Steel Plate and its Manufacturing Method

Abstract

The object of this invention is to provide a steel plate with excellent ductility and expandability, and a yield strength (YP) of 450 MPa or higher, and a method for manufacturing the same. A steel plate comprising, by mass%, C: 0.05% or more and 0.13% or less, Si: 0.01% or more and 0.04% or less, Mn: 0.1% or more and 0.6% or less, P: 0.02% or less, S: 0.02% or less, Al: 0.01% or more and 0.10% or less, N: 0.0005% or more and 0.0040% or less, Ti: 0.005% or more and 0.030% or less. The composition consists of Nb: 0.005% or more and 0.030% or less, Mo: 0.01% or more and 0.05% or less, B: 0.0005% or more and 0.0050% or less, with the balance being iron and unavoidable impurities. The microstructure is ferrite with an area ratio of 90% or more. The total amount of Ti and Nb, in the form of carbonitrides, is 0.005% or more by mass.

Description

Technical Field

[0001] This invention relates to a high-strength steel plate for tank applications with excellent expandability, a yield stress (YP) of 450 MPa or more, an elongation of 20% or more, and a method for manufacturing the same. Background Technology

[0002] In recent years, efforts have been made to reduce the cost of steel plates for tanks, with a focus on thinning steel plates using high-strength materials. Specifically, research is underway to apply high-strength thin steel plates with a YP of 450 MPa or higher to tank applications. When such high strength is required, DR (double-rolled) materials, which undergo a second cold rolling process after annealing, are sometimes used instead of SR (single-rolled) materials, which were previously used for tank steel plates. However, while a second cold rolling process achieves high strength, it reduces ductility, making it unsuitable for three-piece irregularly shaped tanks requiring expansion. Furthermore, the cost is higher than that of SR materials due to the two rolling processes.

[0003] To address such requirements, for example, Patent Document 1 discloses a can steel plate with high strength and excellent workability, which contains, by mass%, C: 0.085% or more and 0.130% or less, Si: 0.04% or less, Mn: 0.10% or more and 0.60% or less, P: 0.02% or less, S: greater than 0.010% and 0.020% or less, Al: 0.02% or more and 0.10% or less, N: 0.0005% or more and 0.0040% or less, and Nb: 0.007%. The composition consists of the following components: B: 0.0010% to 0.0050%; B / N ratio (B / N) of 0.80 or more; and the balance being Fe and unavoidable impurities. It also has a ferrite structure containing 1.0% or more pearlite in terms of area fraction, a yield stress of 500 MPa or more, a tensile strength of 550 MPa or more, a uniform elongation of 10% or more, and a yield elongation of 5.0% or less.

[0004] Patent document 2 discloses a can steel plate with low yield elongation, high ductility, and high strength. In this steel, the composition (by mass%) contains one or more of the following: C: 0.03-0.13%, Si: less than 0.03%, Mn: 0.3-0.6%, P: less than 0.02%, Al: less than 0.1%, N: less than 0.012%, Nb: 0.005-0.05%, Ti: 0.005-0.05%, and B: 0.0005-0.005%. After hot rolling, the steel is cooled at a cooling rate of less than 40°C / s and coiled at a temperature of 550°C or higher. This promotes the precipitation of cementite after cold rolling and recrystallization annealing, resulting in a yield elongation of less than 5%.

[0005] Existing technical documents

[0006] Patent documents

[0007] Patent Document 1: International Publication No. 2020 / 105406

[0008] Patent Document 2: Japanese Patent Application Publication No. 2008-274332 Summary of the Invention

[0009] The problem that the invention aims to solve

[0010] However, in Patent Document 1, the lower limit of elongation is 10%. At such a level of ductility, the ductility is insufficient, making it difficult to use for irregularly shaped cans that require expansion.

[0011] Furthermore, although strength and elongation are mentioned in Patent Document 2, no evaluation of tank expansion capability is conducted, and the quality of tank expansion capability is unclear.

[0012] In view of the problems of the prior art, the object of the present invention is to provide a steel plate with excellent ductility and expandability and a yield strength (YP) of 450 MPa or more, and a method for manufacturing the same.

