JP2012172211A - METHOD OF MANUFACTURING LOW Ni AUSTENITIC STAINLESS STEEL SHEET - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve hot-rolling processability of Ni-saving austenitic stainless steel with reduced Mn, as well as bend-and-return processability of a cold-rolled annealed steel sheet, in a simple manner.SOLUTION: The cold-rolling annealing process including the steps of "cold rolling at 35% or higher rolling rate and annealing at 1,050°C or higher and also within an austenite single phase temperature range" is performed for two or more cycles on a hot-rolled annealed steel sheet made of steel containing, by mass, C: 0.030-0.300%, Si: 0.01-2.00%, Mn: 2.00-3.50%, P: 0.060% or less, S: 0.005% or less, Ni: 1.00-5.00%, Cr: 15.00-19.00%, N: 0.030-0.300%, Cu: 2.00-3.50%, V: 0.050-0.300%, Mo: 0-2.0%, B: 0-0.010%, Ca: 0-0.010%, and Al: 0-1.00%, with the balance being Fe and inevitable impurities.

Description

  The present invention is an austenitic stainless steel cold-rolled annealed steel sheet with reduced Ni content and reduced Mn content, and is particularly capable of withstanding severe forming such that bending back is added to the bent portion. The present invention relates to a method for producing a workable steel sheet.

  Work hardening type metastable austenitic stainless steel represented by SUS304 and SUS301 is widely used for various applications by taking advantage of good properties such as workability and corrosion resistance. However, since these steel types contain a large amount of Ni, there is a drawback that the raw material cost is high. In particular, in recent years, the problem that the balance between supply and demand tends to become unstable due to the fluctuation of the Ni raw material price has become apparent. For these reasons, there is an increasing need for “Ni-saving” austenitic stainless steel with a reduced Ni content.

  As Ni-saving austenitic stainless steels, steel types are known in which Mn is contained in a large amount of 4% or more as an austenite forming element instead of reducing the Ni content (Patent Documents 1 to 4). However, the inclusion of a large amount of Mn in this way causes scattering of Mn oxide fine particles (Mn fume) in the steel making process, and special measures are required from the viewpoint of environmental protection. The refractory wear of the container containing the molten steel is also increased. Moreover, due to the high Mn content in the steel, the surface quality of the steel sheet is likely to be lowered, and the productivity may be impaired in the steel sheet manufacturing process such as annealing and bright annealing. Furthermore, a large amount of Mn content tends to be a factor of reducing the corrosion resistance of the steel sheet product.

  In the Ni-saving austenitic stainless steel, when Mn is reduced to less than about 4%, a large amount of δ ferrite is likely to be generated during casting. Casting slabs of austenitic stainless steel containing a large amount of δ-ferrite have poor hot workability. When hot rolling is performed under the same general conditions as SUS304, SUS301, etc., ear cracks occur, which is a major operational problem. It becomes. Therefore, it is not always easy to reduce the Mn content in this type of steel.

  Patent Document 5 discloses a Ni-saving austenitic stainless steel having good hot workability and capable of reducing the Mn content to 3%. In this technique, a method of suppressing the amount of δ ferrite produced by a limiting formula between components is adopted, thereby avoiding the ear cracking in hot rolling. However, even if the Mn content is reduced to an area of less than 4%, further improvement is often desired for problems of surface quality deterioration and corrosion resistance deterioration.

  On the other hand, various Ni-saving austenitic stainless steels whose Mn content has been reduced to an area of 3% or less have been proposed mainly for the purpose of improving the formability (Patent Documents 6 to 13). When the Mn content is reduced to this level, problems related to environmental conservation and refractory life in the steelmaking process, surface quality of materials, and corrosion resistance are generally solved. However, no drastic improvement measures have been clarified regarding the decrease in hot workability caused by reducing Mn. In order to reduce the cracking in hot rolling, it is necessary to apply production conditions different from those of general-purpose steel grades. For example, there are significant restrictions on production, and the cost merit of reducing Ni is productivity. In some cases, it may not be fully utilized due to a decrease in the level of the material.

  Under such circumstances, the present applicant has developed a manufacturing technique for suppressing ear cracking in hot rolling for Ni-saving austenitic stainless steel in which both Ni and Mn are reduced, and disclosed in Patent Document 14. The technique is to appropriately control the amount and grain size of the δ ferrite phase existing in the vicinity of the slab edge by heating and maintaining the cast slab in the austenite single phase temperature range. It has also been disclosed that increasing the average cooling rate during casting and reducing the amount of δ ferrite produced is effective in suppressing hot-ear cracks.

