HOT-ROLLED AND ANNEALED FERRITIC STAINLESS STEEL SHEET AND METHOD FOR PRODUCING THE SAME

Abstract

To provide a hot-rolled and annealed ferritic stainless steel sheet having sufficient corrosion resistance and excellent punching workability, a predetermined dimensional accuracy being obtained without cracking when the steel sheet is formed into a thick-walled flange by punching work, and a method for producing the hot-rolled and annealed ferritic stainless steel sheet. The hot-rolled and annealed ferritic stainless steel sheet has a chemical composition containing, on a mass percent basis, C: 0.001% to 0.020%, Si: 0.05% to 1.00%, Mn: 0.05% to 1.00%, P: 0.04% or less, S: 0.01% or less, Al: 0.01% to 0.10%, Cr: 10.0% to 20.0%, Ni: 0.50% to 2.00%, Ti: 0.10% to 0.40%, and N: 0.001% to 0.020%, the balance being Fe and incidental impurities; and has a metal microstructure which is a single ferrite phase microstructure having an average grain size of 5 to 20 μm.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This is the U.S. National Phase application of PCT/JP2019/037430, filed Sep. 25, 2019, which claims priority to Japanese Patent Application No. 2018-200479, filed Oct. 25, 2018, the disclosures of these applications being incorporated herein by reference in their entireties for all purposes.

FIELD OF THE INVENTION

The present invention relates to a hot-rolled and annealed ferritic stainless steel sheet having excellent workability and being suitably used for flanges and so forth and to a method for producing the hot-rolled and annealed ferritic stainless steel sheet.

BACKGROUND OF THE INVENTION

In recent years, laws and regulations regarding exhaust gases from automobiles have been increasingly tightened in order to reduce the amount of emission of CO₂, which is a greenhouse effect gas. An improvement in fuel economy is effective in reducing the amount of emission of CO₂ in automotive exhaust gases. Thus, studies have been made of an increase in combustion temperature in engines.

Exhaust gases generated by engines are released to the atmosphere through exhaust system components, such as exhaust gas recirculation (EGR) systems and mufflers. These components of automotive exhaust systems are fastened with flanges in order to prevent gas leakage. Flanges used for exhaust system components need to have dimensional accuracy sufficient for fastening components.

Hitherto, such thick-wall flanges have been composed of plain carbon steel. In recent years, however, as a further improvement in the fuel economy of automobiles is required, combustion temperatures in engines and the temperatures of exhaust gases from engines have been further increased. Thus, flanges are required to have higher high-temperature strength and corrosion resistance than before. From such a background, in recent years, stainless steel, which has higher high-temperature strength and corrosion resistance than plain carbon steel, in particular, a high-strength ferritic stainless steel sheet, which has a relatively low coefficient of thermal expansion and being less likely to generate thermal stress (for example, a thick sheet composed of ASTM A240/240M-S40975 (11 mass % Cr—Ti—Ni steel)) (for example, a sheet thickness of 5 mm or more), has been increasingly used.

However, since flanges used in exhaust systems have thick walls (often 5 mm or more), there is a problem that cracking may occur during punching work in producing flanges to fail to appropriately produce flange components. A thick ferritic stainless steel sheet excellent in punching workability is strongly demanded.

In response to the demands of the market, for example, Patent Literature 1 discloses a hot-rolled ferritic stainless steel sheet containing, on a mass percent basis, C: 0.015% or less, Si: 0.01% to 0.4%, Mn: 0.01% to 0.8%, P: 0.04% or less, S: 0.01% or less, Cr: 14.0% to less than 18.0%, Ni: 0.05% to 1%, Nb: 0.3% to 0.6%, Ti: 0.05% or less, N: 0.020% or less, Al: 0.10% or less, and B: 0.0002% to 0.0020%, the balance being Fe and incidental impurities, the Nb, C, and N contents satisfying Nb/(C+N)≥16, the Charpy impact value being 10 J/cm² or more at 0° C., the thickness of the sheet being 5.0 to 9.0 mm.

Patent Literature

PTL 1: International Publication No. 2014/157576

SUMMARY OF THE INVENTION

The inventors produced experimentally a ferritic stainless steel sheet having a thickness of 10 mm and containing steel components in compliance with ASTM A240/240M-S40975 by a method disclosed in Patent Literature 1 and then produced flanges having holes with a diameter of 20 mm by punching work with a clearance of 10%. The results demonstrated that although no cracking occurred during the punching in any of the flanges, the circumference and/or center hole dimensions of each flange sometimes exceeded the tolerance of the component, which indicated that the steel sheet had not sufficient performance for the thick-wall flange.

Aspects of the present invention aim to solve the foregoing problems and provide a hot-rolled and annealed ferritic stainless steel sheet having sufficient corrosion resistance and excellent punching workability, a predetermined dimensional accuracy being obtained without cracking when the steel sheet is formed into a thick-wall flange by punching work, and a method for producing the hot-rolled and annealed ferritic stainless steel sheet.

The inventors have conducted detailed studies in order to solve the foregoing problems and have found that in order to obtain a predetermined dimensional accuracy without cracking during punching work, it is sufficient that the metal microstructure of a steel sheet is controlled to be a single ferrite phase microstructure having an average grain size of 5 to 20 μm.

Additionally, the inventors have also found that the metal microstructure can be controlled to be a ferrite single phase having an average grain size of 5 to 20 μm by subjecting a ferritic stainless steel containing appropriate composition to hot rolling and subjecting the resulting hot-rolled steel sheet to hot-rolled steel sheet annealing under appropriate conditions that lead to a single ferrite phase, specifically, holding the hot-rolled steel sheet at 600° C. or higher and lower than 750° C. for 1 minute to 24 hours.

These findings have led to the present invention. The gist of aspects of the present invention are described below.

[1] A hot-rolled and annealed ferritic stainless steel sheet has a chemical composition containing, on a mass percent basis, C: 0.001% to 0.020%, Si: 0.05% to 1.00%, Mn: 0.05% to 1.00%, P: 0.04% or less, S: 0.01% or less, Al: 0.01% to 0.10%, Cr: 10.0% to 20.0%, Ni: 0.50% to 2.00%, Ti: 0.10% to 0.40%, and N: 0.001% to 0.020%, the balance being Fe and incidental impurities; and has a metal microstructure is a single ferrite phase microstructure having an average grain size of 5 to 20 μm.

