JPH01293909A - Manufacture of seamless steel tube containing chrome - Google Patents

Abstract

PURPOSE:To restrain generation of flaws on the tube inside surface by specifying both a shape and a setting position of a plug for piercing a round billet by use of inclined rolls and a plug. CONSTITUTION:A continuously cast billet containing, by wt., 2-27% Cr and balance Fe with inevitable impurities is used as a stock because inside surface detects are easily generated when a seamless steel tube having Cr as addition agent to improve wear and high temp. oxidation resistances is pierced and rolled without changing conventional sectional shape. A continuously cast billet having a rectangular section whose ratio of a long side to a short side is 1.6-3.0 is formed into a round billet 1 with normal rolling. For piercing the billet 1 by use of inclined rolls 2 and a plug 3, an absolute value ¦1epsilonn¦1 of a strain epsilonn during piercing defined by an equation I is set that ¦1epsilonn¦1<=0.015 from a position of 10mm behind from the top of the plug 3 toward the rear end of the plug 3. After that, a seamless steel tube free of inside surface flaws is manufactured by mandrel mill rolling and hot outside diameter elongation work.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a method for manufacturing a Cr-containing seamless steel pipe containing 2% or more of Cr by weight, and particularly to a boiler.

2 1/4 Crlliil is used in a wide variety of applications such as oil wells, gas wells, chemical plants, and even mechanical structures.
SIJS 304.316.321 and other methods for manufacturing seamless stainless steel pipes.

<Conventional technology> Seamless steel pipes are produced by perforating a solid round Vilent using inclined rolls and plugs, and then rolling using a mandrel bar called the mandrel mill rolling method, which consists of three hole-shaped rolls. It is formed by a process that combines diameter reduction and stretching using a hot drawing device, or it is rolled using two hole-shaped rolls and a plug called the plug mill method, a milled tube is formed using an inclined roll and a plug, and two hole-shaped rolls. It is generally formed by diameter reduction processing.

The solid round billet that is used as this material is currently manufactured using a continuous casting method, and the method is to roll a rectangular material (hereinafter referred to as bloom) into a round shape. There are two ways to do this: , or use a round material (hereinafter referred to as round billet) as cast.

When manufacturing seamless steel pipes using the above method, ordinary carbon steel or low alloy steel can be made with almost no problems, but C
In the case of high Cr materials with an r content of 2% or more by weight,
Continuously cast plumes or round billets tend to have larger cavities in the center compared to ordinary steel or low alloy steel, which remain as they are in the case of round billets, and in the case of plumes, they are not sufficiently crimped during rolling, resulting in When drilling a hole using a filled round billet, a large gap is created in the center of the billet due to a rice crack caused by the inclined roll (hereinafter referred to as Mannesmann crack) before the hole is drilled by the plug, and this part is separated by the plug and the inclined roll. Even after rolling, internal defects often remain as defects.

Due to Mannesmann cracking during drilling, conventional Cr
It is considered difficult to drill holes in continuously cast materials with a high content without defects, so steel ingot materials that are less likely to form cavities in the center have been used. However, with recent advances in continuous casting technology, the occurrence of cavities in the center has been significantly suppressed, so continuous casting materials are now being used. case,
- A fairly high rate of internal defects is unavoidable in boat fishing, and these must be removed through maintenance to create the final product.

For example, Japanese Patent Application Laid-Open No. 140301/1984 discloses a method for preventing inner surface defects during drilling when continuous casting material is used when manufacturing such a seamless steel pipe with a stainless steel composition using the Mannesmann rolling method. ing.

