JP2001009509A - Manufacturing method of non-heat treated high strength seamless steel pipe - Google Patents

Abstract

(57) [Problem] To provide a non-refined high-strength seamless structure capable of stably securing a steel pipe with a high product yield without bending, even if a material having high strength is used. It is intended to provide a method for manufacturing a steel pipe. A non-heat-treated high-strength seamless steel pipe in which a high-strength steel slab heated to 1200 ° C. or higher is sequentially passed through a piercer, a mandrel mill, a reheating furnace and a stretch reducer to be rolled into a tubular body. In the manufacturing method described above, the rolling of the steel slab is completed in the range of Ar3 to Ar1 by the mandrel mill, and thereafter, the steel slab is heated to the temperature of Ar3 in the reheating furnace, and then rolled by the stretch reducer.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a non-heat treated high strength seamless steel pipe.

[0002]

2. Description of the Related Art Piles for retaining (hereinafter referred to as retaining piles) are sometimes used to prevent collapse of slopes such as mountains, valleys, and roadsides. In general, a non-heat-treated (not heat-treated) seamless steel pipe having a steel type of SAE1541-SH has been adopted as a steel pipe used at the tip of the earth retaining pile. However, recently, due to the demand for diversification of the pile size, a steel pipe having a smaller diameter has been used, and accordingly, it has become necessary to increase the strength of the steel pipe. Therefore, the steel type of the material was changed to a high-strength non-heat treated steel exhibiting higher strength than before, and a seamless steel pipe was manufactured in the same manufacturing process as before. The steps are as follows.

[0003] First, as shown in FIG.
After being heated to 00 to 1300 ° C., the billet 1 is passed through a piercing and rolling mill, and a plug 4 having a shell-like appearance is pressed into the center of the billet 1 to form a hollow shell 5. Subsequently, the mandrel bar 6 is inserted into the hole of the raw tube 5 and expanded by passing through a rolling mill, which is a mandrel mill 7 for rolling. At this time, the temperature of the raw tube 5 is 700
° C, the temperature of the raw tube 5 is set to 90 in the reheating furnace 8.
After raising the temperature to 0 ° C., the outer diameter is adjusted by a multi-high rolling mill including a stretch reducer 9, and cooled to room temperature by a cooling floor (not shown) to obtain a steel pipe 10 of a product.

However, if a high-strength non-heat treated steel having a higher strength than that of the conventional material is used as the raw material 1, there is a problem that the steel pipe to be a product on the cooling floor is warped in the longitudinal direction and bent as shown in FIG. Occurred. As a product, the maximum bending must be suppressed to 1 mm or less per 1 m of the steel pipe, but depending on the product size, it bends 2 to 7 mm per 1 m. In addition, if the bending is to be corrected, the product will crack. In this case, the product yield is low, the productivity is reduced, and production is not economically feasible.

[0005]

SUMMARY OF THE INVENTION In view of the above-mentioned circumstances, the present invention provides a non-adjusting method that can stably secure a steel pipe with a high product yield without bending even if a material having high strength is used. It is an object of the present invention to provide a method for manufacturing a high-quality, high-strength seamless steel pipe.

[0006]

Means for Solving the Problems The inventor has conducted intensive studies to achieve the above object and has embodied the results in the present invention.

[0007] That is, the present invention provides a non-heat-treated non-heat-treated steel slab that has been heated to 1200 ° C or higher and passed through a piercer, a mandrel mill, a reheating furnace, and a stretch reducer in order. In the method for producing a high-strength seamless steel pipe, the mandrel mill terminates the rolling of the steel slab in the range of Ar3 to Ar1, and then heats the steel slab to the temperature of Ar3 in a reheating furnace. This is a method for producing a non-heat treated high-strength seamless steel pipe characterized by rolling.

According to the present invention, the metal structure of the steel pipe which has passed through the stretch reducer 9 and cooled is a bainite phase, a mixed phase of a ferrite phase and a pearlite phase, and the ratio of the ferrite phase is larger than that of the conventional one. Become. That is, once cooled to a two-phase region of Ar3 or less, ferrite is precipitated and the number of crystal grains is increased. Therefore, when reheating is performed subsequently, the austenite crystal grains are fine, and the crystal sites in the ferrite are increased by cooling after the laser rolling. As a result, the strength of the product steel pipe does not become excessive, the elongation is improved, and the occurrence of bending is significantly reduced.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The embodiments of the present invention will be described below on the basis of the circumstances leading to the invention.

First, a steel slab (billet 1) made of high-strength non-heat treated steel was rolled using a conventional production method to produce a non-heat treated high-strength seamless steel pipe.
The product steel pipe 10 was bent. Therefore, the inventor
The cause of the occurrence of the bending was studied intensely, and it was presumed that if the temperature was not uniform in the circumferential direction of the material during rolling, partial plastic deformation would occur during cooling. In particular, when a temperature difference occurs in the circumferential direction around the starting temperature of bainite transformation, that is, around 500 ° C., the stress generated due to the difference in heat shrinkage increases, and the degree of bending increases. In addition, (tube cross-sectional area / tube outer peripheral length)
It has also been found that a steel pipe having a small dimensional coefficient defined by (1) has a high cooling rate and a very high product strength, while a steel pipe having a large dimensional coefficient has a low cooling rate and therefore has a lower strength than the former. Also, cracks in correcting the bending were examined, and a steel pipe manufactured with such a steel type has too high strength and low toughness.

