JPH0674458B2 - Method for producing high strength steel sheet excellent in hydrogen sulfide resistance and toughness - Google Patents

Description

The present invention relates to the production of high-strength steel sheets, and is particularly suitable as a material for petroleum and natural gas transportation line pipes containing wet hydrogen sulfide (H ₂ S). To a high-strength steel sheet having excellent hydrogen sulfide resistance and toughness.

(Prior art) With the depletion of minable oil wells in recent years, so-called sour oil fields and sour gas fields containing a large amount of H ₂ S have been actively developed, and the production of crude oil and natural gas Demand for line pipes for transportation is also increasing. However, in such a sour crude oil or sour gas transportation line pipe, cracking due to wet H ₂ S may occur,
It is an important issue because of the great risk of destruction.

Hydrogen-induced cracking (HIC) and sulfide stress corrosion cracking (SSC) are known as cracks in steel due to wet H ₂ S. Since HIC occurs regardless of material strength and even when external stress does not exist, it is a big problem even for relatively strong materials. This is because hydrogen generated on the steel surface by the corrosion reaction due to H ₂ S penetrates into the steel and diffuses and precipitates as molecular hydrogen at the interface between the non-metallic inclusions and the base iron, increasing the internal pressure at the interface, resulting in cracking. When some of these cracks occur adjacent to each other, they grow by connecting to each other and penetrate through the entire thickness of the steel material.

On the other hand, SSC is a crack that occurs when stress is applied to a relatively high-strength material, and often causes problems in the hardened region of the weld heat-affected zone in line pipes. When it has a structurally non-uniform structure, SSC may occur in the base metal part.

Further, as a recent trend of line pipes, there is a growing demand for higher wall thickness and higher strength for increasing operating pressure and transport efficiency, and higher toughness for cold regions.

(Problems to be Solved by the Invention) The following measures have been taken conventionally for HIC and SSC in the above-mentioned wet H ₂ S environment.

Method to reduce sulfur In this method, cracks such as HIC and SSC often occur along the sulfide inclusions expanded by rolling. It is intended to However, even if the sulfur content is reduced to about S ≦ 0.003%, it is still impossible to completely prevent the formation of wrought sulfides in the segregated portion.

Method by controlling the shape of inclusions This method is to make the sulfide-based inclusions, which are the starting points of cracking, spherical so that cracks are less likely to occur.Specifically, it is a method of adding Ca or a rare earth element. Yes (Japanese Patent Laid-Open No. 51-
114318). However, when a large amount of these is added, a large amount and agglomeration of sulfides and oxides of Ca and rare earth elements are formed, which causes cracking. Therefore,
Strict control of addition amount and low sulfur content are essential.

Method of forming protective coating on steel surface This is the addition of Cu in order to control the generation of hydrogen due to corrosion and the penetration of hydrogen into the steel (Japanese Patent Laid-Open No. 50975975).
No.), Co is added (JP-A-58-133350). However, it is ineffective in an acidic environment with a pH of about 4.5 or less, and there is a problem that hot workability and weldability are deteriorated, which makes the material expensive.

Method by removal of abnormal structure In this method, cracks easily propagate along the band structure of a low temperature transformation product (martensite or lower bainite) formed in a portion where C, Mn, P, etc. are concentrated and segregated, Since it grows, it is intended to prevent the generation of such abnormal tissue, and (1) C and Mn are reduced (Japanese Patent Laid-Open No. 56-33459).
No.) and (2) quenching and tempering (Japanese Patent Laid-Open No. 50-108119).
No.), (3) or uniform bainitic steel (JP-A-53)
-52223) and other methods have been proposed. But,
The method (1) cannot increase the strength, and the method (2) has a problem that the energy consumption increases and the production efficiency decreases. Further, the ultra low carbon bainitic steel obtained by the method (3) has the advantages of high strength and excellent hydrogen sulfide resistance and toughness as compared with conventional ferrite / pearlite steel, but as rolled However, when the bainite structure is used, there are still the following problems.

That is, since the properties of bainitic steel are strongly influenced by the old γ grain size, it is necessary to refine the recrystallized γ grain before entering the non-recrystallization zone rolling. However, in ultra-low carbon steel, Nb carbonitride is hard to precipitate even in Nb-added steel, and it is difficult to refine γ grains in the recrystallization region.