[0013] In this invention, excellent ductility refers to an elongation (EL) of 20% or more.

[0014] Methods for solving problems

[0015] To address the aforementioned problems, the inventors conducted in-depth research. The results showed that by adjusting the steel composition, the ferrite in the metal microstructure, and the amount of Nb and Ti precipitated in the ferrite as carbonitrides, high-strength steel plates with excellent ductility and can-expanding properties, and a YP value of 450 MPa or higher, can be obtained. By controlling the amount of Ti and Nb precipitated as carbonitrides, the strength of ferrite can be increased, and the strength difference between ferrite and other microstructures can be reduced. Therefore, crack initiation can be suppressed during can-expanding, resulting in high-strength steel plates most suitable for, for example, the can body of a three-piece irregularly shaped can undergoing can-expanding. Furthermore, it was found that, as manufacturing conditions, the amount of Ti and Nb precipitated as carbonitrides can be adjusted by strictly controlling the coiling temperature of the hot rolling process and the heating rate of the annealing process.

[0016] The present invention was made to solve the above-mentioned problems, and its main purpose is as follows.

[0017] [1] A steel plate having, by mass percent, the following components: C: 0.05% or more and 0.13% or less, Si: 0.01% or more and 0.04% or less, Mn: 0.1% or more and 0.6% or less, P: 0.02% or less, S: 0.02% or less, Al: 0.01% or more and 0.10% or less, N: 0.0005% or more and 0.0040% or less, Ti: 0.005% or more and 0.030% or less. The composition consists of iron and unavoidable impurities, with Nb content of 0.005% to 0.030%, Mo content of 0.01% to 0.05%, B content of 0.0005% to 0.0050%, and the balance being ferrite and unavoidable impurities. It has a ferrite microstructure with an area ratio of 90% or more, and the total amount of Ti and Nb in the form of carbonitrides is 0.005% or more.

[0018] [2] The steel plate according to [1], wherein, based on the composition, it further contains, by mass%, one or two of the following: Cr: 0.005% or more and 0.100% or less, Ni: 0.005% or more and 0.150% or less.

[0019] [3] A method for manufacturing a steel plate, comprising: The hot rolling process, wherein the steel billet having the composition described in [1] or [2] above is hot rolled at a finishing temperature of 800°C or higher and 950°C or lower and a coiling temperature of 550°C or higher and 750°C or lower. The cold rolling process involves cold rolling of the hot-rolled sheet after the hot rolling process with a reduction rate of 85% or more. The annealing process includes heating the cold-rolled sheet after the cold rolling process to an annealing temperature of 640°C or higher and 760°C or lower at an average heating rate of 5 to 40°C / second, holding it at this annealing temperature, and then cooling it to a cooling stop temperature of 450°C or higher and 650°C or lower at an average cooling rate of 15°C / second or higher; and The leveling rolling process, wherein the leveling rolling rate is 0.5% or more and 5.0% or less for the annealed plate after the annealing process.

[0020] Invention Effects

[0021] According to the present invention, steel plates with a yield strength of over 450 MPa, excellent can-expanding properties, and good ductility can be provided. Due to its excellent ductility, the steel plate of the present invention is suitable for applications involving cans with complex shapes, such as three-piece irregularly shaped cans undergoing can-expanding processing. Furthermore, by applying components manufactured according to the present invention to cans, higher strength and lighter weight are further achieved, greatly contributing to industrial development. Detailed Implementation

[0022] The following describes the suitable range of the composition and structure of the steel plate of the present invention and the reasons for limiting them. It should be noted that, unless otherwise specified, the "%" indicating the composition refers to "mass %". Additionally, the case of excellent ductility and expandability is sometimes simply referred to as excellent processability.

