JPH0578751A - Manufacture of ferritic stainless steel sheet - Google Patents

Abstract

(57) [Summary] [Object] To provide a method for producing a ferritic stainless steel sheet capable of stably producing a softened material without extremely lowering the slab heating temperature in the hot rolling step. [Structure] In a method for producing a ferritic stainless steel sheet by hot rolling a ferritic stainless steel slab, followed by hot-rolled sheet annealing, cold rolling and cold-rolled sheet finish annealing steps, addition of Ti and Nb and further slab 1040
A method for producing a ferritic stainless steel sheet, which is characterized by carrying out soaking and holding in the range of to 1200 ° C, hot rolling, and then performing hot-rolled sheet annealing and cold-rolled sheet finish annealing in the range of 950 to 1050 ° C.

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 by softening a ferritic stainless steel to improve formability.

[0002]

2. Description of the Related Art Conventionally, as seen in JP-A-58-71356 and JP-A-63-219527, the slab heating temperature (SRT) is lowered and the carbon-nitrogen solid solution elements such as Ti and Nb are dissolved. Addition is performed to prevent carbonitrides from precipitating and preventing grain boundaries from moving to prevent coarsening of crystal grains, and aiming for uniform fineness to improve Rankford value (hereinafter referred to as “r value”) and ridging property. Attempts have been made to improve moldability.

Further, Japanese Patent Application No. 02-426 filed by the present applicant.
As seen in No. 50, the composite addition of Nb and Ti is attempted, and the SRT in hot rolling is reduced to 960 to 1040 ° C. to make the precipitation distribution of carbonitride sparse and facilitate the movement of dislocations. Therefore, there is a method for softening the ferritic stainless steel.

Further, Japanese Patent Application No. 03-90 filed by the present applicant.
Nb, and in some cases Ti, as seen in No. 710.
In addition to the ferritic stainless steel containing 1% or 2 types of Zr or V that forms a carbonitride more stable than Ti, the precipitation distribution of carbonitrides can be increased by setting the SRT to 960 to 1170 ° C. Attempts have been made to soften the sparsely.

[0005]

However, JP-A-58-58
No. 71356 and Japanese Patent Laid-Open No. 63-219527 are based on the viewpoint of press moldability, and have not been studied from the viewpoint of softening. In recent years, there is a tendency to shift from ordinary steel to stainless steel in a wide variety of materials with the trend toward higher maintenance, labor cost reduction measures, and maintenance-free measures as a measure for shortage of special skill workers. Moreover, even if stainless steel has been used conventionally, higher corrosion resistance is required and high C
In many cases, it is necessary to change the material such as r. In this way, when changing the material from ordinary steel to stainless steel or from low Cr stainless steel to high Cr stainless steel,
Since it becomes hard, wear of the mold becomes remarkable in press molding in a conventional processing machine, and problems such as difficulty in accurate processing have arisen. Further, industrially, the operation at a slab heating temperature of 1040 ° C. or lower has a high yield due to surface scratches and large hot rolling resistance, resulting in poor plate thickness accuracy, and it is desired to raise the slab heating temperature.

It is an object of the present invention to provide a method for producing a ferritic stainless steel sheet capable of stably producing a softening material without extremely lowering the slab heating temperature in the hot rolling step.

[0007]

To achieve the above object, according to the present invention, a ferritic stainless steel slab is hot-rolled, followed by hot-rolled sheet annealing, cold-rolling and cold-rolled sheet finish annealing steps. In the method for producing a ferritic stainless steel sheet, C: 0.02% by weight or less N: 0.02% by weight or less Cr: 12 to 25% by weight Al: 0.1% by weight or less Ti or Nb alone or in combination with Ti ^* (Wt%) = Ti (wt%)-(47.9 / 14) N (wt%) X = (Ti ^* (wt%) / 47.9 + Nb (wt%) / 92.9) / (C (wt%) / A slab containing a variable X represented by 12) having a value of 2 to 8 and the balance being Fe and unavoidable impurities is soak-maintained in the range of 1040 to 1200 ° C. and hot-rolled. It is characterized in that hot-rolled sheet annealing and cold-rolled sheet finish annealing are performed in the range of 950 to 1050 ° C. Method for producing a ferrite stainless steel sheet is provided.

The present invention will be described in more detail below. According to the prior art, it is said that the hot rolled sheet can be softened by controlling the structure of the hot rolled sheet so that the distribution of precipitates becomes sparse. ,
As a result of repeated research by varying the hot-rolled sheet annealing condition and the cold-rolled sheet annealing condition, even if the precipitate distribution in the hot-rolled sheet is sparse, it may not always be possible depending on the conditions of the subsequent hot-rolled sheet annealing and cold-rolled sheet annealing. I found that it did not soften.