The contents are: C: 0.32wt% or less, N: 0
．． Due to the stainless steel composition limited to 4wt% or less, the round billet for seamless stainless steel pipes to be subjected to the inclined rolling method pipe manufacturing process has a rectangular cross section that is more than three times the cross-sectional area of the round billet, and the long and short sides of the round billet are length ratio is 2
．． 5 or more, the length of the long side is 1100 mm or less, and the length of the short side is at least 40 mm compared to the outer diameter of the round billet to be made. Through hot rolling under conditions of 120mm/pass or more, a plume with a rectangular cross section is created with a short side that is at least 30 mm larger than the outside diameter of the round billet.This bloom is then hot rolled to form a round billet. This is a method for producing a round billet for seamless stainless steel pipes using an inclined rolling method, which is characterized by processing it into a round billet for seamless stainless steel pipes.

<Problems to be Solved by the Invention> However, when a round billet is manufactured by the method shown in JP-A-61-140301, defects on the inner surface of the tube during drilling can be prevented, but defects on the long side length a
Since the lower limit of the ratio a/b of the short side length b is as large as 2.5, the problem of buckling during width reduction during round billet rolling remains completely unresolved. There is a problem in that the dimensions of the manufactured round billet are limited because there is a limit to the rolling reduction ratio when rolling into a round billet.

An object of the present invention is to provide a method for manufacturing a Cr-containing seamless steel pipe that solves the above problems.

<Means for Solving the Problems> In order to solve the above problems, the present inventors have conducted extensive experiments and research, and have found that the strain distribution in the material to be drilled due to the inclined rolls and plugs during drilling is such that the defects on the inner surface of the tube It was discovered that this has a great influence on the occurrence of , and based on this knowledge, the present invention was completed.

That is, the gist of the present invention is that the weight ratio of Cr
: 2 to 27%, and the balance is Fe and unavoidable impurities.
Using a continuously cast slab having a rectangular cross-sectional shape of 0,
This slab is made into a round billet by normal rolling, and when this round billet is perforated with an inclined roll and a plug, the absolute value of strain εn during perforation defined by the following formula is 1ε,
1 at a position 10 mm from the tip of the plug toward the rear end of the plug. ．． A method for manufacturing a seamless steel pipe containing Cr, characterized in that 1≦0.015, εn-m(1,/1.,-+) where, after the plug from a position 10 mm from the tip of the L-1' plug. Thickness (mm) at an arbitrary position WP, -1 toward the end Lゎ Thickness (mm) at a position P0 1 mm away from position Pn-1 toward the rear end of the plug.

<Function> FIG. 1 is a schematic diagram illustrating the definition of strain during drilling of a material to be drilled.

In the figure, a solid round billet 1 is rolled onto a pair of inclined rolls 2.
, 2 and the plug 3, the position Hp of the tip 3a of the plug 3 on the pass line PL. The wall thickness when drilling is t. Assuming that the wall thickness at two arbitrary positions Pl+-1 and P at an interval of 1 mm on the pass line PL is tII-It th, then the position P. The strain ε in is defined by the following equation (1).

ε, = 1. (t, / tII-1)
--- (+) According to experiments conducted by the present inventors, the maximum strain amount of the absolute value 1εn1 of strain εn at all positions on the path line PL from the tip 3a to the rear end 3b of the plug 3 is the inner surface of the tube. It was found that there is a deep causal relationship with the occurrence of defects. In other words, by optimizing the plug shape and settings during drilling, the ratio of the long side to short side of the continuous casting material can be reduced to 1.
．． If it is 6 or more, it is related to the rolling reduction ratio during round billet rolling (
It was confirmed that it is possible to manufacture seamless steel pipes containing 2 to 27% Cr with a significantly low incidence of internal defects.

The reasons for limiting the Cr content in the present invention will be explained below.

Cr is added in a wide range to improve corrosion resistance, high-temperature oxidation resistance, and high-temperature hydrogen attack resistance.The addition of Cr makes it possible to use continuous casting materials with conventional cross-sectional shapes and perforate them using the conventional Mannesmann rolling method. When rolled, internal defects are likely to occur in seamless steel pipes, but if it is less than 2%, no internal defects will occur even with conventional cross-sectional shapes and drilling methods, so there is no need to apply the present invention, and 27
%, thermal stress cracking that is different from cavities tends to occur, and this defect cannot be prevented by the present invention, so it is limited to a range of 2 to 27%.

