JPH0359124B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to hot coils used for line pipes, particularly those used in cold regions and in humid environments containing hydrogen sulfide and carbon dioxide (hereinafter referred to as sour environments). The aim is to significantly improve crackability and low-temperature toughness. (Prior Art) It is known that cracks called hydrogen-induced cracking (hereinafter referred to as HIC) occur in steel materials such as line pipes used in sour environments, which can cause leakage and burst accidents. The mechanism of HIC occurrence is
Atomic hydrogen generated by corrosion of the steel surface in a sour environment penetrates into the steel, causing
It is thought that cracks occur when they accumulate around layered inclusions such as MnS or oxide cluster inclusions. HIC that originates from inclusions propagates along parts of the steel material that are heterogeneous in composition, structure, hardness, etc.
grow up. This inhomogeneous portion is particularly likely to occur in the final solidified part of the slab, that is, a position corresponding to the center of the continuously cast slab solidified by uniform cooling (hereinafter referred to as the center segregation zone). This position is due to the coexistence of inclusions such as MnS and a heterogeneous region called the central segregation zone.
Most likely to cause HIC. Furthermore, in recent years, due to the depletion of natural resources, the development of gas fields with high contents of hydrogen sulfide and carbon dioxide, and the increase in resource development in cold regions, both HIC resistance and low temperature toughness have increased. Demand for steel sheets with special properties is increasing. In order to manufacture HIC-resistant steel as described above, conventional
(1) A method of suppressing corrosion on the steel surface or adding elements such as Cu, Ni, etc. that form a stable film on the surface to reduce the amount of hydrogen penetrating into the steel due to corrosion. The method disclosed in Publication No. 54-38572. (2) Reduction of S content or Ca,
This is a method of suppressing the occurrence of HIC by reducing MnS or controlling its morphology into less harmful spherical inclusions by adding REM, etc.
Publication No. 14747, Special Publication No. 57-14747
Ca addition method. (3) Reduce the content of C, Mn, P, etc.
Alternatively, the slab is soaked and diffused to dilute the concentrated components in the central segregation zone, thereby suppressing the propagation and growth of HIC.
Methods disclosed in JP-A-58-221260, JP-A-58-221261, and JP-A-55-49129.
(4) A method for suppressing the propagation and growth of HIC by uniformizing the structure and hardness of the steel material using an appropriate hot rolling method, such as the method disclosed in JP-A-57-47827. (Problems to be Solved by the Invention) In oil and natural gas line pipes, during periodic internal cleaning, the inner surface of the pipe may be damaged by the instrument (pig) used to pass through the pipe. Therefore, even if a stable corrosion film is formed on the inner surface of the pipe using Cu, Ni, etc., the film will peel off at the scratched areas and new local corrosion will occur, so it is impossible to completely prevent hydrogen intrusion. It's impossible. Therefore, as described in (2) to (4) of the conventional technology,
It is necessary to reduce the origin of HIC and suppress its propagation and growth. As a starting point for HIC, MnS elongated by rolling is the most harmful, and if MnS can be completely eliminated, HIC will hardly occur. However, industrially, S of molten steel
Even if Ca is added to 0.0010% or less, components will be concentrated during the solidification process of molten steel, and at the final solidification position such as the central segregation area, concentration of Mn and S will occur.
Precipitation of MnS is unavoidable, and the origin of HIC cannot be completely eliminated. Therefore, the most important issue is to suppress the spread and growth of HIC.
The upper limit regulation on the amount of Mn described in Publication No. 58-221261 alone cannot avoid the generation of abnormal tissues that serve as a route for the propagation and growth of HIC. Also, JP-A-57-
As in Publication No. 104653, if C≦0.05%, hot cracking of the weld metal during on-site welding is likely to occur.
In addition, in order to suppress the propagation and growth of HIC by uniformly heating the slab, it is necessary to heat the slab to 1300℃ for 30 minutes to 10 hours, as shown in Japanese Patent Publication No. 55-49129.
