JPH0673450A - Production of high strength steel sheet excellent in hydrogen induced cracking resistance - Google Patents

Abstract

PURPOSE:To obtain a high strength steel sheet excellent in hydrogen induced cracking resistance by heating the slab of steel contg. specified amounts of C, Si, Mn, S and Ca and thereafter rolling it under specified conditions to form its structure into a one of polygonal bainite. CONSTITUTION:The slab of steel contg., by weight, 0.02 to 0.06% C, 0.03 to 0.50% Si, 0.5 to 2.5% Mn <=0.001% S and 0.0005 to 0.008% Ca, and the balance Fe is heated. The subsequent rolling is executed under the conditions in which the finishing temp, is regulated to the recrystallization lower limit temp. of austenite or above, and after that, cooling is executed to form its structure into a one of polygonal bainite. Furthermore, the same steel may be incorporated with one or more kinds among each 0.5% Cu, Ni and Cr and/or one or more kinds among <=0.5% Mo, <=0.10% Nb, <=0.15% V and 0.005 to 0.10% Ti.

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 a high strength steel sheet having excellent resistance to hydrogen induced cracking.

[0002]

2. Description of the Related Art In recent years, along with the change in energy situation, mining of crude oil and natural gas containing hydrogen sulfide (H ₂ S) has been actively conducted, and the demand for line pipes used for transporting them has also increased. is doing. However, under these wet hydrogen sulfide environment (hereinafter referred to as sour environment), hydrogen-induced sulfide (HIC) and sulfide stress corrosion cracking (SSC) may occur under the influence of hydrogen sulfide, which may lead to a destruction accident. Have sex. Therefore, steel materials having high HIC resistance and SSC resistance are required under the sour environment.

By the way, HIC is a crack that is generated by the internal pressure of atomic hydrogen generated on the surface of the steel due to the corrosion reaction invading the steel and accumulating at the interface of inclusions such as MnS. It is known to propagate along hard low temperature transformation products such as bainite. And
The following method is adopted to prevent the occurrence of HIC. Method of controlling the shape of inclusions such as MnS by adding Ca or REM [JP-A-54-110119] Method of forming a protective film on the surface by adding Cu to reduce the amount of hydrogen penetrating into steel A method for homogenizing the structure by controlling low-temperature transformation products, heat treatment (QT, T), etc. [JP-A-54-12782]
1] A method for controlling the structure by controlling rolling conditions (heating temperature, rolling end temperature, reduction rate) and cooling conditions (cooling rate, cooling stop temperature) [JP-A-61-16520]
7, JP-A-3-236420]

[0004]

As described above, various hydrogen-induced cracking preventive measures have been taken in the prior art, but it is difficult to completely prevent cracking. For example,
Is a method of adding rare earth elements such as Ca to the steel to control the morphology of inclusions that are the starting point of HIC. However, even with this method, HIC cannot be completely removed, and if the addition amount becomes excessive, There is a disadvantage that B-type inclusions are generated, and the HIC resistance performance is deteriorated.

Regarding the suppression of the amount of invading hydrogen by the addition of Cu as described above, the Cu film is formed only in a mild sour environment having a pH of 5 or more, and under the severe environment which is required recently ( Cu in a pH 4 or less sour environment)
Since the film dissolves, a sufficient effect cannot be obtained for suppressing the intrusion of hydrogen.

Further, the above-mentioned method is a method of making the structure uniform, and as the strength of the steel becomes higher, the susceptibility to cracking becomes greater, and the susceptibility becomes extremely large especially in the central portion where segregation of Mn, P, etc. occurs. Therefore, it is a method of reducing the cracking susceptibility by heat treatment such as quenching and tempering and improving the HIC resistance performance, but the disadvantages such as the reduction of production efficiency and the increase of cost cannot be avoided.

