation may be 40.0% or less or 35.0% or less. The total elongation is measured by taking a JIS No. 5 test specimen from the direction in which the longitudinal direction of the test specimen is parallel to the direction perpendicular to the rolling direction of the steel sheet (direction C), and performing a tensile test in accordance with JIS Z 2241:2011.

[0069] [Bending crack resistance] According to the steel sheet of this embodiment, in addition to high tensile strength, bending crack resistance can be improved, and more specifically, in the bending test described later, a critical bending R / t of 2.0 or less can be achieved.

[0070] [Limit Bending (R / t)] Bending crack resistance is evaluated by the limit bending (R / t) at which no cracks occur during bending. Here, R is the bending radius (mm) and t is the plate thickness (mm). According to the steel plate having the above chemical composition and metal structure, an excellent limit bending, specifically 2.0 or less, can be achieved. The limit bending is preferably 1.9 or less. Note that the limit bending (R / t) becomes more difficult as the bending radius R (mm) decreases and the plate thickness t (mm) increases.

[0071] The limit bending is determined by a bending test using the following method. First, a strip-shaped test piece measuring 100 mm x 30 mm is taken from the 1 / 2 position in the width direction of the steel plate. Next, tests are performed in accordance with the V-block method of JIS Z 2248:2022 (bending angle θ is 90°) for bending where the bending ridge is parallel to the rolling direction (L direction) (L-axis bending) and bending where the bending ridge is parallel to the rolling direction (C direction) (C-axis bending) to determine the minimum bending radius at which no crack occurs. The average of the obtained minimum bending radii for L-axis bending and C-axis bending is taken as the bending radius R (mm). The limit bending (R / t) is determined by dividing the bending radius R (mm) by the plate thickness t (mm). Here, regarding the presence or absence of cracks, the test piece after the bending test is cut with a plane perpendicular to the bending ridge and perpendicular to the plate surface, the cross-section is mirror-polished, and cracks are observed with an optical microscope. If the length of the crack observed on the inside of the bend of the test piece exceeds 30 μm, it is determined that a crack is present.

[0072] The steel sheet according to the embodiment of the present invention can be used, for example, as various automobile parts as described above. The sample collection locations in this case are as follows.

[0073] [Sampling Locations] When sampling from a wound coil, the outermost edge of the coil may have a changed surface condition. Therefore, samples should be taken from the outermost edge (1st turn) of the coil, starting from the 3rd turn and beyond, avoiding the area within 100 mm of the widthwise end. On the other hand, when sampling from automotive parts, the following locations (i) to (iv) should be avoided: (i) within 20 mm of the toe of a spot weld, and within 20 mm of the bead toe of an arc / laser weld (ii) processed areas with a radius of curvature less than 15 mm, and within 5 mm of such processed areas (iii) the end within 5 mm of the cut end face of the part (iv) within 5 mm of any area where red rust is visible

[0074] <Method for Manufacturing Steel Sheets> Next, preferred methods for manufacturing steel sheets according to embodiments of the present invention will be described. The following description is intended to illustrate characteristic methods for manufacturing steel sheets according to embodiments of the present invention, and is not intended to limit the steel sheets to those manufactured by the manufacturing methods described below. More specifically, the following describes the manufacturing of hot-rolled steel sheets, but the steel sheets according to embodiments of the present invention include any steel sheets having the chemical composition and metal structure described above, i.e., not only hot-rolled steel sheets, but also cold-rolled steel sheets, plated steel sheets, etc. Therefore, the following description merely describes preferred manufacturing methods when the steel sheets according to embodiments of the present invention are hot-rolled steel sheets.

