TW201843316A - Hot-rolled and annealed ferritic stainless steel sheet, and method for manufacturing same - Google Patents

Abstract

Provided are: a hot-rolled and annealed ferritic stainless steel sheet having a sufficient corrosion resistance, and capable of suppressing cracking while performing a punching process on a thick flange; and a method for manufacturing the same. The hot-rolled and annealed ferritic stainless steel sheet contains 0.00-0.020 mass% of C, 0.05-1.00 mass% of Si, 0.05-1.00 mass% of Mn, at most 0.04 mass% of P, at most 0.01 mass% of S, 0.001-0.100 mass% of Al, 10.0-19.0 mass% of Cr, 0.65-1.50 mass% of Ni, 0.10-0.40 mass% of Ti, and 0.001-0.020 mass% of N, and the remainder comprising Fe and inevitable impurities, wherein the threshold stress intensity factor KIC is at least 35 MPa.m1/2.

Description

   Ferritic iron-based stainless steel hot-rolled annealed steel sheet and manufacturing method thereof   

The present invention relates to a hot-rolled and annealed steel sheet of ferritic iron-based stainless steel with excellent processability suitable for flanges and the like, and a manufacturing method thereof.

In recent years, laws and regulations on automobile exhaust gas have been continuously strengthened, and the improvement of fuel efficiency has become a top priority. Therefore, the application of the exhaust gas recirculation (EGR) system that uses the exhaust gas generated from the automobile engine as the intake air of the engine is being continuously developed. The exhaust gas generated from the engine is supplied to the engine again after passing through an EGR cooler for reducing the gas temperature. When the exhaust gas is circulated, each exhaust system part is fastened via a flange to prevent gas leakage. The flanges used in such exhaust system parts must be sufficiently rigid. Therefore, a thick-walled flange (for example, a plate thickness of 5 mm or more) is applied to such an exhaust system part.

Conventionally, thick steel flanges have always used ordinary steel. However, flanges corresponding to parts for passing high-temperature exhaust gas such as an EGR system require sufficient corrosion resistance. Therefore, the application of stainless steels with superior corrosion resistance compared to ordinary steels, especially ferrous iron-based stainless steels with relatively small thermal expansion coefficients that are not prone to thermal stress, has been studied, and it is strongly required that they can be applied to thicker flanges with larger plate thicknesses. (For example, a plate thickness of 5 mm or more) a ferrous iron-based stainless steel plate.

In response to this market demand, for example, Patent Document 1 discloses a ferritic iron-based stainless steel hot-rolled steel sheet, which, in terms of mass%, contains: C: 0.015% or less, Si: 0.01-0.4%, and Mn: 0.01-0.8 %, 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, B: 0.0002 to 0.0020% and the remainder is Fe and inevitable impurities. The content of Nb, C, and N satisfies Nb / (C + N) ≧ 16, and the Charpy impact value at 0 ℃ is 10J. / cm ² or more, the thickness is 5.0 ~ 9.0mm.

    [Prior technical literature]         [Patent Literature]    

Patent Document 1: International Publication No. 2014/157576

However, the present inventors have attempted to use a ferrous-iron-based stainless steel hot-rolled steel sheet described in Patent Document 1 to form a thick-walled flange shape having a deburred portion, and have found that, although the steel sheet has a sufficient Charpy impact Value, but there are cases where cracking occurs in the burring processed portion, especially in the center of the plate thickness, and a predetermined flange shape cannot be obtained, which is not sufficient for thick-walled flanges.

An object of the present invention is to solve this problem, and to provide a ferritic iron-based stainless steel hot-rolled and annealed steel sheet having sufficient corrosion resistance and capable of suppressing cracking when punched into a thick-walled flange and a method for manufacturing the same.

The present inventors conducted detailed studies in order to solve the above-mentioned problems, and as a result, found that in order to prevent cracking and process into a thick-walled flange having a deburred portion, it is only necessary to increase the low limit stress intensity factor K _{IC of the} steel sheet. Specifically, it was found that by setting the low limit stress intensity factor K _IC to 35 MPa. M ^1/2 or more can effectively suppress the occurrence of cracks in the deburring portion when being processed into a thick-walled flange having a deburring portion, and can be fully practically applied to a thick-walled flange having a deburring portion.

Furthermore, it was found that by performing hot-rolled sheet annealing on the following hot-rolled steel sheet at an appropriate temperature, the low limiting stress intensity factor K _{IC was} improved. The hot-rolled steel sheet is a ferritic iron-based stainless steel with an appropriate composition, and the appropriate control includes 3 The cumulative rolling reduction rate of the final 3 passes in the multi-pass finishing hot rolling (= 100- (final sheet thickness / plate thickness before the rolling start of the last 3 passes) × 100 [%]) is obtained. This invention is based on the above knowledge, The summary is as follows.

[1] A ferritic iron-based stainless steel hot-rolled annealed steel sheet, which has a mass% of: 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: 0.001 to 0.100%, Cr: 10.0 to 19.0%, Ni: 0.65 to 1.50%, Ti: 0.10 to 0.40%, N: 0.001 to 0.020% and the remainder contains Fe and unavoidable impurities The composition of the composition, the low limit stress intensity factor K _IC is 35MPa. m ^1/2 or more.

[2] The ferritic iron-based stainless steel hot-rolled annealed steel sheet according to [1], wherein the above-mentioned component composition is further selected from Cu: 0.01 to 1.00%, Mo: 0.01 to 2.00%, and W: 0.01 to 0.20%, Co: One or more of 0.01 to 0.20%.

[3] The ferritic iron-based stainless steel hot-rolled annealed steel sheet according to [1] or [2], wherein the above-mentioned component composition is measured in mass% and further contains: selected from V: 0.01 to 0.20%, Nb: 0.01 to 0.10 %, Zr: 0.01 ~ 0.20%, REM: 0.001 ~ 0.100%, B: 0.0002 ~ 0.0025%, Mg: 0.0005 ~ 0.0030%, Ca: 0.0003 ~ 0.0030%.

[4] A method for manufacturing a hot-rolled and annealed steel sheet of a ferrous iron-based stainless steel, which is a method for manufacturing a hot-rolled and annealed steel sheet of a ferrous iron-based stainless steel as described in any one of [1] to [3], which includes: 3 or more hot rolling steps for finishing rolling; and a hot-rolled sheet annealing step of hot-rolled sheet annealing of the hot-rolled steel sheet obtained by the hot-rolling step at 600 ° C to 1100 ° C. In the above-mentioned hot-rolling step, For the last three passes of finishing rolling, the temperature is set to 800 to 1100 ° C and the cumulative rolling reduction rate of the last three passes is set to 25% or more.

