JPS6035201B2 - Rolling method for continuously cast slabs - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing steel products with excellent mechanical properties by rolling with high productivity.

In recent years, as the amount of steel used has increased, coin coins used for rolling have tended to become larger, mainly with the intention of reducing manufacturing costs and increasing productivity.

Manufacturing of continuously cast coin coins (hereinafter referred to as CC coin coins) using continuous casting equipment (hereinafter referred to as CC), which is particularly superior in terms of productivity, product yield, and manufacturing cost, is becoming mainstream. Furthermore, the coin thickness is 30mm for higher productivity.
Large CC coin coins that exceed the enemy's size are also being manufactured. On the other hand, in order to reduce the weight of the product, the recent trend is to reduce the thickness of the product and supplement the strength by using higher tensile strength steel.

That is, if the thickness of the CC coin is the same, the amount of rolling required to produce a product will increase. CC coin thickness D and product thickness d of steel materials manufactured in recent years due to the above two reasons
The ratio, that is, the value of the rolling reduction ratio, tends to increase.

Conventionally, it was thought that there was a certain amount of machining to be added to the CC key piece, and that the mechanical properties of the steel material improved as the rolling reduction ratio increased, but in reality, the mechanical properties of the steel material improve as the rolling reduction ratio increases. There are cases in which steel materials are being manufactured that have deteriorated properties or frequently suffer from deep ultrasonic flaws. In addition, as offshore development has progressed recently, the amount of steel materials used for offshore structures has rapidly increased, but many of the offshore structures are subject to loads in the thickness direction of the steel materials, requiring particularly severe lamellar tear resistance. . Steel materials with excellent lamellar tear resistance properties are usually manufactured by adding special elements, but they have the drawback of significantly increasing manufacturing costs. Based on the above-mentioned current situation, the inventor of the present invention has conducted a detailed study on the influence of processing in the rolling process on the mechanical properties of steel materials, and has determined that the present inventors can achieve low cost, high productivity, and excellent mechanical properties, among others. have invented a method for producing steel materials with lamellar tear resistance and base material rutting properties by devising a rolling method.

A detailed explanation of the present invention will be given below. The present inventor investigated the factors that deteriorate the quality of the material in the as-cast state, and investigated in detail how these factors could be removed during the rolling process.

It has been conventionally recognized that as the rolling reduction ratio increases, the cross-sectional area reduction rate ■z in a tensile test in the thickness direction of steel material decreases, and it is also known that this is due to sulfides, particularly MnS. However, as mentioned above, in the conventional manufacturing method, in order to obtain ballability, a large reduction ratio must be used, and the relationship between the straddle z and the sulfur content is not quantitatively known. The present inventors first discovered that manganese (Mn) and sulfur (S), which are unavoidable components of ordinary steel materials, crystallize as MnS during the solidification process of molten steel. It was revealed that the results show that there is a specific arrangement.

Therefore, in the as-cast state, once the cracks are formed, the M
It has been clarified that nS easily propagates to the crystallized surface and significantly impairs the material properties of CC coins, especially the toughness of the base material.

