JPH0665643A - Production of high yield ratio sus420j 1 series 13% cr steel pipe - Google Patents

Abstract

(57) [Summary] [Purpose] By increasing the yield ratio of 13% Cr steel pipe, the rate of re-heat treatment based on strength deviation is reduced. [Structure] Warm sizing (drawing) with a cross-sectional reduction rate of 1 to 2% is applied in a temperature range of 500 to 700 ° C during cooling after tempering. [Effect] With this method, the yield ratio of 13% Cr steel pipe is several%.
These improvements are made, and the target strength range is greatly expanded compared to the conventional method. As a result, deviation in strength is reduced, and manufacturing stability is improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a SUS420J ₁ type 13% Cr steel pipe, and more particularly to a manufacturing technique for a seamless steel pipe capable of improving the yield ratio (yield strength / tensile strength) without impairing toughness. .

[0002]

2. Description of the Related Art SUS420J ₁ type 13% Cr steel pipe (components are C: 0.16 to 0.25% by weight, Si
<1.00%, Mn <1.00%, P <0.040%,
S <0.03%, Cr: 12.00 to 14.00%, with the balance being essentially Fe. Below, SUS420
The expression of J ₁ system 13% Cr steel pipe includes such components. ) Is often used as an oil country tubular goods, and is hardened to satisfy certain strength and toughness +
The material is usually adjusted by tempering. Regarding the strength, the L-80 grade of the API standard specified for low alloy steel is often applied as it is. In other words, the yield strength range is 552-655N / m
m ² and tensile strength of 655 N / mm ² or more are required. On the other hand, from the viewpoint of sour resistance guarantee, the upper limit of hardness (HRC ≦ 2
It is also customary that 3) is requested at the same time. The tensile strength corresponding to this hardness (HRC ≤ 23) is about 805 N / mm
^{Is 2} . These ranges are indicated by hatched areas in FIG. 1 described later.

Therefore, in the production of the steel pipe, care is taken to carefully select the heat treatment conditions, especially the tempering conditions, to control the strength within a narrow range. As a typical example, the tempering conditions for the production of L-80 grade SUS420J ₁ are as follows: tempering parameter [TP = heating temperature × (20 + log holding time)] so that the tempering temperature is 20 to 20.5 × 10 ^3. Time and has been selected. Where the unit of temperature is °
K, that for retention time is hrs.

[0004]

The actual level of the yield ratio is shown in FIG. 1 in the standard strength range of API standard L-80 class and the strength regulation corresponding to the hardness regulation. As is clear from the figure, a low yield ratio means that both the yield strength and the tensile strength can be controlled within a required range at the same time in order to control both strengths within a required range. In other words, despite the fact that the yield strength and the tensile strength are each allowed to have a somewhat wide allowable range of the bending angle, only an extremely narrow range can be used in the production of the steel pipe of the present invention. This often causes the product strength to deviate from the standard, increases the reheat treatment rate for the relief, and causes a big problem in terms of productivity and manufacturing cost.

In order to solve the above problems, it is important to increase the yield ratio of the steel to 80 to 85%. In a steel having a tempered martensitic structure, a method of increasing the yield ratio is to reduce the composition of carbon, Si, Mn, Ni, Mo.
It is a place where it is possible to easily add such as or use precipitation hardening elements such as Nb. However, in the case of the steel, these methods deviate from the standard composition range, deterioration of pipe formability due to δ ferrite formation, It cannot be adopted easily because it involves various problems such as increased costs. The present invention provides a method for producing a high yield ratio SUS420J ₁ series 13% Cr steel pipe without such problems.

[0006]

[Means for Solving the Problems] The present inventors have studied various means for increasing the yield ratio of a 13% Cr steel pipe in the current production line in such a manner that the influence and restrictions on others are minimized.
As a result, they found that warm sizing (drawing) under specific conditions after tempering is effective for improving the yield ratio as a feature of this steel pipe. This method uses a kind of strain age hardening, but in the case of ordinary steel, it is not used as a measure for improving strength because it involves deterioration of toughness. When strain hardening is unavoidable, stress relief annealing after processing is usually required. However, when the present steel pipe is subjected to the above treatment, it is characterized in that only the yield ratio is increased with almost no deterioration in toughness. In other words, the yield strength is much higher than the tensile strength, and the toughness hardly changes as long as the treatment is carried out within the limited range.

On the other hand, the sizing process is a process originally applied for the purpose of ensuring the dimensional accuracy of steel pipe products.
It was carried out before the heat treatment and was never used for the purpose of material adjustment. The metal structure obtained by quenching and tempering this steel is tempered martensite, and the yield ratio in the L-80 grade strength region is about 75%.

