JP2000246303A - Hot rolling method of stainless steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To perform buildup welding efficiently, reduce the amount of buildup welded metal remaining on a steel plate after rolling, and reduce trimming margin by specifying the dimensions of a buildup welded metallic layer on the slab side which is provided to prevent edge cracking when a stainless steel is hot-rolled. SOLUTION: Stainless steel slab 1 is provided on its side face C with a welded metallic layer 2. Y is distance (mm) between the buildup welded metallic layer 2 and a ridgeline 3 at which the side face C intersects with a top surface A or a bottom surface B of the slab 1, and Ym is maximum allowable value (mm) of Y, and the distance Y is to satisfy Ym/5<=Y<=Ym. In the case of W/H>=3.2, however, Ym=0.8 H2/W is satisfied, and in the case of W/H<3.2, Ym=0.25 H is satisfied. W is slab width (mm) and H slab thickness (mm). This method is effectively applied for stainless steel difficult to be hot-rolled; e.g. stainless steel containing lots of B, S, Mo, N, or the like. The thickness of the buildup welded metallic layer 2 is 2 mm or more.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for hot rolling stainless steel, which can prevent ear cracks generated during hot rolling by providing a build-up weld metal layer on the side surface of a slab.

[0002]

2. Description of the Related Art Neutron absorption capacity of stainless steel, free cutting,
In order to improve properties such as corrosion resistance and high strength, B, S,
Stainless steel containing a large amount of elements such as Mo and N has been developed and used. However, these special stainless steels have high deformation resistance and are difficult to work. Therefore, such hard-to-work stainless steel tends to have ear cracks and scabs due to insufficient deformability during hot rolling.

[0003] Stainless steel exhibits melt brittleness when its temperature is raised to near its melting point, and at temperatures below 1100 ° C, it exhibits grain boundary brittleness due to the concentration of segregated elements. In steel containing a large amount of B or S, (Cr, Fe) ₂ B or (Cr,
Mn) A structure in which nonmetallic inclusions such as S are mixed is formed, voids are generated from the boundary with the inclusions, and the voids propagate to cracks.

Until now, in order to avoid the occurrence of such hot brittleness and to produce a hot-worked product with a high yield, it has been necessary to adjust the chemical composition and clad a slab surface with a metal having good workability, The heating temperature and the working temperature during the final hot working have been optimized.

[0005] The above-mentioned special stainless steel has been mainly used for a method of casting an ingot and hot rolling it as a slab by hot forging, because defects are likely to occur during casting.

However, in recent years, demands for lower prices have increased,
The continuous casting of these special stainless steels has progressed, and it has become more difficult to ensure the hot deformability of the continuous casting slab, and improving the yield has become an important issue.

[0007] As a means for preventing edge cracks of steels whose hot deformability cannot be improved by adjusting the chemical composition, there is a method of improving the workability of the surface layer of the workpiece. As an example of a B-containing stainless steel, JP-A-63-76703 discloses that
A method of cladding or overlay welding of steel having good deformability to the side surface of a billet is disclosed.

[0008] Japanese Patent Publication No. 3-54007 discloses a method in which four surfaces of a steel ingot are packed in an iron tube, and then compacted by slab rolling or forging.

[0009] Japanese Patent Application Laid-Open No. 9-269398 discloses that a build-up welding layer having a B content of 0.3% or less and containing 3 to 12% by volume of δ ferrite and having a thickness of 3 mm or more is provided on the side surface of a slab. For hot rolling of austenitic stainless steel provided with
And a method of rolling by heating.

[0010] The basic idea of these methods is to prevent the occurrence of hot working cracks by coating a steel slab having poor hot deformability with steel having good hot deformability.

However, as a result of conducting various tests to evaluate the manufacturing cost of the above-mentioned methods, these methods are effective in preventing cracks in the ears. It has been found that the method of cladding four surfaces does not necessarily reduce the manufacturing cost.

For example, in the method of cladding steel having good deformability on the side surface of a billet, the machining cost for providing a welding groove to a billet having a thickness of 100 mm or more is extremely high, and the subsequent welding work cost is reduced. I can't afford to ignore it. The method of providing the build-up weld metal layer is relatively inexpensive, but it is necessary to remove the build-up weld metal by a method such as trim in order to leave the build-up weld metal on the hot-rolled product. 5
It became clear that it reached 0 mm and was comparable to the amount of edge cracking when the overlay welding was not performed.

