JP2012188748A - Hot rolled ferritic stainless steel sheet, method for producing the same, and method for producing ferritic stainless steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hot rolled ferritic stainless steel sheet, a method for producing the same, and a method for producing a ferritic stainless steel sheet.SOLUTION: The hot rolled ferritic stainless steel sheet has a steel composition which contains, by mass, 0.02% or less of C, 0.02% or less of N, 0.1 to 1.5% of Si, 1.5% or less of Mn, 0.035% less of P, 0.010% or less of S, 1.5% or less of Ni, 10 to 20% of Cr, 1.0 to 3.0% of Cu, 0.08% to 0.3% of Ti, 0.3% or less of Al, 0.3% or less of V and 0.0002 to 0.0030% of B with the balance comprising Fe and unavoidable impurities. The hot rolled ferritic stainless steel sheet has Vickers hardness of less than 235 Hv.

Description

  The present invention relates to a ferritic stainless steel hot-rolled steel sheet and a method for producing the same, and a method for producing a ferritic stainless steel sheet.

  Generally, stainless steel that is excellent in oxidation resistance and corrosion resistance is used as a member used in an automobile exhaust gas path. In particular, high-temperature exhaust gas exhaust from the engine is passed through exhaust system upstream components such as exhaust manifolds, catalytic converters, front pipes, etc., where the operating temperature is high. Various characteristics such as strength and heat fatigue resistance are required.

Conventionally, as described in Patent Documents 1 to 6, materials for automobile exhaust systems as described above are made of material SUS429 (14Cr-Nb steel) in which high temperature strength is increased by adding Nb, and in addition to Nb. For example, a material SUS444 (19Cr—Nb—Mo steel) to which Mo is added has been used. All materials are premised on the addition of Nb. This is to increase the high temperature strength by solid solution strengthening or precipitation strengthening with Nb or Mo.
Since SUS429 steel is a relatively low alloy stainless steel, it is excellent in workability, but its use environment was limited to a portion where the maximum temperature reached 750 ° C. or less. In addition, SUS444 steel has a high high-temperature strength that can withstand a maximum temperature of 850 ° C., but has a problem that workability is inferior to SUS429 steel.

  Therefore, in recent years, as disclosed in Patent Documents 7 and 8, as an intermediate grade material of SUS429 steel and SUS444 steel, the heat resistance, which was a problem of SUS429 steel, has been improved, and the decrease in workability has been minimized. Nb—Cu and Nb—Ti—Cu composite added steels have also been developed. The characteristics of such composite added steel are to improve the high-temperature strength by utilizing the solid solution strengthening and precipitation strengthening of Cu, while reducing the amount of Nb and Mo added compared to SUS444. It is in improving.

  Here, the precipitation strengthening of Cu as described above is manifested during use in an environment where the use temperature becomes high, such as a member for exhaust system, after processing the composite added steel. When processing into an exhaust system member or the like, Cu is generally formed into a solution (solid solution). For this reason, Cu-added steel is advantageous in workability compared to Nb-added steel in which it is difficult to completely precipitate precipitates. Mo, like Cu, is easy to be completely solutionized in the manufacturing process, but has a higher solid solution strengthening ability at room temperature than Cu and is disadvantageous in workability compared to Cu. Furthermore, since both Mo and Nb are expensive elements compared to Cu, substituting with Cu also reduces alloy costs.

Generally, ferritic stainless steel has lower toughness than ordinary steel, so after unrolling the hot-rolled coil, cold-roll the thin plate and pass it through each process such as pickling and annealing. In addition, cold cracks such as ear cracks and plate breaks may occur. Therefore, in order to ensure the toughness of the hot-rolled sheet, the hot-rolling winding conditions are optimized. In addition, in the stainless steel containing Nb and Mo, the hot-rolled sheet toughness is obtained by a precipitate having a precipitation nose of 650 to 700 ° C., for example, a Laves phase (Fe ₂ Nb, Fe ₂ Mo) or Fe ₃ Nb ₃ C. Since it falls, it is common to wind up at the temperature of 550 degrees C or less.

Further, even in steel to which 1% or more of Cu is added, a decrease in toughness due to Cu precipitates is a problem.
For example, Patent Document 9 discloses a technique for improving toughness by setting a coiling temperature to 550 ° C. or less in a non-oriented electrical steel sheet to which Cu is added. As a specific example, it is described that toughness is improved by winding at 500 ° C., 520 ° C., and 540 ° C.

On the other hand, the material of the Cu-added steel has also been studied focusing on carbon steel.
For example, Non-Patent Document 1 shows the influence of Cu on the material properties of a Ti-added ultra-low carbon steel sheet. Specifically, in a steel containing 1.3% of Cu, the coiling temperature of the hot-rolled sheet is set to R.P. T.A. It is described that when the temperature is set to (room temperature), the Rankford value (r value) becomes the highest, and the r value decreases in the order of 550 ° C. winding and 780 ° C. winding. In addition, with respect to the texture at that time, the influence of the winding temperature on the (222) orientation is not recognized, but the (211) and (200) orientations have the winding temperature of R.P. T.A. It is shown to be the lowest when

Japanese Patent No. 2880839 Japanese Patent No. 30216656 Japanese Patent No. 2959934 Japanese Patent No. 2803538 Japanese Patent No. 2696584 Japanese Patent No. 2562740 International Publication WO2003 / 004714 JP 2008-240143 A JP 2010-24509 A

Iron and Steel, No. 76 (1990), No. 5, pp 759-766

  The present inventors have developed a material that reduces the addition of expensive Nb and Mo by mainly utilizing the high temperature strength improvement by Cu addition. As a result, due to the reduction of Nb and Mo, combined precipitation of the Laves phase and Cu, which are considered to be the cause of the decrease in hot-rolled sheet toughness, is suppressed, and further, when Cu is finely precipitated, Nb and Mo are not added. Alternatively, even when added in a small amount, the heat resistance and high temperature strength can be increased.

  However, even in the production of the steel sheet to which Cu is added, the conditions of Patent Document 9 are satisfied if the conditions are the general hot rolling conditions of exhaust materials for automobiles, and the problem of toughness is Although it was thought that it did not occur, what was actually produced had low toughness, and it was difficult to pass through subsequent processes such as rolling, pickling and annealing in cold conditions. That is, the conventionally known technique cannot improve the toughness of stainless steel to which Cu is added for heat resistance.

Moreover, the problem of workability deterioration was also recognized compared with conventional steel. If the technical thinking of Non-Patent Document 1 can be applied to stainless steel as well, T.A. It was thought that the r value was improved even with stainless steel by winding at a temperature close to, but in reality, a sufficient r value could not be obtained.
That is, the conventionally known manufacturing technique for improving the workability of the Cu-added steel sheet is not sufficiently effective, and requires further improvement.

  Accordingly, the present invention has been made in view of the above circumstances, and improves the high-temperature characteristics by finely dispersing Cu precipitates, and further controls the hardness to control ferritic stainless steel hot rolled with excellent toughness. It aims at providing the manufacturing method of the ferritic stainless steel plate using the steel plate, its manufacturing method, and the said ferritic stainless steel hot-rolled steel plate.

  In order to solve the above-mentioned problems, the inventors have carried out the precipitation behavior and hardness of Cu-based precipitates at about 300 ° C. to 700 ° C. in a hot-rolled steel sheet of Cu-added ferritic stainless steel not containing a large amount of Nb and Mo. The toughness was investigated in detail. And as a result of repeating various examinations in order to achieve the said objective, the following knowledge was acquired.

As a result of the above investigation, it was found that in the case of Cu-added ferritic stainless steel, nano-order Cu-rich clusters are precipitated in the temperature range of 450 to 600 ° C., and the toughness is extremely lowered. That is, it was found that toughness can be improved by preventing the precipitation of Cu-rich clusters.
Here, there are the following two methods as means for preventing the precipitation of Cu-rich clusters.

The first method is a method in which Cu is precipitated as ε-Cu and the hardness is less than 235 Hv by setting the coiling temperature to 620 ° C. or higher. ε-Cu is basically harmless to hot rolled sheet toughness. In the process in which the Cu-based precipitate becomes ε-Cu, it is considered that a Cu-rich cluster is formed. For example, when the winding temperature is 650 ° C., the retention time is 10 minutes or longer, and 700 ° C. takes 60 seconds or longer. As a result, a substantial amount of the solid solution Cu becomes ε-Cu, and a toughness at a level that allows a subsequent process to pass through in a cold state (normal temperature) is obtained. At this time, although the hardness of the hot-rolled sheet after winding is softened to less than 235 Hv, compared to a state where Cu is completely dissolved, it is hardened by precipitation hardening due to Cu-based precipitates. The hardness becomes 200 Hv or more.
In addition, by setting the coiling temperature to 620 ° C. or higher in this way, there is little Cu precipitated in the temperature rising process in the annealing (cold rolled sheet annealing) process after cold rolling, and recrystallization having {222} plane orientation Since the texture can be sufficiently developed, it is possible to produce a steel sheet having excellent workability.

