JP3274013B2 - Method for producing sour resistant high strength steel sheet having excellent low temperature toughness - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a sour-resistant high-strength linepipe steel sheet having excellent low-temperature toughness.
(PI) strength of X60 or more, plate thickness of 15 mm or more), and is desirably applied to a thick plate mill in the steel industry.

[0002]

2. Description of the Related Art Large diameter line pipes for transporting crude oil and natural gas in cold regions and offshore are required to have high strength, excellent low-temperature toughness and on-site weldability. More recently, sourcing of oil and gas wells by injecting seawater and sourcing of pipelines due to the development of poor resources have progressed.
Hydrogen-induced cracking resistance has been required. Conventionally, excellent hydrogen-induced cracking resistance has been achieved by improving the purity and purity of steel,
It has been achieved by making full use of technologies such as morphological control by adding Ca to sulfide-based inclusions, reduction of central segregation under light pressure during continuous casting, and improvement of the microstructure of the central segregation part by accelerated cooling (for example, JP-B-63-001369, JP-A-62-112722). In particular, the application of accelerated cooling is a very effective means for improving the microstructure including the center segregation and improving the resistance to hydrogen-induced cracking. At this time, since accelerated cooling from a temperature higher than Ar ₃ (transformation start temperature) is essential, the rolling end temperature is necessarily restricted to Ar ₃ or higher. In order to simultaneously satisfy excellent low-temperature toughness and high strength under such a limitation of the rolling end temperature, it is important to optimize the rolling method. Rolling in the austenite non-recrystallized region in controlled rolling increases ferrite nucleation sites by elongating austenite grains and actively introducing deformation bands into grains, thereby reducing ferrite grains after transformation. It is extremely effective for improving low-temperature toughness and strength. As a method of increasing the number of ferrite nucleation sites in rolling in the austenite unrecrystallized region, for example, Proceedings of Microalloying 75
(1975), it is effective to increase the cumulative rolling reduction as known in p120, for example, to reduce the rolling temperature as known in Iron and Steel 60 (1974) 11, S557. Therefore, in order to refine the ferrite grains, it is desirable to increase the cumulative rolling reduction by isothermal rolling in the austenite extremely low temperature region just above Ar ₃ .

[0003] In a general plate mill, the rolling reduction per pass is at most about 15% at most. In such conventional rolling, the temperature drop of the steel sheet becomes large during rolling. It was difficult to perform high-strength rolling in a narrow temperature range just above Ar ₃ without heating.
For example, Japanese Patent Application Laid-Open No. 63-050426 defines a rolling temperature range of 150 ° C. such as Ar _{3 to} Ar ₃ + 150 ° C. Therefore, in the conventional rolling, Ar ₃
Since the amount of cumulative reduction in the very low temperature region of austenite immediately above cannot be sufficiently large, there is a limit to the refinement of ferrite grains, and it has been difficult to obtain excellent low-temperature toughness in a sour-resistant high-strength steel sheet.

[0004]

SUMMARY OF THE INVENTION The present invention provides a method for producing a high-strength steel sheet of API5L-X60 or higher having excellent low-temperature toughness and resistance to hydrogen-induced cracking.

[0005]

Means for Solving the Problems The gist of the present invention is that the weight%
C: 0.02 to 0.12%, Si: 0.6% or less, M
n: 0.6 to 1.5%, P: 0.015% or less, S:
0.001% or less, Al: 0.06% or less, Ti: 0.
005-0.03%, Nb: 0.01-0.1%, C
a: 0.001 to 0.005%, N: 0.001 to 0.
005%, O: 0.003% or less, and 0.5 ≦ [Ca] (1-124 [O]) / 1.25 [S] ≦ 7.0, and if necessary Ni: 0.5% or less, C
r: less than 0.5%, Mo: 0.5% or less, Cu: 0.5
%, V: 0.1% or less of steel containing at least one of iron and unavoidable impurities.
After heating to a temperature range of 1300 ° C., the cumulative reduction at 950 ° C. or more was made 30% or more, and then Ar ₃ to Ar ₃ +10
After rolling at 60 ° C. or more at 0 ° C. and rolling at a rolling reduction of 15% or more per pass for 60% or more of the total number of passes, 5 to 40 ° C. from a temperature of Ar ₃ or more. Accelerated cooling to a temperature of 350 to 550 ° C. at a cooling rate of / sec, and then allowed to cool.

