JPH07268467A - Production of hot coil for steel tube having high toughness and sour resistance - Google Patents

Abstract

PURPOSE:To produce a steel tube stock excellent in sour resistance and having superior strength and toughness by specifying a chemical composition of a steel stock and further specifying its rolling conditions and controlled cooling conditions, respectively. CONSTITUTION:A steel stock, having a composition consisting of, by weight, 0.05-0.12% C, 0.10-0.40% Si, 0.50-1.20% Mn, 0.005-0.10% Al, <=0.006% P, <=0.0009% S, 0.0020-0.0060% Ca, further one or >=2 kinds among <=0.60% Ni, <=0.60% Cu, <=1.00% Cr, <=0.60% Mo, <=0.10% Nb, <=0.10% V, <=0.10% Zr, and <=0.10% Ti, and the balance essentially Fe, is used. A continuously cast slab is subjected to rolling reduction at <=950 deg.C at 10-50%, cooled at >=2 deg.C/sec surface cooling rate until a surface temp. not higher than the Ar3 point is reached, recuperated for <250sec, and rolled at >=50% at a temp. in the unrecrystallization region. Rolling is finished at 720-820 deg.C. After cooling at (5 to 30) deg.C/sec average cooling rate, coiling performed at 400-600 deg.C. By this method, the hot coil for steel tube having high toughness and sour resistance can be produced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot coil used for line pipes, particularly one used in a cold environment and in a wet environment containing hydrogen sulfide and carbon dioxide (hereinafter referred to as a sour environment). It is intended to remarkably improve hydrogen-induced cracking property and low temperature toughness.

[0002]

2. Description of the Related Art Cracks called hydrogen-induced cracks (hereinafter referred to as HICs) occur in steel materials such as line pipes used in sour environments, which cause leakage and burst accidents. The mechanism of HIC generation is that atomic hydrogen generated by corrosion of the steel surface that occurs in a sour environment penetrates into the steel material and has a layered spread such as MnS in the steel material and sulfide-based cluster-like inclusions. Accumulate around inclusions,
It is believed that cracking will occur.

The HIC starting from inclusions propagates and grows along inhomogeneous portions such as components, structures and hardness in the steel material. This inhomogeneous portion is particularly likely to occur at a position corresponding to the final solidified portion of the cast, that is, the center of the continuously cast cast solidified by uniform cooling (hereinafter referred to as the center segregation zone). At this position, inclusions such as MnS and the heterogeneous portion of the central segregation zone coexist, so that HIC is most likely to occur. Furthermore, in recent years, along with the depletion of natural resources, there has been an increasing demand for the development of gas fields having higher hydrogen sulfide and carbon dioxide gas contents and for steel sheets having both properties in cold regions.

In order to produce the HIC resistant steel as described above, the conventional (1) corrosion of the steel surface is suppressed, or corrosion is caused by adding Cu, Ni, etc. which are elements forming a stable film on the surface. The method of reducing the amount of hydrogen penetrating into the steel accompanying (2) S content reduction or addition of Ca, REM, etc., reduces MnS or controls the morphology into spherical inclusions with a low degree of harmfulness. In a way to suppress the occurrence,
For example, Japanese Patent Publication No. 57-16184 and Japanese Patent Publication No. 57-
14747, Japanese Patent Publication No. 57-14747, Ca addition method, (3) The content of C, Mn, P, etc. is reduced, and the slab is subjected to uniform heat diffusion treatment to form a center segregation zone. A method of diluting a concentrated component to suppress the propagation and growth of HIC, for example, the method disclosed in JP-A-58-221261 and JP-B-55-49129, (4) Appropriate hot rolling Method to make the structure and hardness of steel uniform,
A method of suppressing the propagation and growth of C is disclosed in, for example, JP-A-57-
It is disclosed in Japanese Patent No. 47827.