[2] The hot-rolled and annealed ferritic stainless steel sheet described in [1] further contains, on a mass percent basis, one or two or more selected from Cu: 0.01% to 1.00%, Mo: 0.01% to 2.00%, W: 0.01% to 0.20%, and Co: 0.01% to 0.20%.

[3] The hot-rolled and annealed ferritic stainless steel sheet described in [1] or [2] further contains, on a mass percent basis, one or two or more selected from V: 0.01% to 0.20%, Nb: 0.01% to 0.10%, Zr: 0.01% to 0.20%, REM: 0.001% to 0.100%, B: 0.0002% to 0.0025%, Mg: 0.0005% to 0.0030%, and Ca: 0.0003% to 0.0030%.

[4] A method for producing the hot-rolled and annealed ferritic stainless steel sheet described in any one of [1] to [3] includes performing hot-rolled steel sheet annealing in which a hot-rolled steel sheet produced through a hot-rolling step is held at 600° C. or higher and lower than 750° C. for 1 minute to 24 hours.

According to aspects of the present invention, a hot-rolled and annealed ferritic stainless steel sheet having sufficient corrosion resistance and excellent punching workability is provided.

The term “sufficient corrosion resistance” used in accordance with aspects of the present invention indicates that when a steel sheet obtained by subjecting surfaces thereof to abrasive finishing with 600-grit emery paper and then sealing end-face portions thereof is subjected to five cycles of a cyclic salt-spray test specified in JIS H 8502 (one cycle of the test consisting of salt spraying (5% by mass NaCl, 35° C., spraying for 2 hours)→drying (60° C., 4 hours, relative humidity: 40%)→wetting (50° C., 2 hours, relative humidity: ≤95%)), the percentage of rusted area of the surfaces of the steel sheet (=rusted area/total area of steel sheet ×100[%]) is 25% or less.

The punching workability is evaluated as follows: Samples measuring 100 mm×100 mm for testing are taken from a hot-rolled and annealed steel sheet. Five test specimens are produced by punching work with a crank press equipped with an upper die (punch) and a lower die (die) so as to form a hole having a diameter of 20 mm (tolerance: ±0.1 mm) in the middle portion of each of the samples, the upper die having a cylindrical edge for punching, the lower die having a hole with a diameter of 20 mm or more. The punching work is performed by selecting the hole diameter of the lower die in accordance with the thickness of the test specimens in such a manner that the clearance between the upper die and the lower die is 10%. Here, the relationships among the clearance (C) [%], the hole diameter of the die (the inside diameter of the die) (Dd) [mm], the diameter of the punch (Dp) [mm], and the sheet thickness (t) [mm] are represented by formula (1) below.

formula  (1)

The “excellent punching workability” in accordance with aspects of the present invention indicates that, with respect to the test specimens thus obtained, when the visual inspection of the appearance of each test specimen and the measurement of the diameter of the hole in the middle portion of the test specimen with a digital caliper are performed, no cracks are observed, and the hole diameter of each of the five test specimens after the punching work is in the range of 19.9 to 20.1 mm.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Embodiments of the present invention will be described below.

A hot-rolled and annealed ferritic stainless steel sheet according to aspects of the present invention has a chemical composition containing, on a mass percent basis, C: 0.001% to 0.020%, Si: 0.05% to 1.00%, Mn: 0.05% to 1.00%, P: 0.04% or less, S: 0.01% or less, Al: 0.01% to 0.10%, Cr: 10.0% to 20.0%, Ni: 0.50% to 2.00%, Ti: 0.10% to 0.40%, and N: 0.001% to 0.020%, the balance being Fe and incidental impurities, in which a metal microstructure is a single ferrite phase microstructure having an average grain size of 5 to 20 μm.

Aspects of the present invention will be described in detail below.

The inventors have produced flanges having holes with a diameter of 20 mm by punching work with a clearance of 10% using various ferritic stainless steel sheets having 10 mm in thick in conformity with ASTM A240/240M-S40975 (a chemical composition containing, on a mass percent basis, C≤0.03%, Si≤1.00%, Mn≤1.00%, P≤0.040%, S≤0.030%, Cr: 10.5% to 11.7%, Ni: 0.50% to 1.00%, N≤0.03%, and Ti: 6×(C+N) to 0.74%, the balance being Fe and incidental impurities). The results revealed that although no cracking occurred during the punching in any of the sheets, the circumference and/or center hole dimensions of each flange sometimes exceeded the tolerance of the component.

The inventors have studied in detail the reason why the levels of the dimensional accuracy in the punching work varied greatly, depending on the steel sheets, and have found that the dimensions of the component after the punching work tended to be smaller than the tolerance when the steel sheet subjected to the punching work had an average grain size of less than 5 μm and that the dimensions of the component after the punching work tended to be larger than the tolerance when the steel sheet had an average grain size of more than 20 μm. The inventors have found out that the reasons why sufficient dimensional accuracy is not stably obtained in the punching work are that an excessively small average grain size results in a small percentage of a sheared surface in the punching work due to an excessively hard steel sheet and that an excessively large average grain size results in the occurrence of large rollover or burrs in the punching work.

The inventors have conducted intensive studies on a method for producing a ferritic stainless steel sheet having a metal microstructure composed of a single ferrite phase having an average grain size of 5 to 20 μm from the viewpoints of steel composition, a hot-rolling process, and a hot-rolled steel sheet annealing process. The following method has been found to be effective: Steel composition, particularly Cr and Ni contents, are controlled to appropriate ranges. In a hot-rolling step, an austenite phase and a ferrite phase are formed, and then hot rolling is performed. Subsequently, the hot-rolled sheet is annealed in an appropriate temperature range in a single ferrite phase temperature region.

