Steel sheet and method for producing same

Abstract

To provide a steel sheet which has high strength and excellent low temperature toughness, and further has reduced surface layer hardness.SOLUTION: The steel sheet has a chemical composition containing, by mass%, C:0.01 to 0.15%, Si: 0.01 to 0.50%, Mn: 0.50 to 3.00%, P: 0.03% or less, S: 0.005% or less, N: 0.0010 to 0.0080%, and Al: 0.008 to 0.10%, with the balance being Fe and an inevitable impurity, and the microstructure has an area ratio of ferrite of 50% or more and an area ratio of a region of KAM values ≤ 1 ° of 50% or more at a 0. 5mm position, and an area ratio of bainite of 60% or more and an area ratio of martensite-austenite constituent of less than 1% at a sheet thickness center.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a steel plate and a manufacturing method thereof, and more particularly to a steel plate having reduced surface layer hardness while maintaining high strength and excellent low-temperature toughness, and a manufacturing method thereof.The steel plate of the present invention can be suitably used as a steel for structures such as tanks for storing liquefied gas on ships and on land. [Background technology]

[0002] When hot-rolled steel plates are used in structures such as liquefied gas storage tanks, the use environment is low, so the steel plates are required to have not only high strength but also excellent toughness at low temperatures. When hot-rolled steel plates are used in tanks for storing liquid ammonia, they must ensure excellent toughness at low temperatures below -34°C, the boiling point of ammonia. If the low-temperature toughness of steel materials is poor, the safety of low-temperature storage structures may not be maintained, so there is a strong demand for improved low-temperature toughness of the steel plates used. Furthermore, in order to increase the size of tanks, there is a strong demand for high-strength steel materials, for example, tensile strength (TS) of 610 MPa or more. From the perspective of preventing stress corrosion cracking due to liquid ammonia, there is also a strong demand for reduced surface hardness.

[0003] Steel plates that are resistant to stress corrosion cracking due to liquid ammonia and have high strength and excellent low-temperature toughness are proposed in Patent Documents 1 and 2, for example.

[0004] The technology described in Patent Document 1 includes the inclusion of predetermined amounts of elements such as C, Si, Mn, and Al, and the metallographic structure is controlled so that the volume fraction of bainite at a depth of 0.5 mm from the surface of the steel sheet is 90% or more. The steel sheet has an average hardness of 230 HV0.1 or less and a hardness variation of 30 HV0.1 or less at a depth of 0.5 mm from the surface. Furthermore, the maximum hardness in the thickness direction is located at a position 1.0 mm or more from the surface and up to ¼ of the thickness of the steel sheet, with a hardness variation of 70 HV1 or less. This results in excellent stress corrosion cracking resistance in a liquid ammonia environment. Patent Document 1 also describes a steel sheet with the above metallographic structure and hardness characteristics obtained through a manufacturing process using a thermomechanical controlled process (TMCP) and an induction heating device.

[0005] In the technology described in Patent Document 2, elements such as C, Si, Mn, Al, etc. are added in predetermined amounts or more, and the metal structure (microstructure) of the steel plate is controlled so that the total volume fraction of the ferrite structure and bainite structure at a position halfway through the plate thickness of the steel plate is 60% or more, thereby achieving the desired low-temperature toughness and strength characteristics.Furthermore, one or more elements selected from Cu, Cr, Sb, and Sn are added in predetermined amounts or more, and the hardness at a position 1.0 mm deep from the surface of the steel plate is controlled to Hv300 or less, thereby obtaining excellent stress corrosion cracking resistance in a liquid ammonia environment. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication No. 2023 / 162571 [Patent Document 2] International Publication No. 2023 / 162507 Summary of the Invention [Problem to be solved by the invention]

[0007] However, in the techniques described in Patent Documents 1 and 2, the use of an induction heating device makes the manufacturing equipment complex, and it becomes necessary to add one or more alloy elements selected from Cu, Cr, Sb, and Sn.

[0008] An object of the present invention is to provide a steel sheet having high strength and excellent low-temperature toughness and further reduced surface layer hardness, and a method for manufacturing the same.

[0009] In the present invention, high strength means that the tensile strength (TS) of the steel plate is 610 MPa or more when a tensile test is carried out in accordance with the provisions of JIS Z 2241:2022.

[0010] In the present invention, the term "excellent low-temperature toughness" refers to the Charpy absorbed energy (vE -50℃ ) is 47J or more. -50℃ ) is determined as follows: A V-notch test piece is taken from the center of the steel plate thickness in accordance with the provisions of JIS Z 2242:2023 so that the longitudinal direction of the test piece coincides with the plate width direction. Using this V-notch test piece, a Charpy impact test is carried out at a test temperature of -50°C in accordance with the provisions of JIS Z 2242:2023. The impact test is carried out on three of the V-notch test pieces for one steel plate, and the average value of the three is taken as the Charpy absorbed energy (vE -50℃ )

[0011] In the present invention, "reduced surface layer hardness" means that the surface layer hardness of the steel sheet is 210 HV10 or less. The surface layer hardness is determined as follows: Vickers hardness is measured at five points 0.5 mm below the surface of the steel sheet with a load of 10 kg, and the average value of the five points is taken as the surface layer hardness of the steel sheet. It can be said that a surface layer hardness of 210 HV10 or less can reduce susceptibility to stress corrosion cracking by liquid ammonia. [Means for solving the problem]

