JPS61157628A - Manufacture of hot coil for high-toughness sour-resistant steel pipe - Google Patents

Abstract

PURPOSE:To manufacture a hot coil for a high-toughness sour-resistant steel pipe by subjecting a continuously cast slab having a specified composition contg. C, Si, Mn, Al, P, S, Ca, etc. to rolling-down, hot working, cooling and coiling under specified conditions. CONSTITUTION:A continuously cast slab consisting of 0.05-0.12% C, 0.10-0.40% Si, 0.5-1.20% Mn, 0.005-0.10% Al, <=0.006% P, <=0.0009% S, 0.0020-0.0060% Ca, one or more 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 Fe with impurities is rolled down at <=950 deg.C and >=50% draft, hot worked at 720-820 deg.C finishing temp., cooled at 5-30 deg.C/sec average cooling rate, and coiled at 400-600 deg.C to obtain a hot coil for a steel pipe having superior resistance to hydrogen induced cracking and superior toughness at low temp. in a cold and sour environment.

Description

[Detailed Description of the Invention] (Industrial Application Field) This invention relates to a hono-F coil used for line pipes, particularly in a cold region and in a humid environment containing hydrogen sulfide and carbon dioxide (hereinafter referred to as a sour environment). The aim is to improve hydrogen-induced cracking resistance 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 generation is that atomic hydrogen generated by corrosion of the steel surface in a sour environment penetrates into the steel material and forms a layered structure such as MnS or oxide-based cluster inclusions in the steel material. It is thought that cracks accumulate around inclusions and live there.

An inclusion? HIC generated at the starting point propagates and grows along parts of the steel material that are heterogeneous in composition, structure, hardness, etc. This inhomogeneous portion is particularly likely to occur in the final solidified part of the slab, that is, in the region R (hereinafter referred to as the central segregation zone) that corresponds to the central portion of the continuous cast slab solidified by uniform cooling. This position is most likely to generate H:(4-) because inclusions such as MnS and a heterogeneous central segregation zone coexist.Moreover, in recent years, due to the depletion of natural resources, hydrogen sulfide and carbon dioxide gas Due to the increase in the development of gas fields with a high content of
There is an increasing demand for steel sheets that have both carbon properties and low-temperature toughness.

In order to manufacture HR c6g42 as described above, conventional methods (1) completely suppress corrosion on the steel surface, or add Cu, Ni, etc., which are elements that form a stable film on the surface, to reduce corrosion in the steel. A method that completely reduces hydrogen intrusion into
For example, the method disclosed in Japanese Patent Publication No. 54-38572. (2) By reducing the S content and adding Ca, REM, etc., MnSi is reduced or the morphology is controlled to less harmful spherical inclusions to suppress the occurrence of HIC. Publication No. 16184, Special Publication No. 1983
- Ca addition method shown in 14747 publication. (3)
The content of C, Mn, P, etc. is completely reduced, or the entire slab is soaked and diffused to completely dilute the concentrated components of the central segregation zone, thereby completely suppressing the propagation and growth of HIC. Publication No. 5'7-104653, Japanese Unexamined Patent Publication No. 58-2212
60, Japanese Patent Application Laid-Open No. 58-221261, and Japanese Patent Publication No. 55-49129.

(4) A method of uniformizing the structure and hardness of the steel material using an appropriate hot rolling method to completely suppress the propagation and growth of HIC, such as the method disclosed in Japanese Patent Laid-Open No. 57-47827.

(Problems to be Solved by the Invention) In line pipes for petroleum and natural gas, during periodic internal cleaning, the inner surface of the pipe may be damaged by the instrument (vig) that passes through the pipe. Therefore, Cu, Ni
Even if a stable corrosion film is completely formed on the inner surface of the pipe by such methods, the film will peel off at the damaged areas and new local corrosion will occur, so it is impossible to completely prevent hydrogen intrusion.

Therefore, it is necessary to reduce the origin of HIC, and to suppress its propagation and growth, as described in (2) to (4) of the prior art.

) The origin of IIC is M that is stretched by rolling.
nS is the most harmful, and if all MnS could be completely eliminated, HIC would hardly occur. However, industrially, molten steel S'i0.0O10%
As shown below, even if Ca f is added, concentration of components occurs during the solidification process of molten steel, and at the final solidification position such as the central segregation zone, precipitation of MnS due to a-contraction of Mn and S is unavoidable, resulting in HIC. It is not possible to completely eliminate the point of origin.