Table 1 shows 18.12% Cr-0.0095% C-0.0073% N-0.25% Ti with a rolling reduction of 84% during hot rolling.
It is a result of steel production conditions and YS and er values under the conditions. YS is high and does not soften under the condition that SRT is as high as 1250 ° C. On the other hand, set the SRT to low temperature (110
0 ° C.) and the cold rolling sheet annealing temperature is 950, 1000, 10
An effect of softening is recognized under the condition of 40 ° C. However, even if the cold-rolled sheet annealing temperature is set to a high temperature of 1100 ° C. even under the condition where the SRT is set to a low temperature of 1100 ° C., YS is high and softening does not occur even though the crystal grains grow. On the other hand, ￣
The r value corresponds to the hot rolled sheet annealing temperature, and the hot rolled sheet annealing temperature 110
1040 ° C or 100 from the one treated at 0 ° C
The 0- ° C value greatly improves the er value. As a result of research conducted by the present inventors, such a phenomenon was found to occur in Ti or Nb.
It was found that it is closely related to the change in the precipitation morphology of the above-mentioned carbides. That is, the carbide is coarsely precipitated by lowering the SRT, and the subsequent hot-rolled sheet annealing is performed at a temperature at which the carbide does not form a solid solution. By setting the above, softening can be achieved, and by combining the above conditions, it is possible to produce a ferritic stainless steel that is soft and has excellent formability.

Based on the above findings, the manufacturing conditions of the present invention are limited as follows. The slab heating temperature affects the grain size of carbonitrides precipitated in the hot-rolled sheet, and if it exceeds 1200 ° C, the precipitates become fine and the distribution becomes dense, so the upper limit is set to 1200 ° C. Further, even if the slab heating temperature is too low, surface scratches and the like are likely to occur, and the hot rolling resistance is large, which deteriorates the plate thickness accuracy, which is not industrially preferable. Therefore, the lower limit is set to 1040 ° C.

When the hot-rolled sheet annealing temperature is higher than 1050 ° C., the amount of re-dissolved carbonitrides precipitated during slab heating increases, and the amount of solid solution elements remaining in the matrix after cooling increases.
The r value is lowered, which is not preferable in terms of moldability. Therefore, the upper limit is set to 1050 ° C. If the temperature is lower than 950 ° C, recrystallization may not be completed completely due to continuous annealing. Therefore, the lower limit is set to 950 ° C.

The upper limit of the cold-rolled sheet annealing temperature is set to 1050 ° C. for the same reason as the limitation of the hot-rolled sheet annealing temperature.
On the other hand, if the temperature is lower than 950 ° C, the recrystallization may not be completely completed by continuous annealing, and at the same time, the recrystallized grain size becomes too fine and does not soften, so the lower limit is 950 ° C.

Next, the reasons for limiting the chemical components will be described.
C and N are elements harmful to formability and corrosion resistance, and T
From the viewpoint of addition of i and Nb, the smaller amount is desirable. Therefore, the upper limit is set to 0.02% by weight.

If Cr is added in an amount of more than 25% by weight, it becomes hard, and the effect of the treatment according to the present invention is small. The lower limit is 12% by weight or more from the viewpoint of obtaining a ferrite single phase structure at the hot rolling temperature and from the viewpoint of corrosion resistance.

When Al is added in a large amount, it causes surface scratches, so the upper limit was made 0.1% by weight.

Both Ti and Nb form stable carbonitrides with C and N and purify the ferrite matrix, and are the most important additive elements in the present invention. Where Ti, N
Since b is more likely to form a nitride thermodynamically more stably than a carbide, Ti ^* (wt%) = Ti (wt%)-(47.9 / 14) N (wt%) X = (Ti ^* (wt %) / 47.9 + Nb (wt%) / 92.9) / (C (wt%) / 12) and the relationship between the variable X and the yield strength YS was investigated. The result is shown in FIG. However, the basic composition is 18 wt% Cr-0.008 wt% N steel, and the addition amounts of Ti and Nb are (Ti ^* (wt%) / 47.9) / (C (wt%) / 12) and (N
b (wt%) / 92.9) / (C (wt%)) / 12) has a composition in which the variables are approximately equal, and the manufacturing is SRT 1100 ° C, hot rolled sheet annealing temperature 1000 ° C, cold rolled sheet annealing It was conducted under the condition of a temperature of 1000 ° C. From this, it is understood that in any of the C contents, it softens as X increases, but has a minimum value in the vicinity of X = 3 to 6, and further increases in X brings about strength increase. Compared with the case where Ti and Nb are not added, the effect of softening is clearly recognized in the region where X = 2 to 8 although it depends on the C content. Further, the addition of Ti and Nb with X = more than 8 is disadvantageous in cost. Therefore, the limited range of Ti and Nb components is Ti ^* (wt%) = Ti (wt%)-(47.9 / 14) N (wt%) X = (Ti ^* (wt%) / 47.9 + Nb (wt%) / 92.9) / (C (wt%) / 12) is defined as a range in which the variable X takes a value of 2-8.

