JP4300634B2 - Manufacturing method of non-tempered high strength seamless steel pipe - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a non-tempered high strength seamless steel pipe.
[0002]
[Prior art]
In order to prevent the collapse of slopes such as mountains, valleys, and roadsides, earth retaining piles (hereinafter referred to as earth retaining piles) are sometimes used. 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. Recently, however, steel pipes with a smaller diameter have been used because of the diversification of pile sizes, and it has become necessary to increase the strength of the narrow steel pipe. Therefore, the steel grade of the material was changed to a high strength non-tempered steel exhibiting higher strength than before, and seamless steel pipes were manufactured in the same manufacturing process as before. The process is as follows.
[0003]
First, as shown in FIG. 4, a high-strength non-heat treated round steel piece (billet 1) as a raw material is heated to 1200 to 1300 ° C. in a heating furnace 2 and then passed through a piercing and rolling machine that is a piercer 3. A hollow shell 5 is formed by pressing and plugging a plug 4 having a bullet-like appearance into the center of the billet 1 to penetrate. Subsequently, the mandrel bar 6 is inserted into the hole of the raw tube 5 and rolled through a mandrel mill 7 for rolling . At this time, since the temperature of the raw tube 5 drops to 700 ° C., the temperature of the raw tube 5 is increased to 900 ° C. in the reheating furnace 8, and then the outer diameter is adjusted with a multi-stage rolling mill consisting of the stretch reducer 9. It cools to normal temperature with a cooling floor (not shown), and makes it the steel pipe 10 of a product.
[0004]
However, when a high-strength non-heat treated steel having a higher strength than that of the conventional material is adopted as the material 1, a problem arises in that the steel pipe to be a product in the cooling bed warps in the longitudinal direction and bends as shown in FIG. As a product, the maximum bend must be suppressed to 1 mm or less per 1 m of the steel pipe, but depending on the product size, it is bent as much as 2 to 7 mm per 1 m. In addition, if the bending is corrected, the product is cracked. In this case, the product yield is low, the productivity is lowered, and the production cannot be realized economically.
[0005]
[Problems to be solved by the invention]
In view of such circumstances, the present invention provides a method for producing a non-tempered high-strength seamless steel pipe that can stably secure a steel pipe at a high product yield without causing bending even when a material exhibiting high strength is used. The purpose is to provide.
[0006]
[Means for Solving the Problems]
The inventor diligently studied to achieve the above object, and the results were embodied in the present invention.
[0007]
That is, the present invention is a non-tempered high-strength seam 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 and rolled into a tubular body. In the steelless pipe manufacturing method, the temperature of the raw pipe before being charged into the reheating furnace is cooled to a range of Ar3 to Ar1, and then heated to a temperature of Ar3 or higher in the reheating furnace, and subsequently with a stretch reducer. It is a manufacturing method of a non-tempered high strength seamless steel pipe characterized by rolling.
[0008]
According to the present invention, the metal structure of the steel pipe that has passed through the stretch reducer 9 and has been cooled is a mixed phase of a bainite phase, a ferrite phase, and a pearlite phase, and the ratio of the ferrite phase is higher than in the prior art. That is, by once cooling to a two-phase region of Ar 3 or less, ferrite is precipitated and the number of crystal grains is increased. Therefore, since the austenite crystal grains are fine when reheated thereafter, the crystal sites to ferrite increase by cooling after reducer 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 remarkably reduced.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below based on the circumstances leading to the invention.
[0010]
First, a steel slab (billet 1) made of high-strength non-tempered steel was rolled using a conventional manufacturing method to produce a non-tempered high-strength seamless steel pipe. 10 was bent. Therefore, the inventor diligently studied the cause of the occurrence of bending, and estimated that if the temperature is not uniform in the circumferential direction of the material during rolling, partial plastic deformation occurs during cooling. In particular, when a temperature difference occurs in the circumferential direction at the start temperature of the bainite transformation, that is, around 500 ° C., the stress generated by the heat shrinkage difference increases and the degree of bending increases. Note that the steel pipe with a small dimensional coefficient defined by (tube cross-sectional area / tube outer circumference length) has a high cooling rate and a very high product strength, while a steel pipe with a large dimensional coefficient has a low cooling rate. It was also found that the strength was lower. Moreover, the crack at the time of correcting a bending is also examined, and the steel pipe manufactured with such a steel type is too strong and has low toughness.
[0011]
Next, the inventor examines measures based on these findings,
a) reducing the circumferential heat deviation before the start of bainite transformation,