When the old γ grains are coarsened in this way, there is a drawback that the toughness and hydrogen sulfide resistance are deteriorated.

An object of the present invention is to solve the above-mentioned drawbacks of the prior art, to provide a method for producing a steel sheet having high strength and excellent ductility, and also having excellent hydrogen sulfide resistance such as HIC resistance and SSC resistance. .

(Means for Solving Problems) In order to achieve the above object, the present inventor has made the composition and rolling conditions for refining the recrystallized γ grains before entering the non-recrystallization region rolling of bainite steel, cooling conditions, and cooling. As a result of repeated research on the conditions, etc., especially when the rolling conditions and the cooling rate after rolling were strictly controlled, the γ grains were refined, and at the same time, an appropriate amount of proeutectoid ferrite was finely precipitated in these γ grain boundaries and grains. Ultra-low carbon bainite structure is obtained, pro-eutectoid ferrite finely precipitated in γ grain boundaries and grains inhibits the development of grain boundary cracks and intragranular cleavage cracks, and hydrogen sulfide cracking resistance can be significantly improved,
It has been found that high strength and excellent ductility can also be imparted.

That is, the gist of the present invention is, by weight ratio (hereinafter, the same), C: 0.005 to 0.08%, Si: 1.0% or less, Mn:
1.0-2.5%, P: 0.02% or less, S: 0.01% or less and Al: 0.005
~ 0.1%, and if necessary, B: 0.005% or less for steel containing 60% or more, 1000 ~ 1000
The rolling reduction between 850 ℃ is 50% or more, and the rolling finishing temperature is 8
After hot rolling under the rolling condition of 50 ° C. or higher, 9 ≦ Ceq.
CR ≦ 15 (2 ≦ Ceq ・ CR ≦ 7 when B is added) (However, C
R: Cooling rate (℃ / S), Ceq = (C + Si / 24 + Mn / 6 + Ni / 40
+ Cr / 5 + Mo / 4 + V / 14)}, the cooling rate is controlled to meet the condition of
A method for producing a high-strength steel sheet excellent in hydrogen sulfide resistance and toughness, characterized by obtaining an extremely low carbon bainite structure in which 5 to 20% of pro-eutectoid ferrite is precipitated in γ grain boundaries and grains.

Hereinafter, the present invention will be described in detail based on examples.

First, the reasons for limiting the chemical composition of steel targeted by the method of the present invention will be described.

C: 0.005 to 0.08% C is an element necessary for obtaining strength, and for that purpose 0.00
5% or more. However, if the C content is too large, the weldability, toughness, and hydrogen sulfide resistance deteriorate, so the upper limit is 0.08%.

Si: 1.0% or less Si is added for deoxidation of molten steel, but if it is too much, weldability and toughness deteriorate, so 1.0% or less is added.

Mn: 1.0 to 2.5% If the Mn content is too low, the ferrite content becomes excessive, so that high strength cannot be obtained, and at the same time, carbon is concentrated in bainite and the toughness and hydrogen sulfide resistance deteriorate. %
The above is added. However, if too much, weldability deteriorates, segregation becomes significant, and hydrogen sulfide resistance also deteriorates, so 2.
The upper limit is 5%.

P: 0.02% or less Since P is an impurity element, the lower the better. If it is too high, segregation becomes remarkable like Mn, and hydrogen sulfide resistance deteriorates. Therefore, it is necessary to suppress it to 0.02% or less.

S: 0.01% or less Since S is an impurity element like P, the lower the content, the better. If it is too high, the amount of sulfide increases and hydrogen sulfide resistance deteriorates, so it is necessary to suppress it to 0.01% or less.

Al: 0.005 to 0.1% Al needs to be added in an amount of 0.005% or more for deoxidation of molten steel, but if it is too high, oxide-based inclusions increase and hydrogen sulfide resistance deteriorates, as well as weldability and toughness. Also deteriorates, so 0.
The upper limit is 1%.

In addition to the above essential elements, in the present invention, the following elements can be added in small amounts, if necessary. Above all, B
Has a great influence on controlling the amount of pro-eutectoid ferrite by cooling after hot rolling, and it is desirable to add it as an effective element for improving strength.

B: 0.005% or less B segregates into γ grains and delays nucleation of proeutectoid ferrite.
Increasing the amount of bainite contributes to the strength improvement, but if added excessively, it causes deterioration of toughness, so 0.
The upper limit is 005%.

