JPH11302776A - High strength steel sheet excellent in sour resistance and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a high strength steel sheet having excellent sour resistance and HAZ toughness at a low cost by subjecting a steel based on a low C-low Mn-Nb-Ti material with strictly controlled amts. of S, Mg, Ca and O to control rolling, then rapidly cooling the rolled sheet to a specified temp. and then slowly cooling. SOLUTION: A steel sheet having the compsn. described below is heated at 1,050 to 1,250 deg.C, rolled at <=950 deg.C under the conditions of >=60% total rolling reduction and 750 to 900 deg.C finish temp., immediately cooled in water to 350 to 600 deg.C at 3 to 40 deg.C/sec cooling rate, and then slowly cooled. The steel consists of, by weight, 0.002 to 0.08% C; <0.5% Si; 0.8 to 1.5% Mn; <=0.010% P; <=0.001% S; 0.01 to 0.05% Nb; 0.005 to 0.03% Ti; 0.02 to 0.05% Al; 0.005 to 0.03% Mg; 0.001 to 0.004% Ca; 0.001 to 0.005% N; 0.001 to 0.003% O, satisfying 1.0<=Ca(1-124 O)/1.25S<=7.0, and if necessary, containing a specified amt. of Ni, Cu, Cr, Mo, V, REM, and the balance Fe and inevitable impurities.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to hydrogen-induced cracking (H
High-strength steel plate for sour pipeline with excellent IC) and sulfide stress corrosion cracking (SSC) resistance (American Petroleum Institute (API) standard X60 or higher, thickness 40 mm or less)
And a method for producing the same.

[0002]

2. Description of the Related Art Large diameter line pipes for transporting crude oil and natural gas in cold regions and offshore areas are required to have high strength, excellent low-temperature toughness and on-site weldability. In recent years, the sourcing of crude oil and gas wells by the injection of seawater and the development of inferior resources have led to the progress of sourcing of pipelines, resulting in excellent resistance to HIC and SSC (resistance to sourcing).
Sex) is required.

Conventionally, line pipes having excellent sour resistance have been obtained by (1) purifying steel, reducing inclusions, (2) controlling the form by adding Ca to sulfide-based inclusions, and (3) continuously casting. It has been manufactured by making full use of technologies such as reduction of center segregation by light reduction at the time of (CC) and (4) control of microstructure by accelerated cooling after rolling (for example, Japanese Patent Publication No. 63-1369, Japanese Patent Application Laid-Open No. JP 112722, JP-A-61-1
No. 24555, Japanese Patent Publication No. 7-5968).

[0004] However, due to an increase in the amount of alloying elements accompanying higher strength and an increase in welding heat input due to extremely thickening, the low temperature toughness of the HAZ of these steels has not always been sufficient. . Tokuhei 7-59
No. 68 discloses a method for producing a steel sheet for the purpose of improving sour resistance and HAZ toughness.
Since the amount is 0.004% or less, the amount of dissolved oxygen in the molten steel increases, and desulfurization reaction hardly occurs. As a result, there is a problem in the production cost because the desulfurization treatment time in the refining process increases. Further, when the welding heat input increases with the increase in thickness, it is impossible to obtain good HAZ toughness even with this steel. For this reason, there has been a strong demand for the development of a sour-resistant line pipe having a significantly higher HAZ toughness than conventional sour-resistant line pipes.

[0005]

SUMMARY OF THE INVENTION The present invention provides a steel sheet having excellent sour resistance and high strength of API standard 5L-X60 or higher, and a method for producing the same.

[0006]

Means for Solving the Problems The gist of the present invention is that the weight%
And C: 0.02 to 0.08%, Si: 0.5% or less,
Mn: 0.8-1.5%, P: 0.010% or less, S:
0.001% or less, Nb: 0.01 to 0.05%, T
i: 0.005 to 0.03%, Al: 0.02 to 0.0
5%, Mg: 0.005 to 0.03%, Ca: 0.00
1 to 0.004%, N: 0.001 to 0.005%,
O: 0.001 to 0.003%, if necessary,
Ni: 0.1 to 1.0%, Cu: 0.1 to 1.0%, C
r: 0.1 to 1.0%, Mo: 0.1 to 1.0%, V:
0.01 to 0.10%, REM: 0.0005 to 0.0
It contains one or more of 0.05% and 1.0 ≦
[Ca] (1-124 [O]) / 1.25 [S] ≦ 7.
Steel sheet whose balance satisfying 0 is composed of iron and inevitable impurities.

