JPH09316534A - Production of high strength steel excellent in toughness at low temperature and having weldability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an ultrahigh strength steel for welding, excellent in balance between strength and toughness at low temp. and easy of site welding, by subjecting a steel slab, in which C content of reduced and a composition and a P value are respectively specified, to reheating, rolling, and cooling under respectively specified conditions. SOLUTION: The steel has a composition which consists of, by weight, 0.05-1.10% C, <=0.6% Si, 1.7-2.2% Mn, <=0.015% P, <=0.003% S, 0.1-1.0% N. 0.15-0.50% Mo, 0.01-0.10% Nb, 0.0003-0.0020% B, 0.005-0.030% Ti, <=0.06% Al, 0.001-0.006% N, and the balance Fe with inevitable impurities and the P value represented by the equation is regulateduced to 2.5-4.0. A slap of steel with this composition is reheated to 950-1250 deg.C at >=700 deg.C steel stock temp. so that cumulative draft at 700-950 deg.C becomes >=50%. Subsequently, cooling is performed and rolled at >=10 deg.C/sec cooling rate down to <=550 deg.C. Then, if necessary, the steel is further tempered at a temp. not higher than the Ac1 , transformation point.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an ultra-high strength steel having a tensile strength (TS) of 950 MPa or more and excellent low temperature toughness and weldability, including line pipes for transporting natural gas and crude oil, and various pressures. Can be widely used as a welding steel material for containers, industrial machinery, etc.

[0002]

2. Description of the Related Art In recent years, line pipes used for long-distance pipelines of crude oil and natural gas are (1) improved in transportation efficiency by increasing pressure and (2) locally reduced in outer diameter and weight of the line pipe. There is a tendency for strength to become even higher in order to improve construction efficiency. To date, the American Petroleum Institute (AP
I) Standard line pipes up to X80 (tensile strength 620 MPa or more) have been put into practical use, but there is a growing need for line pipes with even higher strength. Current,
Research on ultra-high-strength linepipe manufacturing methods is based on conventional X80 linepipe manufacturing technology (for example, NKK Technical Report No. 138 (199).
2), pp24-31 and The 7th Offshore Mechanics and A
rctic Engineering (1988), Volume V, pp179-185) is being studied, but this is at most X100.
(Tensile strength of 760 MPa or more) The production of line pipes is considered to be the limit. The ultra-high-strength pipeline has many problems such as the balance between strength and low-temperature toughness, as well as the weld heat-affected zone (HAZ) toughness, local weldability, and softening of joints. There is a demand for early development of strength line pipes (X100 or more).

[0003]

The present invention has a good balance between strength and low temperature toughness, and has a tensile strength that facilitates on-site welding.
It is intended to provide a method for producing an ultra-high strength steel for welding having a pressure of 50 MPa or more (API standard X100 or more).

[0004]

[Means for Solving the Problems] The present inventors have found that the chemical composition (composition) of a steel material and its composition for obtaining an ultra-high strength steel having a tensile strength of 950 MPa or more and excellent low temperature toughness and field weldability. Through intensive research on the microstructure, the inventors have invented a new method for producing ultra-high strength welding steel.

That is, the gist of the present invention is that, in weight%, C: 0.05 to 0.10%, Si: 0.6%.
Hereinafter, Mn: 1.7 to 2.2%, P: 0.015% or less, S: 0.003% or less, Ni: 0.1 to 1.0%
%, Mo: 0.15 to 0.50%, Nb: 0.01 to
0.10%, B: 0.0003 to 0.0020%, T
i: 0.005 to 0.030%, Al: 0.06% or less, N: 0.001 to 0.006%, V: 0.01 to 0.10%, Cu: 0.1-
1.0%, Cr: 0.1-0.6%, 1 or 2 or more kinds, or further added Ca: 0.001-
0.006, REM: 0.001-0.002%, M
g: 0.001 to 0.006% of 1 type or 2 types or more, and P = 2.7C + 0.4Si + Mn + 0.8Cr + 0.4
5 (Ni + Cu) +2 Mo satisfies 2.5 ≦ P ≦ 4.0, and the balance of steel consisting of iron and inevitable impurities is reheated to 950 to 1250 ° C., and the cumulative reduction at 700 to 950 ° C. After rolling at a steel material temperature of 700 ° C or more so that the amount becomes 50% or more, 1
A method for producing a weldable high-strength steel having excellent low-temperature toughness, which comprises cooling to 550 ° C or less at a cooling rate of 0 ° C / sec or more. Further, it is a method for producing a weldable high-strength steel having excellent low-temperature toughness, which is characterized by tempering this steel at a temperature of A _c1 point or less as required.