However, as a problem when the coiling temperature is set to 620 ° C. or higher, the temperature drop at the innermost winding part (top part) and the outermost winding part (bottom part) of the hot rolled coil becomes large after winding. There is. As a result, the toughness of each part in the hot-rolled coil is lowered, and there is a possibility that a difference in toughness occurs in each part in the hot-rolled coil (specifically, each of the top part, middle part, and bottom part). . And if it winds at 700 degreeC or more, since the required holding time is as short as 60 seconds, it seems that there is no problem about the temperature fall of a top part or a bottom part, but if it winds at the temperature over 750 degreeC, it is hot-rolled. Oxidation of the plate proceeds, and in the next pickling after winding, there is a problem that it takes a long time to remove the oxide scale on the surface of the hot rolled plate.
In addition, when the coil is wound at a temperature lower than 650 ° C., the above-mentioned problem of removing the oxide scale can be solved, but the temperature drop at the top and bottom portions is feared. Such a temperature drop varies depending on the hot rolling winder, the cooling method after winding, etc., so it cannot be said that it is generally a problem, but there is a difference in toughness due to the temperature drop of each part in the hot rolled coil. If there is a possibility that it will occur, for example, when water-cooling the hot-rolled steel sheet after finish rolling, cooling is controlled by appropriately adjusting the cooling conditions for the top and bottom portions of the hot-rolled coil By adjusting the temperature distribution of the hot-rolled steel sheet so that the part that becomes the top part and the bottom part becomes hotter than the part that becomes the middle part, and then take measures such as winding in such a temperature distribution state Thus, the temperature drop at the top part and the bottom part can be reduced, and the variation in toughness of each part in the hot-rolled coil can be suppressed. That is, it is effective to satisfy the following formula (1) in the temperature range of 620 to 750 ° C. over the entire length of the hot rolled coil.
T (20.24 + log (t)) ≧ 17963 (1)
T: Hot rolled steel sheet temperature (K), t: Holding time (h)
In this way, by optimizing the coiling temperature after hot rolling and further controlling the temperature history in the hot rolled coil after winding, toughness variation in the hot rolled coil is suppressed and good hot rolling is achieved. It was found that plate toughness can be obtained. Furthermore, after cold rolling annealing, it discovered that {222} plane orientation advantageous to workability developed, and discovered that workability was improved.

The second method for improving the hot-rolled sheet toughness by preventing the precipitation of Cu-rich clusters is to cool a temperature range of 800 to 500 ° C. at a rate of 10 ° C./second or more after hot rolling, Winding is performed at a temperature of 450 ° C. or lower. This is a method for obtaining good hot rolled sheet toughness by dissolving Cu in solid solution. However, when the coiling temperature is less than 350 ° C., the solid solution C and solid solution N are not sufficiently fixed as carbonitrides such as Ti and Nb, and therefore, during cold rolling annealing (cold rolled sheet annealing) , {222} plane recrystallization texture development is inhibited. As a result, the Rankford value is lowered, and the workability may be impaired. Therefore, when the toughness is improved by dissolving Cu, it is necessary to set the coiling temperature to 350 ° C. or higher and 450 ° C. or lower for compatibility with the workability of the product.
Thus, it discovered that high hot-rolled sheet toughness could be obtained by optimizing the coiling temperature after hot rolling and controlling the form of Cu-based precipitates. Furthermore, it has been found that, depending on the winding conditions, a {222} plane orientation that is advantageous for workability develops after cold rolling annealing, thereby improving workability.

  The present invention has been made based on these findings, and the gist of the present invention for solving the above problems is as follows.

(1) In mass%,
C: 0.02% or less,
N: 0.02% or less,
Si: 0.1 to 1.5%,
Mn: 1.5% or less,
P: 0.035% or less,
S: 0.010% or less,
Ni: 1.5% or less,
Cr: 10 to 20%,
Cu: 1.0-3.0%,
Ti: 0.08 to 0.30%,
Al: 0.3% or less,
Each containing
The balance has a steel composition consisting of Fe and inevitable impurities,
A ferritic stainless steel hot-rolled steel sheet having a Vickers hardness of less than 235 Hv.
(2) Furthermore, in mass%,
Nb: 0.3% or less,
Mo: 0.3% or less,
Zr: 0.3% or less,
Sn: 0.5% or less,
V: 0.3% or less,
B: 0.0002% to 0.0030%,
The ferritic stainless steel hot-rolled steel sheet according to (1) above, comprising at least one of the above.

(3) A hot-rolled steel sheet is obtained by subjecting a steel piece obtained by casting the ferritic stainless steel having the steel composition described in (1) or (2) above to hot rolling to obtain a hot-rolled steel sheet. Is rolled up at a winding temperature of 620 ° C. or higher and 750 ° C. or lower, and a method for producing a ferritic stainless steel hot-rolled steel sheet.
(4) After winding the hot-rolled steel sheet according to (3) above, the hot-rolled steel sheet temperature T (K) and the holding time t (h) so that the following formula (1) is satisfied in the entire hot-rolled coil. The method for producing a ferritic stainless steel hot-rolled steel sheet according to [3] above, wherein the hot-rolled coil is heated or cooled while controlling the temperature.
T (20.24 + log (t)) ≧ 17963 (1)
(5) For the steel slab having the steel composition described in (1) or (2) above, the average cooling rate between 850 ° C. and 450 ° C. is 10 ° C./second or more after the finish rolling of hot rolling. In addition, a method for producing a ferritic stainless steel hot-rolled steel sheet, which is wound at a coiling temperature of 350 ° C. to 450 ° C.

(6) Hot-rolled steel sheet produced by the method according to any one of (3), (4), and (5) above is hot-rolled sheet pickled, cold-rolled, cold-rolled sheet annealed, cold-rolled sheet acid A method for producing a ferritic stainless steel sheet, characterized by washing.
(7) Hot-rolled sheet steel manufactured by the method according to any one of (3), (4), and (5) above is subjected to hot-rolled sheet annealing, hot-rolled sheet pickling, cold rolling, and cold-rolled sheet annealing. A method for producing a ferritic stainless steel sheet, characterized by performing pickling of cold rolled sheets.
(8) The method for producing a ferritic stainless steel sheet according to (6) or (7) above, wherein a rolled work roll having a roll diameter of 400 mm or more is used when the cold rolling is performed.

As described above, according to the present invention, in ferritic stainless steel with excellent heat resistance to which Cu is added, the coiling temperature in hot rolling is optimized, the form of Cu-based precipitates is controlled, and the hardness is adjusted. By doing so, deterioration of toughness, which has been a conventional problem, can be prevented.
Moreover, by controlling the coiling temperature, it is possible to optimize the form of the Cu-based precipitates, and to develop a {222} plane orientation advantageous for workability after cold-rolled sheet annealing, which is a process after winding. it can. As a result, the workability of the steel sheet can be improved.
In particular, by applying the ferritic stainless steel hot-rolled steel sheet according to the present invention to an exhaust system member such as an automobile, a great effect can be obtained for environmental measures and cost reduction of parts.

It is a graph which shows the influence of the heat processing temperature which gives to the Vickers hardness of the ferritic stainless steel hot-rolled steel plate in 1st embodiment, and the absorbed energy of the Charpy impact test in 20 degreeC. The heat treatment temperature shown in FIG. 1 is a simulation of the coiling temperature. It is a graph which shows the influence of the heat processing temperature on the ductility-brittle transition temperature of the Charpy impact test of the ferritic stainless steel hot-rolled steel plate in 1st embodiment. The heat treatment temperature shown in FIG. 2 is a simulation of the coiling temperature. In the ferritic stainless steel hot-rolled steel sheet in the first embodiment, it is a diagram showing the results of observation of the precipitation state of Cu-based precipitates with a transmission electron microscope after heat treatment at various temperatures. It is a graph which shows the influence of L value which gives to the impact value of the Charpy impact test in 20 degreeC of the ferritic stainless steel hot-rolled steel plate in 1st embodiment. It is a graph which shows the influence which the heat processing temperature of the ferritic stainless steel hot-rolled steel sheet in 1st embodiment has on the Rankford value of a cold-rolled annealing board. Note that the heat treatment temperature in FIG. 5 is a simulation of the coiling temperature. It is a graph which shows the influence which the average cooling rate to 850-450 degreeC has on the impact value of the Charpy impact test in 20 degreeC when the ferritic stainless steel hot-rolled steel sheet in 2nd embodiment is wound up at 430 degreeC. is there. It is a graph which shows the relationship between the coiling temperature and the impact value of the Charpy impact test in 20 degreeC of a hot rolled coil bottom part in the ferritic stainless steel hot-rolled steel plate in 2nd embodiment. It is a graph which shows the influence which the winding temperature of the ferritic stainless steel hot-rolled steel sheet in 2nd embodiment has on the Rankford value after cold-rolled sheet annealing.

(Ferrite stainless steel hot-rolled steel sheet)
Below, the ferritic stainless steel hot-rolled steel sheet of this embodiment is demonstrated in detail.

The ferritic stainless steel hot-rolled steel sheet of this embodiment is in mass%, C: 0.02% or less, N: 0.02% or less, Si: 0.1-1.5%, Mn: 1.5% Hereinafter, P: 0.035% or less, S: 0.010% or less, Ni: 1.5% or less, Cr: 10-20%, Cu: 1.0-3.0%, Ti: 0.08- Each steel contains 0.30% and Al: 0.3% or less, and has a steel composition composed of the balance Fe and inevitable impurities, and has a Vickers hardness of less than 235 Hv.
Hereinafter, the reason which limited the steel composition of the ferritic stainless steel hot-rolled steel sheet of this embodiment is demonstrated. In addition, the description of% about a composition means the mass% unless there is particular notice.

C: 0.02% or less Since C deteriorates formability, corrosion resistance, and hot-rolled sheet toughness, the lower the content, the more preferable, so the upper limit is made 0.02%. However, excessive reduction leads to an increase in refining costs, and from the viewpoint of corrosion resistance, it is desirable to be 0.001% to 0.009%.

N: 0.02% or less N, like C, deteriorates formability, corrosion resistance, and hot-rolled sheet toughness. Therefore, the smaller the content, the more preferable, so 0.02% or less. However, excessive reduction leads to an increase in refining cost, so 0.003% to 0.015% is desirable.

Si: 0.1% to 1.5%
Si is an element that is also useful as a deoxidizer, and is an element that improves high-temperature strength and oxidation resistance. The high-temperature strength up to about 800 ° C. increases with an increase in the amount of Si, and the effect is manifested at 0.1% or more, so the lower limit is made 0.1%. However, excessive addition reduces room temperature ductility, so the upper limit is made 1.5%. In consideration of oxidation resistance, 0.2% to 1.0% is desirable.