And C: 0.02-0.08 by weight%
%, Si: 0.5% or less, Mn: 0.8 to 1.5%,
P: 0.010% or less, S: 0.001% or less, Nb:
0.01-0.05%, Ti: 0.005-0.03
%, Al: 0.02-0.05%, Mg: 0.005-
0.03%, Ca: 0.001 to 0.004%, N:
0.001 to 0.005%, O: 0.001 to 0.00
3%, and if necessary, Ni: 0.1 to 1.0.
%, Cu: 0.1 to 1.0%, Cr: 0.1 to 1.0
%, Mo: 0.1 to 1.0%, V: 0.01 to 0.10
%, REM: 0.0005 to 0.005%, one or two or more, and 1.0 ≦ [Ca] (1-124
[O]) / 1.25 [S] ≦ A steel slab consisting of iron and unavoidable impurities with a balance of 7.0 or less was set to 1050 to 125.
After heating to a temperature range of 0 ° C. and rolling at a rolling reduction temperature of 750 to 900 ° C. with a cumulative reduction of 60% or more of 950 ° C. or less, immediately cooling at a cooling rate of 3 to 40 ° C./sec.
Water cooling to 600 ° C. and then cooling.

Hereinafter, the present invention will be described in detail. The features of the present invention include (1) a steel in which the amounts of S, Mg, Ca and O are strictly restricted based on a low C-low Mn-Nb-Ti system and (2) controlled rolling, and then accelerated. This is because cooling is performed, so that sour resistance and excellent HAZ toughness can be simultaneously achieved. By reducing the amounts of C, Mn, and P, the center segregation of the CC slab is improved and the HI
Generation and propagation of C can be prevented. In addition, by strictly limiting the amounts of S, Mg, Ca and O, it is possible to prevent the generation of MnS, which can be a starting point of HIC, and to refine the Ca-based inclusions by adding Mg. Sourness can be obtained.

The HAZ toughness of a low alloy steel is as follows: (1) the size of crystal grains, (2) high carbon island martensite (M *
), The dispersed state of a hardened phase such as upper bainite (Bu), (3) the presence or absence of grain boundary embrittlement, and (4) microsegregation of elements, which are governed by various metallurgical factors. Among them, it is known that the size and M ^* of the crystal grains of HAZ have a great influence on the low-temperature toughness. In the present invention, the HAZ is formed by forming a fine oxide containing Mg or a composite of the oxide and a carbonitride such as TiN in steel by adding Mg.
Of austenite (γ) grains in H
Improves AZ toughness. In particular, when the thickness is extremely increased, the amount of heat input to welding is inevitably increased, and coarsening of γ grains in the HAZ and the subsequent cooling rate are slowed, so that the HAZ structure is coarsened and the HAZ toughness is deteriorated. However, even in such a case, due to the action of a fine oxide containing Mg or a composite of the oxide and a carbonitride such as TiN, γ in the HAZ is reduced.
Grain coarsening is remarkably suppressed, and good HAZ toughness can be obtained.

That is, the role of Mg in the present invention is as follows: (1) S is fixed as MgS similarly to Ca,
Preventing the generation of MnS harmful to C; (2) finely dispersing Ca-based inclusions; (3) suppressing the coarsening of γ grains by oxides containing Mg in HAZ;
The purpose is to improve the Z toughness. As described above, Mg has an effect of improving sour resistance and an effect of improving HAZ toughness. Ca-based inclusions such as CaS and CaO are generated in a sour resistant steel in which MnS is suppressed by adding Ca. However, Ca-based oxides such as CaO tend to be coarsened, and the coarsened oxides are a starting point of HIC.

