JPH04210425A - Production of ferritic stainless steel excellent in ridging resistance - Google Patents

Abstract

PURPOSE:To shorten the annealing stage for a hot rolled plate and to produce a ferritic stainless steel excellent in ridging resistance by specifying rolling reduction at roughing and strain rate, respectively, at the time of hot-rolling a ferritic stainless steel slab. CONSTITUTION:At the time of hot-rolling a slab of ferritic stainless steel, at least one or more passes in roughing are performed at >=20% rolling reduction and also low strain rate rolling of <=2.0s<-1> strain rate is exerted. Subsequently, finish rolling is done by means of a continuous stand to form a hot rolled steel strip, and box annealing or continuous annealing is exerted or annealing is omitted, and successively, a cold rolled steel strip is produced by combining cold rolling with cold rolled sheet annealing. By this method, the ferritic stainless steel sheet excellent in ridging resistance can be obtained at a low cost while obviating the necessity of increasing manufacturing stages.

Description

[Detailed description of the invention]

[00011

[Industrial Application Field] The present invention relates to a method of manufacturing ferritic stainless steel with excellent ridging resistance, and aims to provide a method of manufacturing ferritic stainless steel with good ridging resistance that is preferable in terms of product characteristics and manufacturing. It is. [0002] [0002] Ferritic stainless steel is cheaper than austenitic stainless steel, and is used in a wide range of applications such as building materials and household equipment. Because it is often used for purposes where aesthetics are important,
Excellent surface properties are required. Therefore, it is necessary to have excellent surface gloss. However, it is generally well known that a characteristic of ferritic stainless steel is that surface irregularities called ridging occur during press molding. Since the formation of ridging significantly impairs the appearance of the product, there is a strong demand for improved ridging resistance in ferritic stainless steel. [0003] Ridging is known to be strongly influenced by coagulation tissue. Improvement methods can be broadly divided into two categories: (1) regulation of hot rolling conditions and annealing conditions;
2) Various studies have been made on improvements by component adjustment. For example, specifying the optimum hot rolling temperature range, reduction rate, and annealing conditions (
(Japanese Patent Publication No. 63-26177), in addition to the above, 0.
Addition of 1%-0.3% AI and regulation of winding temperature (Japanese Patent Publication No. 1-60531), 0.08%-0.5% A1
A method of reheating the contained steel during hot rolling (Patent Publication No. 2)
-1211 publication), method of applying bending strain to 0.03%-0.2% A1-containing steel between rough rolling and finish rolling (JP-A-62-136525 publication), 0.01%-0
, a method in which the rolling temperature and reduction rate are specified for 2% AI, and the cooling conditions during annealing are specified (Japanese Patent Publication No. 59-4397
7), a method of heat-treating a slab prior to hot rolling (Japanese Patent Publication No. 2-409, Japanese Patent Publication No. 2-411)
etc. have been proposed. [0004]

Problems to be Solved by the Invention Among the conventional manufacturing methods described above, those based on component adjustment are based on the addition of At. However, the addition of AI results in the formation of hard Al2O3 in the steel. As a result, surface flaws due to rolling flaws can easily occur during hot rolling or cold rolling. This is undesirable because it causes deterioration of surface properties. In addition, methods that are based on specified manufacturing conditions include methods of heat treating the slab, methods of applying reheating during hot rolling, and methods of applying bending strain, but all of these methods increase the number of manufacturing steps. Increase manufacturing costs. The method of specifying the optimum hot rolling temperature range, rolling reduction rate, and annealing conditions is as follows: First, the rough rolling start temperature is set to 1150°C or lower, and the finish rolling start temperature is set to 900°C or higher.
The second finish rolling reduction ratio is 25% or more,
There is a concern that the load of finish rolling will increase due to large reduction. Further, although the annealing condition temperature range is specified as 900 to 1100°C, such high-temperature annealing causes rough skin in the pickling process and deteriorates product characteristics. [0005] As described above, all of the conventional methods have problems in terms of product characteristics or manufacturability. With conventional methods, it is possible to inexpensively produce ferritic stainless steel with excellent ridging resistance without requiring special component adjustments that would degrade the product's surface properties and without significantly changing the conventional manufacturing process. Can not. [00061