[0075] A method for manufacturing a steel sheet according to an embodiment of the present invention includes: (A) a slab heating and extraction step that includes heating and extracting a slab having the chemical composition described above in relation to a steel sheet, satisfying the following conditions (A1) to (A3): (A1) the slab heating temperature is 1200 to less than 1270°C, (A2) the slab heating time is 3000 to 14400 seconds, and (A3) after slab extraction, high-pressure water descaling is performed at a surface temperature of 1150 to 1270°C; (B) a hot rolling step that includes heating the high-pressure water descaled slab to 1150°C or higher, performing high-pressure water descaling and rough rolling, and then high-pressure water descaling and finish rolling, satisfying the following conditions (B1) to (B3); and (B1) when heating to 1150°C or higher, the residence time in the temperature range of 1150°C or higher is less than 100 seconds. (B2) The maximum temperature reached from rough rolling to high-pressure water descaling before finish rolling is 870 to 1130°C, (B3) The temperature at the exit of finish rolling in the final stage of finish rolling is 870 to less than 1000°C, and (C) The process includes a cooling step in which the finish-rolled steel sheet is accelerated to 200°C or less at an average cooling rate of 30°C / second or more.

[0076] [(A) Slab heating and extraction process] [(A1) Slab heating temperature: less than 1200 to 1270°C] [(A2) Slab heating and holding time: 3000 to 14400 seconds] First, a slab having the chemical composition described above in relation to the steel plate is heated. From the viewpoint of productivity, the slab to be used is preferably cast by the continuous casting method, but it may also be manufactured by the ingot casting method or the thin slab casting method. The slab to be used contains a relatively large amount of alloying elements in order to obtain a high-strength steel plate. For this reason, it is necessary to heat the slab before subjecting it to hot rolling to solid dissolve the alloying elements in the slab.

[0077] Furthermore, in steel sheets containing a relatively large amount of Si, more specifically, steel sheets containing 0.30% or more Si, Si oxide forms at the interface between the Fe oxide (also called scale) generated on the steel sheet surface and the steel. This Si oxide may remain on the surface, and may also remain as Si oxide on the surface after pickling. In such cases, this Si oxide can become the starting point for cracks, worsening the resistance to internal bending cracks. Therefore, it is important to control the slab heating temperature to less than 1200-1270°C and the slab heating holding time to 3000-14400 seconds in the slab heating extraction process. It is believed that such control can create an interface between the steel and Si oxide that is easily removed by high-pressure water descaling, making it easier to remove the Si oxide by subsequent high-pressure water descaling. If the slab heating temperature is less than 1200°C or the slab heating and holding time is less than 3000 seconds, the interface between the steel and the Si oxide becomes non-uniform, preventing sufficient removal of the Si oxide by subsequent high-pressure water descaling. As a result, the number density of Si oxide particles with an equivalent circle diameter of 5 μm or more on the surface of the final steel sheet is 60 particles / mm². ２ The above may occur. Therefore, the slab heating temperature should be 1200°C or higher. Similarly, the slab heating and holding time should be 3000 seconds or higher. On the other hand, if the slab heating temperature is 1270°C or higher, or if the slab heating and holding time exceeds 14400 seconds, Si oxides will penetrate deep into the grain boundaries, and the Si oxides cannot be sufficiently removed by subsequent high-pressure water descaling. As a result, the number density of Si oxides on the surface of the final steel sheet will be 60 particles / mm². ２ The above may occur. Therefore, the slab heating temperature should be less than 1270°C. Similarly, the slab heating and holding time should be 14400 seconds or less.

[0078] [(A3) Surface temperature of steel plate in high-pressure water descaling: 1150-1270°C] In this manufacturing method, the heated and extracted slab is subjected to high-pressure water descaling at a surface temperature of 1150-1270°C. High-pressure water descaling is carried out using high-pressure water with a spraying pressure of 10-50 MPa. By using such high-pressure water, it is possible to sufficiently remove scale and Si oxides generated when the slab is heated and extracted. If the surface temperature in high-pressure water descaling is less than 1150°C, the interface between the steel and Si oxide becomes non-uniform, and Si oxides cannot be sufficiently removed by high-pressure water descaling. As a result, the number density of Si oxides with a circular equivalent diameter of 5 μm or more on the surface of the final steel plate is 60 particles / mm². ２ The above may be the case. Therefore, after slab extraction, the surface temperature in high-pressure water descaling should be 1150°C or higher. The surface temperature should be 1270°C or lower, and may be, for example, 1250°C or lower or 1200°C or lower.