In addition, if necessary, Si is used as a deoxidizing agent.
Mn may be added as a desulfurizing agent, and Mo and Cu may be added in appropriate amounts to improve corrosion resistance.

[0018]

EXAMPLES The present invention will be specifically described below based on examples. (Example 1) Inventive steels and comparative steels having the compositions shown in Table 2 were melted in a high-frequency vacuum melting furnace to 100 k each.
g of steel. These steel ingots were subjected to hot rolling (finishing thickness 4 mm, rolling reduction 84%), hot-rolled plate continuous annealing, cold rolling (rolling reduction 75%) and finishing plate continuous annealing under the conditions shown in Table 3 to obtain a finished thickness. A cold-rolled steel sheet having a thickness of 1 mm was produced. From this, JIS13B test pieces were cut out in three directions of 0 °, 45 °, and 90 ° with respect to the rolling direction, and a tensile test was performed to investigate the mechanical properties. Table 4 shows an example of the result.

When the C content is the same (0.01% by weight) and the manufacturing methods are constant (condition 1 of the present invention),
With respect to the YS value and the r-value (shown in Comparative Example 2) of Comparative Steel 2 not containing Ti and Nb, Ti and Nb were added in combination (Invention Examples 1 and 2) and Ti alone was added (Invention Example 3). And Nb
In each of those added alone (Invention Example 4), YS was lowered and the er value was increased, and the effect is recognized. However, when Ti and Nb are excessively added (Comparative Example 3),
Increase in YS, which is considered to be due to solid solution hardening of these elements, causes deterioration of mechanical properties. When Ti and Nb are added so that X is almost the same, the lower the C content, the lower the Y content.
S and a high value are obtained, and good mechanical properties are obtained (Comparative Example 1, Inventive Example 2, Inventive Example 5). On the other hand, even if the steel satisfies the composition of the present invention and the production conditions thereof do not satisfy the scope of the present invention (Comparative Examples 4, 5, and 6), those satisfying the production conditions of the present invention (Invention Example 2, Compared with 6 and 7), both have a high YS value or a low −r value, and it can be seen that the effect required by the present invention cannot be achieved. This tendency is due to the Ti alone addition system (Invention Examples 3 and 8 and Comparative Examples 7 and 8), the Nb alone addition system (Invention Examples 4 and 9, and Comparative Examples 9 and 10) and the low C content system (Invention Examples The same is found in Examples 5 and 10 and Comparative Examples 11 and 12).

[0020]

[0021]

[0022]

[0023]

[0024]

EFFECT OF THE INVENTION Since the present invention is constructed as described above, stainless steel is used in building materials and interior / exterior decoration members such as roofs and sheds which have conventionally used ordinary steel plates, and in recent years, corrosion resistance has been improved. However, since the material became harder as the amount of Cr added to automobile exhaust system members increased, it was possible to solve the problems that were difficult to manufacture with conventional processing technology and processing equipment, and at the same time, a softening material for stainless steel. In producing, the softening material can be stably produced without extremely lowering the slab heating temperature in the hot rolling process.

[Brief description of drawings]

FIG. 1 is a graph showing a relationship between a variable X and a yield strength YS.

Claims (1)

[Claims] 1. A method for producing a ferritic stainless steel sheet by hot rolling a ferritic stainless steel slab, followed by hot-rolled sheet annealing, cold rolling and cold-rolled sheet finish annealing, wherein C: 0.02% by weight. Below N: 0.02 wt% or less Cr: 12 to 25 wt% Al: 0.1 wt% or less Ti or Nb alone or in combination Ti ^* (wt%) = Ti (wt%)-(47.9 / 14) N (wt%) X = (Ti ^* (wt%) / 47.9 + Nb (wt%) / 92.9) / (C (wt%) / 12) The range in which the variable X takes a value of 2 to 8 Of which the balance is Fe and unavoidable impurities as the balance, is soak-maintained in the range of 1050 to 1200 ° C., hot-rolled, and further annealed in the hot-rolled sheet and finish-annealed in the cold-rolled sheet in the range of 950 to 1050 ° C. A method for producing a ferritic stainless steel sheet, comprising: