JPH11140580A - Continuously cast slab for high strength steel excellent in toughness at low temperature, its production, and high strength steel excellent in toughness at low temperature - Google Patents

Abstract

PROBLEM TO BE SOLVED: To inhibit the coarsening of austenite on reheating of a cast slab and to obtain a high strength hot rolled steel plate having fine-grained structure and excellent toughness at low temperature by specifying the area ratio of transgranular transformed ferrite in martensitic and bainitic structures of a cast slab. SOLUTION: A steel composition, consisting of, by mass ratio, 0.03-0.10% C, <0.6% Si, 1.2 2.5% Mn, <0.005% P, <0.003% S, 0.1-1.0% Ni, 0.15-0.60% Mo, 0.005-0.10% Nb, 0.001-0.006% N, 0.005-0.006% Ti, and the balance essentially Fe and containing, if necessary, one or >=2 kinds among <0.10% Cr, <1.0% Cu, <0.10% V, 0.0005-0.0020% B, <0.006% Ca, <0.02% REM, <0.006% Mg, and <0.10% Zn, is provided. Simultaneously, a cast slab, in which the area ratio of transgranular transformed ferrite in martensitic and bainitic structures is regulated to >=10%, is provided. Further, this cast slab is formed into steel plate by controlled rolling.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a tensile strength of 77
The present invention relates to a continuous cast slab used for producing a hot-rolled steel material having a tensile strength (TS) of 0 MPa or more and excellent in low-temperature toughness, and a method for producing the same, and produced from such a continuous cast slab. Steel can be widely used as a weldable steel material for various pressure vessels, steel structures, etc., including line pipes for transporting natural gas and crude oil.

[0002]

2. Description of the Related Art High-strength steels generally contain a large amount of alloying elements and have good quenchability, so that the structure of continuous cast steel slabs used for the production thereof is suppressed in the formation of ferrite, and has a coarse structure corresponding to the cast structure. Martensite, bainite structure. When the slab having such a structure is reheated to the austenite region for hot working, when the heating rate is high, many nuclei are formed as in the usual transformation from ferrite to austenite, and fine austenite grains are obtained. Can be On the other hand, it is known that when the heating rate is low, the austenite grains corresponding to the original coarse cast austenite grains are transformed massively, and the crystal grains upon reheating become coarse grains similar to the cast structure. (Transactions ISIJ, Vol.25, 198
5, p311-317).

[0003] Generally, the heating rate during reheating of billets is low,
10 ° C./min or less. At such a heating rate, the structure at the time of reheating is several hundred μm inheriting the coarse grains of the cast structure.
To several thousand μm. Such coarse grains are difficult to recrystallize in the hot working step, and flat coarse grains remain after rolling without recrystallization.
It is well known that the presence of coarse grains in high-strength steel significantly lowers low-temperature toughness. Therefore, in order to improve the low-temperature toughness, it is essential to refine the crystal grains.

[0004] In order to recrystallize coarse grains by hot working to make them finer, it is necessary to perform one-pass large rolling with a rolling reduction of several tens% or more. However, it is difficult to perform such a large reduction with a large continuous cast slab, and a very powerful rolling mill or the like is required. On the other hand, it is also difficult to increase the heating rate of the steel slab in a normal atmosphere heating furnace.

[0005]

SUMMARY OF THE INVENTION According to the present invention, fine grained austenite is obtained at the time of reheating without using the equipment as described above, and as a result, recrystallization easily occurs during hot working,
As a result, the present invention provides a continuous cast slab, a method for producing the same, and a high-strength steel excellent in low-temperature toughness, from which high-strength steel having a fine-grained structure and excellent low-temperature toughness can be obtained.

[0006]

The present inventors have conducted intensive studies on the relationship between the structure of the continuous cast slab and the crystal grain size at the reheating temperature for hot working. It is found that if the intragranular transformed ferrite is present in the martensite and bainite in an area ratio of 10% or more, a large number of nuclei are generated during the reheating, and the crystal grains are refined at the reheating temperature. Was.