Here, the low limit stress intensity factor K _IC refers to a CT test piece according to ASTM E399 taken from the center of the plate width by means of fatigue pre-splitting to become a right-angle direction of rolling and a stress axis to a parallel direction of rolling, and according to ASTM E399 The stress intensity factor obtained from the test.

According to the present invention, a ferritic iron-based stainless steel hot-rolled annealed steel sheet having sufficient corrosion resistance and excellent toughness capable of suppressing cracking when punched into a thick-walled flange can be obtained.

In addition, the sufficient corrosion resistance in the present invention refers to a steel plate formed by sealing the end surface after grinding and finishing the surface with # 600 sandpaper, and subjected to a salt water spray cycle test specified in JIS H 8502 (spray salt water ( 5 mass% NaCl, 35 ° C, spray for 2hr) → dry (60 ° C, 4hr, relative humidity 40%) → wet (50 ° C, 2hr, relative humidity ≧ 95%)) as a cycle test) 5 cycles In this case, the rust area ratio of the steel plate surface (= rust area / total steel plate area × 100 [%]) is 25% or less.

In addition, the excellent toughness capable of suppressing cracking when punching into a thick-walled flange refers to the basis from the center of the plate width by means of fatigue pre-splitting to become the right-angle direction of rolling and the stress axis to be the parallel direction of rolling. The low limit stress intensity factor K _IC of the CT test piece of ASTM E399 and the test according to ASTM E399 is 35MPa. m ^1/2 or more.

Hereinafter, embodiments of the present invention will be described. The present invention is not limited to the following embodiments.

The ferritic iron-based stainless steel hot-rolled and annealed steel sheet of the present invention has a mass percentage of C: 0.001 to 0.020%, Si: 0.05 to 1.00%, Mn: 0.05 to 1.00%, P: 0.04% or less, and S: 0.01. % Or less, Al: 0.001 to 0.100%, Cr: 10.0 to 19.0%, Ni: 0.65 to 1.50%, Ti: 0.10 to 0.40%, N: 0.001 to 0.020%, and the remainder contains Fe and unavoidable impurity composition , The low limit stress intensity factor K _IC is 35MPa. m ^1/2 or more.

The lower limit stress intensity factor K _IC refers to a CT test piece according to ASTM E399 from the center of the plate width by means of fatigue pre-cracking to become a right-angle direction of rolling and a stress axis to a parallel direction of rolling, and the test is performed according to ASTM E399. The obtained stress intensity factor.

Hereinafter, the present invention will be described in detail.

The present inventors have studied in detail the causes of cracking when forming various ferritic iron-based stainless steel plates with a thickness of 5.0 mm into a flange having a thickness of 30 mm The flange hole portion is pulled from the surface of the steel plate of the blank by a deburring processing portion of 10 mm. As a result, it was found that in the above-mentioned steel sheet where cracks occurred, the minute cracks generated near the central portion of the thickness of the punched end face significantly expanded during deburring and cracked.

The inventors of the present invention conducted a detailed study on the relationship between the significant expansion of the tiny cracks and the material properties. As a result, it has been found that the propagation of minute cracks tends to occur more easily as the lower the lower limit stress intensity factor of the steel sheet. Therefore, attempts have been made to form this flange using various ferritic iron-based stainless steel hot-rolled annealed steel plates (sheet thickness 5.0mm). As a result, it was found that the cracks caused by the propagation of microcracks were caused by the low limit stress intensity obtained by a predetermined measurement method. The factor is lower than 35MPa. It is particularly easy to occur in a steel plate of m ^1/2 .

Furthermore, the present inventors have conducted detailed investigations into the cracked portion of the steel sheet in order to make the cause of the low stress stress factor of the steel sheet cracked during the forming of the flange clearer. As a result, it was found that, in the steel sheet where cracks occurred, cracks generated near the central part of the plate thickness of the punched end face significantly expanded in the grain boundaries near the central part of the plate thickness.

Furthermore, the SEM / EBSD method [scanning electron microscope (SEM) / Electron Backscatter Diffraction (EBSD) method] was used to investigate and analyze the structure of the steel sheet, and as a result, the turtle was identified. Although the crystal grains at the locations where the cracks are significantly expanded are independent crystal grains, so-called colonies (groups of crystal grains with similar crystal orientations) having approximately the same crystal orientation as adjacent crystal grains are formed. Generally speaking, crystal grains have different crystal orientations from adjacent crystal grains. When cracks propagate on grain boundaries, grain boundaries with different orientations play a barrier function as crack propagation. However, the crystal orientations of adjacent crystal grains in the settlement are approximately equal, so the effect of suppressing the crack propagation from the grain boundary between the crystal grains in the settlement becomes smaller. As a result, it was found that in the steel sheet on which the settlements were formed, the low-limit stress intensity factor decreased, and cracking occurred when the flange was formed into the flange.

Therefore, the present inventors have worked hard on a method for improving the low-limit stress intensity factor in a hot-rolled and annealed steel sheet of ferritic iron-based stainless steel. As a result, it was found that by performing hot-rolled sheet annealing on the following hot-rolled steel sheets at 600 ° C to 1100 ° C, the aggregation can be effectively destroyed, and 35 MPa was obtained. Low limit stress intensity factor K _IC above m ^1/2 , the hot-rolled steel sheet has a final rolling temperature of 800 ~ 1100 ° C and a cumulative rolling reduction of the final 3 passes (= 100- (Final sheet thickness / thickness before the start of rolling in the last 3 passes) × 100 [%]) is obtained by hot-rolling a ferrous iron-based stainless steel with an appropriate composition under conditions of 25% or more.

In addition, the thickness of the ferritic iron-based stainless steel hot-rolled annealed steel sheet of the present invention is not particularly limited, and it is preferable that the sheet thickness can be applied to a thick-walled flange. The lower limit is preferably 5.0 mm or more. More preferably, it is 9.0 mm or more. The upper limit is preferably 15.0 mm or less. More preferably, it is 10.0 mm or less.

The reason for promoting the destruction of the settlement by the above method will be described below.