JP 2006-111932 A JP 2007-197806 A JP-A-11-241145 JP-A-7-70700 JP 2007-63632 A Japanese Patent Publication No. 60-33186 JP 2006-22369 A JP 2009-41072 A JP 2009-221553 A JP 2009-221554 A JP 2010-189719 A JP 2010-196103 A JP 2009-30128 A JP 2010-121162 A

  In recent years, machine parts and the like produced by processing a steel plate material have been increasingly manufactured by integral molding from one material as much as possible in order to reduce manufacturing costs. Therefore, in order to endure the processing to a complicated shape, the steel plate material tends to require superior workability as compared with the conventional steel material. Austenitic stainless steel generally exhibits good bending workability, but once it has undergone bending, work hardening is large, and cracking is likely to occur when bent in the reverse direction at or near the bent part. There are drawbacks. This drawback is particularly problematic in Ni-saving austenitic stainless steels. Nevertheless, great expectations are placed on the replacement (material change) to Ni-saving austenitic stainless steel as one of the parts cost reduction means. In particular, Ni-saving austenitic stainless steel with a Mn content significantly lower than 4% has few problems in terms of environmental conservation, product corrosion resistance, surface properties, etc., and future needs are expected to increase.

  On the other hand, regarding the hot workability of Ni-saving austenitic stainless steel whose Mn content is greatly reduced from 4%, the above-mentioned patent adopts a method of increasing the average cooling rate during casting and a method of careful slab heating. We saw a temporary solution by the technique of Reference 14. However, increasing the average cooling rate at the time of casting leads to a reduction in line speed in continuous casting, which causes a reduction in productivity. If it is desired to avoid a reduction in the line speed of continuous casting, it is necessary to heat the casting slab more carefully before hot rolling, and in this case also the productivity is lowered.

  In view of such a current situation, the present invention is SUS304, SUS301 in Ni-saving austenitic stainless steel (hereinafter referred to as “low-Mn-reduced Ni-saving austenitic stainless steel”) in which the Mn content is significantly reduced from 4%. It is possible to produce a good hot-rolled steel sheet without causing the problem of ear cracking by a simple method that does not significantly reduce productivity compared to general-purpose steel grades such as, and with bending deformation in the reverse direction after bending It is an object of the present invention to provide a technique capable of remarkably improving the workability in the case (hereinafter referred to as “bend back workability”).

  The inventors have conducted detailed studies on chemical composition and production conditions as a technique for improving the hot workability of Ni-reducing austenitic stainless steel with a low Mn content. As a result, it has been found that it is extremely effective for improving hot workability to contain V and to prevent the cooling rate during casting from becoming too small. Further, it has been found that the bending workability is remarkably improved by a manufacturing process in which cold rolling / annealing under specific conditions is performed twice or more on a hot-rolled annealed steel sheet of the steel type containing V. The present invention has been completed based on such findings.

The above-mentioned object is mass%, C: 0.030 to 0.300%, Si: 0.01 to 2.00%, Mn: 2.00 to 3.50%, P: 0.060% or less, S : 0.005% or less, Ni: 1.00 to 5.00%, Cr: 15.00 to 19.00%, N: 0.030 to 0.300%, Cu: 2.00 to 3.50% V: 0.050 to 0.300%, Mo: 0 to 2.0%, B: 0 to 0.010%, Ca: 0 to 0.010%, Al: 0 to 1.00%, balance Fe And when manufacturing cold-rolled annealed steel sheets from hot-rolled annealed steel sheets of unavoidable impurities,
This is achieved by subjecting the hot-rolled annealed steel sheet to cold-rolling annealing consisting of “cold rolling with a rolling rate of 35% or more and annealing at 1050 ° C. or more and an austenite single-phase temperature range” twice or more.

  Here, Mo, B, Ca, and Al are arbitrarily added elements, and the content may be 0% (less than the analytical limit in ordinary steelmaking). S is an impurity element inevitably mixed in normal steelmaking. In this specification, a so-called “hot-rolled” steel sheet that has not been heat-treated after hot rolling is referred to as a “hot-rolled sheet”, and a steel sheet obtained by annealing the hot-rolled sheet is referred to as “hot-rolled annealing”. It is called “steel plate”.

As a method of obtaining the above hot-rolled annealed steel sheet,
Injecting molten steel into a mold for slab casting, and casting so that the average cooling rate from the solidification start temperature to 1250 ° C. at the slab edge at the center of the slab thickness is 25 ° C./min or more,
Heating the obtained cast slab in a heating furnace at 1150 to 1250 ° C. for 1.5 hours or more,
A step of hot rolling the slab after heating to obtain a hot-rolled sheet,
A step of subjecting the hot-rolled sheet to annealing (hot-rolled sheet annealing) to form a hot-rolled annealed steel sheet,
Manufacturing conditions having the following can be adopted. Thereby, generation | occurrence | production of an ear crack is prevented effectively and a hot-rolled annealing steel plate can be obtained. It is more preferable to employ the conditions of heating to 1050 ° C. or higher and the austenite single-phase temperature range as the hot-rolled sheet annealing.
In addition, the temperature of the annealing in hot-rolled sheet annealing and the said cold-rolling annealing process all means the ultimate temperature of material.

