JPH09111344A - Production of high strength and low yield ratio seamless steel pipe - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method or producing a high toughness seamless steel pipe having high strength and low yield ratio, furthermore excellent in SSC (suphide stress cracking) resistance or more over having a fine-grained structure by regulating the steel components and rolling-heat treating conditions. SOLUTION: A slab contd., by weight, 0.02 to 0.2% C, 0.01 to 0.5% Si, 0.15 to 2.5% Mn, <=0.02% P, <=0.01% S, 0.005 to 0.1% Al, <=0.01% N and 0.005 to 0.1% Ti and furthermore contg. one or >=two kinds among Cr, Mo, Ni, V, B, rare earth metals, Ca, Co and Cu according to necessity is formed into a hollow pipe stock by hot piercing continuous rolling, which is next subjected to finish rolling to produce a finished steel pipe. This steel pipe is subjected to quenching treatment of executing rapid cooling from the Ar3 point or above, is thereafter heated to between the Ac1 point and the Ac3 point, is rapidly cooled and is then subjected to tempering treatment of executing heating to the Ac1 point or below and executing cooling.

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 high strength and a low yield ratio.

[0002]

2. Description of the Related Art Seamless steel pipes used for excavation of oil wells or line pipes for transporting oil or natural gas are required to have a low yield ratio from the viewpoint of improving safety against fracture. In order to make a low yield ratio steel pipe with a hot-rolled seamless steel pipe, there is a method in which it is structurally made into a two-layer structure of ferrite and pearlite by performing a normalizing treatment that is as-rolled or reheated to the austenite region and air-cooled after rolling. is there. However, in any of these methods, AP
High strength of IX grade cannot be obtained. On the other hand, there is a problem that a low yield ratio cannot be achieved when quenching-tempering treatment is performed to increase strength.

In order to solve such a problem, Japanese Unexamined Patent Publication (Kokai) 2
In Japanese Patent No. 2824227, steel having a mixed structure of ferrite and pearlite at normal temperature is heated from a temperature of Ac ₁ transformation point + 10 ° C. to a temperature of Ac ₁ transformation point + 90 ° C., subjected to rapid cooling treatment after holding, and then tempered. A method for producing a high-strength steel pipe having a maximum hardness and a low yield ratio is disclosed. In addition, "Materials and Processes" (No.3, Vol.4, 1991 Japan Iron and Steel Institute, No. 1
For the 21st (Spring) Lecture Meeting Proposal 552), Nb-Mo steel pipe rolled in a seamless process was quenched (930 ° C x 10 min).
It has been proposed to obtain a low yield ratio material by heating from a (water cooling) treated material (main structure of bainite) to a two-layer temperature range, water cooling and tempering. Further, in Japanese Patent Laid-Open No. 3-64415, Ti
Disclosed is a method for producing a high-toughness seamless steel pipe having a fine grain structure by piercing-rolling a steel slab containing Nb and Nb, reheating a temperature-reduced raw pipe to finish rolling, and then quenching and tempering the steel pipe. .

[0004]

However, in hot seamless rolling, the austenite grain size after rolling is AS
Since there is a large variation between TM Nos. 1 to 6, there was a problem that a seamless steel pipe with stable strength and yield ratio could not be obtained even if the above heat treatment was performed. The present invention solves such a problem, and an object of the present invention is to provide a method for manufacturing a seamless steel pipe having stable high strength and low yield ratio by controlling steel components and rolling / heat treatment conditions. .

[0005]

That is, the gist of the present invention is as follows. As weight%, C: 0.02% to 0.20%, Si: 0.01% to 0.5%, Mn: 0.15% to 2.5%, P: 0.020% or less, S: 0.010% or less, Al: 0.005% to 0.1%, Ti: 0.005% to 0.1%, N: 0.01% or less, and, if necessary, Cr: 0. 1% to 1.5%, Mo: 0.05% to 0.5%, Ni: 0.1% to 2.0%, V: 0.01% to 0.1% B: 0.0003% ~ 0.0030% Group 1 element and rare earth element: 0.001% ~
0.05%, Ca: 0.001% to 0.02%, Co:
It is a second group element consisting of 0.05% to 0.5% and Cu: 0.1% to 0.5%, contains at least one selected from both groups, and the balance is substantially Fe. The resulting steel slab is processed at a reduction rate of ₃ to 15% at a temperature of 900 to 700 ° C. in the final step of hot piercing continuous rolling, and Ar ₃ point −10.
A hollow shell that has been lowered to a temperature of 0 ° C. to Ar ₃ points + 50 ° C. is heated to a temperature of 900 to 1000 ° C., which is higher than the temperature of the shell, and then hot finishing at a finishing temperature of Ar ₃ points + 50 ° C. or more. After rolling, the obtained finished steel pipe is subjected to a quenching treatment for quenching from a temperature of Ar ₃ points or higher, then heated to a temperature between Ac _{1 to} Ac ₃ points, and then suddenly cooled, and then the Ac ₁ point or lower. A method for producing a seamless steel pipe having a high strength and a low yield ratio, which is characterized by performing a tempering treatment of heating and cooling to a temperature.

