JPH0737650B2 - Method for manufacturing high-strength steel plate with excellent resistance to sulfide corrosion cracking - Google Patents

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 plate having excellent resistance to sulfide corrosion cracking, which is used for oil / natural gas oil pipes and LPG storage tanks in a wet hydrogen sulfide corrosive environment. In the case of producing molten steel, the molten steel is made into a steel slab by continuous casting or a large steel ingot-segmentation process, and then hot-rolled to produce the steel, the molten steel composition is a continuously cast slab or a large steel ingot. In consideration of segregation in the center, more specifically, in the center segregated part where sulfide corrosion cracking susceptibility is the highest, the cracking is adjusted to a specific range where it does not pose a problem, and hot rolling with a plate mill is more resistant to HIC. (Hydorogen In
duced Cracking = Hydrogen induced cracking and SSC (Sulf)
The present invention relates to a method for producing a high-strength steel plate having excellent ide stress cracking (sulfide stress corrosion cracking) properties.

[0002]

2. Description of the Related Art Conventionally, HIC is generated by hydrogen generated on the surface of steel penetrating into the steel and gathering at an inclusion / matrix interface through a diffusion process, resulting in hydrogen embrittlement of the matrix and pressure due to hydrogen. It is said to be caused by. Therefore, for HIC resistant steel, (a) prevention of hydrogen intrusion, (b) reduction of inclusions or control of morphology that is the origin of cracking, (c) reduction of hydrogen embrittlement sensitivity of matrix, etc. Measures are taken. Regarding the above (a), there is known a method of forming an anticorrosion coating on the steel surface by adding Cu, for example.

However, in a corrosive environment where the pH of the solution is low, the formation of the coating becomes insufficient, and there is a drawback that hydrogen intrusion cannot be sufficiently prevented. Regarding (b) above, RE
M-treated steel (for example, JP-A-54-31020), Ca-treated steel (for example, JP-A-54-38214) and the like have been proposed and come down. Although they show great effects in low strength materials, they are sufficient in steel strength materials. The improvement effect may not be obtained. The above
With regard to (c), the segregation part becomes a particular problem. That is, since the segregated portion has a large amount of alloying elements such as Mn, Cr, Mo, etc., a low temperature transformation product that is hard and has a high sensitivity to hydrogen embrittlement is likely to occur. Therefore, in order to prevent the occurrence of such a structure, a method such as quenching and tempering a steel sheet after rolling to change these structures to tempered mantensite or bainite with low hydrogen cracking susceptibility is adopted ( For example, JP-A-53-1389
16).

[0004]

However, all of these conventional methods have many problems in terms of productivity and economic efficiency. Further, SSC is a crack that occurs when a hydrogen embrittled steel is subjected to a stress of a certain value (critical stress: σ _th ) or more. In this case, the critical stress for cracking is remarkably reduced in the presence of a hard, hydrogen-embrittled structure. Besides this,
In the case of a line pipe, the amount of inclusions, the shape, and the amount of hydrogen in steel cause SSC. That is, the difference between HIC and SSC is essentially the presence or absence of stress, and there is essentially no difference, and it can be considered that the countermeasures for HIC are effective for SSC and the problems are the same.

By the way, in the production of high-strength steel, it is generally practiced to increase the content of alloying elements such as Mn, Cr and Mo in order to increase the strength. However, when molten steel containing these alloying elements is solidified, especially when slabs, billets, pools, etc. are cast by continuous casting, or when large steel ingots (weight 20 tons or more) are used, these casts or It is known that the alloy element is concentrated in the central portion of the steel ingot and a so-called central segregation portion is easily formed. Therefore, a steel material rolled from such a steel slab, or a steel material manufactured from such a steel ingot through a slab-rolling process, or a steel material subjected to a heat treatment if necessary to these steel materials,
In both cases, the central segregation portion is said to enhance the sensitivity of HIC and SSC.

On the other hand, the composition of the conventional steel material has a uniform composition throughout the steel material (or, although it is known that the segregation of alloying elements in the central portion is not taken into consideration), the molten steel stage The composition is adjusted so as to match that of steel. However, in the case of a fracture such as HIC or SSC in which a crack of a steel material is highly susceptible to cracking and the crack propagates and spreads, such a macroscopic idea alone is insufficient.