The above-mentioned controlled rolling, controlled cooling,
That is, it is a method for making the structure uniform and fine by TMCP technology. For example, finish rolling is performed at a temperature of Ar ₃ transformation point or higher, and controlled cooling is performed at a cooling rate of 30 ° C. or higher to a temperature range of 350 to 550 ° C. [JP-A-61-165207]
However, the susceptibility to hydrogen-induced cracking increases as the strength of the steel material increases, and it is difficult to completely prevent HIC by these rolling methods.

[0008]

In view of the above-mentioned problems of the prior art, the present invention has high strength and good HI resistance.
The inventors have found that it is effective to form a polygonal bainite structure by performing finish rolling in a recrystallized austenite region, and have completed the present invention. It seems that.

(1) C: 0.02 to 0.06 wt.%, Si: 0.03 to 0.50 wt.%, Mn: 0.5 to 2.5 wt.%, S: 0.001 wt.% Or less , Ca: 0.0005 to 0.008 wt.%, With the balance being Fe and unavoidable impurities, after heating a slab of steel, the subsequent rolling is performed under the condition that the rolling finish temperature is the recrystallization lower limit temperature of austenite or higher. A method for producing a high-strength steel sheet having excellent resistance to hydrogen-induced cracking, characterized in that the steel structure is made into a polygonal bainite after being cooled.

(2) C: 0.02 to 0.06 wt.%, Si: 0.03 to 0.50 wt.%, Mn: 0.5 to 2.5 wt.%, S: 0.001 wt.% Or less , Ca: 0.0005 to 0.008 wt.%, And at least one element from the group consisting of Cu, Ni, Cr, Cu: 0.5 wt.% Or less, Ni: 0.5 wt.% Or less, Cr: 0 After heating the slab of steel containing 0.5 wt.% Or less, and the balance being Fe and unavoidable impurities, the subsequent rolling is performed under the condition that the rolling finish temperature is the recrystallization lower limit temperature of austenite or higher, and then the steel is cooled. A method for producing a high-strength steel sheet excellent in hydrogen-induced cracking resistance, characterized in that the structure of No. 2 is a polygonal bainite.

(3) C: 0.02 to 0.06 wt.%, Si: 0.03 to 0.50 wt.%, Mn: 0.5 to 2.5 wt.%, S: 0.001 wt.% Or less , Ca: 0.0005 to 0.008 wt.%, And at least one element from the group consisting of Mo, Nb, V, and Ti, Mo: 0.5 wt.% Or less, Nb: 0.10 wt.% Or less, V: 0.15 wt.% Or less, Ti: 0.005 to 0.10 wt.%, The balance being Fe and unavoidable impurities, after heating a steel slab, the subsequent rolling is recrystallization of rolling austenite A method for producing a high-strength steel sheet excellent in hydrogen-induced cracking resistance, characterized in that the steel structure is made into a polygonal bainite after being cooled under a condition of a lower limit temperature or more and then cooled.

(4) C: 0.02 to 0.06 wt.%, Si: 0.03 to 0.50 wt.%, Mn: 0.5 to 2.5 wt.%, S: 0.001 wt.% Or less , Ca: 0.0005 to 0.008 wt.%, And at least one element from the group consisting of Cu, Ni, Cr, Mo, Nb, V, and Ti, Cu: 0.5 wt.% Or less, Ni: 0. 5 wt.% Or less, Cr: 0.5 wt.% Or less, Mo: 0.5 wt.% Or less, Nb: 0.10 wt.% Or less, V: 0.15 wt.% Or less, Ti: 0.005 to 0.10 wt. After heating the slab of steel containing 0.1% and the balance of Fe and unavoidable impurities, the subsequent rolling is performed under conditions where the rolling finish temperature is at or above the lower limit of recrystallization of austenite, and then the steel structure is cooled to the polygonal structure. Of high strength steel sheet excellent in hydrogen-induced cracking resistance, characterized in that it is made of bainite.