One of the significances of applying rolling processing is to change the arrangement of MnS crystallized between these dendrites, and the amount of rolling required to change the arrangement can be as light as a reduction ratio of about 2. In addition, it was found that processing with a reduction ratio of at least 2 is necessary to eliminate the adverse effects of crystallized MnS. This fact is contrary to conventional wisdom and is one of the most distinctive points of the present invention based on detailed research. In other words, the amount of processing required to remove material deterioration due to MnS is only a rolling reduction ratio of 2, so it is possible to perform the most effective rolling by manufacturing CC coin coins that are twice as thick as the product thickness. become. In more detail, material deterioration due to the two-dimensional arrangement of MnS manifests itself as a significant decrease in the upper shelf absorbed energy in the Shallie test, which is another indicator of base material toughness. It was found that austenite grain size was the main factor determining the fracture surface transition temperature, with no significant effect on the rock surface transition temperature. In the current rolling operation, the austenite crystal grain size is reduced from 0 to -1 (AST
In order to obtain a sufficiently good fracture surface transition temperature, it is essential to refine the austenite crystal grain size by rolling, and it is said that a rolling reduction ratio of 6 or more must be applied. There is. However, as shown in Figure 2, the present inventor has proposed effective utilization of precipitates or CC
By controlling the heating temperature of the coin piece, the austenite grain size of the CC coin piece before rolling is made finer than No. 2, preferably No. 4 or more, which is necessary to obtain a sufficiently good fracture surface transition temperature. It has been found that light processing with a reduction ratio of about 2 is sufficient, and as mentioned above, processing with a reduction ratio of at least 2 is necessary in order to remove the adverse effects of crystallized MnS. From the perspective of saving the energy required to manufacture steel materials, from now on, after molten steel is cast using a continuous casting machine, the C
It is thought that a process will be implemented in which the C coin piece is directly rolled without cooling and reheating, but in that case, the precipitates will be effectively used to sufficiently increase the austenite grain size of the CC mirror piece before rolling. It is efficient to keep it detailed. In the present invention, it is essential to make the austenite crystal grain size of the CC coin coins sufficiently fine before rolling, but there is no restriction on the method or means of grain refinement. C.C.
It is well known that MnS, which is unavoidably contained in mirror pieces, elongates in the rolling direction during the rolling process.

For this reason, the mechanical properties of the manufactured steel material vary significantly, and in particular, the properties in the thickness direction of the steel material are extremely deteriorated.
In recent years, loads are often applied to steel materials in the thickness direction, not only in offshore structures, but even if the base metal quality in the rolling direction is sufficient, the properties of the steel materials are insufficient. A particularly important characteristic of steel materials in the thickness direction is lamellar tear resistance.

Although this lamellar tear resistance has a close relationship with the elongation of MnS, a quantitative relationship has not been obtained, and this is unexplored as a rolling technology. The present inventors conducted detailed research on the relationship between sulfur content and rolling conditions, and found that the same characteristics were determined by the rolling ratio as a guideline for the amount of processing given to CC coin coins and the sulfur contained in CC coin coins. It has been found that this is mainly controlled by the amount of steel material, and that it has a specific relationship with the tensile test section reduction rate (?z) in the thickness direction required for steel materials. This relationship is D/d:C; vessel; (1-◇Z)3 ......'1}
It is expressed as

This is the second most distinctive feature of the present invention. '1
In the formula, D/d is the rolling reduction ratio, and CS is the maximum sulfur content in the CC coin expressed in weight %. Note that z is the original cross-sectional area A of the test piece. and the cross-sectional bond A after the test, medium z=
(A.-A)/A. is given by By the way, lamellar tear resistance has a high correlation with z, and the larger z is, the better the lamellar tear resistance is, for example, according to "Steel Research" No. 28.
No. 6 (1975), page 82.

When CS is constant and the required 0z is given, the rolling reduction ratio is uniquely determined by the formula {1}.Since the expansion of MnS due to processing is the factor that lowers z, it is determined by the formula m. The rolling reduction ratio corresponds to the upper limit. Therefore, if the purpose is to ensure lamellar tear resistance, D/d≦CS≧
etc.

, (1-◇Z)3.

In addition, normally, the thickness of each CC is constant, and when the product board thickness is specified, the reduction ratio is inevitably specified, but in order to secure the required z, it is necessary to set a limit on the CS. There are cases.

In this case as well, the lower the CS, the better the lamellar tear resistance;
The value of is always the upper limit, so D/d≦ is considered a biological damage; (・-
It is expressed as guZ)3.

In addition, due to the productivity of rolling and the difficulty in elongating MnS,
The rolling temperature is preferably 90,000 or higher.

FIG. 1 shows the maximum possible values of D/d when Maz is 15% (dashed line) and 25% (dotted chain line). Also, the solid line shown at D/d=2 is, as mentioned above,
This represents the lower limit of D/d because the reduction ratio required to change the arrangement of MnS crystallized between dendrites and the reduction ratio necessary to eliminate the adverse effects of crystallized MnS is 2. The reason why we chose 15% as medium z is that not all steel materials are required to have perfect lamellar tear resistance, and it is necessary to be able to manufacture steel materials with various product thicknesses from CC coin coins of the same shape. There are two main reasons why it is more convenient to have a wider allowable reduction ratio range.

