JPH05125441A - Manufacture of acceleratedly cooled steel plate excellent in hydrogen induced cracking resistance - Google Patents

Abstract

PURPOSE:To manufacture an acceleratedly cooled steel plate excellent in hydrogen induced cracking resistance, at the time of rolling a slab having a specified compsn. into a steel plate, by controlling the preceding rolling under specified conditions before normal thick plate rolling and the conditions of heating, rolling and cooling at the time of the subsequent thick plate rolling. CONSTITUTION:A slab having a compsn. contg., by weight, 0.03 to 0.20% C, 0.05 to 0.60% Si, 0.50 to 2.50% Mn, <0.02% P, <0.003% S, 0.005 to 0.10% Al, 0.0005 to 0.0050% Ca and <0.0080% N or furthermore contg. one or more kinds among Nb, V, Cu, Ni, Cr, Mo and Ti is held at Ta deg.C of >=1200 deg.C for (ta) time of >=3hr and is thereafter subjected to rolling-down in which the rolling shape factor expressed by formula [I] is regulated to >=0.5 by at least two passes. Next, it is held at Tb deg.C of >=1150 deg.C for (tb) time of >=0.25hr and is thereafter subjected to hot rolling in such a manner that its finishing temp. is regulated to >=(Ar3-30) deg.C into a thick plate. After the completion of the rolling, it is subjected to accelerated cooling to <600 to >=400 deg.C at >=3 deg.C/sec cooling rate; where the above acceleration for two times is executed under the conditions satisfying formula [II] or formula [III].

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 an accelerated cooling steel sheet having excellent resistance to hydrogen-induced cracking, and more specifically,
The present invention relates to a method for producing an accelerated cooling steel sheet having a yield strength of 317 to 482 N / mm ² class and excellent in hydrogen-induced cracking resistance, which is used for oil and gas pipelines and the like that operate in a wet hydrogen sulfide environment.

[0002]

2. Description of the Related Art In recent years, in equipment used in a wet hydrogen sulfide atmosphere, for example, line pipes and petroleum refining equipment for transporting crude oil or natural gas containing hydrogen sulfide, it is caused by so-called hydrogen-induced cracking (HIC). There is a strong demand for steels that have many accidents and excellent resistance to hydrogen-induced cracking.

In this hydrogen-induced cracking, hydrogen generated by the reaction between steel and hydrogen sulfide containing water penetrates and diffuses into the steel in an atomic state, and accumulates at the interface between inclusions in the steel and the base metal, and molecules. It is said that the hydrogen gas is generated by the pressure of hydrogen gas generated by the liquefaction and propagates along the layered hardening structure or the like generated in the segregated portion in the steel.

Many methods have been proposed so far as measures against this hydrogen-induced cracking. That is, as to the method for suppressing the invasion and diffusion of hydrogen into the steel, see Japanese Patent Laid-Open No. 0-097515.
Japanese Patent Publication has been proposed. Regarding the reduction of inclusions, especially the A-type inclusions having a large notch effect at the tip and the shape control method, JP-A-51-114318, JP-A-55-128536 and JP-A-54-031020 are known. Etc. have been proposed. Regarding the method of reducing segregation and suppressing the formation of hardened structure,
JP-A-58-199813 and JP-A-57-073162 have been proposed.

[0005]

However, the above-mentioned Japanese Patent Laid-Open No.
The method proposed in JP-A-50-097515 to form a corrosion-resistant film on the steel surface by the addition of Cu and suppress the intrusion of hydrogen is not effective in a severe environment where the pH is close to 3. The generation of hydrogen-induced cracking cannot be suppressed.

A method proposed in JP-A-51-114318 for controlling the shape and number of A-type inclusions, and JP-A-55-55
-128536 and Japanese Patent Laid-Open No. 54-031020 propose a method for controlling the morphology of A-type inclusions by Ca and REM.
The strength level of the steel sheet is high, and it is difficult to completely prevent the occurrence of hydrogen-induced cracking in a severe environment.

Further, the method of reducing P to 0.002% or less, which is proposed in JP-A-58-199813, has a problem in cost, and is also proposed in JP-A-57-073162. The method of setting the hardness of the hardened structure to HV ≦ 350 is p
It is difficult to eliminate the occurrence of hydrogen-induced cracking of high-strength steel in a severe environment with low H.

Of course, these methods are used in combination to manufacture hydrogen-induced cracking-resistant steel, but it is difficult to manufacture steel that does not cause hydrogen-induced cracking in a severe environment where the pH is close to 3. However, when it can be manufactured, it cannot be said that it is sufficient in terms of productivity as an industrial product and manufacturing cost.