[0013]

The reasons for limiting the chemical composition of the steel sheet according to the present invention as described above are as follows. (1) C (carbon) The C content must be 0.02% or more in order to secure the strength and effectively utilize the precipitation hardening of Nb, V, Ti and the like. However, if the C content exceeds 0.06%, the low temperature transformation product generated during accelerated cooling causes deterioration of HIC characteristics. Therefore, the C content is 0.02 to 0.06%
And

(2) Si (Si) Si is added for deoxidation. If the Si content is less than 0.03%, a sufficient deoxidizing effect cannot be obtained, and the Si content is less than 0.03%.
If it exceeds 5%, the toughness deteriorates. Therefore, the Si content is set to the range of 0.03 to 0.5%.

(3) Mn (manganese) Mn is also added as a deoxidizing element like Si, and the Mn content is
If it is less than 0.5%, a sufficient deoxidizing effect cannot be obtained, and if the Mn content exceeds 2.5%, the weldability deteriorates. Therefore, Mn
The content was in the range of 0.5 to 2.5%.

(4) S (Sulfur) S is an alloy element inevitable as an impurity.
If the content is 0.001% or less, there is no actual harm. But,
If this S content exceeds 0.001%, the sulfide inclusions that are the starting points of HIC increase, and the HIC resistance is deteriorated.
Therefore, the S content is set to 0.001% or less.

(5) Cu (Copper) Cu is an element that improves the corrosion resistance, but if the Cu content exceeds 0.5%, the toughness of the welded portion deteriorates. Therefore,
The Cu content should be below 0.5%.

(6) Ni (Nickel) Ni is an element effective in corrosion resistance and also an element having an action of improving strength and toughness. But this Ni
If the content exceeds 0.5%, it is economically disadvantageous, so Ni
The content is 0.5%.

(7) Cr (Chromium) Cr is an element that improves the corrosion resistance, but if the Cr content exceeds 0.5%, the toughness of the welded portion deteriorates, so Cr
The content was 0.5% or less.

(8) Mo (Molybdenum) Mo is an element that improves strength and toughness, but if the Mo content exceeds 0.5%, it is economically disadvantageous, so the Mo content is 0.5. % Or less.

(9) Nb (niobium) Nb is an element that improves the strength, but the Nb content is
If it exceeds 0.1%, the toughness of the weld and the base metal deteriorates, so the upper limit was made 0.1%.

(10) V (Vanadium) V is an element that improves strength and toughness. However, if the V content exceeds 0.15%, the toughness of the base metal and the welded portion deteriorates, so the V content was set to 0.15% or less.

(11) Ti (Titanium) Ti has the function of preventing coarsening of austenite during heating of the slab and is an element that improves strength and toughness, but if the Ti content is less than 0.005%. These actions are insufficient. Further, if the Ti content exceeds 0.1%, the toughness of the welded portion deteriorates. Therefore, the Ti content is 0.005-0.1%
Should be.

(12) Ca (Calcium) Ca has the action of changing the morphology of sulfide inclusions and reducing the origin of HIC. However, the Ca content is 0.000
If it is less than 5%, this effect cannot be obtained, and 0.008%
On the contrary, if the value exceeds the limit, the amount of inclusions is increased and the HIC resistance is increased.
It causes deterioration of sex. From these, the Ca content was set to 0.0005 to 0.008%.

The reason for limiting the steel plate manufacturing conditions for the steel having the above-described composition is as follows. First, the point of the present invention regarding the rolling conditions is the rolling end temperature (T _R ).
Is limited to the recrystallization lower limit temperature or more, and the inventors of the present invention have investigated the influence of the rolling end temperature (accelerated cooling start temperature) on the HIC characteristics by using various test steels having chemical composition within the range of the present invention. When the experiment shown below was conducted, the value shown in Formula (1) was used as the lower limit temperature for recrystallization,
It has been found that hydrogen-induced cracking does not occur when rolling is completed after that.

That is, a sample steel (slab) having a chemical composition within the scope of the present invention shown in Table 1 below was heated to 1150 ° C., then rolled at a rolling end temperature of 750 to 950 ° C., and then rolled. Accelerated cooling was performed from the rolling end temperature to 200 ° C at a cooling rate of 20 ° C / sec. Then, the obtained test steel sheet was immersed in a solution of 5 wt% sodium chloride-0.5% acetic acid saturated with hydrogen sulfide according to NACE standard (TMO177-90) under stress-free condition for 96 hours, and then the test piece CLR (Clack Length Ratio:
(Σa) / (W) × 100 (%) was measured).

