JPH03111515A - Rolling method of alloy tool steel - Google Patents

Abstract

PURPOSE:To roll and alloy tool steel having a good inner quality without developing crack by rolling the specific steel composition of C, Si, Mn, P, S, Cr and Fe under a specific draft rate, temp., and a total rolling reduction ratio after executing a specific soaking. CONSTITUTION:Soaking is executed to a steel ingot or slab composed of 0.75-2.50wt.% C, <=1.50% Si, <=2.00% Mn, <=0.030% P, <=0.020% S, 4.0-14.0% Cr and, if necessary, one or more kinds of <=2.50% Mo, <=1.50% V, <=2.00% W and the balance Fe at 1100-1200 deg.C heating temp. for >=1.5min/mm of the thick of steel ingot or slab thereof. Successively, the steel ingot or slab is rolled so as to become at >=10% draft rate per one pass in >=950 deg.C rolling finish temp. and at >=8 in total the rolling reduction ratio (ingot thickness/product thickness). By this method, the alloy tool steel having high quality is rolled without developing inner defect and crack.

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method for rolling alloy tool steel with good internal quality and without cracking. (Prior art and problems to be solved) Alloy tool steel used for gauges, dies, press dies, etc. requires high hardness and wear resistance due to its composition. , it has a composition system containing a large amount of alloying elements such as C and Cr that improve hardenability. In such a composition system, due to the presence of hard and brittle giant Cr carbides that are inevitably generated during casting, the rolled steel sheet may have internal cracks (ultrasonic defects) or
Surface cracking of the steel plate as shown in Figures 1 and 3 is likely to occur. For this reason, when manufacturing steel plates, small steel ingots are used and rolled or forged in small mills, but productivity and yield are low.
Furthermore, it has a drawback in terms of industrial production in that large steel plates cannot be manufactured at low cost. SUMMARY OF THE INVENTION An object of the present invention is to provide a method for rolling alloy tool steel with good internal quality and without cracking during rolling, overcoming the drawbacks of the prior art described above. (Means for Solving the Problems) In order to achieve the stated purpose, the present inventors have conducted intensive research on the material properties and hot rolling conditions of alloy tool steel, and have hereby accomplished the present invention. be. That is, the method for rolling alloy tool steel according to the present invention includes C:
0.75-2.50%, Si: 1.50% or less, Mn:
2.00% or less, P: 0.030% or less, Sho. 020% or less and Cr: 4.0 to 14.0%, and if necessary further Mo: 2.50% or less, V: 1.50%
1100% steel ingot or slab containing one or more of the following and W: 2.00% or less, the balance being Fe.
Steel ingot or slab thickness (mo+) at heating temperature of ~1200℃
Xl, after soaking for more than 5 minutes, the reduction rate per pass is 10
% or less, the rolling end temperature is 950° C. or higher, and the rolling is performed so that the total rolling reduction ratio ((Ill block thickness)/(product thickness)) is 8 or higher. The present invention will be explained in more detail below. (Function) The present invention enables the production of high C-high Cr alloy tool steel by selecting appropriate hot rolling conditions under predetermined composition adjustment of the steel, thereby eliminating internal defects and cracks. The present invention relates to a method for rolling steel sheet products. Traditionally, steel sheets have been processed by 1. after product rolling. Because high hardness and high strength can be obtained through quenching and tempering, it is used in many applications that require wear resistance, such as press molds and gauges. However, as shown in FIG. 2, such steel types have extremely poor hot workability. This is because the hard and brittle Cr carbide produced during the solidification process and its surroundings are susceptible to cracks and voids due to processing (rolling). Conventionally, for these steel types, the processing conditions that do not cause defects have not been clearly understood, and based on experience, manufacturing methods that produce fewer defects, such as small steel ingot → small mill/small press forging, have been used to manufacture steel plates. However, there were drawbacks in terms of productivity, cost, and product size (board width and board length). Therefore, as a result of detailed investigation of the material properties and hot working conditions of this steel type, the present inventors found that if (1) heating conditions, (2) working temperature range, and (3) working rate are appropriately controlled, It has been discovered that the steel plate can be manufactured without defects even in large factory equipment (for example, a several thousand ton rolling mill) where strict heat control is difficult. Next, the reasons for limiting the chemical components and manufacturing conditions in the present invention will be explained. First, the reason for limiting the chemical components is as follows. C: C is an important element for heat refining, and in order to obtain HRC50 or more required for alloy tool steel, 0.75
% or more is required. However, if it exceeds 2.50%, the amount of carbide precipitation will increase and the toughness will deteriorate. Therefore, the amount of C is set in the range of 0.75 to 2.50%. Si: SL is a deoxidizing element necessary to obtain cleanliness of the steel plate. It is also added to improve hardenability.
Addition of more than 1.50% promotes segregation of steel ingots,
1.50% or less. Mn: Mn also has the same effect as Si, but if it exceeds 2.0%, the amount of MnS produced increases and the cleanliness is inhibited.
2.0% or less. P: P segregates at grain boundaries and promotes embrittlement after quenching and tempering, and also causes hot cracking, so it is desirable to reduce it as much as possible, but excessive reduction is not recommended in industrial production. It is desirable to keep the content to 0.030% or less, as this will lead to a rise in costs. S: S is an element that promotes high-temperature cracking more than P, and from the viewpoint of preventing cracking, it must be reduced as much as possible, but it is desirable to reduce it to 0.020% or less so as not to cause a large cost increase in industrial production. Cr: Cr is an element that improves hardenability and is essential for alloy tool steels. Hardenability increases in proportion to the amount added, but if it exceeds 14.0%, embrittlement due to excessive Cr carbide precipitation becomes noticeable, and quality characteristics such as hardness and hardenability that are commensurate with the cost of addition cannot be obtained. Also, 4.0%
