JPS607006B2 - Manufacturing method of low yield ratio ERW steel pipe for oil wells - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method of manufacturing a low yield ratio electric resistance welded steel pipe for oil wells.

In recent years, there has been an increasing demand for low yield ratio oil country tubular goods (ERW) similar to seamless steel pipes.

It is for the safety and reliability of the oil well. However, Denso 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 hot-formed seamless steel pipes, cold-formed lightning-stitched steel pipes have a larger amount of work hardening and tend to have a higher yield ratio. Furthermore, since the cooling rate of a hot-rolled coil for a seam-stitched steel pipe is significantly higher than the cooling rate after forming a seamless steel pipe, a high yield ratio tends to occur. Furthermore, since the cooling rate of hot-rolled coils for lightning-seamed steel pipes is high, the strength would be too high if they had the same composition as seamless steel pipes, so C and Mn are generally lower for exposed-seamed steel pipes than for seamless steel pipes. This also results in a high yield ratio. As described above, open-seamed steel pipes have a higher yield ratio than seamless steel pipes, and due to the difference in manufacturing methods, it has been difficult to produce electrical resistance welded steel pipes for oil wells with a low yield ratio.

Therefore, in a conventionally known method, after forming an ERW steel pipe, it is heated to a two-phase temperature above the transformation point and below the A3 transformation point, and then heated at 10qC/
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: 0.26-0.48%, Mn
:1.30~1.70%, Si:0.15~0.30%
A steel consisting of a basic composition of Fe and unavoidable impurities is hot-rolled, the rolling is finished at a temperature of 700 to 820 qo, and the steel is released without water injection for at least 1 second and within 3 minutes from the end temperature of the hot rolling. The average cooling rate from cooling to winding is 50
C/sec or more, winding at 450 qC or more and 650 qC or less, and forming the pipe so that the material width Wo at the time of subsequent pipe forming is 1.5% or less. This is a method for manufacturing low yield ratio electric steel pipes for oil wells without heat treatment.

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 increase during cold forming, it is possible to manufacture electrical steel pipes for oil wells with a low yield ratio without heat treatment. The constituent elements of the present invention will be explained below.

First, regarding the components of the material, 0.26% of C is necessary to obtain the necessary strength, and if it exceeds 0.48%, the ductility and toughness deteriorate, which is undesirable.

Mn is required at 1.30% to obtain the necessary strength.
．． If it exceeds 70%, it is undesirable as it leads to deterioration of ductility and increase in cost of the steel pipe. 0.15% of Si is also required to ensure strength, and the upper limit is set to 0.30% due to the problem of acidity.
Steel having the above-mentioned components is melted in either a converter, open hearth, or electric furnace, and then processed into ingots, blooming, or continuous casting to produce steel billets and billets, which are then sent to a hot rolling process.
In hot rolling, the hot rolling end temperature is limited to 700 to 820 oC, and as is clear from FIG. 1, the hot rolling end temperature has a significant effect on the yield ratio.

In other words, when the temperature exceeds 82,000, strain due to rolling does not accumulate 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, if it is less than 70,000, the generated ferrite will be strained, enter a recrystallized state, and have a high yield ratio, which is not preferable. Next, the reason for cooling without water injection for 1 second or more and within 3 silica sands from the end temperature of hot rolling is that phelite can be uniformly generated during the cooling period after austenite rolling. , this ferrite plays a role in lowering the yield ratio.

However, the cooling of the sand/super steel in step 3 is not preferable from the viewpoint of the length restriction of the hot rolling cooling zone and productivity. After cooling in this way, the average cooling rate until winding is set to 5°C/sec or more, and the film is wound at a temperature of 45,000 to 65,000. When the cooling rate is low, such as less than 5° C./sec, it becomes coarse par**lite, which causes a decrease in tensile strength and a high yield ratio, and further deteriorates ductility, corrosivity, and weldability, which is not preferable. Also,
There is no need to particularly limit the upper limit of the cooling rate, but
The limit is considered to be 100 qC'sec due to equipment capacity constraints. If the coiling temperature is higher than 650 oo, the coil itself will undergo self-annealing, resulting in coarse barlite similar to when the cooling rate is slow, resulting in a decrease in tensile strength and a high yield ratio, resulting in ductility, corrosion resistance, and weldability. deteriorates and is not desirable.

If the coiling temperature is less than 45,000, martensite will be generated during cooling, which will significantly deteriorate ductility and hardness, which is not preferable. 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 condition is that the pipe is formed such that the material width Wo and the elongation rate in the longitudinal direction of the pipe are 1.5% or less.

The reason for this is that, as is clear from Figure 2, if the longitudinal elongation rate of the pipe is 1.5 or less, the yield ratio increases by △Y
It is possible to suppress R to 5% or less and prevent the yield ratio from increasing. The longitudinal elongation rate of the pipe 3 is determined by determining the material width Wo, so 3 is set to 1.5%.
If Wo is selected as shown below, an electric resistance welded steel pipe for oil wells with a stable low yield ratio can be manufactured.

Describing the determination of the material width Wo, it is calculated from the following equation. That is, 1: reduction rate in the pipe circumferential direction (%), 2: increase rate in the pipe wall thickness direction (%), 3: elongation rate in the longitudinal direction of the pipe (%), D: outer diameter, t: wall thickness. ,Wo
: Material width is 3 = {thing. ;; Urgent master; writing X1
00ku%))-----[1're=(chihanmu)(xm
o (%))...[2) Please. = {W station sky de} (X100(%
))---・--{The formula 31 holds true, and the material width Wo is determined from the above equation so that 3 is 1.54% or less.

Note that z3 and ', are theoretical formulas, but d2 is an empirical formula that includes Mill's unique constants. Next, Table 1 shows examples of the present invention.

Table 1 (Pipe size 114.3mm x 5.21) *Does not include slow cooling in the first stage, only cooling and back side cooling in the latter stage ** Although the method of the present invention was used as shown in the examples with insufficient tensile strength or more, 2. For example, it is possible to stably manufacture electrical steel pipes for oil wells with a low yield ratio without the need for pipe body heat treatment or the addition of particularly expensive elements, and its industrial value is extremely high.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a chart showing the relationship between the yield ratio and the end temperature of hot rolling, and FIG. 2 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

Claims (1)

[Claims] 1C: 0.26-0.48%, Mn: 1.30-1.
70%, Si: 0.15 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 820°C, and the hot rolling is completed. Allow to cool without pouring water for 1 to 30 seconds from temperature, then set the average cooling rate until coiling to be 5℃/sec or more, coil at 450℃ or higher and 650℃ or lower, and then adjust the material width during pipe forming. A method for producing an electric resistance welded steel pipe for oil wells with a low yield ratio without heat treatment, characterized in that W_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.