This requires extremely high temperature or long-time soaking and diffusion at 1150° C. for 5 to 150 hours, which is problematic from the viewpoint of manufacturing cost and energy saving. Therefore, as shown in JP-A No. 57-47827, a method of controlling the structure by rolling is effective, but if the hot working end temperature is 870°C or higher, high toughness at low temperatures cannot be obtained. I can't. In addition, when comparing hot coils and thick plates, hot coils undergo large reductions during the continuous rolling process, so they are more likely to form distraction inclusions that cause HIC, and a layered structure that serves as a path for HIC propagation and growth. Furthermore, since hot coils require a winding process that is not present in controlled cooling of thick plates, the cooling rate after water cooling is stopped is significantly lower than that of controlled coolant for thick plates. Therefore, grain boundary embrittlement and coarsening of precipitates are likely to occur due to increased grain boundary segregation of CP, etc., and these are
This causes deterioration of HIC properties. Therefore, it is not possible to improve the material quality of hot coils simply by applying controlled cooling technology to thick plates. From the above, the problem to be solved by the present invention is to solve the problems peculiar to hot coils and to obtain a hot coil that simultaneously exhibits both HIC resistance and low-temperature toughness, which have not been achieved with conventional techniques. It is in. (Means for solving the problems) The gist of the present invention is that C: 0.05 to 0.12%, Si: 0.10%
~0.40%, Mn: 0.5~1.20%, Al: 0.005~0.10%,
P≦0.010%, S≦0.0009%, Ca: 0.0020-0.0060
%, further Ni≦0.60%, Cu≦0.60%, Cr:≦1.00
%, Mo: ≦0.60%, Nb≦0.10%, V≦0.10%,
One or two of Zr: ≦0.10%, Ti: ≦0.10%
A slab containing at least 100% of iron and impurities with the remainder being iron and impurities is hot worked in the austenite region at 1200°C or less with a cross-section reduction rate of 20% or more, and then the center temperature of the slab is maintained at 1100 to 1250°C for 30 minutes or more. After holding for less than 2 hours, reduce the pressure by 50% or more at 950℃ or less,
After finishing hot working in the range of 720 to 820°C and subsequently cooling at an average cooling rate of 5 to 30°C/sec,
This is a method for manufacturing a hot coil for high-toughness, sour-resistant steel pipe, which is characterized by winding at a temperature in the range of 400 to 600°C. Next, the reasons for limiting the components and rolling conditions of the present invention will be described. C is an important element as a strength element, but 0.12
If it exceeds 0.05%, the toughness will deteriorate, and if it is less than 0.05%, it will not only be impossible to secure the necessary strength.
Because hot cracking is more likely to occur during on-site welding,
It was set at 0.05-0.12%. Si is added as a deoxidizing agent, and if it is less than 0.1%, there is no deoxidizing effect, and if it exceeds 0.4%, the toughness deteriorates, so it was set at 0.10 to 0.40%. Mn is also necessary as a deoxidizing agent, but like C, it is an important element as a strength element, and if it is less than 0.5%, the necessary strength cannot be secured;
If it exceeds 0.5 to 1.20, the HIC resistance will deteriorate.
%. Al is necessary for deoxidation and also has the 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 will deteriorate.
It was set at 0.005 to 0.10%. According to the inventors' research, in steels with S≧0.0010%, HIC is dependent on the hot working end temperature, which worsens as the hot working end temperature of rolling decreases, and the improvement in toughness is particularly large. It was discovered that this tendency is significant at temperatures below 820°C, but in steel with S≦0.0009%, the above tendency is completely absent. Therefore, it was set as S≦0.0009%. Even if the structure is controlled by cooling rate and coiling temperature, which will be described later, if segregation is large, the segregated portion will harden and HIC will propagate and grow. However, if the structure is controlled and the P content of molten steel is P≦0.006%, HIC
We found that it is possible to prevent the spread and growth of
(see figure). Furthermore, by hot working the continuously cast slab in the austenite region at temperatures below 1200°C with a reduction in area of 20% or more, the diffusion coefficient of P increases.