Hereinafter, the present invention will be described in detail. In order to simultaneously achieve excellent hydrogen-induced cracking resistance, low-temperature toughness, strength and field weldability, the chemical composition of steel and the production method must be optimized. As for the chemical components, C, Mn, and P, which are likely to be segregated in the center of the slab from the viewpoint of resistance to hydrogen-induced cracking, are reduced.
It is necessary to reduce sulfide inclusions and control morphology by addition. Regarding the manufacturing method, the austenite recrystallization zone rolling in controlled rolling uniformly refines the recrystallized austenite grains, and the subsequent austenite non-recrystallization zone rolling elongates the austenite grains and introduces a deformation zone into the grains. It is necessary to increase the number of nucleation sites, improve the microstructure including the center segregation portion by accelerated cooling from Ar ₃ or more after rolling, and increase the strength.

[0007] The technical idea of the present invention is to control the rolling temperature by using the rolling reduction per pass in the rolling in the austenite non-recrystallized region, so that the strength is reduced in the austenite cryogenic region just above Ar _3. To significantly increase the number of ferrite nucleation sites and refine the ferrite grains to the limit, thereby producing a sour-resistant high-strength steel sheet having excellent low-temperature toughness. As a result of extensive studies by the inventors on the relationship between the rolling temperature and the rolling reduction per pass in the austenite non-recrystallized region below 950 ° C., the rolling reduction per pass was increased to 15% or more as shown in FIG. As a result, the temperature drop of the steel sheet during rolling is significantly reduced,
It became clear that rolling in a narrow temperature range of 0 ° C. or less was possible.

[0008] The reason why such rolling with a small temperature drop becomes possible is that the amount of heat removed from the roll due to the decrease in the number of passes and the amount of heat generated during processing increase. We used a small rolling of such temperature drop, Ar ₃ to Ar
_{The present} inventors have invented a method for obtaining extremely fine ferrite grains by performing rolling at a strength such that the cumulative rolling reduction is 60% or more within a narrow temperature range of austenite extremely low temperature range of ₃ + 100 ° C.

The reasons for limiting the chemical components will be described below. Although the C content inevitably increases in high-strength steels of X60 or more, an increase in the C content strengthens the segregation of Mn and P in the center segregation of the slab and significantly deteriorates the resistance to hydrogen-induced cracking. It was 12%. The lower limit is set to 0.02% to ensure strength and low-temperature toughness. The upper limits of the amounts of Mn and P are set to 1.5% and 0.010%, respectively, in order to reduce center segregation and secure resistance to hydrogen-induced cracking. The lower limit of the Mn content is set at 0. 0 to ensure the strength and low-temperature toughness of the base metal and the welded portion.
6%. On the other hand, the smaller the amount of P, the better the resistance to hydrogen-induced cracking.

[0010] Nb and Ti are essential elements in the steel of the present invention. Nb contributes to refinement of austenite structure and precipitation strengthening in controlled rolling, and strengthens steel. Ti forms fine TiN, suppresses coarsening of heated austenite grains during slab heating and welding, and improves base material toughness and HAZ.
Improve toughness. The lower limit of the amount of Nb and Ti is the minimum amount for these elements to exhibit their effects, and the upper limit is H.
This is the limit of the amount of addition that does not deteriorate the AZ toughness or the on-site weldability.

When a large amount of Si is added, on-site weldability, HAZ
In order to deteriorate toughness, the upper limit was set to 0.6%. Al and Ti alone are sufficient for deoxidizing steel, and Si need not always be added. In the steel of the present invention, the content of S, which is an impurity, is set to 0.001% or less, and Ca is added.
0.5 ≦ [Ca] (1-124 [O]) / 1.25
[S] ≦ 7.0. S forms MnS-based inclusions,
MnS elongates by rolling and becomes a starting point of HIC. To prevent this, while reducing the absolute amount of inclusions,
By controlling the sulfide morphology, CaS (-
O). Therefore, the amount of S is 0.001%
The following, the amount of Ca is added 0.001 to 0.005%,
In order to sufficiently control the sulfide morphology by Ca, ESS
P = [Ca] (1-124 [O]) / 1.25 [S] ≧
0.5. However, if the ESSP is too large, Ca-based inclusions increase and become a starting point of HIC. Therefore, 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 for reducing the oxide-based inclusions that are the starting points of HIC and controlling the sulfide morphology with the amount of Ca.
Al is an element contained in steel as a deoxidizing element, but deoxidizing is also possible with Ti or Si, and it is not always necessary to add it. When the amount of Al is 0.06% or more, Al-based nonmetallic inclusions increase and impair the cleanliness of the steel. Therefore, the upper limit is set to 0.06%.