[0013]

SUMMARY OF THE INVENTION An object of the present invention is to reduce the production cost in a method for producing a hot-rolled steel sheet by hot-rolling a stainless steel slab provided with a build-up welding layer on the side. . Specifically, the present invention provides a method capable of efficiently performing overlay welding, reducing the amount of overlay welding metal remaining on the steel sheet after rolling, and reducing the trim allowance.

[0014]

The gist of the present invention is as follows.

[0015] In a method in which a build-up weld metal layer is provided on the side surface of a stainless steel slab and hot-rolled, the build-up weld metal layer satisfies the following formula (1). Rolling method.

Ym / 5 ≦ Y ≦ Ym (1) However, when W / H ≧ 3.2, Ym = 0.8H ² / W When W / H <3.2, Ym = 0.25H Here, Y: Each distance (mm) between each ridge line where the upper surface or lower surface of the slab intersects the side and the weld overlay metal layer Ym: Maximum allowable value of Y (mm) W: Slab Width (mm) H: Slab thickness (mm) The present inventors have found that a method of providing a build-up weld metal layer (hereinafter simply referred to as a weld metal layer) on the side of the slab is effective in preventing ear cracks. Focus on low production cost,
The following tests were conducted with the aim of improving the efficiency of the overlay welding work and reducing the trim allowance for removing the weld metal remaining on the steel sheet after hot rolling. The study started by investigating deformation behavior and edge cracking behavior in hot rolling near the side end face of hard-working stainless steel slab in detail.

An austenitic stainless steel containing 1% B was selected as a stainless steel having very poor hot deformability. As a rolling test piece, a width of 14
Ten pieces of 0-mm, 70-mm-thick, 200-mm-long steel pieces were cut out.

A 5 mm thick weld metal layer was provided on one side surface of the billet by TIG two-layer welding using a 3 mm diameter welding wire made of austenitic stainless steel SUS308. No weld metal layer was provided on the other side (hereinafter, referred to as a solid side).

TIG welding conditions are as follows: current 160A, voltage 1
7 V, welding speed 10 cm / min.

SUS308 is the most commonly used welding material for welding austenitic stainless steel, and it was confirmed by die check that there was no defect in the weld metal layer.

Using the rolled test piece, the following rolling test was conducted in a reverse hot rolling mill having a work roll diameter of 450 mm.

The heating temperature of the rolled test specimen was 1180 ° C. in order to avoid melt embrittlement. After heating, it was rolled according to the following 10-pass schedule. The rolling was stopped at each pass to obtain nine intermediate rolled materials and one rolled material rolled to the final pass. The test piece surface temperature after each pass was also measured.

70 → 57 → 45 → 35 → 28 → 21 → 1
After the rolling of 6 → 12 → 9 → 7 → 5 (mm), the oxide scale formed on the surface of the test piece was removed, and the shape change on the side surface of the test piece and the generation behavior of hot working crack were observed.

As a result, the bulging of the side surface of the steel slab occurs in the width direction as the rolling progresses and the width increases, and as the rolling progresses, the side surface near the upper surface or the lower surface of the test piece rolls. Had wrapped around the surface.

This width spread increased in about 5 passes from the start of rolling. The temperature drop in this initial rolling pass was not significant, and was approximately 1080 ° C. (the heating temperature of the slab −10 ° C.).
0 ° C.). No cracks were seen on the solid side. In rolling after 6 passes, the surface area of the steel sheet increased and the temperature drop increased.

After 6 to 7 passes, ear cracks were generated on the free deformation surface (side surface not wrapping around the work roll surface) on the solid side surface.

That is, the starting point of the edge crack did not occur in the portion of the side surface of the test piece which went around the rolling surface. In subsequent passes, stress concentrates on the side surface (free deformation surface) of the test piece, and ear cracks expand.
It had developed to a length of 0 to 15 mm. A trim margin of 20 mm was required to remove the ear cracks.

On the side surface on which one of the weld metal layers is provided, the amount of wraparound of the weld metal to the upper or lower surface is 19 m on the upper surface.
m and 16 mm on the lower surface. After the rolling, a trim margin of 20 mm was required again to remove the dissimilar weld metal layer.

From these results, it is not necessary to provide a weld metal layer on the side surface which goes from the side surface to the rolling surface at the beginning of rolling from the viewpoint of preventing ear cracks. Rather, such a weld metal layer is provided. And the fact that it is harmful in terms of minimizing the trim amount of hot-rolled products. Based on this finding, the following experiment was further performed.

From the above continuous cast slab, the thickness (H): 7
0-21 mm, length (L): 200 mm, width (W): 7
Rolled test pieces of 0 to 210 mm were cut out. The reason for changing the thickness and width of the test piece is to examine the influence of the slab dimensions before rolling.