The present inventors have conducted intensive studies and found that M
By adding g, the Ca-based oxide can be finely dispersed, and the fine oxide containing Mg acts as a generation site for carbonitrides such as TiN, and the fine HAZ toughness can be obtained by finely dispersing TiN. And excellent HIC resistance. In order to exhibit these effects, 0.005% or more of Mg must be added. If too much, Mg-based oxides increase,
Since the low-temperature toughness is deteriorated and the HIC resistance is also deteriorated, the upper limit is limited to 0.03%.

In the present invention, the amount of S, which is an impurity element, is set to 0.1.
001% or less, and adding Ca, 1.0 ≦ [C
a] (1-124 [O]) / 1.25 [S] ≦ 7.0. S forms MnS-based inclusions, and MnS elongates by rolling to become a starting point of HIC. In order to prevent this, it is necessary to reduce the absolute amount of inclusions and control the form of sulfide to make CaS (-O) which is difficult to elongate by rolling. Therefore, the amount of S is made 0.001% or less,
To add 0.001-0.004% of Ca and sufficiently control the form of sulfide by Ca, [Ca] (1-12
4 [O]) / 1.25 [S] The ESSP value was 1.0 or more. However, if the ESSP value is too large,
As Ca-based inclusions increase and become the starting point of HIC,
The upper limit was set to 7.0.

In relation to the above, the amount of O is limited to 0.003% or less. This is because the sulfide morphology is controlled by Ca and Mg by reducing the oxide-based inclusions that are the starting points of HIC. The lower limit of the O content of 0.001% is a minimum value for generating a fine oxide serving as a TiN generation site. In order to obtain excellent sour resistance, C, Mn,
It is necessary to limit the amount of P. The reason for this is to improve the center segregation of the CC slab and prevent the generation and propagation of HIC. In the case of high-strength steels of X60 or higher, the C content is inevitably high. However, the increase in the C content strengthens the segregation of Mn and P in the central segregation zone at the time of solidification of the CC slab, promotes the formation of a hardened structure, and improves the resistance to sour -Significantly deteriorates properties.

To prevent this, the upper limit of the C content is 0.08
%. The lower limit of 0.02% of the C content is the minimum amount for securing strength and low-temperature toughness. In addition to reducing the C content, further reducing the Mn and P contents is effective in reducing center segregation, that is, in suppressing the formation of a hardened structure. Therefore, the upper limits of Mn and P amounts are 1.5% and 0.01%, respectively.
Limited to 0%. The lower limit of 0.8% of the Mn content is a minimum value for securing the strength of the base material and the welded portion. On the other hand, the lower the P content, the better the sour resistance.

In the steel of the present invention, Nb: 0.
01-0.05%, Ti: 0.005-0.030%. Nb contributes to refinement of crystal grains and precipitation hardening in controlled rolling, and has an effect of toughening steel. However, if Nb is added at 0.05% or more, on-site weldability and HAZ
Since the toughness is adversely affected, the upper limit is set to 0.05%. Also, the addition of Ti forms fine TiN, suppresses coarsening of γ grains in the slab during reheating and in the welded HAZ, refines the microstructure, and improves the low-temperature toughness of the base metal and the HAZ. In order to exhibit such an effect of TiN,
A minimum of 0.005% Ti addition is required. But Ti
If the amount is too large, coarsening of TiN and precipitation hardening due to TiC occur, and the low-temperature toughness deteriorates.
Must be limited to 3%.

Next, the reasons for limiting other elements will be described. Si is an element added for deoxidation and improvement of strength, but if added in a large amount, the on-site weldability and HAZ toughness are deteriorated, so the upper limit was made 0.5%. Al alone is sufficient for deoxidizing steel, and Si need not always be added.
Al is contained in steel as a deoxidizing element, and in the present invention, the addition of Al has the effect of reducing the oxygen concentration in steel and promoting the subsequent desulfurization reaction. If the amount of Al is too small, the desulfurization reaction is delayed, the processing time in the refining process is lengthened, and the production cost is increased.
And On the other hand, if the Al content is 0.05% or more, the amount of Al-based nonmetallic inclusions increases and impairs the cleanliness of the steel.