Mn: 1.5% or less Mn is an element added as a deoxidizer and an element contributing to an increase in high-temperature strength in the middle temperature range. In addition, Mn-based oxides are formed on the surface layer during long-time use, and are elements that contribute to the adhesion of scale (oxide) and the effect of suppressing abnormal oxidation.
On the other hand, excessive addition causes a decrease in hot-rolled sheet toughness due to precipitation of γ phase (austenite phase) and also forms MnS to reduce corrosion resistance, so the upper limit is made 1.5%. In consideration of high temperature ductility, scale adhesion, and suppression of abnormal oxidation, 0.1 to 1.0% is desirable.

P: 0.035% or less P is an element having a large solid solution strengthening ability, but it is a ferrite stabilizing element and is also an element harmful to corrosion resistance and toughness.
P is contained as an impurity in ferrochrome, which is a raw material of stainless steel. However, it is very difficult to remove P from molten stainless steel, so 0.010% or more is preferable. The P content is almost determined by the purity and amount of the ferrochrome raw material to be used. However, since P is a harmful element, it is preferable that the purity of P in the ferrochrome raw material is low. However, since low P ferrochrome is expensive, it is 0.035% or less, which is a range in which the material and corrosion resistance are not greatly deteriorated. . In addition, Preferably it is 0.030% or less.

S: 0.010% or less S forms sulfide inclusions and degrades the general corrosion resistance (entire corrosion and pitting corrosion) of steel materials. Therefore, the upper limit of the content is preferably small. %. Further, the smaller the S content, the better the corrosion resistance. However, since the desulfurization load increases and the production cost increases for lowering the S content, the lower limit is preferably made 0.001%. In addition, Preferably it is 0.001-0.008%.

Ni: 1.5% or less Ni is mixed as an inevitable impurity in the ferritic stainless steel alloy raw material and is generally contained in the range of 0.03 to 0.10%. Moreover, it is an element effective in suppressing the progress of pitting corrosion, and the effect is stably exhibited by addition of 0.05% or more, so the lower limit is preferably made 0.01%.
On the other hand, since a large amount of addition may cause material hardening due to solid solution strengthening, the upper limit is made 1.5%. In consideration of the alloy cost, 0.05 to 1.0% is desirable.

Cr: 10-20%
In the present invention, Cr is an essential element for ensuring oxidation resistance and corrosion resistance. If it is less than 10%, these effects are not exhibited. On the other hand, if it exceeds 20%, the workability is deteriorated and the toughness is deteriorated. In consideration of manufacturability and high temperature ductility, 10% to 18% is desirable.

Cu: 1.0 to 3.0%
Cu is an element necessary for increasing the high-temperature strength required for use as a member for a high-temperature environment typified by a high-temperature exhaust system of an automobile. Cu mainly exhibits precipitation strengthening ability at 500 to 750 ° C., and at higher temperatures, suppresses plastic deformation of the material by solid solution strengthening and exhibits a function of improving thermal fatigue characteristics. Such an effect is a precipitation hardening action due to the formation of Cu precipitates, and is manifested by addition of 1.0% or more. On the other hand, excessive addition causes a decrease in high-temperature strength, so the upper limit is made 3.0%. In addition, 1.0% to 1.5% is desirable in consideration of solid solution of Cu at the time of cold rolling annealing to suppress a decrease in workability.

Ti: 0.08% to 0.30%
Ti is an element that combines with C, N, and S to improve corrosion resistance, intergranular corrosion resistance, room temperature ductility and deep drawability. Since the amount of Ti is determined from the amount of C, N, and S that can be economically reduced, the lower limit is set to 0.08%. However, excessive addition of Ti increases the surface defects of the slab due to TiN crystallized in the molten steel during continuous casting, so the upper limit is made 0.30%. In addition, since the corrosion-resistance improvement effect by solid solution Ti and the hot-rolled sheet toughness and press workability fall by large precipitate TiN may arise, it is desirable to set it as 0.10%-0.18%.

Al: 0.3% or less In addition to being added as a deoxidizing element, Al is an element that improves oxidation resistance. Moreover, it is useful for the strength improvement in 600-700 degreeC as a solid solution strengthening element. Since the action is stably expressed from 0.01%, the lower limit is preferably set to 0.01%.
On the other hand, excessive addition hardens and significantly reduces the uniform elongation, and also significantly reduces the toughness, so the upper limit is made 0.3%. Furthermore, if generation of surface defects, weldability, and manufacturability are taken into consideration, 0.01% to 0.07% is desirable.

In this embodiment, in addition to the above elements, V: 0.3% or less, B: 0.0002% to 0.0030%, Nb: 0.3% or less, Mo: 0.3% or less, Zr : One or more of 0.3% or less and Sn: 0.5% or less are preferably added.

V: 0.3% or less V forms fine carbonitride and has an effect of causing precipitation strengthening action and contributing to improvement of high-temperature strength. Therefore, V is added as necessary. Since the effect is stably manifested by addition of 0.03% or more, the lower limit is preferably 0.03%.
On the other hand, if added excessively, the precipitates may be coarsened. As a result, the hot-rolled sheet toughness decreases, so the upper limit is made 0.3%. In view of manufacturing cost and manufacturability, it is desirable that the content be 0.03% to 0.1%.

B: 0.0002% to 0.0030%
B is an element that improves the secondary workability during the press working of the product, and also has the effect of improving the high-temperature strength of the Cu-added steel, so is added as necessary. The effect is manifested at 0.0002% or more. However, excessive addition may result in precipitation of Cr ₂ B, (Cr, Fe) ₂₃ (C, B) ₆ , which may impair toughness and corrosion resistance and may also impair weldability. .0002% to 0.0030%. In view of workability and manufacturing cost, 0.0003% to 0.0015% is desirable.

Nb may be added as necessary in order to improve the high temperature strength and thermal fatigue characteristics. In order to exert these effects, the lower limit is preferably made 0.01%.
On the other hand, excessive addition causes the generation of a Laves phase, and as a result, suppresses the precipitation strengthening ability due to Cu precipitation, which is undesirable. In addition, when hot rolling at 630 ° C. or higher is performed by hot rolling, there is a risk that hot rolled sheet toughness is reduced due to the Laves phase. Considering these, the upper limit of Nb is set to 0.3%. Furthermore, from the viewpoint of productivity and manufacturability, 0.01% to 0.2% is desirable.

Mo may be added as necessary in order to improve the high temperature strength and thermal fatigue characteristics. In order to exhibit these effects, the lower limit is preferably made 0.01%.
On the other hand, excessive addition is not desirable because, like Nb, it generates a Laves phase and suppresses the precipitation strengthening ability due to Cu precipitation. In addition, when high temperature winding at 630 ° C. or higher is performed by hot rolling, there is a risk that hot rolled sheet toughness is reduced due to the Laves phase. Considering these, the upper limit of Mo is set to 0.3%. Furthermore, from the viewpoint of productivity and manufacturability, 0.01% to 0.2% is desirable.

Zr, like Ti and Nb, is a carbonitride-forming element and contributes to improving high-temperature strength and oxidation resistance by increasing the amount of solute Ti and Nb. Therefore, Zr may be added as necessary. . Since these effects are stably exhibited by addition of 0.05% or more, the lower limit is preferably set to 0.1%.
However, excessive addition causes significant deterioration in manufacturability, so the upper limit is made 0.3%. In view of cost and surface quality, 0.1% to 0.2% is more desirable.

Sn, like Mo, is an element effective for improving corrosion resistance and high-temperature strength. Moreover, since there exists an effect which does not deteriorate a mechanical characteristic of normal temperature largely, you may add as needed. The contribution to the high-temperature strength is stable when added at 0.05% or more, so the lower limit is preferably 0.05%.
On the other hand, if added excessively, manufacturability and weldability deteriorate significantly, so the upper limit is made 0.5%. In consideration of oxidation resistance and the like, 0.1% to 0.3% is desirable.

(Method for producing ferritic stainless steel hot-rolled steel sheet (first embodiment))
Next, the manufacturing method of the ferritic stainless steel hot-rolled steel sheet in this embodiment is demonstrated.
The method for producing a ferritic stainless steel hot-rolled steel sheet according to the first embodiment is to produce a ferritic stainless steel having the steel composition described above, and after the steel making, hot rolling is performed on the cast steel piece (slab). After finishing rolling to obtain a hot-rolled steel sheet, the hot-rolled steel sheet is wound at a coiling temperature of 620 ° C. or higher and 750 ° C. or lower.

In steelmaking according to the present embodiment, a method of melting the steel containing the essential components and components added as necessary in a converter and subsequently performing secondary refining is preferable.
Next, the molten steel is made into a slab according to a known casting method (continuous casting). Then, the slab is heated to a predetermined temperature, and then hot-rolled to a predetermined plate thickness so that the slab is a hot-rolled steel sheet (hot-rolled sheet). In addition, the finish rolling finish temperature (finishing temperature) of hot rolling shall be in the range of 800 degreeC-980 degreeC.
Next, after the finish rolling, the hot-rolled steel sheet is cooled and wound into a coil to form a hot-rolled coil.
Here, after finish rolling, the temperature at which the hot-rolled steel sheet is wound in a coil shape (winding temperature) greatly affects the hot-rolled sheet toughness.
The reason for limiting the coiling temperature in the present embodiment will be described below.