From this point of view, the present invention provides a steel product manufactured through a continuous casting process in which central segregation is likely to develop or a large steel ingot having a weight of 20 tons or more, which has the highest susceptibility to sulfide cracking, that is, the central part. It is intended to adjust the composition of molten steel so that cracking does not occur even in the segregated portion.

[0008]

[Means for Solving the Problems]
We prepare molten steel with chemical components such as n, Si, and P changed over a wide range, and determine the composition from samples sampled from the molten steel, and then weld these molten steels into large steel ingots, slabs, and rolling (heat treatment if necessary). Or continuous casting-rolling (heat treatment if necessary) macroetching the steel material cross section,
The central segregation portion in the steel ingot or the cast piece was exposed, and the content of each element in that portion was determined by a microanalyzer.
In this way, the content of each element in the segregated portion can be known, and the segregation coefficient of the central segregated portion can be obtained as the ratio of the content in the molten steel. Table 1 shows the segregation coefficient of each element obtained by such a method.

[0009]

[Table 1]

From Table 1, the distribution of each element is not uniform in a large steel ingot of 20 tons or more or a continuously cast slab, and P is 5 times that in molten steel and Mn and Mo are 2. 5
It can be seen that C and Cr are concentrated to 1.5 times. It was also confirmed that not all the elements segregate in the central segregation part, and elements other than the above, such as Cu, Ni and Nb, are at almost the same level as the composition during molten steel.

Next, the inventors conducted a detailed study on a large number of steels corresponding to segregated parts. The steel equivalent to the segregation part referred to here is a steel product manufactured through a large ingot or a continuous casting process as described above, and sulfide corrosion cracking is from the central segregation part with the highest susceptibility to sulfide corrosion cracking. Based on the knowledge that it occurs, for the purpose of preventing or reducing cracks at the relevant part, it is a steel in the sense that the entire steel material is first investigated based on the steel having the same composition as the part corresponding to the central segregation part. That is, the steel material used was melted in the test blast furnace.
A small ingot of about 50 kg was made and then rolled (heat treatment if necessary), and it is considered that there is no segregation throughout the steel material (that is, the ingot). Conditions for obtaining steel having excellent sulfide corrosion cracking resistance for such segregated portion equivalent steel are obtained, and then, according to this and the segregation coefficient of Table 1, continuously cast slabs or large steel ingots-slab rolling It seeks a molten steel composition for improving the sulfide corrosion cracking resistance of a steel material (having a central segregation portion) manufactured in the process.

In the method for producing a high-strength steel plate having excellent resistance to sulfide corrosion cracking according to the first aspect of the present invention, C:
0.03 to 0.20%, Si: 0.1 to 1.0%, Mn: 0.5 to 2.5%, P: 0.
03% or less, S: 0.005% or less, Sol.Al: 0.01 to 0.05%,
Ni: 1.0% or less, Cr: 1.0% or less, Mo: 1.0% or less, Nb: 0.01
.About.0.08%, V: 0.01 to 0.15%, Ti: 0.01 to 0.15%, one or more kinds, Ca: 0.0005 to 0.01%, the balance consisting of iron and unavoidable impurities, and 115-200 × P ^* -50C _eq ^* ≧ 70 [However, P ^* = 5P, C _eq ^* = 1.5C + 2.5Mn / 6 + Ni / 15 + (1.5Cr + 2.5Mo + V) / 5] This is a method in which a steel piece is formed by continuous casting or a large steel ingot-agglomeration process, and then the final rolling finish temperature is controlled and rolled by a plate mill at 680 to 800 ° C.

Further, in the method for producing a high-strength thick steel sheet excellent in sulfide corrosion cracking resistance according to the second aspect of the present invention, C: 0.03 to 0.20%, Si: 0.1 to 1.0%, Mn: 0.5 to 2.5
%, P: 0.03% or less, S: 0.005% or less, Sol.Al: 0.01 to
0.05%, Ni: 1.0% or less, Cr: 1.0% or less, Mo: 1.0% or less, Nb: 0.01 to 0.08%, V: 0.01 to 0.15%, Ti: 0.01 to
0.15% of one or more, Cu: 1.0 or less, and Ca: 0.0005 to 0.01%, the balance consisting of iron and unavoidable impurities, and 115-200 × P ^* -50C _eq ^* ≧ 70 [however, _{^{, P * = 5P, C eq}} * = 1.5C + 2.5Mn / 6 + (Cu + Ni) / 15 + (1.5Cr + 2.5Mo + V) / 5] continuous casting or large the adjusted molten steel so as to satisfy After the steel ingot is formed into a slab by the ingot-agglomeration process, it is controlled and rolled by a plate mill at a final rolling finish temperature of 680 to 800 ° C.

