WO2012108479A1

WO2012108479A1 - Hot rolled ferritic stainless steel sheet, method for producing same, and method for producing ferritic stainless steel sheet

Info

Publication number: WO2012108479A1
Application number: PCT/JP2012/052901
Authority: WO
Inventors: 慎一寺岡; 小林　雅明; 祐司小山; 濱田　純一; 憲博神野; 井上　宜治; 木村　謙; 高橋　淳; 後藤　茂之
Original assignee: 新日鐵住金ステンレス株式会社
Priority date: 2011-02-08
Filing date: 2012-02-08
Publication date: 2012-08-16

Abstract

This hot rolled ferritic stainless steel sheet has a steel composition which contains, in mass%, 0.02% or less of C, 0.02% or less of N, 0.1-1.5% of Si, 1.5% or less of Mn, 0.035% or less of P, 0.010% or less of S, 1.5% or less of Ni, 10-20% of Cr, 1.0-3.0% of Cu, 0.08-0.30% of Ti and 0.3% or less of Al, with the balance made up of Fe and unavoidable impurities. The hot rolled ferritic stainless steel sheet has a Vickers hardness of less than 235 Hv.

Description

Ferritic stainless steel hot-rolled steel sheet and method for producing the same, and method for producing ferritic stainless steel sheet

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.
The present application includes Japanese Patent Application No. 2011-024872 filed in Japan on February 8, 2011, Japanese Patent Application No. 2011-026277 filed in Japan on February 9, 2011, and February 24, 2011. Claiming priority based on Japanese Patent Application No. 2011-038252 filed in Japan and Japanese Patent Application No. 2012-024544 filed in Japan on February 7, 2012, the contents of which are incorporated herein by reference. To do.

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, the material for automobile exhaust system as described above is a 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 reached 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.

In general, 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. Then, in order to ensure the toughness of a hot-rolled sheet, the hot-rolling winding conditions are optimized. In stainless steel containing Nb and Mo, the hot rolled sheet toughness is increased 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 the 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

For the purpose of improving the above properties, ferritic stainless steel sheets to which elements mainly composed of Cr and Mo have been added have been developed so far, but as described above, steel sheets to which Cu has been added have recently been developed. Yes.

Patent Document 10 discloses stainless steel cooling for automobile exhaust system parts to which Cu is added in an amount of 1% by weight or more in order to utilize precipitation strengthening by Cu precipitates in a medium temperature region and solid solution strengthening by solid solution Cu in a high temperature region. A rolled steel sheet is disclosed.

However, generally, when manufacturing such a steel sheet to which a large amount of Cu is added, cold cracking may occur, and the poor productivity resulting from this is cited as a problem. Note that cold cracking means that when a hot-rolled coil is unwound and then passed through a continuous pickling line or a continuous annealing pickling line, the toughness of the hot-rolled coil is insufficient, resulting in ear cracking or plate breakage. Refers to the phenomenon that occurs.

Patent Document 11 discloses a technique regarding a cold rolled annealed sheet of ferritic stainless steel containing 2.0% by mass or less of Cu, but does not mention the toughness of the hot rolled sheet. On the other hand, in order to suppress the formation of precipitates in the cold-rolled sheet, it is described that the winding process is performed by water cooling immediately after hot rolling.
However, there is no disclosure of the coiling temperature, etc., and it is difficult to cool to room temperature after hot rolling due to the capacity of the cooling equipment, and the end temperature of water cooling is not clearly defined, and the conditions that can be actually applied are clear Was not.

Ferritic stainless steels for which hot-rolled sheet toughness is an issue include steel types with a high Cr content and steel types to which Al is added. Patent Documents 12 to 14 are known as means for solving these hot-rolled sheet toughnesses. .

Patent Document 12 discloses a technique for winding up at 400 to 600 ° C. and immediately quenching at a cooling rate equal to or higher than water cooling as a technique for improving the toughness value of a hot-rolled sheet of a steel type added with 25 to 35 wt% of Cr. ing.
Patent Document 13 discloses a technique in which ferritic stainless steel containing 3 to 7% by weight of Al is rapidly cooled after winding.

Patent Document 14 discloses a method in which a winding temperature is set to 550 to 650 ° C. to form a winding coil shape, and then immersed in a water tank within 3 hours.

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 JP 2000-297355 A JP 2002-194507 A JP-A-5-320764 JP-A 64-56822 JP 2001-26826 A

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 adding Cu. 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 the workability fall was 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.

Also, as described above, the techniques of Patent Documents 3 and 5 are disclosed as techniques for improving hot rolled sheet toughness. However, when the present inventors applied the above-mentioned conventional knowledge to a steel type containing 1% or more of Cu, cold cracking may occur, which is not necessarily effective for improving toughness. I understood. In other words, the conventionally known technique for improving the toughness of Cu-added steel sheets is not sufficiently effective in hot rolled sheets of ferritic stainless steel containing a large amount of Cu of 1% or more, and further improvement is required. It was to be done.

Therefore, 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.
Moreover, an object of this invention is to provide the ferritic stainless steel hot-rolled steel plate excellent in cold cracking property, and its manufacturing method.

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 without adding 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 precipitate in the temperature range of 450 to 600 ° C., and the toughness is extremely lowered. That is, it has been 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 the coiling temperature is set to 620 ° C. or more, so that Cu is precipitated as ε-Cu and the hardness is made less than 235 Hv. ε-Cu is essentially harmless to hot rolled sheet toughness. In the process where the Cu-based precipitate becomes ε-Cu, it is considered that Cu-rich clusters are formed. For example, when the coiling temperature is 650 ° C., the retention time is 10 minutes or more, and at 700 ° C., the retention time is 60 seconds or more. As a result, a substantial amount of the solid solution Cu becomes ε-Cu, and a toughness level that can be passed through a subsequent process in a cold (normal temperature) can be 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 the 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 the solid solution N are not sufficiently fixed as carbonitrides such as Ti and Nb. Therefore, during cold rolling annealing (cold rolled sheet annealing) , {222} plane recrystallization texture development is inhibited. As a result, the Rankford value may decrease, 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.

Furthermore, the present inventors investigated the relationship between hot-rolling conditions of ferritic stainless steel and toughness of hot-rolled sheets in order to solve the above-described problems.
First, after ferritic stainless steel with varying Cu content is hot rolled to a thickness of 5 mm in the laboratory, the winding temperature is changed in the range of 300 to 600 ° C., and the winding treatment time is changed in the range of 0.1 to 100 h. The winding process was performed. And after this winding process, it cooled to room temperature by water cooling, and produced the hot rolled sheet steel. A Charpy test was performed from the obtained hot-rolled steel sheet, and the toughness at room temperature (25 ° C.) was evaluated.

