JPS61165207A - Manufacture of unrefined steel plate excellent in sour-resistant property - Google Patents

Abstract

PURPOSE:To obtain a steel plate excellent in sour-resistant properties, strength and toughness by heating a steel of specific composition to a specific temperature and hot rolling, cooling and naturally cooling it under the specific conditions to form it into a fine bainite structure uniform in its thickness direction. CONSTITUTION:A steel is heated up to 1000-1200 deg.C, which consists of by weight %, as the basic components 0.02-0.15 C, 0.1-0.6 Si, 0.5-1.5 Mn, <=0.0015 P, <=0.01 S, 0.005-0.1 Al, 0.005-0.025 Ti, and one or more kinds of the following groups <=0.1 Nb, <=0.005B, <=1.0 Ni, <=1.0 Cu, <=1.25 Cr, <=0.005 Ca, and the balance Fe with inevitable impurities. The steel is rolled thereafter under the draft of >=60% at <=850 deg.C and at the finishing temperature of >=Ar3 transformation point. Next, the steel is cooled at a cooling speed of >=30 deg.C/sec. from the temperature of >=Ar3 transformation point to the temperature range of 350-550 deg.C and then cooled naturally. In this way, a steel plate excellent in sour-resistant properties having the fine bainite structure uniform in the plate- thickness direction is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention is applicable to a humid hydrogen sulfide environment (hereinafter referred to as a sour environment), particularly a high concentration of hydrogen sulfide or even carbon dioxide.
Provides excellent hydrogen-induced cracking resistance and toughness in a sour environment containing chloride ions with a pH of 5 or less (API
Standard X80~X? The present invention relates to a method for producing high-strength steel sheets (equivalent to O).

(Prior art) In recent years, pipelines have been frequently constructed to transport crude oil and natural gas, but as oil wells have become deeper, these crude oil and natural gas often contain hydrogen sulfide gas. As a result, deterioration due to line pipe corrosion has become a problem. In particular, in a humid sour environment containing hydrogen sulfide and carbon dioxide, H2S and GO2 coexist with water, and atomic hydrogen generated by a corrosion reaction on the surface of the steel sheet enters the steel, causing damage to the steel. It accumulates around nonmetallic inclusions such as MnS and oxides, embrittles the base steel, and causes so-called hydrogen-induced cracking. Moreover, since such inclusions often exist in the central segregation zone, hydrogen-induced cracks that originate from the inclusions may propagate along the hard low-temperature transformation products that occur in the central segregation zone. Are known.

In order to prevent such cracks from occurring, elements that form a stable film on the steel surface, such as Cu and Or, are added.
Method for preventing hydrogen intrusion caused by corrosion reactions, ■
Method of controlling the shape of MnS by adding Ca and REM, ■ After blooming the slab, soaking it to remove P in the center segregation area.
A method of promoting the diffusion of steel sheets, ■ A method of reheating after rolling, and uniformizing the steel sheet structure by quenching, tempering, or normalizing. Cooling start temperature after controlled rolling.

A method has been adopted in which low-temperature transformation products are controlled by performing accelerated cooling with controlled cooling rate and cooling stop temperature. (For example, I# Publication No. 54-118325) (Problem to be solved by the invention) However, the corrosive environment conditions are considered to be the most severe so far, 0.5 g acetic acid + 5X NaC1 + H2S
c7) In a pH 3,0 (7) solution (NACE solution),
It is difficult to prevent hydrogen-induced cracking in a steel plate having a strength of about 70 or more, and the methods (1) to (4) above each have the following problems. ■ Formation of a stable corrosion film by Cu and Cr on the steel plate surface is possible up to an environment with a PL of 5, and below that pH the corrosion products of Cu and Cr will no longer dissolve, suppressing hydrogen intrusion. I can't. ■Even if Ca and REM are added while maintaining the cleanliness of the steel plate,
It is impossible to completely spheroidize MnS in the center of a steel sheet. Heat treatment steps such as (1) and (2) reduce the hydrogen-induced cracking susceptibility, but both lower production efficiency and increase energy consumption, so they must be considered disadvantageous methods. When the process of method (2) is adopted, it is difficult to stably obtain a strength equivalent to or higher than X70 due to cooling rate limitations. Furthermore, if the cooling rate is set to 20°C/S or higher, as pointed out in JP-A-54-118325, a band-like martensitic structure or a block-like coarse bainite structure appears, and the microstructure changes. At the same time, it increases the susceptibility to hydrogen-induced cracking.