[0079] [(B) Hot Rolling Process] [(B1) Residence time in the temperature range above 1150°C when heating to 1150°C or higher: less than 100 seconds] In this manufacturing method, the steel sheet that has been descaled with high-pressure water is heated to 1150°C or higher, descaled with high-pressure water, and roughly rolled. Here, when heating to 1150°C or higher, the residence time in the temperature range above 1150°C is less than 100 seconds. Furthermore, the high-pressure water descaling is carried out using high-pressure water with a sprayed water pressure of 10 to 50 MPa. By heating the steel sheet to 1150°C or higher to form an interface between steel and Si oxide that is easily removed by high-pressure water descaling, and then descaling with high-pressure water, it becomes possible to sufficiently remove the Si oxide. If the residence time is 100 seconds or more, the Si oxide penetrates deep into the grain boundaries, and the Si oxide cannot be sufficiently removed by high-pressure water descaling, resulting in a number density of Si oxide with a circle equivalent diameter of 5 μm or more on the surface of the final steel sheet being 60 particles / mm ２The above may occur. Therefore, the residence time shall be less than 100 seconds. The lower limit of the residence time is not particularly limited, but may be, for example, 5 seconds or more, 10 seconds or more, or 15 seconds or more. Also, the upper limit of the heating temperature when heating to 1150°C or higher is not particularly limited, but may be, for example, 1170°C or lower.

[0080] [Rough Rolling] In this manufacturing method, the steel sheet that has been descaled with high-pressure water is subjected to rough rolling for purposes such as adjusting the sheet thickness. The rough rolling is only required to ensure the desired sheet bar dimensions are achieved, and the conditions are not particularly limited.

[0081] [(B2) Maximum temperature reached from rough rolling to descaling before finish rolling: 870-1130°C] Roughly rolled steel sheets are subjected to high-pressure water descaling and then finish rolled. In this manufacturing method, the maximum temperature reached from rough rolling to high-pressure water descaling before finish rolling is controlled to 870-1130°C. This control is important because it allows the difference between the maximum and minimum surface roughness Ra of the final steel sheet to be controlled to 1.5 μm or less. More specifically, by controlling the maximum temperature reached from rough rolling to descaling before finish rolling to 870-1130°C, excessive growth of Si oxide is suppressed, the step difference at the boundary between the peeled and non-peeled parts during descaling can be reduced, and thereby the difference between the maximum and minimum surface roughness Ra can be controlled to 1.5 μm or less. If the maximum temperature reached is less than 870°C, the finish rolling temperature will be too low, which may result in a high bainite area ratio in the final steel sheet. Therefore, the maximum temperature reached from rough rolling to descaleing before finish rolling should be 870°C or higher. On the other hand, if the maximum temperature reached exceeds 1130°C, the Si oxide will grow excessively, and the step difference at the boundary between the delaminated and non-delaminated parts during descaling will become large, which may result in a difference of 1.5 μm or more between the maximum and minimum surface roughness Ra in the final steel sheet. Therefore, the maximum temperature reached from rough rolling to descaleing before finish rolling should be 1130°C or lower.

[0082] [(B3) Finish rolling exit temperature at the final stage of finish rolling: 870 to less than 1000°C] Steel sheets that have undergone high-pressure water descaling before finish rolling are subjected to finish rolling. In this manufacturing method, it is preferable to perform finish rolling using a tandem rolling mill consisting of, for example, three or more rolling stands, more specifically, five to eight rolling stands. In this manufacturing method, during finish rolling, the finish rolling exit temperature at the final stage of the tandem rolling mill is controlled to 870 to less than 1000°C. By performing finish rolling in the unrecrystallized region, appropriate dislocations can be introduced, and bainite is formed with these dislocations as nuclei, so that the desired microstructure fraction can be achieved in the final steel sheet. If the finish rolling exit temperature at the final stage of finish rolling is less than 870°C, the phase transformation during cooling is promoted, so the ferrite transformation is excessively promoted in the final steel sheet, the area ratio of martensite in the final steel sheet becomes low, and as a result, strength may not be obtained. Therefore, the exit temperature at the final stage of finish rolling should be 870°C or higher. On the other hand, if the exit temperature at the final stage of finish rolling is 1000°C or higher, dislocations are relaxed by recrystallization, and as a result, dislocations are not sufficiently introduced, which may result in a lower bainite area ratio in the final steel sheet. Therefore, the exit temperature at the final stage of finish rolling should be less than 1000°C.

