JP2005314740A - Ferritic stainless steel having excellent heat resistance and workability and its production method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide ferritic stainless steel having excellent heat resistance and workability, and to provide its production method. <P>SOLUTION: The ferritic stainless steel sheet has components satisfying, by mass, ≤0.015% C, 0.10 to 0.25% Si, 0.10 to 0.3% Mn (wherein, Si≤Mn), ≤0.04% P, ≤0.01% S, 0.001 to 0.2% Al, ≤0.015% N, 15 to 20% Cr, ≤0.5% Ni, 1.0 to 2.5% Mo, ≤0.2% V, Ti: 3X(C+N) to 0.25%, and 0.3 to 1% Nb (wherein, C+N: ≤0.02%), and the balance Fe with inevitable impurities. The steel sheet may comprise 0.1 to 1.0% Cu and 0.0003 to 0.005% B. It is also possible that the steel has a sheet thickness of 1 to 3 mm, a 0.2% proof stress of ≥15 MPa at 950°C, a mean elongation value of ≥30% at ordinary temperature, and a mean r value of ≥1.3. The production method is characterized in that, in a process where an ingot or a slab is cast, and is subjected to hot rolling, hot rolled sheet annealing, pickling, cold rolling, final annealing and pickling to obtain a product, the temperature in the hot rolled sheet annealing is controlled to 800 to 950°C. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a ferritic stainless steel excellent in heat resistance and workability used for automobile exhaust system members such as a muffler and an exhaust manifold, and a method for producing the same.

Due to the recent increase in environmental problems, the automobile industry is making efforts to deal with environmental problems from both an attempt to reduce the amount of exhaust gas emitted from automobiles and an attempt to purify the exhaust gas itself. The former attempt is, for example, an improvement in fuel consumption of an automobile engine and an attempt to reduce the weight of the vehicle body. In the latter attempt, for example, a catalyst such as platinum or rhodium is used to oxidize CO, HC, NOx, which are typical exhaust harmful substances in exhaust gas, and CO and HC are oxidized to CO ₂ and H ₂ O (water NOx is reduced to nitrogen (N ₂ ) to make it harmless.

  Stainless steel is excellent in heat resistance (high temperature strength and oxidation resistance) and is a commonly used material for automotive exhaust system members through which exhaust gas, which is high temperature corrosive gas, passes. Among them, the exhaust manifold, which is one of the parts exposed to the highest temperatures, has a higher operating temperature due to the reaction efficiency of the catalytic reaction for exhaust gas purification. Therefore, excellent heat resistance is required.

  As a material for a conventional automobile exhaust system member, a steel type having a corresponding temperature of 950 ° C. has been developed. For example, Patent Document 1 discloses Cr: 18-22%, Mo: 1.0-2. An invention of a stainless steel containing 0%, Nb: 0.1-1.0% is disclosed. At present, ferritic stainless steel such as SUS444 (19% Cr-2% Mo) is used as an exhaust manifold material compatible with 950 ° C.

  On the other hand, automobile exhaust pipe members such as exhaust manifolds may have complicated shapes in order to effectively use the limited space in the vehicle body, and are often subjected to severe processing, and excellent workability is also required. It is done. In recent years, in particular, due to the trend toward weight reduction, the shape has become increasingly complex, and the demand for workability on the steel sheet used has become stricter.

  However, steel sheets with high heat resistance usually have low workability because the amount of alloy elements such as Nb and Mo increases. In particular, a thick steel plate having a thickness of 1.5 mm or more used for an exhaust manifold or the like has a relatively low workability because the rolling ratio in cold rolling cannot be increased. Therefore, improving the workability of high alloy steel plates with high heat resistance has been a major issue.

In order to solve such a problem, for example, Patent Document 2 discloses a manufacturing method for obtaining a Cr-containing heat-resistant and corrosion-resistant steel sheet having excellent formability by defining a hot rolling end temperature, a hot rolled sheet annealing temperature, and a final annealing temperature. A method invention is disclosed.
In Patent Document 3, ferrite stainless steel containing a large amount of Nb is subjected to precipitation treatment at 700 to 850 ° C. for 25 hours or less before final annealing, thereby providing a ferrite with excellent workability with controlled precipitates and texture. An invention of a stainless steel plate and a method for producing the same is disclosed.
Japanese Patent Laid-Open No. 06-100990 JP 2002-30346 A JP 2002-194508 A

However, the invention described in Patent Document 2 has a problem that the temperature at which hot rolling is performed is as low as 690 to 1020 ° C. according to the embodiment, and a great load is applied to the hot rolling equipment. Moreover, in the invention described in Patent Document 3, an additional precipitation treatment step is necessary, and the cost increase due to the increase in the number of steps has been a problem. Thus, a ferritic stainless steel sheet having excellent heat resistance and excellent workability that can be manufactured at as low a cost as possible has not been found heretofore.
Accordingly, an object of the present invention is to provide a ferritic stainless steel excellent in heat resistance and workability, which is useful for an automobile exhaust system member, particularly an exhaust manifold, and a method for producing the same.

