JP3423815B2 - Method for producing ferritic stainless steel to prevent surface flaws from occurring during hot rolling - Google Patents

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 stainless steel, and more particularly to a method for producing ferritic stainless steel which prevents the occurrence of surface defects during hot rolling.

[0002]

2. Description of the Related Art Generally, surface defects are apt to occur at the edges of hot-rolled steel sheets, and surface defects are a serious problem in terms of quality in stainless steel, which is one of its characteristics. Further, not only the quality is deteriorated due to the occurrence of the surface defects, but also the yield is lowered in the manufacturing process, and the cost increase is largely caused by the necessity of a flaw grinding step and a repickling step. In particular, it is known that ferritic stainless steel represented by SUS430, which has a large amount of production as well as SUS304, is apt to cause surface defects called line bald defects and edge seam defects.

[0003] The line bald marks and the edge seam marks are linear wrinkles that occur about 100 mm from the edges of the hot-rolled steel sheet. However, since these surface defects have a depth of about 100 µm, they are involved in flaw grinding. The cost is high and the yield is greatly reduced. From the past, many studies have been made to reduce line bald defects and edge seam defects, for example, JP-A-4-350.
No. 123, Japanese Patent Laid-Open No. 63-123516, and the like.

In Japanese Laid-Open Patent Publication No. 4-350123, the shape of the slab, the work roll size, and the rolling reduction are specified from the viewpoint of preventing the cracks from being cracked in the surface layer portion. In terms of preventing cracking, there are great restrictions during hot rolling, and slab shape control and work roll regulation will bring about a significant increase in cost.

Further, Japanese Patent Application Laid-Open No. 63-123516 discloses a scale flaw prevention technique in which the scale-off amount is controlled by defining heating conditions from the viewpoint of preventing scale-induced flaws. However, ferritic stainless steel also has many component systems, and the scale generation amount varies greatly depending on the steel type with different components even under the same heating conditions, so it cannot be said that scale flaws can be completely prevented, and heating to generate scale is not possible. When the time is long, there is a big problem that the slab structure becomes large.

Further, Japanese Patent Application Laid-Open No. 4-279202 and Japanese Patent Publication No. 6-241 disclose techniques relating to the reduction of edge seam flaws. 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 range of occurrence of edge seams and is not a fundamental solution to edge seam flaws. Japanese Patent Fair 6-24
Japanese Patent No. 1 discloses a method of prescribing a reduction ratio during vertical rolling and smoothing wrinkles at the end of a hot-rolled steel sheet that occurs during horizontal rolling by vertical rolling, but chicks are unavoidable.

[0007]

DISCLOSURE OF THE INVENTION The present invention is intended to prevent the line bleeding flaws and edge seam flaws that occur during hot rolling, which is peculiar to the above-mentioned ferritic stainless steel, and to prevent surface defects without increasing the process load. It is an object of the present invention to provide a manufacturing method that prevents the occurrence of

[0008]

DISCLOSURE OF THE INVENTION The inventors of the present invention have investigated the process of occurrence of line bald defects and edge seam defects in ferritic stainless steel. As a result, the line bald defects and the edge seam defects are generated along with the deformation of the crystal grains of the slab edge during hot rolling, and are particularly affected by the grain size of the columnar crystals of the slab surface layer portion, and It was found that the amount of unevenness (irregularity depth) as a unit is greatly related to the occurrence of defects. Regarding the method of preventing the occurrence of irregularities during hot rolling, refining the crystal grains is effective, and the finer the crystal grains are, the less likely irregularities will be during hot rolling. It was clarified that line bald defects and edge seam defects are less likely to occur in the rolling stage.

Since ferritic stainless steel such as SUS430 steel has only a partial γ transformation (α → γ) after solidification, it does not have a complete transformation such as δ → γ transformation and γ → α transformation like carbon steel. It is difficult to refine the cast slab or the microstructure before rolling due to transformation, and in the ferritic stainless steel containing 16% or more of Cr, especially in the continuous cast slab, the refinement of the solidification structure is an important point for the refinement of the microstructure before rolling. Becomes

As a solidification structure control technique for ferritic stainless steel, about 0.3% of Ti is added in order to prevent surface waviness, which is called ridging, which occurs when the product is processed. The technology to make it known is known. However, in the above method, it is necessary to add a large amount of Ti, which increases the cost by adding a large amount of Ti, slab placement cracking, Ti streak caused by Ti-based inclusions, and a color tone called bluing during annealing. There was a problem, and it was not the optimal method.

