JPH04358024A - Production of high strength seamless steel tube reduced in yield ratio - Google Patents

Abstract

PURPOSE:To produce a steel tube reduced in yield ratio from the consideration of safety to large earthquake and usable in the fields of building, bridge, etc. CONSTITUTION:After a strain of 3-10% is applied at 900-750 deg.C to a tube stock in the course of manufacture by the ordinary rolling system, the tube stock is subjected, without cooling down to <=650 deg.C, to reheating up to 900-1150 deg.C, to sizing, and to air cooling. Then, the resulting tube is heated and held to and at a temp. of Ac1 transformation point +(10-130 deg.C) to undergo preferential alpha-gamma transformation and cooled rapidly or further subjected to tempering treatment, by which the high strength seamless steel tube reduced in yield ratio can be produced.

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 steel pipes used in the construction and bridge fields, and particularly to a method for manufacturing steel with a low yield ratio in consideration of safety against large earthquakes.

0002

[Prior Art] Seamless steel pipes are manufactured using rolling methods such as Mannesmann plug mills and Mannesmann Assel mills, which are suitable for mass production of steel pipes with good inner and outer skin and small wall thickness unevenness. Products are subjected to various heat treatments to improve their physical properties. For the heat treatment, for example, Japanese Patent Application Laid-open No. 57-89436
``A steel pipe is heated at a rate of 1 to 30 degrees Celsius/second in the temperature range from Ac1 transformation point to Ac1 transformation point + 50 degrees Celsius, and from the highest heating temperature from that temperature to the austenite grain coarsening starting temperature. Heat cycle for hardening
, “Steel pipe 94
"Thermal cycle of heating to 0 to 1050°C followed by rapid cooling and then tempering at a temperature below the Ac1 transformation point" and JP-A-59
-232220
Quenching from below the grain coarsening temperature, followed by Ac3
There are various types of thermal cycles, such as a thermal cycle in which the material is heated to a temperature below the transformation point to the crystal grain coarsening temperature, then quenched again, and then tempered at a temperature below the Ac1 transformation point.

[0003]More recently, in the fields of architecture and bridges,
To ensure safety against large earthquakes, seamless steel pipes with high strength, sufficient plastic deformability, and a low yield ratio are required. In general, it is known from dislocation theory that high-strength steel can be made to have a mixed structure of ferrite with a low dislocation density and martensite and bainite with a high dislocation density to lower the yield ratio. However, since seamless steel pipes are manufactured through a complicated pipe-making process that involves repeating heating and rolling, it is difficult to stably obtain such a mixed structure. Moreover, it is also difficult to obtain a mixed structure in the heat treatment method of quenching at a high temperature above the Ac3 transformation point as described above.

0004

[Problems to be Solved by the Invention] In view of the current state of technology and demands, an object of the present invention is to provide a manufacturing method that can stably obtain a high-strength seamless steel pipe with a low yield ratio. .

[0005]

[Means for solving the problem] In order to achieve the above purpose,
As a result of various studies, the present inventors have found that by changing the structure before quenching into a coarse mixed structure consisting mainly of ferrite (α) and pearlite in the heat treatment during the manufacturing process of seamless steel pipes as described above, strength, weldability, etc. We obtained the knowledge that the desired seamless steel pipe can be easily manufactured without impairing various properties. The present invention has been constructed based on this knowledge, and its gist is that 3% to 10% of After imparting strain, the pipe is reheated to a temperature of 900°C to 1150°C without lowering the temperature to 650°C or lower, subjected to shape rolling, and then cooled in air to produce a seamless steel pipe having a mixed structure consisting mainly of ferrite and pearlite. Ac1 transformation point +10℃~Ac1 transformation point +1
This is a method for producing high-strength seamless steel pipes with a low yield ratio, in which the pipe is heated to a temperature of 30°C, maintained, and then rapidly cooled, or further tempered at a temperature of 650°C or lower.

The present invention will be explained in detail below. A small strain is imparted to a seamless steel pipe produced by the above-described normal rolling method during pipe-making rolling, and soaking is subsequently performed before shaping rolling without lowering the temperature. The steel tube that has been soaked and heated in this way grows into a coarse austenite grain structure, and is then naturally cooled after rolling to a shape, resulting in a mixture of mainly coarse ferrite and pearlite, except for special alloys (high alloy steel tubes). Become an organization. This structure has a microscopic difference in carbon concentration. When a steel pipe having such a structure with a difference in carbon concentration is heated and held at a temperature equal to or higher than the Ac1 transformation point in the next step, α-γ transformation occurs preferentially from α grain boundaries and locations with high carbon concentration. Therefore, by heating and holding at an appropriate temperature above the Ac1 transformation point, an ideal mixed structure of γ and α can be obtained, and by subsequent rapid cooling treatment, γ transforms into a low-temperature transformation-generated structure and imparts strength. do. At the same time, α influences the yield strength, and the larger the α grains and the higher the α area ratio, the more
It was found that the yield strength was reduced. Therefore, as a method of lowering the yield ratio, it is more effective to make the α grains coarser.