[0006]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the contents of the present invention will be described in detail. The features of the present invention are (1) Ni-Mo-Nb.
-A trace amount B-a low carbon amount and a high Mn type substance in which a small amount of Ti is added in combination, and (2) to obtain a structure mainly composed of fine martensite by selecting specific manufacturing conditions. Conventionally, extremely low carbon-high Mn-Nb- (Mo)-(Ni)-
Trace B-trace Ti steel is known as a steel for line pipes having a fine bainite-based structure, but the upper limit of its tensile strength was 750 MPa at the most.

In order to achieve an ultrahigh strength of 950 MPa or more, it is necessary to suppress the formation of ferrite by making the microstructure of steel a structure mainly composed of martensite. Good low temperature toughness cannot be secured. That is, it is important to combine an appropriate chemical component and a manufacturing method that can obtain a desired microstructure with the chemical component.

First, the reasons for limiting the constituent elements of this embodiment will be described below. The amount of C is limited to 0.05 to 0.10%. Carbon is extremely effective in improving the strength of steel, and at least 0.05% is necessary to obtain the target strength in the martensitic structure. However, if the amount of C is too large, the low temperature toughness of the base material and HAZ and the field weldability will be significantly deteriorated, so the upper limit was made 0.10%. Further, the upper limit is preferably 0.08%.

[0009] Si is an element added for deoxidation and strength improvement, but if added in a large amount, HAZ toughness and field weldability are significantly deteriorated, so the upper limit was made 0.6%. Deoxidation of steel is sufficiently possible with Al or Ti, and Si is not necessarily added. Mn is an essential element for ensuring a good balance between strength and low temperature toughness by making the microstructure of the steel of the present invention a structure mainly composed of martensite, and the lower limit thereof is 1.7%. However, if Mn is too much, not only the hardenability of the steel increases and HAZ toughness and field weldability deteriorate, but also the center segregation of the continuously cast steel slab is promoted and the low temperature toughness of the base material also deteriorates. It was set to 2.2%.

The purpose of adding Ni is to improve the low carbon steel of the present invention without deteriorating the low temperature toughness and field weldability. Compared to Mn, Cr, and Mo additions, addition of Ni is less likely to form a rolling structure (especially the central segregation zone of continuously cast steel pieces), especially a hardened structure detrimental to low temperature toughness. It has been found that the addition of a small amount of Ni is also effective in improving the HAZ toughness (the amount of Ni added which is particularly effective in terms of HAZ toughness is 0.3% or more). However, if the addition amount is too large, not only the economical efficiency but also the HAZ toughness and on-site weldability are deteriorated, so the upper limit was made 1.0%. Further, addition of Ni is also effective for preventing Cu cracking during continuous casting and hot rolling. In this case, Ni is Cu
It is necessary to add 1/3 or more of the amount.

The reason for adding Mo is to improve the hardenability of steel and to obtain the target structure mainly composed of martensite. In the B-added steel, the effect of improving the hardenability of Mo is enhanced, and the multiple of Mo in the P value described later is 1 for the non-B-added steel and 2 for the B-added steel.
o addition is particularly effective. Further, Mo coexists with Nb to suppress recrystallization of austenite during controlled rolling, and is also effective for refining the austenite structure. In order to obtain such effects, Mo must be at least 0.15%.
However, excessive addition of Mo deteriorates the HAZ toughness and field weldability, and the quenching property improving effect of B may disappear, so the upper limit was made 0.5%.

[0012] B is an essential element in the steel of the present invention in order to dramatically improve the hardenability of the steel in a very small amount and to obtain the target structure mainly composed of martensite. There is an effect equivalent to 1 in the P value described later, that is, equivalent to 1% Mn. Further, B enhances the hardenability improving effect of Mo and, together with Nb, synergistically increases the hardenability. In order to obtain such an effect, B is at least 0.
0003% is required. On the other hand, if added excessively, not only the low temperature toughness is deteriorated, but also the hardenability improving effect of B may be lost, so the upper limit is set to 0.
It was set to 0020%.

