JPH01321012A - Surface treating method for continuous casting slab of stainless steel - Google Patents

Abstract

PURPOSE:To efficiently remove defects on the surface and under surface affecting the surface quality caused by generating the work layer of microcracks on the surface of the continuous casting slab of stainless steel by treating through blasting and scaling off through hot rolling. CONSTITUTION:At first, the microcrack worked layer 8 generating nearly uniform and numerous microcracks 4' along with the surface of the continuous casting slab of stainless steel is formed. Then, it is heated >=1,100 deg.C, the heat is transferred to the microcracks 4' on this microcrack worked layer 8, oxidation is promoted rapidly to form an oxide layer 8'. Further, oxidation is advanced into the deep part to form the oxide scale layer 6 and sub-scale layer 6. Therefore, the formed oxide layer 9 consists of the microcrack oxide layer 8 and oxide scale layer 6 and only the thickness of the microcrack layer 8 is thickened and because it contains the deepest part of the under surface defect, by scaling off the oxide scale layer, each defect on the surface of the continuous casting slab is removed completely.

Description

Detailed Description of the Invention [Industrial Application Field] The present invention provides thermal treatment by treating the surface of continuously cast stainless steel slabs by a combination of blasting treatment such as shot blasting and heat treatment accompanied by hot rolling. Surface care of continuous cast stainless steel slabs to efficiently remove surface defects of slabs that affect the surface quality of rolled products after inter-rolling, especially subcutaneous defects formed in the concavities of uneven oscillation marks on the slab surface. It is about the method.

[Conventional technology]

In general, steel plates, sections, etc. are manufactured by directly rolling rolled materials such as slabs, but these rolled materials have traditionally been produced by blooming and rolling steel ingots manufactured in steel factories. In recent years, manufacturing using the continuous casting method (hereinafter, slabs manufactured in this way will be referred to as continuous casting slabs) has become mainstream, as it is comprehensively superior in terms of productivity, workability, quality, yield, manufacturing cost, etc. It is becoming.

The outline of this continuous casting method is as follows. That is, the molten steel transferred from the ladle to the tundish.

It is injected from above into a vertical mold that is cooled with cooling water, solidifies sequentially from the surface layer in contact with the inner surface of the mold, and is continuously drawn out from the bottom of the mold, then cooled by water spray, and then released. After passing through the cold zone and solidifying to the center, it is cut into slabs. In the process of injecting molten steel into a mold and solidifying it using such a continuous casting method, a vertical vibration method (oscillation method) of the mold is employed to promote the downward movement of the solidifying molten steel.

Now, continuous cast stainless steel slabs are also manufactured by the above-mentioned continuous casting method, but these continuous cast slabs have various defects on the surface and under the surface (under assembly). For example, as shown in Fig. 2, oscillation marks (05M and (sometimes abbreviated) and cracks associated with this. In addition, as an assembly defect, component segregation parts 3 (Ni, SL acid components, etc., especially Cr
-Conspicuous in the case of Ni-based stainless steel slabs), local microcracks 4, and mold powder entrapment portions 5, which will be explained later.

Various continuous casting countermeasure techniques have been adopted to suppress the occurrence of such surface and assembly defects in continuously cast stainless steel slabs. In other words, measures to consider the chemical composition of the casting slab, equipment measures such as casting equipment and related equipment, including the material and two-dimensional shape of the inner surface of the mold, and reduction of friction between the inner surface of the mold and the solidified shell and the upper part of the molten steel. Mainly Ca and S are sprayed on the bay steel inside the mold to prevent atmospheric oxidation and to retain heat.
i, C.

Improving the molding powder itself, which consists of Na, and measures such as the amount and method of spraying, as well as casting temperature and speed.

By using cooling conditions, surface reduction rolling of the solidified hot slab, short stroke and high cycle of vertical vibration of the mold, and other countermeasures for continuous casting conditions, it is possible to not only form the continuous cast slab into an accurate rectangular shape, but also to improve its surface O8M. The surface quality of continuous cast slabs has been significantly improved by eliminating as much of the difference between peaks and valleys as possible, smoothing the surface, and reducing the depth of defects under assembly.

As a result of this improvement, some methods have been implemented in which continuously cast slabs are directly sent to a heating furnace without any surface treatment, heated in the furnace, and hot rolled. fact.

