JPH0819461B2 - High-tensile steel plate manufacturing method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a high-strength steel plate having excellent weldability and low-temperature toughness.
It can be applied to hot coils. The steel produced by this method is characterized by being excellent in low temperature toughness and being inexpensive, and can be mainly used for large diameter steel pipes such as line pipes in general.

(Prior art) With the recent increase in energy demand, the construction of line pipes as an economical means of transporting oil and natural gas has been activated, and a large number of large oil fields have been developed in the cold regions such as the Arctic coast and Siberia. With the discovery of the gas field, today's linepipes were premised on long-distance, mass transportation.

For this reason, from the point of economic efficiency, line pipes continue to increase in diameter and pressure and become thinner, and APIX70, X80
(TS60-70kg f / mm ² ) class high strength and excellent low temperature toughness are now required. In addition, automatic welding with small heat input is becoming popular to improve the efficiency of field welding locally.
Requirements for weldability have become stricter from the viewpoint of hardening and crack prevention of welds.

Conventionally, line pipe materials used in cold regions have been manufactured by the controlled rolling of ferrite / pearlite steel containing Nb, V and other fine-grained and precipitation hardening elements. However, in contrast to the trend toward higher strength, higher toughness and stricter weldability of line pipes, conventional ferrite-pearlite steels containing Nb, V, etc. must satisfy the required quality special characteristics, especially strength and weldability. Has become difficult.

In order to deal with this, the Pearlite Red is designed to improve weldability and toughness by reducing the C content and reducing the amount of pearlite.
uced Steel (abbreviated as PRS) and Acicular Ferrite steel (hereinafter referred to as AF steel) that contains low carbon and high Mn and contains Nb and Mo have been developed and put to practical use. However, all of these steels have a ferrite / pearlite structure and a strength of 15 mm
The limit was 60 kg f / mm ² , and weldability was not fully satisfactory.

Therefore, in order to obtain strength of TS60 to 70 kg f / mm ² (APIX70 to 80 class) and excellent low temperature toughness, Nb-B composite additive steel has been developed which aims to make the steel structure fine bainite.
58-77528).

However, in this steel, in order to obtain a fine bainite structure, it was necessary to secure a sufficient amount of solute Nb during reheating of the slab. As a result, C has to be made extremely low, resulting in an increase in steelmaking cost.

Therefore, it has been strongly desired to develop an inexpensive steel material having high strength, excellent low temperature toughness, and capable of mass production.

(Problems to be Solved by the Invention) The present invention is to provide a high-strength steel material for a large-diameter steel pipe such as a line pipe at a low cost. The steel produced by this method is characterized by high strength, excellent low temperature toughness, and low cost, and is suitable for industrial mass production.

(Means for Solving the Problems) The gist of the present invention is, by weight%, C: 0.04 to 0.12%, Si: 0.5% or less, Mn: 1.0 to 2.0%, P: 0.03% or less, S: 0.01% or less, Ti. :
0.03-0.10%, B: 0.0005-0.0020%, Al: 0.05% or less,
N: 0.005% or less, Cr: 0.05 to 0.30%, Cu:
A steel slab containing 0.05 to 0.30% of one or two kinds, the balance of which is iron and unavoidable impurities, is heated to a temperature range of 1100 to 1250 ° C, and the cumulative reduction amount of 900 ° C or less is 60% or more, and the end temperature. After rolling at 680-800 ℃, air cooling or cooling rate
Accelerated cooling to a temperature of 550 to 350 ° C at 10 to 40 ° C / sec, followed by air cooling.

(Operation) Hereinafter, the present invention will be described in detail.

The remarkable features of the steel of the present invention are: (1) Ti is used in place of Nb, and a trace amount of B is added in combination to bainite the rolled structure and the strength and toughness of the base metal and welded portion due to fine precipitation TiC and TiN. Improved, (2) Ti is used because it is more likely to form a solid solution in the γ (austenite) phase than Nb, so high C can be achieved, and the steelmaking cost is reduced due to the small amount of alloy components other than Ti. (3) After heating There is a refinement of rolling structure by CR.

With the conventional ferrite / pearlite structure, it is impossible to obtain high strength at low cost, and it is necessary to improve the strength by making the rolling structure bainite. For this purpose, the use of B, which has the effect of improving hardenability, is extremely effective.

A small amount (5 to 20 ppm) of B is effective for bainizing a rolled structure and is inexpensive, so it is an essential element for the steel of the present invention. However, since B is extremely harmful to the weld toughness and weldability, it is necessary to pay particular attention to the B content. When the amount of B is more than 0.0020%, not only the HAZ is hardened, but also the B compound is formed at the austenite grain boundary. Therefore, it is necessary to set the upper limit of the amount of B to 0.0020%.