The gist of the present invention is as follows.
(1) In mass%,
C: 0.015% or less, Si: 0.10 to 0.25%,
Mn: 0.10 to 0.30%, P: 0.040% or less,
S: 0.020% or less, Al: 0.001 to 0.20%,
N: 0.015% or less, Cr: 15.0-20.0%,
Ni: 0.5% or less, Mo: 1.0-2.5%,
V: 0.2% or less, Ti: 3 × (C + N) to 0.25%,
Nb: 0.3 to 1.0%
Containing
Furthermore, said C and N satisfy | fill the relationship of C + N: 0.020% or less,
Further, the Si and Mn satisfy the relationship of Si ≦ Mn,
A ferritic stainless steel excellent in heat resistance and workability, characterized by comprising the balance Fe and inevitable impurities.
(2) The ferritic stainless steel excellent in heat resistance and workability as described in (1) above, further containing, by mass%, Cu: 0.1 to 1.0%.
(3) Ferritic stainless steel excellent in heat resistance and workability as described in (1) or (2) above, further comprising, by mass%, B: 0.0003 to 0.0050% or less steel.
(4) The ferritic stainless steel has a plate thickness of 1 to 3 mm, a 0.2% proof stress at 950 ° C. of 15 MPa or more, an average elongation value at room temperature of 30% or more, an average r value (average The ferritic stainless steel excellent in heat resistance and workability according to any one of the above (1) to (3), characterized in that it is a ferritic stainless steel sheet having a Rankford value of 1.3 or more. steel.
(5) The ferritic stainless steel having excellent heat resistance and workability as described in (4) above, wherein the crystal grain size of the ferritic stainless steel is a crystal grain size number 5-8.
(6) The crystal orientation of the rolled surface at a half depth of the plate thickness is 5 or more in terms of an integrated intensity ratio defined by the following formula, as described in (4) or (5) above: Ferritic stainless steel with excellent heat resistance and workability.
Integral intensity ratio = 111 random diffraction intensity of X-ray diffraction / 200 random diffraction intensity of X-ray diffraction

(7) Performing hot rolling, hot-rolled sheet annealing, pickling, cold rolling, final annealing, pickling the steel slab of the steel component according to any one of (1) to (3) above When manufacturing the ferritic stainless steel excellent in heat resistance and workability described in any one of (4) to (6) above, the annealing temperature of the hot-rolled sheet annealing is set to 800 to 950 ° C. A method for producing ferritic stainless steel having excellent heat resistance and workability.
(8) The method for producing a ferritic stainless steel excellent in heat resistance and workability according to (7) above, wherein a work roll having a diameter of 300 mm or more is used in the cold rolling.

  According to the present invention, it is possible to provide a ferritic stainless steel excellent in heat resistance and workability that is useful for automobile exhaust system members, particularly for exhaust manifolds, which is great not only for manufacturers but also for those who use this steel. Profits can be obtained and the industrial value is extremely high.

The best mode and limiting conditions for carrying out the present invention will be described in detail.
The present inventors have studied an automobile exhaust system member, particularly an exhaust manifold member having a maximum temperature of about 1000 ° C., which has optimum characteristics. The characteristics required for the exhaust manifold material are heat resistance (high temperature strength, oxidation resistance) and workability.
As a target value of required characteristics,
(A) High temperature strength: 0.2% proof stress at 950 ° C. of 15 MPa or more,
(B) Oxidation resistance: In a continuous oxidation test in air at 950 ° C. for 200 hours, the amount of scale peeling is 0.5 mg / cm ² or less,
(C) Workability: Average elongation value at room temperature is 30% or more, average r value is 1.2 or more, preferably 1.3 or more,
It was decided to satisfy all of the above.

In order to achieve these goals, the component elements were optimized and manufacturing processes were established.
as a result,
(1) In order to satisfy the high temperature strength, it is essential to add 0.3% or more of Nb and 1.0% or more of Mo (high Nb and high Mo).
(2) In order to satisfy oxidation resistance and improve workability, it has the effect of enhancing oxidation resistance by improving the oxidation resistance and setting the Cr content with little deterioration in workability to 15% or more. The amount of Si and Mn, which significantly deteriorates workability, was reduced.
(3) In order to satisfy workability under high Nb and high Mo addition, C and N were reduced as much as possible, and high elongation and high r value were achieved by optimization of Ti addition and hot-rolled sheet annealing. .

