JPH0931536A - Production of ultrahigh strength steel plate excellent in low temperature toughness - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce an ultrahigh strength steel plate excellent in low temp. toughness and weldability by subjecting a low C high Mn-Nb-Ti-B slab having a specified compsn. to hot rolling and cooling under specified temp. conditions. SOLUTION: A slab having a compsn. contg., by weight, 0.05 to 0.10% C, <0.6% Si, 1.7 to 2.5% Mn, 0.1 to 1.0% Ni, 0.15 to 0.60% Mo, 0.01 to 0.10% Nb, 0.005 to 0.030% Ti, 0.0003 to 0.0020% B or the like and value of P defined by the formula is regulated to 2.5 to 4.0 is reheated to the low temp. region of austenite to hold the initial austenitic grains small and is subjected to hot rolling in such a manner that the cumulative rolling draft at <=800 deg.C is regulated to >=70%, the cumulative rolling draft in the two phase region of ferrite- austenite at the Ar3 point to the Ar1 point is regulated to 50 to 100% and the rolling finishing temp. is regulated to 650 to 800 deg.C to form into a hot rolled plate, which is thereafter air-cooled or cooled to <=500 deg.C at a cooling rate of >=10 deg.C/sec. The steel plate for an ultrahigh strength line pipe excellent in low temp. toughness and spot weldability and having >=950N/mm<2> tensile strength can be produced.

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 in low temperature toughness and weldability, and includes various line pipes for transporting natural gas and crude oil. It can be widely used as a welding steel material for pressure vessels and industrial machinery.

[0002]

[Prior Art] Line pipes used for pipelines for long-distance transportation of crude oil and natural gas tend to have higher tensile strength to improve transportation efficiency by increasing pressure and to improve welding efficiency at the site by reducing wall thickness. It is in. Until now, line pipes up to X80 (yield strength 551 MPa or more, tensile strength 620 MPa or more) according to American Petroleum Institute (API) standards have been put into practical use, but the need for line pipes with even higher strength has become stronger. Came.

At present, research on a method for manufacturing an ultra-high strength line pipe is carried out by a conventional X80 line pipe manufacturing technology (for example, N
KK Technical Report No. 138 (1992), pp24-31
And The 7th Offshore Mechanics and Arctic Engine
ering (1988), Volume V, pp179-
185), but this is at best
X100 (yield strength 689 MPa or more, tensile strength 760
Manufacturing 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, weld heat affected zone (HAZ) toughness, on-site weldability, and softening of joints. There is a demand for early development of a typical ultra-high strength line pipe (X100 or more).

[0005]

SUMMARY OF THE INVENTION The present invention has a tensile strength of 950 N / mm ² or more (API standard X1
The invention provides a manufacturing technology for a steel plate for ultra high strength line pipes (greater than 00).

[0006]

The gist of the present invention is as follows. % By weight, C: 0.05-0.
10%, Si: 0.6% or less, Mn: 1.7 to
2.5%, P: 0.015% or less, S:
0.003% or less, Ni: 0.1 to 1.0
%, Mo: 0.15 to 0.60%, Nb: 0.0
1 to 0.10%, Ti: 0.005 to 0.030%,
B: 0.0003 to 0.0020% Al: 0.06% or less, N: 0.001
To 0.006%, O: 0.003% or less, and if necessary, V: 0.01 to 0.10%, Cu: 0.1
~ 0.7%, Cr: 0.1-0.6%, Ca: 0.00
1 to 0.005% of one kind or two kinds or more, the balance consisting of Fe and unavoidable impurities, and P = 2.1
C + 0.4Si + Mn + 0.8Cr + 0.45 (Ni +
Cu) + 2Mo is 2.5 ≦ P ≦ 4.0 After reheating steel slabs satisfying a temperature of 950 to 1050 ° C. The, 800 ° C. or less of the cumulative reduction ratio is 70% or more, and A _r3 point to A _r1 point Of ferrite / austenite in the two-phase region is 50% or more, and rolling is performed so that the rolling end temperature is 650 to 800 ° C., and then air cooling or cooling at a cooling rate of 10 ° C. seconds or more is performed.
A method for producing an ultra-high strength steel sheet excellent in low temperature toughness, which comprises cooling to an arbitrary temperature of 00 ° C or less.