In this embodiment, the coiling temperature is 620 to 750 ° C.
By winding within such a winding temperature range, Cu can be precipitated as ε-Cu, and the hardness of the hot-rolled steel sheet after winding can be made less than 235 Hv.
The deposited ε-Cu is basically harmless to hot rolled sheet toughness as described above. Further, in the process where the Cu-based precipitate becomes ε-Cu, it is considered that a Cu-rich cluster is formed. However, after winding, solid solution Cu is obtained by keeping heat for a predetermined time according to the winding temperature. Can be deposited as ε-Cu. As a result, it is possible to obtain the toughness of a hot-rolled sheet that can be passed through a subsequent process at room temperature (cold). In addition, after winding a hot-rolled steel plate into a hot-rolled coil, the time for keeping the hot-rolled coil is referred to as a holding time t.
In addition, by winding within such a winding temperature range, less Cu precipitates in the temperature rising process in the subsequent cold-rolled sheet annealing, and the recrystallized texture having {222} plane orientation is well developed. It is possible to produce a cold-rolled steel sheet having excellent workability.
However, if it winds below 620 degreeC, the temperature drop of the top part or bottom part of the hot-rolled coil after winding will become large, and there exists a possibility that sufficient holding time t cannot be ensured. If the holding time t cannot be ensured in this way, ε-Cu cannot be sufficiently precipitated, so that the toughness is lowered at each of the top part and the bottom part, and the toughness is reduced at each part in the hot rolled coil. There may be a difference.
Moreover, if it winds above 750 degreeC, the oxidation of a hot-rolled coil will advance and it will take a long time in order to remove the oxidation scale on the surface of a hot-rolled steel sheet in the hot-rolled sheet pickling which is the next process. Therefore, in this embodiment, the coiling temperature is set to 620 to 750 ° C.

Moreover, in this embodiment, after making a hot-rolled steel sheet into a wound hot-rolled coil, the hot-rolled steel sheet temperature T (K) and the holding time t are set so that the following formula (1) is satisfied in the entire hot-rolled coil length. It is preferable to heat-hold or cool the hot-rolled coil while controlling (h). In this way, by controlling the temperature history over the entire length of the hot-rolled coil so as to satisfy the following formula (1), it is possible to prevent variation in toughness in each part in the hot-rolled coil, and a good hot-rolled sheet Toughness can be obtained.
T (20.24 + log (t)) ≧ 17963 (1)
Hereinafter, Formula (1) will be described. Note that T (20.24 + log (t)) in the above equation (1) is referred to as an L value.

  In general, in the cooling step after the hot-rolled steel sheet is wound into a hot-rolled coil, the cooling rate of the top portion and the bottom portion of the hot-rolled coil is increased. For this reason, the temperature drop at the top and bottom portions in the hot-rolled coil is larger than that in the middle portion, and the toughness of the top and bottom portions deteriorates, and the toughness of each part in the hot-rolled coil may vary. There is. Furthermore, the temperature drop of the top part and the bottom part in such a hot-rolled coil becomes more concerned as the coiling temperature becomes lower. However, such a temperature drop varies depending on the hot rolling winder used, the method of cooling the hot rolled coil after winding, and the like. Therefore, it cannot be said that it becomes a general problem, but when the deterioration of toughness due to the temperature drop in the hot rolled coil becomes a problem, the temperature history over the entire length of the hot rolled coil is in the temperature range of 620 to 750 ° C. It is preferable to control the L value so as to satisfy (1). That is, the temperature (hot-rolled steel sheet temperature T) at each part of the hot-rolled coil after winding is controlled, and further, the holding time t under the hot-rolled steel sheet temperature T is adjusted at each part, while maintaining the hot-rolled coil. Heating or cooling is preferably performed.

Here, the method of controlling the L value is not particularly limited, and can be appropriately selected from commonly used methods and conditions. For example, when cooling a hot-rolled steel sheet after finish rolling to the above winding temperature range by water injection, cooling is controlled by appropriately adjusting the cooling conditions for the top and bottom portions of the hot-rolled coil. To do. Thereby, the temperature distribution of the hot-rolled steel sheet before winding is adjusted so that the site | part used as a top part and a bottom part becomes high temperature rather than the site | part used as a middle part. Thereafter, the hot-rolled steel sheet having such a temperature distribution state is taken up as a hot-rolled coil. In other words, in the cooling process after making the hot rolled coil, even if the temperature of the top part or the bottom part has dropped, it is controlled to be higher than the middle part within the winding temperature range, The holding time t can be secured, and the above formula (1) can be satisfied over the entire length of the hot-rolled coil.
Hereinafter, investigation results for explaining in detail such a winding temperature and the reason for limitation of the above formula (1) are shown. In addition, the evaluation method of hot-rolled sheet toughness demonstrated below makes a sample number three, performs a Charpy impact test at 20 degreeC, and calculates | requires absorbed energy. And it evaluated by the minimum value of the obtained result.

In FIG. 1, the ferritic stainless steel according to the present embodiment was hot rolled to a plate thickness of 5 mm at a finishing temperature of 850 ° C. to obtain a hot rolled sheet. Thereafter, the average cooling rate up to 400 ° C. was set to 100 ° C./second, and cooling was performed by water cooling, and thereafter cooling by air cooling.
Next, in order to investigate the influence of the winding temperature at the time of winding after hot rolling using the obtained hot rolled sheet, in order to reproduce the temperature history at the time of winding, 1 hour at various temperatures. The heat treatment was performed.
Next, while measuring the Vickers hardness of the hot-rolled sheet after heat treatment (heat-treated sheet), three samples were taken from the hot-rolled sheet as the Charpy impact test piece (sub-size as it is). A Charpy impact test was conducted at 20 ° C. to evaluate hot rolled sheet toughness. In addition, the minimum value of the absorbed energy at various temperatures is shown in FIG.
As is apparent from FIG. 1, when the heat treatment temperature is between 450 ° C. and 600 ° C., the hardness of the hot-rolled sheet rapidly increases to 235 Hv or more, while the toughness greatly decreases. This is presumably because Cu-rich clusters were precipitated. However, it can be seen that when the heat treatment temperature is 620 ° C. or higher, the hardness is softened to less than 235 Hv, the absorbed energy increases rapidly, and the toughness increases greatly.
The steel composition of the ferritic stainless steel used to investigate the relationship shown in FIG. 1 is 14% Cr-0.5% Si-0.5% Mn-0.005% C-0.010% N- 0.15% Ti-1.2% Cu-0.0005% B.

In FIG. 2, the result of having performed the Charpy impact test in the range of -40 degreeC-140 degreeC of the heat processing board manufactured by the method similar to the case of FIG. 1 is shown in FIG.
As is apparent from FIG. 2, it can be seen that those subjected to heat treatment at 450 to 550 ° C. have increased the ductile-brittle transition temperature to nearly 100 ° C. On the other hand, those heat-treated at 650 ° C. and 700 ° C. have a ductile-brittle transition temperature of 20 ° C. or lower, indicating that the toughness is equal to or higher than that of the unheated hot-rolled sheet.
Note that the steel components of the ferritic stainless steel used to investigate the relationship shown in FIG. 2 were 14% Cr-0.9% Si-0.5% Mn-0.005% C-0.010% N- 0.15% Ti-1.5% Cu-0.0005% B.

In order to clarify the cause of the large change in the toughness of the hot-rolled sheet at the heat treatment temperature as shown in FIG. 2, Cu precipitates in the heat treated material shown in FIG. 2 were observed with a transmission electron microscope. In addition, the heat processing material observed has three types, an unheated hot-rolled sheet (as Hot material), 550 degreeC heat processing material, and 700 degreeC heat processing material. The observation results are shown in FIGS. 3A shows an as Hot material, FIG. 3B shows a 550 ° C. heat-treated material, and FIG. 3C shows a 700 ° C. heat-treated material.
As is clear from FIG. 3A, Cu precipitates are not observed on the unheated hot-rolled sheet. On the other hand, in the 550 ° C. heat treatment material shown in FIG. 3B, it can be confirmed that fine Cu having a size of several nm is deposited. It can be seen that this fine Cu is considered to be a Cu-rich cluster, is relatively large on dislocations, and is more finely precipitated elsewhere. Moreover, in the 700 degreeC heat processing material shown in FIG.3 (c), it can observe that (epsilon) -Cu has precipitated and the observed size was 30-100 nm.
Although the cause of the decrease in toughness due to the Cu-rich cluster is not clear, the uniform elongation was about 10% when the tensile test was carried out, so that it was considered that brittle fracture occurred due to poor ductility at room temperature. In this case, since the precipitates are very finely dispersed, it is presumed that the high-speed movement of dislocations is inhibited and brittle fracture occurs.

In FIG. 4, a hot-rolled sheet manufactured by the same method as in FIG. 1 was rapidly heated to 620-750 ° C. using a salt bath, heat-treated for various times, and then cooled by water cooling. Thereafter, hot rolled sheet toughness was investigated. The heating temperature and the heat treatment time are arranged by L value (T (20.24 + log (t))) and shown in FIG. Even if it heat-processes at 620-750 degreeC, it turns out that toughness falls in a short time. From this result, in this embodiment, after winding the hot-rolled plate, it is preferable to heat-hold or cool the hot-rolled plate so as to satisfy the above formula (3) over the entire length of the coil.
The steel composition of the ferritic stainless steel used to investigate the relationship shown in FIG. 4 is 14% Cr-0.5% Si-0.3% Mn-0.005% C-0.010% N- 0.15% Ti-1.2% Cu-0.0005% B.

Here, in this embodiment, the reason why the temperature history of the hot-rolled coil after winding is defined by the L value will be described.
Precipitation of ε-Cu in the steel sheet proceeds in a shorter time in a higher temperature range in the temperature range of 620 to 750 ° C. near the precipitation nose of Cu. In addition, since the precipitation phenomenon is an atomic diffusion law, it is arranged by the product of the logarithm of the steel sheet temperature and the retention time. Therefore, when the test results in FIG. 4 were organized by L value, it was found that good hot-rolled sheet toughness was obtained under conditions where the L value was 17963 or more. Thus, in this embodiment, the lower limit of the L value is 17963. In view of the difficulty of operation management, it is more preferable to set the L value to 18240 or more.