Further, since it is used at low temperatures in cold regions, it is necessary to have excellent toughness, and as an effective means therefor, there is a technique for improving toughness by controlling crystal grains. In order to improve the low temperature toughness, it is effective to reduce the ferrite grain size after transformation, and for that purpose, it is known that it is effective to refine the austenite grains before transformation.
There are many proposals for this method, for example, Japanese Patent Laid-Open No.
As described in JP-A No. 9-47323, there is a method of heating at a low temperature to increase the processing amount in the non-recrystallized region.

[0006] Conventionally, Japanese Patent Laid-Open No. 58-58 has been used for refining steel materials.
Ni and Nb as disclosed in Japanese Patent Publication No. 19431
The alloying elements such as the above are used, and the toughness of the base material is obtained by the Charpy impact test with the vTrs value of −50 ° C. to −70 ° C. Further, Japanese Patent Publication No. 60-169516 discloses a method for producing a low temperature steel having excellent toughness in a welded portion. In this method, it is heated to 1250 to 1350 ° C. for 60 minutes or more and allowed to cool or roll to cool to a temperature not higher than the Ar ₃ transformation point, and again heated to 900 to 1150 ° C. and a reduction rate of 800 ° C. or lower is 30%. The above rolling is performed and the temperature up to 300 ° C is 10 to 5
There is also a method of cooling at 0 ° C./second and then heating to 400 to 650 ° C. to temper. Further, as a method for improving these methods, there is a method in which high temperature heat treatment is omitted as described in JP-A-1-14668.

However, each of the above-mentioned proposals has various problems inherent in practical use, and their improvements are awaited. As disclosed in Japanese Patent Application Laid-Open No. 59-47323, heating at a low temperature increases the amount of processing in the non-recrystallized region, controlled cooling is essential, and rapid cooling after rolling produces fine ferrite and martensite. In the method, since the heating temperature of the target slab is different from the heating temperature of the other slab, it takes time to adjust the heating operation conditions before and after this. Further, a significant decrease in heating efficiency is unavoidable, and further, since the amount of processing in the non-recrystallized region is increased, the temperature waiting time during controlled rolling becomes extremely long, and the reduction in rolling efficiency,
There are many problems in the current situation of pursuing the improvement of productivity because of the cost increase associated with reheating and controlled cooling.

In addition, Japanese Patent Laid-Open No. 58-19431 discloses a high-strength steel excellent in arrest characteristics in addition to Ni and Nb.
It is indispensable to reheat after rolling to completely transform into austenite, and incidental equipment, cost increase and productivity decrease due to reheating cannot be avoided. Further, Ni and Nb are expensive alloy components, and their addition significantly increases the cost of steel materials. Nevertheless, vTrs in the Charpy impact test, which shows the toughness of the base metal, is only at the level of -50 ° C to -70 ° C.

Further, the method disclosed in Japanese Patent Publication No. 169516/1985 requires heat treatment at a high temperature in order to secure toughness and, after reheating, requires reduction at 800 ° C. or lower. The decrease in sex is remarkable. Further, in the method disclosed in JP-A-1-14668, after heating to a temperature of 1250 ° C. or higher for segregation diffusion for ensuring toughness, a reduction amount of 50% or more from the recrystallization end temperature to the Ar ₃ point temperature is not re-set. Since rolling in the crystal region is indispensable, increase in temperature waiting time until cooling to a predetermined temperature range, especially for thick materials, reduction in rolling ton / hr, and further reduction in rolling unit consumption due to temperature reduction Is greatly lost.

[0010]

DISCLOSURE OF THE INVENTION The present invention solves the above-mentioned drawbacks of the conventional manufacturing method, and makes it possible to produce a steel pipe material excellent in HIC resistance, strength and toughness with high productivity without lowering rolling efficiency. It is an object of the present invention to provide a method for economically manufacturing.

