JP2006281298A - Bloom rolling method of austenitic stainless steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent occurrence of center defects of an austenitic stainless steel. <P>SOLUTION: In a bloom rolling method for manufacturing an austenitic stainless steel by the pass schedule including a first to i-th passes, a void pressure bonding parameter Gm+ is obtained for each of the first to i-th passes, and the pass schedule is constituted so that the accumulated value of ΣGm+i of the void pressure bonding parameter Gm+ of each of the first to i-th passes is not less than the predetermined threshold. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a split rolling method for producing an austenitic stainless steel material, and in particular, a fraction of an austenitic stainless steel material that can prevent the occurrence of center defects by having a predetermined pass schedule in the split rolling pass. The present invention relates to a lump rolling method.

  Regardless of steel ingot casting or continuous casting, a central defect caused by the shrinkage at the end of solidification occurs in steel during steelmaking. In the bulk rolling pass, it is important in terms of quality to eliminate the central defect existing in the slab / steel slab by plastic working such as rolling / forging.

  In order to eliminate the center defect, it is conceivable to increase the forging ratio during plastic working. However, increasing the forging ratio leads to an increase in the size of various manufacturing equipment, which increases the cost as well as the layout. It may not be possible to introduce. Therefore, attempts have been made to eliminate the central defect by improving the pass schedule of the split rolling pass.

For example, in Patent Document 1, by using the gap compression parameter Gm used in free forging, it is possible to extract and evaluate only the energy in the compression direction necessary for actual gap compression, and the value of Gm is 0.2 or more. Thus, a rolling pass method in which no central defect occurs is disclosed. In this rolling pass method, in a rolling pass with a total area reduction rate of 97.7% or less, a pass satisfying Gm ≧ 0.20 from a shape having a cross-sectional area of 120,000 mm ² or less, or a pass having a 40% increase in the finished cross-sectional area. Is manufactured according to a pass schedule including one or more passes.
JP 2002-346604 A

  However, the conventional rolling pass method of Patent Document 1 has the following problems.

  In the method in Patent Document 1, it is indispensable to insert one pass under high pressure during the pass schedule. However, when a difficult-to-work material such as high alloy steel with high hot deformation resistance is rolled, it is difficult to reduce the material with a high pressure pass.

  Moreover, although the method in patent document 1 can evaluate the deformation | transformation of the space | gap part of each pass in a rolling pass schedule, since it does not consider the influence of a steel type, it is applied with difficult-to-work materials, such as an austenitic stainless steel material. The problem remains.

  The present invention has been made to solve such a conventional problem. In particular, the steel type is limited to austenitic stainless steel, and the generation of a central defect is prevented by having a pass schedule that matches the austenitic stainless steel material. It is an object of the present invention to provide an austenitic stainless steel partial rolling method that makes it possible.

  The present invention is a split rolling method for producing an austenitic stainless steel material according to a pass schedule including the first to i-th passes, wherein the gap compression parameter Gm + is set for each of the first to i-th passes. The path schedule is configured such that the cumulative value ΣGm + i of the gap compression parameters Gm + of each of the first to i-th passes is equal to or greater than a predetermined threshold value.

  According to the ingot rolling method of the austenitic stainless steel material of the present invention, the cumulative value ΣGm + i of the gap compression parameter Gm + in each of the first to i-th passes is equal to or greater than a predetermined threshold value. The occurrence of internal cracks in the product can be avoided, and the productivity and quality are dramatically improved.

  Hereinafter, the ingot rolling method which is an embodiment of the present invention will be described in detail with reference to the drawings.

  The pass schedule in the block rolling method of the embodiment of the present invention includes first to i-th passes (i is an integer of 2 or more), and is configured by a plurality of passes of i or more. Each pass has a predetermined hole shape and a predetermined area reduction.

  Each of the first to i-th passes of the present embodiment has a predetermined gap compression parameter. By using the gap compression parameter Gm + used in free forging, it is possible to extract and evaluate only the energy in the compression direction necessary for actual gap compression, and the reduction effect of each pass can be calculated clearly. Become.

  The gap pressure bonding parameter Gm + used in this embodiment can be expressed by the following formula 2.

Here, σm: hydrostatic stress, σ _eq : equivalent stress, ε: equivalent strain, C: constant term (C = 0.024 in the case of a block rolling pass).

  σm is a hydrostatic pressure stress, and when this value is increased, the air gap is easily closed. Σm / σeq obtained by dividing this by the equivalent stress σeq is an index called stress triaxiality, and represents the magnitude of the tension / compression tendency of the stress at each position. Furthermore, since the deformation amount due to strain can also be an index for closing the gap, it is possible to accurately express the closing behavior of the gap by integrating both.

  The present inventors have found that by using Gm +, only the energy in the compression direction necessary for actual gap crimping can be extracted and evaluated, and thus it matches well with the actual behavior. By applying the gap pressure bonding parameter Gm + to rolling, it becomes possible to examine the success or failure of the gap pressure bonding during rolling. The parameter Gm + of the present embodiment more closely matches the actual behavior because the area reduction rate of only the air gap can be taken into consideration.