Cu: 0.5% or less Addition of Cu is effective in preventing corrosion and hydrogen invasion in sour environments with relatively high pH. However, if the added amount is too large, the hot workability and weldability deteriorate, so 0.5% is made the upper limit.

Ni: 0.5% or less Ni is an element effective for improving strength and toughness and preventing deterioration of hot workability due to addition of Cu. However, excessive addition is not only economically disadvantageous, but also
Since it deteriorates the SSC property, 0.5% is the upper limit.

Cr: 1.0% or less Cr is an element effective for improving strength and corrosion resistance, but since weldability deteriorates if the amount of Cr is too large, the upper limit is 1.0%.

Mo: 0.5% or less Mo is an element effective for improving strength, toughness and corrosion resistance, but if the amount of Mo is too large, the weldability deteriorates, so 0.5% is the upper limit.

Nb: 0.1% or less Addition of Nb improves strength by carbonitride precipitation, but
Even if added excessively, the effect is saturated and it is economically disadvantageous, so 0.1% is made the upper limit.

V: 0.2% or less V addition, like Nb, improves strength by carbonitride precipitation, but the effect is saturated even if added excessively and it is economically disadvantageous, so 0.2% is the upper limit. .

Ti: 0.05% or less Ti fixes N as TiN in the B-added steel and effectively acts on the pro-eutectoid ferrite blocking effect of B. But,
If the amount of Ti is too large, coarse TiN is formed, which is HIC and SSC.
The upper limit is 0.05%.

Ca: 0.005% or less Addition of Ca is effective for controlling the shape of sulfides, but if it is contained in excess, oxide inclusions increase and toughness and hydrogen sulfide resistance deteriorate, so 0.005% is the upper limit. .

REM (rare earth element): 0.02% or less Addition of REM is effective for controlling the shape of sulfides, but if it is included in excess, oxide inclusions increase and toughness and hydrogen sulfide resistance deteriorate, so 0.02% Is the upper limit.

For the steel having the above chemical composition, in the present invention, by particularly regulating the hot rolling conditions and the cooling conditions after rolling, 5 to 20% of proeutectoid ferrite is finely precipitated in the γ grain boundaries and grains, The rest is an ultra-low carbon bainite structure.

For that purpose, at the time of hot rolling, it is necessary to control the rolling temperature and the rolling reduction to make the recrystallized γ grains finer and ferrite finely precipitated. For that, 1150
-Reduction of 60% or more between 1000 ℃ and refining the recrystallized γ grains before entering the non-recrystallization zone rolling, and then reducing between 1000 and 850 ℃ to finely precipitate ferrite in the grains. Rolling is performed with a rate of 50% or more. At that time, it is necessary to maintain the rolling finish temperature at 850 ° C. or higher in order to prevent the two-phase region rolling.

After rolling, the cooling rate is strictly controlled so that the amount of pro-eutectoid ferrite is 5 to 20%, and quenching is stopped between 500 and 650 ° C.
This cooling rate CR (° C / S) is, according to the experiment of the present inventor,
Carbon equivalent Ceq (= C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4
It was found that the regulatory conditions depend on + V / 14) (%) and whether B is added or not.

That is, in the case of B-free steel, the relational expression of 9 ≦ Ceq · CR ≦ 15, and in the case of B-added steel, 2 ≦ Ceq · CR ≦
The cooling rate CR after rolling is controlled by the relational expression of 7. Ceq
・ CR depends on the quenching stop temperature, but the amount of pro-eutectoid ferrite decreases at cooling rates above the upper limit, and conversely the amount of pro-eutectoid ferrite increases too much at cooling rates below the lower limit. If the quenching stop temperature is higher than 650 ° C, a large amount of pro-eutectoid ferrite will be precipitated, and if it is lower than 500 ° C, martensite and lower bainite will be generated. Stop temperature is 500-650
It shall be in the range of ° C.

Thus, the obtained steel sheet has a γ grain boundary and a grain content of 5 to 20%.
Of the above eutectoid ferrite is finely precipitated, and the rest has an extremely low carbon bainite structure. This proeutectoid ferrite has the function of preventing the development of intergranular cracks and intragranular cleavage cracks and increasing the resistance to crack propagation, but if it is less than 5%, it has no effect.
%, Precipitation of a large amount will result in a decrease in strength.