Further, in FIG. 5, a hot-rolled sheet manufactured by the same method as in FIG. 1 is heat-treated at 400 to 750 ° C. for 1 hour, then air-cooled, recrystallization annealing is omitted, and a thickness of 5.0 mm to 2 mm. Cold-rolled to 0.0 mm and annealed in the range of 880 to 920 ° C. In addition, the average temperature increase rate in cold-rolled sheet annealing was 4 ° C./s. FIG. 5 shows the relationship between the Rankford value (r value) measured using the obtained cold-rolled annealed plate and the heat treatment temperature applied to the hot-rolled plate. Note that the heat treatment temperature is used to reproduce the winding temperature in the present embodiment.
As can be seen from FIG. 5, it can be seen that the Rankford value increases in the temperature range of 620 to 750 ° C. and the highest value at 700 ° C. That is, it turned out that the workability of a cold-rolled sheet improves by setting a coiling temperature to 620-750 degreeC.

Moreover, in the production of the ferritic stainless steel hot-rolled steel sheet of the present embodiment, the hot-rolled sheet annealing usually performed after hot rolling may be performed, but it is preferable not to perform from the viewpoint of productivity improvement. . Since normal Nb-added steel is hot-rolled steel sheet, it is subjected to hot-rolled sheet annealing before cold rolling, but the steel sheet according to this embodiment does not add Nb or is added in a small amount. Further, annealing of the hot-rolled steel sheet can be omitted, and the manufacturing cost can be reduced.
Moreover, by omitting the annealing of the hot rolled sheet, it is possible to maintain and precipitate the ε-Cu deposited at the time of winding during the cold rolling and the temperature rising process during the cold rolled sheet annealing. It becomes possible. For this reason, the texture after cold rolling and cold-rolled sheet annealing develops, and press formability can be improved by improving the r value and reducing the anisotropy.

Moreover, when performing the cold rolling which is a post process of the manufacturing method of the ferritic stainless steel hot-rolled steel sheet in this embodiment, it is preferable to use the rolling work roll whose roll diameter is 400 mm or more.
Here, the cold rolling of the stainless steel plate is usually reverse-rolled by a Sendzimir rolling mill having a work roll diameter (roll diameter) of about 60 to 100 mm, or by a tandem rolling mill having a work roll diameter of 400 mm or more. Either one-way rolled. In addition, all are rolled by multiple passes.

In this embodiment, in order to increase the r value that is an index of workability, it is preferable to perform cold rolling with a tandem rolling mill having a roll diameter of 400 mm or more. For example, when a small roll having a roll diameter of 100 mm or less is used, a large amount of shear strain is introduced in the vicinity of the steel sheet surface layer during cold rolling, and {222} and {554} during cold rolling annealing (recrystallization annealing) in the next process The development of crystal orientation is suppressed, and it is difficult to improve the r value. However, by cold rolling with a large diameter roll, the crystal orientation is remarkably developed by suppressing shear strain, and the r value can be further improved. Tandem rolling is unidirectional rolling, and has fewer rolling passes than Sendzimir rolling, and is excellent in productivity.
If the rolling reduction in the cold rolling process is low, a recrystallized structure cannot be obtained after cold-rolled sheet annealing, or the mechanical properties deteriorate due to excessive coarsening, so the rolling reduction in the cold rolling process. Is preferably 50% or more.

In the present embodiment, the other manufacturing steps are not particularly defined, but the thickness of the hot-rolled plate, the cold-rolled plate annealing temperature, the cold-rolled plate annealing atmosphere, and the like may be appropriately selected. As preferable conditions, the thickness of the hot rolled sheet is 3.0 to 5.0 mm, the cold rolled sheet annealing temperature is 860 to 960 ° C., and the cold rolled sheet annealing atmosphere is a combustion gas atmosphere, Alternatively, a mixed atmosphere of hydrogen and nitrogen is desirable. Moreover, you may give temper rolling and a tension leveler after cold rolling and cold-rolled sheet annealing. Further, the product (cold rolled steel sheet) thickness may be selected according to the required member thickness.
In addition, since Nb additive-free or content is low in this invention, the cold-rolled sheet annealing temperature after cold rolling can be made into 850-970 degreeC low temperature. However, in the cooling process, it is desirable to cool at a cooling rate of 10 ° C./s or more in order to prevent hardening due to precipitation of Cu-rich clusters.
As described above, according to the ferritic stainless steel hot-rolled steel sheet according to the present invention, since Cu is precipitated as ε-Cu, the hardness of the steel sheet can be made less than 235 Hv. As a result, it is possible to obtain the toughness of a hot-rolled sheet that can be passed through a subsequent process at room temperature (cold).

According to the method for producing a ferritic stainless steel hot-rolled steel sheet according to the present invention, by optimizing the coiling temperature in hot rolling, controlling the form of Cu-based precipitates, and adjusting the hardness, It is possible to prevent deterioration of toughness.
In addition, by controlling the temperature history of the entire hot-rolled steel sheet after winding, variation in toughness can be suppressed inside the coil after winding of the hot-rolled steel sheet, and as a result, good hot-rolled sheet toughness Can be secured.

  Further, by controlling the coiling temperature and the temperature history after coiling, the form of the Cu-based precipitate can be optimized, and after the cold-rolled sheet annealing, which is a process after coiling, is advantageous for workability {222 } The plane orientation can be developed. As a result, the workability of the steel sheet can be improved.

  Moreover, since the ferritic stainless steel hot-rolled steel sheet according to the present invention replaces expensive alloy elements such as Nb and Mo with Cu, when applied to exhaust system members such as automobiles, A great effect can be obtained for cost reduction of parts.

(Method for producing ferritic stainless steel hot-rolled steel sheet (second embodiment))
Next, the manufacturing method of the ferritic stainless steel hot-rolled steel sheet which is 2nd embodiment of this invention is demonstrated.
The method for producing a ferritic stainless steel hot-rolled steel sheet according to the present embodiment is made of ferritic stainless steel having the above steel composition, and after steel making, hot rolled finish rolling of the cast steel slab (slab). Then, while the average cooling rate between 850 degreeC-450 degreeC shall be 10 degrees C / second or more, the coiling temperature is 350 degreeC-450 degreeC, and the hot rolling process wound up is performed.
In addition, although the manufacturing method of this embodiment has a difference in the cooling conditions after finish rolling in the manufacturing method of the first embodiment, and the coiling temperature, even if the manufacturing method of both embodiments is adopted, The effects as described above can be achieved.

In steelmaking according to the present embodiment, a method of melting the steel containing the essential components and components added as necessary in a converter and subsequently performing secondary refining is preferable.
Next, the molten steel is made into a slab according to a known casting method (continuous casting). Then, this slab is heated to a predetermined temperature and hot-rolled to a predetermined plate thickness, and the slab is used as a hot-rolled steel sheet (hot-rolled sheet). In addition, the finish rolling finish temperature (finishing temperature) of hot rolling shall be in the range of 800 degreeC-980 degreeC.
Next, after finish rolling, the hot-rolled steel sheet is cooled by water cooling and wound into a coil shape.
Here, the cooling conditions after finish rolling, and the temperature at which the hot-rolled steel sheet is subsequently wound (winding temperature) greatly affect the hot-rolled sheet toughness.
Below, the cooling conditions in this embodiment and the reason for limitation of coiling temperature are demonstrated.

First, the reasons for limiting the cooling conditions will be described.
In the present embodiment, after finish rolling, the average cooling rate between 850 ° C. and 450 ° C. is set to 10 ° C./second or more.
As described above, according to the investigation by the present inventors, in the case of Cu-added ferritic stainless steel, a nano-order Cu-rich cluster in the temperature range of ~ 450 ° C (particularly 600 ° C to 450 ° C) after finish rolling. It was found that the toughness was extremely lowered. That is, by increasing the cooling rate in such a temperature range, it is possible to prevent the precipitation of Cu-rich clusters. Since such an effect is stably exhibited at an average cooling rate of 10 ° C./second or more, the average cooling rate between 850 ° C. and 450 ° C. is set to 10 ° C./second or more after finish rolling. In consideration of improvement in toughness, it is preferably 20 ° C./second or more.

Next, the reason for limiting the coiling temperature will be described.
In this embodiment, the coiling temperature is set to 350 ° C to 450 ° C.
If the coiling temperature is too low, solid solution C and solid solution N are not sufficiently fixed as carbonitrides such as Ti and Nb, so that the recrystallized texture development of the {222} plane occurs during cold-rolled sheet annealing. Will be disturbed. As a result, workability may be deteriorated. On the other hand, when the coiling temperature is too high, Cu-rich clusters are precipitated, and the hot-rolled sheet toughness may be lowered. Therefore, in order to achieve both improvement of workability and hot rolled sheet toughness, the winding temperature is set to 350 ° C. to 450 ° C. in the present embodiment. In consideration of temperature variation in each part in the coil, the winding temperature is preferably set to 380 ° C. to 430 ° C. to improve toughness.
Below, the investigation result for explaining in detail the reasons for limiting such cooling conditions and coiling temperature is shown. The hot rolled sheet toughness evaluation method described below uses three samples as in the first embodiment, and performs a Charpy impact test at 20 ° C. to obtain the absorbed energy. And it evaluated by the minimum value of the obtained result.

As described in the first embodiment, as is clear from FIG. 1, it can be seen that when the heat treatment temperature is higher than 450 ° C. to 600 ° C., the hardness increases while the toughness greatly decreases. This is presumably because Cu-rich clusters were precipitated.
The steel composition of the ferritic stainless steel used to investigate the relationship shown in FIG. 1 is 14% Cr-0.5% Si-0.5% Mn-0.005% C-0.010% N- 0.15% Ti-1.2% Cu-0.0005% B.

