JP2551692B2 - Manufacturing method of low alloy seamless steel pipe with fine grain structure. - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a low alloy seamless steel pipe having a fine grain structure.

[0002]

2. Description of the Related Art In order to obtain a low alloy high tensile seamless steel pipe having a fine grain structure with a hot-rolled seamless steel pipe, see, for example, Japanese Patent Laid-Open No.
As described in JP-A-77813, the hollow shell that has been hot-roughly rolled is forcibly cooled once to a temperature of Ar ₁ point or lower and then heated again to the austenitizing temperature, and immediately after completion of the subsequent finish rolling, rapid cooling ( There is a method of quenching) -tempering or reheating quenching-tempering after completion of usual finish rolling.

[0003]

However, each of the above-mentioned methods has a drawback that the manufacturing process is complicated in addition to the problem of thermal efficiency. On the other hand, in the direct quenching process after the hot rolling seamless rolling so far, the austenite grain size is ASTM No. There is a problem that a low alloy seamless steel pipe having a fine grain structure cannot be obtained because the grain size is 1 to 6 and the variation is large and the variation is large.

The present invention solves the above problems, and by combining the components and hot rolling conditions to form a fine-grained structure, it has high strength, toughness, and seamlessness excellent in stress corrosion cracking resistance. It is intended to provide a method for manufacturing a steel pipe.

[0005]

DISCLOSURE OF THE INVENTION The inventors of the present invention have developed a low alloy seamless steel pipe having a fine grain structure and an SML (seamless) steel pipe excellent in sulfide stress cracking resistance (hereinafter referred to as SSC resistance). As a result of conducting many experiments for the purpose of manufacturing, it was found that a low alloy seamless steel pipe having a fine grain structure can be manufactured by controlling the steel composition and hot rolling conditions.

The present invention is constructed on the basis of this finding, and the gist thereof is as follows: C: 0.05 to 0.35%, Si: 0.01 to 0.5%, Mn: 0.15 as weight%. -2.5%, S: 0.01% or less, P: 0.02% or less, Mo: 0.05-0.4%, Al: 0.005-0.1%, Ti: 0.005- 0.1%, Nb: 0.005 to 0.1%, and if necessary Cr: 0.1 to 1.5%, Ni: 0.1 to 2.0%, V: 0. 01 to 0.1%, B: 0.0003 to 0.003%, one or more of, and rare earth elements: 0.001 to 0.05%, Ca: 0.00
1 to 0.02%, Co: 0.05 to 0.5%, Cu: 0.1 to 0.5%, a steel slab containing one or more kinds and the balance being substantially Fe. After heating excessively at 1200 ° C. or higher, a hollow shell is produced by hot piercing continuous rolling, and the shell cooled to a temperature of 850 ° C. to Ar ₁ point is heated to 900 to 100 higher than the temperature.
Quenching is performed by quenching a finished steel pipe obtained by heating to 0 ° C and hot finishing rolling at a finishing temperature of Ar ₃ points + 50 ° C or higher from a temperature of Ar ₃ points or higher, followed by Ac ₁ point or lower. Is a method for producing a low alloy seamless steel pipe having a fine grain structure, which is subjected to a tempering treatment of heating to the temperature of and cooling.

[0007]