N forms TiN and suppresses coarsening of γ grains during slab reheating and welding, thereby improving the low-temperature toughness of the base material and HAZ. The minimum amount required for this is 0.00
1%. However, if it is too large, it causes the HAZ toughness to be degraded due to slab surface flaws and solid solution N.
005%. Next, the selected element N
The reason for adding i, Cu, Cr, Mo, V, and REM will be described. The main purpose of adding these elements to the basic components is to improve properties such as strength and low-temperature toughness without impairing the excellent characteristics of the steel sheet obtained by the present invention. Therefore, the amount of addition is of a nature restricted by itself.

The purpose of adding Ni is to improve the strength of the low carbon steel of the present invention without deteriorating the low-temperature toughness and the on-site weldability. Addition of Ni is less likely to form a hardened structure that is harmful to low-temperature toughness and sour resistance in a rolled structure (especially the central segregation zone of the slab) as compared with Mn, and increases the strength. However, if the addition amount is too large, not only economic efficiency, but also on-site weldability and HAZ toughness are deteriorated.
The upper limit was set to 1.0%. Ni is also effective in preventing Cu cracks during continuous casting and hot rolling.

Cu is 0.1% or more and H is almost the same as Ni.
It has the effect of improving strength and low-temperature toughness without significantly affecting AZ toughness, and also has an effect on improving corrosion resistance and hydrogen-induced cracking resistance. Also, the strength is greatly increased by Cu precipitation hardening. However, if added in excess, precipitation hardening causes a decrease in the toughness of the base material and HAZ and Cu cracks during hot rolling, so the upper limit was made 1.0%. Cr is M
Compared with n, the CC slab hardly segregates in the center, and is effective in increasing the strength without impairing the low-temperature toughness and the sour resistance. To achieve this effect, 0.1
% Or more is required. However, if it is too large, the on-site weldability and the HAZ toughness are remarkably deteriorated. Therefore, the upper limit of the Cr content is set to 1.0%.

Mo, like Cr, is less likely to cause segregation in the center of the CC slab than Mn, and is an effective element for increasing strength without impairing low-temperature toughness and sour resistance. In order to obtain such an effect, Mo should be at least 0.1.
%is necessary. However, excessive Mo addition degrades HAZ toughness and on-site weldability, so the upper limit was made 1.0%. V has almost the same effect as Nb, and is effective in improving the low-temperature toughness by increasing the microstructure, increasing the hardenability, and increasing the strength by precipitation hardening. However, if the addition amount is too large, on-site weldability and HAZ toughness deteriorate, so the upper limit was made 0.10%. The lower limit of the added amount of V is the minimum amount for exhibiting the above-mentioned effect. REM, like Ca, is effective in controlling the morphology of MnS. In order to exhibit this effect, 0.0005% or more must be added.
On the other hand, if the addition amount is too large, the amount of REM-based oxide increases, and
0.005% of upper limit value to degrade IC performance
And

In order to improve the low-temperature toughness of the base material in the steel sheet obtained by the present invention as described above, a steel sheet manufacturing method must be further appropriate. For this reason, it is necessary to limit the reheating, rolling, and cooling conditions of the billet (slab).
First, the reheating temperature is limited to the range of 1050C to 1250C. The reheating temperature must be 1050 ° C. or higher in order to dissolve the Nb precipitates and secure the rolling end temperature (a desirable reheating temperature is 1150 to 1200 ° C.). However, when the reheating temperature exceeds 1250 ° C., γ
Since the grains are extremely coarse and cannot be completely refined even by rolling, excellent low-temperature toughness cannot be obtained. Therefore, the upper limit of the reheating temperature is set to 1250 ° C.

Further, the cumulative rolling reduction at 950 ° C. or less is reduced by 60%
As described above, the rolling end temperature must be 750 to 900 ° C. (preferably, the Ar ₃ transformation point or higher). This is to elongate the γ grains refined by recrystallization zone rolling by low-temperature rolling,
This is for improving the low-temperature toughness by thoroughly refining the crystal grains. If the cumulative rolling reduction is less than 60%, elongation of the γ structure is insufficient, and fine crystal grains cannot be obtained. If the rolling end temperature is 900 ° C. or higher, for example, the cumulative rolling reduction is 60%.
% Or more cannot achieve fine crystal grains. However, the temperature at the end of rolling decreases, and water cooling from the (γ + α) two-phase region makes it difficult to control the structure, resulting in deterioration of HIC resistance and strength / low-temperature toughness. Therefore, the lower limit of the rolling end temperature was set to 750 ° C.