Next, the inventor studied countermeasures based on these findings, a) reducing the heat deflection in the circumferential direction before the start of bainite transformation, and b) reducing the strength after rolling with a stretch reducer. Inspired.

However, it is difficult to implement the method of reducing the temperature deviation in a) in terms of equipment, and it is expected that the effect of b) will be greater if implemented. Therefore, the inventor has embodied the idea of b), and in the manufacturing process, adjusts the temperature at which the raw tube is charged into the reheating furnace 8 within the range of Ar3 to Ar1, and adjusts the reheating temperature. The temperature was set to Ar3, and rolling was performed by the reducer 9.

As a result, the metal structure of the material to be rolled is adjusted during the production, and the metal structure of the finally obtained steel pipe is rich in the ferrite phase. In the present invention, if the charging temperature is higher than the above temperature, the effect of the invention becomes insufficient, and if the charging temperature is too low, the productivity decreases. On the other hand, if the reheating temperature is higher than the above-mentioned temperature, the crystal grains grow too much and the effect of the invention is lost, and if the reheating temperature is too low, it becomes difficult to roll with a reducer.

[0014]

EXAMPLE A billet made of high-strength non-heat treated steel was used as a raw material 1, and the outer diameter was 54.00 mm, the thickness was 16.50 mm, and the length was 6375 m (dimension coefficient: 1.8) by the production method according to the present invention.
The seamless steel pipe of 2) was manufactured. This material has 7 Ar3
A high-strength non-heat treated steel with 20 ° C. and Ar1 of 400 ° C.,
It is a tensile strength (Ts = 900 N / mm class). The manufacturing process is the same as that shown in FIG. In addition, seamless steel pipes having various dimensional coefficients were manufactured by the conventional manufacturing method using the same material.

The main manufacturing conditions are as follows: the charging temperature of the material into the reheating furnace is 450 to 500 ° C .;
The oven time in the reheating furnace at 0 ° C. is 8 minutes. In addition, when implementing the conventional method, the charging temperature of the raw material to the reheating furnace was set to 700 to 750 ° C.

FIGS. 1 and 2 show how the temperature and the metal structure of a raw pipe or a steel pipe being manufactured by the method according to the present invention are changed in comparison with the conventional manufacturing method. FIG.
By employing the temperature pattern according to the present invention, it can be easily understood that the metal structure of the final product is rich in the ferrite phase. In FIG. 1, RHF is a heating furnace, PCM
Is a piercer mill, MDM is a man-esman mill, WBF
Indicates the time when the material to be rolled is present in the reheating furnace and HSR is in the hot stretch reducer.

FIGS. 3 (a) and 3 (b) show the results of measuring the strength and elongation of the steel pipe manufactured by these methods after cooling.
Shown in From FIGS. 3A and 3B, according to the present invention,
Strength (yield point Ys, tensile Ts) of the final product
Is lower than that of the conventional method shown by 〇, and it is clear that the elongation shown by 〇 is larger than that of the conventional method. In other words, the metal structure is rich in the ferrite phase that softens the steel material, and the strength over specification by the conventional method is eliminated.

When the bending of the product was measured, the maximum bending of the product measured by the manufacturing method according to the present invention was 2 to 3 mm on average for a total length of 6375 mm. I was satisfied. In addition,
With the conventional manufacturing method, the average was 13 mm.

Further, among the steel pipes obtained by the production method according to the present invention, those having a maximum bend of more than 3 mm were straightened in a cold state. However, there was no crack caused by this straightening.

[0020]

As described above, according to the present invention, even when a material exhibiting high strength is used, it does not cause bending or cracking at the time of straightening, and has a high product yield and a small diameter non-refined high-strength seam. Steel-free pipes can be manufactured stably.

[Brief description of the drawings]

FIG. 1 is a diagram showing a temperature pattern of a material to be rolled when a manufacturing method according to the present invention is performed.

FIG. 2 is a diagram showing a change in metal structure of a material to be rolled when a manufacturing method according to the present invention is performed.

FIGS. 3A and 3B are diagrams showing the results of a survey on steel pipes manufactured by the present invention and the conventional method, wherein FIG. 3A shows the strength and FIG. 3B shows the elongation.

FIG. 4 is a diagram showing a manufacturing process of a seamless steel pipe.

FIG. 5 is a diagram illustrating bending of a product steel pipe.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Round steel slab (billet) 2 Heating furnace 3 Pierce 4 Plug 5 Raw tube 6 Mandrel bar 7 Mandrel mill 8 Reheating furnace 9 Stretch reducer (reducer) 10 Product (steel pipe)

Claims (1)

[Claims] 1. A non-heat-treated high-strength seamless slab in which a high-strength steel slab heated to 1200 ° C. or higher is passed through a piercer, a mandrel mill, a reheating furnace and a stretch reducer to be rolled into a tube. In the method for manufacturing a steel pipe, the temperature of the steel slab is Ar3 to A by the mandrel mill.
Rolling is completed in the range of r1, and then Ar3
A method for producing a non-heat-treated high-strength seamless steel pipe, comprising heating the steel pipe to a temperature equal to the temperature and subsequently rolling with a stretch reducer.