KR20100070639A - Steel with excellent low-temperature toughness for construction and manufacturing method thereof - Google Patents

Abstract

PURPOSE: A steel for construction with a superior low temperature toughness and a manufacturing method thereof are provided to prevent the decrease of the low temperature toughness property. CONSTITUTION: A steel for construction comprises less than C 0.02~0.10 weight%, Si 0.01~0.6 weight%, Mn 1.8~2.5 weight%, less than P 0.02 weight% less than S 0.01 weight%, Al 0.005~0.5 weight%, Nb 0.005~0.1 weight%, B 5~40ppm, Ti 0.005~0.1 weight%, N 15~150ppm and the rest Fe and other inevitable impurities. The steel for construction is a micro-structure and sets a bainitic ferrite to a main organization.

Description

Steel with excellent low-temperature toughness and manufacturing method thereof {Steel with Excellent Low-Temperature Toughness for Construction and Manufacturing Method Thereof}

The present invention relates to a high-strength steel having excellent low-temperature toughness characteristics and a method for manufacturing the same. More particularly, the high-strength steel which can improve the low-temperature toughness by constructing a base structure of the steel as a bainitic ferrite structure and controlling a second phase. It relates to a manufacturing method.

Recently, structures such as various buildings, bridges, etc. are gradually becoming larger, ultra tall, and long spand, and according to these trends, demand for high strength steels and necessity of research are increasing.

The use of high-strength steel in construction or structural materials has high permissible stresses, which makes it easier to rationalize and lighten building and bridge structures, thereby making construction work economically feasible and making the steel sheet relatively thin. There is an advantage that the construction work such as mechanical processing or welding such as cutting, drilling, etc. is facilitated.

However, as the strength of steel increases, low-temperature toughness generally decreases, making it difficult to be used for building structures in areas such as cold regions. Considering the fact that the recent development of technology, many buildings and structures are constructed in the cold regions, such as the Arctic, such low temperature toughness is an important characteristic that can never be overlooked.

Therefore, the steel for construction is important to have a high strength, but at the same time it is necessary to also have excellent low-temperature toughness characteristics.

Japanese Patent Laid-Open No. 1999-264017 has been introduced as a prior art regarding a steel having excellent low temperature toughness while securing a tensile strength of 800 MPa or more. The prior art discloses a method of improving the strength of a steel sheet by appropriately adjusting the components of steel and rolling conditions. For this purpose, 1.0 to 2.0% by weight of Cu is added, and heat treatment is performed at 500 to 650 ° C. after rolling. By using the reinforcement mechanism by precipitation hardening is used. However, the prior art has a problem in that the production cost is increased by adding a large amount of expensive alloy elements for precipitation hardening, and a separate heat treatment process is required after rolling, thereby increasing the processing time and cost.

The present invention is to solve the above-mentioned problems of the prior art, and to provide a high strength steel and a method of manufacturing the same having a yield strength of 690MPa or more and a tensile strength of 800MPa or more and excellent low temperature toughness.

The present invention, in weight percent, C: 0.02 to 0.10%, Si: 0.01 to 0.6%, Mn: 1.8 to 2.5%, P: 0.02% or less, S: 0.01% or less, Al: 0.005 to 0.5%, Nb: 0.005 ~ 0.1%, B: 5 ~ 40ppm, Ti: 0.005 ~ 0.1%, N: 15 ~ 150ppm, Remnant Fe Other unavoidable impurities. MA is a microstructure with bainitic ferrite as the main structure and the average size is 1㎛ or less It relates to a steel for construction containing less than 1% of the structure (martensite / austenite). The steel for construction is one or two or more selected from the group consisting of Cr: 0.05 ~ 1.0%, Mo: 0.01 ~ 1.0%, Ni: 0.01 ~ 2.0%, Cu: 0.01 ~ 1.0% and V: 0.005 ~ 0.3% It may further comprise an alloying element, and may exhibit a yield strength of at least 690 MPa and a ductile brittle transition temperature (DBTT) of at most -40 ° C.

Furthermore, this invention is reheating at 1050-1250 degreeC with respect to the steel slab of the said composition, rough rolling at the temperature of 1250 degreeC-Tnr, finishing rolling at the temperature of Tnr-Bs, and quenching to the cooling end temperature of 250-400 degreeC. It provides a method for manufacturing steel for construction. In this case, the quenching treatment is carried out at a cooling rate of 5 ° C / s or more to provide a method for manufacturing steel for construction to effectively control the internal structure of the steel.

