JPH06256842A - Production of light-gaged high-strength steel sheet having excellent sour resistance - Google Patents

Abstract

PURPOSE:To produce the light-gaged high-strength steel sheet having excellent sour resistance at a low cost by hot rolling and cooling a low-carbon steel having a specific compsn. contg. Nb and Ti as essential components under prescribed conditions. CONSTITUTION:A steel slab composed of the compsn. contg., by weight %, 0.02 to 0.10% C, <0.5% Si, 0.8 to 1.5% Mn, <0.010% P, <0.001% S, <0.05% Al, 0.001 to 0.005% both Ca and N and <0.0025% O, satisfying 1.0<=[Ca](1-124[O])/1.25 [S]<=7.0, contg. 0.005 to 0.03% Ti and 0.01 to 0.08% Nb as essential components or further contg. specific low ratios of >=1 kinds among Ni, Mo, Cr, Cu and V is heated at 1050 to 1300 deg.C, is subjected to the hot rolling of executing >=3 passes of rolling down at 760% cumulative draft at <=100 deg.C, at >15% draft per pass in the final 5 passes of rolling and ending the rolling at 980 deg.C and is then cooled with water at 3 to 40 deg.C/sec cooling rate in a 350 to 600 deg.C temp. range from >=Ar3 point temp.; thereafter, the rolled steel is allowed to cool.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to hydrogen-induced cracking (HI).
C) High-strength steel sheet for sour line pipes with excellent resistance to sulfide stress corrosion cracking (SSC) (American Petroleum Institute (A
PI) Strength of standard X60 or more, thickness 15 mm or less).

[0002]

2. Description of the Related Art A large diameter line pipe for transporting crude oil and natural gas in cold regions and offshore is required to have high strength as well as excellent low temperature toughness and field weldability. Furthermore, in recent years, with the sourization of crude oil and gas wells by the injection of seawater and the development of inferior resources, the sourization of pipelines has progressed, and excellent resistance to HIC and SSC (sour resistance)
Came to be demanded.

Conventionally, line pipes having excellent sour resistance are (1) high-purity steel, reduction of inclusions, (2) morphology control by adding Ca of sulfide-based inclusions, (3) during continuous casting It has been manufactured by making full use of the technology of improving the center segregation by light pressure reduction and accelerated cooling (for example, JP-B-63-00).
1369, JP-A-62-112722).

The application of accelerated cooling is a very effective means for improving the microstructure including center segregation and for improving the HIC resistance. For this purpose, it is essential to start cooling from a temperature of Ar ₃ or higher. . However, with conventional rolling, the plate thickness is 1
The thin steel plate of 5 mm or less has a large decrease in the temperature of the steel plate during rolling, and in order to start accelerated cooling from Ar ₃ or more, the start temperature of finish rolling must be increased to raise the end temperature of rolling. . Such an increase in the rolling temperature range reduces the cumulative rolling reduction in the γ low temperature range, which is disadvantageous for the refinement of the structure and deteriorates the toughness. Therefore, in the past, in the production of thin sour-resistant steel sheets, from the viewpoint of achieving both sour resistance and toughness, sour resistance was secured by soaking and diffusion treatment of the slab, and toughness was secured by lowering the rolling end temperature. (For example, JP-A-61-279621). However, the soaking and diffusing treatment of the slab has a problem that it causes a significant increase in cost because it takes a long time at a high temperature.

[0005]

DISCLOSURE OF THE INVENTION The present invention provides a method for producing a steel sheet for thin steel pipes (electric resistance welded pipes, UOE steel pipes, etc.) having high strength of API standard 5L-X60 or more, which is excellent in sour resistance. is there.

[0006]

SUMMARY OF THE INVENTION The gist of the present invention is the weight%
C: 0.02 to 0.10%, Si: 0.5% or less, M
n: 0.8 to 1.5%, P: 0.010% or less, S:
0.001% or less, Al: 0.05% or less, Ti: 0.
005 to 0.03%, Nb: 0.01 to 0.08%, C
a: 0.001 to 0.005%, N: 0.001 to 0.
005%, O: 0.0025% or less, and 1.
0 ≦ [Ca] (1-124 [O]) / 1.25 [S] ≦
Satisfies 7.0, and if necessary, Ni: 0.1 to 0.1
0.5%, Mo: 0.1 to 0.5%, Cr: 0.1
0.5%, Cu: 0.1 to 0.5%, V: 0.01 to
Steel containing 0.1% of one kind or two kinds or more and the balance consisting of iron and unavoidable impurities is heated to a temperature range of 1050 to 1300 ° C., and a cumulative reduction amount of 1000 ° C. or less is set to 60.
%, And in the final 5 passes of rolling, the reduction rate of 15% or more per pass was performed 3 times or more, and 980 ° C.
After the rolling is completed below, water cooling is performed from a temperature of Ar ₃ or higher at a cooling rate of ₃ to 40 ° C./second to 350 to 600 ° C., and then, cooling is performed.

