JP2781285B2 - Manufacturing method of stainless clad steel sheet - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a stainless steel clad steel sheet, especially a low carbon aluminum killed steel core material (base material).
The present invention relates to a method for producing a three-layer stainless steel clad steel sheet having excellent press formability using austenitic stainless steel as a composite material.

[0002]

2. Description of the Related Art Generally, a stainless clad steel sheet uses a casting method in which one of a stainless steel to be clad and a mild steel is cast with a molten steel, and a rolling method in which both materials are joined by rolling, or an explosive. A two-layer or three-layer clad material is made by a method such as an explosion method, and the clad material is manufactured by a method of performing hot rolling, hot-rolled sheet annealing, cold rolling and finish annealing.

[0003] For example, a stainless clad steel sheet made of mild steel as a core material and an austenitic stainless steel as a composite material, which is the object of the present invention, has better heat insulation than mild steel and has better heat conductivity than stainless steel. There is a characteristic that it is excellent. Moreover, since this stainless clad steel sheet can be used as a material for electromagnetic cookers,
Widely used as a pot material. However, in the case of such a stainless steel clad steel sheet (cold rolled steel sheet), it is pressed and surface-polished to provide a final product in order to provide the above-mentioned use, but recently, the functionality and design of the pot have been improved. Such demands are becoming stricter, so that better press formability is required.

[0004]

However, there is a problem that such a stainless steel clad steel sheet has poor press formability as compared with the stainless steel sheet itself and can be applied only to a design (pattern) having a relatively shallow drawing. Was. In addition, even if the pressing process is weak, cracks do not occur, but local "cracks" are likely to occur, resulting in a continuous streak pattern, which impairs the appearance of the product.

On the other hand, as a method of raising the press forming limit, a warm press method of heating a die of a press apparatus and cooling a punch has been well known (Kawasaki Steel Technical Report 17 (1985) 3, 315-322), and is also applied to press work of stainless clad. However, in the case of stainless clad steel, there is a problem that not only the occurrence of the local cracked pattern described above cannot be solved but also becomes remarkable.

It is an object of the present invention to improve the press formability of austenitic stainless steel clad steel and to propose a technique effective for preventing the occurrence of a crack-like pattern generated as a surface defect during press forming.

[0007]

In earnest research toward realizing the above-mentioned object, the present inventor has found that most of the above-mentioned various problems of the prior art have been achieved by performing a required aging heat treatment after finish annealing. The present inventors have found out that it can be overcome by applying, and have arrived at the present invention. That is, the present invention relates to C: 0.01 to 0.05 wt.
% Of aluminum-killed steel, Cr: 16-20wt
%, Ni: in the production of clad steel sheet using austenitic stainless steel containing 6-11 wt%
The finish annealing of the cold-rolled clad material obtained by the process according to the usual method is performed under the condition that the temperature is maintained within a temperature range of 900 to 1100 ° C. for 3 minutes or less, and subsequently, the temperature is reduced to a temperature of 300 ° C. or less.
A method for producing a stainless steel clad steel sheet having excellent press formability, comprising performing heat treatment at a temperature of 0 to 450 ° C. for 5 seconds to 10 minutes.

[0008]

[Function] The finish annealing of general austenitic stainless steel sheet must be performed at a temperature of 1100 ° C. or more from the viewpoint of formability and corrosion resistance. On the other hand, the low-carbon aluminum-killed steel used as the core material undergoes transformation to the austenite phase during annealing at a temperature of about 730 ° C or higher, so the final annealing is performed immediately below the transformation temperature and the solid solution C In order to reduce the temperature, the temperature is gradually decreased, and there are some inconsistencies between the two production methods.

That is, in the production of the stainless steel clad steel, which is the subject of the present invention, for example, it is necessary to use a steel
When the finish annealing is performed at 700 ° C., not only does the softening of the austenitic stainless steel not occur, but also
Chromium carbide precipitates, causing a so-called sensitization phenomenon, leading to a problem of deteriorating corrosion resistance. For these reasons, conventionally, the finish annealing conditions of the clad material of austenitic stainless steel and mild steel have to meet the requirements of austenitic stainless steel material, about 1100 ℃
It was usual to carry out at the temperature.