[0023] C: Above 0.05% and below 0.13%

[0024] Carbon (C) is an element that contributes to strength. It dissolves in steel or precipitates as carbides, thereby increasing the strength of the steel. To obtain the desired yield strength by utilizing these effects, a content of 0.05% or more is required. Therefore, the C content is set to 0.05% or more. Preferably, the C content is 0.06% or more, more preferably 0.08% or more. On the other hand, when the content exceeds 0.13%, the increased yield strength may sometimes lead to decreased ductility and impaired weldability. Therefore, the C content is set to 0.13% or less. Preferably, it is 0.12% or less, more preferably 0.11% or less. Even more preferably, it is 0.10% or less.

[0025] Si: ≥0.01% and ≤0.04%

[0026] Si contributes to the high strength of steel through solid solution strengthening. To obtain the desired yield strength by utilizing these effects, a Si content of 0.01% or more is required. Therefore, the Si content is set to 0.01% or more. On the other hand, a content exceeding 0.04% may cause significant problems in terms of corrosion resistance and surface properties. Therefore, the Si content is set to 0.04% or less. Preferably, it is 0.03% or less, and more preferably 0.02% or less.

[0027] Mn: ≥0.1% and ≤0.6%

[0028] Mn is an element that contributes to strength, and its solid solution in steel contributes to high strength. To obtain the strength desired by this invention, it needs to contain 0.1% or more. Therefore, the Mn content is set to 0.1% or more. Preferably, it is 0.2% or more, more preferably 0.3% or more, and even more preferably 0.4% or more. On the other hand, a content exceeding 0.6% leads to a decrease in ductility. Therefore, the Mn content is set to 0.6% or less.

[0029] P: below 0.02%

[0030] Phosphorus (P) is an element that inevitably mixes into steel and is effective in strengthening it; therefore, it is preferable to contain 0.001% or more. More preferably, the P content is 0.002% or more, and even more preferably 0.004% or more. On the other hand, P reduces weldability, so the P content is set to 0.02% or less. Preferably, it is 0.018% or less. More preferably, it is 0.017% or less, and even more preferably 0.016% or less.

[0031] S: below 0.02%

[0032] Sulfur (S) is an element that inevitably mixes into steel, forming coarse inclusions such as MnS, which significantly reduces local ductility. Therefore, the S content is set to 0.02% or less, preferably 0.018% or less. It should be noted that refining the steel to achieve an S content below 0.001% would incur excessive costs. Therefore, the S content is preferably set to 0.001% or more. More preferably, it is set to 0.003% or more, and even more preferably, it is set to 0.005% or more.

[0033] Al: Above 0.01% and below 0.10%

[0034] Al acts as a deoxidizer, and to achieve this effect, it needs to contain 0.01% or more. Therefore, the Al content is set to 0.01% or more, preferably 0.03% or more. On the other hand, adding large amounts, especially more than 0.10% Al, would make manufacturing costs too high. Therefore, the Al content is set to 0.10% or less, preferably 0.08% or less, and more preferably 0.07% or less.

[0035] N: Above 0.0005% and below 0.0040%

[0036] Nitrogen (N) combines with carbonitride-forming elements such as Al, Nb, and Ti to form precipitates, which contributes to increased strength. To achieve this effect, the N content needs to be 0.0005% or more. Preferably, the N content is 0.0006% or more. More preferably, it is 0.0007% or more, and even more preferably, it is 0.0010% or more. On the other hand, if N exceeds 0.0040% and is present in large quantities, it will reduce aging resistance. Therefore, the N content is set to 0.0040% or less. Preferably, it is 0.0035% or less. More preferably, it is 0.0034% or less, even more preferably, it is 0.0033% or less, and most preferably, it is 0.0032% or less.

[0037] Ti: ≥0.005% and ≤0.030%

[0038] Ti combines with C and N to form carbonitrides, which contributes to increased strength, and is therefore one of the important additive elements in this invention. Furthermore, it can suppress the formation of BN and enhance the strength-enhancing effect caused by B grain boundary segregation. To achieve this effect, it needs to contain 0.005% or more. Therefore, the Ti content is set to 0.005% or more. Preferably, the Ti content is 0.010% or more. More preferably, the Ti content is 0.011% or more, and even more preferably, 0.012% or more. On the other hand, when Ti exceeds 0.030%, it leads to a decrease in ductility. Therefore, the Ti content is set to 0.030% or less. Preferably, it is 0.025% or less. More preferably, it is 0.024% or less, even more preferably, 0.023% or less, and most preferably, 0.022% or less.