At the central part of the thickness of the slab before hot rolling of the ferrous iron-based stainless steel, coarse and extended settlements (groups of crystal grains with similar crystal orientation) are continuously distributed along the casting direction. On the other hand, when the steel sheet is rolled, the steel sheet is deformed from the surface layer portion and elongated. Therefore, when the reduction ratio is small, the amount of deformation in the center portion of the plate thickness becomes small, and rolling strain is hardly introduced in the center portion of the plate thickness. As a result, in the hot rolling using the conventional technology, the rolling grains in the central portion of the thickness of the steel sheet were not sufficiently introduced with rolling strain, and the recrystallization position in the subsequent hot rolling annealing was insufficient, and the hot rolling was annealed. At this time, although recrystallization occurred near the center of the plate thickness, the settlement was not separated and easily remained, and the 35 MPa required by the present invention could not be obtained. Low limit stress intensity factor K _IC above m ^1/2 .

Furthermore, ferritic iron-based stainless steel hardly undergoes dynamic recrystallization during hot rolling, and tends to easily recover the processing strain due to rolling. Therefore, in the hot rolling using the conventional technology, excessive recovery of the processing strain introduced by rolling is generated, and the processing strain cannot be effectively maintained until after the hot rolling. As a result, the recrystallization position becomes insufficient, and in the hot-rolled sheet annealing in the subsequent step, the settlement is not effectively destroyed, and the predetermined low-limit stress intensity factor K _IC cannot be obtained.

Therefore, the present inventors have worked hard on two aspects of the steel composition and the hot rolling method for a method for effectively reducing the accumulation of residuals after annealing of the hot rolled sheet. As a result, it was found that it is effective to control the content of steel components, especially Cr and Ni to an appropriate range, to generate a predetermined amount of Wastfield iron phase in the hot rolling step, and to finish hot rolling in the hot rolling step. The final 3 passes are managed in an appropriate temperature range, and on this basis, rolling is performed with a larger cumulative rolling reduction rate.

This makes it possible to destroy the aggregates formed during casting by the generation of the vostian iron phase, and to suppress the recovery of the rolling process strain during hot rolling, and to sufficiently and effectively introduce the rolling process strain to the center of the plate thickness. . Thereby, it is possible to obtain a hot-rolled sheet remaining after hot rolling, which has fewer settlements formed during casting than conventional techniques, and which allows sufficient residual rolling strain at the recrystallization position in the annealing of the hot-rolled sheet in the subsequent steps. organization. As a result, in the hot-rolled sheet annealing in the subsequent step, the settlement is more effectively eliminated, and an excellent low-limit stress intensity factor is obtained.

Specifically, it is proposed that for steels whose Cr content is adjusted to be in the range of 10.0 to 19.0% and Ni content is to be adjusted to be in the range of 0.65 to 1.50% in order to generate a Wastfield iron phase during heating before hot rolling, The temperature of the last 3 passes including 3 or more finish hot rolling becomes 800 ~ 1100 ° C and the cumulative rolling reduction of the last 3 passes (= 100- (final sheet thickness / plate thickness before the start of rolling of the last 3 passes) × 100 [%]) is controlled to be 25% or more, and hot rolling is performed by appropriate control.

In addition, the present inventors have also worked hard on better conditions for annealing the hot-rolled sheet in the subsequent steps. Hot-rolled sheet annealing is a step of recrystallizing a processed structure formed by hot rolling. Therefore, annealing must be performed at a temperature that produces sufficient recrystallization. However, in the case where the hot-rolled sheet is annealed at an excessively high temperature, although recrystallization occurs, recrystallization grains are significantly coarsened. Although the significantly coarse recrystallized grains are independent single crystal grains, the grain boundary length becomes significantly longer. Therefore, it was found that the effect of suppressing crack propagation from grain boundaries with different orientations was reduced as in the case of settlement, and it was impossible to obtain a predetermined low limit stress intensity factor.

Therefore, the present inventors investigated the relationship between the particle diameter of the recrystallized grains and the annealing temperature in detail. As a result, it was found that by controlling the annealing temperature of the hot-rolled sheet to 1100 ° C. or less, the generation of coarse recrystallized grains can be suppressed, thereby obtaining a good low-limit stress intensity factor.

Next, the component composition of the ferritic iron-based stainless steel hot-rolled annealed steel sheet of the present invention will be described.

Hereinafter, unless otherwise specified, the unit "%" of the content of the ingredients means "mass%".

C: 0.001 ~ 0.020%

When C is contained in an amount exceeding 0.020%, a decrease in workability and a decrease in corrosion resistance of a welded portion become significant. From the viewpoint of corrosion resistance and processability, the smaller the C content, the better, but in order to make the C content less than 0.001%, it takes time for refining, and it is not good for manufacturing. Therefore, the C content is set in the range of 0.001 to 0.020%. Regarding the lower limit, the C content is preferably 0.003% or more, and more preferably 0.004% or more. The upper limit is preferably 0.015% or less, and more preferably 0.012% or less.

Si: 0.05 ~ 1.00%

Si is an element which is condensed into an oxide film formed during welding to improve the corrosion resistance of the welded portion, and is also an element useful as a deoxidizing element in a steel making step. These effects are obtained by containing more than 0.05% of Si, and the greater the content, the greater these effects. However, if the Si content exceeds 1.00%, the increase in rolling load and significant scale formation in the hot rolling step, and the decrease in pickling properties caused by the formation of a Si-concentrated layer on the steel sheet surface during the annealing step, respectively. , Resulting in increased surface defects or manufacturing costs, so it is not good. Therefore, the Si content is set to 0.05 to 1.00%. The lower limit is preferably 0.15% or more, and more preferably 0.20% or more. The upper limit is preferably 0.60% or less, and further preferably 0.40% or less.

Mn: 0.05 ~ 1.00%

Mn has the effect of increasing the strength of steel, and also has the function of a deoxidizer. In order to obtain these effects, it is necessary to contain Mn of 0.05% or more. However, if the Mn content exceeds 1.00%, the precipitation of MnS, which is the starting point of corrosion, is promoted, and the corrosion resistance is reduced. Therefore, the Mn content is set to 0.05 to 1.00%. The lower limit is preferably 0.10% or more, and more preferably 0.20% or more. The upper limit is preferably 0.60% or less, and further preferably 0.40% or less.