[0010] The present inventor has also conducted research on the press working of stainless clad steel, in particular, the surface defects when the above-mentioned warm pressing is performed, and the cause of press cracking caused by the surface defects. As a result, the above surface defects,
Press cracking was found to be due to finish annealing. That is, in the clad steel as in the present invention,
The mild steel used as the core material must also be subjected to finish annealing at a temperature higher than its transformation temperature, whereby the mild steel entirely becomes an austenite phase during annealing, and almost all of C forms a solid solution. However, in the generally performed finish annealing in the continuous annealing method, since the cooling rate is high, the solid solution C in the ferrite phase transformed from the austenite phase in the cooling process does not sufficiently precipitate as cementite, and the solid solution C is extremely high. It becomes a product in the state. Therefore, if such a material is warm-pressed, the amount of solid solution C in mild steel is high, so that dynamic strain aging is likely to occur, and deformation during pressing becomes uneven, so-called surface defects as described above. It was determined that. Here, as is generally known, the dynamic strain aging means that when carbon steel is worked in a temperature range of about 80 to 400 ° C., the dislocations that move are fixed to solid solution C and immediately become immobile dislocations. This is a phenomenon in which the dislocation density rapidly increases as the processing proceeds, and the ductility decreases. It is to be noted that such a press failure is likely to occur not only in a warm press but also in a conventional press because, even in general press working, the punch is not cooled, and the temperature of the mold is reduced due to the processing heat. This is because it has risen and is actually processed at about 100 ° C.

In order to prevent the occurrence of the dynamic strain aging of the stainless steel clad steel, it is effective to reduce the solute C of mild steel.
A method of adding a stabilizing element such as b or Ti can be considered. However, in order to sufficiently reduce the dynamic strain aging sensitivity by reducing the amount of C, for example, the amount of C must be made extremely low carbon steel of 0.001 wt% or less (hereinafter, simply indicated by “%”) or less. There is. However,
Simply reducing the carbon content to an extremely low C causes a problem that the crystal grains become coarse and orange peel is generated by molding. On the other hand, by adding Nb or Ti without significantly reducing the amount of C, it is possible to reduce the amount of solute C and prevent coarsening of crystal grains by the Nb and Ti carbides. However, even if C is fixed by adding Nb, for example,
Since the solid solution temperature of Nb C is as low as about 800 ° C., it has no effect on preventing the crystal grains from being coarsened during the finish annealing, and when Ti is added, the bonding strength of the clad is reduced by the formation of Ti oxide. The problem arises.

Further, the present inventor has studied a method capable of preventing dynamic strain aging during press working and improving press formability without causing the above-mentioned problems. as a result,
It has been found that performing aging treatment at a low temperature after finish annealing is extremely effective. That is, in the present invention, by aging at about 400 ° C. after the finish annealing, C remaining in solid solution after the finish annealing is precipitated as cementite, thereby preventing dynamic strain aging at the time of press working and simultaneously forming solid solution C. , Thereby improving the elongation and the yield strength to improve the press formability.

Next, the reasons for limitation of the present invention will be described. In the present invention, as the core material, basically, the applied material specified in JIS G3601 can be used as it is, but especially for the carbon content, aluminum killed steel containing C: 0.01 to 0.05% is used. For example, if the product is made of ordinary steel only,
It is a steel type corresponding to standards such as PCC, SPCD and SPCE. The reason for using such a low-carbon aluminum-killed steel is that if C is less than 0.01%, the crystal grains become coarse during the finish annealing, and orange peel occurs during press forming. On the other hand, when C exceeds 0.05%, ductility is reduced and press formability is deteriorated. In addition, since solid solution N is also harmful to dynamic strain aging, it is necessary to use aluminum killed steel.