[0039] Nb: Above 0.005% and below 0.030%

[0040] Nb is one of the important additive elements in this invention, which combines with C and N to form carbonitrides, thereby contributing to increased strength. To achieve this effect, it needs to contain 0.005% or more. Therefore, the Nb content is set to 0.005% or more. Preferably, it is 0.010% or more. More preferably, it is 0.011% or more, and even more preferably, it is 0.012% or more. On the other hand, if Nb exceeds 0.030% and is present in large quantities, it will lead to a decrease in ductility. Therefore, the Nb content is set to 0.030% or less. Preferably, it is 0.025% or less. More preferably, it is 0.024% or less, and even more preferably, it is 0.023% or less.

[0041] Mo: ≥0.01% and ≤0.05%

[0042] Mo, either dissolved in steel or precipitated as carbides, contributes to increased strength. To achieve this effect, a content of 0.01% or more is required. Therefore, the Mo content is set to 0.01% or more, preferably 0.02% or more, and more preferably 0.021% or more. On the other hand, Mo is a very expensive element, and excessive content leads to increased costs. Therefore, the Mo content is set to 0.05% or less, more preferably 0.047% or less, and even more preferably 0.045% or less.

[0043] B: Above 0.0005% and below 0.0050%

[0044] Boron (B) contributes to increased strength through segregation at grain boundaries. To achieve this effect, a content of 0.0005% or more is required. Therefore, the B content is set to 0.0005% or more, preferably 0.0010% or more, more preferably 0.0012% or more, further preferably 0.0013% or more, and most preferably 0.0014% or more. On the other hand, even if B is present in large quantities exceeding 0.0050%, its effect becomes saturated. Therefore, the B content is set to 0.0050% or less, preferably 0.0040% or less, more preferably 0.0038% or less, further preferably 0.0037% or less, and most preferably 0.0036% or less.

[0045] With the above-mentioned essential elements, the steel plate of the present invention can achieve the target characteristics, but in addition to the above-mentioned essential elements, the following elements may also be included as needed.

[0046] Selected from one or two of the following: Cr: ≥0.005% and ≤0.100%; Ni: ≥0.005% and ≤0.150%.

[0047] Cr and Ni improve hardenability, thus they are useful as strengthening elements in steel. To effectively utilize this effect, it is preferable that Cr and Ni each contain 0.005% or more. Therefore, when Cr and Ni are present, the Cr content is set to 0.005% or more, and the Ni content is set to 0.005% or more. Preferably, the Cr content is 0.010% or more, and the Ni content is 0.010% or more. More preferably, the Cr content is 0.011% or more, and the Ni content is 0.011% or more, and even more preferably, the Cr content is 0.012% or more, and the Ni content is 0.012% or more. On the other hand, Cr and Ni are expensive elements, and exceeding their respective upper limits does not yield further improvements in effect. Therefore, when Cr and Ni are present, the Cr content is set to 0.100% or less, and the Ni content is set to 0.150% or less. Preferably, the Cr content is 0.090% or less, and the Ni content is 0.130% or less, and more preferably, the Cr content is 0.085% or less, and the Ni content is 0.100% or less.

[0048] According to one embodiment of the present invention, a steel plate has a composition containing the above-described components, with the balance being Fe and unavoidable impurities. Unavoidable impurities refer to impurities that are unavoidably introduced from raw materials, manufacturing processes, or manufacturing equipment, and are permitted to be present to a extent that does not hinder the purpose of the present invention. Examples of raw materials include iron ore, reduced iron, or scrap steel. Examples of unavoidable impurities include Ca, O, H, V, Sn, Co, W, Zn, Pb, As, Sb, and Bi.

[0049] Next, the microstructure, an important condition of the steel sheet of the present invention, will be described. It should be noted that the area ratios below refer to the area ratios relative to the overall microstructure of the steel sheet.