Also, focusing on fine precipitates such as Cu-rich clusters (hereinafter also simply referred to as Cu clusters) existing in the hot-rolled steel sheets produced under the various conditions described above, the relationship with toughness was investigated. This can be presumed that Cu-based precipitates greatly affect the toughness of the Cu-added steel sheet, but single nano-order fine precipitates such as Cu clusters have been difficult to observe in the past, This is because the relationship between toughness is not clear and the method for controlling such a fine precipitation process has not been known. These studies are conducted and the findings obtained are listed below.

<1> toughness of the resulting hot rolled steel sheet was varied in the range of ^{10J / cm 2 ~ 100J / cm} 2 by the production conditions.
<2> When the metal structure of the obtained hot-rolled steel sheet was observed with an optical microscope, all of them were unrecrystallized structures of ferrite. In addition, Cu precipitates could not be found by observing with either a scanning electron microscope (SEM) or a transmission electron microscope (TEM). That is, it has been found that there are good toughness and poor toughness even though the formation of Cu precipitates is sufficiently suppressed.
Then, when investigating with a three-dimensional atom probe in order to investigate a finer state, many fine clusters (Cu clusters) made of Cu were observed in a hot-rolled steel sheet having a toughness of less than 20 J / cm ² . On the other hand, in a hot-rolled steel sheet having a toughness of 20 J / cm ² or more, such fine Cu clusters were not recognized or the density was very low.
Usually, Cu precipitates are recognized as precipitates by gathering Cu atoms and forming a crystal structure such as BCC, 9R or FCC. Moreover, the deposit confirmed by the conventional TEM observation is a size of several tens nm or more.
In the present invention, a “Cu-rich cluster (Cu cluster)” is defined as an aggregate of Cu atoms having a maximum diameter of 5 nm or less, which is confirmed by a three-dimensional atom probe. In addition, the crystal structure of the Cu cluster defined in the present invention is not particularly limited, and includes a precipitate having a crystal structure such as BCC or 9R, or a precursor state of the precipitate, if any. . On the other hand, the toughness of the hot-rolled steel sheet was found to be closely related to the density of “Cu clusters” defined as described above.
<3> FIG. 9 is a graph showing the relationship between the winding temperature of 1.2% Cu-added steel, the time until the 1.2% Cu-added steel after winding is immersed in a water tank, and toughness. Reference numerals in the graph, ○: Charpy impact value ≧ 20J / cm ^2, ×: a Charpy impact value <20J / cm ^2.
As is apparent from the graph of FIG. 9, at a coiling temperature of 500 ° C. or less, the Charpy impact value (toughness value) decreases as the time until the 1.2% Cu-added steel is immersed in the water tank decreases, and a certain time It was found that the toughness value would be lower than 20 J / cm ^{2 after the time} was exceeded.
Moreover, even when the conditions for the coiling temperature and the time until dipping in the water tank are the same, the toughness is low when the time for dipping the 1.2% Cu-added steel in the water tank (immersion time) is shorter than 1 h. Turned out to be. That is, the toughness of the hot-rolled steel sheet is a factor affected by the coiling temperature, the time until the hot-rolled steel sheet is immersed in the water tank, and the immersion time, and good toughness can be obtained by controlling these factors. I found out.

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) The ferritic stainless steel hot-rolled steel sheet according to the first embodiment of the present invention is 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: 10-20%,
Cu: 1.0 to 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, and has a Vickers hardness of less than 235 Hv.
(2) The ferritic stainless steel hot-rolled steel sheet described in (1) above is further 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%,
One or more of these may be included.

(3) A method for producing a ferritic stainless steel hot-rolled steel sheet according to the first embodiment of the present invention includes a steel piece obtained by casting ferritic stainless steel having the steel composition described in (1) or (2) above. On the other hand, after hot rolling finish rolling is performed 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.
(4) In the method for producing a ferritic stainless steel hot-rolled steel sheet described in (3) above, after winding the hot-rolled steel sheet described in (3) above, The hot rolled coil may be heated or cooled while controlling the hot rolled steel sheet temperature T (K) and the holding time t (h) so as to satisfy.
T (20.24 + log (t)) ≧ 17963 (Equation 1)

(5) A method for producing a ferritic stainless steel hot-rolled steel sheet according to the first embodiment of the present invention includes hot rolling with respect to a steel piece having the steel composition described in (1) or (2). After the finish rolling, the average cooling rate between 850 ° C. and 450 ° C. is 10 ° C./second or more, and the winding temperature is 350 ° C. to 450 ° C.

(6) The manufacturing method of the ferritic stainless steel sheet according to the first embodiment of the present invention is a hot-rolled sheet pickling of the hot-rolled steel sheet manufactured by the method described in (3), (4), (5) above. , Cold rolling, cold rolled sheet annealing, cold rolled sheet pickling.
(7) The manufacturing method of the ferritic stainless steel sheet according to the first embodiment of the present invention is a method of annealing a hot-rolled steel sheet manufactured by the method described in (3), (4) and (5) above. Rolled sheet pickling, cold rolling, cold rolled sheet annealing, and cold rolled sheet pickling are performed.
(8) In the method for producing a ferritic stainless steel sheet described in (6) or (7) above, a rolled work roll having a roll diameter of 400 mm or more may be used when the cold rolling is performed.

(9) The ferritic stainless steel hot-rolled steel sheet according to the second embodiment of the present invention is mass%,
C: 0.0010% to 0.010%,
Si: 0.01% to 1.0%
Mn: 0.01% to 2.00%
P: less than 0.040%,
S: 0.010% or less,
Cr: 10.0% to 30.0%,
Cu: 1.0 to 2.0%,
Al: 0.001% to 0.10%,
N: 0.0030% to 0.0200%
Each containing
The balance has a steel composition composed of Fe and inevitable impurities, and the number density of Cu clusters having a maximum diameter of 5 nm or less made of Cu is less than 2 × 10 ¹³ / mm ^{3 in} the crystal grains.
(10) In the ferritic stainless steel hot-rolled steel sheet described in (9) above, further, in mass%,
Nb: 0.10% to 0.70% or less,
Ti: 0.05% to 0.30% or less,
1 type or 2 types or more may be included so that the following (Formula 2) may be satisfied.
Nb / 93 + Ti / 48 ≧ C / 12 + N / 14 (Expression 2)
(11) In the ferritic stainless steel hot-rolled steel sheet according to (9) or (10) above, further, in mass%,
Mo: 0.1% to 1.0%,
Ni: 0.1% to 1.0%
Al: 0.50% to 3.0%
1 type or 2 types or more may be included.
(12) In the ferritic stainless steel hot-rolled steel sheet according to any one of (9) to (11) above, further, in mass%,
B: 0.0001% to 0.0025%,
May be included.