[0012] In order to solve the above problems, the present inventors have conducted extensive research into the chemical composition and structure of steel sheets and have obtained the following findings. (1) To obtain high strength, the area ratio of bainite in the structure at the center of the plate thickness must be 60% or more. To obtain such a structure, the average cooling rate in the temperature range of 700°C to 500°C at the center of the plate thickness in the cooling process after hot rolling, while satisfying the specified chemical composition, must be 3°C / s or more and 100°C / s or less, and the cooling stop temperature must be 300°C or less. (2) Furthermore, to ensure low-temperature toughness, the area ratio of island martensite in the structure at the center of the plate thickness is set to less than 1%. To achieve this, the tempering temperature is set to 650°C or higher and Ac1 point or lower while satisfying the specified chemical composition. (3) For the purpose of reducing stress corrosion cracking susceptibility, the area ratio of ferrite in the structure 0.5 mm below the steel plate surface is set to 50% or more in order to reduce the surface hardness, and the area ratio of the region with a KAM value of 1° or less is set to 50% or more. To achieve this, the rolling end temperature in the hot rolling process is set to Ar3 point + 30°C or more, the inter-pass time in rolling passes between the rolling end temperature and the rolling end temperature + 100°C or less is set to 30 seconds or less, the heating temperature in the tempering process is set to 650°C or more and Ac1 point or less, and tempering is performed under conditions such that the tempering parameter λ is 18.7 or more.

[0013] The present invention has been completed based on the above findings, and the gist of the present invention is as follows. [1] In mass%, C: 0.01 to 0.15%, Si: 0.01 to 0.50%, Mn: 0.50 to 3.00% P: 0.03% or less, S: 0.005% or less, N: 0.0010 to 0.0080%, and Al: 0.008 to 0.10% and the balance being Fe and unavoidable impurities, The organization At a position 0.5 mm below the surface of the steel sheet, the area ratio of ferrite is 50% or more and the area ratio of the region where the KAM value is ≦ 1° is 50% or more, A steel plate having an area fraction of bainite of 60% or more and an area fraction of island martensite of less than 1% at the center of the plate thickness. [2] The steel sheet according to [1], wherein the chemical composition further contains, in mass %, one or two of the following groups A and B: Group A: Ni: 3.0% or less, Cr: 1.00% or less, Mo: 0.50% or less Cu: 1.00% or less, Nb: 0.05% or less, V: 0.10% or less, Ti: 0.03% or less, B: 0.0030% or less, and Sn: 0.50% or less One or more selected from the following Group B: Ca: 0.005% or less, REM: 0.010% or less, and Mg: 0.005% or less One or more selected from the following [3] A heating step of heating a steel material having the component composition according to [1] or [2]; Next, a hot rolling step of hot rolling the steel material to form a hot-rolled steel sheet; Next, a cooling step of cooling the hot-rolled steel sheet; Then, a tempering process is provided in which the hot-rolled steel sheet after the cooling process is tempered. In the hot rolling process, the temperature of the steel sheet surface is the rolling end temperature: Ar3 point + 30°C or more, and the inter-pass time in the rolling passes is the rolling end temperature or more and the rolling end temperature + 100°C or less, and is 30 seconds or less, In the cooling step, the average cooling rate in the temperature range of 700°C to 500°C at the center of the plate thickness of the hot-rolled steel plate is 3°C / s or more and 100°C / s or less, and the cooling stop temperature is 300°C or less, In the tempering step, the steel sheet is tempered at a heating temperature of 650° C. or higher and Ac1 point or lower, and the tempering parameter λ in the following formula (1) is 18.7 or higher. (λ×1000)=(T+273)×(log(t)+20) ···(1) In the formula (1), T is the heating temperature (° C.) in the tempering step, and t is the holding time (hours) at the heating temperature T. [Effects of the Invention]

[0014] According to the present invention, it is possible to provide a steel sheet having high strength and excellent low-temperature toughness and further reduced surface layer hardness, and a method for manufacturing the same.

[0015] According to the present invention, it is possible to provide a steel sheet having high strength and excellent low-temperature toughness and further reduced surface layer hardness, and a manufacturing method thereof, more simply, specifically, without using a manufacturing process using an induction heating device and without necessarily adding alloy elements such as Cu, Cr, Sb, and Sn.

[0016] According to the present invention, it is possible to provide a steel plate for low temperature use that has reduced surface hardness and reduced liquid ammonia stress corrosion cracking susceptibility while ensuring high strength and low temperature toughness. By using the steel plate of the present invention for steel structures such as tanks for storing liquefied gas, the safety of the steel structures can be improved, which brings about significant industrial effects. DETAILED DESCRIPTION OF THE INVENTION

[0017] Hereinafter, embodiments of the present invention will be specifically described. Note that the following description shows preferred embodiments of the present invention, and the present invention is not limited thereto.