According to the present invention, it is possible to provide a steel material excellent in both the strength characteristics and low temperature toughness characteristics can be used in building structures and the like usefully even in extreme environments, and because it does not require a separate tempering treatment, it is excellent in economy and productivity.

In the present invention, to control the microstructure of the steel, and to do this to optimize the component system and the manufacturing conditions to provide a steel that can exhibit both high strength and excellent low-temperature toughness without a separate temper treatment.

The low temperature toughness of steel is a measure of how much steel can withstand brittle fracture in cryogenic environments, and often the soft brittle transition temperature (DBTT curve) is used as a criterion. In particular, the present invention aims to provide a steel material having a DBTT condition of -40 ° C or less.

Hereinafter, the component system contained in the steel of the present invention will be described in more detail. However,% of the component system below means weight%.

C: 0.02 ~ 0.10%

C plays a role in forming bainitic ferrite in the matrix and is added at least 0.02% because it is an important element that determines the size and fraction of the formed martensite. If the content of C is less than 0.02%, the formation of acicular type bainitic ferrite may be difficult, and the strength may be reduced. If the content of C is more than 0.10%, the area fraction of phase martensite in the tissue is over 1%, and the low temperature toughness is reduced. This can be degraded. Therefore, in the present invention, the range of C is limited to 0.02 to 0.10%. Moreover, in view of weldability, Preferably it can limit to 0.03 to 0.08%.

Si: 0.01 ~ 0.6%

Si is used as the deoxidizer of molten steel, and since it is effective in improving the strength in steel, it is added at 0.01% or more. However, if the content of Si exceeds 0.6%, it is not good for low temperature toughness and weldability. Furthermore, since Si has a property of increasing the stability of the phase martensite, at least a phase C martensite having a large amount of C may be formed, and thus, the content of Si needs to be controlled, and a more preferable range of Si is 0.1 to 0.4%.

Mn: 1.8 ~ 2.5%

Mn is an element having a solid solution strengthening characteristic, which contributes to strength improvement, and therefore, is added in the present invention at 1.8% or more. However, when excessively added in excess of 2.5%, the toughness of the weld can be greatly reduced due to an increase in the hardenability, it is limited to 1.8 to 2.5%, preferably to 1.8 to 2.2%.

P: 0.02% or less

Although P plays a role of improving strength and improving corrosion resistance, it is not preferable in view of improving toughness because it is an element that significantly impairs impact toughness. Therefore, it is advantageous to control P as low as possible and the upper limit is made 0.02%.

S: 0.01% or less

Since S is an impurity element that can significantly inhibit impact toughness by forming MnS or the like, it is necessary to manage S as low as possible, and control the upper limit to 0.01%.

Al: 0.005 ~ 0.5%

Al, like Si, is an element used to deoxidize molten steel. In the present invention, the content is more than 0.005% for the deoxidation effect, but if the content exceeds 0.5%, it may cause clogging of the nozzle during continuous casting, so that it is controlled to include 0.005 to 0.5%, preferably 0.01 to 0.05%. Is stable. In addition, since the dissolved Al encourages the formation of phase martensite, a large amount of phase martensite can be formed even with a small amount of C, and therefore, it is necessary to control for improved strength and toughness.

Nb: 0.005 ~ 0.1%

Nb is an element that plays a very important role in the TMCP manufacturing process. Nb is precipitated in the form of NbC or NbCN since these precipitates greatly improve the strength of the base metal and the welded portion in the welding operation. In addition, Nb dissolved in the step of reheating at a high temperature suppresses the recrystallization of austenite and also suppresses the transformation of ferrite or bainite, thereby miniaturizing the structure. Nb enables bainite formation even at low cooling rates when the slab is cooled after rough rolling, and greatly improves the stability of austenite during the cooling process after the final rolling to promote the generation of martensite at low speeds. do. Therefore, in the present invention, Nb is added at 0.005% or more, but if the amount is excessively excessive, brittle cracks may be caused at the edges of the steel, so the upper limit is limited to 0.1%.