[0011]

The gist of the present invention is as follows. (1) C: 0.05 to 0.12% by weight, Si:
0.10 to 0.40%, Mn: 0.50 to 1.20%,
Al: 0.005 to 0.10%, P: ≤ 0.006%,
S: ≤ 0.0009%, Ca: 0.0020 to 0.00
60%, Ni: ≦ 0.60%, Cu: ≦ 0.60
%, Cr: ≦ 1.00%, Mo: ≦ 0.60%, Nb:
≤0.10%, V: ≤0.10%, Zr: ≤0.10
%, Ti: ≦ 0.10%, a continuous cast slab containing one or more of 950 and the balance of iron and impurities for 950
A reduction of 10% or more and 50% or less is performed at a temperature of ℃ or less, and then the surface is cooled to a temperature of Ar ₃ or less at a cooling rate of 2 ℃ / sec or more, and after re-heating for less than 250 seconds, it is not re-heated. Roll at 50% or more in the crystal region, finish rolling in the range of 720 to 820 ° C, and subsequently average cooling rate of 5 to 30 ° C.
After cooling at a speed of 1 / sec, the material is wound in the range of 400 to 600 ° C.

(2) C: 0.05 to 0.12 by weight%
%, Si: 0.10 to 0.40%, Mn: 0.50
1.20%, Al: 0.005 to 0.10%, P: ≤0.
006%, S: ≤ 0.0009%, Ca: 0.0020
˜0.0060%, Ni: ≦ 0.60%, Cu:
≤0.60%, Cr: ≤1.00%, Mo: ≤0.60
%, Nb: ≦ 0.10%, V: ≦ 0.10%, Zr: ≦
0.10%, Ti: ≤0.10%, one or more of which are continuously cast slabs made of iron and impurities, and the balance of which is 20% or less in the austenite region.
% Hot working, then slab center temperature 1
After holding at 100 to 1250 ° C. for 30 minutes or more and less than 2 hours, a reduction of 10% or more and 50% or less is performed at 950 ° C. or less, and the surface cooling rate is 2 ° C./sec or more and the surface temperature is Ar ₃ or less. After cooling to the temperature and recuperating for less than 250 seconds, 50% or more rolling is performed in the unrecrystallized region to obtain 720
Rolling is completed in the range of up to 820 ° C, followed by cooling at an average cooling rate of 5 to 30 ° C / sec, and then winding in the range of 400 to 600 ° C.

[0013]

The steel material for steel pipes which is the subject of the present invention is, as described below, an operation that has been conventionally confirmed in practical use in the field of the art in order to obtain the required material for the steel for ordinary welded structures.
The same action and effect can be obtained by similarly using the types and amounts of the additional elements defined based on the effect relationship. Therefore, the steel containing these is the subject steel of the present invention. The reason and amount of addition of each of these component elements are shown below.
C is an important element as a strength element, but 0.12%
If it exceeds 0.05, the toughness is deteriorated, and if it is less than 0.05%, not only the required strength cannot be secured, but also hot cracking during welding tends to occur on site, so that 0.05 to 0.
It was set to 12%.

Si is necessary as a deoxidizing element for molten steel,
Although it is added as a strength-increasing element, if it is less than 0.1%, the deoxidizing effect is insufficient, and if it is added in excess of 0.4%, the toughness is deteriorated, so the content was made 0.10 to 0.40%. . Mn is also necessary as a deoxidizing component element, and is an important element for improving similarly to C. If it is less than 0.5%, the required strength cannot be secured, and if it exceeds 1.20%, 0.50 to 1.2 to deteriorate the HIC resistance
It was set to 0%.

Al is necessary for deoxidation and also has an effect of preventing coarsening of crystal grains. If it is less than 0.005%, there is no deoxidizing effect, and if it exceeds 0.10%, the toughness is deteriorated, so that it is less than 0.1%.
It was set to 005 to 0.10%. S is an element that affects the HIC resistance. If S is added in excess of 0.009%, the HIC resistance is adversely affected. That is, since it deteriorates with a decrease in the hot working finish temperature of rolling for improving the toughness, the upper limit range is set to 0.0009% so that the hot working finish temperature dependency is not observed.