Next, in FIG. 6, the ferritic stainless steel according to the present embodiment was hot-rolled at a finishing temperature of 850 ° C. to a plate thickness of 5 mm. Then, it cooled by either furnace cooling, air cooling, air-water cooling, or water cooling, changing the average cooling rate to 450 degreeC, and after cooling, it wound up at 430 degreeC and was set as the hot rolled coil. The result of evaluating the hot-rolled sheet toughness after winding at 20 ° C. is shown in FIG.
As is clear from FIG. 6, the impact value increased as the average cooling rate increased. In addition, when the average cooling rate was 10 ° C./s or more, the impact value exceeded 20 J / cm ² , and it was determined that it was possible to pass through in subsequent processes such as cold rolling at normal temperature and pickling treatment.
This is considered to be because when the average cooling rate is less than 10 ° C./s, Cu-rich clusters are precipitated and hardened in the cooling process.
The steel composition of the ferritic stainless steel used to investigate the relationship shown in FIG. 6 is 17% Cr-0.1% Si-0.2% Mn-0.005% C-0.010% N- 0.15% Ti-1.2% Cu-0.0005% B.

In FIG. 7, the ferritic stainless steel according to the present embodiment was hot-rolled to a plate thickness of 5 mm at a finishing temperature of 850 ° C. Subsequently, after winding up by changing winding temperature from 300 degreeC-800 degreeC, the sample was extract | collected from the bottom part of the obtained hot-rolled coil, and the result of having evaluated hot-rolled sheet toughness is shown in FIG.
As is apparent from FIG. 7, it can be seen that the impact value at the bottom portion is less than 20 J / cm ² when the coiling temperature is 500 ° C. to 700 ° C.
As in the graph shown in FIG. 1, it is considered that when the coiling temperature is in the range of 500 ° C. to 700 ° C., Cu-rich clusters are precipitated at the bottom portion, and thus the toughness is lowered. In such a case, the temperature history over the entire length of the hot-rolled coil is controlled so as to satisfy the above formula (1), thereby eliminating the variation in toughness at each part in the hot-rolled coil. Is possible.
Moreover, the steel composition of the ferritic stainless steel used to investigate the relationship shown in FIG. 7 is 14% Cr-0.9% Si-0.5% Mn-0.005% C-0.010% N- 0.15% Ti-1.2% Cu-0.0005% B.

In FIG. 8, the ferritic stainless steel according to the present embodiment was hot-rolled to a plate thickness of 5 mm with a finishing temperature of 830 ° C. Thereafter, the winding temperature was changed from 30 ° C. to 550 ° C., and winding was performed.
Next, after removing the scale of the hot-rolled coil by pickling, it was rolled from a plate thickness of 5 mm to a plate thickness of 2 mm by cold rolling, and then cold-rolled plate annealed at 900 ° C. In addition, the average temperature increase rate in cold-rolled sheet annealing was performed at 7 ° C./s. FIG. 8 shows the relationship between the Rankford value measured using the obtained cold-rolled sheet and the coiling temperature.
As is apparent from FIG. 8, the Rankford value showed a maximum value when the coiling temperature was between 350 ° C. and 450 ° C. That is, it turned out that the workability of a cold-rolled sheet improves by making coiling temperature between 350 to 450 degreeC. On the other hand, the decrease in Rankford value at a coiling temperature exceeding 450 ° C. is due to precipitation of Cu-rich clusters, and the decrease in Rankford value below 350 ° C. is due to an increase in solid solution C and N. It is thought to be.
In addition, the steel component of the ferritic stainless steel used for investigating the relationship shown in FIG. 8 is 14% Cr-0.5% Si-0.5% Mn-0.005% C-0.010% N- 0.15% Ti-1.2% Cu-0.0005% B.
Here, in this embodiment, the coiling temperature is defined as 350 to 450 ° C. within the low temperature range. Thus, when the coiling temperature is on the low temperature side, the average rate of temperature increase in cold-rolled sheet annealing is preferably 5 ° C./s or more. If the rate of temperature increase is too slow, ε-Cu deposited during winding may grow into a Cu-rich cluster. Therefore, by setting the average rate of temperature increase in cold-rolled sheet annealing to 5 ° C./s or more, the formation of Cu-rich clusters can be suppressed, and as a result, the decrease in r value can be further suppressed.

Moreover, in the manufacture of the ferritic stainless steel sheet of the present embodiment, the hot-rolled sheet annealing usually performed after hot rolling may be performed, but it is preferable not to perform from the viewpoint of improving productivity.
Since normal Nb-added steel is hot-rolled steel sheet, it is subjected to hot-rolled sheet annealing before cold rolling, but the steel sheet according to this embodiment does not add Nb or is added in a small amount. Further, annealing of the hot-rolled steel sheet can be omitted, and the manufacturing cost can be reduced.

  In addition, in manufacture of the ferritic stainless steel plate of this embodiment, you may perform hot-rolled sheet annealing between hot rolling and hot-rolled sheet pickling. As described above, in the manufacturing method according to this embodiment, the hot-rolled sheet annealing step can be omitted, but when performing the hot-rolled sheet annealing, the hot-rolled sheet annealing temperature is set to 880. It is preferable to use a combustion gas atmosphere as the atmosphere in this case. This is due to manufacturing costs and productivity.

  Moreover, in the manufacturing method of the ferritic stainless steel plate in the present embodiment, it is preferable to use a rolled work roll having a roll diameter of 400 mm or more when performing cold rolling, as in the first embodiment. In order to increase the r value that is an index of the above, it is preferable to perform cold rolling with a tandem rolling mill having a roll diameter of 400 mm or more.

  If the rolling reduction in the cold rolling process is low, a recrystallized structure cannot be obtained after cold-rolled sheet annealing, or the mechanical properties deteriorate due to excessive coarsening, so the rolling reduction in the cold rolling process. Is preferably 50% or more.

Also, in the present embodiment, as in the first embodiment, other manufacturing processes are not particularly specified, but the hot-rolled sheet thickness, the cold-rolled sheet annealing temperature, the cold-rolled sheet annealing atmosphere, and the like can be appropriately selected. good. As preferable conditions, the hot rolled sheet thickness is 3.0 to 5.0 mm, the cold rolled sheet annealing temperature is 860 to 960 ° C., and the cold rolled sheet annealing atmosphere is a combustion gas atmosphere or hydrogen. A mixed atmosphere of nitrogen and nitrogen is desirable. However, in the cooling process after cold-rolled sheet annealing, it is desirable to cool at a cooling rate equal to or higher than air cooling in order to prevent hardening due to precipitation of Cu-rich clusters.
Moreover, you may give temper rolling and a tension leveler after cold rolling and cold-rolled sheet annealing. Further, the product plate thickness may be selected according to the required member thickness.

As described above, according to the method for producing a ferritic stainless steel sheet according to the present invention, toughness, which has been a conventional problem, is achieved by optimizing the coiling temperature after hot rolling and controlling the form of Cu-based precipitates. Can be prevented. Further, the amount of solute C and the amount of solute N can be controlled, and workability can be improved.
Further, by optimizing the coiling temperature and controlling the average cooling rate after hot rolling, Cu can be dissolved, and as a result, good toughness can be ensured.

Moreover, since the ferritic stainless steel plate according to the present invention substitutes expensive alloy elements such as Nb and Mo with Cu, when applied to exhaust system members such as automobiles, environmental measures and low parts are required. A great effect can be obtained for cost reduction and the like.

  Hereinafter, the effects of the present invention will be described with reference to examples, but the present invention is not limited to the conditions used in the following examples.

Example 1
In this example, first, steels having the component compositions shown in Tables 1 and 2 were melted and cast into slabs. After this slab was heated to 1190 ° C., the finishing temperature was in the range of 800 to 950 ° C. and hot rolled to a thickness of 5 mm to obtain a hot rolled steel sheet.
Next, the average cooling rate was set to 10 to 100 ° C./s, and air cooling and water cooling were properly used according to the cooling rate, and the coils were cooled to the respective coiling temperatures shown in Tables 3 and 4. Then, it was set as the winding hot-rolling coil at the predetermined winding temperature shown in Tables 3 and 4. The hot-rolled steel sheet temperature after hot rolling was measured while monitoring with a radiation thermometer.

Subsequently, the scale was removed by pickling the hot-rolled coil and cold-rolled to a thickness of 2 mm to obtain a cold-rolled plate. In the case of cold rolling, rolling work rolls as shown in Tables 3 and 4 were used. Here, with respect to test numbers P58 to P63 in Tables 3 and 4, before performing the pickling, hot-rolled sheet annealing was performed with an annealing temperature of 950 ° C., an annealing time of 120 seconds, and an atmosphere as a combustion gas atmosphere. .
After cold rolling, after cold-rolled sheet annealing was performed in a combustion gas atmosphere, pickling was performed at a sheeting speed such that the pickling time was 140 seconds to obtain a product plate. In addition, the average temperature increase rate in cold-rolled sheet annealing was 4 ° C./s.
In cold rolling, unidirectional multipass rolling was performed with a rolling mill having a large diameter roll (diameter 400 mm), or reverse multipass rolling was performed with a rolling mill having a small diameter roll (diameter 100 mm).
Moreover, the cold-rolled sheet annealing temperature was set to a range of 880 to 950 ° C. in order to make the crystal grain size number about 6 to 8. In addition, about the comparative example from which Nb content remove | deviates the upper limit of this invention, the cold-rolled sheet annealing temperature was made into the range of 1000-1050 degreeC.
No. in Table 1 0A to 0C and 1 to 24 are examples of the present invention, No. 1 in Table 2. 25 to 44 are comparative examples.

