JP3298730B2 - Manufacturing method of austenitic stainless steel sheet with few surface defects - 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 a stainless steel sheet having few surface defects, and in particular, to reduce surface flaws generated during hot rolling of an austenitic stainless steel sheet by defining the hot rolling conditions. It is about the method.

[0002]

2. Description of the Related Art In general, surface flaws are easily generated at the edge of a hot-rolled stainless steel sheet, and scale is generated particularly in the inside of the surface flaws caused by cracks and surface irregularities of the steel sheet, so that pickling in a subsequent step is performed. Sometimes, they are not sufficiently removed and become serious surface defects after cold rolling, resulting in a decrease in product yield. In particular,
Surface defects called scabs caused by micro-cracks during hot rolling are subjected to a cold rolling process without being removed in a pickling process because scales generated after the occurrence of cracks bite into the interior by rolling. In this case, a linear defect that is long in the rolling direction is formed, and the degree of reduction in yield is particularly large. In addition, a steel type in which the yield is particularly problematic due to the surface defect caused by the minute cracks is an austenitic stainless steel in which the surface aesthetics of the product is likely to be a problem and the minute cracks are easily generated by hot rolling.

Accordingly, various techniques for reducing surface defects during hot rolling have been devised mainly for stainless steel. For example, JP-A-57-1615
No. 3 discloses a technique in which the components of the austenitic stainless steel are specified to ensure hot workability, and that the steel is reduced in nicks and burrs. JP-A-2-1580
No. 6 discloses a technique for removing surface defects (pinholes) of a stainless steel slab and eliminating the occurrence of barge defects. However, this technique cannot prevent minute cracks generated during hot rolling.

Japanese Patent Application Laid-Open Nos. 58-138502 and 3-207551 disclose techniques for reducing the occurrence of hot rolling flaws in a slab shape. Both disclose a technique for reducing the edge seam flaw of ferritic stainless steel by depressing the center of the short side of the slab. However, this technique cannot prevent the above-described minute cracks during hot rolling. Furthermore, Japanese Patent Publication No. 2-9651 discloses a technique of performing a shot blast treatment before inserting an austenitic stainless steel slab into a heating furnace to prevent surface defects during hot rolling. However, this technique cannot completely prevent micro-cracks during hot rolling, and inevitably increases production costs due to shot blasting.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to provide a stainless steel having improved surface flaws without any particular increase in process load in improving surface flaws generated during hot rolling of stainless steel. .

[0006]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention specifies rolling conditions for hot rolling. The gist of the present invention is to perform rolling by vertical rolls before performing initial rolling by horizontal rolls. And the rolling reduction is set to 5% or less, and the inter-pass time from the vertical roll rolling to the initial rolling by the horizontal roll is set to t seconds or more represented by the following formula. t = 2.9 × 10 ⁻¹⁰ × r ⁻² × exp (25000 / T) Here, r means a reduction rate by the vertical roll rolling,
From the slab width w before rolling and the width w ′ after rolling, r = 1−1
It is obtained as w '/ w. T indicates the absolute temperature of the rolling.

[0007]

The present invention will be described below in detail. The present inventors have carefully investigated the relationship between flaws generated during hot rolling and various conditions related to hot rolling, and completed the present invention. First, the present inventors examined the relationship between the occurrence of hot rolling flaws and the slab shape in detail, and found that the occurrence of hot rolling flaws was greater in the recessed slab of FIG. 2 than in the rectangular slab of FIG. A rectangular slab is a slab obtained when bulging does not occur during casting, as shown in FIG. 1, and h and w in the figure mean slab thickness and slab width, respectively. As shown in FIG. 2, a slab having a concave portion is a slab having a concave portion formed on a long side surface of the slab by bulging or the like during casting. The location of the concave portion is usually formed in the range of 10 mm to 300 mm from the slab edge. In the drawing, Δh represents the maximum concave depth (the depth from the horizontal plane at the center of the slab), and Δw represents the maximum concave depth from the slab edge. Hc indicates the average thickness at the center of the slab.

It is generally difficult for a conventional continuous casting machine to completely eliminate the concave portion and make the slab cross section completely rectangular. In particular, recently, there has been a tendency to increase the casting speed during continuous casting in order to improve productivity, and as a result, it is difficult to obtain a complete rectangular slab, for example, bulging tends to occur.

Further, it has been found that the frequency of occurrence of hot rolling flaws increases as the rectangular slab approaches the slab edge, and that the hot rolling flaws occur in the concave portions with a frequency higher than that of the rectangular slab in the concave slab. In addition, the frequency of occurrence of hot rolling flaws in the slab where the concave portions are generated tends to depend on the maximum concave depth △ h, and the larger the △ h, the higher the frequency of the flaws.
That is, when the slab having the concave portion is used, hot rolling flaws are easily generated inside the edge, and the yield of the final product is significantly reduced.