P: 0.04% or less

The elements inevitably contained in the P-based steel are preferably elements that are detrimental to corrosion resistance and workability. In particular, if the P content exceeds 0.04%, the workability is significantly reduced due to solid solution strengthening. Therefore, the P content is set to 0.04% or less. The P content is preferably 0.03% or less. Furthermore, excessively reducing the P content requires excessive manufacturing costs. Therefore, considering manufacturing costs, the P content is preferably 0.01% or more.

S: 0.01% or less

S and P are elements unavoidably contained in steel as well as P are elements that are harmful to corrosion resistance and workability. Therefore, it is preferable to reduce S as much as possible. In particular, if the S content exceeds 0.01%, the corrosion resistance significantly decreases. Therefore, the S content is set to 0.01% or less. The S content is preferably 0.008% or less. More preferably, the S content is 0.003% or less. Furthermore, excessively reducing the S content requires excessive manufacturing costs. Therefore, considering manufacturing costs, the S content is preferably 0.001% or more.

Al: 0.001 ~ 0.100%

Al is an effective deoxidizer. Furthermore, the affinity of Al and nitrogen is stronger than that of Cr. Therefore, when nitrogen is invaded in the welded portion, it has the effect of suppressing sensitization by precipitating nitrogen in the form of Al nitride instead of Cr nitride. These effects are obtained by containing 0.001% or more of Al. However, if the Al content is more than 0.100%, the penetration properties during welding are reduced and the welding workability is reduced, which is not satisfactory. Therefore, the Al content is set in the range of 0.001 to 0.100%. The lower limit is preferably 0.010% or more, and more preferably 0.020% or more. The upper limit is preferably 0.080% or less, and more preferably 0.060% or less.

Cr: 10.0 ~ 19.0%

Cr is the most important element for ensuring the corrosion resistance of stainless steel. If the content is less than 10.0%, sufficient corrosion resistance cannot be obtained in an automobile exhaust gas environment. On the other hand, if the Cr content exceeds 19.0%, even if a predetermined amount of Ni is contained, a predetermined amount of Vostian iron phases will not be generated during heating in the hot rolling step, so that sufficient settlement failure effect cannot be obtained, Unable to obtain the established low limit stress intensity factor. Therefore, the Cr content is set in the range of 10.0 to 19.0%. The lower limit is preferably 10.5% or more, and more preferably 11.0% or more. The upper limit is preferably in a range of 16.5% or less, more preferably 12.5% or less, and still more preferably 11.5% or less.

Ni: 0.65 ~ 1.50%

Ni is a Vosstian iron generating element, which has the effect of increasing the amount of Vosstian iron generated during heating before rolling in the hot rolling step. In the present invention, by controlling the contents of Cr and Ni to a predetermined amount, a Wastfield iron phase is generated during heating in the hot rolling step. Due to the generation of the Vostian iron phase, the aggregation of the fertile iron phase formed during casting is destroyed. In addition, at the heating temperature before hot rolling, the metal structure becomes a two-phase structure of a ferrous iron phase + a Wastfield iron phase. In the case where the metal structure becomes a two-phase structure of the ferrous iron phase + Vostian iron phase, the heterophase interface between the ferrous iron phase existing before heating and the Vostian iron phase generated during heating functions as crystal grain growth Because of its barrier function, the metal structure is refined before hot rolling. As a result, the metal structure of the hot-rolled sheet after hot rolling and subsequent steps is refined, and a more excellent toughness improving effect can be exhibited. Furthermore, depending on the steel composition, the metal structure may become a single phase of Vosstian iron at the heating temperature before hot rolling. The same effect as described above is obtained for the destruction of the settlement caused by the generation of the Vosstian iron phase, and the coarsening of crystal grains is not easy to occur in the heating temperature region of the slab before the hot-rolling iron phase. The ferritic iron-based stainless steel based on the conventional technology becomes fine, and the toughness improvement effect by miniaturizing the crystal grains is obtained in the same manner as described above. These effects are obtained by containing 0.65% or more of Ni. When the Ni content is 0.65% or more, 35 MPa can be obtained by these effects. Low limit stress intensity factor above m ^1/2 . On the other hand, if the Ni content exceeds 1.50%, the effect of improving the low stress intensity factor is saturated and the workability is reduced. Furthermore, stress corrosion cracking is likely to occur. Therefore, the Ni content is set to 0.65 to 1.50%. The lower limit is preferably 0.70% or more, and more preferably 0.75% or more. The upper limit is preferably 1.00% or less, and more preferably the Ni content is 0.90% or less.

Ti: 0.10 ~ 0.40%

In the present invention, Ti is an extremely important element. Ti has the effects of preferentially bonding with C and N to suppress the precipitation of Cr carbonitrides, thereby lowering the recrystallization temperature, and suppressing the decrease in corrosion resistance due to sensitization caused by the precipitation of Cr carbonitrides. In order to obtain these effects, Ti must be contained in an amount of 0.10% or more. However, if the Ti content exceeds 0.40%, the amount of solid solution Ti is excessively increased, and consequently the recrystallization temperature is increased, so that the technique of the present invention cannot be applied. In addition, if Ti is contained in excess of 0.40%, coarse Ti carbonitrides are formed in the casting step and surface defects are caused, which is also not good in manufacturing. Therefore, the Ti content is set to 0.10 to 0.40%. The lower limit is preferably 0.15% or more, and more preferably 0.20% or more. Furthermore, it is more preferably 0.25% or more. The upper limit is preferably 0.35% or less, and more preferably 0.30% or less. Furthermore, from the viewpoint of corrosion resistance of the welded portion, it is preferable to satisfy the formula: Ti / (C + N) ≧ 8 (the content (mass%) of each element of Ti, C, and N in the formula) Of Ti content.

N: 0.001 ~ 0.020%

When the N content exceeds 0.020%, a decrease in workability and a decrease in corrosion resistance of the welded portion become significant. From the viewpoint of corrosion resistance, the lower the N content is, the better it is. However, reducing the N content to less than 0.001% requires long-term refining, which leads to an increase in manufacturing costs and a decrease in productivity, which is not good. Therefore, the N content is set in the range of 0.001 to 0.020%. The lower limit is preferably 0.005% or more, and more preferably 0.007% or more. The upper limit is preferably 0.015% or less, and more preferably 0.012% or less.