Next, the ratio a of the long side a to the short side of the continuous casting rectangular cross-sectional material
The reason for limiting /b will be explained.

Figure 2 shows a weight ratio of C: 0.20% and Si: 0.4.
5%, Mn: 0.50%, Cr: 13.1%
, P: 0.018%, S: o, oo 2%, the balance is Fe and unavoidable impurities, using a continuously cast rectangular cross-sectional material with varying a/b, and rolled to a diameter of 11.
The absolute value of εn defined by the above formula (1) is 1ε at all positions from a position 10 mm from the tip of the plug toward the rear end of the plug during drilling with an inclined roll and a plug as a round billet with a diameter of 0. , a / b when the maximum value of 1 is 0.0139, and then a seamless steel pipe with an outer diameter of 42.7 + nmφ and a wall thickness of 4.5 mmt is manufactured by mandrel mill rolling and hot outer diameter drawing and stretching. This figure shows the relationship between the internal defect incidence rate.

First, the lower limit of a/b was limited to 1.6 because of the second
As shown in the figure, under the drilling conditions of the present invention, a/b is 1.
If it is 6 or more, the occurrence of internal defects will be drastically reduced, but if it is less than 1.6, internal defects will occur frequently.

Next, the upper limit of a/b was limited to 3.0 because, as is clear from Figure 2, the effect of a/b on internal defects is saturated at 2.4, and when a/b is 3.0. If the billet exceeds 100%, wrinkles due to buckling are likely to occur at the center of the width when the long sides are rolled down in width in the step of rolling the billet into a round billet.

For the above two reasons, a/b was limited to a range of 1.6 to 3.0.

Furthermore, when a round billet is perforated by an inclined roll and a plug, the absolute value of the strain ε during perforation, 1ε0) defined by the above formula ( ), is from a position 10 mm from the tip of the plug,
161≦0.015 at all positions toward the rear end of the plug
The reason for limiting the value to 0 is 1ε as shown in Figure 3.
From l≦0.0150, the incidence of tube inner surface defects decreases dramatically, and 1
This is because when εl >0.015, the incidence of internal defects is high.

Here, in Fig. 3, C: 0.20%, Si+
0.45%, Mn: 0.50%, Cr: 1
3.1%, P: 0.018%.

A continuous casting material with a/b = 1.87 containing S: 0.002% and the balance Fe and unavoidable impurities was rolled into a round billet with a diameter of 110 mmφ.
During drilling with the inclined roll and the plug, the maximum value of the absolute value I8゜1 of εn defined by the above formula (1) at all positions from the position 10+mn from the tip of the plug toward the rear end of the plug is changed. After that, it was made into a diameter of 42.7 by mandrel mill rolling and hot outer diameter drawing and stretching.
This figure shows the relationship between a/b, the internal defect occurrence rate, and the maximum value of 18°1 when a seamless steel pipe with mmφ and wall thickness of 4.5 mmt was manufactured.

In addition, the position at which the maximum value of 1ε□1 occurs during drilling is limited from 10 mm from the tip of the plug toward the rear end of the plug because the temperature during drilling is high at a position within 10 mm from the tip of the plug. This is because even if the strain in the range is quite large, it will not lead to the occurrence of internal defects.

However, from a position 10 mm from the tip of the plug to the rear end of the plug, the temperature of the material to be drilled during drilling decreases due to heat removal to the plug, and the deformability deteriorates, so distortion in this range is limited by this patent. within the range of distortion
That is, 1ε, l≦0.0150.

<Example> Examples of the present invention will be described below.