Even if the structure is controlled by the cooling rate and the coiling temperature, which will be described later, if the segregation is large, the segregation portion becomes hard and the HIC propagates and grows. However, in order to reduce the hardening structure of the central segregation zone, the P content of the molten steel is set to P ≦ 0.
It was found that HIC can be prevented from propagating and growing in a severe sour environment with a pH of less than 4.0 at 006%. Further, the continuous casting slab is hot-worked at a cross-section reduction rate of 20% or more in the austenite region of 1200 ° C. or lower, so that the diffusion coefficient of P increases, and therefore 1100 to 1
Since the diffusion effect of P segregation can be obtained by soaking at a relatively low temperature of 250 ° C. for 30 minutes or more and less than 2 hours, the upper limit of P can be increased to 0.010% by adding this step.

Ca is added to control the morphology of Al ₂ O ₃ to increase the size and make MnS spherically harmless, but 0.00
If it is 20% or less, there is no effect, and if it exceeds 0.0060%, Ca-based cluster-like inclusions are formed and HIC resistance is deteriorated, so the content was made 0.0020 to 0.0060%. Ni is effective in improving the corrosion resistance, increasing the strength and improving the toughness, but if it exceeds 0.6%, the local corrosion increases, so it was made 0.6% or less.

Cu is effective for improving the corrosion resistance and increasing the strength, but if it exceeds 0.6%, rolling defects are likely to occur, so the content is made 0.6% or less. Cr can increase strength without deteriorating HIC resistance and toughness, but 1.0
If it exceeds 0%, the toughness is deteriorated, so the content is set to 1.00% or less. Mo has the effect of improving hardenability and strength, but if added in excess of 0.60%, it causes deterioration of toughness, so 0.60% was made the upper limit. Nb, V and Zr are Mo
Although the same effect as above is obtained, if it exceeds 0.10%, toughness is deteriorated, so 0.10% is made the upper limit. Ti has the effect of improving the toughness of the weld heat affected zone, but if it exceeds 0.10%, it adversely deteriorates the toughness, so 0.10% was made the upper limit.

The inventors of the present invention have solved the problems of the above-mentioned prior art and, in order to achieve the objects of the present invention, repeated various experimental studies using general steel having the chemical composition defined in the present invention. . In order to establish a method of obtaining strength or toughness equivalent to or higher than that obtained by the prior art, without using retention / standby for rolling adjustment, and further reheating rolling in a low temperature range, etc. The examination was repeated.

The hot rolling conditions are such that the austenite grains before transformation are refined and strain is accumulated by working, and the transformation structure is refined.
%. The reason why the rolling reduction is limited to 950 ° C. or less is that the effect of strain accumulation in the recrystallized austenite grain size and the non-recrystallized austenite grain becomes remarkable in the hot rolling at 950 ° C. or less. If the rolling reduction at 950 ° C. or lower is less than 10%, the effect of rolling is insufficient, so the lower limit was made 10%.
If the reduction rate is further increased, it is advantageous for the refinement of the structure before finish rolling, but if this reduction rate is too large, it may not be possible to secure a sufficient reduction amount for the refinement of ferrite in the subsequent finish rolling. Therefore, it was determined based on the result of the basic experiment as a reduction ratio suitable for the final refinement of the texture of the surface layer portion, and was set to 50%.

In order to suppress the grain growth of the refined austenite after sufficiently refining the austenite grains under the above conditions, and to shorten the enormous temperature waiting time between the rough rolling and the finish rolling. The conditions were examined.
As a result, it is necessary to cool at a cooling rate of 2 ° C./second or more to an Ar ₃ point or less. If the cooling rate is less than 2 ° C / sec, this means that the grain growth control effect of austenite refined by hot rolling is not sufficiently exerted, and if the cooling stop temperature is Ar ₃ or more, a temperature waiting time occurs. , Can not enjoy the substantial benefits. The recuperation time is 250
The time is set to less than a second, and the reason is to secure the temperature for finish rolling and substantially improve productivity. Further, in order to secure the low temperature toughness, the rolling reduction in the unrecrystallized region was set to 50% or more.