Hereinafter, the present invention will be described in detail. First,
The method for producing the ultra-high strength steel sheet of the present invention will be described. The features of the method of the present invention are (1) low C-high Mn-Nb-B-Ti.
After heating the system steel to the low temperature range of (2) austenite,
(3) Strictly controlled rolling in the austenite-ferrite two-phase region is followed by air cooling or accelerated cooling to form a fine microstructured ferrite + martensite / bainite mixed structure, which results in ultra-high strength and excellent It achieves low temperature toughness and field weldability at the same time. Further, the mixed structure of worked ferrite + martensite / bainite is also effective for softening the weld.

Conventionally, low C-high Mn-Nb-trace B-
A small amount of Ti steel is well known as a steel for line pipes having a fine bainite-based structure, but the upper limit of the tensile strength was 750 MPa at the most. In order to further increase the strength, it is necessary to (1) increase the amount of C and the amount of alloying elements, and (2) quench-temper from a high temperature of 900 ° C. or higher. The low temperature toughness is insufficient.

The present inventors have found that in Nb-B steel, ultrahigh strength and excellent low temperature toughness can be achieved by strictly controlling the chemical composition and heating / rolling / cooling conditions. The features of the steel of the present invention are (1) that excellent ultra-high strength and low temperature toughness can be obtained without tempering, (2) yield ratio is lower than that of quenched / tempered steel, formability of steel pipe, low temperature Remarkably excellent toughness and the like.
Further, in the present invention, even if the yield strength is low in the state of the steel sheet,
Yield strength is increased by steel pipe forming, and it is possible to obtain a target yield strength.

That is, in order to achieve an ultrahigh strength of 950 MPa or more in tensile strength, the microstructure of the steel material is set to a certain amount or more as a mixed structure of martensite bainite and ferrite, and work ferrite is introduced to dislocation strengthening. ,
Subgrains need to be strengthened.

Further, in order to achieve excellent low temperature toughness, the microstructure is refined and the introduction of processed ferrite causes separation on the fracture surface of the test piece such as the Charpy impact test, resulting in a dramatic transition temperature transition. Fall to. Here, the separation is a delamination phenomenon parallel to the plate surface that occurs during an impact test, and is considered to improve the propagation stopping property of the brittle crack by reducing the triaxial stress at the tip of the brittle crack. ing. From the above, the martensite of Nb-B steel, which was conventionally thought to be inferior in low temperature toughness,
We have succeeded in greatly improving the balance between strength and low temperature toughness of bainite and ferrite hard-soft mixed structure.

Next, the reasons for limiting the manufacturing conditions of the present invention will be described. In the method of the present invention, the steel pieces are 950 to 1050.
After reheating to a temperature range of ° C., 800 ° C. or less of the cumulative reduction ratio is 70% or more, and a ferrite of A _r3 point to A _r1 point - in the 50-100% cumulative reduction ratio of austenite 2-phase region, rolling end temperature Is 650 to 800 ° C.
After that, air cooling or cooling at a cooling rate of 10 ° C / sec or more,
Cool to below ℃.

The reheating temperature of the steel slab is 950 to 950.
It needs to be 1050 ° C. This is because the initial austenite grains during the reheating of the steel slab are kept small and the rolling structure is refined. Furthermore, the smaller the initial austenite grains, the easier the two-phase organization of fine ferrite-martensite occurs. 1050 ° C. is the upper limit temperature at which the austenite grains at the time of reheating do not become coarse. on the other hand,
If the heating temperature is too low, the alloying elements will not be sufficiently solutionized,
The desired material cannot be obtained. Further, since it requires a long period of time to uniformly heat the steel slab, and further, the deformation resistance during rolling increases, the energy cost increases, which is not preferable. Therefore, the lower limit of the reheating temperature is set to 950 ° C.