[0014]

In the present invention, the lower limit of C in the composition of molten steel is set to 0.
The reason why the amount is made 03% or less is that the strength required for this type of steel cannot be imparted below that, and the upper limit is also set.
The reason for setting it to 0.20% is that if it exceeds this value, the weld hardenability and crack susceptibility are remarkably increased.

The lower limit of Si is set to 0.1% because if it is less than this, it is extremely disadvantageous in steel making, and the upper limit is set to 0.1%.
The reason for setting it to 1.0% is that if it exceeds that, the toughness is significantly impaired.

Mn is an element necessary for imparting strength and ductility to the steel, but the lower limit is 0.5% because the effect becomes small at less than that and the upper limit is set.
If 2.5% is set, the weld hardenability is significantly increased, and HI
This is because the sensitivity of C is significantly increased.

Although P is contained in the steel as an impurity, it is extremely segregated, and particularly if it exceeds 0.03%, the sensitivity of HIC is remarkably increased.

In S, sulfide-based nonmetallic inclusions are HI
It is known to be the starting point of C and SSC, and it is not desirable to exceed 0.005% from the viewpoint of shelf energy, so the upper limit was made this.

At least 0.01% of sol.Al is necessary for deoxidation of steel, and if it exceeds 0.05%, this effect is saturated, so the upper limit was made this value.

Cu is mainly added when corrosion resistance is required, but if it exceeds 1.0%, Cu defects frequently occur on the surface of the steel sheet, and the toughness and hardenability of the weld heat affected zone (HAZ) are high. This also has an adverse effect on this, so the upper limit was set.

Ni, Cr and Mo improve the strength and toughness of the base metal without affecting the toughness and hardenability of the HAZ, but when these are added, the amount exceeding 1.0% is still the HAZ. This is not preferable for toughness and curability, and this is the upper limit from the economical point of view.

Nb, V, and Ti are also added to improve strength and toughness. In that case, one of these is 0.01
%, No effect is observed on strength and toughness, so the lower limit was set. Moreover, since the toughness of the base material and HAZ is deteriorated if the addition amount of these is large,
The upper limit of Nb was 0.08% and the upper limit of V, Ti was 0.15%.

In the case of adding Ca, if it is less than 0.0005%, the effect of controlling inclusions is not recognized, and therefore the sensitivity of HIC cannot be reduced, so this is the lower limit. Further, when it exceeds 0.01%, large inclusions are formed and the HIC sensitivity is rather increased.

Next, the condition of the molten steel composition rules in the present invention _{^{115 - 200 × P * -50C eq}} * ≧ 70 [ _{^{where, P * = 5P, C eq}} * = 1.5C + 2.5Mn / 6 + (Cu + Ni) / 15 + (1.5Cr + 2.5Mo + V) / 5] will be described.

FIG. 1 shows the ductile fracture surface ratio at room temperature (hereinafter referred to as S
And that A _RT), a diagram showing the relationship between the crack length ratio (hereinafter referred CLR) in HIC test. As is clear from FIG. 1, when the SA _RT value increases, CL
It can be seen that the R value decreases and cracks do not occur at all when the SA _RT value exceeds 70%. This tendency is observed not only in the as-rolled material but also in the heat-treated material. The inventors
Further, as a result of studying various metallurgical factors affecting the SA _RT , it was found that the strengthening elements such as C and Mn, and especially the amount of P give a great shadow. Based on such experimental data, SA _RT
When a regression equation of the value, the P value, and the C _eq value was obtained, SA _RT (%) = 115−200 × P−50 C _eq (1) was obtained. Here, P is the amount of P (%), C _eq is the carbon equivalent of Lloyd, and C _eq (%) = C + Mn / 6 + (Cn + Ni) / 15 + (Cr + Mo + V) / 5 (2 ) The formula (each component is% by weight).