Continuously cast slabs of ordinary steel, etc. have traditionally been treated with chipping, scarfing, etc., but since the amount of scale-off (depth) due to the heating that accompanies rolling reaches the depth that includes the assembly defects, , these maintenance operations for eliminating defects are in most cases unnecessary.

However, in the case of continuously cast stainless steel slabs, partial or complete maintenance has been performed by cold grinding with a slab grinder or by grinding with a blade of a milling machine for the reasons described below.

b) Stainless steel has excellent oxidation resistance.

The layer (depth) that is scaled off by heating in a heating furnace and hot rolling is shallow, and defects on the surface and under the assembly remain unremoved, and these remain on the surface of the rolled steel strip after hot rolling. Uneven skin, rough skin, patterns caused by 05M, remaining as surface patterns and surface flaws such as sludge marks, mouth) These rolled steel strips cannot be made into products as they are, and such rolled steel strips cannot be used as raw materials. Because the strict surface quality requirements cannot be met when producing the final cold-rolled steel strip or steel plate.

In order to remove surface patterns and flaws during manufacturing, the surface of the copper strip had to be ground using a coil grinder that uses an endless grinding belt.

The above-mentioned partial maintenance of continuous cast slabs is a method of removing only macroscopic defects such as surface cracks and surface roughness caused by disordered 08M, but this method does not improve the yield somewhat compared to the full-scale maintenance described later. However, assembly defects such as mold powder entrainment areas, local microcracks, and component segregation areas that occur due to 08M on the surface remain, and if these are left as they are and manufactured, the final cold-rolled steel sheet will be damaged. It appears as surface flaws and patterns, and has the disadvantage that the required surface quality cannot be satisfied.

In addition, cleaning the entire surface can remove all of these assembly defects, but since most assembly defects occur in the ○SM valleys, it is possible to remove all of these assembly defects from the healthy parts of the peaks to the valleys. Since a large amount of assembling defects are removed by grinding at a depth greater than the depth, there is a drawback that even if the surface quality requirements are satisfied, the manufacturing cost increases due to a decrease in yield due to cleaning.

[Problem to be solved by the invention]

In place of the conventional care method which has various drawbacks as described above, the present invention minimizes the reduction in production yield due to care and improves the surface and its properties that affect the surface quality of each rolled product after hot rolling. It is an object of the present invention to provide a method for treating the surface of a continuously cast stainless steel slab so that defects in the assembling, particularly defects formed in the valleys of oscillation marks on the surface can be efficiently removed.

[Means to solve the problem]

Blast processing such as shot blasting is generally used for descaling, etching, peening, polishing, general cleaning, etc.For slabs, it is conventionally used to level out the surface grinding marks after cleaning with a grinder to smooth the surface. This method has only been used to adjust the surface roughness, but has never been used to remove defects on the surface of continuous cast slabs or in the assemblies themselves. As a result of various studies, the inventors of the present invention have found that by such blasting, a micro-cracked layer with a certain depth or more is created on the surface, and then it can be cleaned by heating it to a certain temperature or more and then hot-rolling it to scale it off. The present invention was completed by researching that each defect can be removed while reducing the decrease in manufacturing yield caused by this.

Hereinafter, a method for treating the surface of a continuously cast stainless steel slab according to the present invention will be explained in detail with reference to the drawings.

Each of FIGS. 1 to 5 is an explanatory cross-sectional view of the surface portion of a continuously cast stainless steel slab along the casting direction, and FIG. 1 is an explanatory cross-sectional view of the surface portion to be scaled off by the method of the present invention;
Figure 2 is an explanatory cross-sectional view showing the occurrence of assembly defects, Figure 3 is an explanatory cross-sectional view of the surface area that is scaled off by heating without maintenance, and Figure 4 is an explanatory cross-sectional view of the surface area that is heated and scaled off after grinding with a slab grinder. FIG. 3 is a cross-sectional explanatory diagram of a surface portion to be scaled off.

FIG. 5 shows the relationship between the heating temperature and the condition of the surface area when heated after blasting, and (a) shows the state before heating.

(B) is a cross-sectional explanatory view showing each state after heating at 900°C, and (C) is after heating at 1100°C or higher. Figure 6 shows the projection conditions of blasting and the depth of the micro-cracked layer. FIG. 3 is a diagram showing the relationship between the amount of scratches and the degree of residual flaws on the surface of the rolled product after rolling.