A weld metal layer was formed on one side surface of the rolled test piece using SUS308 steel under the above-described welding conditions.

FIG. 1 is a perspective view of a rolled test piece provided with a weld metal layer. As shown in FIG. 1, T is applied to a portion excluding a range from Y (0 to 20 mm) in the thickness direction from each ridgeline 3, 3 a where the upper surface A and the lower surface B and the side surface C of the test piece 1 intersect.
A 3 mm thick weld metal layer 2 was provided by IG single layer welding.

This test piece was heated to 1180 ° C. and hot-rolled to a thickness of 5 mm by the above-mentioned hot rolling mill. Then, the trim allowance D for removing the ear cracks on the solid side and the trim allowance E required for completely removing the weld metal and the ear cracks on the side provided with the weld metal layer were measured, and the magnitude relation was compared. .

In the example of a test piece having a thickness of 70 mm and a width of 140 m, the values of D and E when the value of Y is changed to 4.5, 9.0, 13.5, 17.5 and 21.0 mm Was as shown in Table 1. When Y is 21.0 mm, D increases because ear cracks have occurred.

[0035]

[Table 1]

When the trim margin is E <D and the occurrence of edge cracks can be prevented, the prevention of edge cracks by the weld metal layer is effective. The larger Y is, the more the trim for removing the weld metal after rolling is obtained. It turned out that the cost was small.

Based on the results of such a test, the relationship between the size of the test piece before rolling and the occurrence of cracks in the Y-edge was arranged, and the following findings were obtained.

A) The maximum allowable value of Y (Ym) is a value that can prevent the occurrence of edge cracks by providing a weld metal layer, and the trim allowance for removing the residual weld metal after rolling is reduced. , Which is smaller than the trim allowance in the case where the slab is rolled in a solid state, and depends on the width (W) and thickness (H) of the slab before rolling.

B) Ym changes W / H from 3.2 to Sakai as follows: When it is 3.2 or more, Ym = 0.8H ² / W When it is less than 3.2, Ym = 0. 25H c) The minimum value of Y is the Y value when the weld metal layer is not provided on the side surface that goes around the upper surface or lower surface of the slab by rolling, and this value is also the width (W) and thickness (W) of the slab before rolling. H), which is 1/5 of Ym.

The present inventors conducted additional experiments with various changes in the types of overlay welding materials and difficult-to-work stainless steels in order to confirm the versatility of the above results. The same results were obtained. .

[0041]

Embodiments of the present invention will be described below.

(1) Stainless Steel Slab The method of the present invention can be applied to any slab such as a general continuous cast slab, a forged slab, and a slab rolled slab. These slabs are rectangular. In hot rolling of a slab, tentering rolling in which the slab is rolled in the width direction may be performed, but the side surface of the slab is a side surface in the main rolling direction (a surface whose long side is the rolling direction). .

The method of the present invention can be applied to any type of hard-working stainless steel which is difficult to hot-roll. Weight% for hard-to-work stainless steel
B: 0.3-3%, S: 0.1-1%, Mo: 1-1
0%, N: 0.1-1%, Si: 2-6%, Cu: 2
An austenitic stainless steel or a duplex stainless steel containing 6%, Nb: 0.5 to 5% or the like can be used.

(2) Weld Metal Layer The weld metal layer provided on the side surface of the slab can be provided by a conventional method such as TIG welding, covered arc welding, band arc welding or the like.

The thickness of the weld metal layer is desirably 2 mm or more in consideration of the oxidation loss in the heating furnace. On the other hand, when the thickness is 10 mm or more, the welding operation becomes longer, and the manufacturing cost increases. In the case of austenitic stainless steel containing 1% B, SUS308L steel, SUS4
36L steel was suitable. SUS308L steel is commonly used as a welding material for austenitic stainless steel.

The range for providing the weld metal layer on the side of the slab is as follows:
The following equation (1) is used.

Ym / 5 ≦ Y ≦ Ym (1) However, when W / H ≧ 3.2, Ym = 0.8H ² / W When W / H <3.2, Ym = 0.25H FIG. 1 is a perspective view of a rolled test piece having the same shape as the slab, but the above symbols will be described by replacing the test piece 1 with a slab.

Y indicates the distance (mm) between each of the straight lines (ridges) 3 and 3a where the upper surface A or lower surface B of the slab intersects the side surface C and the weld metal layer 2, and Ym indicates the maximum allowable value of Y (mm). ),
W is the slab width (mm), and H is the slab thickness (mm).