After continuous casting of the material with the chemical composition shown in Table 1 into a rectangular or round steel piece with a/b shown in Table 2, the rectangular cross section is rolled, and the round cross section is left as cast. A billet material for seamless steel pipe with a diameter of 110 nu++φ is used, and when this billet is perforated with an inclined roll and a plug, the plug is inserted from a position 10 mm from the tip of the plug where the absolute value of strain εn defined by the above equation (1) is 1εn1. Holes were drilled so that the maximum value at all positions toward the rear end was the value shown in Table 2, and then, by mandrel mill rolling and hot outer diameter drawing and stretching, the outer diameter was 42.7 mm.
A seamless steel pipe with φ and wall thickness of 4.5 mmt was manufactured.

Table 3 shows the shape of the plug used during drilling, the settings, and the maximum value of 1εnI mentioned above. Each dimension shown in Table 3 is schematically shown in FIG.

Table 2 also shows the incidence of internal defects caused by cavities and segregation that occurred in these seamless steel pipes during continuous casting.

Among the preferred conditions of the present invention, the maximum value of 1εn1 satisfies the range of the present invention, but a/b is outside the range of the present invention as Comparative Example I, and a/b is within the range of the present invention. However, those in which the maximum value of 18n1 was outside the range of the present invention are shown as Comparative Examples (■) in Table 2.

As is clear from Table 2, all of the examples of the present invention in which both the cross-sectional shape of the continuously cast steel slab and the maximum value of 18 nl are within the range of the present invention have significantly smaller inner defects due to cavities and segregation. I know that there is.

Therefore, even if the reduction ratio is within the range of the reduction ratio that was determined to be defective due to tube inner surface defects in the above-mentioned Japanese Patent Application Laid-Open No. 61-140301, the inner surface defects can be reduced to a level where there is no problem at all by implementing the method of the present invention. I can see that it is.

21] <Effects of the Invention> As explained above, according to the present invention, the Cr content is 2
When drilling ~27% round billets, it is possible to suppress defects that occur on the inner surface of the tube.

Therefore, it is possible to provide seamless steel pipes at low manufacturing costs for energy-related applications such as boilers, oil wells, gas wells, and chemical plants, as well as steel pipes for corrosive fluid transportation and mechanical structural steel pipes, for which demand is expected to increase significantly in the future. The industrial value of the present invention, which also makes this possible, is great.

[Brief explanation of the drawing]

Fig. 1 is a schematic diagram explaining the definition of strain during drilling of the material to be perforated used in the present invention, and Fig. 2 is a schematic diagram illustrating the internal defects caused by cavities and segregation of the continuous casting material during piercing and rolling of seamless steel pipes. Figure 3 shows the effect of the cross-sectional shape of the continuous casting material on the incidence of internal defects caused by cavities and segregation during piercing and rolling of seamless steel pipes.
B, FIG. 4 is a characteristic diagram showing the influence of the maximum value of l, and is a schematic diagram showing the shape of the piercer plug and the dimensional relationship of each part. 1... Round billet 2... (slanted roll. 3... Plug. Patent applicant Kawasaki Steel Corporation

Claims (1)

[Claims] When manufacturing a seamless steel pipe using a continuously cast slab as a material containing 2 to 27% Cr by weight, with the remainder consisting of Fe and unavoidable impurities, this material has long and short sides. When a continuously cast slab having a rectangular cross-sectional shape with a side ratio of 1.6 to 3.0 is used, this slab is normally rolled into a round billet, and this round billet is perforated with an inclined roll and a plug. Then, the absolute value of strain ε_n during drilling defined by the following formula |ε_n| is |ε_n|≦0.01 at a position from a position 10 mm from the tip of the plug toward the rear end of the plug.
5. A method for producing a Cr-containing seamless steel pipe. ε_n=l_n(t_n/t_n_-_1) where, t_n_-_1: Thickness (mm) at an arbitrary position P_n_-_1 from a position 10 mm from the tip of the plug toward the rear end of the plug. t_n: From position P_n_-_1 to the back of the plug. 1mm on the end side
Thickness (w) at distant position P_n