JPS6154086B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for supplying a slab slab to a hot rolling process, particularly in a direct rolling process in which a slab after continuous casting is rolled as it is, or in a hot charge rolling process in which a slab is kept warm and then hot rolled. The present invention relates to a method for supplying slabs from a continuous casting process to a hot rolling process with a predetermined width and no cracks on the surface or inside. In direct rolling or hot charge rolling, in order to maintain the heat retained during continuous casting and transfer it to the hot rolling process, compared to the normally adopted process of continuous casting - cold care - heating - hot rolling, Care steps and heating steps can be omitted. Therefore, it can be said that direct rolling or hot charge rolling is an extremely excellent process industrially in terms of improving productivity by omitting manufacturing steps and saving energy by reducing unit heat energy consumption. However, when carrying out the above-mentioned direct rolling or hot charge rolling, the problems are the consistency of the continuous casting process and the hot rolling process, and the quality (particularly cracking) of the cast slab. In other words, regarding consistency, in the normal hot rolling process, the width size of the material that is most suitable for producing the desired product with a high yield is determined, and since the width sizes of the finished products are diverse, the width size of the material to be supplied is determined. The width size of the continuous casting mold must also be varied to accommodate this, but from the viewpoint of productivity and workability, it is preferable that the continuous casting mold has a constant width size. It is often not possible to respond to requests. Conventionally, in order to solve such inconsistencies, a casting method using a dimensionally variable mold that allows the slab size to be changed has been proposed, but the mold structure becomes complicated and workability and maintainability deteriorate. Since this is unavoidable, this method is not necessarily advantageous. For this reason, it is thought to be advantageous to adopt a method in which cast slabs are produced with a constant width, then subjected to a hot rolling process in which the slabs are sized to the desired width, and then supplied to the normal hot rolling process, and various studies have been conducted. ing. However, in this hot rolling process, it is important to complete the width rolling as quickly and accurately as possible.This is done by preventing the temperature of the material from dropping and by improving various conditions to completely prevent the occurrence of cracks and ensure continuity. It is necessary to maintain a high level of workability and productivity in hot rolling, and to ensure consistency efficiently. Therefore, in the hot rolling process of the present invention, it is essential that the width reduction amount is 50 mm/pass or more per side edge of the slab slab, and the width rolling is performed at a strain rate of 3/sec or less. In practice, vertical rolls with upper and lower brim are used to reduce the slab in the width direction by 50mm/pass and more than one side to transform it into a dogbone shape, and then horizontal rolls are used to eliminate dogbones. These are then subjected to reversible rolling as necessary to perform multiple passes to obtain a steel billet of a desired width with a minimum number of passes. Therefore, when the width is greatly reduced, the slab stress distribution becomes non-uniform, and compressive or tensile stress is distributed in a complicated manner along the width and longitudinal direction of the slab. In particular, the strain near the dogbone becomes large, and the flaws on the edge of the slab are enlarged (flaws in the direction perpendicular to the rolling direction become noticeable), which appear as defects on the edge of the slab. In addition, during width reduction, a large tensile stress acts inside the central part of the slab in the width direction, which enlarges internal cracks in the slab and makes it difficult to compress in the subsequent rolling process, causing material deterioration. In the invention, the strain rate during width reduction is set to 3/sec or less to prevent the occurrence of the various types of cracks mentioned above. The content of the present invention will be explained in detail below. Figure 1 schematically shows the temperature history and processing history of a continuously cast slab or ingot during direct rolling (Figure 1, 1) and hot charge rolling (Figure 1, 2). Indicated. Direct rolling and hot charge rolling differ from the traditional reheating/rolling process (Fig. 1, 3) in that the billets are hot rolled or charged into a heating furnace and then rolled without being cooled to room temperature. There is. Therefore, the width rolling method targeted by the present invention is as follows:
This is the normal rolling temperature range for such hot rolling.
Rolling is carried out at a temperature range of 1200 to 900°C with a minimum number of passes of 50 mm/pass and width reduction on one side or more to a specified width dimension, and at that time, horizontal cracks are generated on the surface of the steel billet in the 1st to 5th passes of rolling. Alternatively, the rolling is carried out at a strain rate of 3/sec or less so as not to cause edge cracks at the edges of the steel billet, and to prevent cracks from occurring during subsequent continuous rolling and remaining as defects in the product. The reason why the strain rate is limited to 3/sec or less in the present invention is based on the following findings. In other words, as shown in the results of the inventor's simulation experiment shown in Figure 2, the cross-sectional shrinkage rate in the temperature range of 1300 to 900°C is
In steels exhibiting 60% or more, the frequency of surface cracking during direct rolling or hot charge rolling becomes extremely low. On the other hand, when the cross-sectional shrinkage rate is less than 60%, surface cracks tend to occur frequently, and the low carbon Al killed steel (marked with ○), low carbon Si killed steel (marked with x), and medium carbon Al killed steel shown in Table 1. The strain rate shown in Figure 3 is the result of a Greeble test (a test using a horizontal tensile tester using electrical heating) by subjecting the test piece prepared from (● mark) to the heat cycle shown in Figure 4. It is based on the relationship with cross-sectional shrinkage.

