JPH07292417A - Production of ferritic stainless steel sheet excellent in formed surface characteristic - Google Patents

Abstract

PURPOSE:To improve surface roughing at the time of cold working while maintaining inherent formability by combinedly adding Nb and Ti as grain refining elements and controlling hot rolling and cold rolling conditions, respectively. CONSTITUTION:The steel is a ferritic stainless steel having a composition consisting of, by weight ratio, <=0.030% C, 0.50% Si, <=0.40% Mn, 13.0-16.0% Cr, 0.10-0.50% Nb, 0.05-0.30% Ti, and the balance Fe with inevitable impurities. A slab of this steel is heated to 1000-1100 deg.C and hot-rolled under the condition of 730-770 deg.C rolling finishing temp. After annealing, the hot rolled plate is furnace-cooled and then cold-rolled at 80-90% draft and further finish-annealed at 790-860 deg.C. By satisfying these heating and hot rolling conditions, annealing conditions, and cold rolling and subsequent annealing conditions, a fine recrystallized structure can be formed and superior surface characteristic can be obtained at the time of cold forming.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a ferritic stainless steel sheet having excellent forming surface properties, and particularly to a method for producing a low Cr ferritic stainless steel sheet having less surface roughness after forming and good surface properties. .

[0002]

2. Description of the Related Art Conventionally, ferritic stainless steel represented by SUS430 has been used in a wide range of products such as automobile parts, kitchen equipment and household electric equipment. This is because ferritic stainless steel has relatively excellent corrosion resistance, and
This is because it is superior to austenitic stainless steel in terms of price because it is not added.

However, although ferritic stainless steel has good workability in the case of working such as overhanging and deep drawing, it has a problem in the surface quality after working and is called orange peel (rough skin due to coarse grains). Causes rough skin.

Therefore, reduction or elimination of surface roughness during forming has become a major issue in the production of ferritic stainless steel, and much research has been conducted on the prevention method thereof. However, until now, no practically satisfactory preventive measures have been found.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for producing a ferritic stainless steel sheet, which has solved the surface roughening during processing which has been conventionally observed.

[0006]

[Means for Solving the Problems] By the way, as a steel which is relatively inexpensive and has excellent formability and corrosion resistance,
An example is high-purity ferritic stainless steel to which Ti, Zr, or Nb has been added. That is, in high-purity ferritic stainless steel, C and N are suppressed together with the suppression of C and N.
It is generally well known that the addition of Ti, Zr, Nb, etc. as a fixed element for the above and as a carbonitride forming element for controlling the texture is effective for improving the corrosion resistance and formability.

Therefore, the effect of adding Ti, Zr, Nb, etc. was examined in detail. Simultaneous addition of Ti and Nb effectively worked for the refinement of crystal grains, and further, due to the refinement of the crystal, the surface texture after processing was improved. We also focused on the improvement.

In other words, by enlarging the crystal grain refining region of ferritic stainless steel, the idea that the crystal grain refining range which solves the surface roughening during the conventional processing can be enlarged is obtained. And repeated further studies.

First, there is a certain critical condition between the conditions in which grain refinement works more effectively due to the effect of the combined addition of Ti and Nb, that is, the cold rolling condition and the hot rolling temperature condition. It was found that the surface roughness during processing can be improved while maintaining the original formability after cold rolling by maintaining the control, and completed the present invention.

That is, the present invention provides a method for producing a ferritic stainless steel sheet for expanding a grain refinement region, which is characterized by a combination of hot rolling conditions and an appropriate cold reduction.

Therefore, the gist of the present invention is as follows.
By weight, C: 0.030% or less, Si: 0.50% or less, M
n: 0.40% or less, Cr: 13.0 to 16.0%, Nb: 0.10 to 0.50
%, Ti: 0.05 to 0.30%, ferritic stainless steel having a steel composition consisting of balance Fe and associated impurities, slab heating temperature 1000 to 1100 ° C, and hot rolling end temperature 730 to
Hot rolling is performed at 770 ° C, followed by annealing and furnace cooling, and then cold rolling to finish at a reduction rate of 80 to 90%, and further finish annealing at 790 to 860 ° C for good forming. A method for producing a low Cr stainless steel sheet having excellent forming surface properties, which is characterized by having various surface properties.

Incidentally, in Japanese Patent Laid-Open No. 4-74852,
Although the same component system as that of the present invention is disclosed, only good heat fatigue resistance is shown, and there is a problem that the surface properties at the time of molding are not good. Therefore, in this invention, the heat resistance fatigue is maintained, and the surface quality at the time of molding is also improved.

[0013]

Next, the reason why the composition ratio of steel is defined as described above in the present invention will be described below.

C: C is a component effective for securing the strength of steel, but if it is contained in excess of 0.030%, it tends to deteriorate the corrosion resistance, oxidation resistance, heat resistance, formability and weldability of ferrite. Therefore, the C content was determined to be 0.030% or less. It is preferably 0.015% or less.