[0018] [Component composition] The steel sheet of the present invention has a predetermined chemical composition. Furthermore, it is preferable that the steel material used to manufacture the steel sheet of the present invention also has the above-mentioned predetermined chemical composition. Hereinafter, each element contained in this chemical composition will be explained. In addition, unless otherwise specified, in this specification, "%" as a unit of the content of each element means "mass%."

[0019] C: 0.01 to 0.15% C is an element that has the effect of improving the strength of steel sheet. To achieve this effect, the C content is set to 0.01% or more. The C content is preferably set to 0.03% or more. On the other hand, if the C content exceeds 0.15%, the low-temperature toughness of the steel sheet may decrease. Therefore, the C content is set to 0.15% or less. The C content is preferably set to 0.12% or less.

[0020] Si: 0.01 to 0.50% Si is an element that contributes to improving the strength of steel sheet and also acts as a deoxidizer. To achieve these effects, the Si content is set to 0.01% or more. The Si content is preferably set to 0.03% or more. On the other hand, if the Si content exceeds 0.50%, low-temperature toughness decreases. Therefore, the Si content is set to 0.50% or less. The Si content is preferably set to 0.30% or less.

[0021] Mn: 0.50 to 3.00% Mn is an element that improves the hardenability of steel and is effective in increasing the strength of steel sheets. To achieve this effect, the Mn content is set to 0.50% or more. The Mn content is preferably set to 0.60% or more. On the other hand, if the Mn content exceeds 3.00%, there is a concern that the temper embrittlement susceptibility will increase and the low-temperature toughness will decrease. Therefore, the Mn content is set to 3.00% or less. The Mn content is preferably set to 2.50% or less, more preferably 2.00% or less.

[0022] P:0.03% or less P is an inevitable impurity and a harmful element that adversely affects the low-temperature toughness of steel sheet. Therefore, the P content is set to 0.03% or less. The P content is preferably set to 0.02% or less. Furthermore, from the viewpoint of low-temperature toughness, the lower the P content, the better, so the lower limit is not particularly limited and may be 0%, but even in this case, it is acceptable for P to be contained as an inevitable impurity. On the other hand, since an excessive reduction in the P content causes an increase in costs, from the viewpoint of cost, the lower limit of the P content is preferably set to 0.001%.

[0023] S: 0.005% or less Since S forms MnS in steel and deteriorates low-temperature toughness, the upper limit of S is set to 0.005%, and it is desirable to reduce it as much as possible. The S content is preferably 0.003% or less. On the other hand, the lower the S content, the better, so there is no particular restriction on the lower limit, and it may be 0%, but even in that case, it is acceptable for S to be contained as an unavoidable impurity. On the other hand, excessive reduction of the S content causes an increase in cost, so from the viewpoint of cost, it is preferable that the lower limit of the S content is 0.001%.

[0024] N: 0.0010~0.0080% N is an element that contributes to the refinement of the base material grains by forming AlN and contributes to improving the low-temperature toughness of the base material. This effect can be achieved by setting the N content to 0.0010% or more. However, N forms precipitates in steel, and if the N content exceeds 0.0080%, it actually causes a decrease in the low-temperature toughness of the base material. Therefore, the N content is set to 0.0010% or more and 0.0080% or less. The N content is preferably set to 0.0020% or more. Furthermore, the N content is preferably set to 0.0060% or less.

[0025] Al: 0.008 to 0.10% Al is an element that acts as a deoxidizer. If the Al content is less than 0.008%, the effect as a deoxidizer is poor. Furthermore, Al is an element that contributes to the refinement of the base material grains by forming AlN, and contributes to improving the low-temperature toughness of the base material. Therefore, the Al content is set to 0.008% or more. The Al content is preferably set to 0.02% or more. On the other hand, if the Al content exceeds 0.10%, the cleanliness of the steel may be impaired, and the low-temperature toughness may actually decrease. Therefore, the Al content is set to 0.10% or less. The Al content is preferably set to 0.08% or less.

[0026] The composition of one embodiment of the present invention may be such that, in addition to the predetermined amounts of the elements described above, the remainder consists of Fe and inevitable impurities. In other words, the present invention does not require the inclusion of alloying elements such as Cu, Cr, Sb, and Sn. Incidentally, inevitable impurities are impurities that are inevitably mixed in from raw materials, the manufacturing process, manufacturing equipment, etc., and are allowed to be included to the extent that they do not impair the objectives of the present invention. Examples of raw materials include iron ore, reduced iron, and scrap.

[0027] In another embodiment of the present invention, the above-mentioned component composition may optionally contain one or more elements selected from the following Group A and Group B. As mentioned above, the inclusion of these elements is not essential in the present invention.

[0028] (Group A) One or more selected from Ni: 3.0% or less, Cr: 1.00% or less, Mo: 0.50% or less, Cu: 1.00% or less, Nb: 0.05% or less, V: 0.10% or less, Ti: 0.03% or less, B: 0.0030% or less, and Sn: 0.50% or less

[0029] Ni: 3.0% or less Ni is an element effective in improving the low-temperature toughness of steel sheet. To achieve this effect, when Ni is contained, the Ni content is preferably 0.2% or more. However, because Ni is an expensive element, the cost of the steel sheet increases as the Ni content increases. Therefore, when Ni is contained, the Ni content is set to 3.0% or less. The Ni content is preferably set to 2.5% or less.