Therefore, the most important issue is to suppress the propagation and growth of HIC.
The upper limit regulation of the amount of Mn described in Publication No. 221261 alone cannot avoid the generation of abnormal tissues that serve as a route for the propagation and growth of HIC.

Also, as in Japanese Patent Application Laid-Open No. 57-104653, C≦0
．． If it is set to 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 IC by soaking and heating the slab,
As shown in Publication No. 129, 1150? for 30 minutes to 10 hours at 1300tZ'. Z” from 5 hours to 150
In terms of time, extremely high temperature or long-time soaking is required, which is problematic from the viewpoint of manufacturing cost and energy saving.

Therefore, as shown in Japanese Patent Application Laid-Open No. 57-47827, a method of controlling the structure by rolling is effective, but
If the hot working end temperature is 870C or higher, high toughness at low temperatures cannot be obtained. In addition, when comparing hot coils and thick plates, it is found that hot coils require a large reduction in the continuous rolling process, and are therefore prone to distraction inclusions that cause HIC.
It is easy to form a layered structure that serves as a propagation and growth route.

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. As a result, grain boundary embrittlement and coarsening of precipitates are likely to occur due to increased grain boundary segregation of C, P, etc., which causes a total deterioration of HIC resistance. Therefore, it is not possible to completely 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 all the problems peculiar to hot coils and to obtain a real coil that simultaneously exhibits both HIC resistance and low-temperature toughness, which have not been obtained with conventional technology. It's about doing.

(Means to solve the problem) The gist of the present invention is that C: 0.05 to 0.12%, Sl: 0
．． 10-0.40%, Mn: 0.5-1.20%
, AA: 0.005-0.10%, P≦0.00
6%, S≦0. '0009%, Ca:0.002
0 to 0.0060, and Ni≦0.60%.

Cu≦0.60%, Cr≦1.00%, Mo≦0.6
0%.

NbS2.10%, ■≦0.10%, Zr≦0.10
%.

A continuous cast slab consisting of one or more types of TiS2.10%, the remainder being iron and impurities, is fully reduced by 50% or more at 950C or less, and hot worked at 720~820℃.
0C, followed by an average cooling rate of 5-30
After cooling at ℃/sec, winding in the range of 400-600C and c:o, os~0.12%, Si
: 0.10-0.40%, Mn: 0.5-1.
20chi, AA: 0.005-0.10%, P≦0
．． 010%, S≦0.0009%, Ca: 0.0
020 to 0.0060%, further Ni≦0.60%,
Cu≦0.60%, Cr:≦100%.

Mo: ≦0.60%, NbS2.10%, ■≦0.
10%, Zr:≦0. The slab contains one or more of lO%, T1: 60.10%, and the remainder is iron and impurities.The slab is an austenite region of 11200C or less, and is fully hot worked with a cross-section reduction rate of 20% or more, and then the slab After holding the center temperature at 1100-1250C for 30 minutes or more but less than 2 hours, complete reduction of 50 inches or more at 9500C or less, finishing hot working in the range of 720-820C, and then cooling at an average cooling rate of 5 This is a method for producing a hot coil for high toughness and sour-resistant steel pipe, which is characterized by cooling at ~30°C/sec and then winding at a temperature in the range of 400 to 600C.

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 if it exceeds 0.12%, it will deteriorate toughness, and if it is less than 0.05%, it will not only be impossible to secure the necessary strength, but also cause hot cracking during on-site welding. 0.05 to 0.1 because
2%.

%Si is added as a deoxidizing agent, and if it is not 0.1% or more, there is no deoxidizing effect, and if it exceeds 0.4%, the toughness will be completely degraded, so it was set to 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 required strength cannot be secured, and if it exceeds 1.20% gold, The content was set at 0.5 to 1.20% in order to completely deteriorate HIC resistance.

M 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;
If it exceeds 0.005 to 0.0% gold, the toughness will deteriorate.
It was set at 10%.

According to the research conducted by the inventors, in steels with S≦0.0010, HIC is dependent on the end temperature of hot working, which worsens as the end temperature of rolling hot working decreases, and in particular, it is found that HIC is dependent on the end temperature of hot working, and is particularly important for improving toughness. This tendency is greater below 820C, where S≦
It has been discovered that the above-mentioned tendency is no longer observed in 0.0009% steel.

Therefore, S≦0.0009% was set.

Even if the structure is controlled by the 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, we have found that by controlling the structure and setting the P content of molten steel to '2p≦0.006%, it is possible to completely prevent the propagation and growth of ) frc in a harsh sour environment with a pH of less than 4.0.

