JP3898814B2 - Continuous cast slab for high strength steel with excellent low temperature toughness and its manufacturing method, and high strength steel with excellent low temperature toughness - Google Patents

Description

【００３０】
【表２】

【００３１】
【発明の効果】
本発明によれば、従来と同じ熱間加工工程で容易に低温靱性に優れた細粒組織の高強度鋼が製造可能になった。
【図面の簡単な説明】
【図１】図１は、鋳片の粒内変態フェライト生成量と、１１００℃加熱後の結晶粒径との関係を示すグラフである。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a continuous cast slab used for producing a hot rolled steel material having a tensile strength (TS) of 770 MPa or more and excellent in low temperature toughness, and a method for producing such a continuous cast slab. Steel produced from cast slabs can be widely used as weldable steel materials such as line pipes for transporting natural gas and crude oil, various pressure vessels, and steel structures.
[0002]
[Prior art]
High-strength steel generally contains a lot of alloying elements and has good hardenability. Therefore, the structure of continuous cast steel pieces used in its production is suppressed in the formation of ferrite, and coarse martensite and bainite corresponding to the cast structure are used. It is an organization. When a slab having such a structure is reheated to the austenite region for hot working, if the heating rate is high, there are many nucleations such as transformation from normal ferrite to austenite, and fine austenite grains are obtained. It is done. On the other hand, it is known that if the heating rate is slow, it transforms like austenite grains corresponding to the original coarse austenite grains, and the crystal grains during reheating become coarse grains similar to the cast structure. (Transactions ISIJ, Vol.25, 1985, p311-317).
[0003]
Generally, the heating rate at the time of reheating the steel slab is slow and is 10 ° C./min or less. At such a heating rate, there are coarse grains of several hundred μm to several thousand μm inheriting the coarse grains of the cast structure in the structure during reheating. Such coarse grains are difficult to recrystallize in a hot working step, and flat coarse crystal grains remain after rolling without being recrystallized. It is well known that low-temperature toughness deteriorates significantly when coarse grains are present in high-strength steel. 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 processing, a one-pass large reduction with a reduction ratio of several tens of percent or more is required. 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 difficult to increase the heating rate of the steel slab in a normal atmosphere heating furnace.
[0005]
[Problems to be solved by the invention]
In the present invention, fine austenite can be obtained during reheating without adapting to the equipment as described above, and as a result, recrystallization easily occurs during hot working, resulting in excellent low temperature toughness of the fine grain structure. The present invention provides a continuous cast slab from which a high strength steel can be obtained, a method for producing the same, and a high strength steel excellent in low temperature toughness.
[0006]
[Means for Solving the Problems]
As a result of intensive investigations on the relationship between the structure of the continuous cast slab and the crystal grain size at the reheating temperature for hot working, the present inventors found that the structure of the continuous cast slab is intragranular in martensite and bainite. It has been found that if the transformed ferrite is present in an area ratio of 10% or more, a large number of nuclei are generated during reheating, and the crystal grains at the reheating temperature become finer.
[0007]
The present invention embodies this new knowledge, and the gist of the present invention is as follows (1) to (6) .
(1) 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: 0.005-0.050%,
Al: <0.06%,
Containing, the balance iron and unavoidable impurities, a thickness of more than 200 mm, and a martensite, bainite area ratio of 70% or more state, and are the area ratio of the intragranular transformation ferrite 10% or more, balance continuous casting slab for which the area ratio 0-20% of polygonal high strength steels above excellent tensile strength 770MPa in low temperature toughness characterized by ferrite der Rukoto.
(2) mass%,
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%,
The continuous cast slab for high-strength steel having a tensile strength of 770 MPa or more and excellent in low-temperature toughness as described in (1) .
(3) A l: <, characterized in that 0.004% (1) or (2) continuous casting slab of a high-strength steel of more excellent tensile strength 770MPa to low-temperature toughness described.
( 4 ) It is a manufacturing method of the continuous cast slab of any one of (1)-(3), Comprising: 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: 0.005-0.050%,
In molten steel containing, balance of iron and unavoidable impurities, Al: after continuous casting with a <0.004% or, the resulting continuous casting slab, between the surface temperature of 800 to 500 ° C. 100 A method for producing a continuous cast slab for high-strength steel having a tensile strength of 770 MPa or more and excellent in low-temperature toughness, characterized by cooling in minutes or less .
(5) in mass%,
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%,
The method for producing a continuous cast slab for high-strength steel having a tensile strength of 770 MPa or more and excellent in low-temperature toughness as described in (4) , characterized by containing one or more of the following.
[0008]
(6) (1) is reheated to the austenite region of the continuous casting slab according to any one of - (3), the controlled rolling, in that it is manufactured and thereafter cooled in air over a cooling rate High strength steel with excellent low temperature toughness .
[0009]