In the steel of the present invention, N is an essential element.
b: 0.01 to 0.10%, Ti: 0.005 to 0.0
Contains 30%. Nb coexists with Mo and not only suppresses recrystallization of austenite during controlled rolling to refine the structure, but also contributes to precipitation hardening and hardenability increase and strengthens steel. In particular, coexistence of Nb and B synergistically enhances the hardenability improving effect. However, if the amount of Nb added is too large,
The HAZ toughness and field weldability are adversely affected, so the upper limit was made 0.1%. On the other hand, Ti addition is fine TiN
To suppress the coarsening of the austenite grains of the HAZ during reheating of the slab and to refine the microstructure and improve the low temperature toughness of the base material and the HAZ. It also has a role of fixing solid solution N, which is harmful to the effect of improving the hardenability of B, as TiN. For this purpose, it is desirable to add Ti in an amount of 3.4 N (each weight%) or more. Further, when the amount of Al is small (for example, 0.005% or less), Ti forms an oxide and acts as an intragranular ferrite formation nucleus in the HAZ, which also has the effect of refining the HAZ structure. In order to bring out such an effect of TiN, at least 0.005
% Ti addition is required. However, if the amount of Ti is too large, coarsening of TiN and precipitation hardening due to TiC occur and the low temperature toughness deteriorates, so the upper limit was set to 0.03%.

Al is an element usually contained in steel as a deoxidizing material, and also has an effect of refining the structure. However, if the amount of Al exceeds 0.06%, Al-based nonmetallic inclusions increase and impair the cleanliness of steel, so the upper limit was made 0.06%. However, deoxidation is also possible with Ti or Si, and Al does not necessarily have to be added. N forms TiN and suppresses coarsening of austenite grains of the HAZ during reheating of the slab and improves low temperature toughness of the base material and HAZ. The minimum required for this is 0.001%. However, if the amount of N is too large, it causes deterioration of the HAZ toughness due to slab surface defects and solid solution N, and a decrease in the hardenability improving effect of B. Therefore, the upper limit must be suppressed to 0.006%. Further, in the present invention, the amounts of P and S which are impurity elements are 0.0
15% and 0.003% or less. The main reason for this is to further improve the low-temperature toughness of the base material and the HAZ. Reduction of the amount of P reduces the center segregation of the continuously cast slab, prevents grain boundary fracture, and improves the low temperature toughness. Further, the reduction of the amount of S has an effect of reducing MnS which is drawn by hot rolling and improving the ductility and toughness.

Next, V, Cu, Cr, Ca, REM,
The purpose of adding Mg will be described. The main purpose of adding these elements to the basic composition is to further improve the strength and toughness and expand the size of steel that can be manufactured without impairing the excellent characteristics of the steel of the present invention. Therefore, the amount added is of a nature that should be limited by itself. V has almost the same effect as Nb, but its effect is weaker than that of Nb. However, the effect of V addition in the ultra-high strength steel is great, and the combined addition of Nb and V makes the excellent characteristics of the steel of the present invention more remarkable. From the viewpoint of HAZ toughness and on-site weldability, the upper limit is 0.10%, but in the range of 0.03 to 0.08% is particularly desirable.

Cu increases the strength of the base material and the welded portion, but if it is too much, it significantly deteriorates HAZ toughness and field weldability. Therefore, the upper limit of the amount of Cu is 1.0%. Cr increases the strength of the base metal and the welded part, but if it is too much, it becomes HAZ.
Remarkably deteriorates toughness and field weldability. Therefore, the upper limit of the amount of Cr is 0.6%.