Examples of the manufacturing process of the cold-rolled annealed steel sheet according to the present invention include the following. Here, the description is omitted except for the process involving reduction of the thermal history or sheet thickness (for example, pickling performed after annealing) (i) casting → hot rolling → hot rolled sheet annealing → cold rolling 1 → annealing 1 → Cold rolling 2 → Annealing 2
(Ii) Casting-> hot rolling-> hot rolled sheet annealing-> cold rolling 1-> annealing 1-> cold rolling 2-> annealing 2-> cold rolling 3-> annealing 3

  In the example of (i), annealing 1 corresponding to so-called “intermediate annealing” is inserted between the cold rolling 1 and the cold rolling 2, and the annealing 2 corresponds to “finish annealing”. In this case, each process part of “cold rolling 1 → annealing 1” and “cold rolling 2 → annealing 2” is “cold rolling with a rolling rate of 35% or more, 1050 ° C. or more and austenite single phase temperature” It is necessary to correspond to the cold rolling annealing process consisting of “annealing in the zone”.

  In the example of (ii), annealing 1 and annealing 2 corresponding to “intermediate annealing” are inserted between the cold rolling 1 and the cold rolling 2 and between the cold rolling 2 and the cold rolling 3, respectively. Annealing 3 corresponds to “finish annealing”. In this case, at least two of the steps of “cold rolling 1 → annealing 1”, “cold rolling 2 → annealing 2” and “cold rolling 3 → annealing 3” It is necessary to correspond to the cold rolling annealing treatment consisting of “inter-rolling and annealing at 1050 ° C. or higher and austenite single phase temperature range”. Of course, all the three process parts may correspond to the “cold rolling annealing process”.

  According to the present invention, in Ni-saving austenitic stainless steel with reduced Mn, it has become possible to significantly improve the “bend-removability” when subjected to bending deformation in the reverse direction after bending. . In addition, it has become possible to effectively suppress the occurrence of ear cracks in hot rolling, which is likely to be a problem with the steel type, by a simple method that does not significantly reduce productivity. Furthermore, the reduction in Mn reduces the burden of environmental protection measures and improves the steel plate quality as compared with the conventional Ni-saving austenitic stainless steel having a high Mn content. Therefore, the Ni-saving austenitic stainless steel sheet according to the present invention greatly contributes to the cost reduction of parts processed into a complicated shape by replacing the conventional general-purpose steel grade.

The graph which shows the influence of V content and the average cooling rate at the time of casting on generation | occurrence | production of a hot-rolled ear crack.

Hereinafter, “%” relating to chemical composition means “% by mass” unless otherwise specified.
[Hot workability]
The inventors have studied in detail the chemical composition and production conditions advantageous for improving the hot workability of the Ni-saving austenitic stainless steel whose Mn content is significantly reduced from 4%. As a result, it was found that it is very effective to contain V. In this case, by controlling so that the average cooling rate at the time of casting does not become too small, it is possible to effectively suppress the occurrence of hot-ear cracks. The effect of V content and the effect of cooling rate on casting on the occurrence of ear cracks in hot rolling for Ni-saving austenitic stainless steel with Mn content reduced to 3% or less is shown below. Introduce the investigation. Here, the result about the steel which made content of elements other than V constant like Table 1 is illustrated.

《Experimental example of hot workability》
Based on the component composition shown in Table 1, steels with various V contents were melted. For each charge, 270 kg of molten steel was cast into a copper mold to form a 120 mm thick slab. During casting, the solidification start temperature at the slab edge (side of the slab) at the center of the slab thickness is adjusted by adjusting the amount of circulating water that cools the mold based on temperature measurement data from several thermocouples attached to the mold. The average cooling rate from 1 to 1250 ° C. was varied in the range of 30 ° C./min or less. A steel strip for hot rolling having a thickness of 60 mm, a width of 80 mm and a length of 120 mm was taken from the obtained cast slab. At that time, the slab edge of the original slab was made to correspond to one side surface (surface of the end portion in the width direction) of the steel strip for hot rolling. A surface derived from this slab edge is called an “evaluation surface”.

  The steel strip for hot rolling was placed in a heating furnace, heated at 1200 ° C. for 2 hours, extracted, and subjected to a hot rolling experiment in the air using a reverse hot rolling mill to obtain a hot rolled sheet. The hot rolling pass schedule is as shown in Table 2. The delay in each pass was about 7 sec, and the rolling speed was about 30 m / min.