The greatest point of the present invention is the above (3), which is how the workability can be improved with the component system set from the required characteristics of (1) and (2). In particular, improvement of r value (Rankford value, plastic strain ratio) was required.
The present inventors have conducted a detailed examination of the component system and the manufacturing process, and found that the formation of the structure before cold rolling is important in order to obtain excellent workability. That is, it has been found that excellent workability can be obtained by subjecting the hot-rolled sheet of the component of the present invention to hot-rolled sheet annealing in the range of 800 to 950 ° C.

FIG. 1 is an example showing the relationship between the hot rolled sheet annealing temperature and the average r value. From the figure, it can be seen that even when Ti is not added, a relatively high average r value is obtained when the annealing temperature of the hot-rolled sheet is 800 to 900 ° C., but when Ti is added, a higher average r value is obtained. .
FIG. 2 shows the relationship between the Ti amount and the average r value. From the figure, when the amount of Ti is 0.005%, the addition effect is hardly seen because the amount of Ti is too small. However, when the Ti amount is 0.05%, a remarkable effect of improving the r value is observed, and an average r value of 1.3 or more is shown in the range of 0.05% to 0.25%.

The reason for the effect of improving the r value by adding Ti is not clear, but is considered as follows.
That is, Mo and Nb are precipitated to some extent as carbonitrides and Ruffes phases during hot-rolled sheet annealing in amounts within the range of the present invention. As a result of the precipitation of Mo and Nb, the amount of solid solution Mo and the amount of solid solution Nb in the parent phase decreased, which was advantageous for the r value as the structure before cold rolling. Furthermore, recrystallization does not occur at this hot-rolled sheet annealing temperature without addition of Ti, but recrystallization temperature decreases due to the addition of Ti, and recrystallization occurs within this hot-rolled sheet annealing temperature range. This recrystallization achieves refinement of the structure before cold rolling, which is advantageous for improving the r value. It is considered that a high r value was obtained in the component system of the present invention by superimposing these two effects.
Based on the above knowledge, the present inventors proceeded with further detailed studies and completed the present invention.

Next, the reason for limitation regarding each component of this invention is described.
Since C deteriorates workability and corrosion resistance, it is preferable that C be as small as possible. Therefore, in the present invention, it is fixed as carbonitride to remove harmful effects, but the upper limit of C content is set to 0.015% or less in order to minimize the addition amount of Ti as a fixing element for that purpose. However, if the C content is less than 0.002%, an excessive cost burden is imposed on refining, so 0.002% or more is preferable.

Si is an element that improves the oxidation resistance, and is usually added to the heat-resistant stainless steel by about 0.3 to 1%. In particular, when the Cr content is about 14%, Si is an essential element for forming a sound Cr ₂ O ₃ scale. However, Si is also an element that hinders the adhesion between the Cr ₂ O ₃ film and the parent phase, and addition of a large amount is not preferable because the scale easily peels off. Therefore, the present inventors have conducted detailed studies and have clarified that when Cr is contained in an amount of 15% or more, sufficient oxidation resistance can be maintained even if the amount of Si added is small. According to this knowledge, since the improvement of workability can be expected as the amount of Si decreases, the upper limit of Si is set to 0.25%. On the other hand, if Si is less than 0.10%, an excessive cost burden is imposed on refining, so 0.10% was made the lower limit. In addition, since there exists a possibility of deteriorating oxidation resistance to make Si amount less than 0.15%, Preferably it is 0.15% or more and 0.25% or less.

Mn is a component that is inevitably contained in the steel, but is considered to be an element that improves oxidation resistance. In particular, when it coexists with Si, it has an effect of suppressing scale peeling by Si. However, Mn is an element that tends to increase the amount of oxidation. When Si is reduced with a Cr content of 15% or more as in the present invention, Mn can also be reduced. As a result, improvement in workability can be expected. Therefore, in the present invention, the upper limit of Mn is set to 0.30% or less in consideration of the allowable range of increase in oxidation. On the other hand, if the amount of Mn is less than 0.10%, an excessive cost burden is imposed on refining, so 0.10% was made the lower limit. Since there exists a possibility of causing deterioration of oxidation resistance when Mn is reduced too much, it is more preferably 0.15 to 0.30%.
In addition, when Si> Mn, there is a concern that the anti-scale peeling effect of Mn is insufficient and scale peeling occurs, and therefore Si ≦ Mn.

P is a component inevitably contained in the steel, but if it exceeds 0.040%, weldability decreases, so 0.040% was made the upper limit.
S is a component inevitably contained in the steel, but if it exceeds 0.020%, the corrosion resistance is lowered due to an increase in the amount of MnS formed, so 0.020% was made the upper limit.