B: 5 ~ 40ppm

B is an inexpensive element, but exhibits strong hardenability, so that even with a small amount of addition, strength can be greatly improved. In addition, in the cooling step after the rough rolling, it is advantageous to form bainite even at low speed cooling, and in the present invention, B is added 5 ppm or more because it has the effect of promoting the formation of phase martensite. However, when B is excessively added, Fe ₂₃ (CB) ₆ is formed, rather the curing ability is lowered, and it is not good for low temperature toughness. Therefore, the upper limit of B is limited to 40 ppm.

Ti: 0.005 ~ 0.1%

Ti is added to 0.005% or more because it can greatly improve the low-temperature toughness by inhibiting the growth of grains when reheating. However, excessive addition of more than 0.1% may cause clogging of the playing nozzle or crystallization of the core, thereby reducing the low temperature toughness of the product, so the Ti content is limited to 0.005 to 0.1%.

N: 15 ~ 150ppm

Since N is an element that increases strength while greatly reducing toughness, its content needs to be limited to 150 ppm or less. However, the extreme control of N to 15ppm or less is not good in terms of productivity and cost due to the increased process load in the steelmaking stage, so the lower limit of N content is set to 15ppm.

The steel of the present invention can obtain a sufficient effect only by the addition of the above-mentioned essential alloy elements, but in order to further improve the properties such as strength and toughness of the steel, toughness of the weld heat affected zone, weldability, etc. 2 or more types can be added.

Cr: 0.05 ~ 1.0%

Cr plays a big role in improving the strength by increasing the hardenability, so to obtain more such strength improving effect, 0.05% or more of Cr is added. However, since the addition of excessive Cr exceeding 1.0% greatly lowers the weldability, the upper limit of the amount of Cr added is limited to 1.0%.

Mo: 0.01 ~ 1.0%

Mo is required to add more than 0.01% because Mo can greatly improve the hardenability and suppress the formation of ferrite because only a small amount can be added. However, excessive addition exceeding 1.0% causes excessive hardness of the weld. The Mo content is limited to 1.0% or less because it may increase slowly and inhibit toughness.

Ni: 0.01 ~ 2.0%

Ni is an effective element that can simultaneously improve the strength and toughness of the base material, and in order to obtain such effects, 0.01% or more must be added. Since Ni is an expensive element, addition of 2.0% or more deteriorates the economic efficiency. Also, weldability is degraded.

Cu: 0.01 ~ 1.0%

Cu is an element that can increase the strength while minimizing the decrease in toughness of the base metal, and therefore 0.01% or more is added for such an effect. However, excessive addition of Cu can greatly impair product surface quality, so the upper limit thereof is limited to 1.0%.

V: 0.005 ~ 0.3%

V has a lower solubility temperature than other fine alloys, and it is added to 0.005% or more because it has an effect of preventing the drop in strength due to precipitation in the weld heat affected zone. However, excessive addition exceeding 0.3% may reduce the toughness, so the content is limited to 0.005 to 0.3%.

Hereinafter, the microstructure constituting the present invention will be described in more detail.

The steel material of the present invention including the alloying elements described above is a steel material having improved hardenability than conventional steels, and can form a desired structure without cooling at a rapid rate. However, as the hardenability of the steel is improved, hard tissues may be easily formed therein, and thus low-temperature toughness may deteriorate. In the present invention, in order to solve these problems and to construct a desirable structure of steel, By doing so, even if the hardenability of the steel is improved, it is possible to prevent the low-temperature toughness property from deteriorating.

Microstructure: Mixed structure of MA tissue with average size less than 1㎛ and residual bainitic ferrite

As shown in FIG. 1, the microstructure of the steel of the present invention includes a MA structure (martensite / austenite mixed tissue) having an average size of 1 μm or less, and the rest is a mixed structure of bainitic ferrite tissue. In this case, the tissue fraction of the MA tissue is 1% or less.

In order to control the MA structure, it is necessary to control the cooling temperature. As shown in FIG. 2, the soft brittle transition temperature DBTT increases as the cooling end temperature increases. This is because, as the cooling end temperature increases, the MA fraction also increases, increasing the area of the matrix and the MA interface and increasing the place where cracks can occur. In addition, when the cooling end temperature is high, the tissue may be formed into granular bainite tissue, thereby reducing the yield strength.