About the obtained hot-rolled sheet (total rolling ratio 94.2%), the edge crack depth at the edge derived from the evaluation surface was measured. If the maximum ear crack depth is 1 mm or less in this hot rolling experiment, it is evaluated that it has good hot workability. Evaluation was as follows, and ○ evaluation was determined to be acceptable.
O: Maximum ear crack depth: 0 mm (no ear cracks) to 1 mm (hot workability; good).
X: Maximum ear crack depth: more than 1 mm (hot workability; poor).
These results are shown in FIG.

  As can be seen from FIG. 1, when the V content is 0.05% or more, the average cooling rate from the solidification start temperature to 1250 ° C. at the slab edge at the center of the slab thickness is controlled to 25 ° C./min or more. Therefore, good hot workability can be realized. FIG. 1 is a summary of the results when the content of elements other than V is constant as shown in Table 1. In the component composition range defined by the present invention, the inventors have similar to the above. It has been confirmed that an effect of improving hot workability can be obtained.

  According to the inventors' investigation, the ear cracking in hot rolling of Ni-reducing austenitic stainless steel with reduced Mn occurs from the interface between the austenite substrate and the δ ferrite phase, and the austenite substrate and δ ferrite The interface is a crack propagation path. Conditions for lowering the cracking sensitivity in hot rolling include a small amount of δ ferrite, a fine δ ferrite phase, and a discontinuous interface between the austenite phase and the δ ferrite phase as much as possible. .

  Although the mechanism by which the hot workability of the steel type is remarkably improved by containing V has not yet been fully elucidated, the following may be considered. When V is contained in an appropriate range, the primary crystal δ and dendrites are refined, and the δ ferrite phase present in the cast slab becomes finer than before. For this reason, the effect of reducing the δ ferrite phase (the effect of promoting the δ → γ transformation) by slab heating before hot rolling is enhanced, which is presumed to be effective in suppressing ear cracks in hot rolling. This means that in steels not containing V, the cooling rate at the time of casting is increased to 50 ° C./min or more, or if the casting slab is not heated more carefully than usual, there will be a problem of hot cracking. While it was difficult to avoid (see Patent Document 14), according to the present invention, the cooling rate during casting was slowed down to 25 ° C./min, and even when the casting slab was heated under general conditions, hot rolling was performed. It is affirmed from the fact that the problem of ear cracking is avoided.

[Bend back workability]
The inventors have made various studies in order to improve the bending back workability, which tends to be a problem in the cold-rolled annealed steel sheet of Ni-saving austenitic stainless steel with reduced Mn. As a result, the hot-rolled annealed steel sheet containing V as described above is subjected to a cold-rolled annealing treatment consisting of “cold rolling with a rolling rate of 35% or more and annealing at 1050 ° C. or more and an austenite single-phase temperature range”. It has been found that by adopting the step of applying twice or more, the bending back workability is remarkably improved. Below, the experimental example which investigated the influence of the manufacturing conditions on the bending back workability of a cold-rolled annealing steel plate about the low Mn reduction Ni austenitic stainless steel containing V is introduced.

<Experimental example of bending workability>
Steel grade B shown in Table 3 was melted as Ni-saving austenitic stainless steel with reduced Mn. 270 kg of molten steel was cast into a copper mold to form a slab having a thickness of 120 mm. During casting, using several thermocouples attached to the mold, the average cooling rate from the solidification start temperature to 1250 ° C. at the slab edge (slab side surface) at the center of the slab thickness is 27 ° C./min. The amount of circulating water for cooling the mold was adjusted.

The obtained cast slab was hot-rolled according to the conditions of the above-mentioned “Experimental example of hot workability” to obtain a hot-rolled sheet having a thickness of 3.5 mm. At that time, no ear cracks occurred. The obtained hot-rolled sheet was annealed under conditions of 1100 ° C. × soaking for 1 min (hot-rolled sheet annealing) to obtain a hot-rolled annealed steel sheet. Thereafter, a cold-rolled annealed steel sheet having a thickness of 0.8 mm was produced by any one of the following production procedures a to c. Pickling was performed after each annealing. In addition, about each used steel, it is confirmed that the temperature of 1100 degreeC exists in an austenite single phase temperature range.
-Production procedure a;-> cold rolling 1 (77%)-> annealing 1
-Production procedure b;-> cold rolling 1 (49%)-> annealing 1-> cold rolling 2 (56%)-> annealing 2
-Production procedure c;-> cold rolling 1 (37%)-> annealing 1-> cold rolling 2 (39%)-> annealing 2-> cold rolling 3 (41%)-> annealing 3
Here, the numerical value (%) of cold rolling means the cold rolling rate. In the production procedure a, annealing 1 corresponds to finish annealing. Similarly, in manufacturing procedures b and c, annealing 2 and 3 correspond to finish annealing, respectively. In the above description, the processes up to the hot-rolled sheet annealing are omitted.

