JPH0211722A - Manufacture of steel plate having excellent hydrogen-induced cracking resistance - Google Patents

Abstract

PURPOSE:To manufacture the title steel plate in the environment of humid hydrogen sulfide by subjecting a steel slab having specific compsn. to hot finish rolling under specific conditions, thereafter subjecting the steel to accelerated cooling at specific cooling speed according to its carbon equivalent and thereafter allowing it to cool. CONSTITUTION:A steel contg., by weight, 0.03 to 0.20% C, 0.02 to 0.60% Si, 0.50 to 2.50% Mn, <0.020% P, <0.003% S, 0.005 to 0.060% Al, <0.005% Ti, 0.0005 to 0.005% Ca and <0.0050% N is subjected to hot rolling at <=900 deg.C at >=60% or >=30% draft rate to finish its final rolling at the temp. of (Ar3 point -30 deg.C) or above. The steel is rapidly cooled at the cooling rate CR ( deg.C/sec) expressed by the Formula II at the time of >=0.785% carbon equivalent CeqS expressed by the Formula I of the steel and at the cooling rate CR expressed by the Formula III at the time of <=0.785% carbon equivalent. After that, the steel is allowed to cool or is tempered at Ac1 to 500 deg.C.

Description

Detailed Description of the Invention (Industrial Field of Application) The present invention relates to a method for manufacturing a steel sheet with excellent hydrogen-induced cracking resistance, and more particularly, to an oil or gas pipeline operating in a humid hydrogen sulfide environment. The present invention relates to a method for producing a non-tempered steel plate or a tempered steel plate with excellent resistance to hydrogen-induced cracking, which is suitable for refining equipment and the like.

(Prior Art) In recent years, equipment used in a humid hydrogen sulfide atmosphere, for example,
So-called hydrogen-induced cracking (HI) occurs in line pipes and oil refinery equipment that transport crude oil and natural gas containing hydrogen sulfide.
There are many accidents caused by C), and a steel plate with excellent hydrogen-induced cracking resistance is desperately needed.

This hydrogen-induced cracking occurs due to the pressure of hydrogen gas generated when hydrogen generated by corrosion of the steel penetrates and diffuses into the steel in an atomic state, accumulates and becomes molecules at the interface between the inclusions and the base steel. This is said to propagate along band-like hardened structures that occur in segregated areas in the steel.

Therefore, the current measures to prevent hydrogen-induced cracking are: (1) Suppression of hydrogen intrusion and diffusion into steel.

(2) Reduction and shape control of inclusions, especially A-based inclusions that have a large notch effect at the tip.

(3) Reducing segregation and suppressing the formation of hardened structures.

The following methods have been adopted.

(Problem to be Solved by the Invention) However, as for the countermeasure for the above (1), for example, as described in Japanese Patent Application Laid-Open No. 50-097515,
There is a method of forming a corrosion-resistant film by adding Cu, but
It has no effect in a harsh environment such as pH 3, and cannot suppress the occurrence of hydrogen-induced cracking.

In addition, regarding the countermeasure for (2) above, please refer to JP-A-51-1
The method of controlling the shape and number of sulfides as shown in Japanese Patent Publication No. 14318, and the method of regulating the shape and number of sulfides as shown in Japanese Patent Application Laid-open No. 55-128536, Japanese Patent Application Laid-open No. 54-031020, etc.
a. There is a method of controlling the morphology of A-based inclusions using REM, but as the strength level of steel plates increases and the environment becomes harsher,
It is difficult to completely prevent hydrogen-induced cracking.

Furthermore, regarding the countermeasure for the above (3), see Japanese Patent Application Laid-Open No. 58-1
As described in Publication No. 99813, the P content is reduced to 0.
．． There is a method to extremely lower the hardness to 0.002% or less, but there is a problem in terms of cost, and as described in Japanese Patent Application Laid-Open No. 57-073162, the hardness of the hardened tissue part is reduced to Hv≦3.
50, but it is difficult to completely eliminate hydrogen-induced cracking in high-strength steel under harsh environments with low pH.

Of course, these methods are often used in combination, but p
It is difficult to completely suppress the occurrence of hydrogen-induced cracking in harsh environments such as H.3, and even if possible, it may not be sufficient in terms of productivity and manufacturing costs of industrial products. This is the actual situation.

The present invention was made under such circumstances, and
It is an object of the present invention to provide a method for producing a steel sheet that is free from hydrogen-induced cracking and has excellent hydrogen-induced cracking resistance even under a harsh environment such as pH=3.

(Means for Solving the Problems) In order to solve the above-mentioned problems, the present inventors have studied the chemical composition of steel, rolling/cooling conditions, etc., and have developed a non-tempered steel sheet with excellent hydrogen-induced cracking resistance. We conducted extensive research to find a method that would allow us to manufacture both heat-treated steel sheets.