  Al is very useful as a deoxidizing element. In the present invention, addition of Al as a deoxidizing element is essential in order to limit Si to an extremely low level. In order to perform sufficient deoxidation, the amount of Al in the steel after deoxidation needs to be 0.001% or more. However, if added excessively, the workability deteriorates and the high temperature fatigue strength due to internal oxidation decreases, so the upper limit is made 0.20%.

  N is an unavoidable impurity contained in the steel and, like C, is deteriorated in workability and weldability, and therefore is preferably as small as possible. Therefore, the upper limit of the allowable amount of N is set to 0.015% or less. Note that if it is less than 0.005%, an excessive cost burden is imposed on refining, so 0.005% or more is preferable.

Cr is an element that forms a protective Cr ₂ O ₃ film and improves oxidation resistance. When Cr is less than 15.0%, it is difficult to form a sound Cr ₂ O ₃ film, and it is necessary to add Si and Mn to enhance the oxidation resistance. Therefore, in the present invention, the lower limit Cr amount that can obtain oxidation resistance without expecting the effects of Si and Mn is set to 15.0%. Moreover, since workability will fall when it contains Cr exceeding 20.0%, an upper limit shall be 20.0%. From the balance of oxidation resistance and workability, it is more preferably 15.5 to 18.5%.

Ni is an inevitable impurity, but it is allowed to contain up to 0.5% which does not deteriorate the workability.
Mo is an element necessary for ensuring high temperature strength. It also has the effect of improving oxidation resistance and corrosion resistance. Therefore, it adds in 1.0 to 2.5% of range. If it is less than 1.0%, sufficient high-temperature strength cannot be obtained, and if it exceeds 2.5%, problems of deterioration of workability and descaleability during pickling occur.
V is an unavoidable impurity, but it is allowed to contain 0.2% or less without the problem of wrinkling during hot rolling.

  Ti is (1) more easily bonded to C and N than Nb to form carbonitrides, so it is effective for high-temperature strength, but can suppress the consumption of expensive Nb, and (2) Recrystallization temperature is reduced by adding Ti. Therefore, in the hot-rolled sheet annealing at a temperature at which Nb-based precipitates are sufficiently precipitated, the addition is performed aiming at two effects that recrystallization can be caused sufficiently. However, if it is less than 3 × (C + N)%, the effect is poor. If it exceeds 0.25%, the solid solution Ti increases and the recrystallization temperature rises, so that 3 × (C + N) to 0.25% or less. . In addition, as shown in FIG. 2, the range of 4 * (C + N)-0.15% which shows the remarkable r value improvement effect is more preferable.

  Nb is an element necessary for securing high temperature strength together with Mo. In addition, it has a function of fixing C and N as carbonitride together with Ti. However, if it is less than 0.3%, the required high temperature strength cannot be ensured. If it exceeds 1.0%, the workability deteriorates even with the production method of the present invention. Therefore, it is 0.3 to 1.0%. In addition, from the balance of high temperature strength and workability, 0.4 to 0.6% is preferable.

Further, when the sum of C and N (the amount of C + N) exceeds 0.020%, workability deteriorates, so the amount of C + N needs to be 0.020% or less. In the present invention, Ti is mainly used to fix C and N as carbonitrides. However, Nb that should be preserved as solid solution Nb also forms carbonitrides with C and N in order to increase high temperature strength. It will be consumed. Therefore, in order to improve the yield of expensive Nb, the C + N amount is preferably 0.015% or less.
With the above component design, it becomes possible to obtain a ferritic stainless steel excellent in heat resistance (high temperature strength, oxidation resistance) and workability.

In addition to these elements, the following elements may be added.
Cu is an element that improves the corrosion resistance, but if less than 0.1%, the effect cannot be expected. On the other hand, if it exceeds 1.0%, workability deteriorates. Therefore, the addition amount of Cu is set to 0.1 to 1.0%. From the balance of corrosion resistance and workability, the range of 0.1 to 0.5% is more preferable.

  B is an element that improves secondary workability, but if less than 0.0003%, the effect cannot be expected. On the other hand, if it exceeds 0.0050%, (primary) workability deteriorates. Therefore, the addition amount of B is set to 0.0003 to 0.0050%. In addition, from the viewpoint of a balance between primary and secondary workability, a range of 0.0003 to 0.0020% is a more preferable range.

  The plate thickness of the steel plate of the present invention is preferably set to 1 to 3 mm for an automobile exhaust system member such as an exhaust manifold. If the thickness is less than 1 mm, excellent processability is easily obtained regardless of the present invention, and if it exceeds 3 mm, it is difficult to obtain excellent processability even with the method of the present invention.