(13) a step of hot-rolling a steel sheet by hot rolling using a steel piece obtained by casting a ferritic stainless steel having the steel composition according to any one of (9) to (12) above; After the hot rolling, the winding temperature T is set to 300 ° C. to 500 ° C., the step of winding the hot-rolled steel sheet in a coil shape, and the hot-rolled steel sheet coiled is immersed in a water bath for 1 hour or more. And after removing the hot-rolled steel sheet from the water tank,
After the step of winding the hot-rolled steel sheet into a coil shape, the hot-rolled steel sheet is immersed in the water tank within a time tc (h) that satisfies the following (Equation 3).
tc = 10 ^ ((452-T) /76.7) (Equation 3)

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.
Also, by controlling the coiling temperature, it is possible to optimize the morphology 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.
Moreover, according to this invention, the number density of the fine Cu cluster which affects the toughness of a hot-rolled steel plate is distributed lower than before. Therefore, a decrease in toughness of the hot-rolled steel sheet can be suppressed, and as a result, cold cracking of the hot-rolled steel sheet can be prevented.
Moreover, according to the ferritic stainless steel hot-rolled steel sheet according to the present invention, cold cracking does not occur even after continuous annealing or hot pickling after hot rolling.
Moreover, according to this invention, the increase in a production yield and the improvement of production efficiency can be brought about by suppressing the cold crack of the ferritic stainless steel hot-rolled steel sheet containing Cu. As a result, it is possible to exert a very useful effect on the industry in terms of manufacturing cost reduction and the like. In addition, energy consumption can be suppressed by improving production efficiency, which can contribute to global environmental conservation.
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. 3 is a graph showing the effect of heat treatment temperature on the ductile-brittle transition temperature of a Charpy impact test of a ferritic stainless steel hot-rolled steel sheet in the first 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. FIG. 5 is a graph showing the effect of the average cooling rate from 850 to 450 ° C. on the impact value of the Charpy impact test at 20 ° C. when the ferritic stainless steel hot-rolled steel sheet in the second embodiment is wound at 430 ° C. FIG. 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. It is a graph which shows the relationship between the time to immerse in the coiling temperature, the water tank, and toughness of the ferritic stainless steel hot rolled steel sheet in this embodiment.

(Ferrite stainless steel hot-rolled steel sheet (first embodiment))
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 to 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 thereof, the better. Therefore, 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 that the content be 0.001% to 0.009%.

N: 0.02% or less N, like C, degrades formability, corrosion resistance, and hot-rolled sheet toughness, so the smaller the content, the more preferable. Therefore, N is made 0.02% or less. However, excessive reduction leads to an increase in refining costs, 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. is improved 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 view 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 an intermediate temperature range. Further, 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 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 for stainless steel, but 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, the purity of P of the ferrochrome raw material is preferably 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 as small as possible. %. 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%. Preferably, the content is 0.001 to 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%. Further, it is an element effective for 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, addition of a large amount may cause material hardening due to solid solution strengthening, so 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 element essential 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%, workability and toughness are 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, it 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%. Note that 1.0% to 1.5% is desirable in consideration of solid solution of Cu during the cold rolling annealing to suppress the deterioration of 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%. It should be noted that the content of 0.10% to 0.18% is desirable because the effect of improving the corrosion resistance by solute Ti and the reduction of hot-rolled sheet toughness and press workability by large precipitates TiN may occur.

Al: 0.3% or less In addition to being added as a deoxidizing element, Al is an element that improves oxidation resistance. Further, it is useful as a solid solution strengthening element for improving the strength at 600 to 700 ° C. Since the effect is stably expressed from 0.01%, the lower limit is preferably set to 0.01%.
On the other hand, excessive addition hardens and remarkably lowers the uniform elongation and also significantly reduces toughness, so the upper limit is made 0.3%. Furthermore, if generation of surface flaws, 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 carbonitrides 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, 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 causes the precipitation of Cr ₂ B, (Cr, Fe) ₂₃ (C, B) ₆ to impair toughness and corrosion resistance, and may also impair weldability. 0002% to 0.0030%. In view of workability and manufacturing cost, it is desirable that the content be 0.0003% to 0.0015%.

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%. Further, from the viewpoint of productivity and manufacturability, it is desirable that the content be 0.01% to 0.2%.

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 dissolved Ti and Nb, so it 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 significantly degrades 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 that is effective in 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 view 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 the present embodiment, steel containing the above essential components and components added as necessary is melted to form 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). Note that the finish rolling finishing temperature (finishing temperature) of hot rolling is in the range of 800 ° C. to 980 ° C.
Next, after the finish rolling, the hot-rolled steel sheet is cooled and wound into a coil to form a hot-rolled coil.
Here, the temperature (winding temperature) at which the hot-rolled steel sheet is wound in a coil shape after finish rolling 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 precipitated 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 coil into a 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 length of the hot-rolled coil. 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.

Generally, in the cooling step after a hot-rolled steel sheet is wound into a hot-rolled coil, the cooling rate of the top and bottom portions of the hot-rolled coil increases. 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. For this reason, although it cannot be said that it is generally a problem, when the deterioration of toughness due to a 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 for controlling the L value is not particularly limited, and can be appropriately selected from commonly used methods and conditions. For example, when cooling the hot-rolled steel sheet after finish rolling to the above coiling 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 higher temperature 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 and 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.
Below, the investigation result for explaining in detail such winding temperature and the reason for limitation of the above formula (1) is shown. In the method for evaluating hot-rolled sheet toughness described below, the number of samples is three, a Charpy impact test is performed at 20 ° C., and the absorbed energy is obtained. 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, cooling was performed with water cooling, and thereafter cooling was performed with 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 winding, it takes 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 a Charpy impact test piece (sub-size with the plate thickness) 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 is greatly reduced. 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.