[Means for Solving the Problems] The present invention has been devised after repeated studies to solve the above-mentioned problems in the conventional methods, and is as follows. When hot rolling a slab of ferritic stainless steel, at least one pass of rough rolling is performed with a reduction amount of 20% or more,
And, after rolling at a low strain rate of 2.0 s-1 or less, followed by finishing rolling on a continuous stand to form a hot rolled steel strip, box annealing or continuous annealing, or omitting annealing, and then A method for producing ferritic stainless steel with excellent ridging resistance, characterized by producing a cold-rolled steel strip by combining cold rolling and cold-rolled plate annealing. That is, it is known that the sliding phenomenon of ferritic stainless steel is mainly caused by the presence of colonies of texture inherited from the solidified structure. Therefore, randomization of this texture is the most effective way to improve ridging resistance, and conventional proposals have been made based on this point. There are various problems that cannot be solved as a manufacturing method. [0007] In view of this situation, the present invention aims to improve ridging resistance from a completely new perspective. In other words, we focused on the influence of the rolling strain rate in hot rolling, and as a result of a thorough investigation of its influence on ridging, we found that by securing a reduction amount of 20% or more per pass and limiting the strain rate to a low rate, The present invention was made based on the new knowledge that the ridging resistance can be significantly improved. In particular, a reduction in the strain rate during rough rolling performed at high temperatures causes concentration of deformation in the ferrite phase during rolling, promoting recrystallization and effectively randomizing the texture that is harmful to ridging. [0008] According to the present invention as described above, it is only necessary to slow down the rolling speed in at least one pass during hot rolling rough rolling,
There is no need to change existing manufacturing processes. The time required for hot rolling is also substantially the same as that of the conventional method by shortening the waiting time until finish rolling, and there is no increase in manufacturing costs. Hot-rolled sheet annealing can be either regular box annealing or continuous annealing.
Alternatively, annealing may be omitted. If the hot-rolled sheet is subjected to conventional box annealing, the ridging resistance of the final product will be significantly improved. In addition, continuous annealing or omission of annealing does not cause deterioration in relative perijig properties compared to conventional products. [00091 The basic requirement of the present invention is implementation of low strain rate rolling during rough hot rolling (reduction amount per pass is 20% or more). It is well known that strain rate affects the high-temperature deformation behavior of metals. In particular, with regard to recrystallization behavior, the influence of strain rate on dynamic and static recrystallization behavior has been variously investigated for ferritic stainless steel [e.g., Tetsu to Hagane, 69 (1983)]
, p1440, Tetsu to Hagane, 70 (1984), p725
, Tetsu to Hagane, 70 (1984), p2152, Tetsu to Hagane,
75 (1989), pH93, Tetsu to Hagane, 68 (19
82), 5539, Japanese Institute of Metals Spring Conference General Lecture Abstracts, (1982-4), p65]. However, the influence of strain rate during hot working on the gluing properties of ferritic stainless steels has never been investigated. The inventors focused on this point and conducted experiments in which the strain rate during hot rolling and rough rolling was varied, and as a result of investigating the ridging properties of the final cold-rolled annealed material, they found that the ridging properties are greatly affected by the strain rate. I found it. FIG. 1 shows the results of an investigation using 5US430, which is a typical stainless steel. It has been shown that by lowering the strain rate during rough hot rolling, the ridging height is reduced and the ridging resistance is improved, and the effect is significant at strain rates of 2.0 or less. Slab thickness is 16
0 ml, the slab heating temperature was 1150° C., and rough rolling was performed for 5 passes at 25% for each pass. In order to equalize the finish rolling start temperature, the strain rate was changed only in the first and second rough rolling passes, and the strain rate was kept constant at 6.0 s for all remaining rolling passes. The annealing conditions for the plate include box type annealing (820°C x 5 hours of soaking followed by slow cooling), continuous annealing (820°C
40°C x 90 seconds) and omitting annealing, the above-mentioned improvement effect was observed in both cases. Note that the gluing height here is cold rolled (cold rolling rate 80%),
A JISS No. test piece was prepared in the rolling direction from the annealed (850°C x 90 seconds) and pressure adjusted (pressure adjustment ratio 1.0%) material, subjected to a 15% strain using a tensile tester, and then tested using a roughness tester. Measure the surface roughness in the direction perpendicular to rolling, and calculate Rz (10-point average roughness).
This was determined as the ridging height. [00101 FIG. 2 shows the relationship between the rolling reduction amount per pass and the rolling height. When the strain rate is high, the reduction amount has almost no effect in the range of 5 to 40%. On the other hand, under conditions of low strain rate such as 1.0 s-', the reduction amount per pass is 20%.
If it exceeds 20%, the effect of improving ridging resistance becomes remarkable. Therefore, it has been shown that the amount of rolling reduction per pass needs to be 20% or more. As mentioned above, implementation of low strain rate rolling during hot rolling rough rolling (reduction amount per pass is 20% or more)
It has been shown that the present invention has the effect of improving ridging resistance. Although it is not necessarily clear what kind of mechanism this effect is based on, it is clear that randomization of the texture by promoting recrystallization is at work. Particularly in a ferritic stainless steel having a component system that forms a two-phase mixed structure of ferrite and austenite 1 during hot rolling, a decrease in the strain rate causes concentration of strain in the soft ferrite phase. It is recognized that this causes recrystallization promotion due to a substantial increase in strain in the ferrite phase and strain-induced transformation of austenite in the ferrite phase, resulting in randomization of the texture of ferrite grains. [00113