Here, martensite and bainite include those in the state where these are tempered.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the contents of the present invention will be described in detail. In the continuous cast steel slab for high-strength steel having a TS exceeding 770 MPa, which is the object of the present invention, the alloy content is high and the hardenability is high. become wells organization, usually ferrite does not generate. Since it is gradually cooled after transformation, martensite and bainite are in a tempered state. When these are heated to 1100 ° C. at a heating rate of 10 ° C./min or less, which corresponds to heating during atmosphere heating with a steel piece having a thickness of 200 mm or more, for example, the structure in the continuous cast slab corresponding to the cast structure is obtained. It becomes a very coarse organization that has been taken over. However, it has been found that when a small amount of ferrite is dispersed in the structure of a continuous cast slab, it becomes finer. In order to examine the relationship between the amount of ferrite produced and the reheated crystal grain size, the following studies were conducted.
[0011]
First, the amount of intragranular transformed ferrite produced was changed by changing the cooling pattern after casting. This was heated to 1100 ° C. at 6 ° C./min, quenched, and the crystal grain size was measured. As is apparent from the results of FIG. 1, when intragranular transformed ferrite is generated, the crystal grain size at the time of reheating becomes finer with a maximum grain size of 250 μm or less. As a result, fine grains can be obtained. This is because austenite is nucleated from the interface between ferrite, martensite, and bainite, and intragranularly transformed ferrite is characterized by being uniformly dispersed in crystal grains. Therefore, the intragranular ferrite in the martensite and bainite structure needs to be 10% or more in terms of area ratio. The upper limit of the production ratio of intragranular transformed ferrite is not particularly specified, but this is because the effect of grain refinement is almost constant at 10% or more, and the rate at which intragranular transformed ferrite can be produced naturally has an upper limit. is there.
[0012]
Polygonal ferrite is difficult to form with the chemical composition for high-strength steel as intended by the present invention, but polygonal ferrite is used when the amount of chemical composition is relatively low or when the cooling rate after casting is relatively slow. Ferrite may also form. Polygonal ferrite coexists with intragranular transformed ferrite and has a slight grain refinement effect. Since this effect is additive, a lower limit value is not particularly defined. Also, the upper limit value is naturally not limited because it is naturally limited, but it is about 50%. When such intragranular ferrite is present in an area ratio of 10% or more, and polygonal ferrite is present in some cases, fine austenite grains are obtained even when reheated at a low heating rate.
[0013]
Since a high-strength steel excellent in low-temperature toughness can be obtained with a specific chemical component, the reason for limiting this chemical component will be described below.
The amount of C 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 amount of C is too large, the base material, HAZ low temperature toughness and on-site weldability are significantly deteriorated, so the upper limit was made 0.10%.
[0014]
Si is an element added for deoxidation and strength improvement, but if added in a large amount, the HAZ toughness and on-site weldability are remarkably deteriorated, so the upper limit was made 0.6%. However, deoxidation of steel can be performed with either Al or Ti, and Si does not necessarily have to be added.
Mn makes the microstructure of the steel of the present invention a structure mainly composed of lower bainite and is an indispensable element for ensuring an excellent balance between strength and low temperature toughness, and its lower limit is 1.2%. However, if Mn is too much, not only the hardenability of the steel will increase and the HAZ toughness and on-site weldability will deteriorate, but also the center segregation of the continuously cast steel slab will be promoted and the low temperature toughness of the base metal will also deteriorate, so the upper limit is set. 2.5%.
[0015]
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, the addition of Ni is less likely to form a hardened structure that is harmful to low temperature toughness in the rolled structure (especially the central segregation zone of the continuously cast steel slab). It has been found that the addition of a trace amount of Ni is also effective in improving the HAZ toughness. However, if the addition amount is too large, not only the economy but also the HAZ toughness and on-site weldability are deteriorated, so the upper limit was made 1.0%. Ni addition is also effective for preventing Cu cracking during continuous casting and hot rolling. In this case, Ni needs to be added by 1/3 or more of the amount of Cu.
[0016]
The reason for adding Mo is to improve the hardenability of the steel and to obtain the target lower bainite-based structure. 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. In addition, Mo coexists with Nb, suppresses recrystallization of austenite during controlled rolling, and is effective in refining the austenite structure. In order to obtain such an effect, Mo needs to be at least 0.15%. However, excessive Mo addition deteriorates the HAZ toughness and field weldability, so the upper limit was made 0.60%.
[0017]
Moreover, in this invention steel, Nb: 0.005-0.10% and Ti: 0.005-0.050% are contained as an essential element. Nb coexists with Mo to suppress recrystallization of austenite during controlled rolling and refine the structure, and also contributes to precipitation hardening and hardenability, thereby strengthening the steel. In particular, when Nb and B coexist, the effect of improving hardenability increases synergistically. However, if the amount of Nb added is too large, the HAZ toughness and on-site weldability are adversely affected, so the upper limit was made 0.10%. On the other hand, addition of Ti forms fine TiN, suppresses the coarsening of the austenite grains of the HAZ during reheating of the slab, refines the microstructure, and improves the low temperature toughness of the base material and the HAZ. Moreover, it has a role which fixes solid solution N harmful to the hardenability improvement effect of B as TiN. For this purpose, it is desirable to add Ti in an amount of 3.4 N (each by weight%) or more. Further, when the amount of Al is small (for example, 0.004% or less), Ti forms an oxide and acts as an intragranular ferrite transformation nucleus, which is particularly effective for the purpose of fine graining for the purpose of the present invention. In order to exhibit such an effect of TiN, it is necessary to add at least 0.005% Ti. However, if the amount of Ti is too large, TiN coarsening and precipitation hardening due to TiC occur and the low temperature toughness is deteriorated, so the upper limit was limited to 0.050%.