[0030] Cr:1.00% or less Cr is an element that can improve the hardenability and strength of the steel sheet. To achieve this effect, when Cr is contained, the Cr content is preferably 0.05% or more. However, if the Cr content exceeds 1.00%, the low-temperature toughness of the steel sheet decreases. Therefore, when Cr is contained, the Cr content is set to 1.00% or less. The Cr content is preferably 0.80% or less, and more preferably 0.60% or less.

[0031] Mo: 0.50% or less Like Cr, Mo is an element that can improve the hardenability and strength of the steel sheet. To achieve this effect, when Mo is contained, the Mo content is preferably 0.05% or more. On the other hand, if the Mo content exceeds 0.50%, the low-temperature toughness decreases. Therefore, when Mo is contained, the Mo content is set to 0.50% or less. The Mo content is preferably set to 0.40% or less.

[0032] Cu: 1.00% or less Cu is an element that has the effect of increasing the strength of steel sheet by improving hardenability. To achieve this effect, when Cu is contained, the Cu content is preferably 0.05% or more. However, if the Cu content exceeds 1.00%, the low-temperature toughness of the steel sheet decreases. Therefore, when Cu is contained, the Cu content is set to 1.00% or less. The Cu content is preferably set to 0.60% or less.

[0033] Nb: 0.05% or less Nb is an effective element for increasing the strength of steel sheets through precipitation strengthening. To achieve this effect, when Nb is contained, the Nb content is preferably 0.01% or more. However, if the Nb content is excessively high, the low-temperature toughness of the steel sheet decreases. Therefore, when Nb is contained, the Nb content is set to 0.05% or less. The Nb content is preferably set to 0.03% or less.

[0034] V: 0.10% or less Like Nb, V is an effective element for increasing the strength of steel sheet by precipitation strengthening. To achieve this effect, when V is contained, the V content is preferably 0.01% or more. However, if the V content is excessively high, the low-temperature toughness of the steel sheet decreases. Therefore, when V is contained, the V content is set to 0.10% or less. The V content is preferably 0.06% or less, more preferably 0.04% or less.

[0035] Ti: 0.03% or less Ti is an element that has the effect of increasing the toughness of the weld without degrading the mechanical properties of the base material when steel plates are welded to form a welded structure. To achieve this effect, if Ti is contained, the Ti content is preferably 0.01% or more. However, if the Ti content is excessively high, low-temperature toughness decreases. Therefore, if Ti is contained, the Ti content is set to 0.03% or less. The Ti content is preferably set to 0.02% or less.

[0036] B: 0.0030% or less B is an element that improves hardenability even when contained in small amounts. To achieve this effect, when B is contained, the B content is preferably 0.0003% or more. On the other hand, if the B content exceeds 0.0030%, low-temperature toughness deteriorates. Therefore, when B is contained, the B content is set to 0.0030% or less. The B content is preferably set to 0.0025% or less.

[0037] Sn: 0.50% or less Sn is an element that may be unavoidably mixed in, and if contained in excess, there is a risk of reducing toughness. Therefore, if Sn is contained, the Sn content is set to 0.50% or less. There is no particular lower limit for the Sn content, and it may be 0%.

[0038] (Group B) One or more selected from Ca: 0.005% or less, REM: 0.010% or less, and Mg: 0.005% or less

[0039] Ca: 0.005% or less Ca is an element that has the effect of improving the low-temperature toughness of steel sheets by controlling the morphology of inclusions in steel. To achieve this effect, when Ca is contained, the Ca content is preferably 0.001% or more. However, an excessive Ca content impairs the cleanliness of the steel. Therefore, when Ca is contained, the Ca content is set to 0.005% or less. The Ca content is preferably set to 0.004% or less.

[0040] REM: 0.010% or less Like Ca, REM (Rare Earth Metal) is an element that has the effect of improving the low-temperature toughness of steel plate by controlling the morphology of inclusions in steel. To achieve this effect, when REM is contained, the REM content is preferably 0.001% or more. However, an excessive REM content impairs the cleanliness of the steel. Therefore, when REM is contained, the REM content is set to 0.010% or less. The REM content is preferably set to 0.008% or less. Here, REM is a collective term for 17 elements, including 15 lanthanoid elements plus Y and Sc, and these elements can be contained alone or in combination. The REM content refers to the total content of these elements.

[0041] Mg: 0.005% or less Like Ca and REM, Mg is an element that has the effect of improving the low-temperature toughness of steel sheets by controlling the morphology of inclusions in steel. To achieve this effect, when Mg is contained, the Mg content is preferably 0.001% or more. However, an excessive Mg content impairs the cleanliness of the steel. Therefore, when Mg is contained, the Mg content is set to 0.005% or less. The Mg content is preferably set to 0.004% or less.

[0042] [Organization] In the steel sheet of the present invention, in the structure at a position 0.5 mm below the surface of the steel sheet (a position 0.5 mm deep from the surface of the steel sheet in the sheet thickness direction), the area fraction of ferrite is 50% or more and the area fraction of regions with a KAM value ≦ 1° is 50% or more. Furthermore, in the structure at the center of the sheet thickness, the area fraction of bainite is 60% or more and the area fraction of island martensite is less than 1%. This makes it possible to realize a steel sheet that has high strength and excellent low-temperature toughness, and also has reduced surface layer hardness (hardness at a position 0.5 mm below the surface of the steel sheet).