The present invention embodies this new finding, and the gist of the present invention is that (1) the area ratio of the intragranular transformed ferrite in the martensite and bainite structure is 10% or more. Continuous cast slab for high-strength steel excellent in low-temperature toughness characterized by (2) martensite, the area ratio of intragranular transformed ferrite in the bainite structure is 10% or more, and polygonal ferrite is generated. Continuous cast slab for high-strength steel with excellent low-temperature toughness,
(3) In mass%, C: 0.03-0.10%, Si: <0.6%, Mn: 1.2-2.5%, P: <0.015%, S: <0. 003%, Ni: 0.1-1.0%, Mo: 0.15-0.60%, Nb: 0.005-0.10%, N: 0.001-0.006%, Ti: 0 0.005% to 0.050%, Al: <0.06%, and, if necessary, Cr: <1.0%, Cu: <1.0%, V: <0.10%, B : 0.0005-0.0020%, Ca: <0.006%, REM: <0.02%, Mg: <0.006%, Zr: <0.10%, one or more of the following: Low-temperature toughness, characterized in that the area ratio of the intragranular transformed ferrite in the martensite and bainite structure is 10% or more in a steel containing iron and the balance being substantially iron. Continuous casting slab for the excellent high-strength steel,
(4) For high-strength steel according to (3), wherein the area ratio of the intragranular transformed ferrite in the martensite and bainite structures is 10% or more and polygonal ferrite is formed. Continuous cast slab of (5)
(3) or (4), a continuous cast slab for high-strength steel excellent in low-temperature toughness, characterized by Al: <0.004%, (6) mass%, C: 0.03% -0.10%, Si: <0.6%, Mn: 1.2-2.5%, P: <0.015%, S: <0.003%, Ni: 0.1-1.0 %, Mo: 0.15 to 0.60%, Nb: 0.005 to 0.10%, N: 0.001 to 0.006%, Ti: 0.005 to 0.050%, If necessary, Cr: <1.0%, Cu: <1.0%, V: <0.10%, B: 0.0005-0.0020%, Ca: <0.006%, REM: <0.02%, Mg: <0.006%, Zr: <0.10%, In a molten steel containing one or more of the following, and the balance being substantially iron, Al: < By the .004%,
A method for producing a continuous cast slab for high-strength steel excellent in low-temperature toughness, characterized in that the area ratio of intragranular transformed ferrite in a martensite and bainite structure is 10% or more.

(7) In (6), the area ratio of the intragranular transformed ferrite in the martensite and bainite structure is reduced by cooling the surface cooling rate during continuous casting between 800 and 500 ° C. for 100 minutes or less. % Or more, a method for producing a continuous cast slab for high-strength steel excellent in low-temperature toughness, wherein the slab of (8), (1) to (5) is reheated to the austenite region and controlled rolling is performed. And then cooled at a cooling rate equal to or higher than air cooling, and is a high-strength steel excellent in low-temperature toughness.

[0009] Here, martensite and bainite include those in a state where they are tempered.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the contents of the present invention will be described in detail. TS of 770MP as the object of the present invention
Continuous cast billets for high-strength steels exceeding a have martensite or bainant structure despite slow cooling rate due to high alloy content and high hardenability, and usually do not generate ferrite. . Since it is gradually cooled even after the transformation, martensite and bainite are in a tempered state. These are heated at a temperature of 10 ° C /
When heated to 1100 ° C. at a heating rate of 1 minute or less, the structure becomes an extremely coarse structure that inherits the structure of the continuous cast slab corresponding to the cast structure. However, it was found that when a small amount of ferrite was dispersed and formed in the structure of the continuous cast slab, it was refined. In order to study the relationship between the ferrite generation amount and the reheated crystal grain size, the following study was conducted.

First, the cooling pattern after casting was changed to change the amount of intragranular transformed ferrite. 6 ℃ /
The temperature was increased to 1100 ° C. per minute, and the mixture was rapidly cooled, and the crystal grain size was measured. As is apparent from the results of FIG. 1, when intragranular transformed ferrite is formed, the maximum crystal grain size upon reheating is 2.
When the grain size is reduced to 50 μm or less and the intragranular ferrite is formed in an area ratio of 10% or more, fine grains can be stably obtained. This is because austenite is nucleated from the interface between ferrite, martensite, and bainite, and intragranular transformed ferrite is uniformly dispersed and formed in crystal grains. Therefore, the intragranular transformed ferrite in the martensite and bainite structures needs to have an area ratio of 10% or more. The upper limit of the intragranular transformed ferrite generation ratio is not particularly specified, but this is because the effect of grain refinement is almost constant at 10% or more, and the ratio at which the intragranular transformed ferrite can be formed naturally has an upper limit. is there.