Therefore, in order to secure sufficient impact toughness and yield strength, it is required to maintain the cooling end temperature at 400 ° C. or lower as shown in FIG. 2, thereby obtaining high strength and improved low temperature toughness.

Hereinafter, the method of manufacturing the steel of the present invention will be described in more detail.

Steel manufacturing conditions of the present invention is made of a reheating, rough rolling, finishing rolling and cooling step of the slab, it is possible to obtain the required physical properties without a separate temper heat treatment. Each step condition in the manufacturing method is controlled as follows.

Slab reheating temperature: 1050 ~ 1250 ℃

In the present invention, the reheating of the steel sheet is 1050 ° C. or more. The purpose of the reheating is to solidify the carbonitride of Ti and / or Nb formed during the casting process, and the lower limit of the temperature for the present invention is 1050 ° C. In addition, carbonitrides of Ti and / or Nb may be sufficiently dissolved upon reheating above 1050 ° C. However, when heating is performed to a high temperature exceeding 1250 ° C., the austenite structure may grow and coarse particles. Therefore, the reheating temperature is set to 1050 to 1250 ° C.

Rough rolling temperature: 1250 ℃ ~ Tnr

The reheated steel sheet is subjected to rough rolling to adjust its shape. In this case, the rough rolling temperature is at least the temperature Tnr at which recrystallization of austenite stops. According to the present rough rolling temperature conditions, casting structures such as dendrites formed during casting can be destroyed, and the effect of miniaturizing austenite can also be obtained.

Finish rolling temperature: Tnr ~ Bs

After rough rolling, finishing rolling is performed by the method for introducing a nonuniform microstructure into an austenite structure. In the present invention, the finishing rolling temperature is set from the austenite recrystallization temperature (Tnr) to the bainite transformation start temperature (Bs) or more.

Cooling condition: Cooling rate over 5 ℃ / s, end of cooling at 250 ~ 400 ℃

Cooling conditions of the present invention is one of the main features of the present invention, it is necessary to control the structure of the steel sheet to form a bainitic ferrite structure as shown in Route 3 of FIG. In this case, cooling is performed at a cooling rate of 5 ° C./s or more, preferably water cooling, and the cooling is terminated in the range of 250 ° C. to 400 ° C. which is equal to or lower than the Bf temperature (meaning the temperature at which the bainite transformation ends). According to such cooling conditions, the microstructure of the steel can form a tissue including MA tissue having a fine average size using bainitic ferrite as a known structure.

If the cooling rate is slower than 5 ° C / s (Route ① in Fig. 3) or if the cooling end temperature is more than Bf (Route ② in Fig. 3) even if the cooling rate is 5 ° C / s or more, It appears in granular ferrite form, so it is difficult to obtain sufficient yield strength of 690 MPa or more.

Hereinafter, the present invention will be described in more detail with reference to the following examples.

(Example)

Inventive material (A1-A6) and comparative material (B1-B4) slab which have the composition of Table 1 were manufactured by the manufacturing method made into object by this invention.

Steel slabs made of the components shown in Table 1 above were rolled and cooled in the same manner as in Table 2.