[0050] Ferrite: 90% or more by area ratio

[0051] Ferrite forms during cooling after annealing, which helps improve the ductility of the steel. When the ferrite content, by area fraction, is less than 90%, it is difficult to ensure the desired ductility. Therefore, the ferrite content, by area fraction, is set to 90% or more. Preferably, it is 91% or more, more preferably 95% or more. Further preferably, it is 96% or more. There is no particular upper limit, but the ferrite content, by area fraction, is preferably 99% or less, more preferably 98% or less.

[0052] The total amount of Ti and Nb precipitated in the form of carbonitrides: ≥0.005% by mass.

[0053] Ti and Nb, by precipitating in ferrite as carbonitrides, can improve the strength of ferrite and reduce the strength difference between ferrite and other phases, thereby suppressing crack initiation and achieving excellent expansion properties during can-expansion processing. When the total amount of Ti and Nb precipitated as carbonitrides is less than 0.005%, the ferrite is not strengthened, thus increasing the strength difference between ferrite and other phases, resulting in poor expansion properties. Furthermore, it is difficult to ensure the desired yield strength. Therefore, the total amount of Ti and Nb precipitated as carbonitrides in the total Ti and Nb content is set to 0.005% or more. The total amount of Ti and Nb precipitated as carbonitrides is preferably 0.010% or more, more preferably 0.015% or more, further preferably 0.017% or more, and most preferably 0.020% or more. There is no particular upper limit, but it is preferably set to 0.040% or less. More preferably, it is 0.039% or less; even more preferably, it is 0.038% or less; and most preferably, it is 0.037% or less. In addition to composite carbonitrides, the carbonitrides in this invention also include carbides and nitrides, and all can achieve the desired effect regardless of their form.

[0054] It should be noted that the amount of Ti and Nb obtained by subtracting the amount of Ti and Nb in the form of carbonitrides from the total amount of Ti and Nb in the steel plate is the amount of Ti and Nb in solid solution.

[0055] It should be noted that there is no particular limitation on the amount of material other than ferrite in the above-described metallic microstructure. For example, the amount may include retained austenite, cementite, pearlite, bainite, martensite, etc. The amount is preferably 5% or less. More preferably, it is 3% or less. The lower limit can be 0%.

[0056] Furthermore, the thickness of the steel plate of the present invention is not particularly limited, but it is preferably 0.10 mm or more. More preferably, it is 0.12 mm or more, and even more preferably, it is 0.13 mm or more. In addition, the thickness is preferably 0.60 mm or less. More preferably, it is 0.50 mm or less, and even more preferably, it is 0.40 mm or less.

[0057] The method for manufacturing the steel plate of the present invention is characterized in that, for a steel billet having the above-mentioned composition, hot rolling is performed (hot rolling process) under the conditions of finishing rolling temperature of 800°C or higher and 950°C or lower, and coiling temperature of 550°C or higher and 750°C or lower; then cold rolling is performed (cold rolling process) with a reduction rate of 85% or higher; the billet is heated to the annealing temperature at an average heating rate of 5 to 40°C / second; after holding at the annealing temperature of 640°C or higher and 760°C or lower, the billet is cooled to a temperature range of 450°C or higher and 650°C or lower than the above-mentioned annealing temperature at an average cooling rate of 15°C / second or higher (annealing process); and leveling rolling is performed (leveling rolling process) with a leveling rolling rate of 0.5% or higher and 5.0% or lower.

[0058] Hot rolling process

[0059] Finishing rolling temperature: above 800℃ and below 950℃

[0060] When the finishing temperature of hot rolling exceeds 950°C, the ferrite structure after hot rolling becomes coarse, and therefore the ferrite structure after annealing also becomes coarse, resulting in a decrease in yield strength. Therefore, the finishing temperature is set to 950°C or below. Preferably, the finishing temperature is 930°C or below, more preferably 900°C or below. Even more preferably, it is 890°C or below. Furthermore, when the finishing temperature is below 800°C, rolling occurs in the dual-phase region of ferrite and austenite, resulting in coarse grains on the surface of the steel plate, thus reducing yield strength. Therefore, the finishing temperature is set to 800°C or above. Preferably, it is 810°C or above, more preferably 850°C or above. Even more preferably, it is 855°C or above. It should be noted that the finishing temperature in this invention refers to the finishing temperature at the end of the finishing process.

