JP2012162795A - Hot rolled ferritic stainless steel sheet excellent in cold cracking nature and method of manufacturing the same - Google Patents

Abstract

A ferritic stainless hot rolled steel sheet having excellent cold cracking properties is provided.
SOLUTION: In mass%, C: 0.0010% to 0.010%, Si: 0.01% to 1.0%, Mn: 0.01% to 2.00%, P: 0.040% Less than 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% respectively, the balance being 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 in the crystal grains is 2 × 10 A ferritic stainless hot-rolled steel sheet having excellent cold cracking characteristics, which is less than ¹³ pieces / mm ³ , is employed.
[Selection figure] None

Description

  The present invention relates to a ferritic stainless hot rolled steel sheet having excellent cold cracking properties and a method for producing the same.

  Since automobile exhaust system members are exposed to the high-temperature exhaust gas environment discharged from the engine, at present, ferritic stainless steel having excellent heat resistance, oxidation resistance, heat fatigue resistance, etc. is generally used. ing.

  For the purpose of improving the above characteristics, ferritic stainless steel sheets to which elements mainly composed of Cr and Mo have been added have been developed so far. Recently, steel sheets to which Cu has been added have been developed.

  In Patent Document 1, 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 2 discloses a technique about 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 down to room temperature after hot rolling because of the capacity of the cooling equipment, and the end temperature of water cooling is not clear, and the conditions that can be actually applied are clear Was not.

  Ferritic stainless steels that are subject to toughness of hot-rolled sheets include steel types with a high Cr content and steel types to which Al is added. Patent Documents 3 to 5 are known as means for solving these hot-rolled sheet toughnesses. .

Patent Document 3 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 steel type to which Cr is added in an amount of 25 to 35% by weight. ing.
Patent Document 4 discloses a technique in which ferritic stainless steel containing 3 to 7% by weight of Al is rapidly cooled after winding.

  Patent Document 5 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.

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

  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.

  Then, this invention is made | formed in view of said situation, Comprising: It aims at providing the ferritic stainless steel hot-rolled steel plate excellent in cold crack property, and its manufacturing method.

In order to solve the above-described problems, the present inventors have investigated the relationship between hot-rolling conditions of ferritic stainless steel and toughness of hot-rolled sheets.
First, after ferritic stainless steel with varying Cu content is hot-rolled to a thickness of 5 mm in a 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 h 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.

  In addition, focusing on fine precipitates such as Cu clusters existing in the hot-rolled steel sheet 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 has a size of several tens of nm or more.
In the present invention, the “Cu cluster” is defined as an aggregate of Cu atoms having a maximum diameter of 5 nm or less, which is confirmed in a survey using 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. . 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. 1 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. 1, at a coiling temperature of 500 ° C. or lower, the Charpy impact value (toughness value) decreases as the time until the 1.2% Cu-added steel is immersed in the water tank decreases, It has been found that the toughness value is lower than 20 J / cm ^{2 after the time} is 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 that is 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 is as follows.

[1] 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: 10.0-30.0%,
Cu: 1.0-2.0%,
Al: 0.001 to 0.10%,
N: 0.0030-0.0200%,
Each of which has a steel composition comprising 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 pieces / mm ^{3 in} the crystal grains. A ferritic stainless hot rolled steel sheet with excellent cold cracking characteristics.
[2] Furthermore, in mass%,
Nb: 0.10 to 0.70%,
Ti: 0.05-0.30%
1 type or 2 types or more are included so that the following formula (1) may be satisfied, The ferritic stainless steel hot rolled steel sheet excellent in cold crack property as described in said [1] characterized by the above-mentioned.
Nb / 93 + Ti / 48 ≧ C / 12 + N / 14 (1)
[3] 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 the above [1] or [2], comprising one or more of them.
[4] Furthermore, in mass%,
B: 0.0001 to 0.0025%
The ferritic stainless hot-rolled steel sheet having excellent cold cracking properties according to any one of [1] to [3] above.
[5] A method for producing a ferritic stainless hot-rolled steel sheet according to any one of [1] to [4], wherein the ferrite has the steel composition according to any one of [1] to [4]. A hot rolled steel sheet by hot rolling using a steel piece cast from a stainless steel, and after the hot rolling, the coiling temperature T is set to 300 ° C. to 500 ° C., and the hot rolled steel sheet is coiled A step of winding the hot-rolled steel sheet in a coil shape into a water tank for at least 1 hour, and taking out the hot-rolled steel sheet from the water tank after the immersion. After the step of winding into a shape, the hot-rolled steel sheet is immersed in the water tank within a time tc (h) that satisfies the following formula (2). A method for producing rolled steel sheets.
tc = 10 ^ ((452-T) /76.7) (2)

As described above, according to the present invention, the number density of fine Cu clusters that affect the toughness of the hot-rolled steel sheet is distributed lower than in the past. Therefore, the fall of the toughness of a hot-rolled steel plate can be suppressed, and as a result, the cold crack of a hot-rolled steel plate 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 plate 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. Moreover, energy consumption can be suppressed by improving production efficiency, which can contribute to global environmental conservation.

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.

  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 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% -30.0%, Cu: 1.0-2.0%, Al: 0.001% -0. 10% and N: 0.0030% to 0.0200%
Each of which 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 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 influence 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. In view of the intergranular corrosion property 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, the smaller one is preferable, and the content is made 0.010% or less. Moreover, the lower one is preferable from a viewpoint of corrosion resistance, and it is preferable to set it as less than 0.0050%.
In recent years, since desulfurization technology has been developed, the lower limit of S is preferably 0.0001%, and the lower limit is more preferably 0.0005% in consideration of stable productivity.

