JPH0277519A - Method for annealing ms-based stainless steel and cr-mo steel - Google Patents

Abstract

PURPOSE:To effectively reduce delta-ferrite in a short time by soaking an Ms-based stainless steel and a Cr-Mo steel at a precified temp., transiently cooling the steel, then again soaking the steel at a specified temp., and annealing the steel. CONSTITUTION:The bloom or billet of an Ma-based stainless steel and a Cr-Mo steel is soaked at >=1200 deg.C preferably at about 1200-1300 deg.C. The steel is then cooled until the surface or center of the steel is cooled to <=1200 deg.C, or preferably to about 1100 deg.C. In this case, the steel is preferably cooled at a rate of >= about 10 deg.C/min. The steel is then again soaked at 1000-1200 deg.C, or preferably at about 1100 deg.C. The delta-ferrite in the steel is effectively reduced by this soaking and diffusion annealing method. When the steel is formed into pipe by the Mannesmann process, the cracking of the inner surface of the pipe due to the delta-ferrite at the center of the material is prevented.

Description

[Detailed Description of the Invention] Industrial Application Field This invention is a method for preventing internal defects in the pipe caused by δ ferrite in the center of the material when manufacturing Mannesmann pipes from Ms stainless steel and CrMo steel billets. This invention relates to a bloom or billet annealing method.

BACKGROUND OF THE INVENTION When manufacturing Mannesmann pipes from Ms stainless steel and Cr-Moil billet, internal defects may occur due to δ ferrite in the center of the material. In order to prevent this pipe internal flaw, a method of reducing δ ferrite is effective.

The following two methods are conventionally known as methods for reducing this δ ferrite.

(A) Long-time soaking diffusion annealing at around 1100℃ (8
) Reduction of Cr content However, these two methods had the following drawbacks.

(As shown in the Cr distribution near δ ferrite shown in Figure 1, A> is determined by the Cr diffusion rate in γ, and in order to reduce the amount of δ ferrite to a level where the product quality is good, it is necessary to As is clear from the relationship between the heating time and the amount of δ ferrite shown in the figure, it is necessary to heat for a long time.

That is, in heating at 1100° C. and 1200° C., the amount of δ ferrite decreases with the passage of time, but it takes a long time.

(B) is undesirable because it reduces the rust prevention effect, which is an excellent characteristic of stainless steel.

Problems to be Solved by the Invention In view of the above-mentioned actual situation, the present invention has been developed to provide a method for effectively reducing δ ferrite in a short time using a soaked diffusion annealing method without reducing the Cr content. This is what we are trying to propose.

Means for Solving the Problems The inventors have conducted various studies on methods for reducing δ ferrite in Ms-based stainless steel and CrMo steel using a soaked diffusion annealing method, and have come up with the following findings.

In the case of Ms-based stainless uA and Cr h:= steel, as shown in Figure 3, when heated above 1200°C, the amount of δ ferrite increases in a short time and reaches about 12%, but
As shown in FIG. 4, the (, r14 degrees) in ferrite is considerably low.

That is, even if the amount of δ ferrite is large, the temperature is high from the viewpoint of atomic diffusion, and it can be said that heating at 1200'C or higher is effective because it adds a large amount of δ ferrite phase, which has a high diffusion rate.

However, heating to 1200'C or higher alone causes a large amount of δ ferrite to remain, causing internal cracks in the pipe.

Therefore, after heating and holding at 1200'C or higher,
We have discovered that if the steel is cooled to a temperature below, the δ→γ transformation can proceed and the amount of δ ferrite can be reduced.

That is, the gist of the present invention is to first heat and hold Ms stainless steel and Cr hoWA at a temperature of 1200'C or higher in the first step, and then bond the surface or center of the
Cool down to a temperature below 00°C, and then heat again at 1°C in the second stage.
The gist is to heat and maintain the temperature at a temperature of 000°C or more and 1200°C or less.

Here, the components of the Ms-based stainless steel and Cr-city steel targeted by this invention include C0.50 wt% or less,
hn 2.00wt% or less.

si 1. OOW[% or less, P 0.20wt% or less.

S 0.20wt% or less, Cr 4.0-20.0
wt%.

Contains.

Effect δ in Ms stainless steel and Cr Mo14
The behavior of ferrite is dominated by Cr diffusion.

Therefore, the temperature needs to be set according to C and behavior.

FIG. 5 is a diagram showing the relationship between Cr concentration and temperature.

That is, the degree of CTa in the δ ferrite when heated at a high temperature must be completely in the austenite-phase region when heated at a low temperature (here, 1100° C.).