The present invention has been made under such circumstances, and manufacture of an accelerated cooling steel sheet excellent in hydrogen-induced cracking resistance and free from hydrogen-induced cracking even under a severe environment such as pH≈3. The purpose is to provide a method.

[0010]

The present inventors have studied the chemical composition of steel, heating / rolling conditions and the like in order to solve the above-mentioned problems, and have accelerated hydrogenated steel sheet excellent in hydrogen-induced cracking resistance. We conducted diligent research to find a method that can manufacture

As a result, by appropriately adjusting the chemical composition of the steel and controlling the heating, rolling and cooling conditions during the preceding rolling under the specific conditions before the normal plate rolling and the subsequent plate rolling, It has been found that it is possible to obtain a desired steel sheet, and the present invention has been completed here.

The first invention is C: 0.03 to 0.20%, Si: 0.
05 to 0.60%, Mn: 0.50 to 2.50%, P: 0.020% or less, S: 0.
003% or less, Al: 0.005-0.100%, Ca: 0.0005-0.0050
%, N: 0.0080% or less, the balance of which is Fe and unavoidable impurities.
After holding ta (h) at (℃) for 3 hours or more,
The rolling shape factor represented by the formula should be at least two passes with a reduction of 0.5 or more, then a temperature Tb of 1150 ° C or more.
After holding tb (h) at (℃) for 0.25 hours or more, plate rolling with a finishing temperature of (Ar ₃ −30) ℃ or more is performed.
When accelerated cooling is performed at a cooling rate of 3 ° C / s or more to a temperature of less than 600 ° C and 400 ° C or more after rolling, the above two heating conditions satisfy the following formula (2) and are excellent in hydrogen-induced cracking resistance. It is a method of manufacturing an accelerated cooling steel sheet. [R (h1-h2)] ^1/2 / [(h1 + h2) / 2] ………… (1) However, h1: Inlet side steel piece thickness h2: Outgoing steel piece thickness R: Rolling roll radius 1.3C + Si / 15 + Mn / 3 + 30P + 2N + 2.2 ≤ (Ta +273) (log (ta + tc) +20) × 10 ^-4 … (2) where tc = 10 ^{((Tb + 323) (logtb + 20) / (Ta + 273) -20)}

The second invention is C: 0.03 to 0.20%, Si: 0.05 to
0.60%, Mn: 0.50 to 2.50%, P: 0.020% or less, S: 0.003
% Or less, Al: 0.005 to 0.100%, Ca: 0.0005 to 0.0050%,
N: 0.0080% or less, Nb: 0.005 to 0.100%,
V: 0.005 to 0.100%, Cu: 0.05 to 1.0%, Ni: 0.05 to 4.
0%, Cr: 0.05 to 0.5%, Mo: 0.05 to 0.5%, Ti: 0.005
Slab or steel slab containing one or more selected from 0.020% and the balance Fe and unavoidable impurities at a temperature Ta (° C) of 1200 ° C or more for 3 hours or more
After holding ta (h), at least 2 passes of rolling with a rolling shape factor represented by the following formula (1) of 0.5 or more are performed, and then at a temperature Tb (° C) of 1150 ° C or more and a time of 0.25 hours or more.
After holding tb (h), the plate is rolled at a finishing temperature of (Ar ₃ -30) ° C or higher, and after rolling is finished, it is less than 600 ° C, 400
When performing accelerated cooling to a temperature of ℃ or higher at a cooling rate of 3 ℃ / s or more, the above-mentioned two heating conditions satisfy the following formula (3) by a method of manufacturing accelerated cooled steel sheet with excellent hydrogen-induced cracking resistance. is there. [R (h1-h2)] ^1/2 / [(h1 + h2) / 2] ………… (1) However, h1: Inlet side steel piece thickness h2: Outgoing steel piece thickness R: Rolling roll radius 1.3C + Si / 15 + Mn / 3 + 30P + (Cu + Ni) / 15 + (Cr + Mo + V) /5+Nb+2N+2.2 ≦ (Ta + 273) (log (ta + tc) +20) × 10 ^-4 ……………… (3) However, tc = 10 ^{((Tb + 323) (logtb + 20) / (Ta + 273) -20)}

[0014]

The present invention will be described in more detail below. First,
The reasons for limiting the chemical components in the present invention will be described.
C is an element necessary to secure the strength, and if it is less than 0.03%, the target strength cannot be obtained.
If it exceeds%, the weld cracking susceptibility becomes high. Therefore, the content is set to the range of 0.03 to 0.20%.

