JP2556643B2 - Low Yield Ratio High Toughness Seamless Steel Pipe Manufacturing Method - 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 seamless steel pipe having a low yield ratio and high toughness.

[0002]

2. Description of the Related Art In recent years, due to depletion of energy resources,
The development of gas and oil wells in the far north has become active. For this reason, a low alloy seamless steel pipe used as a transportation device for products can be used in a high pressure operation in a cold region, and thus has a low yield ratio, high toughness (guaranteed at -60 ° C), and high strength (X52).
There is a tendency for properties that combine the above) to be required. To give a high yield strength a low yield ratio, for example, "iron and steel"('
87-S1315), it is reported that the amount of C increases or the tempering temperature after quenching decreases. However, an increase in the amount of C remarkably lowers the weldability which is the basic use performance of the line pipe material, and in this case preheating is required before welding, but the workability is remarkably lowered when laying the line pipe. On the other hand, since the low temperature toughness of the line pipe material manufactured by the low temperature tempering process becomes unstable, its use in cold regions is restricted.

[0003]

SUMMARY OF THE INVENTION The present invention is not limited to the above-mentioned technical restrictions, and by adjusting the steel composition and hot rolling conditions, a seamless steel pipe having a fine structure and a low yield ratio and high toughness can be obtained. The purpose is to provide a method of manufacturing.

[0004]

The present invention is constructed to achieve the above object, and the gist thereof is as follows: C: 0.05 to 0.35% by weight, Si: 0.01 to 2 .5
%, Mn: 0.15 to 2.5%, S: 0.01% or less, P: 0.02% or less, Al: 0.005 to 0.1
%, Ti: 0.005-0.1%, Nb: 0.005-
0.1%, N: 0.01% or less, if necessary, Cr: 0.1-1.5%, Mo: 0.05-0.4%,
Ni: 0.1-2.0%, V: 0.01-0.1%,
B: 0.0003 to 0.0033%, one kind or two or more kinds, and if necessary, a 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% of one or more kinds is contained, and the balance is substantially Fe.
After heating the steel slab consisting of the above to 1200 degreeC or more, it is forcibly cooled to 1100 degreeC-900 degreeC during the hot-piercing continuous rolling,
Successively, hot continuous rolling of 15% or more is performed at a wall thickness cross-section reduction rate, the hollow shell lowered to a temperature of 850 ° C to Ar ₁ point is heated to 900 to 1000 ° C higher than the temperature, and the finishing temperature is Ar ₃ It is a method for producing a seamless steel pipe having a low yield ratio and high toughness, which is characterized by performing hot finish rolling at a point of + 50 ° C. or higher and subjecting the finished steel pipe thus obtained to an air cooling treatment at the temperature.

[0005]

The operation of the present invention will be described in detail below.
First, the reason why the present invention is limited to the above steel components will be described. C and Mn are important for securing strength and for atomization. If the amount is too small, the effect is not obtained, and if the amount is too large, the weldability is deteriorated. Therefore, the amounts were made 0.05 to 0.35% and 0.15 to 2.5%, respectively. Si is a residual deoxidizer and is an effective component for increasing strength. If it is too small, the effect will not be obtained, and if it is too large, inclusions will increase and the properties of the steel will become brittle.
It was set to 2.5%.

The content of P is 0.02% or less as a harmful component which easily causes cracks during processing due to segregation of grain boundaries. S forms MnS-based inclusions and is stretched by hot rolling, and its content is 0.02% as a component harmful to low temperature toughness.
Below. Al, like Si, has a deoxidizing agent remaining, and binds with N contained as an impurity component in the steel to suppress the growth of crystal grains, lower the transition temperature of the steel, and improve the low temperature toughness. If it is too small, the effect will not be obtained, and if it is too large, inclusions will increase and the properties of the steel will become brittle.
It was set to 0.1%.