[0027]

[Table 1]

CL of three cross sections obtained as described above
The average value of R was used as the CLR of the test piece. FIG. 2 shows the relationship between the CLR thus obtained and the rolling end temperature. As can be seen from the above, the steel sheet finish-rolled in the recrystallization region exhibited good HIC resistance, and therefore the rolling end temperature was set to the recrystallization lower limit temperature or higher.

As a result of various studies on the lower limit temperature of recrystallization, good HIC resistance was obtained when the finish rolling was performed in the recrystallization region if the lower limit temperature was calculated by the following formula. FIG. 3 shows the microstructures when the rolling end temperature is in the recrystallization temperature range and the non-recrystallization temperature range.

[0030]

[Formula 1] T _R = 2.3 × 10 ³ [Nb]% + 7.1 × 10 ² [Ti]% + 1.5 × 10 ² [Mo]% + 1.2 × 10 ² [V]% ＋825 (℃) ・.. (1)

The cooling rate from the rolling end temperature as described above is preferably 5 to 30 ° C./s in order to form a bainite structure. This is because the structure becomes a mixed structure of ferrite and pearlite at a cooling rate of 5 ° C or less, and the HIC resistance deteriorates. At a cooling rate of 30 ° C / s or more, strength increases due to the increase of island martensite (MA). This is because the HIC resistance increases and the HIC resistance performance deteriorates.

[0032]

EXAMPLES Tables 2 and 3 below show the chemical compositions of the steels (1-28) of the present invention and the comparative steels (29-33) (both slabs) adopted by the present inventors and determined by the formula (1). The minimum recrystallization temperature was shown.

[0033]

[Table 2]

[0034]

[Table 3]

The manufacturing method carried out on the slab of each steel as described above, that is, the heating temperature: 1000 to 120
0 ° C., rolling finishing temperature: recrystallization temperature range, cooling rate after rolling: 5 to 30 ° C./sec. 6, Table 7
As shown in.

[0036]

[Table 4]

[0037]

[Table 5]

[0038]

[Table 6]

[0039]

[Table 7]

Further, Table 8 below shows the steel N of the present invention of Table 3 above.
No. 26, the mechanical properties and HIC characteristics of a steel plate finished to a plate thickness of 25 mm by the manufacturing method according to the present invention and the manufacturing method according to the present invention are shown.

[0041]

[Table 8]

According to the above-mentioned Tables 4, 5, 6 and 7, those having the chemical composition within the scope of the present invention and the manufacturing conditions according to the present invention are shown in Tables 4 and 5. As in the example, both strength and HIC resistance are good. In contrast,
The steels 29 to 33 having chemical compositions different from those of the present invention have tensile strength of 500 N / mm ² or more or HIC resistance as shown in the comparative example of Table 5 even if the manufacturing conditions are according to the present invention. The performance is inferior, and even if the chemical composition is within the range of the present invention, the production conditions deviate from the conditions of the present invention. There is.

Further, according to Table 8 above, even if the steel has the chemical composition defined by the present invention, good strength and HIC resistance can be obtained if the production conditions deviate from the production method of the present invention. It turns out that it has not been obtained. That is, the comparative materials B and C have a finishing temperature which is out of the preferred production method of the present invention, and the cooling rates of D and E are out of the preferred production method of the present invention, and thus are inferior in strength or HIC resistance to the inventive material A. That is, a preferable product can be obtained only when both the steel composition and the manufacturing conditions satisfy the conditions of the present invention.

[0044]

As described above, according to the present invention, it is possible to manufacture a steel sheet having high strength and excellent HIC resistance, which leads to cost reduction due to reduction of the amount of steel material and high pressure operation. It is obvious that the above effects can be obtained, and it is an invention that has a large effect industrially.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing a measurement procedure of a crack length ratio (CLR).

