JP2537317B2 - Hot working method for stainless steel slab - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot working method for an austenitic stainless steel slab. For more information,
The present invention relates to a technique of pretreating a slab before heating before heating the stainless slab by a continuous casting method to 1100 ° C. or higher and hot working.

[0002]

2. Description of the Related Art Due to the rapid spread of continuous casting processes in recent years, the process of continuous casting, hot rolling and cold rolling has become common even for stainless steel sheets. Stainless steel sheets are often used as exterior plates compared to other steel sheets because of their beautiful surfaces, and the requirements for surface quality are particularly strict. However, in the continuous casting method,
The vertical vibration (oscillation) of the mold causes the powder to flow between the mold and the slab, so that periodic irregularities (oscillation marks) are formed on the slab and the surface segregation (Ni, Ni, Due to (P concentration), a grain-like defect occurs after rolling and pickling, which is a problem. For this reason, not only measures for reducing the oscillation marks by optimizing the vibration conditions of the mold but also, for example, JP-A-1-321012 and JP-A-61-1.
Attempts have been made to reduce wood grain pattern defects by optimizing the conditioning conditions by blasting a slab, as disclosed in Japanese Patent No. 57627.

[0003]

However, conventionally, in determining the conditions for the slab blast treatment, there is no index for determining the blast treatment conditions corresponding to the segregation depth of the slab, and the treatment conditions are set in each process line. From experience, I had no choice but to decide by trial and error.

[0004]

The present inventors have found that the oxide film formed in the heating furnace is greatly affected by the strain distribution given to the surface layer of the slab before heating. That is, as the amount of strain in the surface layer of the cast slab, in particular, the depth under strain, the oxide film becomes thicker, and the cast slab surface defects are easily removed in the descaling step after heating. As a result of detailed investigation, it was found that this phenomenon is prominently exhibited in austenitic stainless steel having a high strain recovery temperature, and in the austenitic stainless steel, an oxide film is formed up to a depth of 340 or more in the micro Vickers altitude (Hv). It was In addition, such appropriate introduction of strain can be achieved by optimizing shot projection density, initial velocity, and grain size during blasting as blasting conditions.

The present invention has been completed on the basis of such findings, and a stainless steel slab made by continuous casting method
When heating to 100 ° C. or higher and hot working, the average depth (mm) of the oscillation mark from the surface of the slab was 0.
Micro Vickers hardness (H
The slab before heating is pretreated so that v) is 340 or more, and then heated and hot-worked.
In this method, it is preferable that the pretreatment is shot blasting under the condition that the following formula (1) is satisfied.

0.3H ≦ 7 × 10 ⁻⁷ W · d · V (1) where H: average depth of oscillation mark (mm) W: particle projection density of blast treatment (kg / m ² ) 200 ˜1000 kg / m ² d: Blast treatment particle diameter (mm) 1.0 to 3.0 mmφ V: Blast treatment particle initial velocity (m / sec) 50 to 200 m / sec.

[0007]

In the method of the present invention, strain is introduced into the surface layer of the slab before the slab is inserted into the heating furnace, and the diffusion rate of metal atoms is increased when the strain is recovered in the subsequent heating furnace. Therefore, the formation of an oxide film is promoted.
The strain recovery temperature of austenitic stainless steel is 900
Since it is as high as ℃ to 1000 ℃ and is close to the oxidation temperature, strain recovery and oxidation proceed at the same time. Therefore, the influence of the strain amount distribution on the thickness of the oxide film, particularly the depth of the region to which the strain is applied, appears remarkably.

On the other hand, as a result of detailed investigation of the surface segregation thickness of the slab, the segregation thickness is determined by the oscillation mark depth H of the slab.
It turns out that can be approximated by a linear function. Therefore, using an oscillation mark depth H that is easy to measure, approximate the average segregation layer thickness to 0.3H,
The amount of strain at the depth is used as a reference by measuring hardness, and processing conditions such as blasting are determined. As a result of detailed laboratory experiments, the micro Vickers hardness (Hv) is 3
It was revealed that oxide scale was formed up to 40 or more. Therefore, in the method of the present invention, 0.3H from the surface of the slab
It was decided to perform processing so that the micro Vickers hardness becomes 340 or more at the position of the depth.