If the addition amount is less than that, it is difficult to stably obtain the hardness required for alloy tool steel. Therefore, the Cr content is set in the range of 4.0 to 14.0%. Mo, V, w: Mo, V, and W all improve hardenability and
Since it improves temper softening resistance, it can be added as necessary. When added, these are expensive elements, and excessive addition increases manufacturing costs, so MO
is 2.5o% or less, ■ is 1.50% or less, W is 2.00
% or less. Note that since these elements have similar effects, there is no difference in their effects even if they are added alone or in combination of two or more. Next, the rolling conditions of the present invention will be explained. Riverbed - Regarding the heating temperature of the steel ingot or slab with the above composition, the minimum temperature is 1100℃ for decomposition or solid solution of huge Cr carbides.
Heating above ℃ is required. However, if the temperature exceeds 1200℃, the steel ingot or slab will melt.
．． It is necessary to limit the temperature to a range of 200'C. In order to ensure the shape of the dish-rolled steel ingot or slab and to prevent the occurrence of cracks, it is desirable that there be no temperature difference between the center and the surface of the steel ingot or slab. In order to uniformly heat the surface and center of the ingot or slab, it is necessary to set the furnace atmosphere temperature in consideration of the temperature rise characteristics of the furnace. In particular, for this steel type, in order to decompose and dissolve the Cr carbide present in the center of the steel ingot or slab, it is extremely important to raise the temperature of the center to 1100°C to 1200°C. For this purpose, as a result of temperature measurement experiments conducted by the present inventors, it was found that the heating capacity of a normal industrial furnace is insufficient for steel ingot or slab thickness (mm)
1. Heating for 5 minutes or more is required, and the Cr carbide is thereby eliminated. Note that, from the viewpoint of heating purposes, this heating time condition should be satisfied for both the steel ingot and the slab. (2) The Cr carbide that disappeared during heating by workers also precipitates during rolling, albeit finely, and causes a decrease in ductility. For this reason,
Defects can be prevented by processing at high temperatures when there is a large amount of austenite, which has good ductility. When we investigated the relationship between rolling finishing temperature (rolling completion temperature) and cracking incidence, we found that the occurrence of internal defects could not be prevented unless rolling was completed at 950°C or higher, as shown in Figure 3. Therefore, the working temperature range conditions are from the heating temperature to the finishing rolling temperature of 950° C. or higher. Reduction amount per pass Regarding cracking of steel plates, the amount of reduction per pass has the greatest influence. Cracking occurs because tensile stress is applied to the steel plate in the thickness, width, and length directions of the steel plate during rolling, and the degree of stress is proportional to the amount of reduction per pass. Fourth
The figure shows the relationship between the amount of reduction per pass and the cracks that occur in the steel plate, and shows that it is necessary to suppress the maximum amount of reduction to 10% or less. ■Total reduction ratio Cracks and internal defects in steel sheets occur in and around Cr carbides, but the inability to maintain a balance of strength and toughness with the matrix is also an indirect cause of their occurrence. Therefore, it is necessary to sufficiently crush the coarse columnar crystals generated during casting to improve strength and toughness. To this end, in the case of this steel type, it was found that the total reduction ratio ((I block thickness)/(product thickness)) necessary for crushing columnar crystals must be 8 or more. Next, examples of the present invention will be shown. (Example) Steel having the chemical components shown in Table 1 was melted and cast by a conventional method, and heated and hot rolled under the conditions shown in Table 2. The evaluation results for internal defects and cracks are also listed in Table 2. In Table 2, Nα1 and &2 compare the effects of heating temperature, and the comparative example &2 has a high heating temperature of 1250°C.
In No. 2, the steel ingot melted during heating and blooming was impossible, but in Invention Example Nα1, in which the heating temperature was 1150° C., good product quality was obtained. Moreover, since the heating temperature of Comparative Example Nα4 was as low as 1000° C., internal defects and cracks were generated, whereas inventive example Nα3 had no internal defects or cracks. Nα5 and &6 indicate the difference in the in-furnace time during heating of the steel ingot, and in the comparative example Nα6 of 1720 minutes, which is less than the steel ingot thickness (mm) X 1 , 5 minutes = 1875 minutes, the internal Although the quality was poor, Invention Example 5 of 1950 minutes was good. In Comparative Example Nα8, when the maximum reduction amount per pass was set to 12%, cracks occurred in the steel plate at the end of this one pass. On the other hand, in the present invention example Nα7, the maximum reduction amount per pass was 7.
Since the amount was limited to 5%, rolling was completed without any problems. Comparative Example 10, in which the rolling finishing temperature was 920°C, produced many small cracks around the four peripheries of the steel plate, but the Nα9 of the present invention example was 970.
Because it was finished at ℃, no cracks were observed at all. In Nα11 and Nα12, the product plate thickness is relatively large and the total reduction ratio {((steel ingot thickness)/(product thickness)) is small, but in comparative example N1112, the total reduction ratio {is 7.4. Due to insufficient flaw detection, defects were observed by ultrasonic flaw detection, but no defects occurred in the present invention example Nα11 with a total reduction ratio of 8.9.