Next, the reasons for limiting the rolling and heat treatment conditions will be described. The temperature at which strain is imparted during rolling must be in the non-recrystallization temperature range; temperatures of 900°C or higher cause recrystallization and grain refinement, which is not suitable for the purpose of the present invention. Furthermore, since it is difficult to impart uniform strain at temperatures below 750°C, the strain imparting temperature range was set to 900°C to 750°C. If the amount of strain applied is too large or too small, the coarsening effect of γ grains will disappear, so the amount of strain applied should be 3% to 10%.
And so. The temperature before reheating is related to the amount of α transformation, but if the amount of α transformation is too large, reheating will cause the γ grains to become finer, and the strain imparted during rolling will disappear, causing the γ grains to become coarser. Since the effect would be lost, the temperature was set at 650°C or higher. There is no problem as long as the reheating temperature before shaping rolling is equal to or higher than the Ac3 transformation point, but if it is too low, the effect of coarsening the γ grains becomes small, so the lower limit temperature was set at 900°C. In addition, the higher the upper limit temperature is, the better for coarsening of γ grains, but at temperatures exceeding 1150°C, the coarsening effect of γ grains reaches a saturation range, resulting in an increase in the generated scale and deterioration of the surface quality of the steel pipe. However, there is a problem that the heat treatment cost increases. For the above reasons, the heating temperature range was set to 900°C to 1150°C.

[0008] Seamless steel pipes that are reheated and maintained at a temperature of 900° C. to 1150° C., rolled into a shape, and then naturally cooled have a mixed structure consisting mostly of coarse ferrite and pearlite, except for special alloys (high alloy steel pipes). Seamless steel pipes manufactured in this way are manufactured at Ac1 transformation point +10°C to Ac1 in order to improve the strength and yield ratio characteristics of steel pipes.
After being heated to and held at a temperature in the α and γ two-phase region of the transformation point +130° C., a quenching treatment is performed in which the steel pipe is rapidly cooled to improve the strength and yield characteristics of the steel pipe. However, at a low temperature below the Ac1 transformation point +10° C., it takes time to generate the two-phase mixed structure, and the amount of transformation to the γ structure is small, so a high-strength steel pipe cannot be obtained even after rapid cooling. On the other hand, at a high temperature exceeding the Ac1 transformation point +130° C., there is a problem in that the transformation to the γ structure increases unnecessarily and the grains become finer, increasing the yield ratio. Further, regarding the cooling rate in this case, the faster the better, in order to maintain the characteristics of the two-phase mixed structure generated at high temperature even at low temperature. Seamless steel pipes manufactured in this way have high strength and low yield ratio, and can be used as construction materials in many fields such as architecture and bridges.

[0009] According to the present invention, the seamless steel pipe produced by rapid cooling as described above is further subjected to a tempering treatment, if necessary. This tempering process removes internal distortion and balances strength, hardness, and yield ratio at 650°C.
It is carried out at the following temperatures: In this case, there is a problem that a temperature exceeding 650° C. causes a significant decrease in strength. Also,
A seamless steel pipe subjected to such tempering treatment has better balance of properties within the objective range of the present invention.

0010

[Example] Next, an example of the present invention will be described. Table 1
The steel materials with the chemical compositions shown in the table were used as test materials; one was a steel material that had been rolled for regular pipe making, and the other was a steel material that had been subjected to austenite grain coarsening treatment by applying strain during hot pipe rolling to intentionally change the alpha grains. , Ac1 transformation point +10℃~Ac1 transformation point +130
After being heated to and held at various temperatures of °C, it was rapidly cooled to room temperature. Next, a tempering treatment was performed for the purpose of material adjustment. The various conditions at this time are shown in Table 2 together with the test results.

[0011] When steel produced by the comparative method and steel produced by the method of the present invention are compared at the same strength level, it is clear that the steel of the present invention has a lower yield ratio.

0012

[Table 1]

[0013]

[Table 2]

[0014]

[Effects of the Invention] As explained above, the steel manufactured by the method of the present invention has high strength and a low yield ratio, so it can be used in fields such as architecture and bridges to ensure sufficient safety. The industrial effect is extremely large.

Claims (2)

[Claims] [Claim 1] The raw pipe is heated at 900°C to 75°C during rolling for pipe making.
After applying a strain of 3% to 10% at a temperature of 0°C, 65
900℃~115℃ without reducing the temperature below 0℃
After reheating to a temperature of 0°C, performing shape rolling, and then air cooling, a seamless steel pipe having a mixed structure consisting mainly of ferrite and pearlite is heated to a temperature of Ac1 transformation point +10°C to Ac1 transformation point +130°C and then held. A method for manufacturing high-strength seamless steel pipes with a low yield ratio characterized by rapid cooling. [Claim 2] The raw pipe is heated at 900°C to 75°C during rolling for pipe making.
After applying a strain of 3% to 10% at a temperature of 0°C, 65
900℃~115℃ without reducing the temperature below 0℃
After reheating to a temperature of 0°C, performing shape rolling, and then air cooling, a seamless steel pipe having a mixed structure consisting mainly of ferrite and pearlite is heated to a temperature of Ac1 transformation point +10°C to Ac1 transformation point +130°C and then held. Rapid cooling,
A method for producing a high-strength seamless steel pipe with a low yield ratio, which comprises subsequently performing a tempering treatment at a temperature of 650°C or lower.