FIG. 2 shows the result of a Charpy impact test conducted on a heat-treated plate produced by the same method as in FIG. 1 in the range of −40 ° C. to 140 ° C.
As can be seen from FIG. 2, the ductile-brittle transition temperature rises to near 100 ° C. when heat-treated at 450 to 550 ° 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 an unheated hot-rolled sheet.
The steel composition of the ferritic stainless steel used to investigate the relationship shown in FIG. 2 is 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, no Cu precipitate is 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, which is relatively large on dislocations and more finely precipitated elsewhere. Further, in the 700 ° C. heat treated material shown in FIG. 3C, it was observed that ε-Cu was precipitated, and the observed size was 30 to 100 nm.
Although the cause of the decrease in toughness due to the Cu-rich cluster is not clear, it was found that the uniform elongation was about 10% when the tensile test was performed. In this case, since the precipitates are extremely finely dispersed, it is presumed that the high-speed movement of dislocations is inhibited and brittle fracture occurred.

In FIG. 4, a hot-rolled sheet produced 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 shown in FIG. 4 organized by L value (T (20.24 + log (t))). It can be seen that even after heat treatment at 620 to 750 ° C., the toughness decreases 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 (1) 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.
The precipitation of ε-Cu in the steel sheet proceeds in a shorter time in a temperature range near 620 to 750 ° C. in the vicinity of the Cu deposition nose. The precipitation phenomenon is controlled by the diffusion of atoms, so 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 arranged 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.

Also, in FIG. 5, a hot-rolled sheet manufactured by the same method as in FIG. 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, the Rankford value increases in the temperature range of 620 to 750 ° C. and reaches the highest value at 700 ° C. That is, it was found that the workability of the cold rolled sheet is improved by setting the coiling temperature to 620 to 750 ° C.

Further, 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 it 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.
Also, 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 with a Sendzimir mill with a work roll diameter (roll diameter) of about 60 to 100 mm, or with a tandem rolling mill with 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 diameter 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} or {554} during cold rolling (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, etc. may be appropriately selected. As preferable conditions, the thickness of the hot-rolled plate is 3.0 to 5.0 mm, the cold-rolled plate annealing temperature is 860 to 960 ° C., and the cold-rolled plate 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 the present invention, since Nb is not added or the content is low, the cold-rolled sheet annealing temperature after cold rolling can be as low as 850 to 970 ° C. 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 the deterioration of toughness.
Also, 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 substitutes 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). Thereafter, a hot rolling process is performed in which the average cooling rate between 850 ° C. and 450 ° C. is 10 ° C./second or more, and the winding temperature is 350 ° C. to 450 ° C.
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 this embodiment, the steel containing the above essential components and components added as necessary 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). Note that the finish rolling finishing temperature (finishing temperature) of hot rolling is in the range of 800 ° C. to 980 ° C.
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 this 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, in the temperature range of up to 450 ° C. (particularly 600 ° C. to 450 ° C.) after finish rolling, a nano-order Cu-rich cluster is used. It has been found that toughness is extremely lowered. That is, by increasing the cooling rate in such a temperature range, precipitation of Cu-rich clusters can be prevented. Since such an effect is stably exhibited when the average cooling rate is 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 winding 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. Will be disturbed. As a result, workability may be deteriorated. On the other hand, if the coiling temperature is too high, Cu-rich clusters may be precipitated and hot rolled sheet toughness may be reduced. 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. in order 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, performs a Charpy impact test at 20 ° C., and obtains absorbed energy. And it evaluated by the minimum value of the obtained result.

As described in the first embodiment, as is apparent from FIG. 1, it can be seen that when the heat treatment temperature is between 450 ° C. and 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 to a plate thickness of 5 mm at a finishing temperature of 850 ° C. Then, it cooled by 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. Next, after winding with the winding temperature changed from 30 ° C. to 800 ° C., a sample was taken from the bottom of the obtained hot rolled coil, and the results of evaluating hot rolled sheet toughness are shown in FIG.
As can be seen from FIG. 7, the impact value at the bottom 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 can be 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, so that the toughness is lowered. Even in such a case, when the coiling temperature is 620 to 750 ° C., the temperature history over the entire length of the hot-rolled coil is controlled so as to satisfy the above formula (1). It is possible to eliminate the variation in toughness in each part.
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. to wind up.
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 was found that the workability of the cold rolled sheet is improved by setting the coiling temperature between 350 ° C. and 450 ° C. On the other hand, the decrease in the Rankford value at a coiling temperature exceeding 450 ° C. is due to the precipitation of Cu-rich clusters, and the decrease in the Rankford value below 350 ° C. is due to an increase in solid solution C and N. It is thought to be.
The steel composition 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 rise 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 that the temperature is in the range of 0 to 1000 ° C., and the atmosphere in this case is a combustion gas atmosphere. 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 higher than that of 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.

(Ferritic stainless steel hot-rolled steel sheet (second embodiment))
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 the present embodiment is, in mass%, C: 0.0010% to 0.010%, Si: 0.01% to 1.0%, Mn: 0.01% to 2. 00%, P: less than 0.040%, S: 0.010% or less, Cr: 10.0% to 30.0%, Cu: 1.0 to 2.0%, Al: 0.001% to 0 .10% and N: 0.0030% to 0.0200%
Each of which has a steel composition consisting of Fe and inevitable impurities, and the number density of Cu clusters with a maximum diameter of 5 nm or less made of Cu is less than 2 × 10 ¹³ pieces / mm ^{3 in} the crystal grains. .
Hereinafter, the reason which limited the steel composition of the hot-rolled steel plate of this embodiment is demonstrated. In addition, the description of% about a composition means the mass% unless there is particular notice.

C: 0.0010 to 0.010%
If C exists in a solid solution state, the intergranular corrosion resistance of the welded portion deteriorates, so a large amount is not preferable, and the upper limit is made 0.010%. Further, in order to reduce the amount of C so as not to affect the intergranular corrosion, the lower limit is made 0.0010% in order to bring about an increase in manufacturing cost such as an increase in refining time. From the viewpoint of intergranular corrosion of the weld and the manufacturing cost, the content is preferably 0.0020 to 0.0070%.

Si: 0.01 to 1.0%
Si is an element that improves oxidation resistance. However, if added in a large amount, the toughness is deteriorated, so the upper limit is made 1.0%. On the other hand, in order to inevitably mix as a deoxidizer, the lower limit is made 0.01%. The range is preferably 0.02% to 0.97%.

Mn: 0.01 to 2.00%
Mn is an element that improves high-temperature strength and oxidation resistance. However, addition of a large amount leads to deterioration of toughness like Si, so the upper limit is made 2.00%. Moreover, since it may inevitably be mixed, the lower limit is made 0.01%. The range is preferably 0.02% to 1.95%.