[0014] reheated steel slab 800 ° C. or less of a cumulative reduction ratio of 70% or more, and a ferrite of A _r3 point to A _r1 point - cumulative reduction ratio of austenite 2-phase region is 50 to 100%
It must be rolled so that the rolling end temperature is 650 to 800 ° C. The reason why the cumulative reduction amount at 800 ° C or less is 70% or more is to strengthen rolling in the austenite unrecrystallized region, to refine the austenite structure before transformation, and to make the structure after transformation ferrite-martensite bainite. This is because it is a mixed tissue of. Tensile strength is 950
The ultra-high-strength line pipe having a pressure of MPa or more requires higher toughness than ever before, especially for safety, so the cumulative reduction amount must be 70%. The larger the cumulative reduction amount is, the more preferable it is, and the upper limit is not limited.

Further, in the method of the present invention, the cumulative reduction amount in the ferrite-austenite two-phase region is set to 50 to 100%, and the rolling end temperature is set to 650 to 800 ° C. This is because the austenite structure finely grained in the unrecrystallized austenite region is further refined, and the ferrite is processed to strengthen the ferrite and facilitate the occurrence of separation during the impact test. When the cumulative reduction amount in the two-phase region is 50% or less, the occurrence of separation is insufficient and the propagation stopping property of brittle cracks cannot be improved. On the other hand, even if the cumulative reduction amount is appropriate, excellent low temperature toughness cannot be achieved if the rolling temperature is inappropriate. When the rolling end temperature is 650 ° C. or less, the embrittlement of ferrite due to working becomes significant, so the lower limit of the rolling end temperature was set to 650 ° C. However, if the rolling end temperature is 800 ° C. or higher, refinement of the austenite structure and occurrence of separation are not sufficient, so the upper limit of the rolling end temperature is limited to 800 ° C.

After completion of rolling, the steel plate is air-cooled or 10
It is necessary to cool to an arbitrary temperature of 500 ° C. or less at a cooling rate of ° C./sec or more. In the steel of the present invention, a mixed structure of martensite / bainite and ferrite can be obtained even if air-cooled after rolling, but in order to further increase the strength, it is cooled to an arbitrary temperature of 500 ° C or less at a cooling rate of 10 ° C / sec or more. It doesn't matter. The reason for cooling at a cooling rate of 10 ° C./second or more is to enhance transformation by forming martensite and refine the structure. If the cooling rate is 10 ° C / sec or less or the water cooling stop temperature is 500 ° C or more,
It cannot be expected that the balance between strength and low temperature toughness will be improved by transformation strengthening.

Next, the reasons for limiting the constituent elements will be described. The lower limit of 0.05% of C is the minimum amount for ensuring the strength and low temperature toughness of the base material and the welded portion, and exerting the precipitation hardening effect by the addition of Nb and V and the refinement of crystal grains. However, if the C content is too large, the low temperature toughness, the field weldability and the sour resistance are significantly deteriorated, so the upper limit was made 0.10%.

Si is an element added to deoxidize and improve the strength, but if added in a large amount, it deteriorates the field weldability and the weld heat affected zone (HAZ) toughness, so the upper limit was made 0.6%. Only Ti or Al is sufficient for deoxidizing steel,
Si need not always be added.

Mn is an essential element for ensuring strength and low temperature toughness, and its lower limit is 1.7%, preferably 1.8%. However, if the Mn content is too high, not only the hardenability of the steel increases and the field weldability and HAZ toughness deteriorate, but also the center segregation of the continuously cast steel slab is promoted and the low temperature toughness also deteriorates, so the upper limit is 2. It was set to 5%.

