JPH02290947A - High toughness steel sheet for resistance welded steel tube having excellent sour resistance - Google Patents

Abstract

PURPOSE:To obtain the steel sheet free from the generation of hydrogen cracking even in the environment of hydrogen sulfide as well as having good low temp. toughness by limiting the components such as Ca, Al, S and O in a molten steel and regulating composite inclusions having specified compsn. as deoxidation products. CONSTITUTION:A steel sheet contg. 0.05 to 0.35% C, 0.02 to 0.5% Si and 0.5 to 2% Mn as well as 0.0005 to 0.008% Ca and 0.005 to 0.1% Al and the balance iron with impurities, in which, under the condition where the contents of S, O and Ca satisfy the relationship in the formula, composite inclusions (CaO)m(Al2O3)n are regulated as deoxidation products and whose molecular constitutional ratio is regulated to the range of <1 (m)/(n) is prepd. If required, one or more kinds among 0.1 to 1% Mo, 0.01 to 0.1% Nb, 0.01 to 0.1% V, 0.001 to 0.05% Ti and 0.0005 to 0.004% B and/or one or more kinds among 0.1 to 0.5% Cu, 0.1 to 3% Ni and 0.1 to 3% Cr are furthermore added to the above components.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a high toughness steel plate for electric resistance welded steel pipes having excellent sour resistance.

(Conventional technology) Oil and natural gas produced in recent years very often contain hydrogen sulfide, and when water such as seawater and fresh water coexists, metal loss due to corrosion on the steel surface alone is insufficient. However, hydrogen generated on the surface of the steel due to corrosion can penetrate into the steel, causing damage, which has become a problem. This fracture differs from sulfide stress corrosion cracking, which has long been observed in high-strength steel, and can occur even without any external stress applied. This destruction occurs when hydrogen that has entered from the environment accumulates and gasifies at the boundary between the steel base and A-based sulfide inclusions such as MnS that are present in the base metal and extend in the rolling direction, and due to the gas pressure, The above-mentioned A-based sulfide inclusions such as MnS form sharp notches, which grow into masked parallel cracks as crack nuclei, and these masked parallel cracks are connected in the thickness direction. be.

Various studies have been conducted on steels with high resistance to this type of cracking (hydrogen blistering cracking), and various steels have been proposed. For example, these technologies utilize crack prevention by adding Cu or Co, reduction of MnS by extremely low S, and fixation of S by adding Ca or rare earth elements. Steel has been developed that can withstand up to

For example, as seen in JP-A-59-76818, the content of S, O, and Ca is 1.0≦(%Ca) {1-72(%O))/1.
It is known to add Ca to satisfy 25(%S)≦2.5.

On the other hand, in recent years, areas where oil and natural gas are produced have expanded to include extremely cold regions such as Alaska, the Soviet Union, and the Arctic Ocean, and when ERW steel pipes are used for line pipes and oil country tubular goods used in these regions, it is natural that However, both the base metal and the weld zone are required to have excellent low-temperature toughness.

In ERW steel pipes, the toughness of the welded part is lower than that of the base metal, so various studies have been conducted on ERW steel pipes that have excellent toughness, including the ERW welded parts, and various methods and is proposed. These include, for example, the finishing temperature in the hot rolling process and. Improving the toughness of the material by limiting the winding temperature, controlling the grain size by heat treating the ERW weld after pipe making and then rapidly cooling it, refining the crystal grains by using Nb and V, and heat treating the pipe body after making the pipe. Using these techniques, electric resistance welded steel pipes with considerably superior toughness have been developed to date.

By the way, as the environment in which these ERW steel pipes are used becomes harsher,
Customer requirements have improved, and for example, there are cases where both sour resistance and low temperature toughness are required. In order to meet these composite property requirements, for example, a method is adopted in which Ca is added to the low-temperature ERW steel pipe in order to impart sour resistance properties. However, as a result of detailed investigation into the toughness of the electric resistance welded joints of these composite characteristic steel pipes, it was found that the toughness of the electric resistance welded joints may be significantly lower than that of the base metal.