Si is an element necessary for deoxidation during steel making,
For that, 0.05% or more is required. But a lot
If it is contained in, the toughness deteriorates. Therefore, its content is 0.
The range is from 05 to 0.60%.

Mn is an element necessary for ensuring the strength, but if it is less than 0.50%, this effect is small, and if it exceeds 2.50%, the weldability is impaired. Therefore, its content should be in the range of 0.50 to 2.50%.

Originally, P increases the hardness of the segregated portion of steel,
Although it is not a preferable element because it deteriorates the hydrogen-induced cracking resistance, the content is not particularly limited as long as the requirements of the present invention are satisfied. However, in order to prevent the toughness of the welded part from decreasing, its content should be 0.020% or less.

S is not a preferable element because it forms an A-based inclusion and impairs hydrogen-induced cracking resistance. Therefore, its content should be 0.003% or less.

Al is useful as a deoxidizing element at the time of steel making, and it is necessary to contain at least 0.005% or more. On the other hand, if it is contained in excess of 0.100%, the toughness deteriorates. Therefore, the content is set to 0.005 to 0.100%.

Ca is an element effective in spheroidizing sulfide inclusions, but if the content is less than 0.0005%, this effect is small, and if it exceeds 0.0050%, the toughness deteriorates. Therefore, the content is set to the range of 0.0005 to 0.0050%.

N is an element that greatly enhances the hardenability of steel and hardens the segregated portion in a solid solution state, so its content is made 0.0080% or less.

In addition to the above components, in the present invention,
If necessary, the following elements Nb, V, Cu, Ni, Cr, Mo
And one or more selected from Ti can be contained.

Nb and V are elements effective in improving the strength, but if each is less than 0.005%, the effect is small, and if each exceeds 0.100%, the toughness of the welded portion deteriorates. Therefore, the content of each is 0.0
The range is from 05 to 0.100%.

Cu is an element effective for improving strength,
If it is less than 0.05%, its effect is not obtained, and if it is contained in excess of 1.0%, the hot workability is deteriorated. Therefore, its content should be in the range of 0.05 to 1.0%.

Ni is an element effective in improving strength and toughness, but if it is less than 0.05%, its effect is small, and 4.
If it is contained in excess of 0%, the economy will be impaired. Therefore, its content should be in the range of 0.05 to 4.0%.

[0026] Cr and Mo are elements effective in increasing the strength, but if each is less than 0.05%, the effect is small, and if each exceeds 0.5%, the welded portion deteriorates. Therefore, the content of each is 0.05 to 0.5%.
The range is.

Ti is an element that is effective as a carbonitride for improving hydrogen-induced cracking resistance, but if it is less than 0.005%, its effect is small, and if it exceeds 0.020%, coarse carbonitride precipitates. However, this serves as a crack generation point and deteriorates hydrogen-induced cracking resistance. Therefore, its content is 0.005-
The range is 0.020%.

Next, the reasons for limiting the heating / rolling conditions in the present invention will be explained. In the present invention, the cast or steel slab having the above chemical composition is subjected to the preceding rolling prior to the usual thick plate rolling. 1200 heating conditions in the preceding rolling
The reason for holding ta (h) at a temperature Ta (° C) of 3 ° C or higher for 3 hours or longer is to diffuse the segregated elements in the steel and reduce the hardness of the segregated portion.

Further, the reason for performing at least two passes of rolling with a rolling shape factor of 0.5 or more is that the Zaku in the cast slab or the steel slab is pressure-bonded in the preceding rolling, and the subsequent heating for thick plate rolling is performed in the steel. This is to make the diffusion of the segregation element more effective.

The plate rolling after the preceding rolling is carried out after the temperature Tb (° C.) of 1150 ° C. or higher and the time tb (h) of 0.25 hours or longer are held. The reason for this limitation is that the segregated elements are diffused in the steel. This is to reduce the hardness of the segregated portion.

The reason why the rolling finishing temperature is set to (Ar ₃ -30) ° C. or higher is to prevent the generation of worked ferrite, which is harmful to hydrogen-induced cracking resistance.

Further, accelerated cooling after rolling is performed at less than 600 ° C., 4
The reason for cooling to a temperature of 00 ° C or higher at a cooling rate of 3 ° C / s or higher is to prevent the formation of pearlite and martensite, which are harmful to hydrogen-induced cracking resistance.