Both Ti and Nb are the most important elements among the components of the present invention as elements for controlling the crystal grain size during seamless rolling. Ti combines with N contained as an impurity component in the steel, suppresses crystal grains during hot piercing rolling, and is forcibly cooled to 1100 ° C. to 900 ° C. during hot continuous rolling, and then during hot continuous rolling. Suppresses crystal grain growth and resists S
In addition to enhancing the SC property and low temperature toughness, it enhances the strength by exerting the hardenability of B due to the action of deoxidation and denitrification. If it is too small, the effect is not obtained, and if it is too large, TiC is precipitated to embrittle the steel, and inclusions increase to embrittle the properties of the steel. Therefore, it is set to 0.01 to 0.1%. On the other hand, Nb controls the crystal grain size after completion of the hot piercing continuous rolling. Γ in the case where the raw tube which has fallen to a temperature of 850 ° C. to Ar ₁ point after completion of the hot piercing continuous rolling is heated to 900 to 1000 ° C. higher than the temperature.
The grain size of the grains is remarkably lowered by the γ-grain coarsening, and the grains are abnormally coarsened at a normal reheating temperature (the reheating temperature required for performing the quenching treatment after the final finish rolling). Nb
Is an important element for suppressing abnormal coarsening of γ grains having such a rolling history. If it is too small, the effect is not obtained, and if it is too large, the effect is saturated, and it is very expensive, so the content was made 0.005 to 0.1%.

In order to further increase the strength of the steel having the above-mentioned composition, Cr, Mo, Ni, V, etc. are selectively added if necessary. If Cr, Mo, Ni, and V are too small, the effect will not be obtained, and if too large, the effect will be saturated, and since they are very expensive, 0.1 to 1.5% and 0.0%, respectively.
5 to 0.40%, 0.1 to 2.0%, 0.01 to 0.1
%.

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. The rare earth element, Ca, is an effective component that makes the inclusions spherical and harmless. If it is too small, the effect will not be obtained, and if it is too large, inclusions will increase and SS resistance will increase.
0.001 to 0.05 because it decreases the C property
%, 0.001 to 0.02%. Co and Cu have an effect of suppressing hydrogen invasion into the steel and effectively act on SSC resistance. If the amount is too small, the effect will not be obtained, and if the amount is too large, the effect will be saturated, so 0.05 to 0.5% and 0.1 to 0.1%, respectively.
It was set 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 heated to a temperature of 1200 ° C. or higher immediately or after being once cooled. The heating temperature must be set sufficiently high so that most of C, Cr, V, Ti, etc. are solid-dissolved before the hot indenting continuous rolling. 1 within the range of the components of the present invention
All the components were solid-solved at a temperature of 200 ° C. or higher, and there was no problem in the hot forming process efficiency, so the heating temperature was 1200 ° C. or higher.

The steel billet heated to a high temperature is conveyed to a hot piercing and rolling mill. The pierce-rolled tube is then forcibly cooled during the hot continuous rolling, and then rolled to a target outer diameter and wall thickness to be roughly formed into a hollow tube.
If the temperature is higher than 00 ° C., the recrystallized grains after rolling remarkably grow and the desired fine grains cannot be obtained. FIG. 1 shows the effect of the rolling temperature after forced cooling on the recrystallized γ grain size after hot continuous rolling. The recrystallized γ grain size after hot continuous rolling is
When the rolling temperature is 1100 ° C or higher, recrystallized grains significantly grow and the ASTM No. It will be about 1 to 2. Therefore, the forced cooling temperature needs to be 1100 ° C. or lower. Further, when the rolling temperature is 900 ° C. or less, the formability of the steel pipe is significantly reduced due to an increase in rolling load, and it becomes difficult to obtain the target outer diameter and wall thickness. Therefore, the forced cooling temperature during hot continuous rolling is 110
It was set to 0 ° C to 900 ° C. If the continuous hot rolling reduction after forced cooling is small, the atomization effect is lost, so the content was made 15% or more.

