JPH07284805A - Manufacture of ferritic stainless steel excellent in surface property - Google Patents

Abstract

PURPOSE:To suppress edge seam flaw and scab by using the slab where the grain size of the prescribed value or over parallel to the surface layer occupy in the surface layer at the longer side and the shorter side of the sectional area of the slab. CONSTITUTION:In performing the hot rolling of the ferritic stainless steel, the slab where the grains whose grain size parallel to the surface layer is >=20mm occupy >=80% of the surface layer part within 100mm from the edge in the surface layer of the longer side part of the slab section perpendicular to the slab rolling direction, and 1/4t (t is the thickness of the slab) from the edge in the surface layer of the shorter side part of the slab section is used. This constitution reduces the surface flaws such as scab or edge seam flaw, improves the yield, and omits the flaw-grinding process and the re-pickling process. The reason why the grain size is set to >=20mm is that the roughness depth after the 30-50% hot rolling is rapidly reduced when the grain size exceeds 20mm to indicate the constant value of about 0.2mm. The yield is improved thereby, and the flaw grinding process and the re-pickling process are omitted.

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 steel sheet having few surface defects such as hedging and edge seam defects during hot rolling of ferritic stainless steel.

[0002]

2. Description of the Related Art Generally, stainless steel is required to have high surface quality, and reduction of surface defects is a big problem. The occurrence of surface defects causes quality deterioration, yield reduction, and cost increase due to the addition of a surface flaw grinding step or a repickling step. There are many types of surface defects, such as scale flaws due to oxide scale generated during slab heating, cracks due to segregation of impurity elements, heavier flaws and edge seam flaws that leave linear wrinkles on the surface, and rough surface due to roll burning. There is.

[0003] Among them, as peculiar to ferritic stainless steel, there are a hedging flaw and an edge seam flaw. The bald defects are linear wrinkles that occur within about 100 mm from the edges of hot-rolled steel sheet and the edge seam defects within about 30 mm from the edges. However, these surface defects are deep with a depth of about 100 μm.
The cost of flaw grinding is also high and the yield is reduced.

Many measures have been taken to reduce this baldness and edge seam flaws. Techniques for preventing bald defects are disclosed in JP-A-4-350123 and JP-A-63-1.
The invention described in Japanese Patent No. 23516 has been proposed. In Japanese Unexamined Patent Publication No. 4-350123, a dent is formed in the center of the thickness of both side surfaces of the slab, or the work roll size in the initial 3-pass hot rolling is set to a small diameter roll of 1000 mm or less, or the initial hot rolling 3 It is specified that the total reduction rate of the pass is 35 to 55%. Further, JP-A-63-123516 defines slab heating conditions characterized by a dew point of 60 ° C. or higher. However, the above technology brings about a significant cost increase due to the shape control of the slab and the regulation of the work roll, and due to the high dew point,
The main body of the heating furnace is greatly damaged, and the cost increases due to the increased number of repairs.

As a method for reducing edge seam flaws,
There are inventions described in JP-A-4-279202 and JP-B-6-241. Japanese Unexamined Patent Publication No. 4-279202 discloses a method of performing lubrication rolling to reduce the amount of bulging at the end portion and reduce the amount of trimming. This only narrows the generation range of edge seams, and is not a fundamental solution to edge seam defects. Japanese Patent Examination 6-241
In the gazette, the reduction ratio at the time of vertical rolling is specified, and it is a method of smoothing the wrinkles at the end of the hot-rolled steel sheet that occurs at the time of horizontal rolling by vertical rolling, but the occurrence of wrinkles is unavoidable and the wrinkle depth is The number of wrinkles decreases, but the number of wrinkles does not decrease.

[0006]

DISCLOSURE OF THE INVENTION The present invention relates to a method for preventing the above described ferritic stainless steels, which are peculiar to hot spots and edge seam flaws that occur during hot rolling, and which are not affected by hot rolling. An object of the present invention is to provide a method for producing a steel sheet having few surface flaws such as flaws and edge seam flaws.