Ni is added to improve the strength of the low carbon steel of the present invention without deteriorating the low temperature toughness and field weldability. It has been found that the addition of Ni rarely forms a hardened structure detrimental to the low temperature toughness and sour resistance in the rolling structure (especially the central segregation zone of the slab) and increases the strength as compared with the addition of Mn, Cr and Mo. did. However, if the addition amount is too large, not only the economical efficiency but also the on-site weldability and HAZ toughness are deteriorated, so the upper limit is 1.0% and the lower limit is 0.
1%. Ni is C during continuous casting and hot rolling
It is also effective in preventing u cracks. In this case, Ni is C
It is necessary to add 1/3 or more of the amount of u.

Mo is added to improve the hardenability of steel. In addition, Mo coexists with Nb and strongly suppresses austenite recrystallization during controlled rolling, and is also effective in refining the austenite structure. In order to obtain such an effect, Mo must be at least 0.15%. However, excessive addition of Mo deteriorates HAZ toughness and field weldability, so the upper limit was made 0.6%.

B is an essential element in the present invention because it dramatically enhances the hardenability of steel with a very small amount. P below
There is an effect equivalent to 1 in value, 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 should be at least 0.00
03% is required. On the other hand, if it is added excessively, it not only deteriorates the low-temperature toughness, but also sometimes loses the effect of improving the hardenability of B.
20%.

Nb contributes to grain refinement and precipitation hardening in controlled rolling, and has the effect of strengthening the steel. However, if Nb is added in an amount of 0.10% or more, on-site weldability and HAZ toughness are adversely affected.
0%.

Ti forms fine TiN, suppresses coarsening of austenite grains in the slab reheating and in the welded HAZ to refine the microstructure, and improves the low temperature toughness of the base metal and HAZ. In order to exert such an effect of TiN, at least 0.005% of Ti must be added. However, if the Ti content is too large, TiN becomes coarse and T
Since precipitation hardening due to iC occurs and low temperature toughness deteriorates,
The upper limit must be limited to 0.030%.

Al is an element usually contained in steel as a deoxidizing agent, and has an effect on the refinement of the structure. However, A
When the amount of l exceeds 0.06%, Al-based nonmetallic inclusions increase and impair the cleanliness of steel, so the upper limit was made 0.06%. Deoxidation can be performed with Ti or Si, and Al need not always be added.

Further, in the present invention, the amounts of P, S and O which are impurity elements are 0.015% or less, respectively.
It is 003% or less and 0.003% or less. The main reason for this is to further improve the low temperature toughness of the base material and HAZ toughness. Reduction of the amount of P reduces center segregation of the continuously cast slab, prevents intergranular fracture, and improves low temperature toughness.
Further, the reduction of the amount of S has the effect of reducing the stretched MnS and improving the sour resistance and ductility. The reduction of the amount of O is effective in improving sour resistance and low-temperature toughness by reducing oxides in steel. Therefore, the lower the amount of P, S, and O, the better.

N forms TiN to suppress coarsening of austenite grains of the welded HAZ during reheating of the slab and improve low temperature toughness of the base metal and HAZ. The minimum required for this is 0.001%. However, if it is too large, it will cause deterioration of the HAZ toughness due to slab surface defects and solid solution N, so the upper limit must be suppressed to 0.006%.

Next, the reason for adding V, Cu, Cr and Ca will be explained. The main purpose of adding these elements to the above-mentioned basic components is to increase the manufacturable plate thickness and improve the properties such as strength and toughness of the base metal without impairing the excellent characteristics of the steel of the present invention. This is because. Therefore, the amount added is of a nature that should be limited by itself.

V has almost the same effect as Nb,
The effect is weak compared to 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. It was found that V was strain-induced precipitated by the processing of ferrite (hot rolling) and remarkably strengthened the ferrite. The lower limit is 0.
01%, the upper limit of which is allowable up to 0.10% from the viewpoint of field weldability and HAZ toughness.