After completion of hot-rolling continuous rolling, 850 ° C. to Ar ₁
The temperature of the raw tube lowered to the temperature of the point is higher than the temperature of 900
Reheat to ~ 1000 ℃ and finish temperature is Ar ₃ points + 50 ℃
The finished steel pipe obtained by subjecting the above hot finish rolling to Ar
Air-cooling is applied from ₃ or more points. FIG. 2 shows the effect of Nb on the ferrite grain size of the steel pipe manufactured by this rolling. Even if Ti is added as a crystal grain control element and fine γ grains are obtained by rolling at 1100 ° C. to 900 ° C.,
The ferrite grain size after air-cooling is extremely coarse when Nb is not added or the amount added is less than 0.005%
o. The γ grain size is about 5. Nb 0.005-0.1
% Addition, grain growth is suppressed. Regarding the influence of such Nb, according to the inventor's speculation, when Nb is not added, the temperature of the raw pipe is unavoidably increased to 850 ° C. to Ar ₁ point in the current hot piercing continuous rolling process. It is conceivable that under the condition that the final process in the descending hot-boring continuous rolling is a relatively low temperature and weak working, specific γ grains move to the boundary γ grains in the surroundings to form a coarse structure. Ferrite grains obtained from such γ grains by air cooling are coarse grains. Nb0.0
Addition of 05% or more plays an important role of suppressing the growth coarsening of γ grains having such rolling history. That is,
It has been found that Nb precipitates as NbC during cooling after hot-piercing continuous rolling and during reheating thereafter, and exerts an important effect for suppressing coarsening of γ grains and obtaining fine ferrite 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 shape of the pipe after finish rolling will be insufficient, but if the temperature is higher than 1000 ℃, a large amount of oxide scale will be generated on the steel surface, which will adversely affect the accuracy of the shape of the steel pipe.
Limited to temperatures of ~ 1000 ° C. The ferrite grains obtained under the above production conditions are effective for producing a low yield ratio, high toughness seamless steel pipe having a fine-grained structure without including coarsening.

[0014]

EXAMPLES Next, examples of the present invention will be described. Table 1
Is a steel slab produced by melting and continuous casting in a converter, followed by hot piercing rolling, forced cooling during hot continuous rolling, reheating after hot continuous rolling, and then hot final rolling and air cooling. The toughness and ferrite grain size of the manufactured steel pipe are shown.

It can be seen that the steel pipe manufactured according to the present invention has finer ferrite grains and higher toughness than the comparative method.

[0016]

[Table 1]

[0017]

[Table 2]

[0018]

The steel pipe manufactured by the method of the present invention as described above has high strength and fine grains, and therefore has excellent low temperature toughness and SSC resistance, and is suitable for cold regions in the far north and sulfide stress corrosion environments. used.

[Brief description of drawings]

FIG. 1 is a diagram showing the effect of rolling temperature after forced cooling on the γ grain size after hot-rolling continuous rolling.

FIG. 2 is a diagram showing the influence of the amount of Nb on the γ grain size after direct quenching.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kumi Sato 1-1, Toibata-cho, Tobata-ku, Kitakyushu-shi, Fukuoka Nippon Steel Corporation Yawata Works (56) Reference JP-A-4-21721 (JP, A) JP-A-3-162524 (JP, A) JP-A-3-64415 (JP, A)

Claims (4)