However, regarding the above two heating conditions,
It is necessary to control within the range shown by the following formula according to the content of the components. That is, 1.3C + Si / 15 + Mn / 3 + 30P + 2N + 2.2 ≤ (Ta +273) (log (ta + tc) +20) × 10 ^-4 … (1) or 1.3C + Si / 15 + Mn /3+30P+(Cu+Ni)/15+(Cr+Mo+V)/5+Nb+2N+2.2 ≦ (Ta +273) (log (ta + tc) +20) × 10 ^-4 ………… (3) However, tc = 10 ^{((Tb + 323) (logtb + 20) / (Ta + 273) -20)}

The above equation will be described below based on the test results conducted by the inventors. The inventors
Hydrogen-induced cracking tests were conducted on steel sheets under different heating, rolling and cooling conditions during the preceding rolling and the plate rolling. In addition,
The test solution was saturated with hydrogen sulfide at initial conditions pH ≒ 3 5% NaCl-
Immersion time is 96 hours in 0.5% acetic acid aqueous solution. The test results are shown in FIG.

FIG. 1 shows the presence / absence of hydrogen-induced cracking occurring in the cross section of the test piece under the chemical composition conditions (1.3C + Si / 15 + Mn / 3 + 30P + (Cu + N
i) / 15 + (Cr + Mo + V) /5+Nb+2N+2.2) and heating conditions ((Ta +27
3) (log (ta + tc) +20) × 10 ^-4 ), where the vertical axis represents heating conditions and the horizontal axis represents chemical composition conditions. ○ in the figure
The mark indicates that there is no crack, and the mark indicates that there is crack, and the straight line in the figure indicates the boundary with and without hydrogen-induced cracking. That is, the straight line shows the heating condition in which hydrogen-induced cracking determined by the chemical composition condition does not occur.

As is apparent from FIG. 1, hydrogen-induced cracking does not occur at all when manufactured under heating conditions that satisfy the above formula (2) or (3), but heating conditions that do not satisfy these formulas. In the case of manufacturing at 1, hydrogen-induced cracking occurs. Therefore, it is essential that the heating conditions satisfy equation (2) or equation (3).

[0037]

EXAMPLES Examples of the present invention will be described below. The test steel plate is made by melting and casting a steel having the chemical composition shown in Table 1 by a conventional method, and the obtained cast piece or steel piece is finished to a plate thickness of 20 mm according to the heating and rolling conditions shown in Table 2. is there.

Test pieces were taken from these steel sheets and subjected to a tensile test and a hydrogen-induced cracking test. The results are shown in Table 3. The hydrogen-induced cracking resistance was evaluated by NAC.
It carried out according to EStandard TM-02-84.

As shown in FIG. 2, test pieces of the hydrogen-induced cracking test were taken from the position where the segregation was considered to be the largest in the steel sheet 1 to be tested, and the front and back surfaces were cut by 1 mm each to obtain a thickness of 18 mm,
The width is 20 mm and the length is 100 mm.

Solutions for the hydrogen-induced cracking test were BP solution (hydrogen sulfide saturated artificial seawater, pH≈5) and NACE solution (hydrogen sulfide saturated 5% NaCl-0.5% acetic acid aqueous solution, pH≈3).
It is a kind.

The above test pieces were immersed in these solutions for 96 hours in an unloaded state, and then the presence or absence of cracks was observed under a microscope on the three specular surfaces shown in FIG. The hydrogen-induced cracking test was repeated 3 times. The presence or absence of hydrogen-induced cracking was judged to be non-occurrence of hydrogen-induced cracking only when no crack was observed in three repeated tests.

Table 1 shows the chemical composition, Table 2 shows the pre-rolling conditions,
The thick plate rolling conditions are shown in Table 3, showing chemical composition conditions, heating conditions and tensile properties, and hydrogen induced cracking resistance.

[0043]

[Table 1]

[0044]

[Table 2]

[0045]

[Table 3]

As is clear from Table 3, the steels produced by the method of the present invention (symbols A-1 to 3, B-1, 2, C-1 to D, D).
Nos. 1 and 2) show good hydrogen-induced cracking resistance, as hydrogen-induced cracking does not occur at all in a BP solution having a pH of 5 as well as in a NACE solution having a pH of 3.

On the other hand, comparative methods A-4 to 7 and B-3
Since the heating time, heating temperature or rolling shape factor deviates below the range of the present invention, the BP solution or NA
Hydrogen-induced cracking has occurred in the CE solution. Also,
In Comparative methods B-4, C-4, and D-3, the heating conditions are out of the range determined by the chemical composition conditions, so that hydrogen-induced cracking occurs in the BP solution or the NACE solution.