The production method of the present invention will be described below in detail. First, the reason why the present invention is limited to the above steel components will be described. C and Mn are important in order to enhance the quenching effect to increase the strength, to stably obtain a high-strength steel having a yield point of 60 to 80 kgf / mm ² and to achieve grain refinement. If it is too small, the effect will not be obtained, and if it is too large, it will cause quench cracking.
%, 0.15 to 2.5%. Si is a residual deoxidizer and is an effective component for increasing strength. If it is too small, the effect is not obtained, and if it is too large, inclusions increase and the properties of the steel become brittle, so the content was made 0.01 to 0.5%. The content of P is 0.02% or less as a harmful component which easily causes cracks during working due to grain boundary segregation. S forms MnS-based inclusions and is drawn by hot rolling, and its content is 0.02% or less as a component harmful to low temperature toughness. Mo is
It is effective for increasing strength and improving toughness. If the amount is too small, the effect will not be obtained, and if the amount is too large, the effect will be saturated, and since it will be very expensive, the content was made 0.05 to 0.4%. Al
Is a residual deoxidizing agent similar to Si, and binds with N contained as an impurity component in steel to suppress the growth of crystal grains, lower the transition temperature of steel, and improve the low temperature toughness. If it is too small, the effect is not obtained, and if it is too large, inclusions increase and the properties of the steel become brittle, so the content was made 0.005 to 0.1%. N
b is the most important element among the components of the present invention. After making a hollow shell by hot piercing continuous rolling, 850 ℃ ~ A
r The temperature of the element pipe dropped to the point ₁
Γ grains when heated to ˜1000 ° C.
Since the grain coarsening temperature is remarkably lowered, the grains are coarsened at a normal reheating temperature (a temperature required for ensuring the quenching temperature after final finish rolling). Nb is an important element that suppresses growth coarsening of γ grains having such a rolling history. If the amount is too small, the effect will not be obtained, and if the amount is too large, the effect will be saturated, and since it is very expensive, 0.005-0.1%
And Ti combines with N, which is contained as an impurity component in the steel, to suppress the growth of crystal grains to enhance the strength and to exhibit the hardenability of B. If the amount is too small, the effect is not obtained. If the amount is too large, TiC is precipitated to embrittle the steel, and inclusions are increased to embrittle the properties of the steel.
%.

In the case of the steel having the above-mentioned composition, to further increase the strength of the steel, the components such as Cr, Ni, V and B are selectively added as required. Cr, Ni, and V are added to enhance the hardenability of steel and the strength. If the amount is too small, the effect will not be obtained, and if the amount is too large, the effect will be saturated.
It was set to 1.5%, 0.1 to 2.0%, and 0.01 to 0.1%. B remarkably improves the hardenability and enhances the strength.
If the amount is too small, the effect will not be obtained, and if the amount is too large, the effect will not change and the toughness and hot workability will be deteriorated.
It was set to 0.003%.

Further, in view of the use environment of the seamless steel pipe in recent years, the present invention relates to the SSC of the steel having the above-mentioned composition.
In order to improve the above, a component such as a rare earth element is selectively added as needed. Rare earth elements and Ca are effective components that make the inclusions spherical and harmless. If it is too small, there is no effect, and if it is too large, inclusions increase and SSC resistance decreases, so 0.001 to 0.0, respectively.
5% and 0.001-0.02%. Co and Cu are
It has the effect of increasing the strength of the steel and suppressing the penetration of hydrogen into the steel, and effectively acts on the SSC resistance. If it is too small, the effect will not be obtained, and if it is too large, the effect will be saturated, so 0.0
It was set to 5 to 0.5% and 0.1 to 0.5%.

Next, the reason for limiting the rolling conditions in the final step of the hot piercing continuous rolling as described above will be explained. The steel having the above composition is melted in a melting furnace such as a converter or an electric furnace or further subjected to a vacuum degassing process, and a steel slab is manufactured by a continuous casting method or an ingot lump method. The billet is immediately or once cooled and then heated to a temperature of 1200 ° C. or higher. The heating temperature must be set sufficiently high so that most of C, Cr, V, Ti, etc. are solid-dissolved before the hot piercing continuous rolling. If the temperature is within the range of the components of the present invention, all of them are solid-solved at a temperature of 1200 ° C. or higher, and there is no hindrance in hot forming work efficiency, so the heating temperature is 1200 ° C. or higher.

The steel slab heated to a high temperature is conveyed to a hot-rolling continuous rolling mill, rolled to a target outer diameter and wall thickness, and roughly formed into a hollow shell. After that, the raw steel pipe which has dropped to a temperature of 850 ° C. to Ar ₁ point is heated to 900 to 1000 ° C. higher than the temperature and subjected to hot finish rolling at a finishing temperature of Ar ₃ points + 50 ° C. or more to obtain a finished steel pipe. Is subjected to a quenching treatment in which it is rapidly cooled from a temperature of Ar ₃ point or higher. The figure shows the effect of Nb on the austenite (hereinafter referred to as γ) grain size after the direct quenching treatment of the steel pipe manufactured by this rolling.