In order to obtain such hardness distribution by shot blasting, the hardness is greatly influenced by the particle diameter d of the blasting treatment, the initial velocity V of the blasting treatment particles and the projection density W of the blasting treatment particles. Therefore, the condition of the above formula (1) was derived.

[0010]

[Example] In continuous casting of SUS304, after cutting the torch, the slab was placed in a water tank at a surface temperature of 700 ° C and cooled to the water temperature. This slab was shot blasted under various conditions shown in Table 1. After that, a part of these cast pieces was sampled, and the hardness at a position at a depth of 0.3 H (-100 μm) from the surface layer portion was measured. On the other hand, the remaining slabs were experimentally heated in a production facility at 1240 ° C. under an oxygen partial pressure (Po ₂ ) of 1%, hot-rolled and pickled, and then the surface defects of the steel sheet were investigated.
The results are summarized in Figures 1 and 2. The reference symbols (A, B, C, D, E, a, b, c, d) entered in FIGS. 1 and 2 are in agreement with Table 1. From FIG. 1, it can be seen that no surface defect occurs when the hardness (Hv) at a depth of 0.3H is 340 or more. Further, as shown in c and d, even if the shot blast is strengthened, that is, even if W · d · V is increased, when the surface segregation of the original slab is deep, the grain pattern defect remains. Therefore, as shown in FIG. 2, the segregation depth is 0.3H.
The relationship between W and d · V is 0.3H <7 ×
It was found that surface defects did not occur under the condition of 10 ⁻⁷ W · d · V.

FIG. 3 is a diagram schematically showing a cross section of the oscillation mark portion on the surface layer of the cast slab 5. The surface segregation layer 3 has a hardness Hv of 34 at a depth of 0.3H from the surface with respect to the difference H between the oscillation mark recess 1 and the oscillation mark protrusion 2.
It was also confirmed that an oxide film was formed up to the position of 0.

[0012]

[Table 1]

[0013]

According to the present invention, when the stainless steel slab is refined, the refinement condition can be selected according to the oscillation mark depth of the slab. As a result, not only unnecessary refining equipment and treatment are not required, but the yield loss due to scale-off can be minimized, defects in the surface layer of the slab can be removed, and product defects can be kept low at low cost. it can.

[Brief description of drawings]

FIG. 1 is a diagram summarizing processing conditions performed in an example of the present invention.

FIG. 2 is a scatter diagram summarizing the results of hardness measurement and the wood grain pattern defects similarly performed in the example.

FIG. 3 is a schematic view conceptually showing a cross section of a surface layer of a cast slab.

[Explanation of symbols]

 1 Oscillation mark concave part 2 Oscillation mark convex part 3 Surface segregation layer 4 Dashed line

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Akira Kawarada No. 1 Kawasaki-cho, Chiba City Kawasaki Steel Co., Ltd. Chiba Works (56) JP, A)

Claims (2)

(57) [Claims] 1. A stainless steel slab produced by a continuous casting method
When heated to 100 ° C. or higher and subjected to hot working, the micro Vickers hardness (Hv) at a depth position of 0.3 times the average depth of oscillation marks from the surface of the slab is 34.
After pretreatment of the slab before heating to 0 or more,
A method for hot working a stainless steel slab, which comprises heating and hot working. 2. The hot working method for a stainless steel slab according to claim 1, wherein the pretreatment is shot blasting under the condition that the following formula (1) is satisfied. 0.3H ≦ 7 × 10 ⁻⁷ W · d · V (1) However, 200 kg / m ² <W <1000 kg / m ² 1.0 mmφ <d <3.0 mmφ 50 m / s <V <200 m / s Where: H: average depth of oscillation mark (mm) W: particle projection density of blast treatment (kg / m ² ) d: particle diameter of blast treatment (mm) V: initial velocity of blast treatment particles (m / sec) ).