As a result of a detailed investigation of this low-temperature toughness deterioration, the present inventors found that the cause was the plate-like inclusions present in and near the electrical resistance welding abutment. Further investigation revealed that these plate-shaped inclusions were caused by nearly spherical inclusions that had already existed in the base metal being heated to near the melting point of the steel due to the heat effect during ERW welding, and squeezed. It was revealed that the material was formed in a plate shape because it was pressurized from both sides by the rolls. In addition, as a result of analyzing the components of this inclusion,
It was found to be a composite inclusion of CaO.7V,O.

(Problems to be Solved by the Invention) Various methods have been proposed to solve the above problems.

For example, as seen in JP-A-63-137144, Zr is added to steel to remove inclusions from ZrO.・A7
, 03 to raise its melting point and prevent it from stretching during electric resistance welding.

However, this addition of Zr is not common in ordinary steel manufacturing operations, and is not only expensive, but also dangerous (highly flammable).

Therefore, the present invention aims to solve this problem at low cost without using such special elements.

(Means for Solving the Problems) In order to solve these problems, the present inventors conducted further research. First, the components of the composite inclusions were investigated because there were differences in the level of toughness even among those to which Ca was added. As a result, inclusions that become plate-like at the electric resistance welding part (Ca
The molecular ratio of O)m(A1z03)n was found to be m/n≧1. In other words, it is a state in which more CaO exists than N z 0 3 in the inclusions.

In addition, it was found that inclusions with a molar ratio of m/n<1 (CaO) m (7Vz(h)n) are not plate-shaped, and therefore, there is no deterioration in toughness.
0. Looking at the equilibrium diagram of (CaO) m (Aj
go3) When the molar ratio of n is m/n≧1, the melting point is about 13
60″C, and it is quite conceivable that the elongation occurs near the electric resistance welding part.On the other hand, (CaO) m (AZ zO+
) When the molar ratio of n is m/n<1, the melting point is 1600°C or higher, and it is considered that stretching near the electric resistance welding part can be avoided. In other words, in order to improve the sour resistance by adding Ca and to avoid deterioration of the toughness of the electric resistance welded joint, the composition of the deoxidized product must be changed (Ca
O) m・(IVz Q 3 ) m in the molar ratio of n
It has been found that it is sufficient to control the ratio to /n<1.

As a result of various studies on controlling the composition of deoxidized products, the present inventors considered controlling the content of Ca and N in molten steel. In other words,
By increasing the M content, A7ZO in inclusions
: + This means increasing the fraction. FIG. 1 shows ERW steel pipes manufactured with various components organized only by the Ca and M contents. As is clear from this figure, Ca
/ Aj < 0. If condition 10 is satisfied, it is possible to control the inclusion composition, prevent the inclusion from stretching, and avoid deterioration of the toughness of the electric resistance weld.

By the way, the present invention employs Ca addition as a means to improve sour resistance. In industrial-scale production processes, it is more advantageous to add Ca and control the morphology of MnS to render it harmless than to reduce the amount of S to the maximum in order to eliminate MnS, which is the starting point for hydrogen-induced cracking. This is based on the idea and experimental results that there is.

In other words, the content of S, O, and Ca is 1.0≦(%Ca) (1-72(XO)) / 1.
Ca is added to satisfy 25(%S)≦2.5. This is because Ca has a stronger affinity for oxygen than S, so the remaining Ca (
This shows that MnS is completely morphologically controlled if effective Ca) is combined with S in an atomic weight ratio, and if there is enough effective Ca to match Sfi. This equation also shows that if Ca is added in excess, a large number of cluster-like inclusions will be produced, which may be harmful and the purpose cannot be achieved.

In other words, the effective Ca content shown in the above formula is controlled between the lower limit for controlling the morphology of MnS and the upper limit for preventing the formation of cluster inclusions, thereby ensuring sour resistance. be.