[Table] In other words, what has been clarified from Figure 3 is that when the strain rate during the Greeble test is 3/sec or less, the cross-sectional shrinkage ratio can be maintained at 60% or more;
sec, the cross-sectional shrinkage rate suddenly decreases to less than 60%. This phenomenon can be seen in the Greeble test when the strain rate is 3/sec or less, stress concentration is alleviated and embrittlement due to precipitates in each test piece can be prevented, but when the strain rate exceeds 3/sec This is thought to be due to the sudden increase in stress concentration, which was accompanied by embrittlement due to precipitates in each specimen. Based on this knowledge, hot rolling was carried out at a strain rate of 3/sec or less, and as shown in Figure 5, the embrittlement temperature range during hot rolling was lowered to the low temperature side below the direct delivery or hot charge temperature. It has been found that the temperature range is reduced and the excellent effect of advantageously expanding the hot rolling temperature control range is obtained. That is, using a test slab with the components shown in Table 2, strain rates of 30/sec (● mark) and 3/sec
The relationship between the degree of crack occurrence and the test slab rolling temperature when hot rolling was performed for each of (marked with ○) is shown in the fifth table.
As shown in the figure. As is clear from this figure, by reducing the strain rate to 3/sec or less, the temperature at which cracking occurs, that is, the embrittlement temperature, decreases to 900℃ or less.
The range Y decreases to 700-900°C. This thing is
Even when rolling is performed with a large width reduction of 50 mm/pass on one side or more, there is no risk of cracking since the rolling temperature is 900° C. or higher, which is the general temperature for direct feed or hot charge rolling. On the other hand, the strain rate is 30/
sec, the embrittlement temperature range X in which cracking occurs is as wide as 700 to 1100°C as shown in the figure, which coincides with the subsequent rolling temperature range, leading to cracking.

[Table] Next, Examples of the present invention are listed in Table 3. Third
In the table No.1, 2, 3, 5, 6, 7, 9,
10, 11, 13, 14, 16, and 17 are examples of the present invention,
Nos. 4, 8, 12, 15, and 18 are comparative examples. Furthermore, the meanings of the symbols in the crack occurrence rate are as follows. 〇: No defects on both surface and internal cracks △: Small surface flaws (3 mm or less) Number of cracks: 2 or less Small internal cracks (number of cracks confirmed only by sulfur print: 10 or less) ×: Defects marked with △ or more

[Table] As shown in Table 3 above, it has been confirmed that when the strain rate satisfies the 3/sec or less specified by the present invention, both surface cracks and internal cracks are prevented from occurring. As explained above, according to the method of the present invention, 50
Even if the slab is subjected to large-reduction hot rolling of mm/pass/one side or more, hot rolling cracks such as surface flaws and internal cracks do not occur, and the continuous casting process and hot rolling process are highly energy-saving. It is possible to combine and match productivity, and the industrial effects brought about by this are extremely large.

[Brief explanation of the drawing]

Figure 1 is a schematic diagram of various manufacturing processes, Figure 2 is an explanatory diagram showing the correlation between simulation experiment results and billet cracking during direct feed or hot charge rolling, and Figure 3 is an illustration of the strain rate and cross-sectional shrinkage rate in the Greeble test. An explanatory graph showing the relationship, Figure 4 is an explanatory graph showing the thermal cycle pattern of the test piece, and Figure 5 is an explanatory graph showing the thermal cycle pattern of the test piece.
The figure is a graph showing the relationship between slab rolling temperature and degree of cracking as the strain rate is varied.

Claims (1)

[Claims] 1 50 pieces of hot slab slab obtained by continuous casting of molten steel
mm/pass・When supplying the hot slab slab to the hot rolling process after hot width rolling on one side or more, the strain rate during the hot width rolling is set to 3/sec or less during the hot width rolling. A method for supplying slab slabs to a hot rolling process, characterized by preventing slab cracking.