Furthermore, in the austenite region of the continuous cast slab e1200C or less, hot working with a cross-section reduction rate of 20% or more increases the P diffusion coefficient.
It was found that the diffusion effect of P segregation can be obtained even by soaking at a relatively low temperature for a short time of 30 minutes or more but less than 2 hours in Or, and by adding gold to this process, the upper limit of P can be raised to 0.010%. .

Ca is added to control the M203t" form and increase its size, making MnS completely harmless. However, if it is less than 0.0020%, it has no effect, and if it exceeds 0.0060%, Ca-based cluster-like inclusions are formed. The Ni content was set at 0.0020-0.0060% to prevent formation and deterioration of HIC resistance.Ni is effective in improving corrosion resistance, strength, and toughness;
If it exceeds 0.6% gold, local corrosion will increase, so 50.6% gold will increase.
%. 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 50.6%. C [can increase strength without deteriorating HIC resistance and toughness, but 1.
If it exceeds 00%, the toughness will completely deteriorate, so 51.00%
And so. Mo is effective in improving hardenability and strength, but if it exceeds 0.601e, it will lead to deterioration of toughness.
．． It was set at 60%. Nb, V and Zr are M.

However, if it exceeds 0.10%, the toughness deteriorates, so it is set at 50.10%.

Ti is effective in improving the toughness of the weld heat affected zone, but 0.
If it exceeds 10%, the toughness will deteriorate completely, 50.1
It was set to 0%.

In the present invention, the rolling reduction ratio is 250% below 950C, but 5
Toughness improves by holding it more than 0%, so 50%
That's all. The final hot working temperature is 720 to 820C, but if S≦0.0009%, the dependence of HIC on the final hot working temperature is eliminated.

The average cooling rate is 5 to 30 C/sec. When the average cooling rate is less than 5 tl'/sec, two-phase separation of ferrite pearlite progresses, and a band-like structure of ferrite pearlite is likely to be formed in the central segregation area, resulting in a cooling rate of 30 C/s.
If it exceeds ec, a hardened bainitic structure is likely to be formed, and the resistance to 1 (IC property deteriorates, so the
And so.

The winding temperature is 400 to 600C. 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 A[1+ transformation point, the two-phase separation of ferrite and pearlite progresses, and a band-like structure of ferrite and pearlite is formed. This tendency is particularly remarkable in the central segregation zone, so HIC resistance
Sexuality deteriorates. Therefore, the upper limit of the winding temperature was set at 600C, at which point the ArL) transformation was completed. Furthermore, the winding temperature is 40
In the region below 0 C, a hardened bainitic structure is likely to be formed and HIC resistance deteriorates, as in the case where the average cooling rate exceeds 30 C/sec. From the above, the winding temperature is 400
~600C.

(Function) The inventors found that S≧0 in a harsh sour environment with a pH of less than 4.0.
．． HIC that occurs in 0.0010% steel is dependent on the hot working end temperature, increasing as the hot working end temperature of rolling decreases, and 820C, which has a particularly large improvement in low temperature toughness, has a dependency on the hot working end temperature.
This tendency is remarkable in the following temperature ranges, but S≦0.000
It has been found that for 9% steel, the above trend is not observed at all.

This discovery has made it possible to obtain full low-temperature toughness without impairing HIC resistance by carrying out a full reduction of 50 inches or more at a temperature below 950C and completing hot working at a temperature of 720 to 820C. However, even if S≦0.0009%, there are parts where MnS precipitation is unavoidable in the central segregation part of the continuously cast slab, and HIC occurs in such places.

In order to completely eliminate such HIC, the segregation of P is reduced by setting P≦0.006, and after the completion of hot working, cooling is continued at a cooling rate of 5 to 30 U/sec, and the coiling temperature is lowered. It was discovered that the structure of the center segregated part can be suppressed only by winding at 400 to 600 C and controlling the structure so that it does not propagate or grow.

Furthermore, even if 0.006% and P≦0.010%, the area reduction rate is 2 in the austenite region of 1200C or less in the entire slab.
0% or more hot working to create all the diffusion starting points, and then maintain the slab center temperature i1100r or more and less than 1250C for 30 minutes or more and less than 2 hours to completely diffuse and eliminate or reduce the unavoidably generated segregation. After that, final hot working is performed, followed by cooling at a cooling rate of 5 to 30°C/sec and winding at a winding temperature of 400 to 600°C.
C. Can be completely prevented.