A hot-rolled steel sheet annealing step is performed by performing holding the hot-rolled steel sheet in the appropriate temperature range in the single ferrite phase temperature region, specifically at 600° C. or higher and lower than 750° C. for 1 minute to 24 hours. Regarding the ferrite phase and a martensite phase that have been present in the metal microstructure after the hot rolling, the hot-rolled steel sheet annealing brings recrystallization of the ferrite phase and transformation of the martensite phase into the ferrite phase, thereby providing a single ferrite-phase microstructure. An annealing temperature of the hot-rolled sheet of lower than 600° C. results in insufficient recrystallization of the ferrite phase and insufficient transformation of the martensite phase into the ferrite phase, thereby easily forming punching cracks due to excessive hardening of the steel sheet. An annealing temperature of higher than 750° C. results in excessively coarse grains having an average grain size of more than 20 μm to easily cause large rollover and burrs during punching work, thereby failing to obtain a predetermined dimensional accuracy during the punching work. A holding time of less than 1 minute results in insufficient recrystallization of the ferrite phase and insufficient transformation of the martensite phase into the ferrite phase, thereby easily forming punching cracks due to excessive hardening of the steel sheet. A holding time of more than 24 hours results in excessively coarse grains having an average grain size of more than 20 μm to easily cause large rollover and burrs during punching work, thereby failing to obtain the predetermined dimensional accuracy during the punching work. Accordingly, in accordance with aspects of the present invention, it is necessary to perform the hot-rolled steel sheet annealing in which holding is performed in a temperature range of 600° C. or higher and lower than 750° C. for 1 minute to 24 hours.

As described above, in accordance with aspects of the present invention, the metal microstructure is a single ferrite phase microstructure, and the single ferrite phase microstructure has an average grain size of 5 to 20 μm. The average grain size is preferably 7 μm or more, more preferably 10 μm or more. The average grain size is preferably 18 μm or less, more preferably 15 μm or less.

The average grain size can be determined as follows: A test piece for microstructure observation is taken from the middle portion of the sheet in the width direction. A section thereof in the rolling direction is mirror-polished. Then measurement and analysis are performed in the field of view including the entire thickness by a SEM/EBSD method. Boundaries with a misorientation of 15° or more are defined as grain boundaries. The average grain size can be determined on the basis of an area method.

The thickness of the hot-rolled and annealed ferritic stainless steel sheet according to aspects of the present invention is preferably, but not necessarily, 5.0 mm or more, more preferably 8.0 mm or more because the sheet thickness is desirably a sheet thickness that can be used for a thick-wall flange. The sheet thickness is preferably 15.0 mm or less, more preferably 13.0 mm or less.

The chemical composition of the hot-rolled and annealed ferritic stainless steel sheet according to aspects of the present invention will be described below.

Hereinafter, the component contents are given in units of “%”, which indicates “% by mass”, unless otherwise specified.

C: 0.001% to 0.020%

When C is contained in an amount of more than 0.020%, the workability and the corrosion resistance in a weld zone deteriorate markedly. A lower C content is more preferable from the viewpoints of the corrosion resistance and the workability. To obtain a C content of less than 0.001%, it takes long time to perform refining, which is not preferred in terms of production. Thus, the C content is in the range of 0.001% to 0.020%. The C content is preferably 0.003% or more, more preferably 0.004% or more. The C content is preferably 0.015% or less, more preferably 0.012% or less.

Si: 0.05% to 1.00%

Si is an element that concentrates at an oxide layer formed during welding, is thus effective in improving the corrosion resistance in a weld zone, and is also effective as a deoxidizing element in the steelmaking process. These effects are obtained at a Si content of 0.05% or more and are enhanced as the Si content increases. However, a Si content of more than 1.00% results in an increase in rolling load and significant formation of scales in the hot-rolling step to induce the increases of surface defects and production costs and thus is not preferred. Accordingly, the Si content is 0.05% to 1.00%. The Si content is preferably 0.10% or more, more preferably 0.15% or more. The Si content is preferably 0.60% or less, more preferably 0.40% or less.

Mn: 0.05% to 1.00%

Mn is an austenite-forming element and effective in increasing the amount of austenite formed during heating before rolling in the hot-rolling step. Additionally, Mn acts as a deoxidizer. To obtain these effects, a Mn content of 0.05% or more is needed. However, a Mn content of more than 1.00% results in the promotion of the precipitation of MnS acting as a starting point of corrosion to deteriorate the corrosion resistance. Accordingly, the Mn content is 0.05% to 1.00%. The Mn content is preferably 0.10% or more, more preferably 0.15% or more. The Mn content is preferably 0.60% or less, more preferably 0.30% or less.

P: 0.04% or Less

P is an element that is unavoidably contained in steel and is also an element harmful to the corrosion resistance and workability; thus, the P content is preferably minimized. In particular, a P content of more than 0.04% results in a significant deterioration in workability due to solution hardening. Accordingly, the P content is 0.04% or less. The P content is preferably 0.03% or less.

S: 0.01% or Less

As with P, S is also an element that is unavoidably contained in steel and is also an element harmful to the corrosion resistance and workability; thus, the S content is preferably minimized. In particular, a S content of more than 0.01% results in a significant deterioration in corrosion resistance. Accordingly, the S content is 0.01% or less. The S content is preferably 0.008% or less. The S content is more preferably 0.003% or less.

Al: 0.01% to 0.10%

Al is an effective deoxidizer. Al has a higher affinity for nitrogen than Cr; thus, Al is effective in precipitating nitrogen in the form of aluminum nitride instead of chromium nitride to suppress sensitization when nitrogen enters a weld zone. These effects are obtained at an Al content of 0.01% or more. However, an Al content of more than 0.10% results in the deterioration of the penetration characteristics during welding to deteriorate the welding workability and thus is not preferred. Accordingly, the Al content is in the range of 0.01% to 0.10%. The Al content is preferably 0.02% or more, more preferably 0.03% or more. The Al content is preferably 0.06% or less, more preferably 0.04% or less.

Cr: 10.0% to 20.0%

Cr is the most important element for ensuring the corrosion resistance of stainless steel. If the Cr content is less than 10.0%, the corrosion resistance in an automobile exhaust gas atmosphere cannot be ensured sufficiently. If the Cr content is more than 20.0%, an amount of an austenite phase formed in the hot-rolling step is insufficient even if a predetermined amount of Ni is contained; thus, the effect of reducing the size of grains in the metal microstructure in the hot-rolling step is not sufficiently obtained to lead to an average grain size of more than 20 μm after the hot-rolled steel sheet annealing, thereby failing to obtain the predetermined dimensional accuracy during the punching work. Accordingly, the Cr content is in the range of 10.0% to 20.0%. The Cr content is preferably in the range of 10.0% to 17.0%. The Cr content is more preferably 10.5% or more, even more preferably 11.2% or more. The Cr content is more preferably 12.0% or less, even more preferably 11.7% or less.