Si: Si has an action of improving the oxidation resistance of steel, but if it is contained in an amount exceeding 0.50%, the thermal fatigue resistance and the formability are markedly deteriorated.
It is set to 50% or less, preferably 0.30% or less.

Mn: Mn is an element necessary as a deoxidizing component at the time of steel making, but if its content exceeds 0.40%, it has an adverse effect on the thermal fatigue properties, so the Mn content is 0.40.
Defined to be less than or equal to%. It is preferably 0.20% or less.

Cr: Cr has an effect of improving oxidation resistance and heat fatigue resistance, but if its content is less than 13%, the desired effect due to the above effect cannot be obtained, while on the other hand, it exceeds 16%. If added as Cr, it causes deterioration of heat fatigue resistance and moldability and is disadvantageous in terms of cost. Therefore, the Cr content is defined as 13.0 to 16.0%. It is preferably 13.0 to 14.0%.

When Ti: Ti is not added, the ingot structure is a columnar crystal, but when Ti: about 0.05% is added, equiaxed crystals which are effective for refinement are formed. Therefore, the Ti content is 0.05% or more, preferably It was set at 0.10% or more. Further, if Ti is contained in excess of 0.30%, the thermal fatigue resistance and the formability are deteriorated. Therefore, the Ti content is set to 0.05 to 0.30%, preferably 0.10 to 030%, and more preferably 0.10 to 0.15%.

Nb: Nb has a function of promoting grain refinement, but if it is less than 0.10%, the desired effect cannot be obtained, and if it exceeds 0.50%, heat fatigue resistance and formability tend to deteriorate rather. Therefore, the Nb content was determined to be 0.10 to 0.50%. It is preferably 0.10 to 0.30%.

S: S is an impurity element that forms a non-metallic inclusion together with oxygen, and is not particularly limited as long as it is contained as an ordinary impurity. However, when the total amount of S and oxygen exceeds 50 ppm, heat fatigue resistance and molding Since the property tends to deteriorate, the S content is preferably 50 ppm or less in total with oxygen (O).

In the ferritic stainless steel according to the present invention, Ni is allowed as long as the amount is about the level of ordinary ferritic steel. Here, the working conditions of the present invention will be described in more detail as follows.

As in the prior art, hot rolling (high-temperature hot rolling with a finishing temperature of 850 ° C.) was performed at a slab heating temperature of 1180 ° C. or higher, and then cold rolling was performed twice or more to obtain finely sized particles. Although steel is obtained, the residual strain energy inside the material is not sufficient due to high temperature hot rolling and intermediate annealing, and recrystallization does not occur at a too low temperature. Therefore, the range of annealing in which a fine structure can be obtained is limited, and it is difficult to obtain a stable refined steel.

Therefore, in the present invention, the conventional slab heating temperature is lowered, and a combination of the low temperature hot rolling and the number of cold rolling is reduced to expand the annealing range where a fine structure can be obtained.

Next, the reason for limiting the processing conditions in the present invention as described above will be explained. Reason for low-temperature hot rolling When using a material with a fine structure, when strain is applied at high temperature, strain energy causes dynamic recrystallization, so that strain energy does not remain inside the material, and it is nucleated during subsequent annealing. Since it is not generated, it is difficult to obtain a fine structure. Therefore, a large amount of residual strain energy is generated inside the material by giving a working strain at a low temperature where the strain energy does not cause dynamic recrystallization, and the subsequent annealing is devised to serve as a core for nucleation and a driving force. It was done.

Therefore, in the present invention, the slab heating temperature is set to 1000 to 1100 ° C., but if it is less than 1000 ° C., the deformation resistance of the material is large and the rolling capacity of the rolling mill is insufficient, so that it is not suitable for hot rolling. On the other hand, when the temperature exceeds 1100 ° C., the strain energy at the time of processing becomes insufficient, and it becomes difficult to make the particles fine in the next step, so that the particles are not finally made fine.

The finishing temperature of hot rolling is 730 to 770 ° C. This is because if it is lower than 730 ° C., the deformation resistance becomes large and the rolling ability of the rolling mill becomes insufficient as in the above case. This is because recrystallization occurs if the temperature exceeds ℃. Although hot rolling was performed even in the past,
The slab heating temperature at that time was relatively high at almost 1180 ° C,
Also, the finishing temperature was about 850 ° C, and high temperature finishing was performed.

The hot-rolled steel sheet thus obtained is annealed and cooled in a furnace for subsequent cold rolling. The annealing conditions in this case are not particularly limited, and, for example, 830 to 850 ° C. can be used in a conventional manner.
Hold for 12 to 14 hours before furnace cooling.

Reasons for Increasing the Reduction Rate There are two methods for increasing the residual strain energy, one is by the above-described low temperature hot rolling, and the other is the reduction rate during cold rolling, that is, To increase the cold working rate. By increasing the processing rate, the residual energy increases.