[0083] [(C) Cooling Process] [Average Cooling Rate: 30°C / second or higher] The finish-rolled steel sheet is accelerated cooling at an average cooling rate of 30°C / second or higher from the start of cooling until it reaches 200°C. If the average cooling rate is less than 30°C / second, excessive bainite may be formed, and the area ratio of bainite in the final steel sheet may be high. Therefore, the average cooling rate should be 30°C / second or higher. On the other hand, there is no particular upper limit to the average cooling rate in accelerated cooling, but it may be, for example, 150°C / second or less or 100°C / second or less.

[0084] The cooled steel sheet is wound at a predetermined winding temperature. The winding temperature may be 200°C or lower, 150°C or lower, or 100°C or lower. Furthermore, there is no particular lower limit to the winding temperature, but for example, the winding temperature may be 25°C (room temperature) or higher.

[0085] According to the steel sheet manufactured by the above manufacturing method, a metallic structure can be obtained that contains bainite: 70-95% and martensite: 5-30% by area percentage. As a result, it is possible to achieve the desired ductility while maintaining high strength. In addition, the resulting steel sheet has a surface number density of 60 Si oxides / mm². ２ Since the surface roughness Ra is controlled to be less than 1.5 μm, and the difference between the maximum and minimum values ​​of the surface roughness Ra is controlled to be 1.5 μm or less, the desired resistance to internal bending cracks can be achieved. Therefore, steel sheets manufactured by the above manufacturing method have high strength, excellent ductility, and resistance to internal bending cracks, making them particularly useful in the automotive sector where these properties are required.

[0086] The present invention will be described in more detail below with reference to examples, but the present invention is not limited in any way to these examples.

[0087] In the following examples, steel sheets according to the embodiment of the present invention were manufactured under various conditions, and the tensile strength, total elongation, and limit bending of the obtained steel sheets were investigated.

[0088] First, molten steel was cast using a continuous casting method to form slabs with various chemical compositions as shown in Tables 1 and 2. These slabs were then heated under the conditions shown in Table 3, descaled with high-pressure water, and subsequently hot-rolled. Hot rolling was carried out by rough rolling, high-pressure water descaling, and finish rolling. The rough rolling conditions were the same for all inventive examples and comparative examples. High-pressure water descaling was performed using high-pressure water with a spraying pressure of 15 MPa, and the maximum heating temperature from the completion of rough rolling to the start of high-pressure water descaling before finish rolling was as shown in Table 3. Next, the finish-rolled steel plates were cooled under the conditions shown in Table 3 to obtain steel plates with a thickness of 2.3 to 3.2 mm.

[0089]

[0090]

[0091]

[0092] The properties of the obtained steel plates were measured and evaluated by the following method.

[0093] [Tensile Strength (TS) and Total Elongation (EL)] Tensile strength (TS) and total elongation (EL) were measured by taking a JIS No. 5 test specimen with a length of 200 mm and a thickness of 2.5 mm from the direction in which the longitudinal direction of the test specimen is parallel to the direction perpendicular to the rolling direction of the steel plate (direction C), and performing a tensile test in accordance with JIS Z 2241:2022. More specifically, the test was performed at room temperature in the range of 10 to 35°C, and a tensile test force was applied to the test specimen, allowing strain to be introduced until fracture occurred.