From the above, HIC and S are the highest among steel materials.
The composition of segregation of SC sensitivity is considered ^{low, 115 - 200 × P * -50C} eq * ≧ 70 ... (3) [ _{^{where, P * = 5P, C eq}} * = 1.5C + 2.5Mn / 6 Satisfaction of + (Cu + Ni) / 15 + (1.5Cr + 2.5Mo + V) / 5] is a target for preventing the occurrence of HIC and SSC. That is, the molten steel composition capable of obtaining a high-strength steel plate excellent in sulfide corrosion cracking resistance through continuous casting or a large steel ingot-agglomeration process must satisfy the above expression (3).

In the present invention, a continuously cast steel piece or a slab of large ingot having the above composition is subjected to controlled rolling at a final finishing temperature of 680 to 800 ° C. by a plate mill to obtain a steel sheet. To do. The reason why the hot rolling by the plate mill is adopted in the present invention is that the use of the present steel plate is the transportation pipe of oil and natural gas and the LPG storage tank as described at the beginning. In other words, hot rolling with a hot strip mill is completely unidirectional rolling, so the anisotropic defect that cracks tend to occur in the rolling direction is unavoidable, and the toughness becomes poor. Since the plate size is limited to a width of 2 to 2.3 m and a thickness of 20 to 25 mm, it is impossible to meet the requirements in terms of material and size for this type of application. On the other hand, in hot rolling with a plate mill, L-direction and C-direction cross rolling is performed, and the rolling anisotropy can be controlled. In terms of dimensions, the width is about 5 m and the thickness is about 150 mm, or It is possible to obtain the above, and it is possible to obtain a wide-width, thick-walled, high-toughness, high-strength steel sheet that satisfies the material size requirements for this type of application.

Further, the lower limit of the final rolling finishing temperature is set to 680 ° C. because the sulfide corrosion cracking resistance (SS
This is because C) deteriorates, the load on the mill increases, and the difficulty in rolling significantly increases, which is not practical. On the other hand, the upper limit of the rolling final finishing temperature was set to 800 ° C because if the temperature exceeds 800 ° C, the toughness of the steel is impaired.

As explained above, even if the steel material obtained by continuous casting in which the central segregation portion easily develops or a slab by a large steel ingot-slab rolling process (having parts having different susceptibility to sulfide corrosion cracking), It becomes possible to manufacture a high-strength steel plate having excellent resistance to sulfide corrosion cracking according to the invention.

[0030]

EXAMPLES Examples of the present invention will be shown below. Table 2 shows the chemical composition, C _eq (Lloyd), C _eq ^* and SA _RT of the molten steel under test, and Table 3 shows the rolling conditions, heat treatment conditions and the measurement results of the characteristic test.

[0031]

[Table 2]

[0032]

[Table 3]

Molten steels C and D in Table 2 are large ingots of 23t and 34t, respectively, molten steel G is a slab made by continuous casting in a mold having a size of 220 × 450 mm, and other molten steels have a size of 220. It is a slab continuously cast with a mold of × 1650 mm.

The BP test is generally used as a test method for evaluating the HIC performance, but in the present invention, the HIC susceptibility was evaluated by conducting the HIC test using a NACE solution under severe corrosion conditions. The NACE solution is 1
It is an aqueous solution of 5% sodium chloride + 0.5% acetic acid in which atmospheric pressure H ₂ S is saturatedly dissolved, and is generally used in the HIC test and the SSC test. Here, CLR and CSR were used as parameters showing HIC sensitivity. These are the values obtained by the following equation.

CLR (%) = (Σa _i / A) 100 CSR (%) = (Σa _i · b _i / A · B) 100 where Σa _i · b _i : total area of step cracks A: test piece width B: Thickness of test piece

At present, there is no established theory about the upper limits of CLR and CSR from the viewpoint of sulfide cracking resistance, so that ARAMCO, which is the most severe standard in the present invention, is currently used.
HIC susceptibility was assessed using the following criteria adopted by SPEC AMSS-16.

CLR <15% CSR = 0 (no step cracking)

Molten steels A to F in Table 2 are comparative steels and P
The SA _RT value based on the following formula (4) SA _RT = 115−200P−50C _eq ^* (4) (4) is a low value of less than 70 in all because the respective components such as are relatively high. ing.