[0012] Polygonal ferrite is not easily produced with the chemical composition for high-strength steel as the object of the present invention. However, when the chemical composition is relatively low or when the cooling rate after casting is relatively low, May also produce polygonal ferrite. Polygonal ferrite coexists with intragranular transformed ferrite and has a slight grain refinement effect. Since this effect is additive, no lower limit is specified. The upper limit is also not specified because it naturally has a limit.
%. If such intragranular transformed ferrite is present in an area ratio of 10% or more, and in some cases polygonal ferrite is present, fine austenite grains are obtained even when reheated at a low heating rate.

Since a high-strength steel excellent in low-temperature toughness can be obtained with a specific chemical composition, the reason for limiting the chemical composition will be described below. C content is limited to 0.03 to 0.10%.
Carbon is extremely effective in improving the strength of steel, and at least 0.03% is necessary to obtain the target strength in the martensite structure. However, if the C content is too large, the low-temperature toughness and the on-site weldability of the base material and HAZ are remarkably deteriorated, so the upper limit is set to 0.10%.

[0014] Si is an element added for deoxidation and improvement of strength, but if added in a large amount, HAZ toughness and on-site weldability are significantly deteriorated, so the upper limit was made 0.6%. However, deoxidation of steel is sufficiently possible with Al or Ti,
Need not necessarily be added. Mn makes the microstructure of the steel of the present invention a structure mainly composed of lower bainite, and has excellent strength and strength.
It is an element indispensable for securing the balance of low-temperature toughness, and its lower limit is 1.2%. However, if the Mn content is too large, the hardenability of the steel is increased to deteriorate not only the HAZ toughness and the on-site weldability, but also the central segregation of the continuously cast steel slab and the low-temperature toughness of the base material. 2.5%
And

The purpose of adding Ni is to improve the low temperature toughness of the low carbon steel of the present invention without deteriorating the on-site weldability. Compared with the addition of Mn, Cr and Mo, Ni addition not only causes less formation of a hardened structure harmful to low-temperature toughness in the rolled structure (especially the central segregation zone of a continuously cast steel slab), but also has a content of 0.1% or more. It has been found that the addition of a small amount of Ni is also effective for improving the HAZ toughness. However, if the addition amount is too large, not only economic efficiency but also HAZ toughness and on-site weldability are degraded, so the upper limit was made 1.0%. Also, N
The addition of i is also effective in preventing Cu cracking during continuous casting and hot rolling. In this case, Ni needs to be added at least 1/3 of the Cu amount.

The reason for adding Mo is to improve the hardenability of steel and obtain the desired structure mainly composed of lower bainite. In the B-added steel, the effect of improving the hardenability of Mo is enhanced, and in the B-added steel, the addition of Mo is particularly effective. Further, Mo coexists with Nb to suppress recrystallization of austenite 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 Mo addition is
Since the AZ toughness and on-site weldability deteriorate, the upper limit is 0.6
0%.

In the steel of the present invention, N is an essential element.
b: 0.005 to 0.10%, Ti: 0.005 to 0.
Contains 050%. Nb coexists with Mo to suppress the recrystallization of austenite during controlled rolling, not only to refine the structure, but also to contribute to precipitation hardening and hardenability, and toughen the steel. In particular, when Nb and B coexist, the effect of improving hardenability increases synergistically. However, if the added amount of Nb is too large, it adversely affects HAZ toughness and on-site weldability.
The upper limit was set to 0.10%. On the other hand, the addition of Ti forms fine TiN, suppresses coarsening of austenite grains in the HAZ during reheating of the slab and refines the microstructure,
Improves low temperature toughness of base metal and HAZ. Also, it has a role of fixing solid solution N harmful to the effect of improving the hardenability of B as TiN. For this purpose, the Ti content is 3.4 N
(Each by weight). Also, A
When the amount of l is small (for example, 0.004% or less), Ti forms an oxide and acts as an intragranular transformed ferrite forming nucleus, which is particularly effective for the grain refinement for the purpose of the present invention. In order to exert 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, deteriorating low-temperature toughness. Therefore, the upper limit is limited to 0.050%.