Ni: 0.50% to 2.00%

Ni is an austenite-forming element and effective in increasing the amount of austenite formed during heating before rolling in the hot-rolling step. In accordance with aspects of the present invention, an austenite phase is formed during heating in the hot-rolling step by controlling the Cr and Ni contents to predetermined values. The formation of the austenite phase results in the grain refinement of the coarse-grained metal microstructure formed during casting.

Additionally, the dynamic and/or static recrystallization of the austenite phase occurs in the hot rolling to lead to further grain refinement of the metal microstructure after the hot rolling, thereby contributing to the grain refinement of the metal microstructure after the hot-rolled steel sheet annealing. These effects are obtained at a Ni content of 0.50% or more. A Ni content of more than 2.00% results in an excess of dissolved Ni. This is liable to cause the formation of punching cracks due to excessive hardening of the hot-rolled and annealed steel sheet. Accordingly, the Ni content is 0.50% to 2.00%. The Ni content is preferably 0.60% or more, more preferably 0.70% or more, even more preferably 0.75% or more. The Ni content is preferably 1.50% or less, more preferably 1.00% or less.

Ti: 0.10% to 0.40%

Ti preferentially combines with C and N and thus is effective in suppressing the precipitation of chromium carbonitride, reducing the recrystallization temperature, and suppressing a deterioration in corrosion resistance due to sensitization caused by the precipitation of chromium carbonitride. To obtain these effects, Ti needs to be contained in an amount of 0.10% or more. However, a Ti content of more than 0.40% results in the formation of coarse titanium carbonitride in a casting step to significantly deteriorate the toughness of the steel sheet and to cause surface defects and thus is not preferred in terms of production. Accordingly, the Ti content is 0.10% to 0.40%. The Ti content is preferably 0.15% or more, more preferably 0.20% or more. The Ti content is preferably 0.35% or less. The Ti content is more preferably 0.30% or less. Preferably, the Ti content is such that the Ti content satisfies the following formula Ti/(C+N)≥8 (where, in the formula, Ti, C, and N are the amounts of the relevant elements contained (% by mass)) in view of the corrosion resistance in a weld zone.

N: 0.001% to 0.020%

At a N content of more than 0.020%, the workability and the corrosion resistance in a weld zone deteriorate markedly. A lower N content is more preferable from the viewpoint of the corrosion resistance. It takes long time to perform refining to reduce the N content to less than 0.001%. Such refining leads to an increase in production cost and a deterioration in productivity, which is not preferred. Accordingly, the N content is in the range of 0.001% to 0.020%. The N content is preferably 0.005% or more, more preferably 0.007% or more. The N content is preferably 0.015% or less. The N content is more preferably 0.012% or less.

Aspects of the present invention provide a ferritic stainless steel featured by containing the foregoing essential components and the balance being Fe and incidental impurities. If necessary, one or two or more selected from Cu, Mo, W, and Co, and/or one or two or more selected from V, Nb, Zr, REM, B, Mg, and Ca may be further contained in ranges described below. The advantageous effects according to aspects of the present invention are not impaired when the elements are contained in amounts of less than the lower limits. Thus, when the elements are contained in amounts of less than the lower limits, the elements are regarded as incidental impurities.

Cu: 0.01% to 1.00%

Cu is an element particularly effective in improving the corrosion resistance of the base metal and a weld zone in an aqueous solution or when slightly acidic water droplets adhere thereto. The effect is obtained at a Cu content of 0.01% or more and is enhanced as the Cu content increases. However, a Cu content of more than 1.00% may result in a deterioration in hot workability to induce surface defects. Furthermore, a difficulty may lie in descaling after annealing. Accordingly, when Cu is contained, the Cu content is preferably in the range of 0.01% to 1.00%. The Cu content is more preferably 0.10% or more, even more preferably 0.30% or more. The Cu content is more preferably 0.60% or less, even more preferably 0.45% or less.

Mo: 0.01% to 2.00%

Mo is an element that markedly improves the corrosion resistance of stainless steel. The effect is obtained at a Mo content of 0.01% or more and is improved as the Mo content increases. However, a Mo content of more than 2.00% may result in an increase in rolling load during hot rolling to deteriorate the productivity and an excessive increase in the strength of the steel sheet. Since Mo is an expensive element, a large amount of Mo contained results in an increase in production cost. Accordingly, when Mo is contained, the Mo content is preferably 0.01% to 2.00%. The Mo content is more preferably 0.10% or more, even more preferably 0.30% or more. The Mo content is more preferably 1.40% or less, even more preferably 0.90% or less.

W: 0.01% to 0.20%

As with Mo, W is effective in improving the corrosion resistance. The effect is obtained at a W content of 0.01% or more. However, a W content of more than 0.20% may result in an increase in strength to lead to a deterioration in productivity due to, for example, an increase in rolling load. Accordingly, when W is contained, the W content is preferably in the range of 0.01% to 0.20%. The W content is more preferably 0.05% or more. The W content is more preferably 0.15% or less.

Co: 0.01% to 0.20%

Co is an element that improves the toughness. The effect is obtained when Co is contained in an amount of 0.01% or more. A Co content of more than 0.20% may result in a deterioration in workability. Accordingly, when Co is contained, the Co content is preferably in the range of 0.01% to 0.20%.

V: 0.01% to 0.20%

V combines with C and N in the form of carbonitride to suppress sensitization during welding, improving the corrosion resistance in the weld zone. The effect is obtained at a V content of 0.01% or more. A V content of more than 0.20% may result in significant deteriorations in workability and toughness. Accordingly, the V content is preferably 0.01% to 0.20%. The V content is more preferably 0.02% or more. The V content is more preferably 0.050% or less.

Nb: 0.01% to 0.10%

Nb is effective in increasing the 0.2% proof stress by refining grains and precipitating in the form of a fine carbonitride. These effects are obtained at a Nb content of 0.01% or more. Nb is also effective in increasing the recrystallization temperature. At a Nb content of more than 0.10%, an excessively high annealing temperature is required for sufficient recrystallization in the hot-rolled steel sheet annealing; thus, a single ferrite phase microstructure having an average grain size of 5 to 20 μm after the hot-rolled steel sheet annealing, which is a requirement according to aspects of the present invention, is not obtained, in some cases. Accordingly, when Nb is contained, the Nb content is preferably in the range of 0.01% to 0.10%. The Nb content is more preferably 0.01% to 0.05%.