[0043] (At a position 0.5 mm below the surface, the area ratio of ferrite is 50% or more and the area ratio of the area with a KAM value of 1° or less is 50% or more) Kernel Average Misorientation (KAM) is a parameter obtained from crystal orientation measured using the EBSD method and is an index that indicates the average misorientation at surrounding measurement points. The KAM value corresponds to the dislocation density of geometrically necessary dislocations, and is therefore considered to have a quantitative correspondence with the plastic strain imparted by dislocation motion. To reduce the surface hardness and reduce stress corrosion cracking susceptibility, the area fraction of ferrite at a position 0.5 mm below the surface must be 50% or more and the area fraction of the region with a KAM value ≦1° must be 50% or more. If either of these requirements is not met, the surface hardness cannot be sufficiently reduced and stress corrosion cracking susceptibility increases. It is preferable that both the area fraction of ferrite and the area fraction of the region with a KAM value ≦1° be 60% or more. The remainder of the structure may be one or more of bainite and tempered martensite. The upper limit of the ferrite area fraction in the structure at a position 0.5 mm below the surface is not particularly limited and may be 100%. The upper limit of the area ratio of the region with a KAM value of 1° or less is not particularly limited and may be 100%. The area ratio of the region with a KAM value of 1° or less can be determined in detail by the method described in the Examples.

[0044] (At the center of the plate thickness, the area ratio of bainite is 60% or more and the area ratio of island martensite is less than 1%) To obtain the desired high strength, the area fraction of bainite in the structure at the center of the sheet thickness is set to 60% or more. The remainder of the structure may be ferrite. If the area fraction of bainite in the structure at the center of the sheet thickness is less than 60%, the desired strength cannot be obtained. The area fraction of bainite in the structure at the center of the sheet thickness is preferably set to 70% or more. Furthermore, there is no particular upper limit to the area fraction of bainite in the structure at the center of the sheet thickness, and it may be 100%.

[0045] Since island martensite reduces low-temperature toughness, its area fraction must be reduced. In the present invention, in order to ensure the desired low-temperature toughness, the area fraction of island martensite at the center of the plate thickness is set to less than 1%. The lower limit of the area fraction of island martensite at the center of the plate thickness is not particularly limited and may be 0%.

[0046] The thickness of the steel plate of the present invention is not particularly limited and can be any thickness. For example, the thickness of the steel plate of the present invention is preferably 10 mm or more and 50 mm or less. The thickness of the steel plate of the present invention is more preferably 12 mm or more.

[0047] The tensile strength (TS) of the steel sheet of the present invention is 610 MPa or more. The TS of the steel sheet of the present invention is preferably 620 MPa or more. There is no particular upper limit to the TS of the steel sheet of the present invention, but as an example, the TS of the steel sheet of the present invention is 700 MPa or less.

[0048] The surface layer hardness of the steel sheet of the present invention is 210 HV10 or less. The surface layer hardness of the steel sheet of the present invention is preferably 200 HV10 or less. There is no particular restriction on the lower limit of the surface layer hardness of the steel sheet of the present invention, but as an example, the surface layer hardness of the steel sheet of the present invention is 160 HV10 or more.

[0049] The steel sheet of the present invention has a Charpy absorbed energy (vE -50℃ ) is 47J or more. -50℃ ) is preferably 100J or more.

[0050] [Manufacturing method] Next, an example of a manufacturing method that can suitably manufacture the steel sheet of the present invention will be described. In the following description, unless otherwise specified, temperature refers to the temperature at the center of the sheet thickness. The temperature at the center of the sheet thickness can be calculated by heat transfer calculation from the surface temperature of the steel sheet measured with a radiation thermometer, for example.

[0051] As a specific example of the manufacturing method, the steel sheet of the present invention can be suitably manufactured by sequentially carrying out the following steps (1) to (4). (1) Heating process of steel material (2) Hot rolling process (3) Cooling process (4) Tempering process

[0052] (1) Heating process of steel material First, a steel material having the above-described composition is preferably heated to a temperature of 900°C or higher and 1250°C or lower. The method for producing the steel material is not particularly limited, but the steel material can be produced, for example, by melting molten steel having the above-described composition by a conventional method and casting it. Melting can be carried out by any method, such as a converter, electric furnace, or induction furnace. From the viewpoint of productivity, casting is preferably carried out by a continuous casting method, but it can also be carried out by an ingot making-blooming rolling method. For example, a steel slab can be used as the steel material. Here, the heating of the steel material may be carried out after the steel material obtained by a method such as casting has been cooled, or the obtained steel material may be heated directly without being cooled.

[0053] If the heating temperature of the steel material is less than 900°C, the deformation resistance of the steel material is high, which increases the load on the rolling mill in the subsequent hot rolling, making it difficult to perform the hot rolling. Therefore, it is preferable that the heating temperature of the steel material be 900°C or higher. On the other hand, if the heating temperature of the steel material is higher than 1250°C, oxidation of the steel becomes significant, and loss due to removing the oxide film caused by oxidation increases, resulting in a decrease in yield. Therefore, it is preferable that the heating temperature of the steel material be 1250°C or lower.