Cu has an effect similar to that of Ni, and also has an effect of improving corrosion resistance and hydrogen-induced cracking resistance. Also, the strength is significantly increased by Cu precipitation hardening. However, if added in excess, the base material, H
Since the toughness of AZ decreases and Cu cracks occur during hot rolling, the upper limit was made 1.0%, preferably 0.7%.

[0031] Cr increases the strength of the base material and the welded portion, but if it is too large, the on-site weldability and HAZ toughness are significantly deteriorated. Therefore, the upper limit of the amount of Cr is 0.6%. C
The lower limit of 0.1% for the amounts of u and Cr is the minimum amount at which the effect of the addition of each element on the material becomes remarkable.

Ca controls the morphology of sulfide (MnS) and improves the low temperature toughness (such as increase of absorbed energy in Charpy test). In particular, in the steel of the present invention utilizing the separation in the impact test, the absorbed energy in the Charpy test and the like tends to decrease, so that the addition of Ca is essential. However, if the amount of Ca is 0.001% or less, there is no practical effect, and if added over 0.005%, a large amount of CaO-CaS is formed to form clusters and large inclusions, which only impairs the cleanliness of steel. Not only that, it also adversely affects the field weldability. Therefore, the Ca addition amount is set to 0.
It was limited to 001 to 0.005%. In the case of ultra-high strength steel, the amounts of S and O were reduced to 0.001% and 0.002% or less, respectively, and ESSP = (Ca) [1-124 (O)] /
It is particularly effective to set 125 (S) to 0.5 ≦ ESSP ≦ 10.0.

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 target balance between strength and low temperature toughness without impairing HAZ toughness and field weldability. The lower limit of P value is 2.5, which is 9
This is to obtain strength of 50 N / mm ² or more and excellent low temperature toughness. Also, the upper limit of P value of 4.0 is excellent HA
This is to maintain Z toughness and field weldability.

[0034]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to embodiments. Steel sheets were manufactured by a converter-continuous casting method from billets of various steel components by various manufacturing methods, and various properties were investigated.
The mechanical properties of the steel sheet were investigated in the direction perpendicular to the rolling. HA
The Z toughness was investigated by applying a reproduced heat cycle equivalent to a heat input of 5 kJ / mm (maximum heating temperature: 1400 ° C, 800 to 50).
Cooling time at 0 ° C .: 25 seconds).

The on-site weldability was evaluated in the Y-slit welding crack test (JIS G3158) at the minimum preheating temperature required to prevent low temperature cracking of HAZ (welding method: gas metal arc welding, welding rod: tensile strength). 100 MPa,
Heat input: 0.5 kJ / mm, Hydrogen content of deposited metal: 3 cc / 100
g).

Examples of the present invention are shown in Table 1 together with comparative examples. Steel sheet manufactured according to the method of the present invention (steel No.
1 to 8) have excellent strength and low temperature toughness. On the other hand, the comparative steels (Steel Nos. 9 to 22) have inadequate chemical composition or steel plate manufacturing conditions and are inferior in any of the properties.

Steel 9 is inferior in low-temperature toughness (Charpy absorbed energy, transition temperature) and HAZ toughness because the C content is too large, and also has a high preheating temperature during welding. Steel 10 is Mn
Since the amount and P value are too high, the base metal and HAZ toughness are inferior, and the preheating temperature during welding is also extremely high. Steel 11 is N
Since b is not added, the strength is slightly lower than the Nb-added steel, the Charpy transition temperature is high (the strength / low temperature toughness balance is poor), and the HAZ toughness is also poor.

Steel 12 does not contain Ti, so that
High Charpy transition temperature and poor HAZ toughness. Steel 13
Since the B content is too large, the low temperature toughness deteriorates.

Since the amount of B in steel 14 is too small, the target strength cannot be achieved. Steel 15 requires preheating during welding because the amount of Mo is too large.