Steels G, H, L, T, V, and X are reference comparative steels of the present invention steels, and SA _RT shows a value of 70 or more, but C
Steels not containing a, I and Y are steels of the present invention, S
A _RT is a steel that shows a value of 70 or more and contains 0.0005 to 0.01% of Ca. The rolling conditions and the characteristic measurement results of each of these test steels are as shown in Table 3, and the slab heating temperature, plate thickness,
Plate material, heat treated material, as-rolled material, accelerated cooling material,
Irrespective of whether the comparative steels have CLR in NACE environment,
All CSR values are reference values (CLR <15%, CSR = 0
%), Whereas in the comparative steels containing Ca shown in A, B, C, E, and F, CL in the NACE environment
R and CSR values are both standard values (CLR <15%, CSR
= 0%), whereas in the case of the present invention steel and the reference comparative steel of the present invention steel, in particular, the present invention steel, even if it is as-controlled rolled, CLR and CSR both have excellent HIC resistance below the standard value. Is shown.

FIG. 2 is a diagram showing the relationship between SA _RT and CLR in the example. That is, in FIG. 2, the SA _RT values of the steels A~Y shown in Table 2 on the horizontal axis and the vertical axis represents the CLR% as shown in Table 3, ○ ones while rolling the baked base △ , ◇ indicates the quenched and tempered materials. As is clear from FIG. 2, in the steel sheet according to the present invention having a SA _RT value of 70 or more,
CLR is less than 15% and CSR = 0% even in as-rolled material
Satisfies the criteria of (no step cracking).

[0041]

As described above, according to the method of the present invention, the wide and thick wall thickness is such that the sulfide cracking does not become a problem even in the central segregation portion where the sulfide corrosion cracking resistance tends to be the largest. Of high toughness and high tensile strength steel plate, which was previously impossible to obtain by heat treatment
Excellent effects as a manufacturing method, such as SC performance obtained as-rolled, can be achieved, and the economic effect is extremely large.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between ductile fracture surface ratio and CLR at room temperature.

FIG. 2 is a diagram showing the relationship between SA _RT and CLR.

Claims (2)

[Claims] 1. On a weight basis, C: 0.03 to 0.20%, Si: 0.
1 to 1.0%, Mn: 0.5 to 2.5%, P: 0.03% or less, S: 0.005
% Or less, Sol.Al: 0.01 to 0.05%, Ni: 1.0% or less, Cr:
1.0% or less, Mo: 1.0% or less, Nb: 0.01 to 0.08%, V: 0.01
~ 0.15%, Ti: 0.01 ~ 0.15%, one or more kinds, C
a: 0.0005 to 0.01%, the balance consisting of iron and inevitable impurities, and 115 −200 × P ^* −50C _eq ^* ≧ 70 [where P ^* = 5P, C _eq ^* = 1.5C + 2.5Mn / 6 + Ni / 15 + (1.5Cr + 2.5Mo + V) / 5], the molten steel adjusted so as to satisfy the requirement of 6 + Ni / 15 + (1.5Cr + 2.5Mo + V) / 5] is formed into a slab by continuous casting or a large steel ingot-agglomeration process, and then the final rolling finish temperature is set by a plate mill. A method for producing a high-strength steel plate with excellent resistance to sulfide corrosion cracking, which comprises controlled rolling at 680 to 800 ° C. 2. On a weight basis, C: 0.03 to 0.20%, Si: 0.
1 to 1.0%, Mn: 0.5 to 2.5%, P: 0.03% or less, S: 0.005
% Or less, Sol.Al: 0.01 to 0.05%, Ni: 1.0% or less, Cr:
1.0% or less, Mo: 1.0% or less, Nb: 0.01 to 0.08%, V: 0.01
.About.0.15%, Ti: 0.01 to 0.15%, one or more kinds, Cu: 1.0 or less, and Ca: 0.0005 to 0.01%, with the balance being iron and inevitable impurities, and 115-200.times.P ^*. −50C _eq ^* ≧ 70 [however, P ^* = 5P, C _eq ^* = 1.5C + 2.5Mn / 6 + (Cu + Ni) / 15 + (1.5Cr + 2.5Mo + V) / 5] Excellent sulphide corrosion cracking resistance, characterized in that the adjusted molten steel is continuously cast or made into billets by a large steel ingot-segmentation process, and then controlled by a plate mill at a final rolling finish temperature of 680 to 800 ° C. Manufacturing method of high-tensile steel plate.