[0054] (2) Hot rolling process After the heating, the heated steel material can be hot-rolled to produce a hot-rolled steel sheet. The final thickness of the hot-rolled steel sheet is not particularly limited, but as mentioned above, it is preferably 10 mm or more and 50 mm or less, and more preferably 12 mm or more. The rolling end temperature of the hot rolling at the surface of the steel sheet is Ar3 point + 30°C or more. If the rolling end temperature of the hot rolling at the surface of the steel sheet is less than Ar3 point + 30°C, the area ratio of the region with a KAM value ≦ 1° in the final surface structure (structure 0.5 mm below the surface of the steel sheet) will be less than 50% due to the influence of the formation of worked ferrite. As a result, the surface hardness may exceed 210 HV, which is considered to be at high risk of liquid ammonia stress corrosion cracking. The Ar3 point (Ar3 transformation point) can be determined by the following formula (2). Ar3 points (℃)=910-310×C-80×Mn-20×Cu-55×Ni-15×Cr-80×Mo...(2) Here, the element symbols in formula (2) represent the content (mass%) of each element in the steel, and if the element is not contained, the content in the steel is taken as 0.

[0055] Furthermore, in the hot rolling process, the inter-pass time between rolling passes at a steel sheet surface temperature of not less than the rolling end temperature but not more than 100°C above the rolling end temperature is set to 30 seconds. Here, when there are multiple inter-pass times at not less than the rolling end temperature but not more than 100°C above the rolling end temperature, the inter-pass time is taken to be the average value of these. Furthermore, when the rolling pass temperature immediately before the final pass is more than 100°C above the rolling end temperature, the inter-pass time is taken to be the inter-pass time between the final pass and the rolling pass immediately before it. That is, for example, assuming that a steel plate having an Ar3 point of 720°C is rolled in five rolling passes with a first rolling pass temperature of 920°C, a second rolling pass temperature of 900°C, a third rolling pass temperature of 870°C, a fourth rolling pass temperature of 820°C, and a final rolling pass temperature of 760°C, the inter-pass time between rolling passes at temperatures equal to or higher than the rolling end temperature and equal to or lower than the rolling end temperature + 100°C is the inter-pass time between the fourth rolling pass and the final rolling pass. Note that the rolling pass temperature is the rolling temperature at that rolling pass. Furthermore, for example, assuming that a steel plate having an Ar3 point of 720°C is rolled using five rolling passes with a first rolling pass temperature of 910°C, a second rolling pass temperature of 870°C, a third rolling pass temperature of 830°C, a fourth rolling pass temperature of 790°C, and a final rolling pass temperature of 760°C, the inter-pass time for rolling passes at temperatures equal to or higher than the rolling end temperature and equal to or lower than the rolling end temperature + 100°C is the average value of the inter-pass time between the third and fourth rolling passes and the inter-pass time between the fourth and final rolling passes. For example, assuming that a steel sheet having an Ar3 point of 720°C undergoes five rolling passes, with a first rolling pass temperature of 950°C, a second rolling pass temperature of 930°C, a third rolling pass temperature of 900°C, a fourth rolling pass temperature of 870°C, and a final rolling pass temperature of 760°C (when the rolling pass temperature immediately before the final rolling pass is greater than the rolling end temperature + 100°C), the inter-pass time between the fourth rolling pass and the final rolling pass is set to 30 seconds or less. If the inter-pass time between the rolling passes at or above the rolling end temperature and less than or equal to the rolling end temperature + 100°C (when the rolling pass temperature immediately before the final rolling pass is greater than the rolling end temperature + 100°C, the inter-pass time between the final pass and the rolling pass immediately before it) exceeds 30 seconds, the formation of a worked structure that promotes recrystallization becomes insufficient, and the desired ferrite fraction cannot be obtained in the final surface structure.Therefore, the inter-rolling pass time is set to 30 seconds or less, preferably 25 seconds or less, and more preferably 15 seconds or less. The lower limit of the inter-rolling pass time is not particularly limited, but can be, for example, 5 seconds or more.

[0056] (3) Cooling process The hot-rolled steel sheet after the hot rolling can be cooled. In the cooling step (accelerated cooling), the average cooling rate in the temperature range of 700°C to 500°C is set to 3°C / s or more and 100°C / s or less, and the cooling stop temperature is set to 300°C or less. By cooling under these conditions, the hot-rolled steel sheet is well quenched, and the desired bainite structure is obtained in the center of the steel sheet thickness, and the desired ferrite structure is easily obtained in the surface layer of the steel sheet (0.5 mm below the surface of the steel sheet) after tempering.

[0057] In the cooling process, if the average cooling rate in the temperature range from 700°C to 500°C is less than 3°C / s, it is difficult to obtain the desired transformed structure and sufficient strength. On the other hand, if the average cooling rate in this temperature range is higher than 100°C / s, it becomes difficult to control the temperature at each position in the steel plate, and material properties tend to vary in the plate width direction and rolling direction. As a result, material properties such as tensile properties and low-temperature toughness tend to vary. Therefore, the average cooling rate in the temperature range is set to be 3°C / s or more and 100°C / s or less.