The present invention is a ferritic iron-based stainless steel characterized by containing the above-mentioned essential components and the remainder containing Fe and unavoidable impurities. Further, if necessary, one or two or more selected from Cu, Mo, W, and Co or / and one or more selected from V, Nb, Zr, REM, B, Mg, and Ca may be contained in the following range. Or 2 or more. In addition, when the lower limit value is within the following range, the effect of the present invention is not impaired even if the following elements are included below the lower limit value. Therefore, the following elements are included below the lower limit value. In this case, the element is an unavoidable impurity.

Cu: 0.01 ~ 1.00%

Cu is an element that is particularly effective in improving the corrosion resistance of the base material and the welded part when a weakly acidic water droplet is attached to the aqueous solution. This effect is obtained by containing 0.01% or more of Cu, and the effect is that the higher the Cu content, the higher. However, when Cu is contained in excess of 1.00%, hot workability may be reduced and surface defects may be caused. Furthermore, it may become difficult to remove the scale after annealing. Therefore, when Cu is contained, the Cu content is preferably in the range of 0.01 to 1.00%. The lower limit is more preferably 0.10% or more, and still more preferably 0.30% or more. The upper limit is more preferably 0.60% or less, and still more preferably 0.45% or less.

Mo: 0.01 ~ 2.00%

Mo is an element that significantly improves the corrosion resistance of stainless steel. This effect is obtained by containing 0.01% or more of Mo, and the more the effect content is, the higher it is. However, if the Mo content exceeds 2.00%, the rolling load at the time of hot rolling may be increased, the manufacturability may be lowered, or the strength of the steel sheet may be excessively increased. In addition, Mo is a relatively expensive element, and therefore, containing a large amount of Mo increases manufacturing costs. Therefore, when Mo is contained, the Mo content is preferably set to 0.01 to 2.00%. The lower limit is more preferably 0.10% or more, and still more preferably 0.30% or more. The upper limit is more preferably 1.40% or less, and still more preferably 0.90% or less.

W: 0.01 ~ 0.20%

W has the same effect as Mo in improving corrosion resistance. This effect is obtained by containing W of 0.01% or more. However, if W is contained in an amount exceeding 0.20%, the strength may be increased, and the manufacturability may be reduced due to an increase in rolling load. Therefore, when W is contained, the W content is preferably set in the range of 0.01 to 0.20%. The lower limit is more preferably 0.05% or more. The upper limit is more preferably 0.15% or less.

Co: 0.01 ~ 0.20%

Co is an element that improves toughness. This effect is obtained by containing Co in an amount of 0.01% or more. On the other hand, if the Co content exceeds 0.20%, the processability may be reduced. Therefore, when Co is contained, the Co content is preferably set in the range of 0.01 to 0.20%.

V: 0.01 ~ 0.20%

V forms carbonitrides with C and N, suppresses the precipitation of Cr carbonitrides, and improves the corrosion resistance of the welded portion. This effect is obtained by setting the V content to 0.01% or more. On the other hand, when the V content exceeds 0.20%, the workability and toughness may be significantly reduced. Therefore, the V content is preferably set to 0.01 to 0.20%. The lower limit is more preferably 0.02% or more. The upper limit is more preferably 0.10% or less.

Nb: 0.01 ~ 0.10%

Nb has the effect of increasing the toughness of the steel sheet by making the crystal grains finer and dissolving in the mother phase. These effects are obtained by containing Nb of 0.01% or more. On the other hand, Nb also has the effect of increasing the recrystallization temperature. If the Nb content exceeds 0.10%, the annealing temperature required to produce sufficient recrystallization by hot-rolled sheet annealing will become too high, and it will occur during annealing. When the crystal grain size is at most 300 μm or more, the recrystallized grains are significantly coarsened, so that a predetermined low limit stress intensity factor cannot be obtained. Therefore, when Nb is contained, the Nb content is preferably set in the range of 0.01 to 0.10%. The lower limit is more preferably 0.02% or more. The upper limit is more preferably 0.08% or less.

Zr: 0.01 ~ 0.20%

V forms carbonitrides with C and N, suppresses the precipitation of Cr carbonitrides, and improves the corrosion resistance of the welded portion. This effect is obtained by containing Zr of 0.01% or more. On the other hand, if the content of Zr exceeds 0.20%, the workability may be significantly reduced. Therefore, when Zr is contained, the Zr content is preferably in the range of 0.01 to 0.20%. The lower limit is more preferably 0.03% or more. The upper limit is more preferably 0.10% or less.

REM: 0.001 ~ 0.100%

Rare earth metals (REM, Rare Earth Metals) have the effect of improving the oxidation resistance, inhibiting the formation of oxide film (weld tempering color) in the welded part, inhibiting the formation of Cr-missing areas directly under the oxide film, thereby improving the corrosion resistance of the welded part Sex. This effect is obtained by containing REM of 0.001% or more. On the other hand, when the REM content exceeds 0.100%, the manufacturability such as pickling properties during cold rolling annealing may be reduced. Therefore, when REM is contained, the REM content is preferably set to the range of 0.001 to 0.100%. The lower limit is more preferably 0.005% or more. The upper limit is more preferably 0.050% or less.

B: 0.0002 ~ 0.0025%

B is an element that effectively improves the secondary processing embrittlement resistance after deep drawing. This effect is obtained by setting the content of B to 0.0002% or more. On the other hand, if the content of B exceeds 0.0025%, workability and toughness may be reduced. Therefore, when B is contained, the B content is preferably set to the range of 0.0002 to 0.0025%. The lower limit is more preferably 0.0003% or more. The upper limit is more preferably 0.0006% or less.

Mg: 0.0005 ~ 0.0030%

Mg is an element effective for increasing the equiaxed crystal ratio of the slab and improving the workability or toughness. Furthermore, in a steel containing Ti as in the present invention, if the Ti carbonitride is coarsened, the toughness is reduced, but Mg also has the effect of suppressing the coarsening of the Ti carbonitride. These effects are obtained by containing Mg of 0.0005% or more. On the other hand, if the Mg content exceeds 0.0030%, the surface properties of the steel may be deteriorated. Therefore, when Mg is contained, the content of Mg is preferably in the range of 0.0005 to 0.0030%. The lower limit is more preferably 0.0010% or more. The upper limit is more preferably 0.0020% or more.