(Example) Using a steel piece having the chemical composition shown in Table 1, a rolling condition and a cooling condition after rolling were changed to manufacture a steel plate having a thickness of 15 mm.

From the position of 1/3 width of the steel plate, tensile test piece (JIS 14 A test piece,
Diameter 6mm, C direction cutout, Charpy test piece (JIS4,
C direction cutting), HIC test piece (length 100 mm, width 20 mm, front and back surface 1 mm cutting), SSC test piece (length 75 mm, width 15 mm, thickness 3 m)
m) was prepared and subjected to each test.

The HIC test was carried out by immersing the solution in an aqueous solution containing 5% salt and 0.5% acetic acid and saturated with hydrogen sulfide for 96 hours under no load. rate Was measured. Criteria were as follows: ○ indicates no cracking, Δ indicates a cracking length ratio of less than 3%, and × indicates a cracking length ratio of 3% or more.

In the SSC test, a test piece was bent by a 4-point bending jig so as to have a bending stress, and then immersed in the same solution as the HIC test for 300 hours. After that, the surface was observed with a 10 × microscope to examine surface cracks. Criteria were as follows: ○ indicates no cracking, Δ indicates cracking, and × indicates remarkable cracking.

Table 2 shows the tensile properties, impact properties, and HIC resistance and SSC resistance of hydrogen sulfide resistance of the steel sheet.

As can be seen from the table, the steel sheet according to the method of the present invention has high strength and excellent toughness and hydrogen sulfide resistance. On the other hand, in the case of the comparative example, at least one of the chemical composition, the rolling condition and the cooling rate is out of the range of the present invention, and therefore the hydrogen sulfide resistance is particularly poor.

(Effects of the Invention) As described in detail above, according to the present invention, with respect to steel having a specific chemical composition, the rolling conditions and the cooling rate after rolling are strictly controlled so that the γ grain boundaries and the grain boundaries have 5 to 20 grains. % Proeutectoid ferrite is deposited and the rest is made to have an ultra-low carbon bainite structure, resulting in high strength, toughness, and hydrogen sulfide resistance (HIC resistance).
Steel sheet having excellent heat resistance and SSC resistance) can be manufactured.
Therefore, it is suitable for the production of steel plates containing wet H ₂ S, such as oil and natural gas transportation line pipes, oil well pipes, and storage containers.

Claims (2)

[Claims] 1. A weight ratio of C: 0.005 to 0.08%, Si: 1.0%
For steels containing Mn: 1.0 to 2.5%, P: 0.02% or less, S: 0.01% or less, and Al: 0.005 to 0.1%, the reduction ratio between 1150 and 1000 ° C is 60% or more and between 1000 and 850 ° C. After performing hot rolling under a rolling condition of 50% or more and a rolling finishing temperature of 850 ° C. or more, 9 ≦ Ceq · CR ≦ 15 {however, CR: cooling rate (° C / S), Ceq = (C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + M
o / 4 + V / 14)} to control the cooling rate to meet the conditions of 5
Hydrogen sulfide resistance and toughness characterized by obtaining an extremely low carbon bainite structure in which 5 to 20% of proeutectoid ferrite is precipitated in γ grain boundaries and grains by quenching at a temperature of 00 to 650 ℃ Of excellent high strength steel sheet. 2. By weight ratio, C: 0.005 to 0.08%, Si: 1.0% or less, Mn: 1.0 to 2.5%, P: 0.02% or less, S: 0.01% or less, Al:
1150 for steels containing 0.005-0.1% and B: 0.005% or less
After the hot rolling under the rolling condition of the rolling reduction between ℃ ~ 1000 ℃ is 60% or more, the rolling reduction between 1000 ~ 850 ℃ is 50% or more, and the rolling finishing temperature is 850 ℃ or more, 2 ≤ Ceq・ CR ≦ 7 {However, CR: Cooling rate (℃ / S), Ceq = (C + Si / 24 + Mn / 6 + N
i / 40 + Cr / 5 + Mo / 4 + V / 14)} to control the cooling rate and stop quenching at a temperature of 500 to 650 ° C. A method for producing a high-strength steel sheet excellent in hydrogen sulfide resistance and toughness, which comprises obtaining an ultra-low carbon bainite structure in which slag is precipitated.