JPH02270915A - Production of martensitic stainless steel billet for seamless steel pipe - Google Patents

Abstract

PURPOSE:To reduce the amount of delta-ferrite and to prevent the occurrence of eruption flaw in a tube making stage by holding a steel billet before finish rolling in an austenite stabilization temp. region for a specific length of time at the time of producing the steel billet by rolling a cast billet of high Cr steel. CONSTITUTION:A cast billet of a martensitic high Cr steel containing >=8wt.% Cr is rolled into a steel billet. In the above rolling stage, the steel billet is held, prior to finish rolling, at a temp. in an austenite stabilization region for >=1hr. It is desirable to regulate the holding time to 5-15hr in consideration of the size of the steel billet in practical use. Further, in the course of the above holding, holding at a constant temp. is not necessarily required and temp. may fluctuate if it stays within the austenite stabilization region. By this method, the amount of delta-ferrite in the steel billet to be passed through a piercing stage can be reduced, by which the occurrence of eruption flaw formed in a tube making stage can be reduced.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to a method for producing a steel billet for producing seamless pipes of martensitic stainless steel containing 8% by weight or more of Cr. , Specifically, when manufacturing seamless pipes, the steel billet used as the raw material is The present invention relates to a method for manufacturing a steel billet in which the metallurgical structure is adjusted in advance.

(Prior technology) Seamless pipes made of so-called high chromium steel containing 8% by weight or more of Cr are used for oil country tubular goods used in highly corrosive environments, boiler pipes that require heat resistance and corrosion resistance, pipes for chemical equipment, etc. Widely used. In order to efficiently manufacture such high chromium steel seamless pipes, an inclined roll type perforating mill (Mannesmann perforator) or an extrusion type perforating machine is usually used. 11 (press piercing mill) is used. However, in the case of high chromium steel, scratches are particularly likely to occur on the inner surface of the tube during the drilling and subsequent stretching reducing process.

The applicant discovered that the above-mentioned scratches were caused by the δ ferrite present in the center of the material steel piece, and the main idea was to heat and hold the steel piece at a predetermined temperature before drilling to make it into a single phase of austenite. We have previously proposed a method for manufacturing seamless high-chromium steel pipes (Japanese Patent Application Laid-Open No. 134630/1983). According to this method, seamless martensitic stainless steel pipes without internal surface scratches can be manufactured. However, since the above-mentioned heating and holding is performed as part of the tube-making process, the entire tube-making mill including the punching machine must be stopped during this time, resulting in a decrease in production efficiency.

(To be solved by the invention! II!i) The present invention aims to process steel pieces with less δ ferrite by processing them under appropriate conditions in the stage before starting the pipe manufacturing process, that is, in the manufacturing process of the steel pieces that serve as raw materials. By manufacturing pieces in advance, it is possible to manufacture seamless pipes without any flaws without reducing pipe manufacturing efficiency.

(Means for Solving the Problems) The gist of the present invention is a method for manufacturing a martensitic stainless steel piece for seamless pipes as described below.

When rolling a slab of martensitic high chromium steel containing 8% by weight or more of Cr obtained by a continuous casting method or an ingot-forming method to produce a steel billet for seamless steel pipe production, the steel before finish rolling is used. A method for manufacturing a martensitic stainless steel piece for seamless pipes, which comprises holding the piece in an austenite stable temperature range for one hour or more, then adjusting the temperature to a finish rolling temperature and performing finish rolling.

The method of the present invention targets 8% by weight of Cr (hereinafter referred to as
martensitic high chromium steel containing more than
Based on Cr steel, 17% Cr steel, and these steels, Mo: 0.3-1.5%, Ni: 1.0%
Below, Nb: 0.02-0.15%, V: 0.
It may be a steel type containing one or more types such as 001 to 0.20%. In terms of standard materials, they are stainless steel pipes for machine structures such as SUS 410T and A, and 5tlS 420JI TKA.

The above-mentioned steel is melted by a conventional method, made into a slab by a continuous casting method or an ingot-forming method (mold casting method), and then rolled into a slab. A feature of the method of the present invention is that before finish rolling in this rolling step, the material is maintained at a temperature in the austenite stability range for a short period of time (at least 1 hour).

FIG. 1 is a heat pattern diagram of the method of the present invention when seamless pipes are manufactured using continuous casting bloom in Examples described later. As shown in FIG. The austenite stability region is, for example, shown in Figure 2 (Bungard
t et al, ^rchiv fur EisenhuLten
Wesen 29 (195B) 197). Therefore, although the temperature range varies depending on the composition of the target steel, especially the Cr content, it is safe to set it in the range of approximately 950''C to 1250°C in the case of 13% Cr1i.

The longer the holding time, the better; however, unnecessarily extending the holding time will only lead to scale loss and increased heating costs. Considering the practical size of the steel billet, it is recommended to hold it for at least 1 hour, preferably 5 to 15 hours. Note that it is not necessarily necessary to maintain the temperature at a constant temperature during this holding period, and it is not necessary to maintain the temperature in the austenite stability range. It goes without saying that even in the austenite stable region, where there is no problem with temperature fluctuations, a higher holding temperature is more effective in terms of diffusion rate.