[0010] Further, the present inventors investigated the hot rolled flaws after the completion of one rough hot rolling pass (horizontal roll rolling) in order to clarify the form of flaw generation. As a result, it was found that all the flaws generated on the slab edge and the slab recess were C-direction cracks (cracks in a direction perpendicular to the rolling direction). The size is about 0.1 to 0.2 mm in the C direction and a small crack with a depth of about 0.1 mm. It is elongated in the L direction (rolling direction) by a subsequent rolling process such as hot rolling or cold rolling. It was confirmed that the product plate became a scalpel flaw that seriously degraded the surface quality. The flaws in the final product were roughly hot-rolled (horizontal roll rolling) based on a comparison between the frequency of flaws in the final product and the frequency of micro-cracks after one pass of rough hot rolling.
It almost corresponded to the micro cracks generated during the initial pass, and it was confirmed that defects such as micro cracks did not occur in the rolling after the second pass.

From the above results, the present inventors have found that by setting the vertical roll rolling before the horizontal roll initial rolling to 5% or more, the slab edge recesses are eliminated, and the rolling direction tension generated during the horizontal roll rolling is reduced. Successfully prevented hot rolling. (See Japanese Patent Application No. 5-84932). However, when a large reduction of 5% or more was carried out by the vertical roll rolling, the vertical roll abrasion was severe and the roll basic unit was reduced.

Therefore, the present inventors have further studied, and
As a result of a detailed investigation of the surface layer structure of the slab and the hot rolled sheet, it was found that the location where microcracks occurred after the initial pass of the horizontal roll corresponded to the location where the solidification was delayed during solidification and solute atoms concentrated. In austenitic stainless steel,
Generally, Ni, Mn, P, S, and the like are concentrated in the solidification delay portion, and are usually formed in the valley portion of the oscillation mark, and are often referred to as a Ni segregation zone. However, as the casting speed increases, in addition to the normal Ni segregation zone, N
Ni segregation lines and the like also appear at the i segregation lines and the oscillation mark peaks. According to the observations of the present inventors, mainly this N
In a portion corresponding to the i-segregation line, grain growth is apt to occur during slab heating, forming coarse grains before the initial pass of the rough hot rolling, and generating fine cracks after the initial pass of the rough hot rolling. After the initial pass of the rough hot rolling, recrystallized grains are generated in the slab surface layer including the old Ni segregation line portion, and the degree of segregation is reduced.

The inventors of the present invention made the change in the structure of the surface layer of the slab edge caused during the initial pass by the rough hot rolling horizontal roll by the vertical roll rolling under light pressure performed before the horizontal roll initial pass. A hot rolling condition to prevent occurrence has been developed. That is, in hot rolling austenitic stainless steel, rolling is performed by a vertical roll before performing initial rolling by a horizontal roll, and the rolling reduction is set to 5%. The time between passes until t is given by t
Seconds or more. t = 2.9 × 10 ⁻¹⁰ × r ⁻² × exp (25000 / T) Here, r means a rolling reduction by vertical roll rolling before an initial pass of horizontal roll rolling, and a slab width w and a rolling before the rolling. It is determined as r = 1−w ′ / w from the subsequent width w ′. T
Indicates the absolute temperature of the rolling.

The reasons for limiting the scope of the claims will be described below.
First, the reason why the rolling by the vertical roll before the horizontal roll rolling is performed is that if the horizontal roll rolling is performed without the vertical roll rolling, minute cracks occur. Here, the horizontal roll rolling in the present invention means applying a reduction to opposing long sides in a slab cross section perpendicular to the rolling direction, and the vertical roll rolling applies a reduction to opposing short sides of the cross section. Means that.

The reason why the reduction ratio r of the vertical roll rolling is set to 5% or less is that, when the reduction ratio exceeds 5%, the effect of improving hot rolling defects is recognized even if the time between passes until the horizontal roll rolling is not specified. (See the entire specification). At the same time, if the rolling reduction is higher than this, the roll wear is severe, and the unit consumption of the roll is reduced. Although the lower limit of the rolling reduction r is not particularly limited, it is preferable that the rolling reduction is at least 0.5% or more. Although the rolling temperature range is not particularly defined in the present invention, the lower limit temperature is up to about 1000 ° C. which is the lower limit of the ordinary rough hot rolling temperature, and the upper limit is up to about 1350 ° C.