As is clear from the above examples, the method for producing an accelerated cooling steel sheet excellent in hydrogen-induced cracking resistance according to the present invention is applicable to yield strength 305 to 485 N / mm ² grade steel sheet
Not only in the case of BP solution of H≈5, but also of N of pH≈3
It is possible to manufacture a steel in which hydrogen-induced cracking does not occur even in an ACE solution.

[0049]

As described above, the method for producing an accelerated cooling steel sheet excellent in hydrogen-induced cracking resistance according to the present invention properly adjusts the chemical composition of the steel, and usually the specific conditions before plate rolling. Since the heating conditions during the preceding rolling below and the subsequent thick plate rolling are controlled, according to the present invention, a steel is produced in which hydrogen-induced cracking does not occur even in a severe environment such as pH≈3. It has an excellent effect of being able to.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a chemical component condition and a heating condition that affect hydrogen-induced cracking.

FIG. 2 is a perspective view showing a sampling position of a hydrogen-induced cracking test piece.

FIG. 3 is a perspective view showing a shape of a hydrogen-induced cracking test piece and a cross-sectional inspection position.

[Explanation of symbols]

 1 ... test steel plate, 2 ... test piece.

Claims (2)

[Claims] 1. C: 0.03 to 0.20%, Si: 0.05 to 0.60%, M
n: 0.50 to 2.50%, P: 0.020% or less, S: 0.003% or less, A
l: 0.005-0.100%, Ca: 0.0005-0.0050%, N: 0.0080
% Or less, with the balance being Fe and unavoidable impurities, the cast or steel slab is held at a temperature Ta (° C) of 1200 ° C or higher for a time of ta (h) for 3 hours or longer. The rolling shape factor shown is at least 0.5 pass, and at least two passes are performed. Then, after holding at a temperature Tb (° C) of 1150 ° C or higher for tb (h) of 0.25 hours or longer, the rolling finish temperature is (Ar ₃
Thick plate rolling to -30) ℃ or more, 600 ℃ after rolling
Below, the above two heating conditions are as follows when performing accelerated cooling at a cooling rate of 3 ° C / s or more to a temperature of 400 ° C or more.
A method for manufacturing an accelerated cooling steel sheet having excellent hydrogen-induced cracking resistance, which satisfies the formula (2). [R (h1-h2)] ^1/2 / [(h1 + h2) / 2] ………… (1) However, h1: Inlet side steel piece thickness h2: Outgoing steel piece thickness R: Rolling roll radius 1.3C + Si / 15 + Mn / 3 + 30P + 2N + 2.2 ≤ (Ta +273) (log (ta + tc) +20) × 10 ^-4 … (2) where tc = 10 ^{((Tb + 323) (logtb + 20) / (Ta + 273) -20)} 2. C: 0.03 to 0.20%, Si: 0.05 to 0.60%, M
n: 0.50 to 2.50%, P: 0.020% or less, S: 0.003% or less, A
l: 0.005-0.100%, Ca: 0.0005-0.0050%, N: 0.0080
% Or less, Nb: 0.005 to 0.100%, V: 0.005
~ 0.100%, Cu: 0.05 ~ 1.0%, Ni: 0.05 ~ 4.0%, C
r: 0.05 to 0.5%, Mo: 0.05 to 0.5%, Ti: 0.005 to 0.020
%, Containing one or more selected from
1200 for slab or steel slab consisting of Fe and unavoidable impurities
After holding ta (h) at a temperature Ta (° C) of 3 ° C or higher for 3 hours or longer, rolling shape factor represented by the following formula (1) is 0.5 or more. At least 2 passes, then 1150 ° C or higher After holding tb (h) at temperature Tb (° C) for 0.25 hours or longer, thick plate rolling with a rolling finishing temperature of (Ar ₃ -30) ° C or higher is performed, and after rolling is completed, less than 600 ° C, 400 ° C or more In performing accelerated cooling at a cooling rate of 3 ° C / s or more up to the temperature of, the accelerated cooling steel sheet with excellent hydrogen-induced cracking resistance is characterized in that the above-mentioned two heating conditions satisfy the following equation (3). Production method. [R (h1-h2)] ^1/2 / [(h1 + h2) / 2] ………… (1) However, h1: Inlet side steel piece thickness h2: Outgoing steel piece thickness R: Rolling roll radius 1.3C + Si / 15 + Mn / 3 + 30P + (Cu + Ni) / 15 + (Cr + Mo + V) /5+Nb+2N+2.2 ≦ (Ta + 273) (log (ta + tc) +20) × 10 ^-4 ……………… (3) However, tc = 10 ^{((Tb + 323) (logtb + 20) / (Ta + 273) -20)}