After rolling, it is essential to accelerate cooling of the steel sheet. Accelerated cooling is effective in improving the microstructure including center segregation, and enables an increase in strength and an improvement in HIC resistance without deteriorating low-temperature toughness. The conditions of accelerated cooling are as follows:
Immediately at a cooling rate of 3 to 40 ° C / sec.
It must be cooled to a temperature below ℃ and then air-cooled. If the cooling rate is too slow or the cooling stop temperature is too high, the effect of accelerated cooling cannot be sufficiently obtained, and an appropriate microstructure cannot be obtained. On the other hand, if the cooling rate is too high,
If the stop temperature is too low, a hardened structure is formed, and the low-temperature toughness and the HIC resistance are significantly deteriorated.

It should be noted that if the steel is manufactured and then reheat-treated at a temperature lower than the Ac ₁ transformation point for the purpose of tempering, dehydrogenation, etc., the characteristics of the present invention are not impaired. For the purpose of energy saving, CC slab is heated in a heating furnace.
Di, you may roll. The present invention is most preferably applied to a thick plate mill, but can also be applied to a hot coil (in this case, the steel sheet after rolling and cooling is wound and cooled). Further, since the steel sheet produced by this method has excellent low-temperature toughness, it can be applied to pressure vessels as well as pipelines in cold regions.

[0025]

Next, embodiments of the present invention will be described. In the converter-continuous casting-thick plate process, steel plates (thickness 16 to 38 mm) of various steel components shown in Table 1 were manufactured under various manufacturing conditions shown in Table 2, and their strength, low-temperature toughness, HAZ toughness, and HIC resistance were obtained. Table 2 shows the results. The steel sheets produced according to the invention (invention steels) all have good properties. On the other hand, the comparative steel not according to the present invention does not have an appropriate chemical composition or steel plate manufacturing conditions, and is inferior in any of strength, low-temperature toughness, HAZ toughness, and HIC resistance.

[0026]

[Table 1]

[0027]

[Table 2]

Steel 13 has an excessively high C content, so that the base material / HA
Poor Z toughness and HIC resistance. Steel 14 has inferior HIC resistance because the Mn content is too high. Steel 15 has too high P content,
Poor HIC resistance. Steel 16 has too high an S content, and therefore has a high H resistance.
Poor IC performance. Since steel 17 does not contain Nb, the strength and toughness of the base material are poor. Since steel 18 does not contain Ti, the base material and HAZ toughness are poor. Since steel 19 does not contain Mg, HAZ toughness and HIC resistance are inferior. Steel 20
Is because the Mg content is too high, the base metal, HAZ toughness, and HIC resistance
Poor nature. Steel 21 is inferior in HIC resistance because Ca is not added and the ESSP value indicating morphological control of sulfide is 0.

Since the steel 22 does not satisfy the ESSP value of 1.0 or more, the HIC resistance is poor. Since Steel 23 does not satisfy the ESSP value of 7.0 or less, the HIC resistance is poor. Steel 2
The composition of steel No. 4 to steel 28 is the same as that of the steel of the present invention, but the base material strength, low-temperature toughness or HI resistance is low due to improper production conditions.
Poor C properties. Steel 24 has a low slab reheating temperature and low base metal strength. Steel 25 has a low cumulative toughness of 950 ° C. or less, and thus has low temperature toughness. Since the steel 26 has a low rolling end temperature, the low-temperature toughness and the HIC resistance are inferior. Steel 27 has a low cooling rate after rolling, and thus has low strength and poor HIC resistance. Since steel 28 has a low water-cooling stop temperature, HIC resistance is poor.

[0030]

As described above, the present invention makes it possible to mass-produce high-strength pipeline steel excellent in sour resistance at low cost. As a result, the safety of the pipeline has been significantly improved.