The lower limit of the amount of V, Cu, Cr is 0.01%, 0.
1% and 0.1% are the minimum amounts at which the effect on the material due to the addition of each element becomes remarkable. Ca and REM control the morphology of sulfide (MnS) and improve low temperature toughness (such as increasing absorbed energy in Charpy test). However, if the amount of Ca or the amount of REM is 0.001% or less, there is no practical effect, and the amount of Ca is 0.006% and the amount of REM is 0.
If added over 02%, CaO-CaS or REM
-CaS is produced in a large amount to form large clusters and large inclusions, which not only impairs the cleanliness of steel but also adversely affects the field weldability. Therefore, the upper limit of the amount of Ca added is set to 0.
The condition of 006% or REM addition amount was limited to 0.02%. In the ultra-high strength line pipe, the S and O contents were reduced to 0.001% and 0.002% or less, respectively, and E
It is particularly effective to set SSP = (Ca) [1-124 (O)] / 1.25S to 0.5 ≦ ESSP ≦ 10.0. Mg forms a finely dispersed oxide, suppresses grain coarsening in the weld heat affected zone, and improves low temperature toughness. 0.0
If it is less than 01%, the toughness is not improved, and if it is 0.006% or more, a coarse oxide is generated and conversely the toughness is deteriorated.

In addition to the above limitation of the individual additive elements, in the present invention, P = 2.7C + 0.4Si + Mn + 0.
8Cr + 0.45 (Ni + Cu) + 2Mo 2.5 ≦ P
Limit to ≤4.0. This is to achieve the desired strength / low temperature toughness balance. Lower limit of P value is 2.5
The reason is to obtain strength of 950 MPa or more and excellent low temperature toughness. The upper limit of the P value is set to 4.0 in order to maintain excellent HAZ toughness and field weldability.

Even with the above-mentioned chemical components, the desired characteristics cannot be obtained unless the production conditions are adequate to obtain a fine martensite-based structure. The principle method to obtain a fine martensite-based structure is to process recrystallized grains in the non-recrystallized temperature region to form flattened austenite grains in the plate thickness direction, which is cooled at a martensite critical cooling rate or higher. To cool.

In the present invention, the steel slab is reheated to 950 to 1250 ° C., rolled at a steel material temperature of 700 ° C. or higher so that the cumulative rolling reduction at 700 to 950 ° C. is 50% or higher, and then 1
Cool to 550 ° C or lower at a cooling rate of 0 ° C or higher. Further, if necessary, tempering is performed at a temperature below the A _c1 transformation point. The reasons for limiting the manufacturing conditions will be described below.

First, the reasons for limiting the reheating temperature of the billet will be described. If the temperature is lower than 950 ° C, the solid solution of elements is not sufficient,
The desired characteristics cannot be obtained and it becomes difficult to secure the rolling temperature. On the other hand, if the temperature exceeds 1250 ° C., the austenite crystal grains are remarkably coarsened, and it becomes difficult to refine the crystal grains even in the subsequent rolling. Therefore, the reheating temperature is 950 to 1250 ℃
And The reheated billet is usually roughly rolled to a thickness suitable for controlled rolling at 950 ° C or lower. At this time, 1050 ℃
When the above heating temperature is selected, the crystal grains are large, so it is desirable to perform rolling under the condition that the reheated grains are recrystallized and are finer than the reheated grains.

Next, the reasons for limiting the rolling conditions will be described. In order to obtain a fine structure, large processing is required under the condition that recrystallization does not occur. The steel of the present invention containing 0.01% or more of Nb and further containing Mo has a non-recrystallization temperature of 950 ° C. or lower. In order to refine the structure, a reduction of 50% or more at 950 ° C. or lower is required. is necessary. Further, the processing needs to be completed at 700 ° C. or higher at which ferrite is not formed. In order to obtain good low temperature toughness, the rolling end temperature is more preferably 800 ° C or lower. In order to obtain a desired structure mainly composed of martensite, it is necessary to cool at a cooling rate of 10 ° C./sec or more after rolling to a temperature of 550 ° C. or less at which the martensitic transformation is almost completed. 10 ° C./sec corresponds to the martensitic transformation critical cooling rate of a material having a small alloy content and a P value close to 2.5, and this cooling rate can be easily achieved. Even if the cooling rate becomes faster, the characteristics are slightly improved at the critical cooling rate or higher, and the upper limit of the cooling rate is determined by the heat conduction of the steel material, so the upper limit value is not particularly limited. At this time, if the cooling stop temperature is controlled to 550 to 350 ° C., it is possible to obtain the same effect as in the case where the following tempering treatment is performed by the double heat after transformation.