It was discovered that the diffusion effect of P segregation can be obtained even by soaking at a relatively low temperature and short time of 30 minutes to 1250 degrees Celsius and less than 2 hours, and by adding this step, the upper limit of P can be raised to 0.010%. (See Figure 3). Ca is made by controlling the morphology of Al ₂ O ₃ to make it larger, and then converting it to MnS
is added to make the spherical shape harmless, but if it is less than 0.0020%, it has no effect, and if it exceeds 0.0060%, it forms Ca-based cluster inclusions and deteriorates HIC resistance, so it was set at 0.0020 to 0.0060%. Ni is effective in improving corrosion resistance, assimilating strength, and improving toughness, but if it exceeds 0.6%, local corrosion will increase, so ≦
It was set at 0.6%. Cu is effective in improving corrosion resistance and increasing strength, but if it exceeds 0.6%, rolling defects are likely to occur, so it was set at ≦0.6%. Cr can increase strength without deteriorating HIC resistance and toughness, but if it exceeds 1.00%, toughness deteriorates, so it was set to ≦1.00%. Mo is effective in improving hardenability and strength, but if it exceeds 0.60%, the toughness deteriorates, so it was set to ≦0.60%. Nb, V and Zr
Although Mo has the same effect as Mo, if it exceeds 0.10%, the toughness deteriorates, so it was set to ≦0.10%. Ti is effective in improving the toughness of the weld heat affected zone, but if it exceeds 0.10%, the toughness deteriorates, so it was set to ≦0.10%. In the present invention, the rolling reduction ratio of 950°C or less is ≧50%,
Toughness improves by taking more than 50%, so
50% or more. Final hot working temperature is 720~820℃
However, if S≦0.0009%, the dependence of HIC on the final hot working temperature is eliminated, so the temperature was set in the range of 720 to 820° C., where low-temperature toughness can be obtained (see FIGS. 1 and 2). The average cooling rate is 5 to 30°C/sec. If the average cooling rate is less than 5℃/sec, the two-phase separation of ferrite pearlite will proceed, so a band-like structure of ferrite will be easily formed in the central segregation area, and the cooling rate will be lower than 30℃/sec.
If the temperature is higher than that, a hardened bainitic structure is likely to be formed and the HIC resistance is deteriorated, so the temperature was set at 5 to 30°C/sec (see Fig. 4). The winding temperature is 400 to 600°C. Because hot coils require a winding process, the cooling rate after water cooling is stopped is extremely slow compared to thick plates. Therefore, when the material is wound at a temperature equal to or higher than the Ar ₁ transformation point, two-phase separation of ferrite and pearlite progresses, and a band-like structure of ferrite and pearlite is formed. This tendency is particularly pronounced in the central segregation zone, resulting in deterioration of HIC resistance. Therefore, the upper limit of the winding temperature is 600℃, which is the temperature at which Ar ₁ transformation is completed.
And so. Furthermore, in the region where the winding temperature is less than 400℃, as in the case where the average cooling rate is over 30℃/sec,
Easy to form a hardened bainite structure, resistant to HIC
sex deteriorates. Based on the above, the winding temperature was set at 400 to 600°C (see Fig. 5). (Effect) The inventors discovered that the S
The HIC that occurs in steel with ≧0.0010% is dependent on the end temperature of hot working, increasing as the end temperature of hot working during rolling decreases, and it increases in the temperature range below 820°C, where low-temperature toughness is particularly improved. Although the trend is remarkable,
It was discovered that the above tendency was not observed at all in steel with S≦0.0009%. This discovery has made it possible to obtain low-temperature toughness without sacrificing HIC resistance by applying 50% or more compression at temperatures below 950°C and completing hot working at temperatures between 720 and 820°C. However, even if S≦0.0009%, there are areas where MnS precipitation is unavoidable in the central segregation area of the continuously cast slab, and HIC occurs in such areas. In order to eliminate such HIC, P≦0.006
%, the segregation of P is reduced, and the cooling rate is 5 to 30℃/sec after the hot working is completed.