The reason is that recrystallization starts from a high-energy portion, and the higher the degree of workability is, the more recrystallized portions, ie, the nuclei are, and the recrystallized grains are scattered everywhere. Is.

The large residual strain energy indicates that the energy for recrystallization is ubiquitous, and that part is considered to be the cause of miniaturization when receiving energy such as heat and nucleating. Because it will be.

According to the present invention, the cold working rate is the reduction rate.
It is set to 80 to 90%, because if it is less than 80%, the strain energy is insufficient, while if it exceeds 90%, mixed grains are formed. It is preferably 85 to 90%.

In the cold working, an intermediate annealing may be performed in some cases, but in that case, since the strain energy in the finish annealing performed thereafter becomes low, the recrystallization starting temperature is high and the grain size is No. 8. It is unsuitable because it becomes large. Preferably, cold rolling is performed once.

After the cold working thus performed, finish annealing is carried out, and the annealing temperature at that time is 790 to 860.
C, and recrystallization does not occur below 790 ° C, while 860
If the temperature exceeds ℃, coarse particles will be formed.
The temperature is to 860 ° C, preferably 800 to 850 ° C.

[0034]

EXAMPLES The present invention will be described in detail below based on examples. (Example 1) The inventors of the present invention described below in Example 2 in a vacuum high-frequency small-scale melting furnace for the purpose of improving the rough surface during molding by obtaining a fine recrystallized structure in ferritic stainless steel. A small ingot of ferritic stainless steel having the No. 3 component of Table 1 in Table 1 was melted, and an ingot was cut out and used as a test material.

After heating this test piece to 1000 to 1200 ° C., 50.0
Rolled from mm to 4.0 mm. The hot rolling finish temperature at this time was 740 to 870 ° C. 4.0 mm thus obtained
The thick hot-rolled sheet was annealed at 830 ° C for 12 hours, cooled in the furnace, and then cold-rolled to give a 0.4-mm-thick cold-rolled sheet.

The rolling ratio at this time was 80 to 95%, and cold rolling was performed to 0.4 mm by one cold rolling, and intermediate annealing was performed at 980 ° C., and 0.4 mm was obtained by two cold rolling. Two types of rolled products were prepared.

The cold-rolled annealed sheets thus obtained were subjected to finish annealing treatment at 760 to 940 ° C., respectively, and then the particle size was measured and the Erichsen test was conducted. From these series of results, it was found that if GS = 9 or more, rough skin does not occur.

Then, in the same manner, the heating temperature was 1044 ° C., the hot rolling end temperature was 742 ° C., the rolling reduction was 89%, and cold rolling was performed once to 0.4 mm thickness, and various finishing annealing temperatures were changed for the cold rolled steel sheet. Then, the particle size was determined, and the results are shown in the graph in FIG.

As for the annealing temperature range, as can be seen from FIG. 1, fine grains of GS = 9 are shown in the range of 790 to 860 ° C. Below that temperature, recrystallization is insufficient and unrecrystallized parts remain. , Or above that temperature, it is unsuitable because of coarsening. In the figure, the conventional condition indicates the case where the finish annealing is performed under the condition that the intermediate annealing is performed.

Example 2 A sample steel having a steel composition shown in Table 1 was hot-rolled and cold-rolled under the conditions shown in Table 2,
A cold rolled steel sheet having a thickness of 0.4 mm was obtained. As shown in Table 2, it is understood that the maximum grain-annealing range is obtained in the test material that satisfies all the heating temperature conditions, rolling conditions, annealing conditions, finish rolling conditions, and finish annealing conditions of the present invention.

[0041]

[Table 1]

[0042]

[Table 2]

[0043]

As is apparent from the above description, according to the expansion of the grain refinement region according to the present invention, the steel components are limited as described above, the above-mentioned low-temperature work heat treatment is performed, and the finish annealing is performed. By making the range appropriate, it is possible to obtain a ferritic stainless steel in which crystal grains are made finer and the molding surface is not roughened.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a finish annealing temperature and a grain size in the present invention.

Claims (1)

[Claims] 1. By weight, C: 0.030% or less, Si: 0.50% or less, Mn: 0.40% or less, Cr: 13.0 to 16.0%, Nb: 0.10 to 0.50%, Ti: 0.05 to 0.30.
%, Ferritic stainless steel having a steel composition consisting of balance Fe and incidental impurities, slab heating temperature 1000 to 1100 ° C,
And the hot rolling finish temperature is 730 ~ 770 ℃, hot rolling is performed, then after annealing and furnace cooling, the rolling reduction is 80 ~ 90%.
A method for producing a low Cr stainless steel sheet having excellent forming surface properties, which is characterized by having a good surface property at the time of forming by performing cold rolling for finishing and further performing finish annealing at 790 to 860 ° C.