Because the exhaust gas temperature has increased to around 950 ° C due to improved fuel economy and higher output of automobiles, the performance at 950 ° C is optimal as an index of heat resistance, and the high-temperature strength from the necessity as an automobile exhaust system member As for 0.2% yield strength in 950 ° C, it is preferred that it is 15 MPa or more. Here, the measurement of the high temperature strength is performed in accordance with JIS G 0567. The direction of the test piece to be measured is the rolling direction (L direction).
Furthermore, the oxidation resistance is a practical problem because it does not lead to a peeling situation in which the metal surface is exposed if the peeling amount of the oxide scale is 0.5 mg / cm ² or less in an atmospheric oxidation test at 950 ° C. for 200 hours. Absent. More preferably, there is no scale peeling.

In addition, about the oxidation increase by the atmospheric oxidation test and evaluation of the amount of peeling, it evaluates as follows.
First, a test piece is formed into a square with a side of 20 mm, and the surface and side surfaces are polished and prepared as a # 400 finish. Next, the test piece subjected to mass measurement before the test is inserted into a furnace heated to 950 ° C. After 200 hours, the test piece taken out from the furnace is immediately stored in a metal container with a lid whose mass is measured in an empty state in advance and air-cooled.
Regarding the mass of the test piece after heating and cooling, first, the whole metal container is subjected to mass measurement, then the test piece is taken out from the metal container, and only the test piece is subjected to mass measurement. From these mass measurement results, the increase in oxidation is evaluated by the value per unit area obtained by subtracting the mass of the test piece before oxidation and the mass of the empty container from the mass of the oxidized test piece in a container and dividing the mass by the surface area of the test piece. The scale peeling amount is evaluated by a value per unit area obtained by subtracting the post-oxidation test piece mass and the empty container mass from the post-oxidation test piece mass contained in the container and dividing by the test piece surface area.

  With respect to workability, room temperature elongation and r value are optimal as indices. The rolling direction of the steel sheet, the rolling direction and the 45 ° direction, the rolling direction and the 90 ° direction are the L, D, and C directions, respectively, and the elongation in each direction is El (L), El (D), El (C), r The average elongation value obtained by the following formula: average elongation value = {El (L) + 2 × El (D) + El (C)} / 4, average r value = (rL + 2 × rD + rC) / 4, where rL, rD, and rC are values. Is preferably 30% or more, and the r value is preferably 1.4 or more. The elongation at normal temperature was measured according to JIS Z 2241, and the r value was measured according to JIS Z 2254. All the test pieces used are No. 13B test pieces defined in JIS Z 2201.

  In addition, the grain size of the steel sheet of the present invention is preferably a grain size number defined by JIS G 0552, which is 5 to 8. If it is less than No. 5, rough skin called orange peel is liable to occur during processing, and if it exceeds No. 8, processability deteriorates. The crystal grain size can be measured according to JIS G 0567.

  The crystal orientation of the rolled surface at a depth of 1/2 the thickness of the steel sheet of the present invention is determined as follows. That is, the integral intensity of 111 diffraction and the integral intensity of 200 diffraction of the surface of 1/2 depth of the steel sheet of the present invention are measured by the X-ray diffraction method. Next, the integral intensity of 111 diffraction and the integral intensity of 200 diffraction of the non-directional sample are measured by the X-ray diffraction method. A value obtained by dividing the 111 diffraction integrated intensity of the steel plate of the present invention by the 111 diffraction integrated intensity of the non-directional material is defined as 111 random diffraction intensity. A similar 200 random diffraction intensity is also obtained. Then, the ratio of the 111 random diffraction intensity to the 200 random diffraction intensity (integral intensity ratio = 111 random diffraction intensity of X-ray diffraction / 200 random diffraction intensity of X-ray diffraction) is obtained at a half depth of the plate thickness of the steel sheet. The crystal orientation of the rolled surface. If the integral intensity ratio is less than 5, it is not preferable because a necessary r value cannot be obtained. Although the upper limit of this ratio is not particularly defined, it is technically very difficult to exceed 40.

  The manufacturing method of ferritic stainless steel excellent in heat resistance and workability of the present invention is usually a steel piece such as an ingot and a slab after being melted in a predetermined component system, hot rolling, hot-rolled sheet annealing, A product is obtained through pickling, cold rolling, final annealing, and pickling.