Zr: 0.01% to 0.20%

Zr combines with C and N and is effective in suppressing sensitization. The effect is obtained at a Zr content of 0.01% or more. A Zr content of more than 0.20% may result in a significant deterioration in workability. Accordingly, when Zr is contained, the Zr content is preferably in the range of 0.01% to 0.20%. The Zr content is more preferably in the range of 0.01% to 0.10%.

REMs: 0.001% to 0.100%

Rare earth metals (REMs) are effective in improving oxidation resistance and suppress the formation of an oxide film in a weld zone (temper color due to welding) to suppress the formation of a Cr-depleted region immediately below the oxide film. The effect is obtained when REMs are contained in amounts of 0.001% or more. When REMs are contained in amounts of more than 0.100%, the hot workability may deteriorate. Accordingly, when REMs are contained, REMs are preferably contained in the range of 0.001% to 0.100%. REMs are more preferably contained in the range of 0.001% to 0.050%.

B: 0.0002% to 0.0025%

B is an element effective in improving resistance to secondary work embrittlement after deep drawing. The effect is obtained at a B content of 0.0002% or more. A B content of more than 0.0025% may result in deteriorations in workability and toughness. Accordingly, when B is contained, the B content is preferably in the range of 0.0002% to 0.0025%. The B content is more preferably 0.0003% or more. The B content is more preferably 0.0006% or less.

Mg: 0.0005% to 0.0030%

Mg is an element effective in improving the fraction of equiaxed crystals in a slab to improve the workability and the toughness. Regarding Ti-containing steel as in accordance with aspects of the present invention, the coarsening of titanium carbonitride deteriorates the toughness. Mg is also effective in suppressing the coarsening of titanium carbonitride. These effects are obtained at a Mg content of 0.0005% or more. A Mg content of 0.0030% may deteriorate the surface properties of steel. Accordingly, when Mg is contained, the Mg content is preferably in the range of 0.0005% to 0.0030%. The Mg content is more preferably 0.0010% or more. The Mg content is more preferably 0.0020% or less.

Ca: 0.0003% to 0.0030%

Ca is an element effective in preventing nozzle clogging due to the crystallization of Ti-based inclusions, which tends to occur during continuous casting. The effect is obtained at a Ca content of 0.0003% or more. However, a Ca content of more than 0.0030% may result in the formation of CaS to deteriorate the corrosion resistance. Accordingly, when Ca is contained, the Ca content is preferably in the range of 0.0003% to 0.0030%. The Ca content is more preferably 0.0005% or more. The Ca content is more preferably 0.0015% or less, even more preferably 0.0010% or less.

A method for producing a hot-rolled and annealed ferritic stainless steel sheet according to aspects of the present invention will be described below.

A hot-rolled and annealed ferritic stainless steel sheet according to aspects of the present invention is produced by subjecting a steel slab having the foregoing chemical composition to the usual hot rolling process to produce a hot-rolled steel sheet and then subjecting the resulting hot-rolled steel sheet to hot-rolled steel sheet annealing in which holding is performed at 600° C. or higher and lower than 750° C. for 1 minute to 24 hours.

First, a molten steel having the foregoing chemical composition is produced by a known method using, for example, a converter, an electric furnace, or a vacuum melting furnace and is formed into a steel (slab) by a continuous casting process or an ingot casting-slabbing process.

The slab is heated at 1,050° C. to 1,250° C. for 1 to 24 hours and then subjected to hot rolling. Alternatively, the as-cast slab is directly subjected to hot rolling before the temperature of the slab after casting is decreased to a temperature bellow the foregoing temperature range. In accordance with aspects of the present invention, the technique and conditions of the hot rolling are not particularly limited. When a coiling process is performed at an excessively low temperature, the hot-rolled steel sheet may be significantly hardened to make it difficult to perform the operation of the subsequent process. Thus, the coiling process is preferably performed at 550° C. or higher.

Hot-Rolled Steel Sheet Annealing: Holding at 600° C. or Higher and Lower Than 750° C. for 1 Minute to 24 Hours

In accordance with aspects of the present invention, after the completion of the hot-rolling step, the hot-rolled steel sheet annealing is performed. In the hot-rolled steel sheet annealing, a rolled deformed microstructure formed in the hot-rolling step is recrystallized without excessively coarsening the metal microstructure, and a martensite phase formed in the hot-rolling step is transformed into a ferrite phase. To obtain the effect, the hot-rolled steel sheet annealing needs to be performed at 600° C. or higher and lower than 750° C. At an annealing temperature of lower than 600° C., insufficient recrystallization results in fine recovered grains from the hot-rolled deformed microstructure. The metal microstructure is excessively refined to fail to obtain the predetermined dimensional accuracy during punching work. Additionally, a deformed microstructure and a martensite phase remain in the metal microstructure after the hot-rolled steel sheet annealing. This may lead to the formation of punching cracks due to excessive hardening of the steel sheet even if the average grain size is within a predetermined range. An annealing temperature of 750° C. or higher results in excessively coarse grains having an average grain size of more than 20 μm, thereby failing to obtain the predetermined dimensional accuracy during punching work. At a holding time of less than 1 minute, a deformed microstructure and a martensite phase remain in the metal microstructure after the hot-rolled steel sheet annealing. This easily leads to the formation of punching cracks due to excessive hardening of the steel sheet even if the average grain size is within a predetermined range. A holding time of more than 24 hours results in excessively coarse grains having an average grain size of more than 20 μm, thereby failing to obtain the predetermined dimensional accuracy during punching work. Accordingly, the hot-rolled steel sheet annealing is performed by performing holding in a temperature range of 600° C. or higher and lower than 750° C. for 1 minute to 24 hours. The temperature of the hot-rolled steel sheet annealing is preferably 600° C. or higher, more preferably 640° C. or higher. The temperature of the hot-rolled steel sheet annealing is preferably 700° C. or lower. The holding time is preferably 1 hour or more, more preferably 6 hours or more. The holding time is preferably 20 hours or less, more preferably 12 hours or less. There is no particular limitation on the process of annealing the hot-rolled steel sheet, i.e., the hot-rolled steel sheet annealing process. Any one of box annealing (batch annealing) and continuous annealing may be employed.

The resulting hot-rolled and annealed steel sheet may be subjected to descaling treatment using, for example, shot blasting or pickling, as needed. To improve surface properties, for example, grinding or polishing may be performed. The hot-rolled and annealed steel sheet according to aspects of the present invention may then be subjected to cold rolling and cold-rolled steel sheet annealing.

EXAMPLES

Aspects of the present invention will be described in detail below by examples.