P: Less than 0.040% P is inevitably mixed from Cr raw materials and the like, so 0.005% is often mixed. However, since ductility and manufacturability are reduced, it is preferable that P be as small as possible. However, excessive dephosphorization is extremely difficult, and the manufacturing cost also increases, so the content is made less than 0.040%.

S: 0.010% or less Since S produces a compound that is easily dissolved and may deteriorate the corrosion resistance, it is preferable that the S content be less than 0.010%. Moreover, the lower one is preferable from a viewpoint of corrosion resistance, and less than 0.0050% is preferable.
In recent years, since desulfurization technology has been developed, the lower limit of S is preferably set to 0.0001%, and the lower limit is more preferably set to 0.0005% in view of stable manufacturability.

Cr: 10.0-30.0%
Cr is a basic element necessary for ensuring corrosion resistance, high temperature strength, and oxidation resistance, and in order to exhibit its effects, addition of 10.0% or more is essential. On the other hand, the addition of a large amount causes deterioration of toughness, so the upper limit is made 30.0%. The Cr content is preferably 20.0% or less because the Cr content increases as the Cr content increases, and the embrittlement phenomenon peculiar to high Cr steel called “475 ° C. embrittlement” tends to occur.

Cu: 1.0 to 2.0%
When Cu is added in an appropriate amount, the strength at a high temperature increases, so that addition to a steel plate for an automobile exhaust system member is suitable. If the addition amount is less than 1.0%, a sufficient amount of strengthening by Cu cannot be obtained, so the lower limit is made 1.0%. Further, it is preferably 1.05% or more. On the other hand, addition of a large amount causes deterioration of toughness in the course of production and in cold-rolled products, so the upper limit is made 2.0%. Further, it is preferably 1.75% or less.

Al: 0.001 to 0.10%,
Since Al is used as a deoxidizing element, an appropriate amount is added. If the addition is less than 0.001%, the deoxidizing ability is insufficient, so this is the lower limit. On the other hand, when the addition amount is 0.10%, the amount of oxygen can be sufficiently reduced, and even when the addition amount exceeds this amount, the deoxidation capacity is almost saturated. Furthermore, since excessive addition may cause deterioration in workability, the upper limit is made 0.10%. Preferably, it is in the range of 0.002% to 0.095%.

N: 0.0030 to 0.0200%
If N is present in the form of a solid solution, as in C, the intergranular corrosion property of the welded portion deteriorates, so that a large amount is not preferable. For this reason, the upper limit is made 0.0200%. In order to reduce the amount of N, the production cost increases such as an increase in refining time, so the lower limit is made 0.0030%. From the viewpoint of intergranular corrosion of the weld and the manufacturing cost, the content is preferably 0.0050 to 0.0120%.

In this embodiment, in addition to the above elements, one or more of Nb: 0.10 to 0.70% and Ti: 0.05 to 0.30% are represented by the following formula (2). It is preferable to add so as to satisfy.
Nb / 93 + Ti / 48 ≧ C / 12 + N / 14 (2)
Nb and Ti have a function of forming precipitates with C and N and reducing solid solution C and N. In addition, when Nb and Ti are present in a solid solution state, the high temperature strength and thermal fatigue characteristics of the member are improved by solid solution strengthening at high temperatures. In order to fix C and N, it is necessary to add Nb: 0.10% and Ti: 0.05% or more, respectively. Further, in order to make all the C and N present in the steel into a precipitated state, it is necessary to satisfy the above formula (2) stoichiometrically.
On the other hand, the addition of a large amount of both Nb and Ti leads to toughness deterioration during production, and the occurrence of surface flaws may become prominent, so the upper limit is Nb: 0.70%, Ti: 0.30% To do.

In the present embodiment, in addition to the above elements, one of Mo: 0.1 to 1.0%, Ni: 0.1 to 1.0%, Al: 0.50 to 3.0%, or It is preferable to add two or more kinds.
Mo, Ni, and Al are elements that increase the high-temperature strength, and may be added as necessary. Since Al is added for the purpose different from the aforementioned deoxidation, the appropriate addition amount is different. Ni also has the effect of improving toughness. The increase in the high-temperature strength becomes remarkable when the addition amounts are Mo: 0.10% or more, Ni: 0.10% or more, and Al: 0.50% or more, respectively. Moreover, since a large amount of addition causes deterioration of toughness during the production and generation of surface flaws, the upper limit is made 1.0%, 1.0% and 3.0%, respectively.

In the present embodiment, it is preferable to add B: 0.0001 to 0.0025% in addition to the above elements.
B is an element that improves secondary workability. When used in applications where secondary workability is required, it may be added as necessary. Since the effect of improving secondary workability is manifested when the addition amount is 0.0001% or more, this is the lower limit. Moreover, since a large amount of addition may reduce workability, the upper limit is made 0.0025%.

In addition, as an important feature of the present embodiment, the size of the Cu cluster made of Cu in the crystal grains is 5 nm or less at the maximum diameter. The size of the Cu cluster is defined as the maximum diameter of the Cu cluster, that is, the diameter when the Cu cluster is spherical, and the diagonal length when it is plate-like. In the present invention, the average value of the measured values of the maximum diameter is defined. Is specified. A method for measuring the maximum diameter of the Cu cluster will be described later.
According to the investigation by the present inventors, it was found that in the sample in which the toughness of the hot-rolled steel sheet was lowered, many Cu clusters having a maximum diameter of 5 nm or less existed. Therefore, in the present invention, in order to suppress a decrease in toughness of the hot-rolled steel sheet, the size of the Cu clusters in the crystal grains is set to 5 nm or less at the maximum diameter.
In the present invention, the lower limit of the size of the Cu cluster is not particularly limited. However, in consideration of the measurement accuracy of the size of the Cu cluster, the maximum diameter is preferably 1 nm or more.
In addition, as described above, such a finely sized Cu cluster is observed for the first time by the three-dimensional atom probe method or the like, and is different from the Cu precipitate disclosed in the prior art. It is considered a state.

Further, as a result of the above investigation, it was also found that there is a relationship between the density of the finely sized Cu clusters as described above and the toughness of the hot-rolled steel sheet. Therefore, in this embodiment, in order to maintain good toughness of the hot-rolled steel sheet, the number density of Cu clusters having a maximum diameter of 5 nm or less needs to be less than 2 × 10 ¹³ pieces / mm ³ .
The number density of Cu clusters greatly affects the strength and toughness of hot-rolled steel sheets. When Cu clusters are present at 2 × 10 ¹³ pieces / mm ³ or more, the toughness of hot-rolled steel sheets is significantly reduced, There are many cases where cracks occur. Such a Cu cluster having a maximum diameter of 5 nm or less becomes a strong pinning site such as dislocation, and the dislocation piles up and stress concentration is likely to occur. Therefore, it is considered that the density of stress concentration sites increases and the toughness decreases as the spatial density of such fine Cu clusters increases, so the number density of Cu clusters is 2 × 10 ¹³ pieces / mm ^3. Less than.

In addition, it is not only the fine Cu clusters as described above but also larger Cu precipitates that affect the toughness of the hot-rolled steel sheet, but within the scope of the present disclosure, such coarse Cu Since the cooling was terminated before the precipitates appeared, no coarse Cu precipitates were observed. That is, it is considered that the toughness of the hot rolled steel sheet according to the present invention is determined by the density of Cu clusters having a maximum diameter of 5 nm or less.

Next, there is a method for measuring the size and number density of fine Cu clusters as described above. Since Cu clusters are smaller than ordinary precipitates, the size and distribution density can be measured with a transmission electron microscope (TEM). It is difficult to measure. Therefore, the size and number density of Cu clusters in the crystal grains of the ferritic stainless steel hot rolled steel sheet according to the present invention are measured by the following procedure using the following three-dimensional atom probe (3D-AP) method.

First, a rod-shaped sample having a size of 0.3 mm × 0.3 mm × 10 mm is cut out from a hot-rolled steel sheet to be measured, and needle-shaped by an electrolytic polishing method. Using this processed needle-like sample, a measurement of 500,000 atoms or more is performed with 3D-AP (manufactured by Oxford Nanoscience) in an arbitrary direction within the crystal grain, and is visualized and quantitatively analyzed with a three-dimensional map.
Such measurement in an arbitrary direction is carried out for 10 or more different crystal grains, and the number density (number of clusters per volume of the observation region) and size of Cu included in each crystal grain are averaged. Ask. Regarding the size of the Cu cluster, the maximum length was measured in any shape such as a spherical shape or a plate shape. In particular, since the shape of a small-sized Cu cluster is often unclear, it is preferable to perform precise size measurement using electrolytic evaporation of a field ion microscope (FIM).
Here, FIM is a method of projecting the electric field distribution on the sample surface two-dimensionally by applying a high voltage to the needle-like sample and introducing an inert gas.
In general, precipitates in steel materials give a brighter or darker contrast than the ferrite matrix. By observing the generation and disappearance of precipitate contrast by performing field evaporation of specific atomic planes one atomic plane at a time, it is possible to accurately estimate the size of the precipitate in the depth direction.

(Method for producing ferritic stainless steel hot-rolled steel sheet (third 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 present embodiment uses a steel piece obtained by casting a ferritic stainless steel having the composition described in the ferritic stainless steel hot-rolled steel sheet (second embodiment). A step of forming a hot-rolled steel sheet by performing hot rolling, a step of winding the hot-rolled steel sheet in a coil shape after the hot rolling at a coiling temperature T of 300 ° C. to 500 ° C., and a hot-rolled steel sheet having a coil shape Is immersed in a water tank for 1 hour or longer, and after the immersion, the step of taking out the hot-rolled steel sheet from the water tank is included. After the step of winding the hot-rolled steel sheet into a coil shape, the hot-rolled steel sheet is expressed by the following formula (3). It is immersed in the water tank within a time tc (h) that satisfies the above.
tc = 10 ^ ((452-T) /76.7) (3)
Below, the manufacturing method of the ferritic stainless steel hot-rolled steel plate in this embodiment is demonstrated in detail.

First, hot rolling is performed using a steel slab cast from ferritic stainless steel containing the above steel composition. Then, after finish rolling, the product is cooled with water and wound into a coil. In this embodiment, the winding temperature T at this time is set to 300 ° C. to 500 ° C. When the winding temperature T is less than 300 ° C., the cooling state before winding tends to be uneven for each part of the steel sheet, and as a result, the shape of the winding coil tends to be poor, which is not preferable. In addition, when the coiling temperature T exceeds 500 ° C., the number density of Cu clusters made of Cu as described above becomes very high, which leads to poor toughness of the hot-rolled steel sheet.

Next, after winding up in a coil shape, it is immersed in a water tank. This is to suppress the formation of Cu clusters.
Here, the time from when the temperature of the hot-rolled steel sheet reaches the coiling temperature due to water cooling after finish rolling, until a Cu cluster having a maximum diameter of 5 nm or less is generated, its number density increases, and toughness starts to decrease. Strongly depends on the temperature change of the hot rolled steel sheet. When winding at a coiling temperature of 300 to 500 ° C. in normal hot rolling, the time from hot rolling to reaching the coiling temperature is within 1 min, and the cooling rate during this time is 3 ° C./sec or more. It is. In such a cooling rate condition, Cu clusters do not precipitate before winding. Further, it does not affect the subsequent winding conditions. In other words, after coiling after reaching the coiling temperature, it is necessary to quickly immerse in a water bath according to the coiling temperature and prevent Cu cluster precipitation before the toughness of the hot-rolled steel sheet decreases. is there. Therefore, together with the winding temperature T described above, the time required to immerse in the water tank after reaching the winding temperature T and winding it in a coil shape becomes important.
As a result of investigation by the present inventors, in this embodiment, after hot rolling and cooling, after winding at a winding temperature T (° C.), the time t (h) required for immersion is expressed by the above formula ( Within tc of 3).

When the time t from reaching the coiling temperature T to immersing in the water tank exceeds tc, the number density of Cu clusters having a size of 5 nm or less increases and exceeds 2 × 10 ¹³ pieces / mm ^3. This is not preferable because the toughness of the steel is lowered. In addition, when the coiling temperature T is high, tc is shortened because the Cu cluster generation start time is early, and when the coiling temperature T is low, tc is long.

Further, in the present embodiment, the time (immersion holding time) for holding in the water tank after being immersed in the water tank is also an important item. In the case of a steel sheet of a component system containing a large amount of Cu of 1% or more, if the immersion holding time in the water tank is as short as less than 1 hour, the cooling becomes insufficient and the formation of Cu clusters is not sufficiently suppressed. As a result, since the toughness of the hot-rolled steel sheet may be poor, the immersion holding time is set to 1 hour or more. In consideration of improvement in toughness, it is preferably 1.2 hours or longer. In addition, in this embodiment, although the minimum of the time hold | maintained in a water tank is not specifically limited, When productivity is considered, it is preferable that the immersion holding time in a water tank shall be less than 48 hours.