In this process, the most important process of the present invention is hot-rolled sheet annealing. Since the stainless steel of the present invention contains high Nb and high Mo, almost no recrystallization occurs in the hot-rolled sheet. Therefore, excellent workability cannot be obtained even if cold rolling and final annealing are performed in this state. Therefore, the structure before cold rolling is formed by hot-rolled sheet annealing. The hot-rolled sheet annealing is performed at 800 to 950 ° C. If it is less than 800 ° C., recrystallization does not occur, so the r value in the product state is lowered and the workability is deteriorated, which is not preferable. If it exceeds 950 ° C., the amount of Nb and Mo deposited will decrease, and the amount of solid solution Nb and solid solution Mo in the matrix will not decrease, so the r value in the product state will decrease and the workability will also deteriorate. Therefore, it is not preferable.
If it is 850-950 degreeC, since it becomes a structure | tissue which is more advantageous for r value improvement in both sides of structure | tissue refinement | miniaturization and the fall of the amount of solid solution Nb and solid solution Mo in a mother phase, it is preferable. Although the annealing time is not particularly defined, 10 seconds to 10 hours are preferable. If it is less than 10 seconds, the variation in the annealing effect becomes large, which is not preferable, and if it exceeds 10 hours, the grain growth proceeds excessively, and the structure tends to be disadvantageous for improving the r value. From the viewpoint of manufacturing cost, it is more preferable to set it to less than 1 hour. The most preferable hot-rolled sheet annealing conditions are an annealing temperature of 850 to 950 ° C. and an annealing time of 10 seconds or more and less than 1 hour.

  Further, cold rolling is preferably performed using a work roll having a diameter of 300 mm or more because the r value is improved. If the diameter is less than 300 mm, the effect of improving the r value is small, which is not preferable. Although the upper limit of the work roll diameter is not particularly defined, the larger the work roll diameter is, the larger the housing, the motor, etc. are required, and the equipment cost is increased, so the diameter is preferably 1000 mm or less.

Other manufacturing conditions are not particularly defined, but the following conditions are preferable.
In the hot rolling, the heating temperature of the ingot or slab is preferably 1100 to 1300 ° C. If the temperature is lower than 1000 ° C., linear wrinkles occur frequently during hot rolling, and if it exceeds 1300 ° C., the scale becomes strong and the pickling property is impaired.
The hot rolling start temperature is preferably 1050 to 1300 ° C. When the temperature is lower than 1050 ° C., linear wrinkles occur frequently, and when it exceeds 1300 ° C., the scale becomes strong and the pickling property is impaired.

  The end temperature of hot rolling is preferably 750 to 900 ° C. When the temperature is lower than 750 ° C., the number of linear wrinkles increases and the deformation resistance of the steel sheet increases, so the load on the hot-rolled roll increases and the roll life is reduced. Moreover, in order to make it over 900 degreeC, it is because the installation which prevents the temperature fall of a steel plate during hot rolling is required, and a manufacturing cost becomes high. 780-880 degreeC is more preferable from a viewpoint of cost minimum.

  As for the hot rolling rate, the hot rolling rate (%) = {(ingot or slab thickness−sheet thickness after hot rolling) / (ingot or slab thickness)} × 100 is defined as 90%. The above is preferable. If this value is less than 90%, the strain accumulated in the hot-rolled sheet after hot rolling is small, and recrystallization hardly occurs in the hot-rolled sheet annealing, which is an important process of the present invention. 95% or more is more preferable for improving workability.

  The cold rolling ratio is a cold rolling ratio (%) = {(sheet thickness before cold rolling−sheet thickness after cold rolling) / (sheet thickness before cold rolling)} × 100 It is preferable that a hot rolling rate is 50 to 80%. It is because it is difficult to obtain the steel plate excellent in workability as it is less than 50% even if it has this invention. In addition, when the sheet thickness after cold rolling is 1 to 3 mm, in order to increase the cold rolling rate to more than 80%, it is necessary to perform cold rolling by increasing the sheet thickness before cold rolling. This is because an excessive load is applied.

  The final annealing temperature is preferably 1000 to 1100 ° C. If the temperature is less than 1000 ° C., the high Nb high Mo steel of the present invention has insufficient grain growth and deteriorates workability, and the Nb and Mo-containing precipitates precipitated by hot-rolled sheet annealing are insufficiently melted. This is because the solid solution Nb and the solid solution Mo for securing the carbon content are insufficient. On the other hand, when the temperature exceeds 1100 ° C., the grain growth is excessive and the crystal grain size becomes too large, causing rough skin during processing.

  Molten steel having the chemical components shown in Table 1 was melted to form a steel piece having a thickness of 200 mm, followed by hot rolling at 1250 ° C. to obtain a hot-rolled sheet having a thickness of 5 mm. At this time, the hot rolling start temperature was 1200 to 1250 ° C, and the hot rolling end temperature was 800 to 880 ° C. Then, the hot-rolled sheet annealing which heated a hot-rolled sheet to 900 degreeC and hold | maintained for 60 seconds was performed. Further, after cold rolling using a work roll having a diameter of 400 mm to form a cold-rolled sheet having a thickness of 2 mm, it is heated to 1050 ° C. and subjected to final annealing for 60 seconds, and pickling with hydrofluoric acid. The obtained steel plate was used as a test steel.