The following three annealing conditions were adopted. In the manufacturing procedures b and c, the annealing conditions in each annealing step (excluding hot-rolled sheet annealing) were made common.
・ Annealing condition P: Cooling outside the furnace immediately after the heating temperature reaches 1000 ° C. and the material temperature reaches 1000 ° C. ・ Annealing condition Q; Cooling outside the furnace immediately after the heating temperature reaches 1050 ° C. and the material temperature reaches 1050 ° C.・ Annealing condition R: heating temperature 1100 ° C., material temperature is 1100 ° C.

  As described above, cold-rolled annealed steel sheets having a thickness of 0.8 mm were obtained under three manufacturing procedures × three annealing conditions = total nine manufacturing conditions. A strip-shaped test piece having a width of 25 mm and a length of 50 mm was taken from each cold-rolled annealed steel sheet so that the width direction coincided with the rolling direction, and subjected to a 90 ° V bending test. The test piece was set so that the bending ridge line was in the rolling direction, and a 90 ° V bending process was performed by applying a load of 20 kN using a mold having a tip R of 0.1 mm. Next, after the bent part is returned to almost flat with a flat jig, the front and back of the test piece are reversed and set in the above mold, and bent in the same manner as described above on the opposite side of the first bend. gave. About the test piece after a process, the bending outer periphery part of the 2nd time was observed with the magnification of 150 time with the microscope, and the presence or absence of the opening by a crack was investigated. The test was conducted with the number of tests n = 3, and no crack was observed in all three specimens (bending-removability: good), and one of the three specimens in which a crack was observed △ ( Bend-back workability: somewhat poor) A crack observed in two or more of the three pieces was evaluated as x (bend-back workability: bad), and a ◯ evaluation was accepted. The results are shown in Table 4.

  As shown in Table 4, in the low Mn-reduced Ni-saving austenitic stainless steel containing V, "cold rolling annealing" combining cold rolling and annealing at 1050 ° C or higher and an austenite single-phase temperature range By manufacturing a cold-rolled annealed steel sheet by a method of performing the above two or more times, the bending back workability of the cold-rolled annealed steel sheet can be improved.

  Although the mechanism of the effect of improving the bending back workability is not necessarily clear at present, the following may be considered. By containing V, the solidified structure of the cast slab surface is refined, and the δ ferrite phase is also finely distributed. For this reason, most of the δ ferrite phase of the slab surface layer portion disappears by heating the cast slab before hot rolling. The δ ferrite disappearance effect suppresses hot-rolled edge cracking. However, when an annealed material is produced by a normal cold rolling annealing process as it is, the bending back workability cannot be stably improved. As the cause, component segregation such as Cr in the old δ ferrite region can be considered. Since elements such as Cr are segregated (concentrated) in the δ ferrite phase, segregation such as Cr remains in the old δ ferrite region in the austenite crystal even after the disappearance of the δ ferrite phase. Such segregation in crystals is difficult to homogenize even by simple heating (soaking) or hot rolling, and segregation of Cr or the like tends to remain in the austenite phase of the cold-rolled annealed steel sheet. This type of segregation remaining near the surface of the cold-rolled annealed steel sheet causes a local difference in the transformation behavior to the work-induced martensite phase during bending. When further deformation (bending return deformation) is applied to the processed portion, cracks are likely to occur due to differences in formability due to local differences in structure. When "cold rolling annealing" by an appropriate combination of cold rolling and annealing according to the present invention is performed a plurality of times, segregation of Cr and the like in the cold rolled annealed steel sheet by sufficiently progressing the homogenization of the component elements As a result, it is surmised that the bending back workability is remarkably improved.

[Chemical composition]
C and N are austenite-generating elements. If the content of these elements is too small, the amount of δ ferrite phase increases and hot workability decreases. These elements are also important for ensuring sufficient ductility due to the TRIP effect. In the present invention, each of C and N requires a content of 0.030% or more. It is more effective to set the content of C to 0.05% or more and N to 0.05% or more. On the other hand, if the content of C and N is too large, the steel becomes excessively hard and becomes a factor that hinders workability. As a result of various studies, C is desirably set to a range of 0.300% or less, and may be controlled to a range of 0.150% or less. N is also preferably in the range of 0.300% or less, and may be managed in the range of 0.150% or less.

  Si is an element that is effective for deoxidation in steelmaking and contributes to solid solution strengthening. Usually, the content is 0.01% or more, and it is more effective to set the content to 0.30% or more. However, when a large amount of Si is contained, it causes a large amount of δ ferrite phase. Moreover, it becomes a factor which hardens steel and impairs bending back workability. As a result of the examination, the Si content is limited to 2.00% or less. You may manage to Si content of 1.00% or less.