[Example] To explain a specific example of the present invention, first, the ferritic stainless steel used by the present inventors is as shown in Table 1 below, and 5US430 is a typical ferritic stainless steel. Starting with 5US
430LX, 5US444.5US410L, 5UH4
09, high Si-containing heat-resistant steel was used. Cr content is 10.
A wide range of steel types is used, ranging from 8 to 18.5%. In addition,
The present invention relates to ferritic stainless steel, including 5US410 series, which is usually called martensitic stainless steel.
It also includes the steel type SUH409, which is a high Cr heat-resistant steel. [001,2] [Table 1] [00131] Table 2 below shows the ridging height of each steel type when manufactured under the conditions of the present invention and when manufactured under comparative conditions. The difference in conditions between the present invention and the comparative example is
The difference lies in the strain rate during rough rolling, but in both cases, after heating the slab, 5 passes of rough rolling (reduction amount 24.30.37.25
．． 33%) and finished into a 3 mm hot rolled steel strip using a 7-stage continuous stand. After hot rolling, the hot rolled steel strip was box-shaped annealed, cold rolled into a cold rolled sheet of 0.8 mm, and after continuous annealing, the pressure adjustment ratio was 1.0.
% pressure adjustment was performed. [0014]

[Table 2] (0015) As shown in Table 2, the comparative example (conventional hot rolling conditions) had a ridging height of 8 to 12 gm, but according to the present invention, the ridging height was 8 to 12 gm for each steel type. The height is reduced by about 50 to 60% to 5.0 μm or less. If the ridging height is improved to this extent, there will be almost no problems when the product is actually press-formed. [00163 The following Table 3 shows the rolling height of the final product when the reduction amount and strain rate of the rough hot rolling conditions were varied.[0017]

[Table 3] [0018] That is, according to the present invention, the ridging height exhibits excellent properties of 4.5 gm or less regardless of whether the hot rolled sheet is annealed by box annealing, continuous annealing, or omitting annealing. . The application of the present invention of low strain rate rolling is
The effect was obtained with each pass. Further, as shown in the comparative example, the rolling properties are not improved when the reduction amount is less than 20% or when the strain rate exceeds 28-1. [0019]

Effects of the Invention According to the present invention as explained above, the rough hot rolling conditions for ferritic stainless steel are set to a rolling reduction of 20% or more and a low strain rate rolling of 2.0 s'' or less. By configuring it to do this, it is possible to shorten the annealing process for hot-rolled sheets without increasing the manufacturing process, and to obtain ferritic stainless steel with excellent ridging resistance at a low cost. This invention has the following effects and has great industrial effects. [00201

[Brief explanation of the drawing]

FIG. 1 shows the effect of rough rolling, which is a component of the present invention, on the strain rate and sliding height. It has been shown that the ridging resistance is improved by lowering the strain rate, and in particular, when the strain rate is 2.0 s'' or less, the ridging height becomes 5 μm or less, and excellent ridging resistance can be obtained.

FIG. 2 shows the influence of the reduction amount per pass of rough rolling, which is a component of the present invention, on the sliding height. Ridging resistance is improved by increasing the reduction amount, especially when the reduction amount is 20% or more, the ridging height is 5.
μm or less, and it has been shown that excellent ridging resistance can be obtained. invention long ago

Claims (1)

[Claims] Claim 1: When hot rolling a slab of ferritic stainless steel, at least one pass of rough rolling is performed with a reduction amount of 20% or more and a strain rate of 2.0 s^-^
Rolling is performed at a low strain rate of 1 or less, followed by finishing rolling on a continuous stand to form a hot rolled steel strip, followed by box annealing or continuous annealing, or omitting annealing, followed by cold rolling and cold rolled sheet. A method for producing ferritic stainless steel with excellent ridging resistance, characterized by producing a cold-rolled steel strip in combination with annealing.