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 exert 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 greater the Cr content, the higher the strength, and the more likely the embrittlement phenomenon peculiar to high Cr steel called “475 ° C. embrittlement” occurs.

Cu: 1.0-2.0%
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 leads to deterioration of toughness during 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 the deoxidation ability is almost saturated even when the addition amount exceeds this amount. Furthermore, since excessive addition may cause deterioration in workability, the upper limit is made 0.10%. In addition, Preferably, it is 0.002%-0.095% of range.

N: 0.0030 to 0.0200%
If N is present in a solid solution state as in C, addition of a large amount is not preferable because the intergranular corrosion resistance of the welded portion deteriorates. For this reason, the upper limit is made 0.0200%. Further, 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%. In view of the intergranular corrosion property 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 (3). It is preferable to add so as to satisfy.
Nb / 93 + Ti / 48 ≧ C / 12 + N / 14 (3)
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 (3) 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 this 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.
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. Further, since a large amount of addition causes deterioration of toughness during production and generation of surface flaws, the upper limits are set to 1.0%, 1.0%, and 3.0%, respectively.

In this 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%.

Further, as an important feature of the present embodiment, the size of the Cu cluster made of Cu in the crystal grains is set to 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 this invention, in order to suppress the fall of the toughness of a hot-rolled steel plate, the size of the Cu cluster in a crystal grain shall be 5 nm or less in 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. Accordingly, it is considered that the density of stress concentration sites increases and the toughness decreases due to an increase in the spatial density of such fine Cu clusters. Therefore, 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 in the present invention are measured by the following procedure using a three-dimensional atom probe (3D-AP) method described below.

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 in 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.

  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 includes a step of hot-rolling a steel strip obtained by casting a ferritic stainless steel having the above composition to form a hot-rolled steel sheet, and after hot rolling The winding temperature T is set to 300 ° C. to 500 ° C., the step of winding the hot-rolled steel sheet into a coil shape, and the hot-rolled steel sheet made into a coil shape is immersed in a water tank for 1 hour or more. After the step of winding the hot-rolled steel sheet into a coil shape, the hot-rolled steel sheet is immersed in a water tank within a time tc (h) that satisfies the following formula (4).
tc = 10 ^ ((452-T) /76.7) (4)
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 piece obtained by casting a 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 for suppressing the formation of Cu clusters.
Here, the time from when the temperature of the hot-rolled steel sheet reaches the coiling temperature by 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. In addition, 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 are not deposited before winding. Moreover, 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 investigations 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 dipping is expressed by the above formula ( Within tc of 4).

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.

  Moreover, in this embodiment, the time (immersion holding time) to hold | maintain in a water tank after being immersed in a 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 invention 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 above components and configuration. . Therefore, the fall of the toughness of a hot-rolled steel plate can be suppressed, and as a result, the cold crack of a hot-rolled steel plate 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 the ferritic stainless steel hot-rolled steel sheet according to the present invention, cold cracking can be suppressed, so that 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 invention, the coil is wound in the coil shape at the winding temperature T as described above, and after the winding, it takes time tc and the immersion to be immersed in the water tank. By controlling the holding time, the number density of Cu clusters can be controlled. As a result, a decrease in toughness of the hot rolled steel sheet can be suppressed.
As a result, it is possible to provide a ferritic stainless hot-rolled steel sheet having excellent cold cracking properties.

  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.

In this example, first, each steel having the components shown in Table 1 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, the steel sheet was cooled to a predetermined coiling temperature T (° C.) shown in Table 2 by water cooling. 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.). Then, as shown in Table 2, the time until dipping in the water tank was set to t (h), and the hot rolled steel sheet wound up 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 2, the hot rolled steel sheet was taken out. The time tc (h) in Table 2 is a value calculated from the above formula (4), 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 of the obtained hot-rolled steel sheets, 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. The measurement results are shown in Table 2. In addition, the number density X in Table 2 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 2. 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 2.

  As apparent from Table 2, 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 deviated 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 numbers 10 and 25, since the coiling temperature T was too high, the formation of Cu clusters could be sufficiently suppressed. 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, the number density of Cu clusters could be kept low, but it was considered that the toughness was lowered because the Cr content in the steel sheet was too large.

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. 1 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 (4), 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.

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

Claims (5)

% 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: 1.0-2.0%,
Al: 0.001% to 0.10%,
And N: 0.0030% to 0.0200%
Each containing
The balance has a steel composition consisting of Fe and inevitable impurities,
A ferritic stainless hot-rolled steel sheet having excellent cold cracking properties, 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 having excellent cold cracking properties according to claim 1, wherein one or more of them are included so as to satisfy the following formula (1).
Nb / 93 + Ti / 48 ≧ C / 12 + N / 14 (1) 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 1 or 2, 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 1 to 3, 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 forming a hot-rolled steel sheet by performing hot rolling using a steel piece obtained by casting the ferritic stainless steel having the steel composition according to any one of claims 1 to 4,
A step of winding the hot-rolled steel sheet into a coil after setting the coiling temperature T to 300C to 500C after hot rolling;
The step of immersing the coiled hot-rolled steel sheet in a water tank for 1 hour or more, and taking out 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 formula (2). A method for producing excellent ferritic stainless steel hot-rolled steel sheets.
tc = 10 ^ ((452-T) /76.7) (2)

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