The present invention will be described in detail below. In order to obtain high strength, excellent low temperature toughness, and excellent sour resistance along with in-situ weldability, it is necessary to first of all limit its chemical composition. Therefore, the amounts of C, Mn, and P are reduced.
The reason for this is to improve the center segregation of the continuous casting (CC) slab and prevent the generation and propagation of HIC. X60
In the above high-strength steels, the C content inevitably increases, but an increase in the C content strengthens Mn and P segregation in the central segregation zone of the CC slab, promotes the formation of a hardened structure, and significantly deteriorates the sour resistance. .

To prevent this, the upper limit of the amount of C is 0.10.
It must be%. The lower limit of 0.02% of the amount of C is the minimum amount for ensuring strength and toughness. In addition to reducing the amount of C, further reducing the amounts of Mn and P is effective in reducing center segregation, that is, suppressing the formation of a hardened structure.

Therefore, the upper limits of the amounts of Mn and P are limited to 1.5% and 0.010%, respectively. Lower limit of Mn amount 0.
8% is the minimum amount 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.

The steel of the present invention contains Nb: 0.0 as an essential element.
1-0.08%, Ti: 0.005-0.03% is contained. Nb contributes to refinement of crystal grains and precipitation hardening in controlled rolling, and strengthens steel. In addition, Ti addition is fine T
iN is formed, and it is effective in improving the base material toughness and HAZ toughness by suppressing γ grain coarsening during slab heating and welding.

It has been found that the addition of Nb and Ti is essential for the steel of the present invention which requires particularly good low temperature toughness.
The lower limits of the amounts of Nb and Ti are the minimum amounts for these elements to exert their effects, and the upper limits thereof are the limits of the addition amounts that do not deteriorate the HAZ toughness and field weldability.

Next, the reasons for limiting other elements will be described. If a large amount of Si is added, the field weldability and HAZ toughness deteriorate, so the upper limit was made 0.5%. Only Al and Ti are sufficient for deoxidizing steel, and Si is not necessarily added. In the steel of the present invention, S which is an impurity
To 0.001% or less, and adding Ca to 1.0
≦ [Ca] (1-124 [O]) / 1.25 [S] ≦
Set to 7.0. S forms MnS inclusions, and MnS is elongated by rolling and becomes a starting point of HIC generation. In order to prevent this, the absolute amount of inclusions must be reduced, and the form of sulfide must be controlled to obtain CaS (-O) that is difficult to be stretched by rolling.

Therefore, the amount of S is set to 0.001% or less and Ca
Is added in an amount of 0.001 to 0.005% to sufficiently control the morphology of sulfide by Ca, therefore ESSP = [Ca] (1
-124 [O]) / 1.25 [S] ≧ 1.0. However, if ESSP is too large, Ca-based inclusions increase and H
Since this is the starting point of IC generation, its upper limit was set to 7.0.
In relation to the above, the amount of O was limited to 0.0025% or less.
This reduces oxide inclusions that are the starting point of HIC, and
This is because the sulfide morphology is controlled by the amount of a. 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. If the Al content is 0.05% or more, Al-based nonmetallic inclusions increase and impair the cleanliness of steel, so the upper limit is set to 0.
It was set to 05%.

N forms TiN and improves the base metal and HAZ toughness by suppressing the coarsening of γ grains during slab reheating and welding. The minimum amount required for this is 0.001%. However, if it is too much, HA will be caused by slab surface defects and solid solution N.
Since it causes deterioration of Z toughness, its upper limit is 0.005%.
It is necessary to keep below.

Next, Ni, Mo, Cr and C which are selective elements
The reason for adding u and V will be described. The main purpose of adding these elements to the basic composition is to improve the properties such as strength and toughness without impairing the excellent characteristics of the steel of the present invention. Therefore, the amount added is of a nature that should be limited by itself, and the lower limit is the minimum amount at which these substantial effects can be obtained.