According to the present invention, in order to obtain a higher yield ratio in a SUS420J ₁ type steel pipe, 500 to 700 ° C. is used during cooling to room temperature after normal quenching and tempering.
SUS420J ₁ system 13%, which is characterized by imparting warm sizing with a cross-sectional reduction rate of 1% to 2%
A method for producing a Cr high yield ratio steel pipe, particularly a seamless steel pipe. Fig. 2 shows the cross-sectional shrinkage ratio in warm sizing {(cross-sectional area of steel pipe before working-cross-sectional area of steel pipe after working) / cross-sectional area of steel pipe before working} and yield strength, tensile strength, yield ratio, notch toughness. Shows the relationship with. This figure gives the basis of the present invention. The outer diameter of SUS420J ₁ series 13% Cr steel is 143 mm.
It is an experimental result showing a change in material due to warm sizing carried out using a steel pipe of φ × wall thickness 8 mm, and the effect of the present invention is clearly and quantitatively suggested. The heat treatment conditions of the steel pipe at this time were 990 ° C. × 20 minutes AC for quenching, 735 ° C. × 55 minutes for tempering, and a warm working temperature was 600 ° C.

As a matter of course, in the method of the present invention, the tensile strength is increased by about 50 N / mm ² at the maximum as compared with the conventional method. The conditions should be set so that Also, warm sizing does not become a major constraint by utilizing a sizing mill installed in a normal seamless steel pipe production line.

The reasons for limiting the warm sizing conditions of the present invention will be described below. First, warm sizing is performed within a temperature range of 500 to 700 ° C during cooling after tempering, but working in a temperature range of less than 500 ° C results in substantial cold working and lower toughness. May occur, or annealing for stress relief may be required, while the working in the temperature range of more than 700 ° C. reduces the strength improving effect. In addition, processing above 700 ° C cannot be realized unless sizing equipment is installed immediately after the exit side of the tempering furnace. Next, the amount of sizing is closely related to the yield ratio improvement margin. If it is less than 1%, the yield ratio is not improved, and if it exceeds 2%, the yield ratio is sufficiently improved. Is too large, these values are set as the lower and upper limits.

[0011]

EXAMPLES Examples of the present invention will be described below to specifically explain the content of the invention. As a raw material, a seamless steel pipe factory commercially produced SUS420J ₁ component steel pipe (size:
With a nominal outer diameter of 150 mmφ and a nominal wall thickness of 7.8 mm,
For quenching, heating temperature = 1000 ° C, holding time = 15
Min., Natural air cooling, and heat treatment for tempering were uniformly performed as heating temperature = 740 ° C. and holding time = 60 minutes. Then, a sizing mill installed at the rear of the tempering furnace was used to manufacture a sample in which the processing temperature and the processing amount were changed.
A mmV notch Charpy test piece was sampled and its strength and toughness were investigated. The results are shown in Table 1.

[0012]

[Table 1]

Test numbers 1 to 5 are manufacturing methods within the scope of the present invention, and 6 to 11 are comparative methods including conventional methods. The comparative method deviates from the constituent conditions of the method of the present invention in some respects. The results in Table 1 show that the absolute value of tensile strength is slightly increased according to the method of the present invention compared with the material in the comparative method, but the yield ratio is improved by 4 to 11% and the change in toughness is extremely large. It shows that it is very small.

[0014]

As described above, 13 of the SUS420J ₁ system is used.
For the% Cr steel pipe, the manufacturing technique of the present invention, in which warm sizing is applied after tempering within a specific condition range, significantly improves the yield ratio, and according to the conventional manufacturing method, API standard L-80 grade steel is stably manufactured. In contrast to the problem of not being able to do so, it is possible to greatly improve the manufacturing stability by increasing the target range of strength. As a result, the reduction of the reheat treatment rate due to the strength deviation during the steel production is achieved.

[Brief description of drawings]

FIG. 1 shows the strength range of API standard L-80 class and the upper limit strength corresponding to hardness regulation (HRC ≦ 23). The dotted line in the figure shows the yield ratio = 0.75, and the chain line shows the yield ratio of 0. It shows the relationship between tensile strength and yield strength at 80 and 0.85.

FIG. 2 is a table summarizing the relationship between the cross-section reduction rate and the material due to warm sizing in a SUS420J ₁ series 13% Cr steel pipe.

Claims (1)

[Claims] 1. After normal quenching and tempering, warm sizing is applied at a cross-sectional reduction rate of 1% to 2% between 500 and 700 ° C. during cooling to room temperature. SUS420J ₁ series 13% Cr high yield ratio steel pipe manufacturing method.