[Brief description of the drawings]

FIG. 1 is a view showing a sampling position of a Charpy test specimen from a double-sided latent arc weld.

Claims (4)

[Claims] 1. C .: 0.02 to 0.08%, Si: 0.5% or less, Mn: 0.8 to 1.5%, P: 0.010% or less, S: 0 by weight% 0.001% or less, Nb: 0.01 to 0.05%, Ti: 0.005 to 0.03%, Al: 0.02 to 0.05%, Mg: 0.005 to 0.03%, Ca : 0.001 to 0.004%, N: 0.001 to 0.005%, O: 0.001 to 0.003%, and 1.0 ≦ [Ca] (1-124 [ O]) / 1.2
A high-strength steel sheet excellent in sour resistance, the balance of which satisfies 5 [S] ≦ 7.0 is composed of iron and unavoidable impurities. 2. In% by weight, C: 0.02 to 0.08%, Si: 0.5% or less, Mn: 0.8 to 1.5%, P: 0.010% or less, S: 0 0.001% or less, Nb: 0.01 to 0.05%, Ti: 0.005 to 0.03%, Al: 0.02 to 0.05%, Mg: 0.005 to 0.03%, Ca : 0.001 to 0.004%, N: 0.001 to 0.005%, O: 0.001 to 0.003%, Ni: 0.1 to 1.0%, if necessary, Cu : 0.1-1.0%, Cr: 0.1-1.0%, Mo: 0.1-1.0%, V: 0.01-0.10%, REM: 0.0005-0 0.001% or more, and 1.0 ≦ [Ca] (1-124
[O]) / High-strength steel sheet excellent in sour resistance, the balance of which satisfies 1.25 [S] ≦ 7.0, the balance being iron and unavoidable impurities. 3. In% by weight, C: 0.02 to 0.08%, Si: 0.5% or less, Mn: 0.8 to 1.5%, P: 0.010% or less, S: 0 0.001% or less, Nb: 0.01 to 0.05%, Ti: 0.005 to 0.03%, Al: 0.02 to 0.05%, Mg: 0.005 to 0.03%, Ca : 0.001 to 0.004%, N: 0.001 to 0.005%, O: 0.001 to 0.003%, and 1.0 ≦ [Ca] (1-124 [O ]) / 1.2
A steel slab, the balance of which satisfies 5 [S] ≦ 7.0, consisting of iron and unavoidable impurities, is heated to a temperature range of 1,050 to 1,250 ° C., and a rolling reduction of 950 ° C. or less 60% or more, and a rolling end temperature 750. A method for producing a high-strength steel sheet excellent in sour resistance, comprising: immediately rolling at a temperature of up to 900 ° C., water cooling to 350 to 600 ° C. at a cooling rate of 3 to 40 ° C./sec, and then cooling. 4. In% by weight, C: 0.02 to 0.08%, Si: 0.5% or less, Mn: 0.8 to 1.5%, P: 0.010% or less, S: 0 0.001% or less, Nb: 0.01 to 0.05%, Ti: 0.005 to 0.03%, Al: 0.02 to 0.05%, Mg: 0.005 to 0.03%, Ca : 0.001 to 0.004%, N: 0.001 to 0.005%, O: 0.001 to 0.003%, Ni: 0.1 to 1.0%, Cu as required : 0.1-1.0%, Cr: 0.1-1.0%, Mo: 0.1-1.0%, V: 0.01-0.10%, REM: 0.0005-0 0.001% or more, and 1.0 ≦ [Ca] (1-124
[O]) / 1.25 [S] ≦ A steel slab consisting of iron and unavoidable impurities with a balance of 7.0 or less was set to 1050 to 125.
After heating to a temperature range of 0 ° C. and rolling at a rolling reduction temperature of 750 to 900 ° C. with a cumulative reduction of 60% or more of 950 ° C. or less, immediately cooling at a cooling rate of 3 to 40 ° C./sec.
A method for producing a high-strength steel sheet having excellent sour resistance, characterized by cooling with water to 600 ° C. and then allowing it to cool.