Molten stainless steels having chemical compositions given in Table 1 were produced with a 100 kg vacuum melting furnace. Steel ingots of those molten steels were heated at 1,100° C. for 1 hour, hot-rolled to the thicknesses given in Table 2 (see “Thickness after completion of hot rolling” in Table 2), and subjected to simulated coiling treatment in which the steel sheets were held at 650° C. for 1 hour and then furnace-cooled, thereby producing hot-rolled steel sheets. Subsequently, the hot-rolled steel sheets were subjected to hot-rolled steel sheet annealing in which the steel sheets were held at the temperatures given in Table 2 (see “Hot-rolled steel sheet annealing temperature” in Table 2) for 8 hours and then slowly cooled, thereby producing hot-rolled and annealed steel sheets.

The thicknesses of the resulting hot-rolled and annealed steel sheets were the same as the respective thicknesses after the completion of the hot rolling.

The hot-rolled and annealed steel sheets thus obtained were evaluated as described below.

(1) Evaluation of Metal Microstructure

Test pieces for microstructure observation were taken from the middle portions of the sheets in the width direction. A section of each test piece in the rolling direction was mirror-polished. Then measurement and analysis were performed in the field of view including the entire thickness by a SEM/EBSD method. Boundaries with a misorientation of 15° or more were defined as grain boundaries. The average grain size was determined on the basis of an area method. A steel sheet having an average grain size of 5 μm or more and 20 μm or less is within the scope according to aspects of the present invention. A steel sheet having an average grain size of less than 5 μm or more than 20 μm is outside the scope according to aspects of the present invention and underlined in Table 2.

Similarly, test pieces for microstructure observation were taken from the middle portions of the sheets in the width direction. A section of each test piece in the rolling direction was mirror-polished. The section was etched for observation with an aqueous solution of picric acid and hydrochloric acid to expose a metal microstructure. The metal microstructure was observed with an optical microscope at a magnification of ×500. Whether the metal microstructure of each steel sheet was a single ferrite phase microstructure was determined by distinguishing a ferrite phase from a martensite phase on the basis of the morphology of the metal microstructure. Specifically, a region in which a uniform and flat morphology was observed in a grain and a relatively bright tone was observed was regarded as a ferrite phase. A region in which surface morphology unique to the martensite phase, such as subboundaries and block boundaries, was observed in a grain and which exhibited a darker tone than the ferrite phase was regarded as a martensite phase. In the table, F indicates that the metal microstructure was a single ferrite phase microstructure.

(2) Evaluation of Corrosion Resistance

Test pieces measuring 60×100 mm were taken from the hot-rolled and annealed steel sheets. Surfaces of each of the test pieces were subjected to abrasive finishing with 600-grit emery paper. Then end-face portions of each test piece were sealed. The test pieces were subjected to a cyclic salt-spray test specified in JIS H 8502. One cycle of the cyclic salt-spray test consisted of salt spraying (5% by mass NaCl, 35° C., spraying for 2 hours)→drying (60° C., 4 hours, relative humidity: 40%)→wetting (50° C., 2 hours, relative humidity: 95%). The cyclic salt-spray test was performed five cycles. After the cyclic salt-spray test was performed five cycles, the surfaces of each test piece were photographed. The rusted area of the surfaces of the test piece was measured by image analysis. The percentage of rusted area ((rusted area of test piece/total area of test piece)×100 [%]) was calculated with respect to the total area of the test piece. A percentage of rusted area of 10% or less, which indicated that the steel sheet had outstanding corrosion resistance, was rated as “acceptable” (⊙). A percentage of rusted area of more than 10% and 25% or less was rated as “acceptable” (◯). A percentage of rusted area of more than 25% was rated as “unacceptable” (×).

(3) Evaluation of Punching Work

Samples measuring 100 mm×100 mm for testing were taken from the hot-rolled and annealed steel sheets. Five test specimens were produced by punching work with a crank press equipped with an upper die (punch) and a lower die (die) so as to form a hole having a diameter of 20 mm (tolerance: ±0.1 mm) in the middle portion of each of the samples, the upper die having a cylindrical edge for punching and a diameter of 20 mm, the lower die having a hole appropriately selected in such a manner that the clearance between the upper die and the lower die was 10%. The relationships among the clearance (C) [%], the hole diameter of the die (the inside diameter of the die) (Dd) [mm], the diameter of the punch (Dp) [mm], and the sheet thickness (t) [mm] are represented by formula (1) below.

C=(Dd−Dp)÷(2×t)×100 formula  (1)

With respect to the test specimens thus obtained, the appearance of each test specimen was visually inspected, and the diameter of the hole in the middle portion of the test specimen was measured with a digital caliper. The case where no cracks were observed and where the hole diameter of each of the five test specimens after the punching work was in the range of 19.9 to 20.1 mm was rated as “acceptable” (◯). The case where any one of them was cracked or had a hole diameter of less than 19.9 mm or more than 20.1 mm was rated as “unacceptable” (×).

Table 2 presents the test results together with hot-rolled steel sheet annealing conditions.