[0018]
Al is an element usually contained in steel as a deoxidizing material, and has an effect on refinement of the structure. However, if the amount of Al exceeds 0.06%, Al-based non-metallic inclusions increase to impair the cleanliness of the steel, so the upper limit was made 0.06%. However, deoxidation can be performed with Ti or Si, and Al need not necessarily be added.
N forms TiN and suppresses coarsening of the austenite grains of HAZ during reheating of the slab and improves the low temperature toughness of the base material and HAZ. The minimum amount required for this is 0.001%. However, if the amount of N is too large, it will cause deterioration of the HAZ toughness due to slab surface defects and solute N, and decrease in the effect of improving the hardenability of B, so the upper limit must be limited to 0.006%.
[0019]
Furthermore, in the present invention, the amounts of impurity elements P and S are made 0.015% and less than 0.003%, respectively. The main reason is to further improve the low temperature toughness. The reduction of the amount of P reduces the center segregation of the continuously cast slab and prevents the grain boundary fracture, thereby improving the low temperature toughness. Further, the reduction of the amount of S has the effect of reducing the MnS stretched by hot rolling and improving the ductility.
[0020]
Next, the purpose of adding V, Cu, Cr, B, Ca, REM, 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 steel size that can be produced without impairing the characteristics. Therefore, the amount of addition is naturally limited.
[0021]
V has almost the same effect as Nb, but the effect is weaker than 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 even more remarkable. The upper limit is allowable up to 0.10% in terms of HAZ toughness and field weldability.
Cu increases the strength of the base metal and the welded portion, but if too much, the HAZ toughness and on-site weldability are remarkably deteriorated. For this reason, the upper limit of the amount of Cu is 1.0%.
[0022]
Cr increases the strength of the base metal and the welded portion, but if too much, the HAZ toughness and on-site weldability are significantly deteriorated. For this reason, the upper limit of the Cr content is 1.0%.
B is an extremely effective element for dramatically increasing the hardenability of steel with a very small amount, suppressing the formation of upper bainite, and obtaining a structure mainly composed of lower bainite. There is an effect corresponding to 1% Mn. Further, B enhances the hardenability improvement effect of Mo, and synergistically increases the hardenability by coexisting with Nb. In order to obtain such an effect, B needs to be at least 0.0005%. On the other hand, if added excessively, not only the low-temperature toughness is deteriorated, but also the effect of improving the hardenability of B may be lost, so the upper limit was made 0.0020%.
[0023]
Ca and REM control the form of sulfide (MnS) and improve low-temperature toughness (such as an increase in absorbed energy in the Charpy test). When Ca content is 0.006% and REM exceeds 0.02%, a large amount of CaO-CaS or REM-CaS is formed, resulting in large clusters and large inclusions, not only harming the cleanliness of the steel, It also adversely affects on-site weldability. For this reason, the upper limit of the Ca addition amount is limited to 0.006% or the REM addition amount condition is limited to 0.02%.
[0024]
Mg forms finely dispersed oxides and suppresses the coarsening of the weld heat affected zone to improve the low temperature toughness. If it is 0.006% or more, a coarse oxide is produced and the toughness is deteriorated conversely, so the upper limit was made 0.006%.
Zr has the effect of forming a P compound to substantially reduce P and improve low temperature toughness. However, if the content exceeds 0.10%, a coarse oxide is formed and the low-temperature toughness is deteriorated.
[0025]
The steel having the chemical composition as described above basically becomes a high strength steel excellent in low temperature toughness.
In such steel, when the Al content is further less than 0.004%, finely dispersed Ti oxide is formed in normal continuous casting, and intragranular transformed ferrite is generated therefrom. When the surface temperature is lowered to 800 to 500 ° C. for 100 minutes or less, intragranular ferrite is particularly likely to be generated.
[0026]
As described above, when the slab of the present invention in which the intragranular ferrite is present in an area ratio of 10% or more is reheated to the austenite region, it becomes fine-grained austenite, and high strength with excellent low-temperature toughness when controlled rolling is performed from now on. Steel is obtained.
[0027]
【Example】
Steels having chemical components shown in Table 1 were melted in a converter, and 250 mm thick slabs were produced by continuous casting. These were reheated to 1050 ° C. (average heating rate from 600 ° C. to 900 ° C. was about 6 ° C./min), controlled rolling from 100 mm to 20 mm at a temperature of 950 ° C. or less, finished rolling at 740 ° C., The steel sheet was manufactured by cooling to 400 ° C. so that the average cooling rate at the thickness center was about 20 ° C./second.
[0028]
About these, the ferrite ratio in a slab, the crystal grain diameter observed by cooling as it is after reheating, the tensile test result (TS) of the steel sheet after rolling, the Charpy test result (vTrs), the crystal grain measured in the thickness direction The diameter is shown in Table 2.
[0029]
[Table 1]