Ca: 0.0003 ~ 0.0030%

Ca is an effective component to prevent nozzle clogging caused by crystallization of Ti-based intermediaries, which is easily generated during continuous casting. This effect is obtained by containing Ca of 0.0003% or more. However, if the Ca content exceeds 0.0030%, the corrosion resistance may decrease due to the generation of CaS. Therefore, when Ca is contained, the Ca content is preferably in the range of 0.0003 to 0.0030%. The lower limit is more preferably 0.0005% or more, and still more preferably 0.0006% or more. The upper limit is more preferably 0.0015% or less, and still more preferably 0.0010% or less.

Next, the manufacturing method of the ferritic iron-based stainless steel hot-rolled annealed steel sheet of the present invention will be described.

The ferritic iron-based stainless steel hot-rolled and annealed steel sheet of the present invention uses a slab having the above-mentioned composition in a hot rolling process including rough rolling and finishing rolling of 3 or more passes at a temperature of 800 to 1100 ° C and finally 3 passes. The final three rolling passes of finishing rolling are performed under the condition that the cumulative rolling reduction rate is 25% or more to obtain a hot-rolled steel sheet, and the hot-rolled steel sheet is annealed at 600 to 1100 ° C.

First, a known method such as a converter, an electric furnace, and a vacuum melting furnace is used to melt a molten steel containing the above-mentioned composition, and a steel material (slab) is produced by a continuous casting method or a billet-opening method.

The slab is heated at 1050 to 1250 ° C for 1 to 24 hours or is directly supplied to the hot rolling as it is without being heated. In the present invention, rough rolling is not particularly limited, but when the cast structure is effectively destroyed in advance before the finish hot rolling, it will have a good effect on the miniaturization of crystal grains in the subsequent finish hot rolling. Therefore, The cumulative rolling reduction during rough rolling is preferably 65% or more. Thereafter, rolling is performed by finishing hot rolling until it reaches a predetermined thickness, and rolling in the last three passes of finishing rolling is set to a temperature range of 800 to 1100 ° C, and the cumulative rolling reduction is set to 25% or more.

The rolling temperature range of the final 3 passes: 800 ~ 1100 ℃

Cumulative rolling reduction in the last 3 passes: more than 25%

During rough rolling before finishing rolling, the coarse cast structure was destroyed, but the crystal grains of the structure were significantly coarse. In order to obtain the predetermined low-limit stress intensity factor after hot-rolled sheet annealing, it is necessary to suppress the recovery of the strain in the rolling by properly controlling the rolling temperature and the cumulative rolling reduction rate of the final 3 passes, especially for the plate thickness. The central portion effectively imparts rolling strain.

The reason why it is necessary to effectively impart rolling strain to the center portion of the plate thickness is as follows. The rolling process is a process of reducing the thickness of the steel sheet by shearing the steel sheet. The amount of the shear strain (hereinafter referred to as rolling strain) in the rolling process becomes smaller from the surface layer toward the center of the plate thickness. Therefore, when the reduction ratio is small, a large rolling strain is given to the vicinity of the surface layer of the steel sheet, and on the other hand, the amount of the rolling strain given to the center portion of the plate thickness is small. The rolling strain becomes the recrystallization position in the subsequent step, that is, the annealing of the hot rolled sheet. However, when the rolling strain applied to the center portion of the sheet thickness is small, the recrystallization of the center portion of the sheet thickness during the annealing of the hot-rolled sheet becomes insufficient, and the metal structure of the annealed sheet of the hot-rolled sheet is in the thickness direction. It becomes non-uniform, and the established low limit stress intensity factor cannot be obtained. Therefore, in order to effectively impart a rolling strain to the center portion of the plate thickness, it is necessary to perform rolling at a specific reduction rate or more until the recovery of the rolling strain occurs.

In order to introduce a sufficient recrystallization position in the subsequent step, ie, hot-rolled sheet annealing, to obtain a predetermined metal structure, the rolling temperature of the final 3 passes must be set to a range of 800 to 1100 ° C, and the cumulative rolling of the final 3 passes must be rolled. The ratio (= 100- (final sheet thickness / thickness before the rolling of the last 3 passes) × 100 [%]) is set to 25% or more to prevent the rolling strain imparted by the final 3 passes from being restored due to recovery. In addition, a rolling strain is effectively given to the center of the plate thickness.

If the cumulative rolling reduction rate in the last 3 passes is less than 25%, rolling stress cannot be effectively imparted to the central part of the sheet thickness, so it will cause residual settlement in the hot-rolled sheet annealing in the subsequent steps, and the established lower limit cannot be obtained. Stress intensity factor. Therefore, the cumulative rolling reduction of the last three passes is set to 25% or more. The cumulative rolling reduction is preferably 30% or more. The cumulative rolling reduction is more preferably 35% or more. In addition, the upper limit of the cumulative rolling reduction rate is not particularly limited, but if the cumulative rolling reduction rate is excessively increased, the rolling load may increase, the manufacturability may decrease, and the surface may be roughened after rolling. It is 60% or less.

When the rolling temperature of the last 3 passes is set to less than 800 ° C, the rolling load increases significantly with the decrease in the temperature of the steel sheet, and therefore it is not good in manufacturing. In addition, there is a case where the surface of the steel sheet is roughened by rolling at a low temperature, and the surface quality may be deteriorated. On the other hand, if the rolling temperature of the final 3 passes exceeds 1100 ° C, recovery of the strain imparted by the rolling will occur, and the recrystallization position after the annealing of the hot-rolled sheet in the subsequent steps will be insufficient, which will result in hot rolling. After the sheet was annealed, the settlement remained, and the predetermined low limit stress intensity factor could not be obtained. Therefore, the rolling temperature of the last three passes is set to a range of 800 to 1100 ° C. The lower limit is preferably 850 ° C or higher. The upper limit is preferably 1050 ° C or lower, and more preferably 1000 ° C or lower. In addition, the rolling temperature of the last 3 passes refers to the rolling end temperature and the rolling start temperature.

Furthermore, in order to prevent an excessive rolling load from being applied to a specific one of the last three passes, it is preferable to set the rolling temperature range of the first one of the last three passes to 950 to 1100 ° C, and to perform the process after the first pass. The rolling temperature range of the second pass is set to 925 to 1075 ° C, and the rolling temperature range of the third pass performed after the second pass is set to 875 to 1050 ° C.