[0061] Winding temperature: 550℃ or higher and 750℃ or lower

[0062] In this invention, by controlling the winding temperature and the heating rate up to the annealing temperature, a desired amount of Nb and Ti-containing carbonitrides is obtained, exhibiting excellent can-expanding properties. To achieve this effect, it is important to control the winding temperature to 550°C or higher and 750°C or lower. When the winding temperature exceeds 750°C, sufficient carbonitrides cannot be generated during winding, making it difficult to ensure can-expanding properties. Furthermore, oxide scale forms on the surface, easily leading to surface defects. Therefore, the winding temperature is set to 750°C or lower, preferably 720°C or lower, more preferably 710°C or lower, and even more preferably 700°C or lower. When the winding temperature is lower than 550°C, sufficient carbonitrides cannot be generated during winding, making it difficult to ensure can-expanding properties. Therefore, the winding temperature is set to 550°C or higher, preferably 600°C or higher, more preferably 610°C or higher, and even more preferably 620°C or higher. After the above process, a hot-rolled sheet is obtained.

[0063] cold rolling process

[0064] Reduction rate in cold rolling: 85% or more

[0065] By controlling the reduction rate during cold rolling, the grains become finer during annealing, which in turn refines the martensite formed during cooling after annealing. To achieve this effect, the reduction rate needs to be set to 85% or more. Preferably, the reduction rate is 87% or more. More preferably, it is 88% or more, and even more preferably, it is 89% or more. On the other hand, when the reduction rate is greater than 95%, the rolling load increases significantly, increasing the load on the rolling mill. Therefore, the reduction rate is preferably 95% or less. More preferably, it is 94% or less, and even more preferably, it is 93% or less. After the above process, a cold-rolled sheet is obtained.

[0066] Annealing process

[0067] Average heating rate up to annealing temperature: 5–40 °C / second

[0068] In this invention, a desired amount of Ti and Nb-containing carbonitrides is obtained by controlling the coiling temperature and the heating rate up to the annealing temperature, exhibiting excellent can-expanding properties. To achieve this effect, it is important to control the average heating rate up to the annealing temperature to be 5–40 °C / second. When the average heating rate is less than 5 °C / second, the carbonitrides obtained during hot rolling dissolve during heating, and a sufficient amount of carbonitride cannot be ensured after annealing. Therefore, the ferrite is not strengthened, the strength difference between ferrite and other phases increases, and the can-expanding properties deteriorate. Furthermore, it is difficult to ensure the desired yield strength. Therefore, the average heating rate is set to 5 °C / second or more. Preferably, it is 6 °C / second or more, more preferably 7 °C / second or more, further preferably 8 °C / second or more, and most preferably 10 °C / second or more. Additionally, when the average heating rate is greater than 40 °C / second, sufficient recrystallization of ferrite does not occur, resulting in excessive residual unrecrystallized grains, thus increasing strength and decreasing ductility. Therefore, the average heating rate up to the annealing temperature is set to 40°C / second or less. Preferably, it is 38°C / second or less, more preferably 37°C / second or less, and even more preferably 36°C / second or less.

[0069] Annealing temperature: above 640℃ and below 760℃

[0070] When the annealing temperature is below 640°C, sufficient recrystallization of ferrite does not occur, resulting in an excessive amount of unrecrystallized grains remaining, thus increasing strength and decreasing ductility. Therefore, the annealing temperature is set to 640°C or higher. Preferably, it is 660°C or higher. More preferably, it is 665°C or higher, and even more preferably, it is 670°C or higher. On the other hand, when the annealing temperature exceeds 760°C, the carbonitrides containing Nb and Ti generated during hot rolling dissolve, thus the desired amount of carbonitrides cannot be obtained after annealing. Therefore, the annealing temperature is set to 760°C or lower. Preferably, it is 740°C or lower. More preferably, it is 735°C or lower, and even more preferably, it is 730°C or lower.

