JPS61227127A - Method for hot working high ni steel - Google Patents

Abstract

PURPOSE:To inhibit cracking and to improve the yield by slowly heating a high Ni steel ingot contg. a specified amount of Ni at a specified heating rate or below in a specified surface temp. range when the ingot is heated. CONSTITUTION:The composition of a high Ni steel ingot is composed of, by weight, 35-50% Ni, <1% Mn, <1% Si, <=0.05% C and the balance Fe with inevitable impurities. When the ingot is heated, it is slowly heated at <=70 deg.C/hr heating rate in the surface temp. range of 300-500 deg.C. In the other temp. range to the forging temp., the ingot is heated at the optimum economical heating rate. The heated ingot is forged at <=4% draft per one pass until the total draft attains to at least 20%.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention deals with high Ni wI4@lump heat +1 which has poor hot workability.
The present invention relates to a method for suppressing the occurrence of cracks in fl processing, improving yield, and improving productivity.

(Prior art) It is well known that high-Ni steel ingots containing about 30% or more of Ni have poor hot workability and are prone to cracking on the surface during rolling. Although the heating temperature increase rate, processing start temperature, rolling reduction rate, processing speed, final temperature, etc. are very strictly controlled during interworking, cracks still often occur and the yield is reduced.

For example, in the case of a 5 ton ingot containing 40% Ni, the maximum surface temperature increase rate is 100°C 7'hr until it reaches 1200°C.
Slowly heated over 0 hours, with a total processing degree of 20
%, the reduction rate per pass is 3%, which is extremely small, and then the 1-pass reduction rate is increased to 5%, and when the surface temperature reaches 950℃, processing is stopped and reheated. This was an extremely inefficient process.

Conventional methods require time for heating and processing, so
Thermal efficiency and productivity were extremely low.

(Problems to be Solved) As described above, the conventional methods have extremely poor thermal efficiency and productivity, and often cracks occur during processing, reducing yield.

Therefore, it has been desired to develop a processing method that is more productive and less likely to cause cracks.

(Means for Solving) The present invention has been made to solve the above problems, and its gist is N i 35J! r-50%,
When heating a high Ni steel ingot consisting of Mn 10% or less, 8iLO% or less, C60,05x and other unavoidable impurities, and the balance being Fe, the surface temperature of the ingot should be at least 300°C or higher.
500°C at a heating rate of 70'C/hr or less, the rest of the temperature range is optimally economical, heating at a heating rate up to the rolling temperature, and then the total working degree is at least 2.0 This is a rolling method for high Ni @ @ lumps, which is characterized by rolling at a rolling reduction rate of 4% or less in one pass until reaching 4%.

Next, the reason for limiting the steps as described above in the present invention will be described.

Regarding components: The reason why the components in the present invention are limited as specified in the claims is that the present invention can be applied to achieve significant effects within at least this range.

Raise the temperature at a slow rate. This is the result obtained by analyzing a number of conventional temperature rise curves and rolling cracking results obtained by actually processing the target steel (Fig. This was difficult to avoid no matter how careful we were during rolling. The reason for this is not clear, but it may be related to the behavior of the Fe-Ni intermetallic compound during this time. 300 If the temperature is below ℃ or above 500℃, even if the temperature is raised at the normal maximum economical heating rate, cracking can be avoided if the forging process is carried out carefully.

Regarding initial machining degree: In the present invention, initial machining (machining degree up to 20%) is regulated to a reduction rate of 4% or less per pass, but
Although this is a significant improvement over the conventional machining rate of less than 3% per pass, as shown in Figure 2.

It can be seen that if the degree of processing per bus is 4% or more, the probability of cracking increases and must be avoided.

(Function) In the present invention, when heating an iron alloy ingot containing 35,150% Ni, heat is removed at a rate of 70°C/hr or less especially in the range of surface temperature from 300°C to 500°C, thereby reducing the amount of Fe generated therebetween. It is imagined that -N i has some favorable influence on the precipitation behavior of the compound.

(!l!Example) A 5 ton ingot of the components shown in Table 1, including N+4096, was placed in a forging heating furnace fueled by light oil and heated to 300°C at a surface temperature increase rate of 600°C/11r, and then heated to 500°C. The ingot was heated slowly at a rate of about 50°C/hr until 1000°C, then rapidly heated to 1000°C at a surface heating rate of about 250°C/11r, and further heated to 1200°C for 8 hours including soaking. . Next, the heated ingot was removed from the kettle and heated to 3.0
With OOL high-speed forging press, the reduction amount per ゛ (l bath) is 2.
011j (equivalent to an average reduction rate of 3%) The total reduction amount is 18 (
m (total reduction ratio of 1g96), the amount of reduction per round was increased to 3Q*g (equivalent to an average reduction ratio of 6%), and from then on, the total reduction ratio of 36g was forged until the surface temperature reached 950℃. When the temperature reached 950°C, it was returned to the heating furnace for reheating. During this processing, no cracks, especially at the corners, or surface cracks, which have conventionally occurred, occurred. In addition, the total heating time was shortened to 15 hours in this example, compared to 20 hours in the conventional case, and productivity and thermal efficiency were improved accordingly.

(Effects) By implementing the present invention, the crack failure rate in forging high Ni steel has been reduced from about 70% in the past to IO, s%, the productivity has been reduced to about 40%, and the heating unit has been reduced to This has improved to 25%.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the relationship between the heating rate of the ingot, the surface temperature of the ingot, and rolling cracks, and FIG. 2 is a diagram showing the relationship between the total working degree, the rolling reduction rate per bath, and rolling cracks. Figure 1 Figure 2 110n1! L

Claims (1)

[Claims] Ni 35% to 50%, Mn 1.0% or less, Si 1.0%
Hereinafter, when heating a high Ni steel ingot that contains C≦0.05% and other unavoidable impurities and the balance is Fe, the surface temperature of the ingot is at least 300°C to 500°C at 70°C/hr.
Raise the temperature at the following heating rate, and for the rest of the temperature range, raise the temperature to the rolling temperature at the optimal economic heating rate, then reduce the reduction rate of 1 bath to 4% or less until the total degree of reduction reaches at least 20%. A method for forging a high-Ni steel ingot, which is characterized by forging a high Ni steel ingot.