It is known that ingots are heated and held at a constant temperature for the purpose of diffusing segregated components, such as in soaking treatment, but usually, in order to maximize the diffusion effect,
Maintain at high temperature, such as above 250°C. However, according to the test results of the present inventors, in the conventional high-temperature high-speed diffusion, the growth and persistence of ferrite grains due to heating exceeding the temperature of the austenite stable single-phase region is caused by subsequent blooming and rolling. Adversely affects the seamless pipe drilling process. That is,
If the steel is held at a temperature above the austenite stability range before finish rolling, defects such as cracks will occur due to deterioration of hot workability due to the generated δ ferrite. Furthermore, when drilling a steel billet after finishing rolling, a center-cover flaw occurs due to δ ferrite.

For the above reasons, even if the diffusion rate is small, it is desirable to maintain the austenite in the stable austenite region (austenite single phase region) where ferrite does not occur. Although the temperature is increased to a predetermined temperature for the purpose of achieving this, the time of exposure to temperatures outside the austenite stability range should be kept to the minimum necessary.

After finish rolling, the rope pieces are sent to the pipe-making process and become raw materials for seamless pipes. Even if pipe manufacturing is performed using a conventional method, a product with extremely low overburden pressure can be obtained. However, the pipe manufacturing method described in JP-A No. 63-134630 proposed by the applicant mentioned above, that is, the steel slab is held at an angle of 950 to 1150 degrees before drilling.
If a method is adopted in which the hole is held in the temperature range C for 30 minutes or more and then adjusted to a predetermined temperature before drilling, the prevention of scratches will be more reliable.

Hereinafter, the present invention will be explained in more detail with reference to Examples.

(Example) A bloom of 13% Cr martensitic stainless steel produced by a continuous casting method was rolled into a round steel piece using the heat pattern shown in FIG. The composition of the above steel is shown in Table 1.

The rolling includes rough rolling heated at 1280°C for 7 hours, and rolling at 127°C.
A process (a) in which finishing rolling is performed at 0°C for 4 hours, followed by heating and holding at 1100°C for 4 hours before finishing rolling (a), and a process in which this heating and holding is not carried out (b)
) was carried out under two conditions.

The 187m5+φ round steel piece obtained by the above rolling process was sent to a seamless pipe manufacturing process, and 73m-φX was processed using the Mannesmann method.
A tube of 5.51 mm t (wall thickness) was manufactured. At that time, the drilling process was carried out under the following two conditions (C) and (d) (first
(see figure).

(C) Before drilling, heat and hold (2) at 1100'CX for 5 hours, then heat to 1180'C and drill (method described in JP-A-63-134630).

(d) Drilling by heating to 1180'c without performing the above heating and holding.

After the above-mentioned perforation step, the diameter was reduced using a sizing mill, stretch rolling was performed using a mandrel mill, and after reheating, the material was reduced and rolled using a stretch reso-acing mill to obtain a predetermined outer diameter and wall thickness.

Table 2 shows the measurement results of the amount of δ ferrite (volume %) by microscopic observation of the steel slab after the completion of the rolling process and the steel slab immediately before perforation.

Table 3 shows the results of a visual UST inspection of the final product, the seamless pipe, to determine the incidence of internal damage. The occurrence rate of internal damage is the ratio (percentage) of the number of cracked pieces to the total number of pieces inspected.

As seen in Table 2 and Table 3, in the steel slab treated in (a) to maintain the austenite stability range before finish rolling, the δ ferrite content was significantly reduced to 0.2%. When drilling holes using this steel piece as a material, even if using the normal drilling method, the occurrence rate of center wear flaws is as low as 9.8% ((a) + (d) in Table 3). , before peeling and drilling 110
When holding at 0°C ((a) in Table 3)
+ (C)) has a - layer effect, and the incidence of middle wear flaws has decreased to 3.6%.

(Effects of the Invention) According to the rolling method according to the present invention, in which the steel slab before finish rolling is held in the austenite stability region for a certain period of time, the amount of δ ferrite in the steel slab to be subjected to the drilling process can be reduced, thereby making it possible to It is possible to reduce the smear spots that occur during the process. As mentioned above, the method of the present invention and JP-A-63-1
Even more remarkable effects can be obtained by combining the pipe-making process shown in Publication No. 34630, but in cases where you want to avoid mill line suspension, even if pipe-making is performed under conventional drilling conditions, there will be no significant damage to the core. It is possible to reduce

[Brief explanation of drawings]

FIG. 1 is a heat pattern diagram showing an example of the method of the present invention. FIG. 2 is a phase diagram of the Fe-Cr-C system when C is 0.1%.

Claims (1)

[Claims] When rolling a slab of martensitic high chromium steel containing 8% by weight or more of Cr obtained by a continuous casting method or an ingot-forming method to produce a steel billet for seamless steel pipe production, the steel before finish rolling is A method for manufacturing a martensitic stainless steel piece for seamless pipes, which comprises holding the piece in an austenite stable temperature range for one hour or more, then adjusting the temperature to a finish rolling temperature and performing finish rolling.