  The rolling pass is analyzed by a known three-dimensional finite element method, the equivalent strain and the stress in each direction at the center defect occurrence location are calculated by the analysis, and the void pressure bonding parameters are calculated to examine each pass. There is also a method to divide the element by actually assuming the central defect, but this considers the size of the material and the size of the defect. It becomes impractical. Therefore, the calculation is performed using the element division of the solid material, and the defect is predicted therefrom.

  The central reduction effect up to each pass is calculated by accumulating the gap compression parameter Gm + obtained by the above-described mathematical formula.

  When the cumulative value ΣGm + i of the gap compression parameter Gm + from the first to the i-th pass (i is an integer of 2 or more) is greater than or equal to a predetermined threshold, it is possible to prevent the occurrence of a center defect. And

  The threshold value can be determined by combining a numerical calculation described later and an actual steel experiment.

  By clearly distinguishing between austenitic stainless steel and other steel types, changing the threshold, and having a pass schedule that matches the austenitic stainless steel, it is possible to prevent the occurrence of center defects.

  As described above, according to the agglomeration rolling method of the austenitic stainless steel material of the present embodiment, the air gap compression parameter Gm + is used to clearly calculate the rolling effect of each pass and accumulate it until each pass. It becomes possible to calculate the central reduction effect. By clearly distinguishing between austenitic stainless steel and other steel types, and changing the threshold value according to the steel type, it is possible to design a path that can be crimped by a gap even in continuous passes that do not include a high-pressure pass Become.

  Next, an example is shown and demonstrated in detail about the method of lump rolling of the austenitic stainless steel material by embodiment of this invention. However, the ingot rolling method of the present invention is not limited to the following examples.

  First, the gap crimping parameter Gm + is obtained for each path in the path schedule by the analysis method described above. Here, the determination method of the constant term C of Formula 2 will be described below.

  The disappearance timing of the central defect was determined by plasticine experiment using multiple pass schedules. As a result, it was estimated that the area where the area reduction rate of the entire rolled section and the area reduction ratio of the void defect coincided with each other was in the vicinity of C = 0.024, and this value was added. However, if the area reduction is too small, the reduction may not reach the center. Therefore, in the case of an area reduction of 2% or less in which no deformation is observed in the center defect, a process of setting C = 0 is performed.

  Next, a method for determining the threshold value of the cumulative value ΣGm + i will be described. FIG. 1 is a diagram showing the relationship between product diameter and cumulative value ΣGm + i in multi-pass rolling.

  Multiple continuous rolling passes were made from a bloom base material heated to 1150 ° C in a 490mm x 380mm cross section, and were determined by combining numerical calculations and actual steel experiments (Fig. 1). First, the value of ΣGm + at each product diameter was determined by analysis using the three-dimensional finite element method. Next, it rolled with real steel and the presence or absence of the space | gap was determined by the ultrasonic test. In the ultrasonic test, a flaw detection wave of 30% or more was regarded as defective at 5 MHz.

  As a result, as shown in FIG. 1 (a), SUS304 could be rolled up to φ210mm product, and as shown in FIG. 1 (b), SUS316L could be rolled up to φ190mm without defects. Therefore, the threshold value was determined to be 0.35 (line A in the figure).

Tables 1 to 4 are tables showing cumulative value ΣGm + i determination results when a predetermined austenitic stainless steel material is rolled by a predetermined pass schedule. Table 5 is a table summarizing the determination results of the center defects between the method of split rolling of this example and the method of the comparative example.

  As is clear from Table 5, according to the agglomeration rolling method of the austenitic stainless steel material of this example, it is possible to prevent the occurrence of center defects in the austenitic stainless steel material as compared with the conventional comparative example. I can see that Therefore, the validity of the split rolling method of this example was proved.

It is a figure which shows the relationship between the product shape and cumulative value (SIGMA) Gm + i in the multipass rolling of the agitation rolling method of the austenitic stainless steel material of a present Example.

Claims (5)

A block rolling method for producing an austenitic stainless steel material by a pass schedule including the first to i-th passes,
Obtaining a gap crimping parameter Gm + for each of the first to i-th passes;
The austenite system is characterized in that the pass schedule is configured such that a cumulative value ΣGm + i of the gap compression parameter Gm + of each of the first to i-th passes is equal to or greater than a predetermined threshold value. A method of ingot rolling of stainless steel. 2. The agglomeration rolling method for an austenitic stainless steel material according to claim 1, wherein the gap compression parameter Gm + is obtained by the following mathematical formula 1.

Here, ε is equivalent strain, σm is hydrostatic stress, σeq is equivalent stress, and C is a constant term.   The constant term C is 0 when the area reduction rate of each of the first to i-th passes is 2% or less, and 0.024 when the area reduction rate is greater than 2%. The method for ingot rolling of an austenitic stainless steel material according to claim 2.   The method for split rolling austenitic stainless steel according to any one of claims 1 to 3, wherein the predetermined threshold value is 0.35 or more.   The austenitic stainless steel material manufactured by the agglomeration rolling method of the austenitic stainless steel material in any one of Claim 1 to 4.

Images