Therefore, the present invention has been made based on the above findings, and the gist thereof is that C: 0. 0 5-0.35%
, Si: 0.02-0.5%, Mn: 0.5-2%
In addition to 0.0005~O. Contains 8% O O and 0.005 to 0.1% Al, and further Mo: 0
．． 1-1%, Nb: 0.01-0.1%, V: 0.0
1-0.1%, Ti:O. OO1~0.05%, B:
0.0005~O. One or more of O 2 O 4% and Cu: 0. 1 ~ 0. 5%, N
i: 0.1-3%, Cr: 0. 1 out of 1-3%
species or two or more species, the remainder consisting of Fe and impurities, S, 0. The content of Ca is 1,0≦(%Ca) (1-72(χ0)) / 12
After satisfying 5(%S)≦2.5, the deoxidized product (
Manufacture of a steel sheet for high-toughness electric resistance welded steel pipes with excellent sour resistance, characterized by having composite inclusions of CaO)m(IV.o.)n and having a molecular composition ratio in the range of m/n<1. It's a method.

Next, in the method of the present invention, the reason why the composition range of the steel is limited as described above will be explained.

(Component composition range) (a) C It is a basic element that improves the strength of steel, and it is necessary to contain 0.05% or more to ensure strength, but 0.
．． If it exceeds 35%, it will have an unfavorable effect on the toughness of the steel, so it is set at 0.05 to 0.35%.

(b) Si It is an element that improves the strength and ductility of steel sheets through solid solution strengthening, so it should be contained at 0.02% or more, but the upper limit should be 0.5% to ensure toughness.

(c) Mn This element is necessary for strength, so 0. The content should be 5% or more, but in order to ensure weldability and toughness, the upper limit content should be 2%.

(d) Ca In order to spheroidize MnS and improve its sour resistance properties, a Ca content of at least 0.0005% is required. On the other hand, if the amount added is too large, the melting point of the composite inclusions will drop too much and the composite inclusions will be stretched into a plate shape at the electric resistance welding part, deteriorating the toughness, so the upper limit should be set to 0. 0 O 8%.

What is important is that the component ratio with N satisfies Ca/A7<0.1.

(e) Aj Necessary for deoxidation in the steelmaking stage, with a lower limit of 0. 0
O was set at 5%. Also, if the content is too large, the absolute amount of inclusions themselves will increase, so the upper limit should be set to 0. It was set at 1%.

What is important is that the component ratio with Ca is Ca/A! <0.1.

Cf>other elements are the basic component system of the present invention, but in the present invention, in addition to these, depending on the use, (A) Mo, Nb,
One or more of V, Ti, B or (B) Cu
, Ni, and Cr, any one or both of (A) and (B) can be contained.

First, MOSNb and V are both elements that improve the strength of steel, MOSN being 0.1% or more, and Nb and V being 0.1% or more. At Ol% or more, the same strength increasing effect is shown, but when Mo exceeds 1%, and when Nb and V exceed 0. If added in excess of 1%, there is a risk of reducing toughness, so MO should be added between 0.1 and 1%.
1%, Nb and ■ were limited to a range of 0.01 to 0.1%.

Ti is an element that combines with nitrogen in steel to form TiN and improves toughness, but if added in excess of 0.05%, there is a risk of decreasing toughness, so the range should be limited to O.
O O was limited to 1 to 0.05%.

B is an element that improves the hardenability of steel, and is essential for manufacturing quenched and tempered ERW steel pipes, but O. O
Since adding more than 4% O may reduce toughness, the range was limited to 0.0005 to 0.005%. Are Cu, Ni, and Cr all elements added to improve the corrosion resistance of the base metal and reduce the amount of hydrogen penetrating into the steel? Ru. Cu is 0. If it is less than 1%, there is no effect, and if it exceeds 0.5%, it will have a negative effect on hot workability, so 0.1 to 0
．． Limited to a range of 5%. If Ni is less than 0.1%, it is ineffective, and if it exceeds 3%, it may induce sulfide stress corrosion cracking, so it is limited to a range of 0.1 to 3%. C
If r is less than 0.1%, it has no effect, and if it exceeds 3%, it reduces the toughness of the base material, so it is limited to a range of 0.1 to 3%.

Hereinafter, the effects of the present invention will be described in more detail with reference to Examples.