On the other hand, at least 0.00 is required to secure the stable hardenability of B.
05% is required, and this amount is larger than that in the usual quenching and tempering process. The most preferable content is 0.0010 to 0.0015
%.

The existing form of B, which has an effect on hardenability, is a state in which it is uniformly segregated at the austenite grain boundaries during cooling after the completion of rolling,
The effect disappears when it becomes a precipitate. Therefore, it is necessary to secure the suitability of solid solution B in order to stably improve the hardenability, but since B easily forms nitride BN, it is necessary to fix N with a stronger nitride forming element. There is. Therefore, it is very effective to add Ti.

Furthermore, since the toughness of bainite steel has a very strong grain size dependency, in order to improve the base metal and HAZ toughness of bainite steel and to secure the low temperature toughness suitable for line pipe materials, the rolling structure and HAZ must be thoroughly It is necessary to reduce the grain size. For this purpose, it is important to make effective use of Ti as an alloying element together with the limitation of heating and rolling conditions described later.

As described above, Ti fixes N as TiN to sufficiently exert the effect of improving the hardenability of B, and TiN finely precipitated in the steel slab,
TiC (0.05μ or less) refines austenite grains during heating (hereinafter referred to as heated γ grains) and is effective for grain refinement of the rolling structure. Fine TiN and TiC present in the steel sheet are
The AZ structure is refined.

However, the coarse TiN produced by the conventional steelmaking process adversely affects the toughness. Therefore, in order to add Ti and to use it to improve the toughness of the base material and HAZ, fine precipitation of TiN is essential for applying this steel to line pipes.

Also, in order to produce high-strength steel in large quantities and at low cost, it is necessary to avoid the addition of other expensive alloy components as much as possible,
It is necessary to add Ti in an amount more than stoichiometrically to fix N to precipitate TiC and secure the strength.

For this purpose, it is effective to limit the amounts of Ti and N together, and the amounts of Ti and N are limited to 0.03 to 0.10% and 0.005% or less, respectively. If the lower limit of Ti is less than that, it is difficult to secure the strength of the base metal due to lack of TiC, and it is the minimum amount necessary to improve the toughness of the base metal and HAZ. On the other hand, the upper limit of the amount of Ti, N is that if it exceeds this, fine TiN cannot be obtained in the ordinary steelmaking process, and excessive TiC precipitates to deteriorate the base metal and HAZ toughness.

 The reasons for limiting the component ranges will be described below.

In the steel of the present invention having the above characteristics, the composition range of the steel of the first invention is C: 0.04 to 0.12%, Si: 0.5% or less, Mn: 1.0 to 2.0%,
P: 0.03% or less, S: 0.01% or less, Ti: 0.03 to 0.10%, B: 0.0
005 to 0.0020%, Al: 0.05% or less, N: 0.005% or less.

 Ti, B and N are as described above.

The reason for limiting the C content to 0.04 to 0.12% is to improve weldability, ensure strength, and reduce steelmaking costs. That is, in the line pipe, field welding with a small heat input is performed due to relay welding, but this weld portion is easily hardened and various weld cracks occur, which may cause the line pipe to break. However, in order to completely repair and weld a myriad of cracks, enormous cost is required, and it is important to consider so as not to occur during welding.

For this purpose, it is important to select a welding rod, welding conditions and the like, but first of all, it is extremely important to use a line pipe material having a low hardening property. Therefore, the upper limit of the C content is 0.12%.

Also, in bainitic steel, a large amount of high-carbon island martensite is generated in the base material and the welded portion, which deteriorates toughness and hydrogen-induced cracking resistance, but reduces the amount of island martensite and finely disperses them. Low C to improve the characteristics of
The conversion is effective.

However, too extreme reduction of the C content weakens the precipitation strengthening and refinement effects of Ti carbonitrides, and it becomes difficult to secure the strength of the base metal and welded parts, which further increases the steelmaking cost. Limited to 0.04%.

Si is an element necessarily contained in steel for deoxidation,
Since Si is an element that is not preferable in terms of weldability and toughness of the weld zone, its upper limit was made 0.5%.