In addition, the method for manufacturing a ferritic-iron-based stainless steel hot-rolled annealed steel sheet according to the present invention is characterized in that a temperature range is controlled in the final three passes including three or more finish hot rolling, and on this basis, a larger rolling reduction is applied. If a larger rolling reduction is performed throughout the final 4 or more passes, even if it is the same cumulative rolling reduction rate, the rolling reduction rate is distributed to each pass, so the strain impartment to the center of the plate thickness becomes ineffective It is sufficient, and the cumulative transfer time between the lanes is increased, so it will facilitate recovery during the transfer between lanes, and reduce the effect of strain imparting. In addition, if the control of the rolling temperature and the cumulative rolling reduction rate of finishing rolling are set to two or less in the end, there will be a large rolling with cumulative rolling reduction of 25% or more in two passes, so the rolling load will increase significantly. The situation that the manufacturability is reduced is not good. Therefore, in the manufacturing method of the ferritic iron-based stainless steel hot-rolled steel sheet of the present invention, the rolling temperature and the cumulative rolling reduction rate of the last three passes of the finish rolling are controlled.

Furthermore, in the manufacturing method of the ferritic-iron-based stainless steel hot-rolled steel sheet of the present invention, it is critical to control the rolling temperature and the cumulative rolling reduction rate of the final 3 passes, as long as the finishing rolling is more than 3 passes, it can be performed arbitrarily. The rolling of the Tao. However, if the maximum number of passes is more than 15, there may be cases in which the manufacturability is reduced or the manufacturing cost is increased. For example, the temperature of the steel sheet is likely to decrease due to the increase in the number of contact with the rolling rolls. It is necessary to apply external heating within a temperature range, so the maximum number of channels is preferably 15 or less. More preferably, the maximum number of channels is 10 or less.

After finishing hot rolling, the steel sheet is cooled, and then the steel sheet is coiled to produce a hot-rolled steel strip. In the present invention, the coiling temperature is not particularly limited. When the coiling temperature is set to exceed 450 to 500 ° C, embrittlement may occur due to embrittlement at 475 ° C. Therefore, the winding temperature is preferably 450 ° C or lower or 500 ° C or higher.

Hot rolled sheet annealing temperature: 600 ~ 1100 ℃

In the present invention, the hot-rolled sheet annealing is performed after the hot-rolling step is completed. In the hot-rolled sheet annealing, the rolled working structure formed in the hot-rolling step is recrystallized. In the present invention, the rolling strain is effectively imparted in the hot rolling step to increase the recrystallization position, thereby promoting the destruction of the settlement during the annealing of the hot rolled sheet. In order to obtain this effect, it is necessary to perform hot-rolled sheet annealing in a range of 600 to 1100 ° C. If the annealing temperature is less than 600 ° C, recrystallization becomes insufficient, and a predetermined low limit stress intensity factor cannot be obtained. On the other hand, if the annealing temperature exceeds 1100 ° C., the recrystallized grains will be significantly coarsened as the maximum grain size becomes 300 μm or more, and a predetermined low limit stress intensity factor cannot be obtained. Therefore, the hot-rolled sheet annealing temperature is set to a range of 600 to 1100 ° C. The lower limit is preferably 650 ° C or higher. It is more preferably 700 ° C or higher. The upper limit is preferably 1050 ° C or lower, and more preferably 900 ° C or lower. In addition, the holding time and method of the hot-rolled sheet annealing are not particularly limited, and can be performed by any of box annealing (batch annealing) and continuous annealing.

If necessary, the obtained hot-rolled annealed steel sheet may be subjected to descaling treatment by shot blasting or pickling. Furthermore, in order to improve surface properties, grinding, polishing, etc. may be performed. In addition, the hot-rolled annealed steel sheet provided by the present invention may be cold-rolled and cold-rolled sheet-annealed afterwards.

    [Example]    

Hereinafter, the present invention will be described in detail through examples.

The stainless steel molten steel with the chemical composition shown in Table 1 was melted and melted by a 150 ton converter and a strong agitation-vacuum oxygen decarburization treatment (SS-VOD). Of billets. Except for No. 36, after heating the slab at 1150 ° C for 1 hour, reverse rough rolling using a three-segment stand as hot rolling was performed to produce a steel plate with a thickness of about 40 mm, followed by the conditions described in Table 2. The final 3 passes (the 5th pass, the 6th pass, and the 7th pass) including 7 finish rolling are made into a hot-rolled steel sheet. No. 36 series was heated at 1300 ° C for 1 hour and then supplied to hot rolling. About the obtained hot-rolled steel sheet, the hot-rolled sheet annealing using box annealing was performed under the conditions described in Table 2 to obtain a hot-rolled annealed sheet. In addition, the thickness at the end of the seventh pass is the thickness of the hot-rolled steel sheet. The obtained hot-rolled annealed steel sheet was evaluated as follows.

(1) Evaluation of low limit stress intensity factor K _IC

A compact tension (CT) compact test piece according to ASTM E399 was taken from the center of the plate width in such a manner that the fatigue pre-cracking became the rolling right angle direction and the stress axis became the rolling parallel direction. For this test piece, a low limit stress intensity factor K _{IC was} obtained in accordance with ASTM E399. Set the lower limit stress intensity factor to 35MPa. If m ^1/2 or more is set to pass, it will be less than 35MPa. m ^{1/2 is regarded} as a failure.

(2) Evaluation of corrosion resistance

A test piece of 60 × 100 mm was taken from the hot-rolled annealed steel sheet, and the surface of the surface to be evaluated was ground and finished with # 600 sandpaper. In order to eliminate the influence from the end surface and the back surface, the end surface and the back surface were sealed to produce a test The test piece was subjected to a salt water spray cycle test specified in JIS H 8502. The salt water spray cycle test system uses salt water spray (5 mass% NaCl, 35 ° C, 2hr spray) → dry (60 ° C, 4hr, relative humidity 40%) → humid (50 ° C, 2hr, relative humidity ≧ 95%) as one 5 cycles were performed. The surface of the test piece on the side to be evaluated after 5 cycles of the salt water spray cycle test was photographed, and the rust area on the surface of the test piece was measured by image analysis. The rust rate was calculated based on the ratio to the total area of the test piece. ((Rust area in test piece / total area of test piece) x 100 [%]). A rust rate of 10% or less is considered to be particularly excellent and has a good corrosion resistance (◎), a rust rate of more than 10% to 25% or less is considered to be a pass (○), and more than 25% is considered to be a fail (×).

The test results are shown in Table 2 together with the hot-rolled and hot-rolled sheet annealing conditions.