No. Rough rolling condition Finish rolling condition Cooling condition Remarks Steel grade Slab
thickness
(mm) Reheat
Extraction temperature
(℃) Crude rolling
End temperature
(℃) Rolling
Start temperature
(℃) Rolling
End temperature
(℃) Cooling
speed
(℃ / s) Cooling
End
Temperature A1 a 244 1065 985 938 898 15.0 383 Inventive Example b 244 1080 1000 918 878 23.0 363 Inventive Example c 220 1120 1040 908 868 15.0 333 Inventive Example d 244 1110 1030 920 880 2.0 343 Under cooling rate e 220 1050 970 1068 1028 7.0 398 Finish rolling temperature> Tnr f 220 1050 970 850 810 9.0 493 Cooling shutdown temperature
High temperature A2 a 244 1070 990 784 744 13.0 375 Inventive Example b 244 1075 995 764 724 21.0 355 Inventive Example c 220 1110 1030 754 714 14.0 325 Inventive Example d 244 1105 1025 820 780 2.0 335 Under cooling rate e 220 1060 980 914 874 8.0 390 Finish rolling temperature> Tnr f 220 1060 980 850 810 11.0 485 Cooling end temperature high temperature A3 a 244 1080 1000 990 950 10.0 362 Inventive Example b 244 1070 990 970 930 25.0 382 Inventive Example c 220 1120 1040 960 920 15.0 352 Inventive Example d 244 1100 1020 820 780 3.0 362 Under cooling rate e 220 1055 975 1120 1080 7.0 377 Finish rolling temperature> Tnr f 220 1055 975 850 810 9.0 512 Cooling end temperature high temperature A4 a 244 1080 1000 919 879 10.0 341 Inventive Example b 244 1070 990 899 859 15.0 361 Inventive Example c 220 1120 1040 889 849 15.0 331 Inventive Example d 244 1100 1020 820 780 2.5 341 Under cooling rate e 220 1155 1105 1029 989 7.0 376 Finish rolling temperature> Tnr f 220 1055 975 850 810 9.0 491 Cooling end temperature high temperature A5 a 244 1080 1000 886 846 10.0 354 Inventive Example b 244 1070 990 866 826 19.0 389 Inventive Example c 220 1120 1040 856 816 15.0 359 Inventive Example d 244 1100 1020 820 780 3.0 369 Under cooling rate e 220 1055 1035 996 956 7.0 384 Finish rolling temperature> Tnr f 220 1055 975 850 810 9.0 519 Cooling end temperature high temperature A5 a 244 1080 1000 1021 981 10.0 356 Inventive Example b 244 1070 990 971 931 17.0 376 Inventive Example c 220 1120 1040 991 951 15.0 346 Inventive Example d 244 1100 1020 820 780 2.0 356 Under cooling rate e 220 1055 1035 1101 1061 7.0 371 Finish rolling temperature> Tnr f 220 1055 975 850 810 9.0 506 Cooling end temperature high temperature B1 a 244 1080 1000 875 835 10.0 487 Comparative example b 244 1070 990 855 815 7.0 467 Comparative example c 220 1120 1040 845 805 15.0 437 Comparative example B2 a 244 1080 1000 963 923 10.0 507 Comparative example b 244 1070 990 943 903 7.0 487 Comparative example c 220 1120 1040 933 893 15.0 457 Comparative example B3 a 244 1080 1000 754 714 10.0 563 Comparative example b 244 1070 990 734 694 7.0 543 Comparative example c 220 1120 1040 724 684 15.0 513 Comparative example B4 a 244 1080 1000 955 915 10.0 444 Comparative example b 244 1070 990 935 895 7.0 424 Comparative example c 220 1120 1040 925 885 15.0 419 Comparative example

The yield strength, tensile strength, MA fraction in the structure and DBTT characteristics of the steel sheets produced by the manufacturing conditions of Table 2 were evaluated, and the results are shown in Table 3 as Inventive Example (A) and Comparative Example (B). .

Steel grade
number Thickness
(mm) Yield strength
(MPa) The tensile strength
(MPa) MA fraction
(%) DBTT
(℃) A1 a 80 711 841 0.7 -50 b 25 760 890 0.5 -54 c 50 743 873 0.4 -59 d 65 678 808 0.4 -57 e 35 664 794 0.9 -44 f 60 597 727 2.2 -30 A2 a 80 707 837 0.6 -54 b 25 757 887 0.5 -58 c 50 744 874 0.3 -64 d 65 683 813 0.4 -61 e 35 674 804 0.8 -49 f 60 613 743 2 -32 A3 a 80 702 832 0.5 -52 b 25 757 887 0.7 -49 c 50 731 861 0.5 -55 d 65 671 801 0.5 -56 e 35 679 809 0.7 -47 f 60 580 710 2.5 -26 A4 a 80 716 846 0.4 -58 b 25 726 856 0.5 -55 c 50 745 875 0.3 -60 d 65 682 812 0.4 -60 e 35 680 810 0.6 -49 f 60 600 730 2.1 -30 A5 a 80 708 838 0.5 -56 b 25 725 855 0.8 -50 c 50 727 857 0.5 -56 d 65 666 796 0.6 -55 e 35 674 804 0.7 -49 f 60 574 704 2.6 -25 A5 a 80 706 836 0.5 -53 b 25 725 855 0.6 -50 c 50 735 865 0.4 -55 d 65 670 800 0.5 -57 e 35 683 813 0.6 -49 f 60 586 716 2.4 -27 B1 a 65 607 737 2.1 -31 b 20 610 740 1.7 -35 c 55 671 801 1.3 -41 B2 a 65 589 719 2.4 -25 b 20 594 724 2.1 -29 c 55 655 785 1.6 -36 B3 a 65 537 667 3.6 -17 b 20 543 673 3.1 -22 c 55 607 737 2.5 -28 B4 a 65 622 752 1.8 -33 b 20 625 755 1.5 -37 c 55 685 815 1.1 -43

Referring to Table 3, the a, b and c steels of A1 to A3 satisfying the component system and manufacturing conditions of the present invention exhibited a MA fraction of 1% or less, and all of the yield strength, tensile strength and DBTT characteristics of the present invention. The purpose was met.