As described above, the present invention makes it possible to manufacture a hot coil that has both excellent IttHIC properties and low-temperature toughness in a harsh sour environment with a pH of less than 4.0.

(Example) In order to fully investigate the effects of S, P and rolling conditions on HIC resistance, the inventors conducted experiments using steels with varying S and P contents. All samples were cast using the continuous casting method, and Ca was a granular alloy. It was carried out by a method of continuous addition to tandates/yu.

Next, the slab manufactured in this manner was rolled in a hot mill, and a HIC resistance evaluation test was conducted using a plate obtained as a hot coil. The HIC resistance evaluation test is the so-called BP
The test was conducted in accordance with the test method. That is, sample '1NA
CE solution (0.5% acetic acid-pentachloride) 96% in an IJium solution saturated with H2S (pH approximately 3.8)
Soaked for an hour.

The presence or absence of HIC generation was evaluated by the ratio of defects to the surface of the specimen (hereinafter referred to as CAR) by inspecting the specimen after immersion using UST.

Examples Kaneko are shown in Tables 1 to 9.

As shown in Table 2, all the steel materials according to the present invention exhibit excellent HIC resistance and low temperature toughness, but P>0.006
% C, D, E and S≧o, ooio% F are HI resistance
Poor C properties. In addition, as shown in Tables 3 to 6, hot working end temperature ≧720 C1 average cooling rate 5 to 30 C/se
c % Exhibits excellent HIC resistance in the range of winding temperatures of 400 to 600C.

(Effects of the Invention) As described above, according to the present invention, it is possible to manufacture a hot coil that has excellent HIC resistance in a harsh sour environment with a pH of less than 4.0, and has excellent low temperature and toughness characteristics, and can be used in a sour environment in a cold region. It is possible to manufacture line pipes that do not cause HIC or burst accidents due to low-temperature brittle fracture.

[Brief explanation of the drawing]

Figure 1 is a graph showing the effect of hot working end temperature on HIC resistance, Figure 2 is a graph showing the effect of hot working ending temperature on toughness, and Figure 3 is 1. P on HIC resistance. Figure 4 is a graph of the influence of content, Figure 4 is a graph of the influence of average cooling rate on 1tHIC resistance, Figure 5 is a graph of the influence of coiling temperature on HIC resistance, and Figure 4 is a graph of the influence of coiling temperature on HIC resistance. The figure shows the influence of the average cooling rate at the center of the length of the coil after winding. Ward (%) WD O fu bonito 0 10 20 JO Kuyu uniform cold #danyou ('℃/sec, ) Figure 5 Tori temperature a (°c) Procedural amendment (method) % formula % 1. Indication Patent Application No. 274296 of 1982 2, Name of the invention Method for manufacturing hot coils for high toughness and sour resistant steel pipes 3, Relationship with the amended case Applicant address 2-6-3 Otemachi, Chiyoda-ku, Tokyo Name (
665) Nippon Steel Corporation Representative Takeshi 1) Yutaka 4, Agent Address 3-3-3-4 Nihonbashi, Chuo-ku, Tokyo, Brief explanation of the drawings

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≦0.006%, S≦0.0009%
, Ca: 0.0020-0.0060%, and Ni≦0
．． 60%, Cu: 0.60%, Cr≦1.00%, Mo≦0.60%, Nb≦0.10%, V≦0.10%, Zr≦0.10%, Ti≦0.10 Continuously cast slabs containing one or more of the following, with the remainder being iron and impurities, are rolled down to 50% or more at 950°C or below, and hot worked to a temperature of 720 to 820°C.
℃ range, followed by an average cooling rate of 5-30℃
A method for producing a hot coil for high toughness and sour-resistant steel pipes, which comprises cooling at a rate of 0.degree. C./sec and then winding at a temperature in the range of 400 to 600.degree. 2, 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 Ni≦0
．． 60%, Cu≦0.60%, Cr:≦1.00%, Mo:≦0.60%, Nb≦0.10%, V≦0.10%, Zr:≦0.10%, Ti: Contains one or more of ≦0.10%,
The remaining part consists of iron and impurities, and the slab is hot-worked in the austenite region below 1200°C with a cross-section reduction rate of 20% or more, and then the center temperature of the slab is reduced to 1100-1250°C.
After holding for 30 minutes or more but less than 2 hours at
, followed by an average cooling rate of 5-30°C
A method for producing a hot coil for a high toughness and sour-resistant steel pipe, which comprises cooling the coil at a temperature of 400 to 600° C. after cooling the coil at a temperature of 400 to 600° C.