Next, the selected elements Ni, Mo, Cr, C
The reason for adding u and V will be described. The main purpose of adding these elements to the basic components is to improve properties such as strength and toughness without impairing the excellent characteristics of the steel of the present invention. Therefore, the addition amount thereof is Ru der of a nature to be itself limited. Ni improves the strength and toughness of the base material without adversely affecting the weldability and HAZ toughness, but the upper limit is set to 0.5% because excessive addition is not preferable for the weldability.

Mo improves both the strength and the toughness of the base material, but an excessive addition causes deterioration of the toughness and weldability of the base material and HAZ, so the upper limit was made 0.5%. Cr does not easily segregate in the center of the CC slab and improves the strength of the base material, but excessive addition deteriorates the toughness and weldability of the base material and HAZ, so the upper limit was made 0.5%. Cu is Ni
The effect is almost the same as that described above, but excessive addition generates Cu-cracks during hot rolling and makes production difficult, so the upper limit was made 0.5%. V has almost the same effect as Nb,
It enables toughness to be improved by microstructural refinement, hardenability to be increased, and strength to be improved by precipitation hardening. But,
Excessive addition causes deterioration of HAZ toughness and weldability, so the upper limit was made 0.1%.

Next, the reasons for limiting the manufacturing method will be described.
The heating temperature of the steel (slab) must be 1000-1300 ° C. This is because Nb is sufficiently dissolved to form a solid solution, and at the same time, coarsening of the heated austenite grains is suppressed. If the heating temperature is lower than 1000 ° C., Nb does not form a solid solution sufficiently, so
The precipitation strengthening by b becomes insufficient, and the low-temperature toughness and strength deteriorate. When the heating temperature exceeds 1300 ° C., the heated austenite grains become coarse, and the ferrite grains are not sufficiently refined, and the low-temperature toughness deteriorates. Desirable heating temperature is 1150-1250 ° C.

In rolling at 950 ° C. or higher, the cumulative rolling reduction must be 30% or higher. This is because uniform and fine austenite grains are obtained by rolling in the austenite recrystallization region. In rolling at 950 ° C. or higher, austenite grains are almost completely recrystallized. 95
If the cumulative rolling reduction at 0 ° C. or more is less than 30%, refining by recrystallization becomes insufficient, and rolling is performed in the austenite unrecrystallized region with some coarse recrystallized grains. A mixed grain structure containing ferrite grains is formed, and the low-temperature toughness deteriorates.

Subsequently, rolling must be performed in which the cumulative rolling reduction at Ar _{3 to} Ar ₃ + 100 ° C. is 60% or more, and the rolling reduction per pass is 15% or more for 60% or more of the total number of passes. No. This is a feature of the present invention, and is a new method for remarkably increasing the number of ferrite nucleation sites of austenite grains by rolling the strength just above Ar ₃ and reducing the size of the ferrite grains to the limit. FIG. 2 shows the effect of the cumulative reduction at Ar _{3 to} Ar ₃ + 100 ° C. on the average ferrite grain size. If the cumulative reduction at Ar _{3 to} Ar ₃ + 100 ° C. is less than 60%, the formation of ferrite nucleation sites becomes insufficient, and the ferrite grains are not sufficiently refined.

When the rolling temperature is lower than Ar ₃ , C is concentrated in the central segregation part as the transformation progresses, and even if accelerated cooling is applied after rolling, a hardened structure is formed in the central segregation part and hydrogen resistance is induced. Cracking property deteriorates. Reduction rate per pass is 15
% Is less than 60%, the temperature drop of the steel sheet during rolling becomes large, and Ar _{3 to} Ar ₃ +
Rolling with a strength at which the cumulative rolling reduction at 100 ° C. becomes 60% or more cannot be performed.