The occurrence of 08M, which is a major surface defect, and the assembly defects that occur along with it are as shown in FIG. The difference in height between peak part 1 and valley part 2 of 08M, that is, 08
The depth of M is usually 150 to 600 mm, and component segregation zones 3 (often noticeable if Ni is present) occur in the valleys 2 at varying depths of 100 to 300 mm. , microcracks 4 and mold powder entrapment portions 5 also occur with varying depths in the range of 50 to 300p. Heat continuously cast stainless steel slabs with such assembly defects without maintenance (for example, at 1230°C for 1 hour)
Then, as shown in FIG. 3, the surface portion becomes an oxide scale layer 6.
When this is hot-rolled, it peels off, that is, scales off, but in the case of stainless steel, the thickness of this oxide scale layer 6 is 100 to 300p, which is thin. A part of 5 remains and often remains as surface flaws on the rolled product after rolling. In addition, as in the prior art, the surface of the continuously cast stainless steel slab is ground with a slab grinder to remove most of the micro cracks 4 and mold powder entrainment portions 5, as shown in FIG. Then, when this is heated, an oxide scale layer 6 is formed as shown in FIG. 4, and by hot rolling, this oxide scale layer 6 is peeled off and no surface flaws remain. The reduction in production yield due to cleaning and oxidized scale is significant.

In the method of the present invention, first, a blasting process is performed on the surface of a continuously cast stainless steel slab by projecting a blasting material such as iron shot, grid, or cut wire to create microcracks 4' with a depth of 50 mm or more on the surface. The slab heated to 1100° C. or higher is hot rolled to scale off.

The reason why each defect is completely removed by this method is as follows. As shown in Figure 1, the blasting process creates almost uniform and numerous microcracks 4' along the surface.
A micro-cracked layer 8 is formed, which is then heated. In this micro-cracked layer 8, oxidation proceeds rapidly through the micro-cracks 4', forming an oxidized layer (
below.

A micro-crack oxide layer 8') is formed, and oxidation similar to heating during no-maintenance or slab grinder grinding progresses toward the deeper part, forming an oxide scale layer 6 and a subscale layer 6'. .

Therefore, the oxide scale layer 9 formed by the method of the present invention is composed of the microcrack oxide layer 8' and the oxide scale layer 6, and its thickness is microscopic compared to the case where the microcrack processed layer 8 is not formed. crack oxide layer 8
The oxide scale layer 9 is thickened by the thickness of the micro-cracked layer 8, which corresponds approximately to When the stainless steel slab is scaled off, all defects are cleanly removed from the surface of the continuously cast stainless steel slab, and furthermore, this cleaning method by scaling off reduces the yield loss.

In order to sufficiently increase the thickness of this oxide scale layer 9 as described above, it was found that the depth of the microcracks 4' constituting the microcracked layer 8 must be 50 mm or more. . The depth of microcracks 4' caused by the blasting process is influenced by the strength of the blasting process. The strength of this blasting treatment is determined by the initial velocity V (m
It is expressed as the product W of the average grain size D (am) and the average grain size D (am), and blasting was performed with various values W to form microcracked layers 8 with microcracks 4' of various depths. After obtaining a continuously cast stainless steel slab and further heating it to 1230°C, hot rolling it and scaling it off, the obtained rolled steel strip was cold rolled as a raw material, and the surface of each cold rolled stainless steel plate obtained was FIG. 6 shows the results of inspection for residual flaws. In addition, the projection density of the above blasting treatment is 1
00kg/rrr.

As shown in Figure 6, in order to prevent the occurrence of residual flaws from the continuous cast stainless steel slab on the surface of the cold rolled stainless steel plate, the surface of the continuous cast stainless steel slab should be
It is necessary to generate microcracks 4' of 0- or more. It can be seen that when the projection conditions of the blasting treatment are W≧50, the depth of the micro-cracks 4' can be made to be 50 Ia or more.

Furthermore, the effect of the degree of heating on the formation of the oxide scale layer 9 after blasting the continuously cast stainless steel slab was investigated.

When the heating temperature after blasting is 1100° C. or higher, the microcracked layer 8 in which microcracks 4' have occurred as shown in FIG. 5(a) is completely oxidized by microcracks as shown in FIG. A layer 8' is formed below the layer 8', and an oxide scale layer 6 is formed thereunder.