The maximum allowable value Ym and the minimum value Ym / 5 of Y depend on the slab width W and the thickness H of the hot-rolled steel slab before rolling.

When Y exceeds the upper limit Ym, ear cracks occur, and when Y is smaller than the lower limit Ym / 5, the effect of reducing the trim amount by the weld metal layer is almost negligible. Therefore, the range of Y is set to Ym / 5 or more and Ym or less.

In an actual slab, the edge of the slab may be chamfered.

FIG. 2 is a partial cross-sectional view of a slab that has been chamfered. The above-mentioned ridges when chamfered are defined as extensions 4a and 4b of the slab upper surface A or lower surface B,
The ridge lines 3 and 3a intersecting with the extension lines 5a and 5b of the side surface C are referred to. Therefore, if the above expression (1) is satisfied in the side projection view, the weld coating of the chamfered portion is unnecessary.

If the weld metal layer has weld cracks or weld defects, ear cracks may occur due to the cracks or weld defects. Therefore, it is preferable to appropriately manage the pretreatment for preheating the steel slab, the welding material, the welding heat input, and the like in order to prevent welding cracks and welding defects.

(3) Hot Rolling Hot rolling refers to hot rolling when producing a thick plate from a slab and hot rolling when producing a hot-rolled steel strip.

The heating temperature of the slab is preferably set to a high temperature within a range in which no melt embrittlement occurs. In many hard-to-work stainless steels, the heating temperature is set to 1 because the alloying elements tend to cause melt embrittlement due to lowering of the solidus temperature.
There are many types of steel that need to be limited to 230 ° C or lower. A higher hot rolling finish temperature is preferable for preventing ear cracks. However, as long as the hot deformability of the weld metal layer allows,
A low-temperature finish of 600 to 900 ° C. is also possible.

[0056]

EXAMPLES Continuous cast slabs of stainless steel having the four chemical compositions shown in Table 2, slabs obtained by slab-rolling the slabs, and slabs obtained by slab-rolling ingots were produced.

[0057]

[Table 2]

Overlay welding material shown in Table 3, Y-308L
Alternatively, the value of Y was variously changed within the range of 0 to 30 mm by band arc welding or TIG welding using SUS436L, and a weld metal layer was provided on the side surface of each slab.

[0059]

[Table 3]

The thickness of each of the weld metal layers was 5 mm. Next, it was rolled to a thickness of 6 mm by hot rolling to obtain a hot-rolled steel sheet.

At this time, the heating temperature of the slab was 1190 ° C. for 1% B steel, 1220 ° C. for 0.3% S steel, and SUS630.
1235 ° C. with steel. Finishing temperatures were as described in Table 3.

The obtained steel sheet was examined for edge cracks and the amount of wrap around of the weld metal layer, and the trim amount B required to remove all the edge cracks was measured. Table 3 shows the results.

As is clear from Table 3, in the present invention example,
Ear cracks are not generated, and the trim amount for removing the weld metal used for preventing ear cracks is less than 10 mm, which is extremely small compared to 23 to 35 mm of the comparative example, and the yield is good.

[0064]

According to the method of the present invention, the welding time for providing the weld metal layer on the side of the slab is reduced, and the trim allowance for the steel sheet after hot rolling is reduced. Yield is improved and difficult-to-work stainless steel for special applications, for which needs are increasing, can be supplied to the market at low cost.

[Brief description of the drawings]

FIG. 1 is a perspective view of a rolled test piece provided with a build-up weld metal layer on a side surface.

FIG. 2 is a partial cross-sectional view of a chamfered slab.

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Kazuhiro Ogawa 4-5-33 Kitahama, Chuo-ku, Osaka-shi, Osaka Sumitomo Metal Industries, Ltd. F-term (reference) 4E002 AA07 AB01 AD01 BC05 BC07 CB04

Claims (1)

[Claims] 1. A method for hot-rolling by providing a build-up weld metal layer on a side surface of a stainless steel slab, wherein the build-up weld metal layer satisfies the following formula (1). Hot rolling method. Ym / 5 ≦ Y ≦ Ym (1) However, when W / H ≧ 3.2, Ym = 0.8H ² / W When W / H <3.2, Ym = 0. .25H Here, Y: Each distance (mm) between each ridge line where the upper or lower surface of the slab intersects the side surface and the weld overlay metal layer Ym: Maximum allowable value of Y (mm) W: Slab width (mm) ) H: Slab thickness (mm)