The tensile test at room temperature was performed according to JIS Z 2241. The direction of the measured specimen is the rolling direction (L direction), the rolling direction and 45 ° direction (D direction), the rolling direction and 90 ° direction (C direction), and the total elongation El (L). , El (D), El (C) and average elongation values were determined. The r value measurement was performed according to JIS Z 2254. The direction of the measured specimen was the same three directions as in the tensile test, and the respective r values rL, rD, rC and the average r value were determined. All the test pieces used were No. 13B test pieces defined in JIS Z 2201.
The high temperature tensile test was performed according to JIS G 0567. The direction of the measured specimen is the rolling direction (L direction).

The oxidation test was conducted in the atmosphere. The shape of the test piece was a square of 20 mm, and the surface and side surfaces were polished to give a # 400 finish. The evaluation method of the amount of increase in oxidation and the amount of peeling was performed as follows.
Insert the test piece whose mass was measured before the test into a furnace heated to 950 ° C., remove it from the furnace after 200 hours, and immediately store it in a metal container with a lid whose mass was measured in advance in an empty state. Air-cool. First, the whole metal container was subjected to mass measurement, and then the test piece was taken out of the metal container, and only the test piece was subjected to mass measurement.
From the measurement results of these masses, the oxidation increase was evaluated by subtracting the pre-oxidation test piece mass and the empty container mass from the mass of the oxidation test piece contained in the container and dividing the result by the surface area of the test piece. Moreover, the scale peeling amount was evaluated by a value obtained by subtracting the post-oxidation test piece mass and the empty container mass from the post-oxidation test piece mass in the container and dividing the result by the test piece surface area.
The grain size number of the test steel was determined by a cutting method in accordance with JIS G 0552.

  The crystal orientation of the rolled surface of the test steel was performed using an X-ray diffraction method. After cutting to 1/2 of the plate thickness, 111 and 200 diffraction intensities are measured using a polished test piece, and the same measurement is performed on a non-directional test piece made from powder. Then, 111 random diffraction integral intensity [I (111)] and 200 random diffraction integral intensity [I (200)] divided by the value are obtained, and the ratio, I (111) / I (200) is obtained, rolling It was used as an index of the crystal orientation of the surface.

Table 2 shows the results of the room temperature tensile test, the high temperature tensile test at 950 ° C., the result of the atmospheric oxidation test at 950 ° C. × 200 hours, and the integral strength ratio [I (111) representing the crystal orientation of the rolling surface of the test steel. ) / I (200)].
Steels A to G are test steels based on 16% Cr-1.5% Mo-0.5% Nb steel with only the Ti content changed. The steels of the present invention, from steel B to steel F, have a high-temperature strength of 15 MPa or more, no scale peeling in the oxidation test, excellent heat resistance, an average elongation value of 30% or more, and an average r value of 1 Excellent workability of 3 or more.
On the other hand, steel A having a small Ti content of 0.001% has a low average elongation value and average r value of 1.1, and the workability is not sufficient. Further, steel G having an excessive amount of Ti of 0.3% has an average r value as low as 1.1 and is not sufficiently workable.

FIG. 2 shows the relationship between the Ti amount and the average r value using A steel to G steel. From the scope of the present invention, it is clear that the average r value is low both when the amount of Ti is too small and when it is too large.
In Table 2, from H steel to R steel, the contained components are changed around the scope of the present invention. The steels of the present invention, from H steel to M steel, exhibit excellent heat resistance and excellent workability.
On the other hand, N steel with a high amount of C and N is not sufficiently workable, O steel with too much Si and Mn is not sufficiently workable, and P steel with a high Ni content is not sufficiently workable. Moreover, Q steel with low Nb and Mo does not have sufficient high-temperature strength. Furthermore, not only is the workability of R steel with Mo too low, but scale residue is generated by pickling after the final annealing.

Steel ingots having the same components as those of steel A and steel D in Table 1 were melted, and steel sheets were produced under the production conditions shown in Table 3. A1-A7 steel is the same component as A steel, D1-D19 steel is the same component as D steel. Thereafter, these were used as test steels and subjected to the same evaluation test as in Example 1. The results are shown in Table 4.
A1 to A7 steel and D1 to D7 steel are obtained by changing the temperature of intermediate annealing. The A series (A1 to A7 steel) is a steel with no Ti content (Ti content: 0.001% of the impurity level), and the D series (D1 to D7 steel) is a steel with 0.15% Ti content.

  In Table 4, both A series and D series have sufficient heat resistance, but there is a difference in workability, particularly r value. FIG. 1 shows the relationship between the intermediate annealing temperature and the average r value. Steel A, which is a Ti-free steel, has a maximum average r value of 1.15, whereas it is a steel of the present invention and a Ti-added steel. In the D series, it is clear that the average r value is 1.3 or more in the intermediate annealing temperature range of 800 ° C. to 950 ° C. and excellent workability is exhibited.