Furthermore, the subscale layer 6' formed under the oxide scale layer 6 consists of a grain boundary oxidation part and a metal/oxide mixed part, and its thickness is 307a or less after no maintenance or after grinding with a slab grinder. This layer is similar to the subscale layer 6' that is produced when heated, and turns into scale on the surface of the rolled product while undergoing oxidation during hot rolling. The micro-crack oxide layer 8' and the oxide scale layer 6 constitute an oxide scale layer 9 which is easy to peel off and which cleanly removes assembly defects.

On the other hand, if the heating temperature is 1100℃ or less, the fifth
As shown in Figure (c), the micro-crack oxide layer 8' is thin and the oxide scale layer 6 hardly develops, and the sub-scale layer 6' develops sparsely along the micro-crack 4' and peels off. becomes thinner, and the effect of the method of the present invention is not apparent.

It is better that the density of the micro-crack 4' itself is sufficient, but if the density is very low, even if the depth of the micro-crack 4' is a predetermined length and the heating temperature is 1100°C or higher, as a result, the fifth As a result of various tests, we found that a situation similar to that shown in Figure (c) may occur.
Below 0 kg/r&, it is difficult to obtain a sufficient density of micro-cracks 4', and at around 1000 kg/r&, the effect does not change, and the preferred projection density range is 100 to 800.
kg/-.

Since the oxide scale layer 9 obtained as described above is extremely fragile, it can be easily removed by hot rolling, which is performed at a temperature of about 1050 to 750°C and a reduction rate of several percent or more while vigorously removing scale from the slab surface. It is peeled off and removed, and does not result in patterns or flaws on the surface of the rolled steel strip after rolling.

〔Example〕

Examples (1 to 8) and comparative examples (1 to 8) according to the method of the present invention
8) was carried out as detailed below, and the results are summarized in the following table.

First, 5 is a representative type of austenitic stainless steel.
Continuously cast US304 molten steel to create dimensions 200cm (thickness) x
A large number of continuously cast slabs measuring 1050 m (width) x 9000 m (length) were produced.

Concerning these continuously cast slabs, those in Comparative Example 1 in the following table were hot-rolled after heating in an untreated state as cast, while those in Comparative Example 2 were polished over the entire surface of the slab using a conventional slab grinder. to a depth of 1 m (1000 m
) and then heated and hot rolled.

The remaining Examples 1 to 8 and Comparative Examples 3 to 8 were all subjected to a blasting process under the conditions described below, and samples were taken from the end of the slab.

Blasting conditions: (A) Projection machine: Iron grid (projection machine) with an average particle size in the range of 0.2 to 2.01.

(B) Initial velocity of projector (V) near 0 and 100 m/s.

(C) Projection density: 50.100 and 200kg/m2
(By adjusting the threading speed of the continuous cast slabs) Next, all the continuous cast slabs were heated to a temperature of 1230.1130.
Heated to 1080°C and 1050°C, rolling temperature range 12
A hot rolled steel strip having a thickness of approximately 4 m was obtained by hot rolling at a temperature of 00 to 800°C and a rolling reduction rate of 5 to 40% per pass.

Using this steel strip as a raw material, it was further subjected to heat treatment, pickling, cold rolling, heat treatment, pickling, and shearing to obtain a cold rolled steel plate with a thickness of 2.0 m.

In order to express the effect of surface care of continuously cast stainless steel slabs in the above, instead of directly evaluating the remaining state of defects on the surface or plate after care, we evaluate the remaining patterns and flaws on the surface of the obtained cold-rolled stainless steel sheets. Evaluation was made according to the condition. To evaluate the presence or absence of surface patterns and surface flaws on cold-rolled stainless steel sheets, calculate the ratio of the number of sheets with surface patterns and surface flaws to the total number of sheets rolled obtained for each continuous cast slab care category as follows: This was done by classifying and displaying them using symbols.

0: Ratio less than 3% (no occurrence) Δ: Ratio 3% or more and less than 15% (some occurrence) In addition to measuring the depth of the micro-cracked layer, the samples were placed on top of a continuously cast slab to be hot rolled, charged into a heating furnace, and heated. After heating, these samples were cooled without hot rolling, and the surface layer The thickness of the oxide scale layer formed in the area was measured. The continuous cast slab of Comparative Example 1 without maintenance and the continuous cast slab of Comparative Example 2 whose entire surface was ground by slab grinder grinding were similarly measured.