Steel 16 has an appropriate chemical composition, but has a high Charpy transition temperature because the billet reheating start temperature in the production conditions is too high. Since the reheating temperature of the steel slab is too low, the steel 17 has insufficient strength and low strength. Steel 18 is 9
The low temperature toughness is a step forward because the cumulative reduction amount of 00 ° C or less is too small.

Steel 19 is austenite-ferrite 2
The Charpy transition temperature is high because the cumulative reduction in the phase region is too small. Steel 20 is inferior in low temperature toughness because there is no rolling in the two-phase region and the rolling end temperature is too high. Steel 21 is inferior in low temperature toughness because the rolling end temperature is too low. Steel 22
The strength is low because the water cooling stop temperature is too high.

[0042]

EFFECTS OF THE INVENTION According to the present invention, it becomes possible to stably manufacture an ultra-high strength steel sheet having excellent low temperature toughness and field weldability. As a result, the safety of the pipeline was significantly improved, and the construction efficiency and transportation efficiency of the pipeline were dramatically improved.

[0043]

[Table 1]

[0044]

[Table 2]

[0045]

[Table 3]

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location C22C 38/54 C22C 38/54 (72) Inventor Takuya Hara 1 Kimitsu, Kimitsu-shi Nippon Steel Kimitsu Works, Ltd.

Claims (2)

[Claims] 1. By weight%, C: 0.05 to 0.10%, Si: 0.6% or less, Mn: 1.7 to 2.5%, P: 0.015.
% Or less, S: 0.003% or less, Ni: 0.1
1.0%, Mo: 0.15 to 0.60%, Nb: 0.01 to
0.10%, Ti: 0.005-0.030%, B: 0.000
3 to 0.0020% Al: 0.06% or less, N: 0.001
~ 0.006%, O: 0.003% or less, the balance consisting of Fe and unavoidable impurities, and a P value defined by the following formula, a P value in the range of 2.5 to 4.0. After reheating to a temperature of 950 to 1050 ° C., 800
℃ following a cumulative reduction ratio of 70% or more, and the A _r3 point to A _r1 point ferrite - cumulative reduction ratio of austenite 2-phase region is 5
Tensile strength characterized by rolling at 0% or more so that the rolling end temperature becomes 650 to 800 ° C, and then cooling by air or at any cooling rate of 500 ° C or less at a cooling rate of 10 ° C / sec or more. Of 950 MPa or more, which is excellent in low temperature toughness, and a method for producing an ultra high strength steel sheet. P = 2.7C + 0.4Si + Mn + 0.8Cr + 0.4
5 (Ni + Cu) + 2Mo 2. By weight%, C: 0.05 to 0.10%, Si: 0.6% or less, Mn: 1.7 to 2.5%, P: 0.015
% Or less, S: 0.003% or less, Ni: 0.1
1.0%, Mo: 0.15 to 0.60%, Nb: 0.01 to
0.10%, Ti: 0.005-0.030%, B: 0.000
3 to 0.0020% Al: 0.06% or less, N: 0.001
-0.006%, O: 0.003% or less, and V: 0.01-0.1.
0%, Cu: 0.1 to 0.7%, Cr: 0.1 to 0.6
%, Ca: 0.001 to 0.005% of one kind or two or more kinds, and the balance of Fe and inevitable impurities, and the P value defined by the following formula is 2.1 to 3.0. After reheating the billet in the range to a temperature of 950 to 1050 ° C, 800
℃ following a cumulative reduction ratio of 70% or more, and the A _r3 point to A _r1 point ferrite - cumulative reduction ratio of austenite 2-phase region is 5
Tensile strength characterized by rolling at 0% or more so that the rolling end temperature becomes 650 to 800 ° C, and then cooling by air or at any cooling rate of 500 ° C or less at a cooling rate of 10 ° C / sec or more. Of 950 MPa or more, which is excellent in low temperature toughness, and a method for producing an ultra high strength steel sheet. P = 2.7C + 0.4Si + Mn + 0.8Cr + 0.4
5 (Ni + Cu) + 2Mo