Ni improves the strength and toughness of the base metal without adversely affecting the weldability and HAZ toughness, but excessive addition is not preferable for weldability, so the upper limit was made 0.5%. Mo improves both strength and toughness of the base material, but excessive addition causes deterioration of the base material, HAZ toughness, and weldability, so the upper limit was made 0.5%.

Cr is not easily segregated in the center of the CC slab and improves the strength of the base material, but excessive addition deteriorates the base material, HAZ toughness and weldability, so the upper limit is set to 0.
It was set to 5%. Cu has almost the same effect as Ni,
Since excessive addition causes Cu-cracks during hot rolling and makes production difficult, the upper limit was made 0.5%. V has almost the same effect as Nb, and makes it possible to improve the toughness by increasing the fineness of the microstructure, increase the hardenability, and improve the strength by precipitation hardening. However, excessive addition causes deterioration of HAZ toughness and weldability, so the upper limit was made 0.1%.

In order to improve the low temperature toughness of the base material in the above steel, the manufacturing method must be appropriate. Therefore, it is necessary to limit reheating, rolling, and cooling conditions for steel (slab). First, set the reheating temperature to 1050-1
Limit to 300 ° C range. The reheating temperature is 1050 in order to form a solid solution with Nb precipitates and to secure the rolling end temperature.
Must be above ℃ (the preferred reheating temperature is 1
150-1250 ° C). However, the reheating temperature is 13
At 00 ° C. or higher, the γ grains are remarkably coarsened and cannot be completely refined by rolling, so that excellent low temperature toughness cannot be obtained. Therefore, the reheating temperature is set to 1300 ° C. or lower.

Further, the cumulative rolling reduction at 1000 ° C. or less is 6
0% or more, and in the final 5 passes of rolling, reduction of 15% or more per pass was performed 3 times or more, and 980
After finishing the rolling at a temperature of ℃ or less, it must be accelerated cooled from a temperature of Ar ₃ or higher. By setting the cumulative reduction amount at 1000 ° C. or less to 60% or more, the γ grains are refined and sized by repeating the recrystallization in the γ recrystallization region, and the γ grains are stretched in the γ unrecrystallized region. It also promotes the introduction of strain into the grains and refines the structure to improve toughness. If the cumulative reduction amount at 1000 ° C. or less is less than 60%, the micronization of the structure is insufficient and the toughness deteriorates.

It is a feature of the present invention that the rolling reduction of 15% or more per pass is performed three or more times in the final five passes of rolling. As a result of the study by the inventors, in such rolling with a large reduction ratio per pass, the steel sheet temperature is decreased due to a decrease in the amount of heat removed from the roll as the number of passes decreases and an increase in the amount of heat generated during processing as the reduction ratio increases. It has been found that it becomes difficult to lower the temperature, and even in the case of a thin steel plate having a plate thickness of 15 mm or less, it is possible to increase the rolling end temperature with the finish biting temperature unchanged.

Further, the increase in the rolling reduction per pass promotes the repetition of γ-recrystallization, the stretching of unrecrystallized γ-grains and the introduction of strain, and the structure becomes finer even if the final rolling temperature rises. It was found that it is possible to secure toughness.
As a result, according to the method of the present invention, it is possible to increase the rolling end temperature without deteriorating the toughness of a thin steel plate having a thickness of 15 mm or less, and it is possible to secure sour resistance by applying accelerated cooling from Ar ₃ or more. became.

FIG. 1 shows a schematic diagram of steel plate temperature history in the rolling and cooling steps of the method of the present invention and the conventional method. If the reduction of 15% or more in the final five passes of the rolling is two times or less, or if the reduction ratio per pass is less than 15%, it is possible to achieve both the high rolling end temperature and the toughness by the refinement of the structure. It is impossible.

In high-temperature finishing where the rolling end temperature exceeds 980 ° C., the cumulative reduction amount of 1000 ° C. or less or 1
Even if the rolling reduction per pass is large, the structure becomes coarse and the toughness deteriorates. The accelerated cooling must be water-cooled from a temperature of Ar ₃ or higher to 350 to 600 ° C. at a cooling rate of ₃ to 40 ° C./second, and then air-cooled. Accelerated cooling is effective in improving the microstructure including the center segregation zone,
It is possible to improve the toughness and increase the strength without impairing the toughness.