The hardness of the hot-rolled coil thus obtained was evaluated by a Vickers hardness test (based on JIS Z 2244), and a value less than 235 Hv was accepted. The test load at this time was 5 kgf, and the hardness test was performed.
Moreover, the V notch Charpy impact test piece was created from the hot rolled coil, the Charpy test was performed at 20 degreeC, and the absorbed energy was measured. In addition, while performing the Charpy test based on JISZ2242, the impact value evaluated 20J / cm < ² > or more as a pass ((circle)) and less than 20 J / cm < ² > as a disqualification (x). The results are shown in Tables 3 and 4.
The test piece in this embodiment is the hot-rolled sheet thickness remains subsize specimen by dividing the absorption energy by the sectional area (unit cm ^2), the toughness of the hot-rolled sheet in each example ( The impact value was compared and evaluated.

  Next, a high-temperature tensile test piece was prepared from the cold-rolled sheet subjected to cold-rolled sheet annealing, and a high-temperature tensile test was performed at 600 ° C. and 800 ° C., and 0.2% proof stress was measured (according to JIS G 0567). ). In the evaluation of the high temperature strength, the 600 ° C. proof stress was 150 MPa or higher and the 800 ° C. proof strength was 30 MPa or higher.

Next, the Rankford value was measured at room temperature (according to JIS Z 2254). In addition, the test piece was extract | collected from 3 directions, respectively parallel (0 degree), 45 degrees, and 90 degrees with respect to the rolling direction of the steel plate surface. In addition, the evaluation of workability was particularly excellent when the average Rankford value of the measured values in the three directions obtained was 1.1 or more, but it is not always necessary to achieve the numerical value, and 0.9 or more If there was, it was judged to be good.
The above production conditions and evaluation results are shown in Tables 3 and 4.

As can be seen from Tables 3 and 4, in the case of the present invention example produced under the component composition to which the present invention is applied and the hot rolling winding condition, the hot rolled sheet toughness is better than that of the comparative example. It can also be seen that the high-temperature strength at 600 ° C. and 800 ° C. is high as well as the Rankford value, which is an index of workability. That is, according to the manufacturing method to which the present invention is applied, a ferritic stainless steel hot-rolled steel sheet having excellent toughness and high-temperature strength can be manufactured. Moreover, even when it cold-rolls using the hot-rolled steel plate concerning this invention, it can be set as a favorable cold-rolled plate, without deterioration of workability.
Moreover, it turns out that the effect similar to the example of this invention which abbreviate | omitted hot-rolled sheet annealing is acquired also in the case of test number P58-60 which performed hot-rolled sheet annealing.

  For test numbers P1 to P4 and P15, since the coiling temperature was less than 450 ° C., Cu in the steel sheet could be dissolved, and as a result, good toughness values could be secured. However, since Cu dissolved in supersaturation as Cu-rich clusters precipitated during the temperature rising process in cold-rolled sheet annealing, the Rankford value decreased and the workability deteriorated.

  For test numbers P5-7 and P12-14, the coiling temperature was in the low temperature range of more than 450 ° C and less than 650 ° C. As a result, Cu-rich clusters were precipitated, and the Vickers hardness was greatly increased. Moreover, the toughness of the hot-rolled sheet was inferior, and the Rankford value was greatly reduced.

  For test numbers P29 and 30, the coiling temperature was increased to over 750 ° C., so the toughness was good, but the pickling property was poor. This is because the coiling temperature was high and the oxidation of the hot-rolled coil progressed, and it took a long time to remove the oxide scale on the surface of the hot-rolled plate in the pickling process of the hot-rolled plate. It is done.

In Test Nos. P38 and 53, the C and N contents were out of the upper limits, respectively, and the toughness of the hot-rolled sheet was lowered due to Cr carbonitride precipitation at the grain boundaries. Furthermore, since there was much content of C and N, the value of Ti / (C + N) became low. That is, since there was too much content of C and N with respect to content of Ti, solid solution C and solid solution N could not fully be fixed as carbonitrides, such as Ti. As a result, at the time of cold-rolled sheet annealing, the recrystallized texture development on the {222} plane was hindered, resulting in a low Rankford value.
For test number P53, the Vickers hardness increased. This is presumably because Cr was deposited and hardened because the N content was too high.

Test No. P39 had a high Si content and a good Rankford value, but its toughness was inferior due to solid solution strengthening.
In Test Nos. P40 and 45, the contents of Mn and Ni were large, and the hot rolled sheet toughness deteriorated due to the precipitation of the γ phase, and the high temperature strength and the Rankford value also deteriorated.

Test No. P41 had a high P content and poor toughness.
Test No. P42 had a high S content, and the high temperature strength was inferior due to an increase in the amount of MnS deposited.

In test number P43, since the Cr content was small, the high temperature oxidation progressed and the high temperature strength was impaired. In addition, the Rankford value of the cold-rolled sheet was inferior due to γ phase precipitation during hot rolling.
On the other hand, Test No. P44 had a high Cr content, so that 475 ° C. brittleness occurred, and the toughness was inferior and the Rankford value was also deteriorated.

In Test No. P46, since the Cu content was small, satisfactory results were obtained in toughness, but sufficient high-temperature strength was not obtained.
On the other hand, in Test No. P47, since Cu was excessively added, the amount of Cu-based precipitates was excessively increased, and the hot-rolled sheet toughness, the Rankford value and the high temperature strength were lowered.

In Test No. P48, since the Ti content was small and solute C and N could not be fixed sufficiently, Cr carbonitride precipitated at the grain boundaries, and the toughness and the Rankford value decreased.
In Test Nos. P49 and P50, the Ti and V contents deviated from the upper limit, so that the precipitates became coarse, and the hot precipitates deteriorated from the coarse precipitates as the starting point.

  Test No. P51 was hardened because the Al content was off the upper limit, and the uniform elongation was significantly reduced. Moreover, the hot-rolled sheet toughness also decreased.

In test number P52, since the B content deviated from the upper limit, a large amount of Cr ₂ B was precipitated, and the hot-rolled sheet toughness decreased.

In Test Nos. P54 and P55, since the Mo and Nb contents exceeded the upper limit, the Laves phase was precipitated on the hot-rolled sheet, and the toughness was deteriorated. Rankford also fell.
In test number P56, since the Zr content exceeded the upper limit, the hot-rolled sheet toughness decreased and the high-temperature strength also decreased.
In Test No. P57, since the Sn content exceeded the upper limit, the toughness decreased due to solid solution strengthening with Sn, and the high-temperature strength also decreased due to the decrease in oxidation resistance.

  Moreover, although test number P61-63 is a case where hot-rolled sheet annealing is performed, it was a low temperature range with coiling temperature more than 450 degreeC and less than 650 degreeC similarly to test number P5-7 and P12-14. . As a result, Cu-rich clusters were precipitated, the Vickers hardness increased greatly, and the hot-rolled sheet toughness also decreased.

(Example 2)
In this example, first, steels having the component compositions shown in Tables 5 and 6 were melted and cast into slabs. This slab was heated to 1190 ° C. in the same manner as in Example 1 and then hot-rolled to a sheet thickness of 5 mm with a finishing temperature in the range of 800 to 950 ° C. to obtain a hot-rolled steel sheet.
Next, the average cooling rate between 850-450 degreeC was made into the predetermined speed as shown in Tables 7 and 8, and the hot-rolled steel plate was cooled by water cooling to each coiling temperature shown in Tables 7 and 8. Then, it was set as the winding hot-rolling coil at the predetermined winding temperature shown in Tables 7 and 8. The steel sheet temperature after hot rolling was measured while monitoring with a radiation thermometer.

Then, it cold-rolled by the method similar to Example 1, and was set as the cold rolled sheet. In the case of cold rolling, rolling work rolls as shown in Tables 7 and 8 were used. Here, with respect to test numbers P58 to P64 in Tables 7 and 8, before performing the pickling, hot-rolled sheet annealing was performed with an annealing temperature of 950 ° C., an annealing time of 120 seconds, and an atmosphere as a combustion gas atmosphere. .
After cold rolling, after cold-rolled sheet annealing in a combustion gas atmosphere, pickling was performed to obtain a product sheet. In this example, the average temperature increase rate in the cold-rolled sheet annealing was set to 7 ° C./s.
The pickling of the hot-rolled coil was performed at a plate passing speed such that the pickling time was 140 seconds. Moreover, as shown in Tables 7 and 8, a product having no remaining scale was regarded as acceptable (◯), and the pickling property of the hot-rolled sheet was evaluated. The remaining state of the scale was confirmed with a loupe.
In cold rolling, unidirectional multi-pass rolling was performed with a rolling mill having a large-diameter roll (diameter 400 mm), or reverse multi-pass rolling was performed with a rolling mill having a small-diameter roll (diameter 100 mm).
Moreover, the cold-rolled sheet annealing temperature was set to a range of 880 to 950 ° C. in order to make the crystal grain size number about 6 to 8. In addition, about the comparative example from which Nb content remove | deviates the upper limit of this invention, the cold-rolled sheet annealing temperature was made into the range of 1000-1050 degreeC.
In Tables 5 and 6, steel types 0A to 0C and 1 to 24 are examples of the present invention, and steel types 25 to 44 are comparative examples.

A V-notch Charpy impact test piece was prepared from the middle part and the bottom part of the hot-rolled coil thus obtained, and a Charpy test was performed at 20 ° C. to measure the absorbed energy. The Charpy test was performed according to JIS Z 2242, and an impact value of 20 J / cm ² or more was evaluated as pass (◯), and less than 20 J / cm ² was evaluated as reject (x).
In addition, since the test piece in a present Example is a subsize test piece with a hot-rolled sheet thickness, the toughness of the hot-rolled sheet in each Example is obtained by dividing the absorbed energy by the cross-sectional area (unit cm ² ). Comparison and evaluation were made.

  Next, a high-temperature tensile test piece was prepared from the cold-rolled sheet subjected to cold-rolled sheet annealing, and a high-temperature tensile test was performed at 600 ° C. and 800 ° C., and 0.2% proof stress was measured (according to JIS G 0567). ). In the evaluation of the high temperature strength, the 600 ° C. proof stress was 150 MPa or higher and the 800 ° C. proof strength was 30 MPa or higher.