When the final working temperature is 820 ° C. or higher, the low temperature toughness is poor, and the rolling finishing temperature is set to 720 to 820 ° C. to secure the low temperature toughness. Average cooling rate is 5 ~ 30 ℃
/ Sec. If the average cooling rate is less than 5 ° C / sec, the two-phase separation of ferrite / pearlite proceeds, so a band structure of ferrite / pearlite tends to be formed at the central segregation portion, and if it exceeds 30 ° C / sec, a hardened bainite-like structure is formed. It is easily formed and the HIC resistance deteriorates.
C / sec.

The winding temperature is 400 to 600 ° C. Since the hot coil has a winding process, the cooling rate after the water cooling stop temperature is extremely low as compared with the thick plate. Therefore, if it is wound at a temperature above the Ar ₁ transformation point, ferrite / pearlite 2
Phase separation proceeds, and a band structure of ferrite / pearlite is formed. Especially in the center segregation zone, this tendency is remarkable, so that the HIC resistance deteriorates. Therefore, the upper limit of the winding temperature is set to 600 ° C. at which Ar ₁ transformation is completed. further,
The average cooling rate is 30 ° C in the region where the winding is less than 400 ° C.
As in the case of more than 1 sec / sec, a hard bainite structure is likely to be formed and the HIC resistance is deteriorated. From the above, the winding temperature was set to 400 to 600 ° C.

For the above reasons, 950 is a steel slab having a limited chemical composition and the balance being Fe and unavoidable impurities.
A reduction of 10% or more and 50% or less is performed at a temperature of ℃ or less, and then the surface is cooled to a temperature of Ar ₃ or less at a cooling rate of 2 ℃ / sec or more, and after reheating for less than 250 seconds, unrecrystallized. Rolling at 50% or more in the zone, finish rolling in the range of 720 to 820 ° C., and then average cooling rate of 5 to 30 ° C.
After cooling at a cooling rate of 10 seconds / second, by winding in the range of 400 to 600 ° C., it is possible to manufacture a hot coil excellent in both HIC resistance and low temperature toughness.

[0025]

EXAMPLES In order to clarify the effects of S, P and rolling conditions on HIC resistance, the present inventors conducted experiments using steels with different S and P contents. All the samples were cast by the continuous casting method, and Ca was performed by the method of continuously adding the granular alloy to the tundish. Next, using the plate obtained by using the slab thus manufactured as a hot coil, the HIC resistance is increased.
A sex evaluation test was conducted.

The HIC resistance evaluation test was carried out by a method based on the so-called BP test method. That is, the sample was immersed in a NACE solution (0.5% acetic acid-5% sodium chloride solution, a solution saturated with H ₂ S and having a pH of about 3.8) for 96 hours. For the presence or absence of HIC, US
By performing flaw detection with T, the rate of defects (hereinafter, CAR) with respect to the surface of the test piece was evaluated.

Table 1 shows the chemical composition of the test steel. Steel A,
B, C, D, E, F, G and H are steels of the present invention, and I, J and K are comparative steels.

[0028]

[Table 1]

Table 2 shows the manufacturing conditions, materials, and HIC resistance examination results. Example No. 1 of the present invention 1 to 16 are HIC resistance characteristics,
The low temperature toughness was good, there was no temperature waiting, and the productivity was also good. On the other hand, Comparative Example No. 17 to 32 each have a problem. No. Nos. 17 to 19 and 21 have ingredients outside the scope of the present invention, and No. No. 17 did not undergo the pre-rolling treatment, so all had problems with HIC resistance. No. Two
0 means that the condition of the treatment before rolling was not satisfied, so H resistance was high.
There was a problem in IC characteristics. No. 20 to 30 do not satisfy the rolling condition or the cooling condition after rolling, and the toughness or H resistance is high.
The IC characteristics did not satisfy the predetermined value. No. 31-33
In the case of, the productivity was significantly deteriorated because cooling was not performed during rolling.