Al is an element usually contained in steel as a deoxidizing material, and also has an effect on refining the structure. However, if the amount of Al exceeds 0.06%, Al-based nonmetallic inclusions increase and impair the cleanliness of the steel, so the upper limit was made 0.06%. However, deoxidation is possible with Ti or Si, and Al need not always be added. N forms TiN and suppresses coarsening of austenite grains in the slab during reheating and in the HAZ, thereby improving the low-temperature toughness of the base material and the HAZ. The minimum required for this is 0.001%. However, if the N amount is too large, it causes deterioration of the HAZ toughness due to slab surface flaws and solid solution N, and a decrease in the effect of improving the hardenability of B, so the upper limit must be suppressed to 0.006%.

Further, in the present invention, the contents of P and S as impurity elements are set to less than 0.015% and 0.003%, respectively. The main reason for this is to further improve the low-temperature toughness. The reduction of the P content reduces the segregation of the center of the continuously cast slab, prevents the grain boundary fracture, and improves the low-temperature toughness. Further, the reduction of the amount of S is achieved by Mn which is stretched by hot rolling.
This has the effect of reducing S and improving ductility.

Next, V, Cu, Cr, B, Ca, RE
The purpose of adding M, Mg, and Zr will be described. The main purpose of adding these elements to the basic components is to further improve the strength and toughness and expand the size of the steel material that can be manufactured without deteriorating the characteristics. Therefore, the amount of addition is of a nature that should be naturally restricted.

V has almost the same effect as Nb, but the effect is weaker than Nb. However, the effect of V addition on ultra-high strength steel is great, and the combined addition of Nb and V makes the excellent features of the steel of the present invention more remarkable. The upper limit is H
From the point of AZ toughness and on-site weldability, 0.10% is acceptable. Cu increases the strength of the base material and the welded portion, but if it is too large, the HAZ toughness and on-site weldability are significantly deteriorated. Therefore, the upper limit of the amount of Cu is 1.0%.

[0022] Cr increases the strength of the base material and the welded portion, but if too much, the HAZ toughness and the on-site weldability are remarkably deteriorated. For this reason, the upper limit of the amount of Cr is 1.0%. B is a very small amount to dramatically enhance the hardenability of steel, suppress the formation of upper bainite, and obtain a structure mainly composed of lower bainite.
It is a very effective element. There is an effect equivalent to 1% Mn. Further, B enhances the effect of improving the hardenability of Mo, and synergistically increases the hardenability together with Nb. In order to obtain such an effect, B must be at least 0.0005%. 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, so the upper limit was made 0.0020%.

Ca and REM control the sulfide (MnS) morphology and improve low-temperature toughness (such as an increase in the energy absorbed in the Charpy test). Ca content is 0.006%,
When REM is added in excess of 0.02%, CaO-CaS
Alternatively, a large amount of REM-CaS is generated to form large clusters and large inclusions, which not only impairs the cleanliness of steel but also adversely affects on-site weldability. Therefore, the upper limit of the amount of Ca added was limited to 0.006%, and the condition of the amount of REM added was limited to 0.02%.

Mg forms a finely dispersed oxide, suppresses grain coarsening of the heat affected zone by welding, and improves low temperature toughness.
If the content is 0.006% or more, a coarse oxide is formed and the toughness is deteriorated, so the upper limit is made 0.006%. Zr has the effect of forming a P compound to substantially reduce P and to improve low-temperature toughness. However, when the content exceeds 0.10%, a coarse oxide is formed, and the low-temperature toughness is rather deteriorated. Therefore, the addition amount is set to 0.10% or less.

A steel having the above-mentioned chemical composition is basically a high-strength steel having excellent low-temperature toughness. In such steels, by further reducing the Al content to less than 0.004%, finely dispersed Ti oxides are formed in ordinary continuous casting, and intragranular transformed ferrite is generated therefrom. When the surface temperature is cooled from 800 to 500 ° C. for 100 minutes or less, intragranular transformed ferrite is particularly likely to be formed.

As described above, when the slab of the present invention in which the intragranular transformed ferrite is present in an area ratio of 10% or more is reheated to the austenite region, the slab becomes fine-grained austenite. High strength steel is obtained.