As a result, in addition to appropriately adjusting the chemical composition of the steel, the rolling reduction rate and rolling finishing temperature in a specific temperature range during hot rolling, depending on whether to obtain non-thermal steel or heat-treated steel, In addition, by controlling the cooling rate after rolling, it is possible to obtain the desired non-tempered steel, and furthermore, by controlling the tempering conditions, it is possible to obtain the desired tempered steel. This discovery led to the creation of the present invention.

That is, in the present invention, C: 0.03 to 0.20%, Si
:0.02-0.60%, Mn:0.50-2.50%
, P: 0.020% or less, S: 0.003% or less, A
Q:0. OO5~0.060%, Ti:0.005%
Below, Ca: 0. OO05-0,0050% and N:0
．． 0050% or less, and if necessary, Nb:0
．． OO5-0, 100%, V: 0. OO5~0°100
%, Cu: 0.05-1.50%, Ni: 0,05
~1.50%, Cr:0.05~0.50% and MO:
In short, when heating and hot rolling a steel billet containing one or more of 0.05 to 0.50%, with the remainder consisting of iron and unavoidable impurities, (1) In the case of quality steel, after finishing rolling at a temperature of 900°C or lower with a rolling reduction of 60% or higher and a finishing temperature of (Ar3-30'c) or higher, the cooling rate CR ('C/S) is set as described below. Equation (1), (
It is characterized by accelerated cooling to a temperature of 450°C or more and less than 600°C within the range shown in 2), and then allowed to cool. The rolling reduction rate is 30% or more, and the rolling finishing temperature is (Ar33
After finishing the rolling to achieve a temperature of 0°C or higher, the cooling rate CR (°C
/s) is 60 in the range shown by formulas (3) and (4) below.
Accelerated cooling to any temperature below 0℃, then AC
It is characterized by tempering in a temperature range of 1 to 500°C.

The present invention will be explained in more detail below.

First, the reasons for limiting the chemical components in the present invention will be explained.

Although 0.03% or more of C is required to ensure strength, if the content exceeds 0.20%, the susceptibility to weld cracking increases. Therefore, the C content is set in the range of 0.03% to 0.20%.

Si is an element necessary for deoxidation, and for that purpose the content needs to be 0.02% or more. However, when contained in large amounts, toughness deteriorates. Therefore, the Si content is 0.02
The range is 0.60%.

Mn is a necessary element to ensure strength, but if the content is less than 0.50%, this effect will be small;
If the content exceeds the above, weldability will be impaired. Therefore, Mn
The content is in the range of 0.50 to 2.50%.

P is inherently undesirable because it increases the hardness of the segregated parts of steel and deteriorates the hydrogen-induced cracking resistance, but as long as the requirements of the present invention are satisfied, there is no need for any particular rules regarding the P content. . However, P
The content is regulated to 0.020% or less.

S is an element that forms A-based inclusions and impairs hydrogen-induced cracking resistance, and is therefore undesirable. .

0.003% or less.

AQ is required as a deoxidizing element in a content of 0.005% or more, but since too much content causes deterioration of toughness, it should be added in a content of 0.060%.
must be the upper limit. Therefore, AQ content is 0.0
The range is 0.05% to 0.060%.

Ti is an element that easily combines with N to form nitride. - If these nitrides precipitate near the segregated areas in the steel sheet, they are likely to become points of occurrence for hydrogen-induced cracking. For this reason, the T1 content is regulated to 0.005% or less.

Ca is an element that is effective in spheroidizing sulfide inclusions, and if the content is less than 0.0005%, this effect is small;
Moreover, if the content exceeds 0.0050%, the toughness will deteriorate. Therefore, the Ca content is 0.0005 to 0.00
The range is 50%.

In a solid solution state, N is an element that greatly increases the hardenability of steel and hardens the segregated parts even in a small amount, or because it is an element that combines with Ti to form precipitates and impairs hydrogen-induced cracking resistance.
0050% or less.

In addition to the above-mentioned components, the following elements Nb, V, Cu, Ni, Cr and MO may be used as necessary in the present invention.
One or more of these may be contained in appropriate amounts.

Nb and V are elements that are effective in improving strength, but if each is contained in an amount less than 0.005%, the effect is small, and if each is contained in an amount exceeding 0.100%, the toughness of the welded part is deteriorated. Therefore, each content of Nb and V(7) is 0.
．． The range is 0.005-0.100%.

Cu is ineffective in improving strength if it is less than 0.05%, and deteriorates hot workability if it is contained in more than 1.50%. Therefore, the Cu content is in the range of 0.05 to 1.50%.

If the Ni content is less than 0.05%, it will have little effect on improving strength and toughness, and if the content exceeds 1.50%, it will impair economic efficiency. Therefore, the Ni content is 0.05 to 1.
The range is 50%.