TABLE 1
Steel	Chemical composition (% by mass)
symbol	C	Si	Mn	P	S	Al	Cr	Ni	Ti	N	Others	Remarks
A1	0.008	0.24	0.34	0.03	0.004	0.05	10.3	0.83	0.23	0.008	—	Example
A2	0.006	0.24	0.30	0.02	0.004	0.04	15.7	0.85	0.25	0.007	—	Example
A3	0.008	0.23	0.29	0.02	0.003	0.03	19.7	0.88	0.23	0.007	—	Example
A4	0.007	0.27	0.27	0.03	0.004	0.06	11.5	0.86	0.25	0.008	—	Example
A5	0.009	0.07	0.29	0.03	0.005	0.05	11.5	0.84	0.24	0.006	—	Example
A6	0.008	0.11	0.30	0.04	0.004	0.03	11.3	0.82	0.26	0.009	—	Example
A7	0.006	0.15	0.28	0.02	0.003	0.03	11.4	0.84	0.25	0.005	—	Example
A8	0.008	0.58	0.32	0.03	0.004	0.04	11.6	0.81	0.25	0.008	—	Example
A9	0.008	0.26	0.06	0.03	0.004	0.04	11.6	0.81	0.23	0.006	—	Example
A10	0.007	0.23	0.10	0.03	0.003	0.03	11.4	0.83	0.21	0.009	—	Example
A11	0.008	0.24	0.17	0.04	0.005	0.03	11.6	0.85	0.20	0.010	—	Example
A12	0.006	0.27	0.97	0.02	0.003	0.06	11.6	0.90	0.22	0.006	—	Example
A13	0.008	0.23	0.26	0.03	0.004	0.04	11.6	0.52	0.25	0.007	—	Example
A14	0.007	0.26	0.27	0.03	0.003	0.06	11.7	0.98	0.24	0.008	—	Example
A15	0.010	0.23	0.25	0.04	0.003	0.05	11.5	1.97	0.21	0.010	—	Example
A16	0.008	0.28	0.25	0.02	0.004	0.04	10.2	0.51	0.24	0.007	—	Example
A17	0.006	0.23	0.34	0.03	0.004	0.04	11.8	0.97	0.24	0.008	—	Example
A18	0.009	0.25	0.35	0.03	0.005	0.01	19.6	1.98	0.23	0.011	—	Example
A19	0.007	0.27	0.33	0.02	0.004	0.04	11.4	0.50	0.24	0.008	Cu: 0.34, B: 0.0011	Example
A20	0.008	0.24	0.31	0.03	0.004	0.04	11.5	0.99	0.26	0.007	Mo: 0.51, Nb: 0.08	Example
A21	0.007	0.28	0.28	0.02	0.004	0.03	11.6	0.83	0.26	0.007	Cu: 0.46	Example
A22	0.009	0.23	0.35	0.03	0.004	0.06	11.6	0.88	0.23	0.008	Mo: 1.22	Example
A23	0.007	0.26	0.27	0.02	0.004	0.06	11.7	0.78	0.22	0.009	W: 0.11	Example
A24	0.008	0.25	0.31	0.03	0.003	0.04	11.6	0.88	0.23	0.007	Co: 0.15	Example
A25	0.007	0.26	0.34	0.02	0.004	0.04	11.5	0.86	0.25	0.006	V: 0.12	Example
A26	0.009	0.27	0.35	0.02	0.005	0.04	11.6	0.78	0.24	0.008	V: 0.05, Nb: 0.09	Example
A27	0.008	0.25	0.26	0.03	0.005	0.05	11.5	0.86	0.24	0.007	Zr: 0.10	Example
A28	0.008	0.24	0.34	0.03	0.004	0.04	11.6	0.86	0.25	0.009	REM: 0.008	Example
A29	0.007	0.27	0.29	0.03	0.005	0.06	11.5	0.83	0.25	0.008	B: 0.0014	Example
A30	0.006	0.27	0.32	0.03	0.004	0.04	11.5	0.80	0.23	0.006	Mg: 0.0007, Ca: 0.0005	Example
B1	0.008	0.27	0.28	0.03	0.003	0.03	11.5	0.21	0.23	0.008	—	Comparative example
B2	0.008	0.25	0.28	0.03	0.005	0.04	11.4	2.08	0.26	0.008	—	Comparative example
B3	0.009	0.28	0.26	0.03	0.003	0.05	9.4	0.79	0.22	0.007	—	Comparative example
B4	0.006	0.28	0.33	0.03	0.003	0.03	20.8	0.81	0.24	0.009	—	Comparative example
B5	0.006	0.25	0.28	0.03	0.004	0.05	11.6	0.86	0.07	0.009	—	Comparative example
The balance other than the component composition described above is Fe and incidental impurities.
Underlined values are outside the scope of the present invention.

TABLE 2
			Hot-rolled steel
		Thickness after	sheet annealing	Metal	Average
	Steel	completion of	temperature	microstructure	grain size	Punching	Corrosion
No.	symbol	hot rolling [mm]	[° C.]	(*)	[μm]	workability	resistance	Remarks
1	A1	10.1	652	F	11	◯	◯	Example
2	A2	9.9	653	F	16	◯	◯	Example
3	A3	10.2	655	F	19	◯	⊙	Example
4	A4	9.8	651	F	12	◯	◯	Example
5	A5	10.0	651	F	9	◯	◯	Example
6	A6	9.9	653	F	11	◯	◯	Example
7	A7	10.2	652	F	12	◯	◯	Example
8	A8	10.2	649	F	17	◯	◯	Example
9	A9	9.9	653	F	16	◯	◯	Example
10	A10	9.9	654	F	10	◯	◯	Example
11	A11	9.9	650	F	18	◯	◯	Example
12	A12	9.8	647	F	8	◯	◯	Example
13	A13	10.3	655	F	5	◯	◯	Example
14	A14	10.0	646	F	19	◯	◯	Example
15	A15	10.0	653	F	8	◯	◯	Example
16	A16	10.1	655	F	18	◯	◯	Example
17	A17	9.8	646	F	17	◯	◯	Example
18	A18	10.0	653	F	9	◯	⊙	Example
19	A19	9.9	650	F	11	◯	⊙	Example
20	A20	9.8	652	F	10	◯	⊙	Example
21	A21	9.8	645	F	13	◯	⊙	Example
22	A22	9.9	652	F	12	◯	⊙	Example
23	A23	10.1	646	F	13	◯	◯	Example
24	A24	9.8	652	F	14	◯	◯	Example
25	A25	10.0	647	F	13	◯	◯	Example
26	A26	10.1	649	F	12	◯	◯	Example
27	A27	9.9	653	F	9	◯	◯	Example
28	A28	10.0	645	F	11	◯	◯	Example
29	A29	10.2	648	F	10	◯	◯	Example
30	A30	10.2	655	F	12	◯	◯	Example
31	A1	5.2	652	F	10	◯	◯	Example
32	A1	8.1	650	F	12	◯	◯	Example
33	A1	13.0	645	F	16	◯	◯	Example
34	A1	14.7	647	F	18	◯	◯	Example
35	A13	10.0	602	F	8	◯	◯	Example
36	A13	9.9	747	F	17	◯	◯	Example
37	B1	9.8	749	F	34	X	◯	Comparative example
38	B2	9.8	652	F	8	X	◯	Comparative example
39	B3	10.1	650	F	11	◯	X	Comparative example
40	B4	9.8	651	F	26	X	⊙	Comparative example
41	B5	10.1	646	F	16	◯	X	Comparative example
43	A26	10.0	803	F	28	X	◯	Comparative example
44	A14	9.9	806	F	34	X	◯	Comparative example
Underlined values are outside the scope of the present invention.
(*) F: ferrite phase

In Nos. 1 to 36 in which the steel compositions and the hot-rolled steel sheet annealing conditions were within the scope according to aspects of the present invention, the predetermined punching workability was obtained because, in addition to the formation of an austenite phase during heating in the hot-rolling step, recrystallization occurred by the predetermined hot-rolled steel sheet annealing without excessive coarsening of grains to obtain the predetermined average grain size. The evaluation results of the corrosion resistance of the resulting hot-rolled and annealed sheets revealed that each sheet had a percentage of rusted area of 25% or less and had sufficient corrosion resistance.