[0007]

DISCLOSURE OF THE INVENTION The present inventors have investigated the generation process of bald defects and edge seam defects in ferritic stainless steel. As a result, bald defects and edge seam defects are generated along with the deformation of the columnar crystal grains in contact with the surface layer of the slab during hot rolling, and the amount of irregularities (concavo-convex depth) in the unit of the columnar crystal grains causes defects. It turned out to be greatly related. In the present invention, as a result of examining a slab surface layer structure in which unevenness is less likely to occur during hot rolling from the above viewpoint, the unevenness depth is found to be the particle size of the surface layer portion (the particle size measuring method is shown in FIG. 1) and the hot rolling reduction ratio. The present invention has been completed by discovering that the unevenness depth during hot rolling decreases when the crystal grains in the surface layer partly exceed a certain critical value and become large, and the summary thereof is as follows.

That is, according to the present invention, when hot-rolling ferritic stainless steel, the edge surface of the slab cross section perpendicular to the slab rolling direction has a surface of 100 mm from the edge.
80% of the surface layer within mm and within 1/4 t (t is the slab thickness) from the edge at the surface of the short side of the slab cross section
The above is a method for producing a steel sheet with few surface defects, which is characterized by using a slab in which particles having a particle diameter of 20 mm or more parallel to the surface layer are used.

Here, the slab and the crystal grain size measuring method in the present invention will be clearly described. FIG. 1A shows a slab cross section perpendicular to the rolling direction. FIG. 7B is a partially enlarged view of FIG. In the slab used in the method of the present invention, the total length of the grain diameters of 20 mm or more is 80, which is the total length of the thick lines at four locations.
It is characterized by occupying at least%. The "grain size parallel to the surface layer" in the present invention means the grain size parallel to the surface layer of the grains in contact with the surface layer in the slab transverse section perpendicular to the rolling direction.

[0010]

The present inventors investigated the generation process of hegging and edge seam flaws in ferritic stainless steel. A typical ferritic stainless steel, SUS430 steel cast slab (slab thickness 250 mm) was interrupted in several steps while rolling to a thickness of 3 mm with a hot rolling mill, and the short side of the cross section of the test piece at each stage was interrupted. The unevenness was investigated. as a result,
Asperity generated on the short side of the cross section of the test piece during rolling has a reduction rate of 3
When it exceeded 0%, it began to occur, and thereafter, the unevenness depth became deeper as the rolling reduction increased (FIG. 2). As shown in FIG. 3, the unevenness depth was measured by drawing a perpendicular line from the straight line connecting the vertices of the adjacent protruding parts to the largest recessed part and defining the length as the unevenness depth.

The irregularities on the short side of the cross section of the hot-rolled steel sheet occur in units of crystal grains, and these irregularities wrap around the surface of the hot-rolled steel sheet and become edge seam flaws when the rolling reduction of the horizontal rolling increases. It was confirmed that the range of wrapping around the long side of the slab cross section was 1/4 t (t is the slab thickness) from the edge of the short side. In addition, it was discovered that the bald defects were left in a linear shape by crushing the surface irregularities generated within about 100 mm from the edge during vertical rolling and wrinkling during horizontal rolling.

[0012] As described above, since both the hedging flaw and the edge seam flaw are wrinkles caused by the unevenness of the surface generated during rolling,
If a slab having a structure in which unevenness is less likely to occur during rolling is used in a range where edge seams and bald defects are generated during hot rolling, edge seam defects and bald defects are suppressed.

Next, the relationship between the structure of the slab before hot rolling and the depth of the unevenness at the edges of the hot-rolled steel sheet generated was investigated. First,
SUS430 as a typical ferritic stainless steel
The columnar grain size was adjusted by changing the heating conditions using steel. After this test piece was rolled by a hot rolling mill by 30 to 50%, the relationship between the grain size before rolling and the depth of the uneven portion on the short side of the cross section of the test piece was investigated. Concavities and convexities are generated on a grain-by-grain basis, and as shown in FIG. 4, the irregularity depth increases as the crystal grain size increases up to a pre-rolling crystal grain size of about 10 mm. When the particle size is about 10 mm, the maximum unevenness depth is about 2 mm. However, when the particle size exceeds 10 mm, the uneven depth sharply decreases, and when the particle size is 20 mm or more, the uneven depth does not depend on the particle size and is substantially constant at about 0.2 mm.

Thus, the uneven depth after 30 to 50% rolling was measured, and then hot rolling was performed up to 98%.
It was confirmed that after the rolling of 30 to 50%, the unevenness depth of 1 mm or more resulted in a bald spot and an edge seam flaw. Therefore, in order to reduce the baldness and edge seam flaws, about 20
It was found that crystal grains having a diameter of mm or more should be aligned on the surface layer.