The steel composition, hot-rolled conditions, and hot-rolled sheet annealing conditions satisfy Nos. 1 to 31 and 45 to 49 of Table 2 within the scope of the present invention. The settlement was effectively destroyed by the predetermined hot-rolled and hot-rolled sheet annealing, and the result obtained was The lower limit stress intensity factor. Furthermore, the corrosion resistance of the obtained hot-rolled annealed sheet was evaluated, and as a result, it was confirmed that the rust rates were all 25%, and they also had sufficient corrosion resistance.

In particular, the rust rate was found in Nos. 7, 9, and 14 of steels A7, A9, and A14 with a Cr content of more than 17%, furthermore, steel A17 with Cu, and steel A18 with Mo, 18 When it is 10% or less, more excellent corrosion resistance is obtained.

In No. 32 in which the rolling temperature of the last 3 passes exceeded the range of the present invention, although the rolling was performed at a predetermined cumulative rolling reduction rate, the rolling temperature was too high, so the processing strain was restored and the recrystallization position occurred. The introduction is not sufficient, so the destruction effect of the settlement in the hot-rolled sheet annealing is insufficient. As a result, a large number of settlements remain after the hot-rolled sheet annealing, and the predetermined low limit stress intensity factor cannot be obtained.

In No. 33 in which the cumulative rolling reduction rate in the last 3 passes was lower than the range of the present invention, the settlement failure effect could not be fully obtained in the annealing step of the hot-rolled sheet. As a result, a large amount of residue remained in the center of the plate thickness after the annealing of the hot-rolled sheet Settle, and the established low limit stress intensity factor cannot be obtained.

In No. 34, where the annealing temperature of the hot-rolled sheet exceeds the range of the present invention, a significant coarsening of the recrystallized grains generated is generated, and as a result, a predetermined low limit stress intensity factor cannot be obtained.

No. 36 is an example in which the slab is heated at 1300 ° C for 1 hour and then supplied to hot rolling, and the rolling temperature range of the last three passes of finishing hot rolling is set to exceed 1100 ° C. In No. 36, the restoration of processing strain occurred during the final 3 passes of rolling and the introduction of the recrystallized position became insufficient. Therefore, the destruction effect of the settlement caused by the annealing of the hot-rolled sheet became insufficient, and as a result, it was impossible. Obtain a predetermined low limit stress intensity factor.

In No. 37 where the rolling temperature range of the last 3 passes was lower than the range of the present invention in 3 passes, the rolling load increased significantly. When the final rolling pass was implemented, the load exceeded the allowable range of the device, so it was impossible Rolling is completed and the evaluation cannot be performed.

In steels B1 to B4 using No. 38 to 41 in which the Ni content is lower than the range of the present invention, although the predetermined hot rolling and hot rolled sheet annealing have been performed, the production capacity of the iron phase of Vostian decreases, resulting in hot rolling. The settlement failure effect in the step is insufficient, and the predetermined low limit stress intensity factor cannot be obtained.

In the steel No. 42 using steel B5 whose Cr content exceeds the range of the present invention, although the predetermined Ni is contained, the generation ability of the Vostian iron phase is reduced due to the excessive Cr content, and as a result, the destruction effect of aggregation in the hot rolling step is caused. Insufficient to obtain the established low limit stress intensity factor.

In steel No. 43 using steel B6 whose Ti content is lower than the range of the present invention, the sensitization caused by the precipitation of Cr carbonitride during the annealing of the hot-rolled sheet, and the predetermined corrosion resistance cannot be obtained. On the other hand, in steel No. 44 using steel B7 whose Ti content exceeds the range of the present invention, the recrystallization temperature increases due to the excessive Ti content, and even if the predetermined hot-rolled sheet annealing is performed, sufficient recrystallization does not occur. Settlement remains, and as a result, the established low limit stress intensity factor cannot be obtained.

In No. 50 where the annealing temperature of the hot-rolled sheet is lower than the range of the present invention, the recrystallization is insufficient, so that the destruction effect of sufficient settlement cannot be obtained, and the predetermined low limit stress intensity factor cannot be obtained.

    (Industrial availability)    

The ferritic iron-based stainless steel hot-rolled annealed steel sheet obtained by the present invention can be particularly suitably used for applications requiring high workability and corrosion resistance, such as flanges having a deburring processing section.

Claims (4)

A hot-rolled and annealed steel sheet of ferrous iron-based stainless steel, which has a mass percentage of: C: 0.001 to 0.020%, Si: 0.05 to 1.00%, Mn: 0.05 to 1.00%, P: 0.04% or less, and S: 0.01 % Or less, Al: 0.001 to 0.100%, Cr: 10.0 to 19.0%, Ni: 0.65 to 1.50%, Ti: 0.10 to 0.40%, N: 0.001 to 0.020%, and the remainder contains Fe and unavoidable impurity composition , And the low limit stress intensity factor K _IC is 35MPa. m ^1/2 or more.     The ferritic iron-based stainless steel hot-rolled annealed steel sheet according to claim 1, wherein the above-mentioned component composition is measured in mass% and further contains a member selected from: Cu: 0.01 to 1.00%, Mo: 0.01 to 2.00%, and W: 0.01 to 0.20% Co: one or more of 0.01 to 0.20%.         For example, the ferritic iron-based stainless steel hot-rolled annealed steel sheet according to claim 1 or 2, wherein the above-mentioned component composition is based on mass% and further contains a member selected from the group consisting of: V: 0.01 to 0.20%, Nb: 0.01 to 0.10%, and Zr: 0.01 to One or more of 0.20%, REM: 0.001 to 0.100%, B: 0.0002 to 0.0025%, Mg: 0.0005 to 0.0030%, Ca: 0.0003 to 0.0030%.         A method for manufacturing a hot-rolled and annealed steel sheet of ferrous-iron-based stainless steel, which is a method for manufacturing a hot-rolled and annealed steel sheet of ferrous-iron-based stainless steel according to any one of claims 1 to 3, which comprises: A hot rolling step; and a hot-rolled sheet annealing step of hot-rolled sheet annealing of the hot-rolled steel sheet obtained by the hot-rolling step at 600 to 1100 ° C. In the above-mentioned hot-rolling step, for the final three passes of finishing rolling, The temperature was set to 800 to 1100 ° C, and the cumulative rolling reduction rate of the last three passes was set to 25% or more.