(Means for Solving Problems 9) The present invention advantageously eliminates the above-mentioned drawbacks, and its gist is as follows: C: 0.02-0.15% Si: 0.02-0.15% by weight 0.1-0.6% Mn: 0.5-1.5% P: 0.015% or less S: 0.010% or less AI + 0.005-0.1% Ti: 0.005-0. In addition, Nb: 0.10% or less, B: 0.005% or less, Ni: 1.0% or less, Cu: 1.025%. OX or less, Cr
: 1.25X or less, Ca: 0.005% or less of one or more types, and the balance is iron and non-habitable impurities, is heated to 1000°C to 1200°C, and then rolled. Reduction rate of 850'C or less when hitting 6
Rolling is carried out at a finishing temperature of 0z or higher and a finishing temperature of Ar3 transformation point or higher,
Next, from the temperature above this Ar3 transformation point, at a cooling rate of 3000/S or more, 350'C or more and 550'C! This is a method for producing a steel sheet with excellent sour resistance properties and having a uniform fine bainite structure in the thickness direction.

According to this method, the structure after cooling becomes uniform and fine upper bainite or ancular ferrite in the sheet thickness direction, and the microstructure in the central segregation area is improved, resulting in a steel sheet with excellent strength, toughness, and sour resistance. is obtained.

Next, the present invention will be explained in detail.

First, heating, rolling, and cooling conditions will be explained. The heating temperature of the slab having the above component range is 1000°C to 120°C.
The reason for limiting the temperature to 0°C is to make the austenite fine during heating and to make the structure fine after cooling. 1200
If the temperature is higher than 0.degree. C., the austenite becomes coarse during heating, and the structure after rolling cannot be made sufficiently fine. Also, the heating temperature is 1000
If the temperature is lower than 0.degree. C., alloy additive elements such as Nb will not be sufficiently solid-dissolved, and no improvement in material quality can be expected.

Further, in finish rolling, a reduction of 60% or more is required in a temperature range of 850° C. or lower in order to obtain strength and toughness at the same time. This is to sufficiently stretch the austenite in the non-recrystallized region, suppress the appearance of coarse block-like bainite after rolling and cooling, and make the microstructure of the steel sheet a fine and uniform bainite structure in the thickness direction. . However, the rolling finish temperature is set to be equal to or higher than the Ar3 transformation point based on the cooling conditions described below. Here, it is assumed that Ar3 is estimated by the following regression equation.

Ar3 = -3980+ 24. l3Si-88,
IMn -36, INi -20,7Cu -24,8
Cr+ 29. The cooling start temperature of EiMo+ 8138°C must be equal to or higher than the Ar3 transformation point in order to obtain a uniform and fine bainite structure. Below the Ar3 transformation point, coarse polygonal ferrite is mixed in the microstructure, making it difficult to obtain good toughness, and C atoms are enriched due to the formation of pro-eutectoid ferrite, which changes the hardenability from untransformed austenite. Martensite is generated and the microstructure becomes non-uniform in the thickness direction, resulting in deterioration of sour resistance.

Limiting the cooling rate and cooling stop temperature conditions is an essential condition for achieving both high strength and toughness as well as sour resistance properties, and the reason for this will be described below.

Cooling must be carried out at a cooling rate of 30° C./S or higher from 350° C. to lower than 550° C. immediately after the end of rolling. The reason for this is that in order to obtain a uniform fine bainite or acicular ferrite structure in the thickness direction from austenite whose hardenability has deteriorated due to reduction of 80% or more at temperatures below 850°C, the cooling rate is set at 30°C or more. It needs to be rapidly cooled. If the cooling rate is less than 30° C./S, the microstructure after cooling becomes polygonal ferrite-pearlite or polygonal ferrite-bainite, and the required strength cannot be obtained. Regarding the cooling stop temperature, in order to suppress the formation of martensite and obtain uniform fine bainite, the upper limit was set to less than 550°C, and the lower limit was set to 350°C.

The components of the steel of the present invention will be described below.

The lower limit of 0.02% of C is the minimum amount in order to ensure the strength of the base metal and welded part and to fully exhibit the effect of Nb precipitation. However, if the C content is too high, island-like martensite will be generated in controlled cooling, which will not only adversely affect ductility but also deteriorate weldability and HAZ toughness, so the upper limit should be set to 0.
．． It was set at 15%.

Si is an element that is naturally included in deoxidized steel, but Si
Also, the upper limit was set at 0.6% because it also deteriorates weldability and HAZ toughness.

If Mn is less than 0.5z, sufficient strength cannot be obtained. Furthermore, if it exceeds 1.5z, coarse block-like bainite or island-like martensite is likely to be formed, which impairs not only the hydrogen-induced cracking resistance but also the toughness.
~1.5z.

For P, the upper limit is set to 0.0 to reduce center segregation.
It was set to 15'X or less.