[0058] Furthermore, if the cooling stop temperature in the cooling step is higher than 300°C, there is a risk of the strength decreasing. Therefore, the cooling stop temperature is set to 300°C or lower. There is no lower limit to the cooling stop temperature, but as an example, it is 50°C or higher.

[0059] The cooling (accelerated cooling) in the cooling step is not particularly limited and can be performed by any method. For example, air cooling and / or water cooling can be used. As for water cooling, any cooling method using water (e.g., spray cooling, mist cooling, laminar cooling, etc.) can be used.

[0060] (4) Tempering process Next, the hot-rolled steel sheet after the cooling process can be subjected to a tempering process. In the tempering process, tempering is performed under conditions where the heating temperature (tempering temperature) is 650°C or higher and the Ac1 point or lower, and the tempering parameter λ in the following formula (1) is 18.7 or higher. If the tempering temperature is lower than 650°C, the desired ferrite structure cannot be obtained in the surface layer (0.5 mm below the surface of the steel sheet), and the surface layer hardness increases. Furthermore, if the tempering temperature exceeds the Ac1 point, island martensite is formed in the center of the sheet thickness after tempering, and low-temperature toughness decreases. The Ac1 point (Ac1 transformation point) can be calculated using the following formula (3). Ac1 point (℃) = 750.8 - 26.6 × C + 17.6 × Si - 11.6 × Mn - 22.9 × Cu - 23 × Ni + 24.1 × Cr + 22.5 × Mo - 39.7 × V - 5.7 × Ti + 232.4 × Nb - 169.4 × Al (3) Here, the element symbols in formula (3) represent the content (mass%) of each element in the steel, and if the element is not contained, the content in the steel is set to 0.

[0061] (λ×1000)=(T+273)×(log(t)+20) ···(1) In the formula (1), T is the heating temperature (° C.) in the tempering process, and t is the holding time (hours) at the heating temperature T in the tempering process. If λ is less than 18.7, the desired ferrite structure will not form in the surface layer of the steel sheet, and the desired surface hardness will not be obtained. Therefore, tempering is performed under conditions where λ is 18.7 or more. In the tempering process, tempering is preferably performed under conditions where λ is 19.0 or more. On the other hand, although there is no particular upper limit for λ, tempering under conditions where λ exceeds 19.5 will deteriorate production efficiency. Therefore, from the viewpoint of production efficiency, tempering is preferably performed under conditions where λ is 19.5 or less, and more preferably under conditions where λ is 19.3 or less.

[0062] Any heating method can be used in the tempering process as long as it can control the heating temperature as described above. One example of the heating method is furnace heating. The furnace heating method is not particularly limited, and a general heat treatment furnace can be used. That is, according to the present invention, a steel sheet having high strength and excellent low-temperature toughness and reduced surface hardness can be obtained more easily without using an induction heating device and without necessarily adding alloy elements such as Cu, Cr, Sb, and Sn. [Example]

[0063] The present invention will be further described below based on examples, but the present invention is not limited to the following examples.

[0064] Steel sheets were manufactured according to the procedure described below, and their properties were evaluated.

[0065] First, molten steel having the chemical composition shown in Table 1 was melted in a converter, and a steel slab (thickness: 250 mm) was produced as a steel material by continuous casting. The Ar3 point (°C) calculated using the above-mentioned formula (2) and the Ac1 point (°C) calculated using the formula (3) are also shown in Table 1. Note that blank cells in Table 1 indicate that the element was not intentionally added, and include not only cases where the element was not contained (0 mass%), but also cases where the element was unavoidably contained.

[0066] [Table 1]

[0067] Next, the obtained steel slab was heated according to the conditions shown in Table 2 (steel material heating step). Next, the steel slab was hot rolled to obtain a hot-rolled steel sheet having each thickness (final thickness) (hot rolling step). Next, according to the conditions shown in Table 2, the obtained hot-rolled steel sheet was subjected to cooling (accelerated cooling) (cooling step). Next, the hot-rolled steel sheet after the cooling step was subjected to tempering (tempering step). In all examples, steel sheets having various thicknesses ranging from 12 mm to 50 mm were obtained. Note that a heat treatment furnace was used for heating in each of the above steps.

[0068] Next, for each of the obtained steel sheets, the structure, tensile strength (TS), surface hardness and Charpy absorbed energy (vE -50℃ ) was evaluated according to the following method.

[0069] [Organization] Test specimens for microstructural observation were taken from each steel sheet, embedded in resin so that the cross section perpendicular to the rolling direction served as the observation surface, and mirror-polished. Next, they were subjected to nital etching, and then observed with an optical microscope at 200x magnification, and images of the microstructure were taken. The resulting images were analyzed to identify each microstructure. Furthermore, after mirror polishing and chemical polishing at a position 0.5 mm below the steel sheet surface, EBSD measurements were performed in a 500 μm × 500 μm area with a step size of 0.2 μm to determine the KAM value and the area ratio of areas with a KAM value ≦1°. The results are also shown in Table 2.