[Left below]

(Effects of the Invention) As detailed above, according to the present invention, since alloy tool steel is heated and rolled under appropriate manufacturing conditions with predetermined composition adjustment, no internal defects or cracks occur. It is possible to roll it into Moreover, since high-quality alloy tool steel plates can be manufactured even using large-scale factory equipment, it is economical and its practical effects are remarkable.

[Brief explanation of drawings]

Figure 1 is a diagram explaining the various types of separation that occur in rolled steel sheets, Figure 2 is a diagram showing the high-temperature ductility of alloy tool steel, and Figure 3 is a diagram showing the rolling finishing temperature and internal defect occurrence rate (defect peak value, F1 ≥ 50
Figure 4 is a diagram showing the relationship between the rolling reduction rate per machine and the number of cracks generated. No.]

Claims (2)

[Claims] (1) In weight% (the same applies hereinafter), C: 0.75 to 2.5
0%, Si: 1.50% or less, Mn: 2.00% or less,
P: 0.030% or less, S: 0.020% or less, and Cr
: After soaking a steel ingot or slab containing 4.0 to 14.0% and the remainder consisting of Fe at a heating temperature of 1100 to 1200°C for more than 1.5 minutes x the thickness of the steel ingot or slab (mm), 1. The rolling reduction per pass is 10% or less, and the rolling end temperature is 950℃
The temperature is higher than that, and the total reduction ratio {(steel ingot thickness)/(product thickness)}
1. A method for rolling alloy tool steel, characterized by rolling the alloy tool steel so that it is 8 or more. (2) The steel ingot or slab further contains one or more of Mo: 2.50% or less, V: 1.50% or less, and W: 2.00% or less. Method described.