Next, the reason why the time between passes between the vertical roll rolling pass and the initial horizontal roll rolling pass is limited to t seconds or more is that the effect of improving the surface structure of the slab edge by the vertical rolling is insufficient when the time between passes is shorter than t seconds. This is because micro-cracks tend to occur during the subsequent horizontal roll rolling. FIG. 4 shows that the lower limit t seconds of the pass time is inversely proportional to the square of the rolling reduction r during this time. FIG. 5 shows a relationship in which the natural logarithm of the lower limit t seconds of the inter-pass time is proportional to the reciprocal of the vertical roll rolling temperature T.

In the present invention, the upper limit of the inter-pass time is not particularly specified. However, in the case where the slab is transferred to the horizontal roll hot rolling using the sensible heat of the slab immediately after the vertical roll rolling as in the normal hot rolling. Is preferably 300 seconds or less from the viewpoint of preventing a temperature drop. However, when reheating after rolling with a vertical rolling roll or the like and then subjecting it to horizontal roll rolling, this is not the only limitation, and it is desirable that the time be up to about 30 minutes from the viewpoint of preventing grain growth.

The reason why the hot rolling flaw can be improved by the technique described in the present invention is not always clear at present, but is considered as follows. As described above, the locations where micro-cracks occur during conventional rough hot rolling generally correspond to the locations where Ni segregation lines occur on the surface layer of the slab, that is, the oscillation valleys, and are coarse grains before rolling. The Ni segregation line portion is a portion where the completion of solidification is delayed, and Ni, S, P, etc. are concentrated in the austenitic stainless steel. Ni is an austenite stabilizing element, and its concentration stabilizes the austenite phase and has the effect of suppressing the crystallization of the δ-Fe phase and promoting the disappearance of the δ-Fe phase. As a result, the amount of the δ-Fe phase in the part is smaller than that of the other parts, the effect of inhibiting the grain growth by δ-Fe (pinning effect) is small, and grain growth of austenite grains tends to occur particularly during slab heating, and It is considered that the grains become coarse before rolling. When the grains become coarse, the grain boundary area is relatively reduced, so that the degree of grain boundary segregation of the grain boundary segregation elements S, P and the like becomes high. In addition, as described above, S and P are concentrated in the Ni segregated portion during solidification, and the degree of grain boundary segregation of S and P in the Ni segregated portion is determined to be sufficiently higher than the others. It is considered that these segregations naturally lower the hot workability and facilitate the occurrence of cracks during hot rolling. That is,
It can be said that the Ni segregation line after slab heating is a locally extremely embrittled portion.

When the locally segregated Mi segregation line is rolled by a horizontal roll, tension is generated in the rolling direction as described above, and fine cracks are generated in the C direction. This rolling direction tension is considered to be caused by a difference in metal flow in the rolling direction between the central portion and the edge portion of the slab. That is, at the edge portion, the width is widened at the time of rolling, and the amount of rotary flow in the rolling direction is smaller than that at the center portion. As a result, the metal at the edge is dragged by the flow of the metal at the center, and tension in the rolling direction is generated at the edge. The above is considered to be the reason why the micro-cracks occurred at the edge even in the conventional rectangular slab. Further, in the slab having concave portions shown in FIG. 2, it is considered that in the concave region where the slab thickness is small and the metal flow in the rolling direction is small, the tension in the rolling direction increases during the initial pass of horizontal roll rolling, and the generation of flaws is considered to be concentrated.

However, in the former Ni segregated portion where no fine cracks occurred in the initial horizontal roll rolling, "recrystallized grains" or "reduction of segregation" occurred after the horizontal roll rolling, and tension was generated during the subsequent horizontal roll rolling. No fine cracks occur. Therefore, if a similar structural change is caused before the initial rolling of the horizontal roll, the toughness of the former Ni segregated portion is recovered, and hot rolling defects (fine cracks) can be prevented. That is, in the vertical roll rolling of the invention of the prior application (see the above-mentioned specification), 5% of the slab edge is removed in order to eliminate the concave portion and further form the convex portion.
It is considered that the above large pressure reduction was necessary. On the other hand, in the case of the vertical roll rolling according to the present invention, an inter-pass time after rolling is required to generate the above-mentioned structural change. Further, once the toughness of the old Ni segregated portion is recovered by the above structural change, even if the slab edge concave portions are insufficiently eliminated by the vertical roll rolling and the rolling direction tension occurs during the horizontal roll rolling,
An austenitic stainless steel sheet having a good surface without fine cracks is obtained. Hereinafter, the present invention will be described in detail with reference to examples.