Further, if necessary, the steel produced by the above method may be tempered at a temperature not _higher than the A _c1 transformation point. The ductility is moderately restored by the tempering treatment. The tempering treatment does not change the microstructure fraction itself, does not impair the excellent features of the present invention, and has the effect of narrowing the softening width of the weld heat affected zone.

[0024]

EXAMPLES Next, examples of the present invention will be described. Slabs of various steel components were produced by laboratory melting (50 kg, 100 mm thick steel ingot) or converter-continuous casting method (240 mm thick). These cast pieces have a thickness of 15 to 25 m under various conditions.
The steel sheet was rolled into a steel sheet of m, and tempered in some cases to investigate various properties and microstructure. Mechanical properties of steel sheet (yield strength: YS, tensile strength: TS, Charpy test -40
The absorbed energy at vC: vE-40 and the 50% fracture surface transition temperature: vTrs) were investigated in the direction perpendicular to the rolling. HAZ toughness (absorbed energy at -40 ° C in Charpy test: v
E-40) was evaluated by the HAZ reproduced by a reproduction heat cycle device (maximum heating temperature: 1400 ° C, 800 to 500 ° C).
Cooling time [Δt800-500]: 25 seconds). The field weldability is the Y slit welding crack test (JIS G315
In 8), the minimum preheating temperature required for preventing low temperature cracking of HAZ was evaluated (welding method: gas metal arc welding, welding rod: tensile strength 100 MPa, heat input: 0.3 kJ / mm,
Hydrogen amount of deposited metal: 3 cc / 100 g metal).

Examples are shown in Tables 1 and 2. The steel sheet produced according to the method of the present invention has an excellent strength / low temperature toughness balance,
Shows HAZ toughness and field weldability. On the other hand, it is clear that the comparative steels are inferior in any one of the properties due to the improper chemical composition or microstructure.

[0026]

[Table 1]

[0027]

[Table 2]

[0028]

Industrial Applicability According to the present invention, it has become possible to stably manufacture a large amount of steel for super-strength line tape (tensile strength of 950 MPa or more, API standard X100 or more) excellent in low temperature toughness and field weldability. As a result, the safety of the pipeline is significantly improved, and the transportation efficiency of the pipeline,
It has become possible to dramatically improve construction efficiency.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yoshio Terada 1 Kimitsu, Kimitsu-shi, Chiba Nippon Steel Corp. Kimitsu Steel Co., Ltd.

Claims (4)

[Claims] 1. By weight%, C: 0.05 to 0.10%, Si: 0.6% or less, Mn: 1.7 to 2.2%, P: 0.015% or less, S: 0 0.003% or less, Ni: 0.1 to 1.0%, Mo: 0.15 to 0.50%, Nb: 0.01 to 0.10%, B: 0.0003 to 0.0020%, Ti : 0.005 to 0.030%, Al: 0.06% or less, N: 0.001 to 0.006%, the balance consisting of iron and unavoidable impurities, and a P value defined by the following formula Of steel in the range of 2.5 or more and 4.0 or less is reheated to 950 to 1250 ° C., and 700 to
Low temperature toughness characterized by being rolled at a steel material temperature of 700 ° C or higher so that the cumulative rolling reduction at a temperature of 950 ° C is 50% or higher, and then cooled to 550 ° C or lower at a cooling rate of 10 ° C / sec or higher. Of excellent weldability of high strength steel. P = 2.7 C + 0.4 Si + Mn + 0.8 Cr + 0.45 (Ni +
Cu) + 2Mo 2. In addition to the steel components of claim 1, in weight%, V: 0.01 to 0.10% Cu: 0.1 to 1.0% Cr: 0.1 to 0.6% 2. The method for producing a weldable high-strength steel with excellent low-temperature toughness according to claim 1, characterized in that it contains one or more of 3. In addition to the components according to claim 1 or 2, further, by weight%, Ca: 0.001 to 0.006%, REM: 0.001 to 0.02%, Mg: 0.001 to 0.006% of 1 type or 2 types or more are contained, The manufacturing method of the weldability high strength steel excellent in the low temperature toughness characterized by the above-mentioned. 4. A method for producing a weldable high-strength steel having excellent low-temperature toughness, which comprises tempering the steel produced by the method of claim 1, 2 or 3 at a temperature not higher than the A _c1 transformation point.