JPS607007B2 - Manufacturing method of low yield ratio, high tensile resistance welded steel pipe - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a low yield ratio, high tensile resistance welded steel pipe without heat treatment.

In recent years, customer demand for low yield ratio, high tensile resistance welded steel pipes has been increasing for the safety and reliability of line pipes, etc., and for the workability of steel pipes for automobiles and material pipes.

However, open-seamed steel pipes generally have a higher yield ratio than seamless steel pipes. The reason for this is the fundamental difference in manufacturing methods.

In other words, compared to seamless steel pipes that are hot-formed, electric resistance welded steel pipes that are cold-formed have a larger amount of work hardening and tend to have a higher yield ratio. Furthermore, since the cooling rate of hot rolled coils for crimped steel pipes is significantly higher than the cooling rate after forming seamless steel pipes, the yield ratio tends to be high. Furthermore, since the cooling rate of hot-rolled coils for open-seamed steel pipes is high, the strength would be too high if they had the same composition as seamless steel pipes.Generally, the C and Mn content of ERW steel pipes is lower than that of seamless steel pipes. It also has a high yield ratio. As described above, ERW steel pipes have a higher yield ratio than seamless steel pipes, and due to the difference in manufacturing methods, it has been difficult to manufacture steel pipes with a low yield ratio.

Therefore, as a conventionally known method, after forming an ERW steel pipe, heating it to a two-phase temperature of A and A3 transformation points and below, 100C/100C/
Although a low yield ratio has been achieved by cooling at a cooling rate of sec or less, it has the disadvantage of increasing the number of steps such as requiring a heating step after forming the steel pipe and increasing manufacturing costs. The present invention advantageously eliminates the above-mentioned drawbacks, and its gist is that C: 0.10-0.20%, Mn
: 0.7 to 1.5%, Si: 0.10 to 0.30% as a basic component, and the balance is Fe and unavoidable impurities. Hot rolling is completed at a temperature of 700 to 8200C, The sand within 1 second or more from the end temperature of the hot rolling is allowed to cool without pouring water, and then the average cooling rate until coiling is 50C.
/sec or more and winding at 500 qo or less.The material Nakatomi Wo during subsequent pipe forming is pipe-formed so that the elongation rate in the longitudinal direction of the pipe is 1.5% or less.No heat treatment is applied. This is a method for manufacturing low yield ratio, high tensile resistance welded steel pipes.

That is, the present invention first manufactures a hot rolled coil with a low yield ratio,
Next, by controlling work hardening so that the amount of work hardening does not become large during line forming, it is possible to manufacture a low yield ratio, high tensile resistance welded steel pipe without heat treatment.

Next, the present invention will be explained in detail. First, regarding the ingredients of the material, C is necessary to ensure the necessary strength, but if it is less than 0.10%, the yield ratio will increase, which is undesirable, and if it is more than 0.20%, it will reduce the ductility and grain quality. Since deterioration is significant and impairs weldability, the content is set in the range of 0.10 to 0.20%.

Mn is also necessary to ensure the necessary strength, but 0.7
If it is less than 0%, the yield ratio will increase, which is undesirable, and 1.5
If it exceeds 0.70 to 1, it deteriorates ductility and impairs weldability.
．． The range is 50%.

Si is necessary to ensure the necessary strength, but Mn/
Due to the relationship with Si, a minimum of 0.10% or more is required, and 0.3
If it exceeds 0%, weldability will be impaired, so 0.10 to 0.30
% range. The molten steel having the above-mentioned basic components may be produced in either an open hearth converter or an electric furnace, and the type of steel may be rimmed steel, capped steel, semi-killed steel, or killed steel. Any of ingot forming, blooming, rolling or continuous casting may be used.

Next, hot rolling conditions will be explained.

The relationship between the hot rolling end temperature and the yield ratio is shown in FIG. 1, and the yield ratio is significantly low in the range of 700 to 82,000, which is defined by the present invention. That is, when the temperature exceeds 82,000, no strain is accumulated due to rolling due to the high temperature, and therefore the generation of many fine ferrites is suppressed, and the structure becomes a bainite structure, resulting in a high yield ratio.