  Mn is an austenite-generating element that is less expensive than Ni, and is added as an alternative element for Ni in Ni-saving austenitic stainless steel. In order to fully utilize its function in the present invention, it is necessary to contain 2.00% or more of Mn. However, when the Mn content increases, the load of environmental conservation measures in the steel making process increases. In addition, since the surface properties of the obtained steel sheet are likely to be lowered, taking measures to solve it may cause a reduction in productivity. Inclusions such as MnS increase to cause deterioration of workability and corrosion resistance. In order to overcome these drawbacks due to the large amount of Mn, the present invention limits the Mn content to 3.50% or less. It is more effective to make it 3.00% or less, and it may be managed to be less than 3.00%.

  S is mixed as an impurity, but if its content increases, workability and other material properties and manufacturability are adversely affected. As a result of the study, the S content is limited to 0.005% or less. Although the lower the S content, the better. However, excessively low S increases the steelmaking load. It is desirable in terms of manufacturing cost that S is left in a range of 0.0005% or more.

  Ni is an essential element for austenitic stainless steel, but in the present invention, component design is performed to minimize the Ni content from the viewpoint of cost reduction, and the upper limit of Ni content is limited to 5.00%. . The Ni content may be controlled to be less than 5.00%, or 4.00% or less, or even 3.00% or less. However, when C, N, and Mn are specified in the above range, the Ni content is 1.00% or more because it is necessary to adjust the components so as to become an austenite single phase in the slab heating temperature range (for example, 1150 to 1250 ° C.). It is necessary to ensure. You may manage to the quantity exceeding 1.00%.

  Since Cu is an austenite-generating element, the degree of freedom in setting the content of other austenite-generating elements increases as the Cu content increases, and component design that suppresses Ni becomes easy. In addition, since Cu has an action of suppressing the formation of a work-induced martensite phase in compression deformation, the inclusion of Cu causes excessive processing inside the bend when bending is performed in the first bending process. Generation of the induced martensite phase is suppressed, and cracking during subsequent bending back deformation is effectively prevented. In order to obtain these effects sufficiently, the present invention contains 2.00% or more of Cu. It is more preferable to set it to 2.50% or more. However, if a large amount of Cu is contained, a low-melting-point alloy phase is formed, which becomes a factor that hinders hot workability. For this reason, Cu content is restrict | limited to 3.50% or less. You may manage to 3.00% or less.

  Cr is an essential element for ensuring the corrosion resistance of stainless steel. In the present invention, steel having a Cr content of 15.00% or more is targeted in order to ensure corrosion resistance that can be applied to alternative uses of general-purpose austenitic stainless steels such as SUS304 and SUS301. However, Cr is a ferrite-forming element, and excessive Cr content is not preferable because it causes a large amount of δ ferrite phase to be generated. As a result of various studies on the balance with the above-described content ranges of C, N, Mn, Ni, and Cu, which are austenite forming elements, the Cr content is set to a range of 19.00% or less.

  V is an extremely important element in the present invention. As described above, V is considered to have the effect of refining the cast structure, which leads to the refinement of the δ ferrite phase, which improves hot workability and bend workability of cold-rolled annealed steel sheets. It is inferred that it has contributed. In order to sufficiently obtain such an action of V, in the present invention, a V content of 0.050% or more is ensured. More preferably, it is 0.080% or more. However, excessive V content causes an increase in manufacturing cost and hardening of the steel. As a result of various studies, the V content is limited to 0.300% or less. You may manage to 0.200% or less.

  P has the effect of refining the primary δ ferrite phase and dendrite during casting. For that purpose, it is more effective that the P content is, for example, 0.010% or more, and it is more effective to set it to 0.020 or more. However, since P also becomes a factor that lowers the toughness of steel, it is necessary to set the content to 0.060% or less. You may manage to 0.050% or less.

  Since Mo exhibits an effect of improving corrosion resistance, etc., it can be added as necessary. It is more effective to secure a Mo content of 0.2% or more. However, since excessive Mo content causes a decrease in hot workability, when Mo is added, it is performed in a range of 2.0% or less.

  B and Ca have an advantageous effect on improving hot workability when added in a small amount, so that one or two of them can be added as necessary. In order to sufficiently obtain the action, it is more effective to secure a content of 0.001% or more in the case of B and 0.001% or more in the case of Ca. However, if these elements are added excessively, conversely, the hot workability is hindered. As a result of the study, when adding one or more of B and Ca, it is necessary to add them in a range of 0.010% or less.

  Al can be added as needed as a deoxidizer. In that case, it is more effective to secure an Al content of 0.01% or more. However, excessive addition of Al becomes a factor that hinders hot workability. Therefore, when Al is added, the content range is 1.00% or less.