As a result of studying the refinement of the solidification structure without adding a large amount of Ti, the inventors of the present invention can achieve the refinement of the solidification structure of ferritic stainless steel by controlling the trace amounts of Ti and N. It was clarified that it is possible to prevent line bleeding flaws and edge seam flaws that are likely to occur at the slab edge portion due to coarse crystal grains.

The present invention has been made based on the above viewpoints, and the gist thereof is as follows. (1) in mass% Cr: In ferritic stainless steel containing 16~35%, Ti: 0.02~0.10
%, N: 0.0070 to 0.05%, Al: 0.15%
Below, Ti (%) × N (%): 1.5 × 10 ⁻⁴ or more, Al (%) / Ti (%): more than 0.10. Molten steel superheat degree during continuous casting ( the continuous casting slab cast a difference) of the molten steel temperature and the liquidus temperature of 50 ° C. or less, the hot
The heating temperature T (° C) before rolling is set to 1050 to 1250 ° C.
In addition, the method for producing ferritic stainless steel for preventing the occurrence of surface flaws in hot rolling, characterized by performing hot rolling after heating within a range where the soaking time t (min) satisfies the following formula . Ferritic stainless steel for preventing the occurrence of surface flaws in hot rolling according to the above item (1 ), characterized in that t ≦ − (6/5) · T + 1560 ( 2 ) Ti is added into the mold for continuous casting. Steel manufacturing method .

[0013]

According to the present invention, it is possible to prevent the occurrence of surface flaws such as line-scratch flaws and edge seam flaws that occur during hot rolling.

The present invention will be described in detail below. The present inventors have examined in detail the generation process of line shaving defects and edge seam defects in ferritic stainless steel. In particular, the relationship between the slab structure and the occurrence of defects was investigated and the organizational factors were clarified.

Continuously cast slabs (slab thickness 250 mm) of SUS430 steel, which is a typical ferritic stainless steel having the components shown in Table 1, were interrupted in several steps while rolling to 3 mm by a hot rolling mill, and each step The unevenness of the short piece portion of the cross-section of the test piece was examined.

[0016]

[Table 1]

As a result, as shown in FIG. 1, the unevenness of the short piece of the slab began to occur when the rolling reduction exceeded 30%, and thereafter the unevenness depth increased as the rolling reduction increased. As a method of measuring the depth of the unevenness, as shown in FIG. 2, a perpendicular was dropped from the straight line connecting the vertices of the adjacent convexes to the concave, and the length was defined as the concave depth. The unevenness of this slab short piece part occurs in units of crystal grains, and this unevenness wraps around the surface of the hot-rolled steel sheet and becomes an edge seam defect when the rolling reduction of the horizontal rolling becomes high, and it becomes an edge seam flaw, and the long side part of the slab cross section. It was confirmed that the wraparound range was ¼t (t is the slab thickness) from the edge of the short piece. It was also clarified that the bald spots had surface irregularities formed within about 100 mm from the edge during vertical rolling and were crushed during horizontal rolling to become wrinkles and remained linearly.

As described above, both the line bald defects and the edge seam defects are wrinkles caused by surface irregularities that occur during rolling, and since the irregularities are generated in crystal grain units, the crystal grains in the surface layer are made finer. It has been found that the amount of unevenness can be reduced if possible, and the occurrence of edge seam flaws and line bald flaws can be prevented.

From the comparison between the unevenness of 50% lab hot rolled material and the occurrence rate of line baldness flaws and edge seam flaws of the actual machine, if the unevenness at the time of lab hot rolling is 800 μm or less, the line baldness flaws and edge seam flaw occurrence rates are It was found to be approximately 0%, and it was found that the width of the columnar crystal as the initial structure and the average grain size of 2 mm or less for the equiaxed crystal were sufficient for this purpose.

From the above-mentioned examination results, the largest structural cause of line bald defects and edge seam defects is the grain size. In the case of columnar crystals, the width of the columnar crystals and in the case of equiaxed grains, the average grain size is coarsened. It was necessary not to do so, and the method for controlling the slab structure was further investigated. As a result of various studies on the above-described points regarding the method of refining the solidification structure, the inventors have found that the surface layer structure can be refined by controlling the amounts of Ti and N.

SUS430 steel having the components shown in Table 2 was vacuum melted in a laboratory at 3 kg, Ti was added to the molten steel, and the molten steel superheat degree ΔT
As a result of investigating the influence of Ti on the ferrite grain size of the cast iron by sucking molten steel with a hollow steel pipe having an inner diameter of 25 mm as shown in FIG. It has been found that there is a 2/3 reduction effect.