[0030]
[Table 2]

[0031]
【The invention's effect】
According to the present invention, a high-strength steel having a fine-grained structure excellent in low-temperature toughness can be easily manufactured in the same hot working process as before.
[Brief description of the drawings]
FIG. 1 is a graph showing the relationship between the amount of intragranular transformed ferrite produced in a slab and the crystal grain size after heating at 1100 ° C. FIG.

Claims (6)

% By 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: 0.005-0.050%,
Al: <0.06%, the balance is made of iron and inevitable impurities, the thickness is 200 mm or more, the area ratio of martensite and bainite is 70% or more, and the area ratio of intragranular transformed ferrite There Ri der least 10%, continuous casting slab for the balance area ratio 0-20% of polygonal high strength steels above excellent tensile strength 770MPa in low temperature toughness characterized by ferrite der Rukoto. % By mass
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%,
The continuous cast slab for high-strength steel having a tensile strength of 770 MPa or more excellent in low-temperature toughness according to claim 1 , further comprising one or more of the following. A l: <continuous casting slab for that claim 1 or 2 excellent tensile strength 770MPa or more high-strength steel low temperature toughness described is characterized in that 0.004%. It is a manufacturing method of the continuous cast piece according to any one of claims 1 to 3, Comprising :
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: 0.005-0.050%,
In molten steel containing, balance of iron and unavoidable impurities, Al: after continuous casting with a <0.004% or, the resulting continuous casting slab, between the surface temperature of 800 to 500 ° C. 100 A method for producing a continuous cast slab for high-strength steel having a tensile strength of 770 MPa or more and excellent in low-temperature toughness, characterized by cooling in minutes or less . In mass%,
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%,
1 or 2 types or more of these are further contained, The manufacturing method of the continuous cast slab for high strength steel excellent in the low temperature toughness of 770 Mpa or more excellent in low-temperature toughness of Claim 4 characterized by the above-mentioned . The low temperature toughness produced by reheating the continuous cast slab according to any one of claims 1 to 3 to an austenite region, performing controlled rolling, and then cooling at a cooling rate higher than air cooling. High strength steel with excellent tensile strength of 770 MPa or more.

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