After rolling, a temperature of 5 to 40 ° C. from a temperature of Ar ₃ or more
It must be accelerated and cooled at a cooling rate of 350/550 ° C./sec, and then allowed to cool. Accelerated cooling improves the microstructure including the central segregation to improve the resistance to hydrogen-induced cracking, and also enables high strength without impairing the low-temperature toughness. If the cooling start temperature is lower than Ar ₃ , the cooling rate is lower than 5 ° C./sec, or the cooling stop temperature is higher than 550 ° C., the carbon is hardened by enrichment of C in the central segregation part as the transformation proceeds. A structure is formed, and the resistance to hydrogen-induced cracking deteriorates. On the other hand, if the cooling rate exceeds 40 ° C./sec or the water cooling stop temperature is less than 350 ° C., low-temperature transformation products are formed, and the hydrogen-induced cracking resistance and low-temperature toughness deteriorate.

It should be noted that tempering the steel sheet to a temperature equal to or lower than Ac ₁ does not impair the properties of the steel sheet of the present invention. Further, the CC slab may be hot-charged into a heating furnace and rolled for energy saving and the like. The steel of the present invention can be applied not only to a sour resistant line pipe in a cold region but also to a sour resistant pressure vessel.

[0021]

EXAMPLES Table 1 shows the chemical composition of the billet. Table 2 shows the manufacturing conditions of the steel sheet. Table 3 shows the mechanical properties and the resistance to hydrogen-induced cracking of the steel sheet.

[0022]

[Table 1]

[0023]

[Table 2]

[0024]

[Table 3]

[0025]

[Table 4]

Steels 1 to 6 in Tables 1, 2 and 3 are steels of the present invention, and steels 7 to 25 are comparative steels. The steel of the present invention has an AP
It has high strength of I5L-X60 or more and excellent low-temperature toughness (vTrs ≦ −140 ° C., BDWTT 85% She)
ar FATT ≦ −50 ° C.) and excellent hydrogen-induced cracking resistance (CAR = 0%) in a NACE solution. On the other hand, the comparative steel is inferior in strength, low-temperature toughness, or resistance to hydrogen-induced cracking due to an inappropriate chemical composition or rolling method. Steels 7, 8, and 9 each have too much C, Mn, and P contents, so that a hardened structure is formed at the center segregation portion, and the resistance to hydrogen-induced cracking is poor.

[0027] Steel 10 has an ESP
(= [Ca] (1-124 [O]) / 1.25 [S])
Is less than 0.5, the morphological control of the sulfide-based inclusions is insufficient, and the hydrogen-induced cracking resistance is poor. Steel 11 is N
Since the amount of b is too small, refinement of the austenite structure by rolling and precipitation strengthening by Nb become insufficient, and the low-temperature toughness and strength are inferior. Since the amount of Ca is too small in the steel 12, the morphological control of the sulfide-based inclusions is insufficient, and the hydrogen-induced cracking resistance is poor. In Steel 13, the amount of Ti is too small, so that the suppression of the growth of heated austenite grains by TiN becomes insufficient, and the ferrite grains are not sufficiently refined due to the coarsening of the heated austenite grains, resulting in poor low-temperature toughness.

Since the heating temperature of steel 14 is lower than 1000 ° C., the amount of solute Nb during heating is small, the austenite grains are not refined by rolling and the precipitation strengthening by Nb is insufficient, and the low-temperature toughness and strength are poor. ing. Since the heating temperature of steel 15 exceeds 1300 ° C., the heated austenite grains are coarsened, and the ferrite grains are not sufficiently refined, and the low-temperature toughness is poor. Since the cumulative reduction in rolling at 950 ° C. or more is less than 30%, the austenite grains are not sufficiently refined by recrystallization, and ferrite grains are mixed to deteriorate the low-temperature toughness. . Since the rolling start temperature of steel 17 at a temperature lower than 950 ° C. is too high, Ar ₃
Cumulative reduction ratio in to Ar ₃ + 100 ° C. is becomes less than 60%, the ferrite grains have poor low-temperature toughness is not sufficiently fine.