(57) [Claims] 1. As a weight%, C: 0.05 to 0.35%, Si: 0.01 to 2.5%, Mn: 0.15 to 2.5%, S: 0.01% or less, P: 0.02% or less, Al: 0.005 to 0.1%, Ti: 0.005 to 0.1%, Nb: 0.005 to 0.1%, N: 0.01% or less, 1200 containing a steel slab containing the balance substantially Fe
After heating to ℃ or more, 1100
℃ to 900 ℃ forced cooling, and then continuously carry out hot continuous rolling of 15% or more at the thickness cross section reduction rate, 850 ℃ to Ar
A hollow shell that has dropped to a temperature of ₁
After heating to 0 to 1000 ° C, the finishing temperature is Ar ₃ points + 50 ° C.
A method for producing a seamless steel pipe with a low yield ratio and high toughness, which comprises subjecting the finished steel pipe thus obtained to the hot finish rolling and subjecting the finished steel pipe thus obtained to an air cooling treatment from the temperature. 2. As weight%, C: 0.05 to 0.35%, Si: 0.01 to 2.5%, Mn: 0.15 to 2.5%, S: 0.01% or less, P: 0.02% or less, Al: 0.005 to 0.1%, Ti: 0.005 to 0.1%, Nb: 0.005 to 0.1%, N: 0.01% or less, In addition, Cr: 0.1 to 1.5%, Mo: 0.05 to 0.4%, Ni: 0.1 to 2.0%, V: 0.01 to 0.1%, B : A steel slab containing 0.0003 to 0.0033% of 1 type or 2 types or more and the balance of which is substantially Fe is heated to 1200 ° C. or higher, and then 1100 ° C. to 900 ° C. during hot rolling continuous rolling. After performing forced cooling to ℃, and then continuously performing hot rolling at 15% or more at a wall thickness cross-section reduction rate, the hollow shell that has dropped to a temperature of 850 ° C. to Ar ₁ point is After being heated to a temperature higher than 900 ° C to 1000 ° C, the finishing temperature is subjected to hot finish rolling at an Ar ₃ point + 50 ° C or higher, and the finished steel pipe thus obtained is subjected to air cooling treatment from that temperature. Low yield ratio High toughness Seamless steel pipe manufacturing method. 3. As weight%, C: 0.05 to 0.35%, Si: 0.01 to 2.5%, Mn: 0.15 to 2.5%, S: 0.01% or less, P: 0.02% or less, Al: 0.005 to 0.1%, Ti: 0.005 to 0.1%, Nb: 0.005 to 0.1%, N: 0.01% or less, In addition, rare earth elements: 0.001 to 0.05%, Ca: 0.001 to 0.02%, Co: 0.05 to 0.5%, Cu: 0.1 to 0.5%, After heating a steel slab containing 1 or 2 or more of the above and the balance substantially consisting of Fe to 1200 ° C. or higher, it is forcibly cooled to 1100 ° C. to 900 ° C. during the continuous hot piercing rolling, and the wall thickness is continued. Hot continuous rolling of 15% or more is performed at a cross-section reduction rate, and a hollow shell that has dropped to a temperature of 850 ° C. to Ar ₁ point is heated to a temperature higher than 900 ° C. After heating to ˜1000 ° C., a finishing temperature is Ar ₃ point + 50 ° C. or more, and hot finishing rolling is performed, and the finished steel pipe thus obtained is subjected to air cooling treatment from that temperature, which is characterized by a high yield ratio. Manufacturing method of toughness seamless steel pipe. 4. As weight%, C: 0.05 to 0.35%, Si: 0.01 to 2.5%, Mn: 0.15 to 2.5%, S: 0.01% or less, P: 0.02% or less, Al: 0.005 to 0.1%, Ti: 0.005 to 0.1%, Nb: 0.005 to 0.1%, N: 0.01% or less, In addition, Cr: 0.1 to 1.5%, Mo: 0.05 to 0.4%, Ni: 0.1 to 2.0%, V: 0.01 to 0.1%, B : 0.0003 to 0.0033% of 1 type or 2 or more types and rare earth elements: 0.001 to 0.05%, Ca: 0.001 to 0.02%, Co: 0.05 to 0.5% , Cu: 0.1 to 0.5%, and a steel slab containing one or more of the above and the balance substantially consisting of Fe is heated to 1200 ° C. or higher, and then 1 Forced cooling to 100 ° C. to 900 ° C., followed by continuous hot rolling of 15% or more at a wall thickness reduction rate, the hollow shell lowered to a temperature of 850 ° C. to Ar ₁ point was heated to 900 to 1000 higher than the temperature. Low yield ratio and high toughness seamless, characterized in that after heating to ℃, the finishing temperature is hot finish rolling with Ar ₃ point + 50 ℃ or more, and the finished steel pipe thus obtained is subjected to air cooling treatment from that temperature. Steel pipe manufacturing method.