[0027]

EXAMPLES Steel having the chemical composition shown in Table 1 was melted in a converter and cast into a 250 mm thick slab by continuous casting. These are 1
Reheat to 050 ° C (average heating rate from 600 ° C to 900 ° C is about 6 ° C / min), control roll from 100mm to 20mm at 950 ° C or lower, finish rolling at 740 ° C,
4 so that the average cooling rate at the center of the thickness is about 20 ° C / sec.
It was cooled to 00 ° C. to produce a steel sheet.

For these, the ferrite ratio in the slab, the crystal grain size observed by cooling as it is after reheating, the tensile test result (TS), the Charpy test result (vTrs) of the rolled steel sheet, and the measurement in the thickness direction Table 2 shows the obtained crystal grain sizes.

[0029]

[Table 1]

[0030]

[Table 2]

[0031]

According to the present invention, a high-strength steel having a fine-grained structure excellent in low-temperature toughness can be easily produced in the same hot working step as in the prior art.

[Brief description of the drawings]

FIG. 1 is a graph showing the amount of intragranular transformed ferrite produced in a slab;
It is a graph which shows the relationship with the crystal grain size after heating at 1100 degreeC.

 ────────────────────────────────────────────────── ─── Continued from the front page (72) Inventor Ryuji Uemori 20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel Corporation Technology Development Division

Claims (8)

[Claims] 1. A continuous cast slab for high-strength steel excellent in low-temperature toughness, characterized in that the area ratio of intragranular transformed ferrite in a martensite or bainite structure is 10% or more. 2. Continuous casting for high-strength steel excellent in low-temperature toughness, characterized in that the area ratio of intragranular transformed ferrite in the martensite and bainite structure is 10% or more and polygonal ferrite is formed. Slab. 3. In mass%, C: 0.03-0.10%, Si: <0.6%, Mn: 1.2-2.5%, P: <0.015%, S: < 0.003%, Ni: 0.1-1.0%, Mo: 0.15-0.60%, Nb: 0.005-0.10%, N: 0.001-0.006%, Ti : 0.005 to 0.050%, Al: <0.06%, and if necessary, Cr: <1.0%, Cu: <1.0%, V: <0.10% , B: 0.0005-0.0020%, Ca: <0.006%, REM: <0.02%, Mg: <0.006%, Zr: <0.10%, one or two of the following: In a steel containing the above and the balance being substantially iron, the area ratio of the intragranular transformed ferrite in the martensite and bainite structure is at least 10%. Continuous cast slab for high strength steel with excellent thermal toughness. 4. The method according to claim 3, wherein the area ratio of the intragranular transformed ferrite in the martensite and bainite structures is 10%.
A continuous cast slab for high-strength steel excellent in low-temperature toughness, characterized in that polygonal ferrite is formed as described above. 5. The method according to claim 3, wherein Al: <
A continuous cast slab for high-strength steel excellent in low-temperature toughness characterized by being 0.004%. 6. In mass%, C: 0.03-0.10%, Si: <0.6%, Mn: 1.2-2.5%, P: <0.015%, S: < 0.003%, Ni: 0.1-1.0%, Mo: 0.15-0.60%, Nb: 0.005-0.10%, N: 0.001-0.006%, Ti : 0.005 to 0.050%, and if necessary, Cr: <1.0%, Cu: <1.0%, V: <0.10%, B: 0.0005-0 .0020%, Ca: <0.006%, REM: <0.02%, Mg: <0.006%, Zr: <0.10%, and the balance is substantially In molten steel, which is primarily iron, by setting Al: <0.004%,
A method for producing a continuous cast slab for high-strength steel excellent in low-temperature toughness, characterized in that the area ratio of intragranular transformed ferrite in a martensite and bainite structure is 10% or more. 7. The area ratio of the intragranular transformed ferrite in the structure of martensite and bainite is reduced by cooling the surface cooling rate during continuous casting between 800 and 500 ° C. for 100 minutes or less. A method for producing a continuous cast slab for high-strength steel excellent in low-temperature toughness, characterized by the above. 8. An excellent low-temperature toughness characterized in that the slab according to any one of claims 1 to 5 is reheated to an austenite region, subjected to controlled rolling, and then cooled at a cooling rate higher than air cooling. High strength steel.