Mn is an extremely important element that lowers the transformation point of the steel of the present invention, enhances the material improvement effect by CR, and bainites the rolled structure to simultaneously improve strength and toughness. However, if it is less than 1.0%, bainite is insufficient and the desired strength and toughness cannot be obtained, so the lower limit is 1.0%.
And

On the other hand, if Mn is too much, hardenability increases, island martensite is formed, not only the toughness of the base metal and HAZ deteriorates, but also the carbon equivalent increases and the weldability is impaired, so the upper limit is set.
It was set to 2.0%. A desirable Mn range is 1.4 to 1.7%.

Al is an element that is inevitably contained in this type of killed steel in terms of deoxidation, but if Al total exceeds 0.05%, the toughness of HAZ deteriorates, so the upper limit was made 0.05%. When heating, Al
If N does not form a solid solution and N is fixed by Al, Al is T
Like i, it helps to improve the hardenability of B.

The reason why S, which is an impurity, is limited to 0.01% or less is that a large-diameter high-pressure gas line pipe used in a cold region requires high absorbed energy in the base material and the welded portion from the viewpoint of preventing unstable ductile fracture.

In the present invention, the rolling is performed in a considerably low temperature range,
Generally, the impact value decreases. Therefore, S is set to 0.01% or less as a measure for improving the impact value. In this case, the lower the S is, the more the toughness is improved, but the content is particularly improved by setting the S to 0.001% or less.

Further, the steel of the present invention contains P as an impurity, but it is usually 0.03
% Or less, and the lower the value, the higher the base metal, weld zone toughness, and weldability.

In the second invention, the composition and manufacturing process of the steel of the first invention further includes Cr: 0.05 to 0.30%, Cu: 0.05 to 0.30.
% Of one type or two types.

The main purpose of containing these elements is to improve the base metal strength and toughness of the steel of the present invention and to increase the plate thickness that can be manufactured, and the content is a property that should be limited by itself. .

Cr is a valuable element because it accelerates bainization of the rolled structure, improves strength and toughness, and is an inexpensive element having corrosion resistance to the environment. However, if it is less than 0.05%, the effect is not sufficiently obtained, and if it is added in a large amount, the hardenability of the welded portion is increased and the toughness and crack resistance are deteriorated, so the upper limit was made 0.30%.

Cu is the strength of the base metal without adversely affecting the weldability,
It is an extremely preferable element that improves the toughness and toughness of the weld zone, and also increases the strength of bainitic steel by precipitation hardening. Further, it is effective in corrosion resistance to the environment and hydrogen-induced cracking resistance. However, if less than 0.05%, the effect is not sufficiently obtained, and if added in a large amount, Cu is added during hot rolling of steel.
-Cracks occur, making manufacturing difficult. Therefore, the upper limit was made 0.30%.

Even if the component system is limited as described above, if the heating and rolling conditions are unsuitable, excellent strength and toughness cannot be obtained, so the heating and rolling conditions are also limited.

As described above, the toughness of bainite steel is highly dependent on the grain size, and unless the rolling structure is sufficiently refined, sufficient low temperature toughness cannot be secured.

For this purpose, the lower limit of the heating temperature was first limited to 1100 ° C. The reason for this is that if the temperature is lower than 1100 ° C., Ti does not form a solid solution sufficiently and the amount of TiC (N) reprecipitated during rolling is small, so that the γ-grain recrystallization suppressing effect cannot be expected.

On the other hand, the upper limit was set to 1250 ° C. Above this temperature, the finely precipitated TiN in the steel slab began to coarsen, and heated γ grains and H
This is because the miniaturization effect of AZ cannot be expected sufficiently.

However, it is difficult to manufacture a steel sheet having high strength and excellent low temperature toughness simply by rolling, no matter how fine the heated γ grains are. Therefore, the rolling conditions are also limited.

In the present invention, as a rolling condition, a cumulative rolling reduction of 900 ° C or less is set to 6
0% or more and finishing temperature limited to 680-800 ℃. If this condition is followed, the strength and toughness of the steel sheet will be greatly improved.

 The reasons for limiting the rolling conditions will be described below.

First, when the cumulative rolling reduction at 900 ° C or lower is 60% or more, the ferrite grains are remarkably refined and the strength and toughness are significantly improved. However, if the cumulative rolling reduction is less than 60%, high strength and excellent toughness cannot be secured. On the other hand, even if the cumulative rolling reduction at 900 ° C or lower is 60% or more, a steel sheet having remarkably excellent strength and toughness cannot be manufactured at a finishing temperature of 800 ° C or higher.

By setting the finishing temperature to less than 800 ° C., the refining of ferrite grains is remarkably promoted, and both strength and toughness can be improved or the strength can be improved without degrading the toughness.