Cr and Mo are elements that are effective in increasing strength, but
If each content is less than 0.05%, the effect will be small, and if each content exceeds 0.50%, weldability will deteriorate. Therefore, each content of Cr and Mo is 0.05 to 0.
The range is 50%.

Next, the reasons for limiting conditions such as rolling and cooling in the present invention will be explained.

In the present invention, a steel slab having the above chemical composition is heated, hot rolled, and cooled. However, if the austenite grains before cooling are large, a large amount of coarse bainite structure is generated on the steel surface during the cooling process, resulting in hydrogen resistance. Detracts from induced cracking. Therefore, in hot rolling before cooling, it is necessary to perform a reduction of 60% or more in a temperature range of 900° C. or lower to refine the austenite grains. However, in the case of tempered steel,
A reduction of 30% or more is performed in a temperature range of 900° C. or more to refine the austenite grains.

Further, the finishing temperature of rolling is set to be higher than (Ar3 transformation point -30°C) in order to fully exhibit the effect of subsequent cooling.

Next is cooling after hot rolling, which is done in order to reduce the hardness of the segregated parts of the steel sheet, depending on the component content and whether it is for non-tempered steel or for tempered steel. Depending on the case, it is necessary to perform accelerated cooling within a range where the cooling rate cR ('C/s) is expressed by the following formula.

That is, in the case of non-tempered steel, the cooling rate CR is set in the range expressed by the following equations (1) and (2).

When CeqS≧0.785%, (13,3XCeqS-8,7)2≦CR≦4-0 ・
-(1) If CeqS<0, 785%, 3≦CR≦40 ・(2) Also,
In the case of tempered steel, the cooling rate CR is within the range defined by the following equations (3) and (4).

When CeqS≧0.935%, (13,3XCeqS-10,7)”≦CR・(3)C
When eqS<0, 935%, 3≦CR...(4) However, C eqS in the above formulas (1), (2), (3), and (4) is defined by the following formula.

+ Nb + 2 As a result of the hydrogen-induced cracking test for hours, it was found that in the case of non-thermal steel, hydrogen-induced cracking occurs when cooled under conditions that satisfy equations (1) and (2) above, as shown in Figure 1. Although no occurrence occurred, it was found that hydrogen-induced cracking occurs when the formulas (1) and (2) are not satisfied. In addition, in the case of tempered steel, as shown in Figure 2, the above (3) and (
4) When a steel plate that has been cooled under conditions that satisfy equation (3) and (4) is tempered at a temperature within the condition range of the present invention, no hydrogen-induced cracking occurs, but these equations (3) and (4) are not satisfied. It was discovered that hydrogen-induced cracking occurs in some cases.

In addition, the water cooling stop temperature is 4 in the case of non-tempered steel.
The temperature needs to be 50°C or higher and lower than 600°C. 60
At temperatures above 0°C, water cooling has little effect in reducing the hardness of segregated parts, and below the lower limit of 450°C, the surface of the steel plate hardens,
This is not preferable because it impairs hydrogen-induced cracking resistance. After this rapid cooling, slow cooling is performed.

On the other hand, in the case of tempered steel, the water-cooling stop temperature may be any temperature range below 600°C; at temperatures above 600°C, the effect of water cooling in reducing the hardness of segregated parts is insufficient, and the hydrogen-induced cracking resistance deteriorates. It is not desirable because it causes harm.

In the case of tempered steel, tempering is further performed, but the temperature needs to be 500° C. or higher in order to reduce the hardness of the segregated portion and improve hydrogen-induced cracking resistance.

(Example) Next, examples of the present invention will be shown.

Lost Seed, melt the steel with the chemical composition shown in Table 1 by a conventional method,
After casting by a continuous casting method or an ingot-forming method, non-tempered steel sheets were manufactured under the rolling and cooling conditions shown in Table 2.

The strength and hydrogen-induced cracking resistance of the obtained steel sheets are also listed in Table 2.

In addition, the evaluation of hydrogen-induced cracking resistance was performed using NACEStand.
It was carried out according to ard TM-0284.

However, the solutions used in the test were ■-■ artificial seawater saturated with 2S (so-called BP solution, pH: 5) and 5% NaCQ+
There are two types: 0.5% acetic acid solution (so-called NACE solution, pH=3).

A test piece taken from each test copper plate was immersed in the above solution for 96 hours under no load, and then the presence or absence of hydrogen-induced cracking was determined using a cross-sectional microscope.

``The test piece used in the hydrogen-induced cracking test described in Section 2 is the test steel plate.''
Samples were taken from the position where the segregation was considered to be the largest in , as shown in Figure 3. The shape of the test piece 2 and the position of the cross-sectional microscope are shown in FIG. The size (mm) of test piece 2 is
t×20tll×100Q.