[0012] The γ grain size after the direct quenching is remarkably coarsened when Nb is not added or the addition amount is 0.005% or less,
ASTM No. It will be about 1. Therefore, 0.005 to 0.1% of Nb is necessary to prevent the coarsening of γ grains. Regarding the influence of such Nb, according to the inventor's speculation, when Nb is not added or the addition amount is 0.005% or less, the temperature of the raw pipe is 850 inevitably in the current hot-piercing continuous rolling process. When the temperature is lowered to ₁ ° C. to Ar ₁ point and then heated to a temperature of Ac ₃ point or higher, the final step in the hot-rolling continuous rolling step is a relatively low temperature and under a small reduction, in the reheating step, γ Grain boundary migration occurs due to non-uniform strain between
It grows to 2-3 times the size of the surrounding gamma particles. Such grains subsequently undergo secondary recrystallization and become coarse γ grains. Addition of 0.005% or more of Nb plays an important role in suppressing the growth coarsening of γ grains having such a rolling history. That is, it has been found that Nb precipitates as Nb (CN) during cooling after hot indentation continuous rolling and during reheating thereafter, and exerts an important effect of suppressing the coarsening of γ grains.

A hollow shell which has been rolled under the above-mentioned compositional elements and rolling conditions and which has been lowered to a temperature of 850 ° C. to Ar ₁ point is used.
Heat to 00-1000 ° C. This heating temperature is 900
If the temperature is lower than ℃, the quenching temperature after hot final finish rolling cannot be secured, and if the temperature is higher than 1000 ° C, a large amount of oxide scale is generated on the steel surface, which adversely affects the securing of the shape accuracy of the steel pipe.
The temperature was limited to 00 to 1000 ° C.

Further, if the hot final finishing temperature is too low, the state of perfect γ at the time of quenching required for obtaining high strength cannot be ensured, so the Ar ₃ point is set to + 50 ° C. The quenching treatment starting temperature was set to ₃ Ar or higher in order to secure a sufficient quenching structure and obtain the required strength. The cooling rate at the time of quenching is not particularly limited, but it is faster than air cooling. The tempering temperature was set to Ac ₁ point or less because it is necessary to ensure the stability of strength and toughness. The heating method is not particularly limited.

The steel obtained under the above production conditions is effective for producing a low alloy seamless steel pipe having a fine grain structure without containing coarse grains.

[0016]

EXAMPLES Next, examples of the present invention will be described. Table 1
Is the strength, toughness, and γ grain size of the steel pipe that is directly quenched and tempered by hot-rolling continuous rolling, reheating the steel slab produced by continuous melting and smelting in a converter, and then performing hot final finishing rolling. And SSC resistance. The SSC resistance is NACE TM0
1-77, σth (Thre
The evaluation was carried out by determining the Hold Stress. "Steel pipe produced according to the present invention, the particle size of the γ compared to have and the conventional method of high strength is fine high toughness is obtained, SSC resistance
It can be seen that the property is improved by 0.2σy or more in σth.

[0017]

[Table 1]

[Table 2]

[0018]

INDUSTRIAL APPLICABILITY As described above, the steel pipe manufactured by the method of the present invention has high strength and fine grains, so that it has excellent low temperature toughness and SSC resistance, and is suitable for cold regions in the north and sulfide stress corrosion environment. Is extremely effective in.

[Brief description of drawings]

FIG. 1 shows the effect of Nb content in hot-piercing continuous rolling on the γ grain size after direct quenching.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kumi Sato 1-1 Tobahata-cho, Tobata-ku, Kitakyushu City, Fukuoka Prefecture Nippon Steel Co., Ltd. Yawata Works (72) Inventor Tomio Terazawa Tobata, Tobata-ku, Kitakyushu City, Fukuoka Prefecture No. 1 in Machi Nippon Steel Co., Ltd. Inside Yawata Works (56) References JP-A-58-136715 (JP, A) JP-A-60-75523 (JP, A)

Claims (4)