Next, the Rankford value was measured at room temperature (according to JIS Z 2254). A test piece was collected in the same manner as in Example 1. In the evaluation of workability, although the average value of the obtained Rankford values in each of the three directions was 1.1 or more, the numerical value was not necessarily achieved. If there was, it was judged to be good.
The above production conditions and evaluation results are shown in Tables 7 and 8.

As is apparent from Tables 7 and 8, in the case of the present invention example produced under the component composition to which the present invention is applied and the hot rolling coiling conditions, the toughness, pickling property, and coldness of the hot rolled sheet compared to the comparative example It can be seen that the high temperature strength and the Rankford value of the rolled annealed sheet are good. That is, according to the manufacturing method to which the present invention is applied, a ferritic stainless steel sheet excellent in workability, toughness, and high-temperature strength can be manufactured.
Moreover, it turns out that the effect similar to the example of this invention which abbreviate | omitted hot-rolled sheet annealing is acquired also in the case of test number P58-61 which gave hot-rolled sheet annealing.

  On the other hand, in a comparative example that deviates from the inventive example, at least one of the Charpy impact value (absorbed energy), the 0.2% proof stress, and the Rankford value was low. Thereby, it turns out that the toughness of the ferritic stainless steel plate in a comparative example, workability, or high temperature strength fell.

  Test numbers P1 to P3 of the comparative examples were temperatures at which the coiling temperature was as low as less than 350 ° C. Therefore, very good results were obtained as hot-rolled sheet toughness, but the Rankford value decreased. This is because solid solution C and solid solution N were not sufficiently fixed as carbonitrides such as Ti, and therefore, the development of the recrystallized texture on the {222} plane was hindered during cold-rolled sheet annealing. As a result, it is thought that the Rankford value decreased and the workability deteriorated.

Test numbers P8 and P9 were in a temperature range where the coiling temperature was higher than 450 ° C and lower than 650 ° C. Therefore, Cu-rich clusters were precipitated and embrittled. As a result, the toughness of the hot-rolled sheet was inferior and the Rankford value was greatly reduced.
In test number P10, the coiling temperature was as high as 650 ° C., and thus a large difference occurred in the temperature drop amount of the middle part and the bottom part of the hot rolled coil. For this reason, the toughness of the middle part of the hot-rolled coil was very good, but the toughness of the bottom part was poor, resulting in a large difference in the toughness of each part of the hot-rolled coil. The Rankford value was also low.
In test numbers P11 and 12, the coiling temperature was 430 ° C., but the average cooling rate until winding was less than 10 ° C./s, so the toughness of the hot-rolled sheet was lowered. This is presumably because Cu-rich clusters were precipitated because the average cooling rate was low. Rankford also fell.

  In Test Nos. P38 and P53, the C and N contents were out of the upper limits, respectively, and the toughness of the hot-rolled sheet was lowered due to Cr carbonitride precipitation at the grain boundaries. Furthermore, since there was much content of C and N, the value of Ti / (C + N) became low. That is, since there was too much content of C and N with respect to content of Ti, solid solution C and solid solution N could not fully be fixed as carbonitrides, such as Ti. As a result, at the time of cold-rolled sheet annealing, the development of the recrystallized texture on the {222} plane was hindered, resulting in a low average Rankford value.

Test No. P39 had a high Si content and a good Rankford value, but its toughness was inferior due to solid solution strengthening.
P40 and P45 each had a high content of Mn and Ni, and the hot rolled sheet toughness deteriorated due to the precipitation of the γ phase, as well as the high temperature strength and the Rankford value.

Test No. P41 had a high P content and poor toughness.
Test No. P42 had a high S content, and the high temperature strength was inferior due to an increase in the amount of MnS deposited.

In test number P43, since the Cr content was small, the high temperature oxidation progressed and the high temperature strength was impaired. Moreover, due to γ phase precipitation during hot rolling, the hot rolled sheet toughness and the Rankford value of the cold rolled sheet were inferior.
On the other hand, since test number P44 had a large Cr content, brittleness occurred at 475 ° C. and toughness was poor.

In Test No. P46, since the Cu content was small, satisfactory results were obtained in toughness, but sufficient high-temperature strength was not obtained.
On the other hand, in Test No. P47, since Cu was excessively added, the amount of Cu-based precipitates was excessively increased, and the hot-rolled sheet toughness, the Rankford value and the high temperature strength were lowered.

In Test No. P48, since the Ti content was small and solute C and N could not be fixed sufficiently, Cr carbonitride precipitated at the grain boundaries, and the toughness and the Rankford value decreased.
Test Nos. P49, P50, P51, and P56 have Ti, V, Al, and Zr contents that deviate from the upper limit, resulting in coarse precipitates. Decreased.

In test number P52, since the B content deviated from the upper limit, a large amount of Cr ₂ B was precipitated, and the hot-rolled sheet toughness decreased.

In Test Nos. P54 and P55, since the Mo and Nb contents exceeded the upper limit, the Laves phase was precipitated on the hot-rolled sheet, and the toughness was deteriorated. In addition, pickling performance and Rankford value also decreased.
In Test No. P57, since the Sn content exceeded the upper limit, the toughness decreased due to solid solution strengthening with Sn, and the high-temperature strength also decreased due to the decrease in oxidation resistance.

  In addition, test numbers P62 to 64 are cases where hot-rolled sheet annealing was performed, but test numbers 62 and 63 were in a temperature range where the coiling temperature was higher than 450 ° C. and lower than 650 ° C., similarly to P8 and 9. It was. As a result, Cu-rich clusters were precipitated, the Vickers hardness increased greatly, and the hot-rolled sheet toughness also decreased. In Test No. 64, the coiling temperature was as high as 650 ° C., so that a large difference occurred in the temperature drop amount of the middle part and the bottom part of the hot rolled coil. For this reason, the toughness of the middle part of the hot-rolled coil was very good, but the toughness of the bottom part was poor, resulting in a large difference in the toughness of each part of the hot-rolled coil.

Among the inventive examples, the coiling temperature is set in the range of 350 ° C. to 450 ° C., and after hot rolling, the average cooling rate of 850 ° C. to 450 ° C. is set to 10 ° C./s or more. The washability, high temperature strength, and rankford value all showed good values.
In addition, test numbers P21 and P25, which are examples of the present invention, used a rolling mill having a small-diameter roll having a diameter of 100 mm when performing cold rolling. For this reason, the Rankford value was within a range of acceptable values, but was slightly lower. Thereby, when performing cold rolling, it turns out that it is more preferable to use the rolling mill which has a large diameter roll with a diameter of 400 mm.

  From these results, the above-mentioned findings could be confirmed, and the grounds for limiting the above-described steel compositions and configurations could be supported.

  As is clear from the above description, according to the method for manufacturing a ferritic stainless steel sheet of the present invention, since an expensive alloy element such as Nb or Mo is replaced by Cu, stainless steel having high high-temperature strength. In a steel plate, it is possible to increase hot rolled sheet toughness. For this reason, highly efficient manufacture is attained. In addition, by applying the material to which the present invention is applied, particularly to exhaust members, it is possible to increase social contributions such as environmental measures by reducing component costs and weight. That is, the present invention has sufficient industrial applicability.

Claims (8)

% By mass
C: 0.02% or less,
N: 0.02% or less,
Si: 0.1 to 1.5%,
Mn: 1.5% or less,
P: 0.035% or less,
S: 0.010% or less,
Ni: 1.5% or less,
Cr: 10 to 20%,
Cu: 1.0-3.0%,
Ti: 0.08 to 0.30%,
Al: 0.3% or less,
Each containing
The balance has a steel composition consisting of Fe and inevitable impurities,
A ferritic stainless steel hot-rolled steel sheet having a Vickers hardness of less than 235 Hv. Furthermore, in mass%,
Nb: 0.3% or less,
Mo: 0.3% or less,
Zr: 0.3% or less,
Sn: 0.5% or less,
V: 0.3% or less,
B: 0.0002% to 0.0030%,
The ferritic stainless steel hot-rolled steel sheet according to claim 1, comprising at least one of the following.   A hot rolled steel sheet is formed by subjecting a steel piece cast with the ferritic stainless steel having the steel composition according to claim 1 or 2 to a hot rolled steel sheet. Is manufactured at 620 ° C. or higher and 750 ° C. or lower, and a method for producing a ferritic stainless steel hot-rolled steel sheet. After winding the hot-rolled steel sheet according to claim 3, the hot-rolled steel sheet temperature T (K) and the holding time t (h) are controlled so that the following formula (1) is satisfied in the entire hot-rolled coil. The method for producing a ferritic stainless steel hot-rolled steel sheet according to claim 3, wherein the hot-rolled coil is heated or cooled.
T (20.24 + log (t)) ≧ 17963 (1)   With respect to the steel slab having the steel composition according to claim 1 or 2, the average cooling rate between 850 ° C. and 450 ° C. after finish rolling of hot rolling is set to 10 ° C./second or more, and the coiling temperature Is manufactured at 350 ° C. to 450 ° C., and a method for producing a ferritic stainless steel hot-rolled steel sheet.   Performing hot-rolled sheet pickling, cold rolling, cold-rolled sheet annealing, and cold-rolled sheet pickling on the hot-rolled steel sheet produced by the method according to any one of claims 3, 4, and 5. A method for producing a ferritic stainless steel sheet.   A hot-rolled steel sheet manufactured by the method according to any one of claims 3, 4, and 5 is subjected to hot-rolled sheet annealing, hot-rolled sheet pickling, cold rolling, cold-rolled sheet annealing, and cold-rolled sheet. A method for producing a ferritic stainless steel sheet, characterized by pickling.   The method for producing a ferritic stainless steel sheet according to claim 6 or 7, wherein a rolled work roll having a roll diameter of 400 mm or more is used when the cold rolling is performed.