From the above-mentioned coating, it is possible to obtain cold-rolled stainless steel sheets with almost no surface patterns or surface defects from continuous cast stainless steel slabs subjected to blasting and heating accompanied by hot rolling under the conditions specified in the method of the present invention. I understand. On the other hand, if the conditions are outside the range specified by the method of the present invention, not only continuous cast slabs without maintenance, but also blasting and heating accompanied by hot rolling, cold rolled stainless steel sheets without surface patterns or defects will be produced. I can't get it. From this, it can be seen that according to the method of the present invention, not only the surface defects of the continuously cast stainless steel slab but also the surface defects thereof can be almost completely removed. Note that when slab grinder grinding is performed, surface patterns and scratches are eliminated, but the total thickness loss due to both grinding and scale-off reaches approximately 1250 tJn, indicating a significant decrease in manufacturing yield.

The embodiment of the surface care method according to the present invention for continuously cast slabs of steel type 5US304 has been described above, but if the surface care method is carried out within the condition range specified in the method of the present invention, it can be applied not only to continuously cast slabs of this steel type. , ferritic and martensitic stainless steels (specifically 5US430 and 5US41) that have similar O3M surface defects and their dish defects.
Similarly, good evaluations and results can be obtained for continuously cast slabs (e.g., 0, etc.).

〔Effect of the invention〕

The surface care method for stainless steel continuously cast slabs according to the present invention described in detail above has the following effects:
It can replace the conventional cleaning methods using whetstone grinding and blade grinding using slab grinders and milling machines, which have problems in terms of productivity, especially manufacturing yield and cost.Moreover, it can be sent directly to the heating furnace without any maintenance, and can be heated directly into the furnace. It has great industrial value as it approaches the ideal method of hot rolling after heating.

b) Not only the O8M surface defects of continuous casting slabs but also various plate defects can be removed with good workability and productivity (1/1/2 of the conventional method).
15 processing times), especially compared to traditional grinding care methods.
%, ensuring a manufacturing yield that is almost maintenance-free, and therefore can be effectively removed at the manufacturing cost of the conventional 1720.

(b) After being treated, the surface quality of the rolled product obtained after heating and hot rolling is as good as that of the conventionally treated product.

[Brief explanation of the drawing]

Each of FIGS. 1 to 5 is an explanatory cross-sectional view of the surface portion of a continuously cast stainless steel slab along the casting direction. Fig. 1 is a cross-sectional explanatory diagram of the surface portion to be scaled off by the method of the present invention, Fig. 2 is a cross-sectional explanatory diagram showing the occurrence of defects on the plate, and Fig. 3 is the surface to be scaled off by heating without maintenance. FIG. 4 is a cross-sectional view of the surface portion which is heated and scaled off after grinding with a slab grinder. FIG. 5 shows the relationship between the heating temperature and the condition of the surface area when heated after blasting, and (a) shows the state before heating. (B) is a cross-sectional explanatory view showing each state after heating at 900°C, and (C) is after heating at 1100°C or higher. Figure 6 shows the projection conditions of blasting and the depth of the micro-cracked layer. FIG. 3 is a diagram showing the relationship between the amount of scratches and the degree of residual flaws on the surface of the rolled product after rolling. In the drawings 1... Peaks of the oscillation mark 2... Valleys of the oscillation mark 3... Component segregation areas 4.4'... Micro cracks 5... of the mold powder Rolling part 6... Oxide scale layer 6'... Subscale layer 7... Grinding part 8... Micro crack processing layer 8'... Micro crack oxide layer

Claims (1)

[Claims] 1. A micro-cracked layer with a depth of 50 μm or more is created on the surface of a continuously cast stainless steel slab by blasting, and then the slab heated to 1100° C. or more is hot rolled to scale off. A method for surface care of continuously cast stainless steel slabs, characterized by the following. 2 The blasting process was carried out under the following conditions (i) initial velocity V (m/sec) of the blasting material and average particle diameter D (mm
) The method for surface care of a continuously cast stainless steel slab according to claim 1, wherein the product W is W≧50 (ii) the projection density is 100 to 800 kg/m^2.