[0006]

BEST MODE FOR CARRYING OUT THE INVENTION The manufacturing method 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 in order to enhance the hardening effect to increase the strength, to stably obtain a high tensile strength steel having a yield point of 40 to 60 kgf / mm ² and to reduce the yield ratio. If it is too small, the effect will not be obtained, and if it is too large, quench cracking will be induced and the hardness will become high, resulting in a decrease in SSC resistance.
It was set to 2.5%.

Si is a residual deoxidizer and is an effective component for increasing the strength and decreasing the yield ratio.
If it is too small, the effect is not obtained, and if it is too large, inclusions increase and the SSC resistance decreases, so the content was made 0.01 to 0.5%.

[0008] P is a harmful component which tends to cause grain boundary segregation to cause cracks during processing, and since it causes deterioration of low temperature toughness, its content is set to 0.020% or less. S is
MnS-based inclusions are formed and stretched by hot continuous rolling to form a layered structure, which improves the fracture propagation performance of steel. 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%.

Al, like Si, has a deoxidizing agent remaining, and is combined with N contained as an impurity component in steel to suppress the growth of crystal grains and improve the fracture propagation performance of steel. 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%.

Ti is the most important element as a crystal grain size controlling element during seamless rolling. Ti combines with N, which is contained as an impurity component in the steel, to control the crystal grains during hot rolling and suppress the growth of crystal grains after hot rolling to improve the fracture propagation performance of the steel, as well as deoxidize, From the effect of denitrification, the hardenability of B described later is exerted to enhance the strength. If it is too small, the effect is not obtained, and if it is too large, TiC precipitates and the steel is embrittled, so the content was made 0.005 to 0.1%. The content of N is 0.01% or less as a harmful component that deteriorates the hardenability of B described later.

In the case of the steel having the above-mentioned composition, to further increase the strength of the steel, Cr and other components are selectively added as required.
Cr, Mo, 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 it is too large, the effect will be saturated, and since it will be very expensive, the upper and lower limits are limited for each, and Cr is 0.1 to 1.5% and Mo is 0. 05-0.5
%, Ni 0.1 to 2.0%, V 0.01 to 0.1%
And Further, 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. On the other hand, the toughness and hot workability will be deteriorated, so the content range was made 0.0003 to 0.0030%. Each element of Cr to B may be selectively added as needed.

Further, in view of the use environment of the seamless steel pipe in recent years, the present invention further provides an SSC resistant steel having the above-mentioned composition.
In order to improve the properties, rare earth elements, Ca, Co, Cu and other components are selectively added as needed. Rare earth element, C
“A” is an effective component that makes the shape of the inclusion spherical and renders it harmless. If the amount is too small, there is no effect, and if the amount is too large, inclusions increase and SSC resistance decreases, so rare earth elements are 0.001 to 0.05%, and Ca is 0.001 to 0.0.
2%. 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 Co will be 0.
05-0.5%, Cu 0.1-0.5%.

Next, the reason why the hot seamless rolling conditions are limited as described above will be explained. Steel having the above-described composition is melted in a melting furnace such as a converter or an electric furnace, or is further subjected to vacuum degassing treatment, and a steel slab is manufactured by a continuous casting method or an ingot agglomeration method. The steel slab is immediately or once cooled and then heated to a high temperature for hot piercing and rolling. The heating temperature must be sufficiently high to facilitate hot piercing and rolling. As long as the steel is within the range of the composition of the present invention, there is no problem even if hot drilling is performed at a temperature of 1200 ° C. or higher.
The heating temperature was 1000 to 1250 ° C.