[0022]

[Table 2]

It has been found that this effect greatly changes depending on the degree of superheating of molten steel, and as shown in FIG. 4, it is remarkable when the degree of superheating of molten steel ΔT (difference between molten steel temperature and solidification temperature) is 50 ° C. or less. . It is known that the refinement of solidification structure of ferritic stainless steel has been conventionally observed when a large amount of Ti having a Ti content of about 0.3% is added. No refinement of stainless steel has been reported. Also, this refinement effect is due to Al
It was also found that when / Ti was 0.1 or more, there was no effect from the addition of Ti to casting. From this, it is considered that this refinement effect is different from the heterogeneous nucleation by TiN in the molten steel which has been conventionally reported.

When it is attempted to prevent line bald defects and edge seam defects as in the present invention, the problem is the grain size of the surface layer of the slab, and in the case of columnar crystals, the width of the columnar crystals and the equiaxed crystal. In the case of, it is necessary not to coarsen the average grain size, and from the viewpoint of reducing the grain size of the surface layer of the cast slab, the addition method of Ti is C
The most desirable method is to add it directly into the C mold with a wire or the like.

Regarding the above-mentioned method for refining the solidified structure, SUS
We investigated various ferritic stainless steels centering on 430 steel and found that the grain size was about 2/3 compared to the Ti-free material.
If the molten steel superheating degree ΔT is 50 ° C or less, the product of Ti and N Ti (%) × N (% ) Is 1.5
It was found that the effect was observed when it was ^{x10 -4} or more.

Further, Cr of 16% or more, which is the object of the present invention,
Since the ferritic stainless steel containing the alloy does not have the transformations of δ → γ and γ → α like ordinary steel below the solidification temperature, the refinement of the structure from casting to heating before rolling is essential except utilizing recrystallization. It is impossible to do so, and when the processing that causes recrystallization is not performed, that is, when the continuously cast slab is directly subjected to hot rolling, it is substantially important to prevent the coarsening of crystal grains. Especially when directly heating a continuous cast slab, the grain growth behavior greatly depends on the heating temperature,
Even if the solidified structure is miniaturized, it coarsens during heating, and it is not possible to prevent the occurrence of line bald defects and edge seam defects.

Therefore, the inventors of the present invention have Ti: 0.03.
%, N: 0.012% material was used to investigate the relationship between soaking temperature and soaking time before heating before rolling and the occurrence of unevenness, and the unevenness at a hot rolling rate of 50% was measured to find that the unevenness was 800 μm. If the excess is sorted as the size of the unevenness, it is 12 as shown in Fig. 6.
It was found that unevenness was remarkable when heated over 50 ° C. or even below 1250 ° C. for a long time, and it was found that the heating condition must satisfy the following equation. t (min) = − (6/5) · T (° C.) + 1560 where t: soaking time (min), T: soaking temperature (° C.), where 1050 ° C. ≦ T ≦ 1250 ° C.

At a heating temperature higher than 1250 ° C., coarsening occurs due to grain growth and it is not possible to prevent line-scratch flaws and edge seam flaws, so the heating temperature must be 1250 ° C. or lower. In terms of grain growth, low temperature heating is desirable, but 1050
If the temperature is lower than 0 ° C, defects such as scales are likely to occur during hot rolling, and the lower limit is 1050 ° C.

The reasons for limitation in the present invention will be described below. The amount of Ti is set to 0.02% or more because, in the ferritic stainless steel containing 16% or more of Cr, which is the object of the present invention, if it is less than 0.02%, the refining effect is hard to appear. Further, if Ti is contained in an amount of more than 0.10%, there is a refining effect, but the frequency of delayed fracture due to thermal stress in the slab becomes remarkable, and hot charge is mainly used to prevent delayed fracture of the slab. It is a factor of cost increase such as the need for hot strip treatment. Further, when hot rolling, a surface flaw called Ti streak is generated, and a color tone problem called bluing also occurs in a product. Therefore, it is not desirable to add a large amount of Ti in view of the above problems. From the above, Ti is added in the range of 0.02 to 0.10% in the present invention.

The reason why N is set to 0.007% or more is that in the ferritic stainless steel containing 16% or more of Cr, which is the object of the present invention, if it is less than 0.007%, the refining effect is hard to appear. Further, the amount of N is set to 500 ppm or less because if Ti content exceeds 500 ppm, coarse TiN is likely to be deposited, which easily becomes a starting point for slab placement cracking and the like, and from the viewpoint of preventing placement cracking. In addition, from the viewpoint of the refinement effect, the product of the contents of Ti and N, Ti
It is necessary that (%) × N (%) be 1.5 × 10 ⁻⁴ or more.