[0094] [Bending crack resistance: Limit bending (R / t)] To determine the limit bending (R / t), a strip-shaped test specimen measuring 100 mm in length and 30 mm in width was taken from the 1 / 2 position in the width direction of the steel plate, with the longitudinal direction of the test specimen parallel to the rolling direction of the steel plate (L direction) and the longitudinal direction of the test specimen perpendicular to the rolling direction (C direction). The V-block method (bending angle θ = 90°) in accordance with JIS Z 2248:2022 was performed for both bending where the bending ridge is parallel to the rolling direction (L direction) (L-axis bending) and bending where the bending ridge is parallel to the rolling direction (C direction) (C-axis bending), and the minimum bending radius without cracking was determined. The bending radius R (mm) was calculated from the average of the obtained minimum bending radii for L-axis bending and C-axis bending. Next, the limit bending (R / t) was calculated by dividing the bending radius R (mm) by the plate thickness t (mm). However, the presence or absence of cracks was determined by mirror-polishing the cross-section of the specimen, which was cut parallel to the bending direction and perpendicular to the plate surface after the bending test, and observing the cracks with an optical microscope. If the length of the crack observed on the inside of the bend of the specimen exceeded 30 μm, it was determined that a crack was present.

[0095] Steel sheets with a tensile strength (TS) of 980 MPa or higher, a total elongation (EL) of 10.0% or higher, and a critical bending radius (R / t) of 2.0 or lower were evaluated as having high strength, excellent ductility, and excellent resistance to internal cracking during bending. The results are shown in Table 4. In Table 4, "Maximum Ra - Minimum Ra" indicates the difference between the maximum and minimum values ​​of the surface roughness Ra.

[0096]

[0097] Referring to Tables 1-4, in Comparative Example 4, the high slab heating temperature allowed Si oxides to penetrate deep into the grain boundaries, and the subsequent high-pressure water descaling could not sufficiently remove the Si oxides. As a result, the number density of coarse Si oxides on the surface of the final steel sheet was excessively high. Consequently, R / t increased. In Comparative Example 5, the long slab heating time allowed Si oxides to penetrate deep into the grain boundaries, and the subsequent high-pressure water descaling could not sufficiently remove the Si oxides. As a result, the number density of coarse Si oxides on the surface of the final steel sheet was excessively high. Consequently, R / t increased. In Comparative Example 6, the low surface temperature of the steel sheet during high-pressure water descaling after slab extraction resulted in a non-uniform interface between the steel and Si oxides. As a result, the high-pressure water descaling could not sufficiently remove the Si oxides. Consequently, the number density of coarse Si oxides on the surface of the final steel sheet was excessively high. As a result, R / t increased.

[0098] In Comparative Examples 7 and 36, the residence time in the high-pressure water descaling before rough rolling was long, resulting in Si oxides penetrating deep into the grain boundaries. This prevented sufficient removal of Si oxides by high-pressure water descaling, and consequently, the number density of coarse Si oxides on the surface of the final steel sheet became excessively high. As a result, R / t increased. In Comparative Example 8, the maximum temperature reached from rough rolling to high-pressure water descaling before finish rolling was high, causing excessive growth of Si oxides. This resulted in a large step difference at the boundary between the delaminated and non-delaminated areas during descaling, and consequently, a large difference between the maximum and minimum surface roughness Ra values ​​in the final steel sheet. As a result, R / t increased.

[0099] In Comparative Example 9, the low finishing rolling temperature likely led to excessive ferrite transformation, resulting in a low martensite area ratio in the final steel sheet. Consequently, the TS decreased. On the other hand, in Comparative Example 10, the high finishing rolling temperature likely led to dislocation relaxation through recrystallization, resulting in insufficient dislocation introduction and a low bainite area ratio in the final steel sheet. Consequently, the EL decreased.

[0100] In Comparative Example 11, the average cooling rate up to 200°C was slow, which likely led to excessive bainite formation. As a result, the area ratio of bainite in the final steel sheet was high, while the area ratio of martensite was low. Consequently, the total surface area (TS) decreased.

[0101] In Comparative Example 29, the low carbon content likely resulted in insufficient martensite hardness. Consequently, the TS (Total Score) decreased. On the other hand, in Comparative Example 30, the high carbon content likely caused the martensite to become excessively hard. As a result, the EL (Energy Level) decreased and the R / t (Resilience / Talency) ratio increased.