Next, the relationship between the coverage rate of grains having a grain size of 20 mm or more and the occurrence of baldness and edge seam flaws was investigated. At this time, the coverage is defined as the ratio of the total length of the grain diameters of 20 mm or more in the range corresponding to the area where the hedging and edge seam flaws occur after hot rolling. After hot-rolling, the area where the hairline and edge seam flaws are generated is 10 from the edge of the long side of the slab cross section.
Considering wrapping from within 0 mm and the short side of the slab cross section, 1/4 t from the edge of the short side of the slab cross section
(T is the slab thickness) (see FIG. 5). SUS4
Using a 250 mm thick cast slab of 30 steel, a grain size of 20 mm
After measuring the coverage of the above-mentioned grains, hot rolling was performed to a thickness of 3 mm, and the occurrence of baldness and edge seam flaws was examined. The results are shown in Table 1. The evaluations of the hedging and edge seams in Table 1 are
⊚: No occurrence (less than 1% of the length occupied by hedging and edge seam flaws occupying the entire plate length), ◯: Slight occurrence (1 to 3)
%), △: Occurrence (3 to 10%), ×: Large amount (10)
% Or more).

[0016]

[Table 1]

A slab having a coverage of 100% in the range of occurrence of baldness and edge seam flaws did not occur at all in baldness and edge seam flaws. The slabs with a coverage of 80% or more had an incidence of 3% or less, and passed the results of hega and edge seam flaws, but the slabs of 70% or less had a high incidence of hege and edge seam flaws and failed. there were.

Therefore, in order to suppress the occurrence of baldness and edge seam flaws, the coverage of crystal grains having a grain size of 20 mm or more occupies 80% or more (preferably 100%) of the range of baldness and edge seam flaw generation. Hot rolling may be performed using a slab of ferritic stainless steel.

The reason why the occurrence of baldness and edge seam flaws is suppressed when the grains of 20 mm or more are present as described above is not clear, but it is considered as follows. Since the irregularities generated during hot rolling are generated on a grain-by-grain basis, it is considered that this is due to the plastic anisotropy of each grain. For example, when the orientations of adjacent grains are different in the short side portion of the slab cross section and the r values of the grains are different, the amount of strain in the plate width direction during hot rolling is different, so surface undulations (irregularities) occur. . As the grain size, which is a unit of the unevenness, increases, the unevenness depth also increases. However, when the grain size exceeds 20 mm, it is considered that the amount of deformation carried by the grains increases and a slip system works to reduce the depth of irregularities, and a flat surface is obtained as a whole.

As described above, it is possible to prevent edge seam flaws and baldness flaws during hot rolling by making 80% or more of the crystal grains having a grain size of 20 mm or more in the range corresponding to the hedging and edge seam flaw generation area. Became. As a method of forming grains of 20 mm or more in the surface layer in this way, SUS43
If there is a second phase that inhibits grain growth, such as the γ phase in 0 steel, heat to a high temperature at which the second phase can be solutionized,
Alternatively, there is a method of promoting grain growth by lengthening the heating time. In addition, a decarburization preventive material is applied to the area other than the area where the hedging and edge seam defects are generated, the heating atmosphere is adjusted to a decarburizing atmosphere, and the area corresponding to the hege and edge seam defects is decarburized to inhibit grain growth. There is a method of suppressing the precipitation of the γ phase and promoting grain growth. There is also a method of promoting grain growth by inducing strain by adding some processing before heating, and it is also effective to carry out reduction with a pinch roll or the like during casting, or to give cold strain to the slab surface layer before heating. Further, increasing the casting temperature during casting or increasing the grain size during casting by slowing the cooling rate is also effective in promoting the coarsening of grain size during heating.

The reasons for limiting the present invention will be described below. Reasons for measuring the grain size within 100 mm from the edge on the long side surface layer of the slab cross section perpendicular to the slab rolling direction and within 1/4 t (t is the slab thickness) from the edge on the short side surface layer of the slab cross section Is that the range of baldness is within about 100 mm from the edge of the hot-rolled steel sheet, and the range from the short side to the long side of the slab cross section during hot rolling is 1/4 t from the edge of the short side. (T is the thickness of the slab).