Therefore, in this invention, the heating holding temperature in the first stage is set to 12
00° C. or higher, then cooled to a temperature of 1200° C. or lower, and then heated and maintained at a temperature of 1000° C. or higher and 1200° C. or lower in the second stage.

In other words, the reason why the heating holding temperature in the first stage was limited to 1200°C or higher was to keep the Cr concentration in the δ-ferrite sufficiently low to prevent the δ→γ transformation during the low temperature (1000°C to 1200°C) heating in the second stage. It is used to raise the vine and keep it in a vine state.

Further, after being heated and maintained at a temperature of 1200° C. or higher, the surface or center portion is cooled to a temperature of 1200° C. or lower, and in a second stage after cooling, it is heated and maintained at a temperature of 1000° C. or higher and 1200° C. or lower. This is the amount of Cr in δ in the first stage! This is to allow the δ→γ transformation to proceed once the degree of oxidation is sufficiently lowered.

Note that when the cooling rate is low, the Cr distribution changes according to the equilibrium of each temperature, so that Cr concentration occurs again at 1100° C., and the effect of reducing the amount of δ ferrite is small. Therefore, the cooling rate is set to a rate that can maintain the Cr of 11 degrees lowered by high-temperature heating.

Figure 6 is a diagram showing the relationship between the cooling rate and the amount of δ ferrite, and in order to reduce the δ ferrite to a level where the product is good, it is necessary to cool at a cooling rate of at least 10"C/min or more. .

An example of the process of this invention is shown in FIG.

That is, the method of this invention can be applied to Ms stainless steel and C
r MalM billet or bloom is heated and maintained at 1200°C or higher, preferably 1300°C in a soaking furnace (1), and after extraction, the surface or center of the material is 1200°C or lower,
Preferably, it is cooled to 1100°C. After that, it is charged again into the soaking furnace (2) and heated to 1000'C or more and 1200'
It is heated to a temperature below 1,100'C, preferably 1100'C, and sent to the extraction and recovery pipe line.

Although this soaking diffusion annealing treatment can be carried out at the blooming stage before billet rolling, it is preferably carried out at the stage of small-diameter billets where it is easy to maintain a sufficient cooling rate as much as possible.

Example A small-diameter billet of Ms-based stainless steel having the components shown in Table 1 was heated at 1300°C for 2 hours, and then air-cooled at a cooling rate of 30' (::/min) until the billet center became 110°C.
After cooling to 0°C and then heating again at 1100°C for 1 hour, the incidence of internal flaws in the pipe produced was calculated as follows:
A comparison is shown in FIG. 8 with a conventional 1100'CX 10 hour heat treatment.

As is clear from Figure 8, the incidence of internal pipe flaws is the same or lower than that of the conventional method, so short-time soaked diffusion annealing treatment can achieve quality equivalent to or better than conventional long-time annealing treatment. I was able to get it.

As described in detail, according to the method of the present invention, the long-term heat treatment that conventionally required 10 hours or more to reduce δ ferrite in Ms-based stainless steel and Cr-Mo steel can be reduced to about 3 hours. It is possible to effectively reduce δ ferrite in a short period of time, which has a great effect on improving quality and productivity.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the Cr concentration distribution in the billet according to the conventional method (A), Figures 2 and 3 are diagrams showing the relationship between the heating temperature and heating time of the billet, and the amount of δ ferrite, and Figure 4 is the diagram according to the invention. Figure 5 is a diagram showing the relationship between the Cr concentration in the billet and temperature, Figure 6 is a diagram showing the relationship between the cooling rate of the billet and the amount of δ ferrite, and Figure 7 is a diagram showing the relationship between the billet cooling rate and the amount of δ ferrite. FIG. 8 is a block diagram showing an example of a process for carrying out the method of this invention, and FIG. 8 is a diagram showing the relationship between the amount of Cr and the rate of occurrence of internal defects in a pipe in an embodiment of the invention. 1.2 Soaking furnace Applicant Sumitomo Metal Industries Co., Ltd. Agent Patent attorney Yoshihisa Oshida Fig. 5 Fig. 6 Cooling rate (°c, forward,.)

Claims (1)

[Claims] Ms stainless steel and Cr-Mo steel at 1200℃
After heating and holding at a temperature above, the surface or center becomes 1
Cool it down to 200℃ or less, then cool it again to 1000℃ or higher1.
A method for annealing Ms-based stainless steel and Cr-Mo steel, characterized in that δ ferrite is reduced by heating and holding at a temperature of 200° C. or lower.