[Production method of hot-rolled sheet]
According to this invention, a hot-rolled sheet can be manufactured in the same process as general-purpose austenitic stainless steels such as SUS304 and SUS301. Specifically, the component-adjusted molten steel is cast continuously or batchwise, and the resulting cast slab is heated and extracted, and then hot-rolled with a continuous hot rolling mill or reverse hot rolling mill. Can be used.

  However, since it is important that the δ ferrite phase is as small and fine as possible in the slab subjected to hot rolling, it is necessary to consider that the δ ferrite phase does not grow excessively during casting. Specifically, molten steel whose components are adjusted as described above is poured into a mold for slab casting, and the average cooling rate from the solidification start temperature to 1250 ° C. at the slab edge at the center of the slab thickness is 25 ° C./min or more. It can be cast so that Since the steel which is the subject of the present invention contains P, the δ ferrite phase in the cast slab is refined from the conventional steel type. For this reason, conventionally, when it was desired to avoid an increase in load in the slab heating process in the subsequent process, the average cooling rate during casting had to be set to 50 ° C./min or more (Patent Document 14). Accordingly, it can be reduced to 25 ° C./min. In continuous casting, the fact that the cooling rate during casting can be reduced means that the time for passing through the mold can be shortened, leading to an improvement in casting speed (line speed). That is, productivity is improved.

  In the cast slab obtained by the above method, since the δ ferrite phase is refined in the as-cast state, it is not necessary to perform the slab heating before hot rolling particularly carefully. Specifically, a condition of heating at 1150 to 1250 ° C. and austenite single phase temperature range for 1.5 hours or more may be employed. Thereafter, if hot rolling is performed in accordance with a general method for producing austenitic stainless steel hot-rolled steel sheets, a healthy hot-rolled sheet can be obtained without causing trouble due to ear cracks.

[Hot rolled sheet annealing]
It is preferable that the hot-rolled sheet annealing performed on the obtained hot-rolled sheet has a condition of heating to a temperature of 1050 ° C. or higher and an austenite single-phase temperature range. When the heating temperature is lowered, the disappearance effect of the δ ferrite phase is reduced. In addition, it tends to be disadvantageous for sufficient recrystallization in a short time. On the other hand, when the heating temperature exceeds the austenite single phase temperature range, precipitation of the ferrite phase is caused. In the case of the steel type targeted in the present invention, the upper limit of the austenite single-phase temperature range is usually higher than 1250 ° C, but it is sufficient that the hot-rolled sheet annealing is performed at a temperature of 1200 ° C or lower. You may manage in the range below 1150 degreeC. The heating time may be set to 0 sec soaking immediately after the material temperature reaches a predetermined temperature within the above temperature range, but is usually maintained for 30 sec or longer (for example, 120 sec or shorter) in the predetermined temperature range within the above temperature range. It is desirable to adjust the heating time in such a manner. After hot-rolled sheet annealing, pickling is usually performed.

[Cold rolling annealing]
In this invention, a cold-rolled annealing steel plate is manufactured by implementing the process of "cold rolling + annealing" on specific conditions with respect to the hot-rolled annealing steel plate which finished hot-rolled sheet annealing twice or more. Specifically, as described above, the cold rolling annealing process including “cold rolling with a rolling rate of 35% or more and annealing at 1050 ° C. or more and an austenite single phase temperature range” is performed twice or more. Thereby, the structure is homogenized, and a cold-rolled annealed steel sheet having good bending back workability is obtained. The thickness of the cold-rolled annealed steel sheet is, for example, about 2 to 0.5 mm. When the rolling rate in cold rolling is smaller than the above, or when the annealing temperature is lower than the above range, homogenization that promotes diffusion of segregating elements such as Cr by annealing using processing strain by cold rolling. Is not fully effective. When the annealing temperature exceeds the austenite single phase region, the ferrite phase is precipitated. In that case, elements such as Cr are easily concentrated in the ferrite phase. Further, when this “cold rolling annealing process” is performed only once, the effect of homogenization becomes insufficient.

  Although the upper limit of the cold rolling rate is limited by the equipment capacity, it is usually set within a range of 70% or less. What is necessary is just to set the upper limit of annealing temperature in the range below 1150 degreeC normally. If the annealing time is too long, the productivity is lowered. Therefore, the time during which the material temperature is maintained in the predetermined range is usually set within a range of 1 min or less. Soaking temperature may be 0 sec. In addition, pickling is normally performed after each annealing.