Next, as for the composite material, basically, the applicable material as the composite material specified in JIS G3601 is applied as it is, but in the present invention, Cr: 16 to 20 is used.
%, Ni: 6 to 11%. That is, the reason for limiting the Cr of the austenitic stainless steel to 16 to 20% is that if it is less than 16%, not only does the corrosion resistance decrease, but also the austenite phase becomes unstable and the press formability decreases,
On the other hand, if it exceeds 20%, the austenite phase becomes too stable and the press formability decreases, so the content is limited to 16 to 20%. Also, if Ni is less than 6%, not only does the corrosion resistance decrease, but also the austenite phase becomes unstable and the press formability decreases, while if it exceeds 11%, the austenite phase becomes too stable and the press formability decreases. Because it drops
Limited to 6-11%.

Next, in the present invention, the reason why the finish annealing temperature after cold rolling is limited to 900 to 1100 ° C. is that if the temperature is lower than 900 ° C., the softening of the austenitic stainless steel is insufficient, and the press formability is insufficient. When the temperature exceeds 1100 ° C., the crystal grains become coarse and orange peel occurs during press molding. Therefore, the temperature was limited to 900 to 1100 ° C. Further, the reason why the holding time of the finish annealing is limited to 3 minutes or less is that if it exceeds 3 minutes, the crystal grains become coarse and orange peel is generated during press molding. This is because it is not always necessary to perform the holding.

Next, the clad steel after the finish annealing is subsequently cooled to 300 ° C. or less. The reason for this is that if the temperature exceeds 300 ° C, the supersaturation degree of solid solution C is insufficient, and the precipitation rate of C in the subsequent overaging treatment decreases. Is necessary. The reason for limiting the temperature range of the heat treatment for aging after the finish annealing to 350 to 450 ° C. in the present invention is that solid solution C does not precipitate below 350 ° C.
When the temperature exceeds 450 ° C., the precipitation amount of solid solution C decreases. Therefore, the temperature range of this heat treatment is limited to 350 to 450 ° C. further,
The reason for limiting the time of the heat treatment performed after this finish annealing to 5 seconds or more and 10 minutes or less is that if it is 5 seconds or less, the precipitation of solid solution C is insufficient, and the effect of the aging treatment is almost saturated in 10 minutes. The time of heat treatment after annealing was limited to 5 seconds or more and 10 minutes or less.

[0017]

EXAMPLE An aluminum-killed steel having the composition shown in Table 1 (C:
A stainless steel-mild steel-stainless steel three-layer clad steel with a core material of 0.030%) and austenitic stainless steel (Cr: 18.2%, Ni: 8.1%) having the composition shown in Table 1 Hot-rolled, hot-rolled sheet annealing, and cold-rolled, and then subjected to finish annealing and aging treatment in the laboratory. The thus obtained clad steel sheet was subjected to a working test with a small press machine to investigate the limit drawing ratio (blank diameter / punch diameter). At this time, a hot press was used to cool the punch and heat the die. The results are shown in Table 2 together with the conditions of the finish annealing and the aging treatment affecting the press formability. As is clear from Table 2, the clad steel sheet manufactured according to the method of the present invention does not undergo dynamic strain aging, and has excellent surface properties after pressing and excellent moldability.

[0018]

[0019]

[0020]

As described above, according to the manufacturing method of the present invention, the cold-rolled stainless steel clad material is subjected to a predetermined finish annealing and then a predetermined aging treatment.
In the past, the problem of press formability deterioration due to poor material quality of mild steel, which was a problem in press working, can be solved, and a stainless clad steel product with excellent surface properties can be obtained, therefore expanding the use of this steel material be able to.

Claims (1)

(57) [Claims] 1. A clad steel sheet comprising, as a base material, an aluminum killed steel containing 0.01% to 0.05% by weight of C and a composite material of an austenitic stainless steel containing 16% to 20% by weight of Cr and 6% to 11% by weight of Ni. Upon production, the finish annealing of the cold rolled clad material obtained by the process according to the usual method is 900-1100
The temperature is maintained within 3 minutes within the temperature range of
A method for producing a stainless steel clad steel sheet having excellent press formability, comprising performing a heat treatment at a temperature of 350 to 450 ° C. for 5 seconds to 10 minutes after cooling to a temperature of 00 ° C. or less.