Since the rolling start temperature of steel 18 at a temperature lower than 950 ° C. is too low, the rolling end temperature is lower than Ar ₃ ,
The microstructure of the center segregation was not improved, and the resistance to hydrogen-induced cracking was poor. Steel 19 is 1 in rolling below 950 ° C.
Since the ratio of the number of passes at which the rolling reduction per pass is 15% or more is less than 60%, the cumulative rolling reduction at Ar _{3 to} Ar ₃ + 100 ° C. is less than 60%, and the ferrite grains are not sufficiently refined. Low temperature toughness. Steel 20 is 950
Since the ratio of the number of passes at which the rolling reduction per pass is 15% or more in rolling at a temperature of not more than 60 ° C. is less than 60%, Ar ₃
The rolling reduction temperature becomes less than Ar ₃ at the same time as the cumulative rolling reduction at ~ Ar ₃ + 100 ° C., and the ferrite grains are not sufficiently refined and the microstructure of the center segregation part is insufficiently improved, resulting in low temperature. Poor toughness and resistance to hydrogen-induced cracking.

Since the start temperature of accelerated cooling of steel 21 is lower than Ar ₃ , the stop temperature of accelerated cooling of steel 22 is 550 ° C.
Since the cooling rate of the accelerated cooling of the steel 23 is less than 5 ° C./sec, the microstructure of the central segregation portion is not improved and the resistance to hydrogen-induced cracking is inferior. Since the stop temperature of accelerated cooling of steel 24 is lower than 350 ° C., the cooling rate of accelerated cooling of steel 25 exceeds 40 ° C./sec, so that a low-temperature transformation product is formed and low-temperature toughness and hydrogen-induced cracking resistance are reduced. Inferior.

[0031]

The sour-resistant high-strength linepipe steel sheet produced by the method of the present invention has extremely low temperature toughness as compared with conventional steel, and is suitable for pipelines in cold and sour environments. The safety of has been greatly improved.

[Brief description of the drawings]

FIG. 1 is a chart showing the effect of the rolling reduction per pass on the rolling temperature in rolling (sheet thickness 15 mm) in which the cumulative rolling reduction below 950 ° C. is 60%.

[2] on the ferrite grain size in average Ar ₃ to Ar ₃
The chart which shows the influence of the cumulative rolling reduction at +100 degreeC.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-2-173208 (JP, A) JP-A-3-236420 (JP, A) JP-A-5-9575 (JP, A) JP-A-2- 217417 (JP, A) JP-A-4-202712 (JP, A) JP-A-5-9573 (JP, A) (58) Fields studied (Int. Cl. ⁷ , DB name) C21D 8/00-8 / 10 C22C 38/00-38/60

Claims (2)

(57) [Claims] C: 0.02 to 0.12%, Si: 0.6% or less, Mn: 0.6 to 1.5%, P: 0.015% by weight
% Or less, S: 0.001% or less, Al: 0.06%
Hereinafter, Ti: 0.005 to 0.03%, Nb: 0.01 to
0.1%, Ca: 0.001 to 0.005%, N: 0.001
0.005%, O: 0.003% or less, and 0.5 ≦ [Ca] (1-124 [O])
/1.25[S]≦7.0 The steel whose balance satisfies 7.0 is composed of iron and unavoidable impurities is heated to a temperature range of 1000 to 1300 ° C., and the cumulative rolling reduction at 950 ° C. or more is 30%.
The rolling as described above is performed, and subsequently, Ar ₃ to Ar ₃ +100
The cumulative reduction at 60 ° C is 60% or more and the total number of passes is 6
After rolling at 0% or more, the rolling reduction per pass is 15% or more, accelerated cooling from a temperature of Ar ₃ or more to 350 to 550 ° C. at a cooling rate of 5 to 40 ° C./sec, and then cooling A method for producing a sour-resistant high-strength steel sheet having excellent low-temperature toughness. However, [Ca]: Ca content (% by weight) [O]: O content (% by weight) [S]: S content (% by weight) 2. A further weight%, Ni: 0.5% or less, Cr: 0 to 0.5%
Less than , Mo: 0.5% or less, Cu: 0 to 0.5%
The method for producing a sour-resistant high-strength steel sheet having excellent low-temperature toughness according to claim 1, wherein one or more of V: 0.1% or less are contained.