After hot rolling, the hot billet is conveyed to a hot continuous rolling mill, rolled to a target outer diameter and wall thickness, and roughly formed into a hollow shell. This rolling has a great influence on the material of the manufactured steel pipe, especially on the grain size and the yield ratio. Figures 1 and 2 show the γ grain size of steel pipes (compositions of steel No. 5 in Table 1) that have been quenched after rolling in order to investigate the grain size of austenite immediately after rolling, and the rolling conditions and the reheating start in the final process of hot-rolling continuous rolling. The relationship between the temperature and the reheating furnace temperature is shown. As shown in the figure, the γ grain size immediately after rolling greatly changes from ASTM No. 0 to 8 depending on each condition.

According to the study of the present invention, the yield ratio of API X grade high-strength steel is affected by the grain size before quenching, and the yield ratio decreases as the grain size becomes relatively large. We have found that the γ grain size that satisfies the yield ratio and the low temperature toughness required for line pipe materials, etc. must be ASTM Nos. 3-5. Also, ASTM No.
In order to obtain a γ grain size of 3 to 5, it is essential to utilize the γ grain coarsening phenomenon caused by secondary recrystallization after strain-induced grain growth that occurs during reheating from the final process of hot-rolling continuous rolling. It started.

Γ grain size control utilizing strain-induced grain growth is
It becomes possible by defining the rolling conditions, the reheating start temperature, and the reheating furnace temperature in the final process of the hot piercing continuous rolling as follows. That is, when the reduction temperature in the final rolling of the hot piercing continuous rolling exceeds 900 ° C., the strain energy introduced by the working is recovered and the strain energy is reduced by recrystallization, so that the driving force for the strain-induced grain growth disappears and the grain size of the crystal grain is reduced. Coarsening does not occur and prevents the yield ratio of the steel pipe from decreasing. Therefore, the reduction temperature in the final process of hot-rolling continuous rolling is 900 to 700 ° C.
Limited to.

In such hot piercing continuous rolling, if the reduction amount is 0 to 2%, the driving force for the strain-induced grain growth is insufficient, and if it is 15% or more, the strain energy accumulated becomes too large and the strain energy is reduced or reduced. Since the driving force for strain-induced grain growth disappears due to the growth of γ grains having no strain energy in the reheating process, the grain size does not become coarse and the yield ratio of the steel pipe is prevented from lowering. Therefore, the amount of reduction in the final process of hot piercing continuous rolling was limited to 3 to 15%.

After the reduction, the hollow crude tube that has dropped to a temperature of 850 ° C. to Ar ₁ point is reheated and finish-rolled. The reheating start temperature is Ar ₃ -100 ° C. to Ar ₃ -150 ° C.
In the case of Ar ₃ + 50 ° C, the driving force for the strain-induced grain growth disappears and moderate γ grain coarsening does not occur, so that the yield ratio of the steel pipe increases. Inhibit the decline. Therefore, the reheating start temperature after the reduction is set to Ar ₃ −100 ° C. to Ar ₃ + 50 ° C. On the other hand, if the reheating temperature is less than 900 ° C., it is not sufficient for the growth of γ grains and the quenching temperature after the hot final finish rolling cannot be secured,
Moreover, when the temperature exceeds 1000 ° C, the γ grains are suddenly coarsened,
The low temperature toughness is significantly deteriorated and a large amount of oxide scale is generated on the steel surface, which adversely affects the surface quality of the steel pipe. Therefore, the reheating temperature during finish rolling is set to a temperature of 900 ° C to 1000 ° C.

The hollow shell heated to 900 to 1000 ° C. is subjected to hot final finish rolling at a finishing temperature of Ar ₃ point + 50 ° C. or higher, and the obtained finished steel pipe is subjected to quenching treatment. The cooling start temperature after the hot final finish rolling has a stable and sufficient quenching structure, secures the required strength, sour resistance and toughness, and is Ac _{3 to} A after the quenching treatment.
The temperature is set to Ar ₃ point + 50 ° C. or higher because it is necessary to secure the uniformity of the structure obtained by the quenching treatment from the c ₁ point.

After the quenching treatment for quenching from the Ar ₃ point + 50 ° C. or higher, it is subsequently reheated to the Ac ₃ to Ac ₁ point and then subjected to the quenching treatment. This treatment is an important step in the present invention for obtaining high strength and low yield ratio of the fine grain structure. The reheating temperature greatly affects the target strength and yield ratio. This temperature is not particularly limited as long as it is in the Ac ₃ to Ac ₁ range, but if the Ac ₁ point is lower than + 50 ° C., the strength is remarkably lowered, and if the Ac ₃ point is −50 ° C. or higher, the yield ratio tends to be high. , Ac ₁ point + 50 ° C. to Ac ₃ point −50 ° C. is preferable. The cooling rate at the time of direct quenching and from the Ac _{3 to} Ac ₁ point is not particularly limited, but it is faster than air cooling.