Region P indicates a range of rolling conditions in which 02, which exhibits sufficient lamellar tear resistance, is 25% or more.

Region P and region Q are considered to have good lamellar tear resistance under rolling conditions where ◇z is 15% or more. In the center of the rust piece produced by the CC method, which is the main manufacturing method for mirror pieces, there is a region with a high concentration of the contained elements, called the so-called center polarization zone.

It is said that the presence of a central segregation zone deteriorates the mechanical properties of steel materials. As a result, the enriched MnS is more elongated, and the lamellar tear resistance of the steel material tends to be rather drastically reduced. That is, for the steel material, excessive processing on the central segregation zone will unnecessarily promote the elongation of MnS, which will have a detrimental effect on the contrary. The sulfur content on the horizontal axis of the graph shown in FIG. 1 means the average concentration in the area that is the measurement point in the normal analysis method and the highest value when there is variation within the CC coin. Therefore, in the case of CC twill, it means the sulfur content in the central segregation zone. Based on the above research results, which are based on new knowledge that was previously unknown, we found that when the sulfur content is high, which produces MnS, which deteriorates the mechanical properties of steel, a reduction ratio of 2 is required. Rolling is rather preferable; the permissible rolling reduction ratio can be expanded to 2 or more only in small cases, and by reducing the austenite grain size of the CC mirror piece before rolling to No. 2 or more, preferably No. 4 or more, it is possible to improve the base material. The present invention has been achieved, which makes it possible to manufacture steel materials with excellent grain tear resistance as well as grain tear resistance at low cost and with high productivity.

Although the above description has mainly been given to steel plates, the present invention can be similarly applied to other steel materials.

The chemical composition of the CC mirror piece targeted by the present invention is the following low-carbon, low-alloy steel. tl} C: 0.05-0.25
%, Sj: 0.05-0.60%, Mn: 0.60-1
．． Standard low carbon silicon manganese steel containing 90%.

■ For the reference low carbon silicon manganese steel in {1) above, 0
．． 1% or less mountain, 0.5% or less Cu, 2% or less N, 0.5% or less Cr, 0.3% or less Mo, 0.2%
Low alloy steel containing one or more of the following V, 0.1% or less Nb, and 0.1% or less Tj.

Next, examples of the present invention will be described.

The basic components of the CC coin coins used are equivalent to JIS standard SM50. The rolling method according to the present invention and the rolling method from the secondary mold were performed, and the mechanical properties were compared. The test results are shown in Table 1.
The tensile test piece was a JIS standard No. 4 round camphor test piece.
In Table 1 Mechanical Properties (1), z indicates the reduction of area in the thickness direction of the steel material.

[Brief explanation of drawings]

Figure 1 is a diagram showing a comparison between the allowable range of rolling conditions determined experimentally to fully satisfy the mechanical properties of steel and the calculation formula shown by the present invention.
, the curve shown by a dashed dotted line shows the permissible limit of rolling conditions where z=25%. FIG. 2 is a diagram showing that if the austenite crystal grain size of the CC slab before rolling is fine, the rolling reduction ratio may be small to obtain the required base metal quality. Figure 1 Figure 2

Claims (1)

[Claims] 1. When manufacturing steel products from continuously cast slabs by rolling, when the continuous cast slab thickness is D and the product thickness is d, the austenite grain size (ASTM) of the continuous cast slab before rolling is
2 or higher, the value of D/d is determined according to the tensile test cross-sectional area reduction rate φ_Z in the plate thickness direction required for the steel material and the maximum sulfur content C_S expressed in weight percent of the continuously cast slab. 2≦
D/d≦(0.31)/(C_S+0.01)(1-φ
＿Z)＾3 A rolling method for continuously cast slabs.