FIG. 2 is a chart showing an influence of a rolling end temperature on the CLR.

FIG. 3 is a micrograph showing a microstructure of a steel sheet subjected to finish rolling in a recrystallized region and a non-recrystallized region of each of the sample steels shown in Table 1.

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[Procedure amendment]

[Submission date] August 5, 1993

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be corrected] Brief description of the drawing

[Correction method] Change

[Correction content]

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing a measurement procedure of a crack length ratio (CLR).

FIG. 2 is a chart showing an influence of a rolling end temperature on the CLR.

FIG. 3 is a photomicrograph showing the metallurgical structure of a steel sheet subjected to finish rolling in the recrystallized region and the non-recrystallized region of the sample steels shown in Table 1.

Front Page Continuation (72) Inventor Osamu Hirano 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Steel Pipe Co., Ltd.

Claims (4)

[Claims] 1. C: 0.02 to 0.06 wt.%, Si: 0.03 to 0.50 wt.%, Mn: 0.5 to 2.5 wt.%, S: 0.001 wt.% Or less, After heating a slab of steel containing Ca: 0.0005 to 0.008 wt.% And the balance being Fe and inevitable impurities, the subsequent rolling is performed under the condition that the rolling finish temperature is equal to or higher than the recrystallization lower limit temperature of austenite. A method for producing a high-strength steel sheet having excellent resistance to hydrogen-induced cracking, characterized in that the steel structure is changed to polygonal bainite after cooling. 2. C: 0.02 to 0.06 wt.%, Si: 0.03 to 0.50 wt.%, Mn: 0.5 to 2.5 wt.%, S: 0.001 wt.% Or less, Ca: 0.0005 to 0.008 wt.%, And at least one element from the group consisting of Cu, Ni, and Cr, Cu: 0.5 wt.% Or less, Ni: 0.5 wt.% Or less, Cr: 0. After heating the slab of steel containing 5 wt.% Or less and the balance being Fe and unavoidable impurities, the subsequent rolling is performed under the condition that the rolling finish temperature is the recrystallization lower limit temperature of austenite or higher, and then cooled. A method for producing a high-strength steel sheet excellent in hydrogen-induced cracking resistance, characterized in that the structure is polygonal bainite. 3. C: 0.02 to 0.06 wt.%, Si: 0.03 to 0.50 wt.%, Mn: 0.5 to 2.5 wt.%, S: 0.001 wt.% Or less, Ca: 0.0005 to 0.008 wt.%, And at least one element from the group consisting of Mo, Nb, V, and Ti, Mo: 0.5 wt.% Or less, Nb: 0.10 wt.% Or less, V : 0.15 wt.% Or less, Ti: 0.005 to 0.10 wt.% And the balance is Fe and unavoidable impurities. After heating a steel slab, the subsequent rolling is performed. A method for producing a high-strength steel sheet excellent in hydrogen-induced cracking resistance, characterized in that the steel structure is made into a polygonal bainite after being cooled under a condition of temperature or higher. 4. C: 0.02 to 0.06 wt.%, Si: 0.03 to 0.50 wt.%, Mn: 0.5 to 2.5 wt.%, S: 0.001 wt.% Or less, Ca: 0.0005 to 0.008 wt.%, And at least one element from the group consisting of Cu, Ni, Cr, Mo, Nb, V, and Ti, Cu: 0.5 wt.% Or less, Ni: 0.5 wt. % Or less, Cr: 0.5 wt.% Or less, Mo: 0.5 wt.% Or less, Nb: 0.10 wt.% Or less, V: 0.15 wt.% Or less, Ti: 0.005 to 0.10 wt. %, The balance is Fe and the slab of steel that is an unavoidable impurity is heated, and then the subsequent rolling is performed under the condition that the rolling finish temperature is at or above the recrystallization lower limit temperature of austenite, and then cooled to make the steel structure polygonal. A method for producing a high-strength steel sheet excellent in hydrogen-induced cracking resistance, characterized by using bainite.