If Cr is less than 16%, α during cooling after solidification
→ γ transformation occurs in a considerable proportion, technology such as refinement by matrix transformation can be used, and C exceeding 35%
The ferritic stainless steel containing r has low toughness due to high Cr and may deteriorate toughness when a certain amount of N is contained as in the present invention, and the lower limit of Cr is 1
6% and the upper limit was 35%.

In addition, in the ferritic stainless steel containing Ti as in the present invention, it is easy to overcome problems such as nozzle clogging due to the oxide of Ti during casting, and deoxidation with Al from the viewpoint of preventing the formation of the oxide of Ti. It is necessary to strengthen the Al content, and the ratio Al / Ti of the Al content to the Ti content is set to 0.
By setting it to 1 or more, problems such as nozzle clogging during casting can be prevented.

[0033]

EXAMPLE A SUS430 steel having the components shown in Table 3 was melted and then continuously cast into a CC slab having a thickness of 250 mm. In the first half of casting, the same composition was used for casting, but in the second half of casting, a Ti wire was inserted into the mold to add 0.025% of Ti. After slab maintenance, heating 1190 before hot rolling
The temperature was soaked at 60 ° C. for 60 minutes, and hot rolling was performed up to 3 mm to compare the occurrence of defects at the hot rolled steel sheet stage. As a result, flaws were observed in the Ti-free portion in the first half of casting, whereas the Ti-added material of the present invention showed no flaws, and a good steel plate was obtained.

[0034]

[Table 3]

[0035]

As described above, according to the present invention, it is possible to obtain a very good steel sheet without causing any defects such as line-heavy flaws and edge seam flaws during hot rolling of ferritic stainless steel, and without burdening the process. Obtainable.

[Brief description of drawings]

FIG. 1 is a schematic view showing a process of generating unevenness of a slab short piece portion that occurs during hot rolling, as viewed from the slab rolling direction, in which (a) is before reduction and (b) is after 50% reduction. ,
(C) shows after 80% reduction.

FIG. 2 shows a method of measuring the unevenness depth of a short piece portion in a slab cross section after hot rolling, in which a dotted line is a straight line connecting adjacent protruding portions, and an arrow represents the unevenness depth. ing.

FIG. 3 Ti on the grain size of SUS430 steel slab
It is a figure which shows the influence of quantity.

FIG. 4 is a diagram showing the degree of superheating of molten steel (difference between the molten steel temperature and the liquidus temperature) which affects the crystal grain size of the slab of SUS430 steel.

FIG. 5 is a diagram showing the relationship between the Ti content and the N content, which affects the refinement of the cast slab structure of various ferritic stainless steels.

FIG. 6 is a diagram showing the influence of heating conditions on the unevenness generation behavior in the Ti-added material of the present invention.

Continuation of front page (51) Int.Cl. ⁷ Identification code FI C21D 8/00 C21D 8/00 E C22C 38/00 301 C22C 38/00 301W 38/18 38/18 38/28 38/28 (72) Invention Toru Suzuki No. 1-1 Toibata-cho, Tobata-ku, Kitakyushu, Fukuoka Prefecture Inside the Yawata Works (72) Inventor Toshihiko Ozeki 20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel Co., Ltd. (56) Reference JP-A 59-166655 (JP, A) JP-A 1-197046 (JP, A) JP-A 52-133827 (JP, A) JP-A 59-59826 (JP, A) JP-A-63-123516 (JP, A) JP-A-4-350123 (JP, A) JP-A-4-279248 (JP, A) JP-A-2-270942 (JP, A) JP-A-1-118341 (JP, A) JP 57-127506 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) B22D 11/00 B21B 3/02 B22D 11/108 B22D 11/12 C21D 8 / 00 C22C 38/00 C22C 38/18 C22C 38/28

Claims (2)

(57) [Claims] In 1. A mass% Cr: In ferritic stainless steel containing 16~35%, Ti: 0.02~0.10%, N: 0.0070~0.05%, Al: 0. Molten steel superheat degree when continuously casting molten steel that is 15% or less and Ti (%) × N (%): 1.5 × 10 ⁻⁴ or more, Al (%) / Ti (%): more than 0.10. (The difference between the molten steel temperature and the liquidus temperature) was 50 ° C. or less, and the continuously cast slab was heated to a heating temperature T (° C.) of 1050 to 1 before hot rolling.
250 ° C and soaking time t (min) satisfies the following formula
A method for producing a ferritic stainless steel for preventing the occurrence of surface flaws in hot rolling, which comprises performing hot rolling after heating within a range . t≤- (6/5) ・ T + 1560 2. The method for producing a ferritic stainless steel according to claim 1, wherein Ti is added into the mold for continuous casting to prevent the occurrence of surface defects during hot rolling.