[0102] In Comparative Example 31, the low Si content likely led to the formation of carbides, resulting in a lower martensite area ratio in the final steel sheet. Consequently, the total stress (TS) decreased. On the other hand, in Comparative Example 32, the high Si content likely caused slab cracking.

[0103] In Comparative Example 33, the low Mn content resulted in insufficient martensite formation, leading to a low martensite area ratio in the final steel sheet. As a result, the TS decreased. On the other hand, in Comparative Example 34, the high Mn content resulted in excessive martensite formation, leading to a low martensite area ratio in the final steel sheet. As a result, the EL decreased.

[0104] In Comparative Example 35, the high Al content stabilized the ferrite, which is thought to have resulted in a higher ferrite area ratio in the final steel sheet. As a result, the TS decreased.

[0105] In contrast, in all the steel sheets according to the invention examples, a predetermined chemical composition is achieved, and by appropriately controlling each condition in the manufacturing method, the metal structure has a metal structure containing bainite: 70-95% and martensite: 5-30% by area percentage, and the number density of Si oxides with a circular equivalent diameter of 5 μm or more on the surface is 60 particles / mm². ２ We were able to obtain steel sheets with a surface roughness Ra of less than 1.5 μm, where the difference between the maximum and minimum values ​​of the surface roughness Ra was 1.5 μm or less. As a result, the sheets had a high tensile strength of 980 MPa, excellent ductility, and excellent resistance to internal cracking during bending. In addition, in all the steel sheets according to the invention examples, the remaining microstructure other than bainite and martensite consisted of at least one of ferrite, pearlite, and retained austenite.

Claims

1. In mass percent, C: 0.045-0.120%, Si: 0.30-3.00%, Mn: 1.20-3.00%, Al: 0.010-0.400%, P: 0.080% or less, S: 0.0100% or less, N: 0.0150% or less, O: 0.0100% or less, Ti: 0-0.180%, Nb: 0-0.100%, V: 0-1.000%, Cu: 0-1.000%, Cr: 0-2.000%, Mo: 0-3.000%, Ni: 0-1.000%, B: 0-0.0100%, Ca: 0-0.0500%, Mg: 0-0.050% The material has a chemical composition consisting of REM: 0-0.100%, Bi: 0-0.100%, Ta: 0-0.100%, Zr: 0-0.500%, Co: 0-3.000%, Zn: 0-0.200%, W: 0-0.200%, Sb: 0-0.500%, As: 0-0.100%, Sn: 0-1.000%, and the remainder being Fe and impurities. At a position 1 / 4 of the plate thickness from the surface, it has a metallic structure containing, in area percent, bainite: 70-95% and martensite: 5-30%. On the surface, the number density of Si oxide particles with an equivalent circle diameter of 5 μm or more is 60 particles / mm². ２ A steel sheet characterized by having a surface roughness Ra of less than 1.5 μm, and the difference between the maximum and minimum values ​​of the surface roughness Ra being 1.5 μm or less.

2. The chemical composition is as follows, in mass%,: Ti: 0.001-0.180%, Nb: 0.001-0.100%, V: 0.001-1.000%, Cu: 0.001-1.000%, Cr: 0.001-2.000%, Mo: 0.001-3.000%, Ni: 0.001-1.000%, B: 0.0001-0.0100%, Ca: 0.0001-0.0500%, Mg: 0.0001-0.050%, REM: 0.0001-0.100%, Bi: 0.001-0.100%, Ta: 0.001-0.100%, The steel sheet according to claim 1, characterized by containing at least one of the following: Zr: 0.001 to 0.500%, Co: 0.001 to 3.000%, Zn: 0.001 to 0.200%, W: 0.001 to 0.200%, Sb: 0.001 to 0.500%, As: 0.001 to 0.100%, and Sn: 0.001 to 1.000%.

3. The steel plate according to claim 1 or 2, characterized by having a tensile strength of 980 MPa or more.

4. A steel plate according to any one of claims 1 to 3, characterized in that it has a plate thickness of 1.0 to 8.0 mm.

5. A component characterized by comprising a steel plate as described in any one of claims 1 to 4.

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