According to the ferritic stainless steel hot-rolled steel sheet according to the present embodiment as described above, the number density of fine Cu clusters that affect the toughness of the hot-rolled steel sheet is distributed lower than before due to the steel composition and configuration. Has been. Therefore, a decrease in toughness of the hot-rolled steel sheet can be suppressed, and as a result, cold cracking of the hot-rolled steel sheet can be prevented.
Moreover, according to the ferritic stainless steel hot-rolled steel sheet according to the present embodiment, cold cracking does not occur even after continuous annealing or hot pickling after hot rolling.

Moreover, according to the ferritic stainless steel hot-rolled steel sheet according to the present embodiment, since cold cracking can be suppressed, an increase in manufacturing yield and an improvement in production efficiency can be brought about. As a result, it is possible to exert a very useful effect on the industry in terms of manufacturing cost reduction and the like.
Moreover, since the energy used in the manufacturing process can be suppressed by improving the production efficiency, it can contribute to the conservation of the global environment.

Moreover, according to the manufacturing method of the ferritic stainless steel hot-rolled steel sheet according to the present embodiment, the time tc required to wind in the coil shape at the winding temperature T as described above and to immerse in the water tank after winding. And the number density of Cu clusters can be controlled by controlling the immersion holding time. As a result, a decrease in toughness of the hot-rolled steel sheet can be suppressed.
Thereby, it becomes possible to provide a ferritic stainless steel hot-rolled steel sheet excellent in cold cracking property.

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. The slab was heated to 1190 ° C. 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 was set to 10 to 100 ° C./s, and air cooling and water cooling were properly used according to the cooling rate, and the cooling was performed 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, for test numbers P58 to P63 in Tables 3 and 4, before 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).
The cold-rolled sheet annealing temperature was in the range of 880 to 950 ° C. in order to make the crystal grain size number about 6 to 8. For the comparative example in which the Nb content deviates from the upper limit of the present invention, the cold-rolled sheet annealing temperature was in the range of 1000 to 1050 ° C.
No. in Table 1 Nos. 0A to 0C and 1 to 24 are Nos. Reference numerals 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 less than 235 Hv was determined to be acceptable. 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. Incidentally, the Charpy test, performs compliant with JIS Z 2242, the impact value passed 20 J / cm ² or more (○), was evaluated less than 20 J / cm ² as unacceptable (×). The results are shown in Tables 3 and 4.
In addition, since the test piece in a present Example is a subsize test piece with 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 ). 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 three directions, respectively parallel (0 degree), 45 degrees, and 90 degrees with respect to the rolling direction of a steel plate surface. In the evaluation of workability, the average rankford value of the measured values in the three directions obtained was particularly excellent when it was 1.1 or more. However, the numerical value does not necessarily need to be achieved and is 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 in 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 excellent in 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.
Further, it can be seen that even in the case of test numbers P58 to 60 subjected to hot-rolled sheet annealing, the same effect as in the present invention example in which hot-rolled sheet annealing is omitted can be obtained.

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, a good toughness value 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.

Test numbers P5 to P7 and P12 to P14 were in a low temperature range where the coiling temperature was higher than 450 ° C and lower 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, since the coiling temperature was increased to over 750 ° C., the toughness was good, but the pickling property was poor. This is because the coiling temperature was high and 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 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 precipitated.

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, good 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 number P48, since the Ti content was small and solute C and N could not be sufficiently fixed, 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 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.

Further, test numbers P61 to P63 were cases where hot-rolled sheet annealing was performed, but the coiling temperature was a low temperature range of more than 450 ° C. and less than 650 ° C. as in test numbers P5 to P7 and P12 to P14. . 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 and 450 ° C. was set to a predetermined rate as shown in Tables 7 and 8, and the hot-rolled steel sheet was cooled to each winding temperature shown in Tables 7 and 8 by water cooling. 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, for test numbers P58 to P64 in Tables 7 and 8, before 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).
The cold-rolled sheet annealing temperature was in the range of 880 to 950 ° C. in order to make the crystal grain size number about 6 to 8. For the comparative example in which the Nb content deviates from the upper limit of the present invention, the cold-rolled sheet annealing temperature was in the range of 1000 to 1050 ° C.
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, 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 manufacturing 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.
Further, it can be seen that even in the case of test numbers P58 to 61 subjected to hot-rolled sheet annealing, the same effects as those of the present invention example in which hot-rolled sheet annealing is omitted can be obtained.

On the other hand, in the 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.

The test numbers P1 to P3 of the comparative examples had a low coiling temperature of 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 carbonitride 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 decreased. 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.

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, 475 ° C. brittleness occurred and the toughness was poor.

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, and these coarse precipitates serve as starting points for 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, the pickling property and the 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 P64 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. as in 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 in the range of 350 ° C. to 450 ° C., and the average cooling rate of 850 ° C. to 450 ° C. is 10 ° C./s or more after hot rolling. The washability, high-temperature strength, and rankford values 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 knowledge could be confirmed, and the grounds for limiting the above-described steel composition and composition could be supported.

(Example 3)
In this example, first, each steel having the components shown in Table 9 was melted to obtain a steel ingot. The steel ingot was ground to a thickness of 90 mm and hot rolled to a thickness of 5 mm to obtain a hot rolled steel sheet. Next, while monitoring the steel plate temperature after rolling with a radiation thermometer, it was cooled by water cooling to a predetermined coiling temperature T (° C.) shown in Table 10. The cooling rate at this time was about 20 ° C./sec.
Next, the hot-rolled steel sheet was wound in a coil shape at a winding temperature T (° C.). Thereafter, as shown in Table 10, the time until dipping in the water tank was set to t (h), and the hot-rolled steel sheet wound in a coil shape was immersed in the water tank.
Subsequently, after being immersed in the water tank for the immersion holding time (h) as shown in Table 10, the hot rolled steel sheet was taken out. In addition, time tc (h) in Table 10 is a value calculated from the above formula (3), and is the upper limit time after winding the hot-rolled steel sheet in order to exert the effect of the present invention. It is necessary to immerse in the water tank within the time tc.