In particular, in each of No. 19 using Cu-containing steel A19, No. 21 using Cu-containing steel A21, No. 20 using Mo-containing steel A20, and No. 22 using Mo-containing steel A22, the percentage of rusted area was 10% or less, and better corrosion resistance was provided.

In each of No. 3 using steel A3 having a high Cr content of 19.7% and No. 18 using steel A18 having a high Cr content of 19.6%, a strong passivation film was formed on the surfaces of the steel sheet; thus, the percentage of rusted area was 10% or less, and better corrosion resistance was provided.

In No. 37 using steel B1 having a Ni content of less than that in the scope according to aspects of the present invention, almost no austenite phase was formed during heating in the hot-rolling step to fail to obtain the effect of refining the metal microstructure. Thus, the average grain size was more than the scope according to aspects of the present invention to fail to obtain the predetermined punching workability.

In No. 38 using steel B2 having a Ni content of more than that in the scope according to aspects of the present invention, although the predetermined average grain size was obtained, the excessive amount of dissolved Ni resulted in the excessively hardened steel sheet to cause cracking during punching work, thereby failing to process the steel sheet into a predetermined shape.

In No. 39 using steel B3 having a Cr content of less than that in the scope according to aspects of the present invention, the predetermined corrosion resistance was not be obtained due to the insufficient Cr content.

In No. 40 using steel B4 having a Cr content of more than that in the scope according to aspects of the present invention, the excessive Cr content resulted in the decrease of the austenite phase formed during heating in the hot-rolling step, even though the predetermined amount of Ni was contained. Thus, the effect of grain refinement due to the formation of the austenite phase in the hot-rolling step was not sufficiently obtained. Thereby, the predetermined average grain size was not obtained, failing to obtain the predetermined punching workability.

In No. 41 using steel B5 having a Ti content of less than that in the scope according to aspects of the present invention, a large amount of chromium carbonitride was precipitated during the hot-rolled steel sheet annealing to cause sensitization, thereby failing to obtain the predetermined corrosion resistance.

In No. 43 in which the hot-rolled steel sheet annealing temperature was higher than that in the scope according to aspects of the present invention, recrystallized grains formed coarsened significantly to fail to provide the predetermined average grain size, thereby failing to obtain the predetermined punching workability.

No. 44 is an example in which steel A14 containing the predetermined steel composition was annealed at 806° C., which is higher than that in the scope according to aspects of the present invention, to increase the average grain size to 34 μm, which is more than that in the scope according to aspects of the present invention. Although the predetermined steel composition was contained, the excessively coarse grains resulted in significant rollover and burrs during the punching work, thereby failing to obtain the predetermined punching workability.

INDUSTRIAL APPLICABILITY

The hot-rolled and annealed ferritic stainless steel sheet according to aspects of the present invention is particularly suitable for applications that require high workability and corrosion resistance, for example, flanges having burring portions.

Claims

1. A hot-rolled and annealed ferritic stainless steel sheet having a chemical composition containing, on a mass percent basis:

C: 0.001% to 0.020%,

Si: 0.05% to 1.00%,

Mn: 0.05% to 1.00%,

P: 0.04% or less,

S: 0.01% or less,

Al: 0.01% to 0.10%,

Cr: 10.0% to 20.0%,

Ni: 0.50% to 2.00%,

Ti: 0.10% to 0.40%, and

N: 0.001% to 0.020%, the balance being Fe and incidental impurities; and

having a metal microstructure which is a single ferrite phase microstructure having an average grain size of 5 to 20 μμm.

2. The hot-rolled and annealed ferritic stainless steel sheet according to claim 1, further containing, on a mass percent basis, one or two or more selected from:

Cu: 0.01% to 1.00%,

Mo: 0.01% to 2.00%,

W: 0.01% to 0.20%, and

Co: 0.01% to 0.20%.

3. The hot-rolled and annealed ferritic stainless steel sheet according to claim 1, further containing, on a mass percent basis, one or two or more selected from:

V: 0.01% to 0.20%,

Nb: 0.01% to 0.10%,

Zr: 0.01% to 0.20%,

REM: 0.001% to 0.100%,

B: 0.0002% to 0.0025%,

Mg: 0.0005% to 0.0030%, and

Ca: 0.0003% to 0.0030%.

4. A method for producing the hot-rolled and annealed ferritic stainless steel sheet according to claim 1, comprising:

performing hot-rolled steel sheet annealing in which a hot-rolled steel sheet produced through a hot-rolling step is held at 600° C. or higher and lower than 750° C. for 1 minute to 24 hours.

5. The hot-rolled and annealed ferritic stainless steel sheet according to claim 2, further containing, on a mass percent basis, one or two or more selected from:

V: 0.01% to 0.20%,

Nb: 0.01% to 0.10%,

Zr: 0.01% to 0.20%,

REM: 0.001% to 0.100%,

B: 0.0002% to 0.0025%,

Mg: 0.0005% to 0.0030%, and

Ca: 0.0003% to 0.0030%.

6. A method for producing the hot-rolled and annealed ferritic stainless steel sheet according to claim 2, comprising:

performing hot-rolled steel sheet annealing in which a hot-rolled steel sheet produced through a hot-rolling step is held at 600° C. or higher and lower than 750° C. for 1 minute to 24 hours.

7. A method for producing the hot-rolled and annealed ferritic stainless steel sheet according to claim 3, comprising:

performing hot-rolled steel sheet annealing in which a hot-rolled steel sheet produced through a hot-rolling step is held at 600° C. or higher and lower than 750° C. for 1 minute to 24 hours.

8. A method for producing the hot-rolled and annealed ferritic stainless steel sheet according to claim 5, comprising:

performing hot-rolled steel sheet annealing in which a hot-rolled steel sheet produced through a hot-rolling step is held at 600° C. or higher and lower than 750° C. for 1 minute to 24 hours.