Next, D8 to D19 steels are test steels manufactured by changing the manufacturing conditions with the components of D steel. D8 to D10 steels are test steels having different final plate thicknesses.
The D8 steel, which has a thick plate thickness of 3.2 mm, has a low cold rolling rate of about 47%, so the average elongation is 30% and the average r value is slightly low at 1.2. D10 steel with a thin plate thickness of 0.8 mm shows excellent workability, but it is possible to obtain this level of value other than the production method of the present invention.
D11 and D12 steels are obtained by changing the diameter of the cold-rolled work roll. D11 steel using a work roll with a diameter of 300 mm, which is a range of the production method of the present invention, shows excellent workability with an average r value of 1.4, but D12 steel using a work roll with a diameter of 100 mm has an average r The value 1.2 is slightly low.

D13 and D14 steels have different final annealing temperatures, but D13 steel with a high final annealing temperature of 1130 ° C has excellent workability, but the grain size number is as small as 5.5. There are concerns about rough skin because of the large crystal grains. The D14 steel having a low final annealing temperature of 950 ° C. has a large grain size number of 8.5 because it is difficult to grow grains, and therefore the average r value is slightly low at 1.2.
The D15 and D16 steels have different ingot thicknesses, and the D15 steel with a thin ingot thickness of 80 mm has a low hot rolling reduction rate of 94%, an average elongation value of 30%, and an average r value of 1.3. However, the D16 steel of 160 mm is sufficient with a rolling rate of 97%, and exhibits an excellent value of an average elongation value of 32% and an average r value of 1.5.
Although D17-D19 steel is what changed hot-rolling conditions, since it exists in the preferable range of this invention, the outstanding workability is obtained.
From the above, it is clear that the ferritic stainless steel of the present invention has excellent heat resistance and excellent workability.

It is a figure which shows the example of a measurement of the influence of Ti addition and the hot-rolled sheet annealing temperature which have on the average r value of a 16% Cr-1.5% Mo-0.5% Nb type stainless steel plate. It is a figure which shows one measurement example of the influence of the amount of Ti which has on the average r value of a 16% Cr-1.5% Mo-0.5% Nb-Ti type stainless steel plate.

Claims (8)

% By mass
C: 0.015% or less,
Si: 0.10 to 0.25%,
Mn: 0.10 to 0.30%,
P: 0.040% or less,
S: 0.020% or less,
Al: 0.001 to 0.20%,
N: 0.015% or less,
Cr: 15.0-20.0%,
Ni: 0.5% or less,
Mo: 1.0-2.5%,
V: 0.2% or less,
Ti: 3 × (C + N) to 0.25%,
Nb: 0.3 to 1.0%
In addition, the C and N are
C + N: 0.020% or less is satisfied, and the Si and Mn are
Si ≦ Mn
Ferritic stainless steel excellent in heat resistance and workability, characterized by satisfying the above relationship and comprising the balance Fe and inevitable impurities. In addition,
Cu: 0.1 to 1.0%
The ferritic stainless steel excellent in heat resistance and workability according to claim 1, characterized by comprising: In addition,
B: Ferritic stainless steel excellent in heat resistance and workability according to claim 1 or 2, characterized by containing 0.0003 to 0.0050% or less. The ferritic stainless steel has a plate thickness of 1 to 3 mm, a 0.2% proof stress at 950 ° C. of 15 MPa or more, an average elongation value at room temperature of 30% or more, an average r value (average Rankford value) 4. The ferritic stainless steel excellent in heat resistance and workability according to claim 1, wherein the ferritic stainless steel plate is a ferritic stainless steel plate having a value of 1.3 or more. The ferritic stainless steel having excellent heat resistance and workability according to claim 4, wherein the ferritic stainless steel has a crystal grain size number of 5 to 8. The heat resistance and workability according to claim 4 or 5, wherein the crystal orientation of the rolled surface at a half depth of the plate thickness is 5 or more in terms of an integrated strength ratio defined by the following formula: Excellent ferritic stainless steel.
Integral intensity ratio = 111 random diffraction intensity of X-ray diffraction / 200 random diffraction intensity of X-ray diffraction The steel slab of any one of claims 1 to 3 is subjected to hot rolling, hot-rolled sheet annealing, pickling, cold rolling, final annealing, and pickling. When manufacturing the ferritic stainless steel excellent in heat resistance and workability described in any one of the items, the annealing temperature of the hot-rolled sheet annealing is set to 800 to 950 ° C. Of ferritic stainless steel with excellent properties. The method for producing ferritic stainless steel excellent in heat resistance and workability according to claim 7, wherein a work roll having a diameter of 300 mm or more is used in the cold rolling.

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