(57) [Claims] 1. As weight%, C: 0.05 to 0.35%, Si: 0.01 to 0.5%, Mn: 0.15 to 2.5%, S: 0.01% or less, P: 0.02% or less, Mo: 0.05 to 0.4%, Al: 0.005 to 0.1%, Ti: 0.005 to 0.1%, Nb: 0.005 to 0.1 %, And the balance is 1200
After heating to ℃ or more, the hollow shell is made by hot piercing continuous rolling, and the shell that has dropped to a temperature of 850 ℃ ~ Ar ₁ point,
Heating to 900 to 1000 ° C., which is higher than the temperature, and hot finishing rolling with a finishing temperature of Ar ₃ points + 50 ° C. or higher, and the finished steel pipe thus obtained is rapidly cooled from the temperature of Ar ₃ points or higher. A method for producing a low alloy seamless steel pipe having a fine grain structure, which comprises performing a quenching treatment, and subsequently performing a tempering treatment of heating to a temperature of Ac ₁ point or lower and cooling. 2. As a weight%, C: 0.05 to 0.35%, Si: 0.01 to 0.5%, Mn: 0.15 to 2.5%, S: 0.01% or less, P: 0.02% or less, Mo: 0.05 to 0.4%, Al: 0.005 to 0.1%, Ti: 0.005 to 0.1%, Nb: 0.005 to 0.1 %, Further, Cr: 0.1 to 1.5%, Ni: 0.1 to 2.0%, V: 0.01 to 0.1%, B: 0.0003 to 0.
After heating a steel slab containing one or two or more of 003% and the balance substantially consisting of Fe to 1200 ° C. or higher, a hollow shell is produced by hot rolling continuous rolling, and 850 ° C. to Ar ₁ The blank pipe that has been lowered to the temperature of the point is heated to 900 to 1000 ° C. higher than the temperature and subjected to hot finish rolling at a finishing temperature of Ar ₃ points + 50 ° C. or more, and the finished steel pipe thus obtained is obtained. , Ar, quenching treatment is performed from the temperature of ₃ points or higher, followed by Ac ₁
A method for producing a low alloy seamless steel pipe having a fine-grained structure, which comprises performing a tempering treatment of heating to a temperature below a point and cooling. 3. As weight%, C: 0.05 to 0.35%, Si: 0.01 to 0.5%, Mn: 0.15 to 2.5%, S: 0.01% or less, P: 0.02% or less, Mo: 0.05 to 0.4%, Al: 0.005 to 0.1%, Ti: 0.005 to 0.1%, Nb: 0.005 to 0.1 %, Further, rare earth element: 0.001 to 0.05%, Ca: 0.00
1 to 0.02%, Co: 0.05 to 0.5%, Cu: 0.1 to 0.5%, a steel slab containing one or more kinds and the balance substantially consisting of 1200 After heating to ℃ or higher, a hollow shell is produced by hot piercing continuous rolling, and the shell that has dropped to a temperature of 850 ℃ ~ Ar ₁ point, 900 ~ 1000 ℃ higher than the temperature
The finished steel pipe thus obtained is subjected to hot finish rolling with a finishing temperature of Ar ₃ points + 50 ° C. or higher by heating to
Quenching is performed by quenching from a temperature of ₃ points or more, and then A
c A method for producing a low alloy seamless steel pipe having a fine grain structure, which comprises performing a tempering treatment of heating to a temperature of ₁ point or less and cooling. 4. As weight%, C: 0.05 to 0.35%, Si: 0.01 to 0.5%, Mn: 0.15 to 2.5%, S: 0.01% or less, P: 0.02% or less, Mo: 0.05 to 0.4%, Al: 0.005 to 0.1%, Ti: 0.005 to 0.1%, Nb: 0.005 to 0.1 %, Further, Cr: 0.1 to 1.5%, Ni: 0.1 to 2.0%, V: 0.01 to 0.1%, B: 0.0003 to 0.003. %, One or more of, and rare earth elements: 0.001 to 0.05%, Ca: 0.00
1 to 0.02%, Co: 0.05 to 0.5%, Cu: 0.1 to 0.5%, a steel slab containing one or more kinds and the balance being substantially Fe. After heating to 1200 ° C. or higher, a hollow shell is manufactured by hot piercing continuous rolling, and the shell that has dropped to a temperature of 850 ° C. to Ar ₁ point is heated to 900 to 1000 ° C. higher than the temperature.
The finished steel pipe thus obtained is subjected to hot finishing rolling at a finishing temperature of Ar ₃ points + 50 ° C. or higher, and the finished steel pipe thus obtained is subjected to Ar ₃
Quenching treatment is performed by quenching from a temperature above the point, followed by Ac
_A method for producing a low alloy seamless steel pipe having a fine-grained structure, which comprises performing a tempering treatment of heating to a temperature of ₁ point or less and cooling.