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 or the stopping temperature is too low, a hardened structure is formed and the low temperature toughness and HIC resistance are significantly deteriorated. It should be noted that the characteristics of the present invention will not be impaired even if the steel is reheated at a temperature of Ac ₁ point or lower for the purpose of tempering, dehydrogenation, etc. after the production.
The CC slab may be hot-charge rolled in a heating furnace for the purpose of energy saving.

Although the present invention is most preferably applied to a thick plate mill, it can also be applied to a hot coil (in this case, the steel plate after rolling and cooling is wound and cooled). Further, since the steel sheet manufactured by this method has excellent low temperature toughness and field weldability, it can be applied not only to pipelines in cold regions but also to pressure vessels.

[0026]

[Examples] Steel sheets (sheet thickness 10 to 15 mm) of various steel components were manufactured in a converter-continuous casting-thick plate process, and the strength, toughness,
The HIC resistance was investigated. Note that the slab was not subjected to soaking and diffusion treatment. Examples are shown in Tables 1 and 2.

[0027]

[Table 1]

[0028]

[Table 2]

[0029]

[Table 3]

[0030]

[Table 4]

The steel sheets manufactured according to the method of the present invention (the steel sheet of the present invention) all have good properties. In contrast, the comparative steels not according to the present invention are inferior in strength, toughness and HIC resistance. In Comparative Steels 9 to 20, Steels 9 and 10 have too high a C content and a Mn content, respectively, so that the toughness or the HIC resistance is high.
Inferior in nature. Steel 11 has a high P and S content and does not satisfy ESSP, which is an index for morphology control of sulfide, of 1.0 or more, and therefore has poor HIC resistance. Steel 12 does not contain Nb and therefore has poor toughness. Steel 13 is inferior in HIC resistance because Ca is not added.

The components of Steels 14 to 20 are the same as those of the present invention, but since the manufacturing conditions are not appropriate, the strength, toughness, and HI resistance are high.
Either of the C characteristics is inferior. Since steel 14 has a low slab reheating temperature, solid solution of Nb is insufficient, and the water cooling start temperature is too low, resulting in poor toughness and HIC resistance. Steel 15 is 1000 ° C
Since the following cumulative reduction amount is small, the toughness is poor.

Steels 16 and 17 have 1 in the final 5 passes of rolling.
Since the reduction rate per pass is small or the number of times of reduction of 15% or more is small, the water cooling start temperature is too low and the H resistance is high.
The IC property is inferior. Steel 18 has too high a rolling end temperature,
Inferior toughness. Since steel 19 does not apply accelerated cooling, its strength and HIC resistance are poor. Steel 20 has an inferior HIC resistance because the water cooling stop temperature is too high.

[0034]

According to the present invention, the soaking and diffusing treatment of the slab, which was conventionally required to manufacture thin sour-resistant high-strength steel sheets, can be omitted, and instead, accelerated cooling with high productivity can be applied. . As a result, it has become possible to manufacture a thin sour-resistant high-strength steel sheet having excellent characteristics at low cost.

[Brief description of drawings]

FIG. 1 is a schematic diagram of a steel plate temperature history in a rolling / cooling process.

Claims (2)

[Claims] 1. By weight%, C: 0.02 to 0.10% Si: 0.5% or less Mn: 0.8 to 1.5% P: 0.010% or less S: 0.001% or less Al : 0.05% or less Ti: 0.005-0.03% Nb: 0.01-0.08% Ca: 0.001-0.005% N: 0.001-0.005% O: 0. 0.25% or less and 1.0 ≦ [Ca] (1-124 [O]) / 1.25
[S] ≦ 7.0 The steel whose balance is iron and unavoidable impurities is
When heated to a temperature range of 1300 ° C., the cumulative rolling reduction of 1000 ° C. or less is 60% or more, and the rolling reduction of 15% or more per pass is performed 3 times or more in the final 5 passes of rolling, and the rolling reduction is performed at 980 ° C. or less. After the rolling is finished, a method for producing a thin high-strength steel sheet excellent in sour resistance, characterized by water cooling from Ar ₃ or higher to 350 to 600 ° C. at a cooling rate of ₃ to 40 ° C./second, and then allowing to cool. . 2. The steel composition further comprises, by weight%, Ni: 0.1-0.5% Mo: 0.1-0.5% Cr: 0.1-0.5% Cu: 0.1. 0.5% V: 0.01-0.1% of 1 type (s) or 2 or more types are contained, The manufacturing method of the thin high strength steel plate excellent in the sour resistance of Claim 1 characterized by the above-mentioned.