On the other hand, below 700 pm, ferrite distortion occurs,
It enters a recrystallized state and the yield ratio increases. Therefore, the hot rolling end temperature is limited to 700 to 82,000.
Next, the reason why the sand within 1 second or more from the hot rolling end temperature is allowed to cool without water injection is that ferrite can be uniformly generated during the cooling period after rolling in the austenier** region. This ferrite plays a role in lowering the yield ratio. However, cooling of more than 3 inkstone sand is not preferable from the viewpoint of length restriction of the hot rolling cooling zone and productivity. Next, after cooling, the average cooling rate until winding was set at 5°C/se.
The reason why the cooling rate is less than 50,000 and the cooling rate is less than 50,000 is as follows. When the cooling rate is slow, such as less than 5°C' sec, coarse pearlite is formed, resulting in a decrease in tensile strength and a high yield ratio, which is not preferable.

Further, although there is no need to particularly limit the upper limit of the cooling rate, 1000 C'sec is considered to be the upper limit due to restrictions in terms of equipment capacity. Figure 2 shows the relationship between coiling temperature and yield ratio. When the coiling temperature is high, the yield strength increases due to the precipitation of solid solution C due to autogenous annealing in the coil itself, and the yield ratio increases. . As described above, in addition to limiting the raw material components and hot rolling conditions, the present invention also includes pipe forming conditions, and the forming conditions will be described below.

The conditions are that the material width Wo during pipe forming is such that the elongation rate in the longitudinal direction of the pipe 3 is 1.5% or less.

The reason for this is, as shown in Figure 3, if Go3 is set to 1.5% or less, the amount of increase in yield ratio △YR can be held to 5% or less, and therefore it is possible not to increase the yield ratio. . Here, 3 is the longitudinal elongation rate (%) of the pipe,
3 is determined by determining the material width Wo. Therefore, if Wo is selected so that 3 is 1.5% or less, a high tensile resistance welded steel pipe with a stable low yield ratio can be manufactured.

Next, the determination of the material width Wo will be described. It is calculated from the following equation.

Go3={character. ;;Takayunushi;Koto｝kuX100(%))...
[11 Go 2: (Academic.

・Cicada) (xl..(%))…■go. = {W Fortress} (X
・oo(%))---...{31D: outer diameter, t: wall thickness, W
o: Material width, ,: Pipe circumferential drawing ratio (%), 2
: Pipe wall thickness increase rate (%), 3: Pipe longitudinal elongation rate (%), 3 and 3 above are theoretical formulas, but z2 is an empirical formula that includes mill-specific constants. be. Next, Table 1 shows examples of the present invention.

Table 1 (Pipe size 114.3 x 5.21) As described above, according to the present invention, it is possible to manufacture high-strength electric resistance welded steel pipes with a low yield ratio without heat treatment, and the heating process is omitted. However, it is extremely advantageous in that the manufacturing cost is low.

[Brief explanation of the drawing]

Figure 1 is a chart showing the relationship between yield ratio and hot rolling end temperature, Figure 2 is a chart showing the relationship between yield ratio and coiling temperature, and Figure 3 is a chart showing the relationship between yield ratio and coiling temperature.
The figure is a chart showing the relationship between the increase in yield ratio and the elongation rate in the longitudinal direction of the pipe. Figure 1 Figure 2 Figure 3

Claims (1)

[Claims] 1C: 0.10-0.20%, Mn: 0.7-1.5
%, Si: 0.10 to 0.30% as a basic component, and the balance is Fe and unavoidable impurities.
Finish rolling at a temperature of 00 to 820 ° C., leave to cool without pouring water for 1 second or more and within 30 seconds from the end temperature of the hot rolling, and then set the average cooling rate until coiling at 5 ° C / sec or more, 5
Material width W during coiling at 00℃ or less and subsequent pipe forming
A method for manufacturing a low yield ratio, high tensile strength, electric resistance welded steel pipe without heat treatment, characterized in that _0 is formed into a pipe such that the elongation rate ε_3 in the longitudinal direction of the pipe is 1.5% or less.