In addition, the thickness of the test piece 2 is 1.2 mm on both the front and back sides of the steel plate. It is cut in mm increments.

Three test pieces were taken for each test solution from each test steel plate,
Only when no hydrogen-induced cracking was observed in any of the test pieces, it was determined that there was no hydrogen-induced cracking (marked O).

As is clear from Table 2, in the case of non-tempered steel sheets manufactured under the conditions of the present invention (method of the present invention), not only in the BP solution with a pH of ~5, but also in the NACE solution with a pH of 3. No hydrogen-induced cracking occurred at all.

On the other hand, in steel sheets whose manufacturing conditions did not satisfy the conditions within the scope of the present invention (comparative method), hydrogen-induced cracking occurred in all of them.

[Left below] Steel having the chemical composition shown in Table 3 is melted by a conventional method,
After casting by continuous casting or ingot-forming, tempered steel plates with a thickness of 25 mm were manufactured under the rolling, cooling, and tempering conditions shown in Table 4.

The strength and hydrogen-induced cracking resistance of the obtained steel sheets are also listed in Table 4. Note that the evaluation of hydrogen-induced cracking resistance was the same as in Example 1.

As is clear from Table 4, in the case of heat-treated steel sheets manufactured under the conditions within the scope of the present invention (method of the present invention), hydrogen is present not only in the BP solution with a pH of 5, but also in the NACE solution with a pH of 3. No induced cracking occurred.

On the other hand, in steel sheets whose manufacturing conditions did not satisfy the conditions within the scope of the present invention (comparative method), hydrogen-induced cracking occurred in all of them.

[Blank below] (Effects of the invention) As explained above, according to the present invention, a steel plate that does not undergo any hydrogen-induced cracking even under a harsh environment such as p It can be manufactured at low cost as a steel plate or as a tempered steel plate.

[Brief explanation of the drawing]

Figures 1 and 2 show the effect of component Ce on hydrogen-induced cracking.
A diagram showing the relationship between qS and cooling rate CR during water cooling. Figure 3 is a perspective view showing the sampling position of the hydrogen-induced cracking test piece. Figure 4 is a perspective view showing the shape and cross-sectional examination position of the hydrogen-induced cracking test piece. It is a diagram. Patent applicant Hisashi Nakamura, patent attorney representing Kobe Steel, Ltd.

Claims (1)

[Claims] (1) In weight% (the same applies hereinafter), C: 0.03 to 0.2
0%, Si: 0.02~0.60%, Mn: 0.50~
2.50%, P: 0.020% or less, S: 0.003%
Below, Al: 0.005-0.060%, Ti: 0.0
05% or less, Ca: 0.0005 to 0.0050% and N: 0.0050% or less, and the balance is iron and unavoidable impurities.
After finishing rolling with a rolling reduction of 60% or more at 900°C or less and a finishing temperature of (Ar_3-30°C) or more, the cooling rate CR (°C/s) is determined by the following formulas (1) and (2). A method for producing a non-annealed steel sheet with excellent hydrogen-induced cracking resistance, characterized by accelerating cooling to a temperature of 450° C. or higher and lower than 600° C. within the indicated range, and then allowing it to cool. In the case of CeqS≧0.785%, (13.3CeqS-8.7)^2≦CR≦40…(1
) When CeqS<0.785%, 3≦CR≦40…(2) However, CeqS=1.3×C+Si/15+Mn/3+30×
P+2×N (%) (2) When heating and hot rolling a steel billet having the chemical composition according to claim 1, the rolling reduction at 900°C or lower is 30% or more, and the rolling finishing temperature is ( Ar_3
-30℃) or higher after finishing rolling, cooling rate CR
(℃/s) is 60 within the range shown by the following formulas (3) and (4).
Accelerated cooling to any temperature below 0°C, then further Ac
A method for producing a steel sheet with excellent hydrogen-induced cracking resistance, characterized by tempering at a temperature range of _1 to 500°C. When CeqS≧0.935%, (13.3CeqS-10.7)^2≦CR…(3)C
When eqS<0.935%, 3≦CR...(4) However, CeqS is the same as in claim 1. (3) In the method according to claim 1 or 2, the steel piece further contains Nb: 0.005 to 0.100% and V: 0.00%.
5-0.0100%, Cu: 0.05-1.50%, N
i: 0.05-1.50%, Cr: 0.05-0.50
% and Mo: 1 or 2 of 0.05 to 0.50%
The above formulas (1) to (2) or (3) to (4)
) is defined by the following formula. CeqS=1.3×C+Si/15+Mn/3+30×
P+(Cu+Ni)/15+(Cr+Mo+V)/5+
Nb+2×N(%)