b) Reducing the strength after rolling with a stretch reducer,
Inspired.
[0012]
However, it is difficult to implement a reduction in heat deviation in a) in terms of equipment, and even if implemented, it can be expected that b) is more effective. Therefore, the inventor embodies the idea of b), and adjusts the temperature at which the raw tube is charged into the reheating furnace 8 in the manufacturing process within the range of Ar3 to Ar1, and sets the reheating temperature. The temperature was set to Ar3 or higher , and rolling was performed by the reducer 9.
[0013]
As a result, the metal structure of the rolling agent is adjusted during the production, and the metal structure of the steel pipe finally obtained is rich in the ferrite phase. In the present invention, charging temperature, the effect of the invention is too high than the above temperature is insufficient, too low and productivity is lowered, also the reheating temperature is reducer When the temperature by Ri too low Rolling becomes difficult.
[0014]
【Example】
Using a billet made of high-strength non-tempered steel as a raw material 1, a seamless steel pipe having an outer diameter of 54.00 mm, a thickness of 16.50 mm, and a length of 6375 m (size factor: 1.82) is manufactured by the manufacturing method according to the present invention. did. This material is a high strength non-tempered steel with Ar3 of 720 ° C. and Ar1 of 400 ° C., and has a tensile strength (Ts = 900 N / mm class). The manufacturing process is the same as that shown in FIG. In addition, seamless steel pipes using the same material and having various dimensional factors were manufactured by a conventional manufacturing method.
[0015]
The main production conditions are that the charging temperature of the material into the reheating furnace is 450 to 500 ° C., the atmospheric temperature of the reheating furnace is 920 ° C., and the in-furnace time in the reheating furnace of the material is 8 minutes. In addition, when implementing the conventional method, the charging temperature of the raw material to a reheating furnace was 700-750 degreeC.
[0016]
FIG. 1 and FIG. 2 show how the temperature of the raw pipe or steel pipe and the metal structure being changed by the method according to the present invention change compared to the case of the conventional manufacturing method. By adopting the temperature pattern according to the present invention of FIG. 1, it can be easily understood that the metal composition of the final product is rich in ferrite phase. In FIG. 1, RHF is a heating furnace, PCM is a piercer mill, MDM is a Manssmann mill, WBF is a reheating furnace, HSR is a hot stretch reducer, etc. ing.
[0017]
The result of having measured the intensity | strength and elongation of the steel pipe manufactured by these methods after cooling is shown to Fig.3 (a) and (b). 3A and 3B, according to the present invention, the strength (yield point Ys, tensile Ts) indicated by Δ of the final product is lower than that by the conventional method indicated by ◯, and the elongation indicated by Δ is It is clear that there was an increase from the conventional O. In other words, the metal structure is rich in ferrite phase that softens the steel material, and the strength over spec by the conventional method is eliminated.
[0018]
Moreover, 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 the average with respect to the total length of 6375 mm, which satisfied the allowable range for the product. It was. In addition, the thing by the conventional manufacturing method was 13 mm on average.
[0019]
Furthermore, among the steel pipes obtained by the production method according to the present invention, those having a maximum bending exceeding 3 mm were straightened in a cold shape. However, none of the cracks caused by this correction.
[0020]
【The invention's effect】
As described above, according to the present invention, even if a material exhibiting high strength is used, it is possible to stabilize a small-diameter non-heat treated high-strength seamless steel pipe with high product yield without causing bending or cracking during correction. Can now be manufactured.
[Brief description of the drawings]
FIG. 1 is a diagram showing a temperature pattern of a material to be rolled when carrying out a manufacturing method according to the present invention.
FIG. 2 is a diagram showing a change in metal composition of a material to be rolled when the manufacturing method according to the present invention is performed.
FIGS. 3A and 3B are diagrams showing the investigation results of steel pipes manufactured by the present invention and the conventional method, in which FIG. 3A shows strength and FIG. 3B shows elongation values.
FIG. 4 is a view showing a manufacturing process of a seamless steel pipe.
FIG. 5 is a diagram illustrating bending of a product steel pipe.
[Explanation of symbols]
1 Round billet
2 Heating furnace 3 Piercer 4 Plug 5 Raw pipe 6 Mandrel bar 7 Mandrel mill 8 Reheating furnace 9 Stretch reducer (reducer)
10 products (steel pipe)

Claims (1)

In a manufacturing method of a non-tempered 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 and rolled into a tubular body. ,
The temperature of the raw tube before charging into the reheating furnace is cooled to a range of Ar3 to Ar1, then heated to a temperature of Ar3 or higher in the reheating furnace, and subsequently rolled with a stretch reducer. Manufacturing method of non-tempered high strength seamless steel pipe.

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