Using each obtained hot-rolled steel sheet, the size (maximum diameter) and number density of Cu clusters in the crystal grains of the hot-rolled steel sheet were measured by the 3D-AP method. Table 10 shows the measurement results. In addition, the number density X in Table 10 represents the number density (× 10 ¹³ / mm ² ) of Cu clusters having a maximum diameter of 5 nm or less.
Furthermore, Charpy test pieces were collected from the obtained hot-rolled steel sheet in the direction perpendicular to the rolling direction, and the Charpy test was performed at 25 ° C. to determine the Charpy impact value. The results are shown in Table 10. Moreover, from the obtained result, the cold cracking property of the hot rolled steel sheet was evaluated by the following method. The Charpy test was conducted in accordance with JIS Z 2242.
In this example, the evaluation method of the cold cracking property is such that when the Charpy impact value is less than 20 J / cm ² , cold cracking or the like occurs in the subsequent process, such as continuous annealing or pickling process, and the yield decreases. Therefore, it was judged as bad. Further, in the case of 20 J / cm ² or more, such a cold crack did not occur.
The above production conditions and evaluation results are shown in Table 10.

As is apparent from Table 10, according to the present invention example to which the present invention is applied, a ferritic stainless hot rolled steel sheet having good toughness of the hot rolled steel sheet, that is, excellent cold cracking property, can be obtained.

On the other hand, in the comparative examples that deviate from the examples of the present invention, the Charpy impact value was low. Thereby, it turns out that the toughness of the hot-rolled steel sheet in the comparative example has decreased.

In Test Nos. 10 and 25, since the coiling temperature T was too high, the formation of Cu clusters could not be sufficiently suppressed, and as a result, the number density became very high. Thereby, it is thought that the toughness of the hot-rolled steel sheet has been lowered.

In

test numbers

2, 5, 6, 9, 14, 15, 17, 17, 21, 24, 25, 26, 31, 34, and 37, the time t until the hot-rolled steel sheet is wound and immersed in the water tank is It was longer than the upper limit time tc. Therefore, the production of Cu clusters progressed during that time, and the number density of Cu clusters increased. As a result, it is considered that the Charpy impact value has decreased.

Test Nos. 3, 5, 12, 18, 21, 28, 33, 34 and 37 all had an immersion holding time of 1 hour, so that the hot-rolled steel sheet was not sufficiently cooled, and the suppression of the formation of Cu clusters was not possible. It was enough. As a result, it is considered that the toughness of the hot-rolled steel sheet has decreased.

In Test Nos. 35 and 36, although the number density of Cu clusters could be kept low, it was considered that the toughness was lowered because the Cr content in the steel sheet was too much.

Moreover, the coiling temperature T was changed variously using J steel, it wound up, and also the result of having evaluated the toughness of what was immersed in the water tank for 2 hours changing various time t until it immersed in a water tank is FIG. Shown in X indicates that the Charpy impact value is less than 20 J / cm ² , which is inferior in toughness, and ○ indicates that the Charpy impact value is 20 J / cm ² or more, which is favorable in toughness.
A straight line indicated by a dotted line in FIG. 9 indicates a boundary between an inferior toughness and a good one, and reaches the winding temperature T and the winding temperature T represented by the above formula (3), The relationship of the upper limit tc of time after winding up and immersing in a water tank is shown. Furthermore, it was found that even if a similar graph was created using other steel types, straight lines showing similar boundaries could be obtained.

As is clear from the above description, according to the method for producing a ferritic stainless steel hot-rolled steel sheet of the present invention, since an expensive alloy element such as Nb or Mo is replaced by Cu, it has high high-temperature strength. In such a stainless steel plate, it is possible to increase the 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

% 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: 10-20%,
Cu: 1.0 to 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 (Equation 1)
The steel slab having the steel composition according to claim 1 or claim 2, wherein the average cooling rate between 850 ° C and 450 ° C after finish rolling of hot rolling is 10 ° C / second or more, and the coiling temperature A method for producing a ferritic stainless steel hot-rolled steel sheet, characterized by winding at 350 ° C. to 450 ° C.
A ferrite comprising hot-rolled sheet pickling, cold-rolling, cold-rolled sheet annealing, and cold-rolled sheet pickling performed on the hot-rolled steel sheet produced by the method according to claim 3, claim 4, or claim 5. Of manufacturing stainless steel sheet.
Performing hot-rolled sheet annealing, 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 claim 3, claim 4, or claim 5. A method for producing a ferritic stainless steel sheet.
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.
% By mass
C: 0.0010% to 0.010%,
Si: 0.01% to 1.0%
Mn: 0.01% to 2.00%
P: less than 0.040%,
S: 0.010% or less,
Cr: 10.0% to 30.0%,
Cu: 1.0 to 2.0%,
Al: 0.001% to 0.10%,
N: 0.0030% to 0.0200%
Each containing
The balance has a steel composition consisting of Fe and inevitable impurities,
A ferritic stainless steel hot-rolled steel sheet, wherein the number density of Cu clusters having a maximum diameter of 5 nm or less made of Cu is less than 2 × 10 13 pieces / mm 3 in crystal grains.
Furthermore, in mass%,
Nb: 0.10% to 0.70% or less,
Ti: 0.05% to 0.30% or less,
The ferritic stainless steel hot-rolled steel sheet according to claim 9, wherein one or more of them are included so as to satisfy the following (formula 2).
Nb / 93 + Ti / 48 ≧ C / 12 + N / 14 (Formula 2)
Furthermore, in mass%,
Mo: 0.1% to 1.0%,
Ni: 0.1% to 1.0%
Al: 0.50% to 3.0%
The ferritic stainless steel hot-rolled steel sheet having excellent cold cracking properties according to claim 9 or 10, wherein one or more of them are included.
Furthermore, in mass%,
B: 0.0001% to 0.0025%,
The ferritic stainless steel hot-rolled steel sheet having excellent cold cracking properties according to any one of claims 9 to 11, characterized by comprising:
A method for producing a ferritic stainless hot-rolled steel sheet according to any one of claims 1 to 4,
A step of hot-rolling a steel sheet by hot rolling using a steel slab cast from the ferritic stainless steel having the steel composition according to any one of claims 9 to 12, and
A step of winding the hot-rolled steel sheet into a coil after setting the coiling temperature T to 300 ° C. to 500 ° C. after hot rolling;
The step of immersing the coiled hot-rolled steel sheet in a water tank for 1 hour or more, and removing the hot-rolled steel sheet from the water tank after the immersion;
Have
After the step of winding the hot-rolled steel sheet into a coil, the hot-rolled steel sheet is immersed in the water tank within a time tc (h) that satisfies the following (Equation 3). A method for producing rolled steel sheets.
tc = 10 ^ ((452-T) /76.7) (Formula 3)