After that, a tempering treatment is carried out by heating to a temperature below the Ac ₁ point and cooling. The tempering temperature is used to ensure stability of strength, sour resistance and toughness,
It should be Ac ₁ or less. The heating method is not particularly limited.

The steel pipe obtained under the above manufacturing conditions is suitable for a seamless steel pipe which is required to have various characteristics of γ grains, a fine grain structure, high strength, low yield ratio, sour resistance, and toughness. Is.

[0023]

EXAMPLE Molten steel having the components shown in Table 1 was refined in a converter, and the molten steel was cast into a slab, which was seamlessly rolled-directly quenched and subsequently Ac _{1 to} Ac ₃ under the following conditions.
A two-phase region treatment of heating and quenching to a spot and a tempering treatment of Ac ₁ or less were performed.

The slab processing conditions of the present invention are as follows. Slab heating temperature: 1200 ° C. Final rolling of hot piercing continuous rolling: 850 ° C. × 5% Reheating start temperature: Ar ₃ −75 ° C. to Ar ₃ + 50 ° C. Final finishing rolling temperature: 850 ° C. Final finishing reduction amount: 20% Direct quenching temperature: 900 ° C. Two-phase region heating temperature: 780-860 ° C. Tempering temperature: 600 ° C. Further, the following treatment was performed as a comparative example. Heating temperature: 1200 ° C Final rolling of hot piercing continuous rolling: 950 ° C x 5% Reheating start temperature: Ar ₃ + 50 ° C Final finishing rolling temperature: 850 ° C Final finishing reduction amount: 20% Reheating quenching temperature: 950 ° C Tempering Temperature: 600 ℃

Table 2 shows the characteristics (yield strength, yield ratio, SCC resistance) of the seamless steel pipe obtained by the above treatment. The SSC resistance was evaluated by obtaining σth (Threshold Stress) by the loading method according to NACE TM01-77. As is apparent from the above, it is clear that the steel pipe manufactured according to the present invention has high strength, low yield ratio and sour resistance.

[0026]

[Table 1]

[0027]

[Table 2]

[0028]

The steel pipe manufactured by the method of the present invention as described above has a high strength yield ratio, excellent sour resistance, and is used in a poor oil well environment.

[Brief description of the drawings]

FIG. 1 is a diagram showing the influence of a reheating start temperature and a final reduction rate of hot-piercing continuous rolling on the austenite grain size after quenching.

FIG. 2 is a diagram showing the effects of a rolling temperature and a reheating furnace temperature on the austenite grain size after rapid cooling.

Claims (3)

[Claims] 1. As weight%, C: 0.02% to 0.20%, Si: 0.01% to 0.5%, Mn: 0.15% to 2.5%, P: 0.020. % Or less, S: 0.010% or less, Al: 0.005% to 0.1%, Ti: 0.005% to 0.1%, N: 0.01% or less, and the balance substantially. A steel slab made of Fe is processed at a temperature of 900 to 700 ° C. at a rolling reduction of ₃ to 15% in the final step of hot piercing continuous rolling, and Ar ₃ point −100 ° C. to Ar.
_After heating the hollow shell lowered to a temperature of ₃ points + 50 ° C between 900 to 1000 ° C higher than the shell temperature, hot finishing rolling is performed at a finishing temperature of Ar ₃ points + 50 ° C or more,
The obtained finished steel pipe is quenched at a temperature of Ar ₃ or higher, quenched, heated to a temperature between Ac ₁ and Ac ₃ and then rapidly cooled, and then heated and cooled to a temperature of Ac ₁ or lower. A method for producing a high-strength, low-yield ratio seamless steel pipe, characterized by performing a tempering treatment. 2. As a billet component, Cr: 0.1% to 1.5%, Mo: 0.05% to 0.5%, Ni: 0.1% to 2.0% in weight%. V: 0.01% -0.1% B: 0.0003% -0.0030% At least 1 sort (s) selected from the group of elements is contained, The high strength of Claim 1 characterized by the above-mentioned. And the manufacturing method of a low yield ratio seamless steel pipe. 3. As a billet component, further as a weight% rare earth element: 0.001% to 0.05%, Ca: 0.001% to 0.02%, Co: 0.05% to 0.5% And Cu: containing at least one element selected from the group consisting of 0.1% to 0.5%, and manufacturing a high-strength, low-yield ratio seamless steel pipe according to claim 1 or 2. Method.