The reason why the coverage of the particle size of 20 mm or more in the above range is set to 80% or more is that from Table 1, the coverage of the length of the hedging and edge seam flaws in the total plate length is less than 3%. Is 80% or more. Particle size 20
As shown in FIG. 4, the reason why the grain size is set to be equal to or larger than 30 mm is that the irregularity depth after 30-50% hot rolling is sharply reduced when the grain size is equal to or larger than 20 mm.
This is because it shows a constant value of 2 mm.

The grain size in the slab surface layer parallel to the surface layer is determined by the crystal grain size measuring method because the grain size hardly changes up to 5 mm inside the surface layer. The representative value was used.

[0024]

[Example] SUS43 which is a ferritic stainless steel
The grain size 2 of the grains contacting the surface layer in the area corresponding to the area where the hedging and edge seam defects are generated before hot rolling using 0 steel
The relationship between the coverage of grains of 0 mm or more and the occurrence of baldness and edge seam defects after hot rolling was investigated. As shown in FIG. 6, a cast slab having a thickness of 250 mm was used to apply a decarburization preventive material (hatching part) to the area other than the area where the hedging and edge seam flaws were generated (hatched area) (FIG. 6) and heated at 1230 ° C. The grain coverage of grains having a grain size of 20 mm or more was adjusted by promoting grain growth of crystal grains in the range corresponding to the region where the edge and edge seam defects were generated and changing the heating time from 30 to 150 minutes.

After measuring the coverage, no grain growth occurs 120.
Reheat at 0 ℃ for 30 minutes, hot-roll to 3.0 mm, and apply heat treatment before hot-rolling and coverage of 20 mm or more of grain size in the area where edge seam flaws occur and after heat-rolling. And the relationship between the occurrence of edge seam flaws was investigated. The results are shown in Table 2.

[0026]

[Table 2]

In the method of the present invention in which the coverage of grains having a grain size of 20 mm or more exceeds 80%, the occurrences of baldness and edge seam flaws are small and are 3% or less, or ◯ is less than 1%. . On the other hand, in the comparative methods in which the coverage of particles of 20 mm or more was less than 80%, the rate of occurrence of beard and edge seams was Δ of 3 to 10% or × of 10% or more.

[0028]

As described in detail above, the effects of the present invention are
By using a predetermined slab during hot rolling of ferritic stainless steel to reduce surface defects such as baldness and edge seam defects, the yield can be improved and the defect grinding and repickling steps can be omitted.

[Brief description of drawings]

FIG. 1 is a diagram clearly showing a slab used in the method of the present invention, in which (a) shows a slab cross section perpendicular to the rolling direction, and the grain size of crystal grains in contact with the thick line portion of the slab surface layer was measured. In the crystal grain size measuring method, as shown in (b), the grain size of the grain in contact with the thick line portion of the surface layer was defined as the grain size of the grain.

FIG. 2 is a schematic diagram showing a process of generating irregularities on a short side portion in a cross section of a slab that occurs during hot rolling, as viewed from the slab rolling direction. (A) is before reduction, (b) is 50
After% reduction, (c) shows after 80% reduction.

FIG. 3 shows a method for measuring the depth of irregularities generated on the short side portion of the slab cross section after hot rolling. The dotted line is a straight line connecting adjacent convex portions, and the arrow indicates the depth of the unevenness.

FIG. 4 is a graph showing the relationship between the unevenness depth in a slab cross section after hot rolling 30 to 50% of SUS430 steel and the grain size before rolling.

FIG. 5 is a schematic view showing a range corresponding to a region where a hairline and an edge seam flaw are generated after hot rolling, and a hatched portion represents the range. The shaded portion is within 100 mm from the edge in the long side portion of the slab cross section, and within 1/4 t from the edge in the short side portion (t is the slab thickness).

FIG. 6 is a schematic view showing a region of SUS430 steel to which a decarburization preventing material is applied, and a hatched portion shows the region.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Masamitsu Tsuchinaga 1-1 Hibahata-cho, Tobata-ku, Kitakyushu, Kitakyushu, Fukuoka New Nippon Steel Corporation Yawata Works

Claims (1)

[Claims] 1. Hot-rolling ferritic stainless steel is within 100 mm from the edge on the long side surface layer of the slab cross section perpendicular to the slab rolling direction and from the edge on the short side surface layer of the slab cross section by 1 mm or less. / 4t (t
Is a slab thickness), and 80% or more of the surface layer portion within the slab is occupied by slabs in which particles having a grain size of 20 mm or more parallel to the surface layer are used.