On the other hand, all the steel materials of the B1 ~ B4 component system is out of the scope of the present invention was not good all yield strength, tensile strength as well as DBTT characteristics. In addition, A1-d, A2-d, A3-d and A4-d steels satisfying the component system but having a cooling rate outside the conditions of the present invention, and A1-e, A2-e, and A3 having a finishing rolling temperature higher than Tnr -e and A4-e exhibited problems in yield strength characteristics, which may be due to problems in constructing the main structure of bainitic ferrite. Furthermore, A1-f, A2-f, A3-f and A4-f having high cooling end temperatures showed problems not only in yield strength but also in tensile strength and DBTT characteristics.

In summary, according to the present invention, it is possible to provide a steel having excellent strength and low temperature toughness, so that it can be usefully used in building structures even in an extreme cold environment, and since it does not require a separate tempering treatment, it is excellent in economy and productivity. Has the property of

1 is a photograph of a tissue of the present invention steel observed with a scanning electron microscope

Figure 2 is a graph showing the yield strength and ductile brittle transition temperature (DBTT) of the steel according to the cooling end temperature provided by the present invention

Figure 3 is a graph schematically showing the temperature behavior inside the steel sheet over time during the manufacturing process of the present invention over time

Claims (7)

By weight%, C: 0.02 to 0.10%, Si: 0.01 to 0.6%, Mn: 1.8 to 2.5%, P: 0.02% or less, S: 0.01% or less, Al: 0.005 to 0.5%, Nb: 0.005 to 0.1% , B: 5 ~ 40ppm, Ti: 0.005 ~ 0.1%, N: 15 ~ 150ppm, balance Fe Other unavoidable impurities, Steel for construction, characterized in that the microstructure is composed of bainitic ferrite as the main structure and the MA structure (martensite / austenite) having an average size of 1 μm or less at 1% or less. According to claim 1, wherein the steel material is selected from the group consisting of Cr: 0.05 ~ 1.0%, Mo: 0.01 ~ 1.0%, Ni: 0.01 ~ 2.0%, Cu: 0.01 ~ 1.0% and V: 0.005 ~ 0.3% Or two or more alloying elements. The construction steel according to claim 1 or 2, wherein the construction steel exhibits a yield strength of 690 MPa or more and a ductile brittle transition temperature (DBTT) of -40 ° C or less. By weight%, C: 0.02 to 0.10%, Si: 0.01 to 0.6%, Mn: 1.8 to 2.5%, P: 0.02% or less, S: 0.01% or less, Al: 0.005 to 0.5%, Nb: 0.005 to 0.1% , B: 5 ~ 40ppm, Ti: 0.005 ~ 0.1%, N: 15 ~ 150ppm, balance Fe For steel slab containing other unavoidable impurities, Reheating at 1050-1250 ° C .; Rough rolling step of rough rolling at a temperature of 1250 ℃ ~ Tnr; A finishing rolling step of finishing rolling at a temperature of Tnr to Bs; And Cooling step for quenching to 250 ~ 400 ℃ cooling end temperature Method of manufacturing a steel for construction, characterized in that it comprises a. The steel slab of claim 4, wherein the steel slab is selected from the group consisting of Cr: 0.05 to 1.0%, Mo: 0.01 to 1.0%, Ni: 0.01 to 2.0%, Cu: 0.01 to 1.0%, and V: 0.005 to 0.3%. A method for producing steel for construction, characterized in that it further comprises a species or two or more alloying elements. The method of claim 4, wherein the quenching treatment is performed at a cooling rate of 5 ° C./s or more. The method of manufacturing a steel for construction according to claim 4 or 6, wherein the quenching treatment is water cooling.

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