[0070] The area ratio of regions with a KAM value ≦1° was calculated as follows. The KAM value of the EBSD measurement data was analyzed using the analysis software OIM Analysis, and the area ratio of regions with a KAM value ≦1° was calculated. Here, the KAM value indicates the average misorientation between six adjacent pixels at each measurement point (regular hexagonal pixels). Because the KAM value corresponds to the dislocation density of geometrically necessary dislocations (GN dislocations), it is said to have a quantitative correspondence with the plastic strain caused by dislocation motion.

[0071] (tensile strength) A JIS No. 4 tensile test specimen was taken from the center of the steel plate thickness so that the longitudinal direction of the test specimen coincided with the plate width direction. Using this tensile test specimen, a tensile test was carried out in accordance with the provisions of JIS Z 2241:2022 to evaluate the tensile strength (TS) of the steel plate. A TS of 610 MPa or more was evaluated as good. The results are shown in Table 2.

[0072] (low temperature toughness) V-notch test pieces were taken from the center of the steel plate thickness in accordance with the provisions of JIS Z 2242:2023 so that the longitudinal direction of the test piece coincided with the plate width direction. Using these V-notch test pieces, Charpy impact tests were carried out on three pieces of each steel plate at a test temperature of -50°C in accordance with the provisions of JIS Z 2242:2023, and the average value of the three pieces was taken as the Charpy absorbed energy (vE -50℃ ) was evaluated as vE -50℃ A value of 47 J or more was evaluated as good. The results are also shown in Table 2.

[0073] (Surface hardness) The Vickers hardness was measured at five points 0.5 mm below the surface of the steel sheet with a load of 10 kg, and the average of the five measurements was evaluated. A Vickers hardness of 210 HV10 or less 0.5 mm below the surface was evaluated as good. The results are also shown in Table 2.

[0074] [Table 2]

[0075] The steel sheets Nos. 1 to 4, 11 to 30, and 43 to 45, which are examples of the invention, have a ferrite area fraction of 50% or more and an area fraction of regions with a KAM value of 1° or less in a structure 0.5 mm below the surface of the steel sheet. Furthermore, the area fraction of bainite in the structure at the center of the sheet thickness is 60% or more and an area fraction of island martensite is less than 1%. It was confirmed that the steel sheets of the examples of the invention ensure high strength and excellent low-temperature toughness, and furthermore, reduced surface layer hardness. On the other hand, the steel sheets Nos. 5 to 10, 31 to 42, and 46 are comparative examples whose structure or chemical composition is outside the range of the present invention, and failed to satisfy one or more of the target performances described above. [Industrial Applicability]

[0076] According to the present invention, a steel sheet can be produced more simply, specifically, using general production equipment without using a production process that uses an induction heating device, and it is possible to obtain a steel sheet that has high strength and low-temperature toughness and has reduced surface hardness without necessarily requiring the addition of alloy elements such as Cu, Cr, Sb, and Sn.

Claims

1. In mass%, C: 0.01-0.15%, Si: 0.01 to 0.50%, Mn: 0.50-3.00%, P: 0.03% or less, S: 0.005% or less, N: 0.0010 to 0.0080%, and Al: 0.008-0.10% and the balance being Fe and unavoidable impurities, The organization At a position 0.5 mm below the surface of the steel sheet, the area ratio of ferrite is 50% or more and the area ratio of the region where the KAM value is ≦1° is 50% or more, A steel plate having an area ratio of bainite of 60% or more and an area ratio of island martensite of less than 1% at the center of the plate thickness.

2. The steel sheet according to claim 1, wherein the chemical composition further contains, in mass %, one or two of the following groups A and B: Group A: Ni: 3.0% or less, Cr: 1.00% or less, Mo: 0.50% or less, Cu: 1.00% or less, Nb: 0.05% or less, V: 0.10% or less, Ti: 0.03% or less, B: 0.0030% or less, and Sn: 0.50% or less One or more selected from the following: Group B: Ca: 0.005% or less, REM: 0.010% or less, and Mg: 0.005% or less One or more selected from the following:

3. a heating step of heating a steel material having the component composition according to claim 1 or 2; Next, a hot rolling step of hot rolling the steel material to form a hot-rolled steel sheet; Next, a cooling step of cooling the hot-rolled steel sheet; Then, a tempering process is provided in which the hot-rolled steel sheet after the cooling process is tempered. In the hot rolling step, the temperature of the steel sheet surface is such that the rolling end temperature is Ar3 point + 30°C or more, and the inter-pass time in rolling passes is from the rolling end temperature to the rolling end temperature + 100°C or less is 30 seconds or less, In the cooling step, at a temperature at the center of the thickness of the hot-rolled steel sheet, an average cooling rate in a temperature range of 700°C to 500°C is set to be 3°C / s or more and 100°C / s or less, and a cooling stop temperature is set to be 300°C or less, In the tempering step, the steel sheet is tempered under conditions where the heating temperature is 650°C or higher and the Ac1 point or lower, and the tempering parameter λ in the following formula (1) is 18.7 or higher. (λ×1000)=(T+273)×(log(t)+20)...(1) In the formula (1), T is the heating temperature (° C.) in the tempering process, and t is the holding time (hours) at the heating temperature T.

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