[0071] Average cooling rate: 15℃ / second or higher

[0072] When the average cooling rate from the annealing temperature to the cooling stop temperature is less than 15°C / second, the desired amount of carbonitrides containing Ti and Nb cannot be obtained, and the expansion properties deteriorate. Therefore, the cooling rate is set to 15°C / second or higher. Preferably, it is set to 20°C / second or higher. More preferably, it is set to 23°C / second or higher, even more preferably, it is set to 24°C / second or higher, and most preferably, it is set to 25°C / second or higher. It should be noted that, regarding this cooling, in addition to gas cooling, one or more of the following methods can be used in combination: mist cooling, roller cooling, and water cooling. There is no particular upper limit, but in order to maintain a good shape of the cooled steel sheet, it is preferably set to 150°C / second or lower. More preferably, it is set to 147°C / second or lower, even more preferably, it is set to 146°C / second or lower, and most preferably, it is set to 145°C / second or lower.

[0073] Cooling stop temperature: below the above annealing temperature and above 450°C but below 650°C

[0074] By setting the cooling stop temperature after annealing to be lower than the aforementioned annealing temperature and between 450°C and 650°C, a desired amount of Ti and Nb-containing carbonitrides can be obtained. When the cooling stop temperature exceeds 650°C, the desired amount of Ti and Nb-containing carbonitrides cannot be obtained, and the ductility deteriorates. Therefore, the cooling stop temperature is set to 650°C or lower. Preferably, it is 600°C or lower. More preferably, it is 595°C or lower, and even more preferably, it is 590°C or lower. On the other hand, when the cooling stop temperature is set below 450°C, the desired amount of ferrite cannot be obtained, and the ductility decreases. Therefore, the cooling stop temperature after annealing is set to 450°C or higher. Preferably, it is 500°C or higher. More preferably, it is 505°C or higher, and even more preferably, it is 510°C or higher. After the above process, an annealed plate is obtained. It should be noted that cooling from the annealing temperature to the cooling stop temperature is preferably performed at least 40°C. That is, the annealing temperature is preferably 40°C or more higher than the cooling stop temperature, and preferably the annealing temperature is cooled to the cooling stop temperature by cooling 40°C or more from the annealing temperature.

[0075] Leveling rolling ratio: ≥0.5% and ≤5.0%

[0076] By performing leveling rolling, surface roughness can be adjusted, shape corrected, and yield strength can be increased by introducing strain into the annealed sheet. To achieve this effect, the leveling rolling rate needs to be set to 0.5% or more. Therefore, the leveling rolling rate is set to 0.5% or more. Preferably, the leveling rolling rate is set to 0.6% or more, more preferably 0.7% or more, even more preferably 0.8% or more, and most preferably 0.9% or more. On the other hand, when the leveling rolling rate is greater than 5.0%, the strain introduced into the steel sheet is excessive, and the elongation decreases. Therefore, the leveling rolling rate is set to 5.0% or less. Preferably, it is 3.0% or less. More preferably, it is 2.5% or less, even more preferably 2.3% or less, and most preferably 2.0% or less. Through the above processes, the high-strength steel sheet of the present invention is obtained.

[0077] Example

[0078] The following describes embodiments of the steel plate and its manufacturing method according to the present invention. The present invention is not limited to the embodiments shown herein.

[0079] Steel with the composition shown in Table 1 was smelted to produce thin slabs with a thickness of 20 mm. These slabs were then hot-rolled under the conditions shown in Table 2. The resulting hot-rolled sheets were then pickled with hydrochloric acid and cold-rolled at the reduction rates shown in Table 2 to produce cold-rolled sheets with a thickness of 0.2 mm.

[0080] Next, the above-mentioned cold-rolled sheet is heated, annealed, held, cooled, and leveled under the heat treatment conditions shown in Table 2 to obtain the finished steel sheet (also known as steel plate).

[0081] For the steel sheet obtained above, the microstructure and mechanical properties of the steel sheet were investigated as shown below. The results are shown in Table 2.