[0021]

EXAMPLE An austenitic stainless steel (SUS304 steel) having the components shown in Table 1 was smelted according to a normal smelting method, and the slab had a center thickness of 165 mm and a slab width of 1250 mm.
And slabs of 1000 mm were cast. Of the cast slabs,
A slab having a shape that easily causes flaws in normal hot rolling was selected. That is, in a slab having an edge recess as shown in FIG. 2, Δh = 1.0 to 1.5 mm and Δw = 120 to 170
mm. These slabs were subjected to hot rolling, and one pass rolling was performed with a vertical roll before horizontal roll rolling, and horizontal roll rolling was performed while changing the time until horizontal roll rolling. The obtained hot-rolled coil was subjected to a normal pickling and cold-rolling process to be a cold-rolled coil, and a scuff test was performed. Table 2 shows the relationship between the flaw inspection results and the vertical roll rolling conditions together with the lower limit inter-pass time t of the present invention. Further, FIG. 3 shows the result of the rolling temperature of 1230 ° C. in Table 2 together with other results. The horizontal axis of the drawing in FIG. 3 indicates the vertical roll rolling reduction ratio r, which is determined as r = 1−w ′ / w from the slab width w before the rolling and the width w ′ after the rolling. The vertical axis in the figure indicates the time between passes from the vertical roll rolling to the initial horizontal roll rolling. The open circles in the figure indicate that the coil hot-rolled under the conditions passed the flaw test, and the crosses indicate that the coil failed the flaw test.

As is apparent from FIG. 3, when the rolling reduction r of the vertical roll rolling increases, the inter-pass time required for reducing the flaws decreases. At this time, the lower limit inter-pass time t for reducing flaws
Is inversely proportional to the square of r. In order to clarify this tendency, FIG. 4 shows the result when the horizontal axis is 1 / r ² . From the figure, it can be seen that the required time t between passes for flaw reduction is inversely proportional to the square of the vertical roll rolling reduction r.
Here, the slope of the straight line in the figure was 4.9 × 10 ⁻³ s.

Further, it is recognized that the inter-pass time t required to reduce the flaws tends to decrease as the vertical roll rolling temperature T increases. To clarify this tendency, FIG. 5 shows an Arrhenius plot of the time between passes when the vertical roll rolling reduction is 1.5%. The horizontal axis is the vertical roll rolling temperature T (absolute temperature)
The vertical axis represents the time between passes from vertical roll rolling to horizontal roll rolling. The white circles in the figure indicate the pass of the flaw inspection, and the x marks indicate the rejection. The straight line in the figure corresponds to the lower limit of the inter-pass time required for flaw reduction, and the slope was 25000K.

4 and 5, the lower limit of the inter-pass time t is represented by the following equation, and the t value obtained from the following equation is shown in Table 2. t = 2.9 × 10 ⁻¹⁰ × r ⁻² × exp (25000 / T) As is clear from Table 2, the vertical roll rolling method according to the present invention is used in comparison with the conventional vertical roll rolling method. It is recognized that generation of flaws during production is small. That is, under each rolling condition, the flaw is improved by setting the time between passes from vertical roll rolling to horizontal roll rolling to be equal to or more than the t value obtained from the above equation.

[0025]

[Table 1]

[0026]

[Table 2]

[0027]

As described in detail above, the effect of the present invention is as follows.
By performing predetermined vertical roll rolling before horizontal roll rolling when producing a steel sheet by hot rolling, it is possible to reduce the surface flaws of the product and improve the product yield.

[Brief description of the drawings]

FIG. 1 shows a cross section of a full rectangular slab obtained when no bulging occurs during casting.

FIG. 2 shows a cross section of a slab when a concave portion is formed on a long side surface of the slab by bulging or the like during casting.

FIG. 3 is a diagram showing the relationship between vertical roll rolling conditions and surface flaw generation performed before initial horizontal roll rolling, and the vertical roll rolling temperature is 1230 ° C.

FIG. 4 is a diagram showing the result of FIG. 3 with the horizontal axis changed to 1 / r ² .

FIG. 5 shows that the vertical roll rolling reduction is 1.5% (r =
It is a figure which shows the Arrhenius plot of the time between passes until the horizontal roll rolling with respect to the said rolling temperature T at the time of 0.015).

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Masaki Ueda 20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel Corporation Technology Development Division (56) References JP-A-6-304606 (JP, A) JP-A-6-292907 (JP, A) JP-A-60-240301 (JP, A) JP-A-7-164001 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) B21B 1/00-3/02 B21B 37/00-37/78

Claims (1)

(57) [Claims] 1. In producing a stainless steel sheet by hot rolling, rolling is performed by a vertical roll before performing initial rolling by a horizontal roll, the rolling reduction r is set to 5% or less, and after the vertical roll rolling, The pass time up to the initial rolling by the horizontal roll is represented by t = 2.9 × 10 ⁻¹⁰ × r ⁻² × exp (25000 / T) (where r means the rolling reduction by the vertical roll rolling and before rolling) From the slab width w and w 'after rolling, r = 1-
w '/ w, where T represents the absolute temperature of the rolling) for at least t seconds.