[0030]

[Table 2]

[0031]

[Table 3]

[0032]

[Table 4]

[0033]

[Table 5]

* HIC resistance evaluation test: ◎ The sample was dipped in a NACE solution (0.5% acetic acid-5% sodium chloride solution saturated with H ₂ S to have a pH of about 3.8) for 96 hours. The presence or absence of HIC was evaluated by the ratio of defects to the surface of the test piece (hereinafter referred to as CAR) by performing flaw detection on the test piece after the immersion with UST. Regarding the evaluation criteria, ◯ indicates no cracking, and x indicates cracking. Note 1) Hold at 1160 ° C for 25 minutes after 25% pressure reduction. Note 2) After 30% pressure reduction at 1150 ° C, hold at 1120 ° C for 20 minutes.

[0035]

EFFECTS OF THE INVENTION According to the present invention, a hot coil excellent in both HIC resistance in a severe sour environment with a pH of less than 4.0 and low temperature toughness can be produced, and H coil in a sour environment in a cold region can be manufactured.
It is possible to manufacture a line pipe in which a burst accident due to IC generation and low temperature brittle fracture does not occur.

Claims (2)

[Claims] 1. By weight%, C: 0.05 to 0.12%, Si: 0.10 to 0.40%, Mn: 0.50 to 1.20%, Al: 0.005 to 0. 10%, P: ≤ 0.006%, S: ≤ 0.0009%, Ca: 0.0020 to 0.0060%, and further Ni: ≤ 0.60%, Cu: ≤ 0.60%, Cr: ≤ 1 0.000%, Mo: ≦ 0.60%, Nb: ≦ 0.10%, V: ≦ 0.10%, Zr: ≦ 0.10%, Ti: ≦ 0.10%, one or two kinds A continuous cast slab containing the above and the balance being iron and impurities is 10% or more 50% at 950 ° C or less.
% Or less, and subsequently, cooling is performed until the surface cooling rate is 2 ° C./sec or more and the surface temperature is Ar ₃ or less,
After recuperating for less than 250 seconds, rolling at 50% or more in the unrecrystallized region, ending rolling in the range of 720 to 820 ° C, and subsequently cooling at an average cooling rate of 5 to 30 ° C / sec. , 40
A method for producing a hot coil for high toughness sour-resistant steel pipe, which comprises winding in the range of 0 to 600 ° C. 2. C .: 0.05 to 0.12%, Si: 0.10 to 0.40%, Mn: 0.50 to 1.20%, Al: 0.005 to 0. 10%, P: ≤ 0.006%, S: ≤ 0.0009%, Ca: 0.0020 to 0.0060%, and further Ni: ≤ 0.60%, Cu: ≤ 0.60%, Cr: ≤ 1 0.000%, Mo: ≦ 0.60%, Nb: ≦ 0.10%, V: ≦ 0.10%, Zr: ≦ 0.10%, Ti: ≦ 0.10%, one or two kinds Including the above, the continuous cast slab consisting of iron and impurities as the balance is hot-worked at a cross-section reduction rate of 20% or more in an austenite region of 1200 ° C or less, and then the center temperature of the slab is 1100 to 1250 ° C for 30 minutes or more. After holding for less than 2 hours, perform 10% or more and 50% or less reduction at 950 ° C or lower, and continue Surface cooling rate is 2 ℃
/ Sec or more, the surface temperature is cooled to a temperature of Ar ₃ or less, and after recuperation for less than 250 seconds, 50% or more of rolling is performed in the unrecrystallized region, and rolling is completed in the range of 720 to 820 ° C. ,
Then, after cooling at an average cooling rate of 5 to 30 ° C./sec,
A method of manufacturing a hot coil for high toughness sour resistant steel pipe, which comprises winding in a range of 400 to 600 ° C.