We discovered that suppression can only be achieved by controlling the structure of the center segregation area to a structure that does not allow HIC to propagate or grow, by cooling it at a temperature of 400 to 600°C and winding it. Furthermore, even if 0.006%<P≦0.010%, the slab
In the austenite region below 1200℃, the cross-section reduction rate is 20
% or more to create a diffusion starting point,
After that, the center temperature of the slab is maintained at 1100°C or more and less than 1250°C for 30 minutes or more and less than 2 hours to diffuse and eliminate or reduce the unavoidable segregation, and then final hot working is performed, followed by cooling. Speed 5-30℃/
HIC can be prevented by cooling at sec and winding at a winding temperature of 400 to 600°C. As described above, the present invention makes it possible to manufacture a hot coil that has both excellent HIC resistance in a harsh sour environment with a pH of less than 4.0 and low-temperature toughness. (Example) In order to clarify the effects of S, R and rolling conditions on HIC resistance, the inventors conducted experiments using steels with varying S and P contents. All samples were cast by continuous casting, and Ca was continuously added to the tundish. Next, the slab manufactured in this way was rolled in a hot mill, and using the plate obtained as a hot coil,
A HIC resistance evaluation test was conducted. The HIC resistance evaluation test was conducted in accordance with the so-called BP test method.
That is, the sample was PH-treated with NACE solution (0.5% acetic acid-5% sodium chloride solution saturated with _H2S ).
3.8) for 96 hours. To check whether HIC has occurred, test the specimen after it has been immersed.
By performing flaw detection with UST, we evaluated the ratio of defects to the specimen surface (hereinafter referred to as CAR). Examples are shown in Tables 1 and 2.

【table】

[Table] * ○No cracks ×Cracks Table 2 shows the toughness and resistance of the invention steel and comparative steel.
Indicates HIC evaluation. As is clear from this table, the invention steels A and B
has superior toughness and durability compared to Comparative Examples C, D, and E.
HIC properties can be obtained. (Effects of the Invention) As described above, according to the present invention, a hot coil can be manufactured that has both excellent HIC resistance in harsh sour environments with a pH of less than 4.0 and low-temperature toughness. It is possible to manufacture line pipes that do not cause burst accidents due to low-temperature brittle fractures or low-temperature brittle fractures.

[Brief explanation of drawings]

Figure 1 is a chart showing the effect of hot working end temperature on HIC resistance, Figure 2 is a chart showing the effect of hot working ending temperature on toughness, and Figure 3 is a chart showing the effect of P content on HIC resistance. Figure 4 is a diagram of the influence of average cooling rate on HIC resistance, Figure 5 is a diagram of the influence of average cooling rate on HIC resistance.
2 is a chart showing the influence of coiling temperature on HIC properties.

Claims (1)

[Claims] 1. C: 0.05-0.12%, Si: 0.10-0.40%, Mn: 0.5-1.20%, Al: 0.005-0.10%, P≦0.010%, S≦0.0009%, Ca: 0.0020～0.0060%, and one of the following: Ni≦0.60%, Cu≦0.60%, Cr≦1.00%, Mo≦0.60%, Nb≦0.10%, V≦0.10%, Zr≦0.10%, Zr≦0.10% Or hot working a slab containing two or more types, the remainder consisting of iron and impurities in the austenite region at 1200°C or less with a cross-section reduction rate of 20% or more,
After that, the center temperature of the slab was maintained at 1100 to 1250℃ for 30 minutes or more but less than 2 hours, and then the reduction was performed at 950℃ or less by 50% or more, and the hot working was completed in the range of 720 to 820℃, and then , average cooling rate 5-30℃/
A method for manufacturing a hot coil for high toughness and sour-resistant steel pipes, which comprises cooling at sec and then winding at a temperature in the range of 400 to 600°C.