Further, if the composition range of the steel of the present invention, the heating and rolling conditions,
The low temperature toughness is good even after rolling a considerable amount in the ferrite / austenite region or in the ferrite region, and it is effective for increasing strength, but the lower limit of the finishing temperature is 680 ° C.
When it is below, work hardening becomes remarkable and toughness starts to deteriorate. Therefore, the finishing temperature was limited to 680-800 ° C.

Regarding cooling after rolling, APIX70 class steel sheets with excellent strength, elongation and toughness can be produced by air cooling, but spray water,
Accelerated cooling with mist or air is effective for bainite and grain refinement of the rolling structure. In this case, a cooling rate of 10-40 ° C / sec is desirable.

However, if the water cooling stop temperature is 550 ° C or higher, it is difficult to secure APIX80 class strength, and if it is 350 ° C or lower, it is difficult to secure toughness. For this reason, in the case of accelerated cooling, if cooling is stopped at 550 to 350 ° C and then air cooling is performed, a steel sheet excellent in strength and toughness can be produced, so the water cooling stop temperature was limited to 550 to 350 ° C.

The method for producing the billet of the present invention may be either an ingot method or a continuous casting method, but the continuous casting method is more preferable because the cooling rate is fast and a large amount of fine TiN can be obtained. Further, as the hot rolling method, hot strip rolling, thick plate rolling, shaped steel rolling and the like are adopted.

(Examples) Table 1 shows the results of examining the mechanical properties of the base metal and welds by producing steel plates (thickness: 15 to 17 mm) of various steel components produced by the converter-continuous continuous casting-rolling process. Show.

Steels 1 to 12 in Table 1 are steels of the present invention, and steels 13 to 18 are comparative steels. Steel 13 has C higher than the upper limit, Steel 14 has Ti less than the lower limit, Steel 15 has B less than the lower limit, Steel 15
16 is steel with a cumulative rolling reduction of 900 ° C or less below the lower limit, steel 17
Indicates that the water-cooling stop temperature is low, and Steel 18 indicates that the cumulative rolling reduction of 900 ° C or lower is lower than the lower limit and the finishing temperature is high.

The toughness of the welded portion is the value at -20 ° C at the portion where the weld metal and the heat-affected zone are 50:50 after performing the inner and outer surface pipe welding corresponding to the heat input of 30 kJ / cm.

The steel of the present invention has good low temperature toughness and weld toughness despite high strength. On the other hand, the comparative steel does not satisfy both strength and toughness.

(Effects of the Invention) The present invention makes it possible to mass-produce large-diameter steel pipe at low cost. As a result, the construction period of line pipes has been shortened and oil and natural gas can be economically transported in cold regions.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hiroshi Taehiro 1 Kimitsu, Chiba Prefecture Kimitsu 1 Nippon Steel Co., Ltd. Kimitsu Works (56) References JP-A-58-77528 (JP, A) JP-A-SHO 61-127814 (JP, A) JP-A-60-56019 (JP, A)

Claims (2)

[Claims] 1. By weight%, C: 0.04 to 0.12%, Si: 0.5% or less, Mn: 1.0 to 2.0%, P: 0.03% or less, S: 0.01% or less, Ti: 0.03 to 0.10%, B: 0.0005 ~ 0.0020%, Al: 0.05% or less, N: 0.005% or less, with the balance being steel and unavoidable impurities.
After heating to a temperature range of 1100 to 1250 ° C and rolling at a cumulative reduction of 60% or more at 900 ° C or less and a finishing temperature of 680 to 800 ° C, air cooling or 550 to 350 at a cooling rate of 10 to 40 ° C / sec. A method for producing a high-strength steel sheet, which comprises accelerating cooling to a temperature of ℃, and then performing air cooling. 2. C: 0.04 to 0.12% by weight, Si: 0.5% or less, Mn: 1.0 to 2.0%, P: 0.03% or less, S: 0.01% or less, Ti: 0.03 to 0.10%, B: 0.0005 ~ 0.0020%, Al: 0.05% or less, N: 0.005% or less, and Cr: 0.05 to 0.30%, Cu: 0.05 to 0.30%, one or two kinds, with the balance being iron and inevitable impurities. The piece is heated to a temperature range of 1100 to 1250 ℃, and the cumulative rolling reduction of 900 ℃ or less is 60% or more, and the end temperature is 680 to 80.
After rolling at 0 ℃, air cooling or cooling rate 10-40 ℃ /
A method for producing a high-strength steel sheet, which comprises accelerating cooling to a temperature of 550 to 350 ° C in sec and then air cooling.