If the S content exceeds 0.010%, hydrogen-induced cracking occurs frequently around sulfide inclusions, so the upper limit is set to o, otox.

Although AI is necessary as a deoxidizing agent for steel, it is not preferable to add more than 0.10% because it contaminates the steel and deteriorates its toughness.

The lower limit of the amount of Ti added is 0.005%, which is the minimum amount that is expected to form fine TiN and refine the rolling structure and HAZ, and the upper limit is 0.025% under the condition that no toughness deterioration due to TiC occurs. And so.

Nb is included to refine the rolling structure, improve hardenability, and precipitation harden, and is an important element that improves both strength and toughness. Even if it is added, it has no effect on the quality of the material and is harmful to weldability and HAZ toughness, so the upper limit was limited to 0.102.

B segregates at austenite grain boundaries during rolling, increases hardenability and facilitates the formation of bainite structure, but at 0.005%
When it becomes super tough, it deteriorates.

Ni is an effective element for corrosion resistance. However, if it exceeds 1%, it is unfavorable for toughness.

As mentioned above, Cu forms a stable film on the steel sheet surface in a sour environment with a p)I"? of about 5.2 and reduces the amount of atomic hydrogen intrusion. However, if it exceeds 1z, cracks occur during rolling. becomes more likely to occur.

Or is an element that improves hardenability, but at 1.25%
If it exceeds this, the toughness will deteriorate.

Ga is an effective element for controlling the shape of sulfide-based inclusions in steel, and its effect becomes apparent at 1.0Q (IIX or higher), but if it exceeds 0.005%, the steel becomes contaminated. Therefore, this is the upper limit.

In addition, in the present invention, Nb, B, Ni, Cu, Cr
, Ca may be contained in one kind or in combination of two or more kinds.

(Example) Next, an example of the present invention will be described. Table 1 shows the chemical components, manufacturing conditions such as slab heating temperature, finish rolling conditions, and cooling conditions, and the resulting mechanical properties, as well as the U after HIG test.
The crack area ratio (CAR) measured by ST is shown.

The material is either an in-situ melted slab or a laboratory-melted material. After heating to 1100°C to 1200°C to austenitize, appropriate rough rolling is performed at a temperature of 850°C or less to obtain various reduction ratios in finish rolling, followed by finish rolling, and then immediately transferred to a cooling zone. The sample was transported to the factory for accelerated cooling.

For the HIG test, the front and back surfaces of the steel plate were cut to a thickness of 1IllI11, with a width of 20fiII11 and a length of 100IIIll.
Has saturation temperature 0.5XCH3 at 25℃ using a test piece of
It was immersed in a COQH+ 5% NaCl aqueous solution for 86 hours.

A-1, B-1, C-1, D-1, E-1 are the steels of the present invention, A-2, A-3゜B-2, B-3, G-2, D-2,
E-2 is a comparison steel.

According to the present invention, A-1, B-1, C-1, El-1, E-
As shown in 1, X? 0 class strength and vTrs of -100°C or less toughness and CAR by UST
Steel sheets with excellent sour resistance properties, called RO, can be produced.

Comparison @ A-2 and D-2 have insufficient rolling reduction of 850"0 or less, comparative steel A-3 has an inappropriate cooling rate, and comparative steel B-2 has an insufficient cooling stop temperature. In comparison steel B-3, the finishing temperature is lower than the Ar3 transformation point, so the toughness and HIc resistance deteriorate, respectively.Furthermore, in comparison steels C-2 and E-2, the cooling rate is too low. Because it is too slow, it is not possible to obtain the strength of the X?O class.

(Effect of the invention)

Claims (1)

[Claims] In weight percent, C: 0.02 to 0.15% Si: 0.1 to 0.6% Mn: 0.5 to 1.5% P: 0.015% or less S: 0. 010% or less Al: 0.005-0.1% Ti: 0.005-0.025% as basic components, further Nb: 0.10% or less, B: 0.005% or less, Ni:
1.0% or less, Cu: 1.0% or less, Cr: 1.25%
Hereinafter, a steel containing one or more types of Ca: 0.005% or less, with the balance consisting of iron and non-deformable impurities, is heated to 1000°C to 1200°C, and then rolled to 850°C. Finishing temperature Ar_ with the following reduction rate of 60% or more
Rolling is carried out at a temperature of 3 transformation point or higher, and then from the temperature of this Ar_3 transformation point or higher, at a cooling rate of 30°C/S or higher, 35
A method for manufacturing a steel sheet with excellent sour resistance properties, which comprises cooling to a temperature in the range of 0° C. or more and less than 550° C., and then allowing it to cool, and having a uniform fine bainite structure in the thickness direction.