[0082] Steel plate structure

[0083] The area fraction of ferrite in the overall microstructure was investigated as follows: A section along the rolling direction was etched with a nitric acid-ethanol solution at half the thickness of the steel plate, and then observed using a scanning electron microscope (SEM). Observations were performed in three randomly selected fields of view. Using cross-sectional micrographs at 2000x magnification, image processing software (Photoshop, Adobe Photoshop) was used to binarize the images, and the area occupied by ferrite within an arbitrarily defined 50μm × 50μm square region was calculated. The average of the three fields of view was taken as the area fraction of ferrite. Black areas observed in blocky shapes were identified as ferrite.

[0084] The amounts of Ti and Nb precipitated in the form of carbonitrides were determined by extraction analysis. The sample was extracted electrolyzed using a 10% AA-based electrolyte. After decomposition with mixed acid, the amounts of Ti and Nb precipitated in the form of carbonitrides were determined by ICP. A 0.2 μm filter was used.

[0085] Mechanical properties

[0086] Regarding mechanical properties (yield strength YP, elongation EL), the tensile test was conducted according to JIS Z 2241, with the rolling direction as the long side (tensile direction). The test piece (JIS No. 5) described in JIS Z 2241 was used for evaluation. Regarding the yield strength (YP) in this invention, the term "upper yield point" indicates the upper yield point if present, and 0.2% of the endurance if no upper yield point is found. The tensile speed was 10 mm / min.

[0087] Expansion tank

[0088] Long strips of blank are cut from the steel sheet of the product, with the rolling direction aligned with the long side of the test piece. These blanks are then welded into a cylindrical shape to expand the can diameter by a maximum of 15%. For the portion that has undergone the 15% expansion, visual inspection is performed. Items without cracks are marked as acceptable (○), while items with one or more cracks are marked as unacceptable (×).

[0089]

[0090] It can be seen that the high-strength steel plate of the present invention has a YP of 450 MPa or more, an E1 of 20% or more, and excellent can-expanding properties. In contrast, the steel plate of the comparative example, which is outside the scope of the present invention, does not reach a satisfactory level in any one of YP, E1, and can-expanding properties, and is significantly worse than the steel plate of the present invention in one or more of the following: strength, elongation, and can-expanding properties.

Claims

1. A steel plate comprising, by mass%, C: 0.05% or more and 0.13% or less, Si: 0.01% or more and 0.04% or less, Mn: 0.1% or more and 0.6% or less, P: 0.02% or less, S: 0.02% or less, Al: 0.01% or more and 0.10% or less, N: 0.0005% or more and 0.0040% or less, Ti: 0.005% or more and 0.030% or less, Nb: 0.005% or more and 0.030% or less, Mo: 0.01% or more and 0.05% or less, B: 0.0005% or more and 0.0050% or less, with the balance being iron and unavoidable impurities. Furthermore, it possesses a ferrite microstructure with an area ratio of over 90%. Of the total Ti and Nb content, the total amount of Ti and Nb existing in the form of carbonitrides is 0.005% or more by mass%.

2. The steel plate according to claim 1, wherein, Based on the aforementioned composition, the ingredients also contain, by mass%, one or two of the following: Cr: 0.005% or more and 0.100% or less, Ni: 0.005% or more and 0.150% or less.

3. A method for manufacturing a steel plate, comprising: The hot rolling process, in which, For a steel billet having the composition described in claim 1 or 2, hot rolling is performed at a finishing rolling temperature of 800°C or higher and 950°C or lower, and a coiling temperature of 550°C or higher and 750°C or lower. The cold rolling process involves cold rolling of the hot-rolled sheet after the hot rolling process with a reduction rate of 85% or more. The annealing process includes heating the cold-rolled sheet after the cold rolling process to an annealing temperature of 640°C or higher and 760°C or lower at an average heating rate of 5 to 40°C / second, holding it at this annealing temperature, and then cooling it to a cooling stop temperature of 450°C or higher and 650°C or lower at an average cooling rate of 15°C / second or higher; and The leveling rolling process, wherein the leveling rolling rate is 0.5% or more and 5.0% or less for the annealed plate after the annealing process.

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