(Example) Steel having the composition shown in Table 1 is melted, hot rolled to a thickness of 6 to 12 mm, and then made into an electric resistance welded steel pipe with an outer diameter of 114.3 to 406.4 mm by a normal process. After quenching and tempering some of the samples, a sour resistance evaluation test was conducted, and the toughness of the electric resistance welded joint was also measured.

As an evaluation test for sour resistance, a test piece was immersed in a 5% NaCl aqueous solution saturated with 11.3%. 5% CI3COO
The specimens were immersed in a solution containing H (pH 2.8 to 3.8) for 96 hours, and cracks were measured. As is clear from Table 1,
In the steel pipes using the steel of the present invention, hydrogen bulge cracking did not occur and the decrease in toughness was very small, whereas in the comparison steel, hydrogen bulge cracks occurred or the toughness significantly decreased. ing.

(Effects of the Invention) As can be seen from the above test results, the present invention makes it possible to provide an electric resistance welded steel pipe that is free from hydrogen blistering cracking even in a hydrogen sulfide environment and has good low-temperature toughness.
There are some extremely significant contributions to the development of industry.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the presence or absence of elongation of inclusions in an electric resistance welded part depending on the amounts of Ca and M added. 1st 0.020 θ, σ4θ A (l degradation amount (%) θ.θlo O.σθ8θ

Claims (1)

[Claims] (1) C: 0.05-0.35%, Si: 0.02-0
．． 5%, Mn: 0.5-2% plus Ca 0.000
5 to 0.008%, and 0.005 to 0.1% of Al, and the balance consists of Fe and impurities, and the content of S, O, and Ca is 1.0≦(%Ca) {1-72( %O)}/1.
After satisfying 25(%S)≦2.5, the deoxidized product is made into a composite inclusion of (CaO)_m(Al_2O_3)_n, and its molecular composition ratio is in the range of m/n<1. A high-toughness ERW steel plate with excellent sour resistance. (2) C: 0.05-0.35%, Si: 0.02-0
．． 5%, Mn: 0.5-2% plus Ca 0.000
5-0.008% and Al 0.005-0.1%, further Mo: 0.1-1%, Nb: 0.01-0.1
%, V: 0.01-0.1%, Ti: 0.001-0.
05%, B: one or more of 0.0005 to 0.004%, the remainder consisting of Fe and impurities,
The content of S, O, and Ca is 1.0≦(%Ca) {1-72(%O)}/1.25(
%S)≦2.5, and then convert the deoxidized product into (Ca
A steel sheet for high-toughness electric resistance welded steel pipes having excellent sour resistance, characterized by having composite inclusions of O)_m(Al_2O_3)_n and having a molecular composition ratio in the range of m/n<1. (3) C: 0.05-0.35%, Si: 0.02-0
．． 5%, Mn: 0.5-2% plus Ca 0.000
5-0.008% and Al 0.005-0.1%, further Cu: 0.1-0.5%, Ni: 0.1-3%
, Cr: 0.1 to 3%, the remainder consists of Fe and impurities, and the content of S, O, and Ca is 1.0≦(%Ca)(1-72( %O))/1.
After satisfying 25(%S)≦2.5, the deoxidized product is made into a composite inclusion of (CaO)_m(Al_2O_3)_n, and its molecular composition ratio is in the range of m/n<1. A high-toughness ERW steel plate with excellent sour resistance. (4) C: 0.05-0.35%, Si: 0.02-0
．． 5%, Mn: 0.5-2% plus Ca 0.000
5-0.008% and Al 0.005-0.1%, further Mo: 0.1-1%, Nb: 0.01-0.1
%, V: 0.01-0.1%, Ti: 0.001-0.
05%, B: one or more of 0.0005 to 0.004% and Cu: 0.1 to 0.5%, Ni: 0
．． 1 to 3%, Cr: 0.1 to 3%, the remainder consists of Fe and impurities, S, O,
The content of Ca is 1.0≦(%Ca)(1-72(%O
))/1.25(%S)≦2.5, the deoxidized product is made into a composite inclusion of (CaO)_m(Al_2O_3)_n, and its molecular composition ratio is m/n<1. A high toughness steel plate for ERW steel pipes with excellent sour resistance.