  The steel shown in Table 5 was melted, and a hot rolling experiment was conducted using the slab edge as an evaluation surface in the same manner as in the above-mentioned “Experimental example of hot workability”. However, here, the average cooling rate from the solidification start temperature during casting to 1250 ° C., the slab heating temperature before hot rolling, and the heating time were varied as shown in Table 6. About the obtained hot-rolled steel sheet (total rolling ratio 94.5%), the ear crack depth at the edge derived from the evaluation surface was measured, the hot workability was evaluated by the above-mentioned evaluation method, and the maximum ear crack depth Of 0-1 mm was evaluated as acceptable.

Next, hot-rolled sheet with good hot workability was subjected to hot-rolled sheet annealing of 1100 ° C. × soaking for 1 min. About the obtained hot-rolled annealed steel plate (thickness 3.5 mm), the cold-rolled annealing steel plate having a final thickness of 0.8 mm is obtained by carrying out a cold-rolling annealing process consisting of cold rolling and annealing once or twice. Obtained. Specifically, the manufacturing procedure a or b described above was employed, and each annealing was performed according to the annealing condition Q described above. Using the test piece collected from the obtained cold-rolled annealed steel sheet, the bending back workability was evaluated in the same manner as in the above-mentioned “Experimental example of bending back workability”. And the thing ((circle) evaluation) whose bending back workability was favorable was determined to be a pass.
These results are shown in Table 6.

  As can be seen from Table 6, when the content of each alloy component is optimized in accordance with the present invention and cooling and slab heating are performed under appropriate conditions, the ears of the Ni-saving austenitic stainless steel with reduced Mn Good hot workability without cracking problems was realized. Moreover, all the cold-rolled annealing steel plates manufactured by the method of performing the above-described cold-rolling annealing treatment according to the present invention twice exhibited good bending back workability.

  However, even in the case of steel having the chemical composition defined in the present invention, good hot workability and bend back workability may not be obtained if the manufacturing conditions are inappropriate. For example, No. 3 which is a comparative example has a slab heating temperature too low, and No. 5 has a slab heating time too short. The interworkability was not improved sufficiently. In No. 4, since the slab heating temperature exceeded the austenite (γ) single phase region and was too high, the δ ferrite phase precipitated, and the hot workability deteriorated. In Nos. 6 and 9, since the cooling rate during casting was too low, the δ ferrite phase present in the cast slab was insufficiently refined, and the hot workability was not sufficiently improved. In Nos. 2 and 8, the cold rolling annealing treatment described above was performed only once, so that the bending back workability of the cold rolled annealing steel sheet was not sufficiently improved.

  Nos. 18 to 24, which are comparative examples, employ steel having a chemical composition that does not satisfy the provisions of the present invention. Of these, No. 18 did not contain V, so hot-ear cracks exceeding 1 mm occurred. Nos. 19, 21, 22, and 23 are austenite-forming elements, but the amount of δ-ferrite phase is increased due to insufficient contents of C, Mn, Ni, and N, and hot workability is improved. There wasn't. No. 20 has an excessive Si content, and No. 24 has an insufficient Cu content. Therefore, none of these has sufficiently improved the bending workability of the cold-rolled annealed steel sheet.

Claims (4)

By mass%, C: 0.030 to 0.300%, Si: 0.01 to 2.00%, Mn: 2.00 to 3.50%, P: 0.060% or less, S: 0.005 %: Ni: 1.00 to 5.00%, Cr: 15.00 to 19.00%, N: 0.030 to 0.300%, Cu: 2.00 to 3.50%, V: 0 In producing a cold-rolled annealed steel sheet from a hot-rolled annealed steel sheet of 0.050 to 0.300%, the balance Fe and unavoidable impurities,
The hot-rolled annealed steel sheet is subjected to cold-rolling annealing consisting of “cold rolling with a rolling rate of 35% or more and annealing at 1050 ° C. or more and an austenite single-phase temperature range” twice or more. A method for producing a low Ni austenitic stainless steel sheet.   The steel further contains one or more of Mo: 2.0% or less, B: 0.010% or less, Ca: 0.010% or less, Al: 1.00% or less. The manufacturing method of the low Ni austenitic stainless steel sheet of description. Injecting molten steel into a mold for slab casting, and casting so that the average cooling rate from the solidification start temperature to 1250 ° C. at the slab edge at the center of the slab thickness is 25 ° C./min or more,
Heating the obtained cast slab in a heating furnace at 1150 to 1250 ° C. for 1.5 hours or more,
A step of hot rolling the slab after heating to obtain a hot-rolled sheet,
A step of subjecting the hot-rolled sheet to annealing (hot-rolled sheet annealing) to form a hot-rolled annealed steel sheet,
The manufacturing method of the low Ni austenitic stainless steel plate of Claim 1 or 2 which has these.   The method for producing a low-Ni austenitic stainless steel sheet according to claim 3, wherein the hot-rolled sheet annealing is performed by heating the hot-rolled sheet to 1050 ° C or higher and an austenite single-phase temperature range.

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