KR20160079165A - High strength thick steel for structure having excellent properties at the center of thickness and method of producing the same - Google Patents

Abstract

The present invention relates to a high strength steel with excellent center properties for a thin steel plate. According to an embodiment of the present invention, the high strength steel with excellent properties of the center thereof for a thin steel plate comprises: 0.02-0.12 wt% of carbon (C); 0.3-2.5 wt% of manganese (Mn); 0.01-0.6 wt% of silicon (Si); 0.02 wt% or less of phosphorus (P); 0.01 wt% or less of sulfur (S); 0.005-0.05 wt% of aluminum (Al); 0.005-0.1 wt% of titanium (Ti); 0.005-0.10 wt% of niobium (Nb); 5-40ppm of boron (B); 15-150ppm of nitrogen (N); and the remaining consisting of iron (Fe) and inevitable impurities. Moreover, the high strength steel with excellent center properties for a thin steel plate has a microstructure including 1 or less area % of massive martensite.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel material having high strength and excellent in core properties,

The present invention relates to a steel material for high strength steel sheet having excellent core property and a method for producing the same.

In recent years, the use of high strength ultra high strength steels tends to increase in accordance with the tendency to become very large. Slabs of usually 250 to 300 mm are commonly used in the production of ultra-fine steel products of 80 mm or more. However, as the thickness of the final product becomes thicker, the reduction amount that can be applied at the time of rough rolling and finish rolling decreases, so that the core pore and segregation band formed at the time of slab manufacturing can not be removed, There arises a problem that the shrinkage ratio (ZRA) is lowered.

In the conventional technique, a special process is required to reduce the pressure during the continuous casting in order to remove the central pore and the segregation zone formed at the center of the slab.

According to this prior art, Patent Document 1 proposes to reduce the seam solidification rate by 85 to 99% to 1 to 25 mm to eliminate segregation in the center of the steel strip thickness. Further, in Patent Document 2, it is proposed to produce the extreme-surface steel sheet by performing surface reduction of 1.1 to 2 times the non-solidified thickness in the region of the center solidification rate of 0.6 or more.

However, such a solution as described above has a problem in that it is not possible to sufficiently remove segregation because a large-scale depressing facility must be installed and the depressing is performed at the end of solidification.

Therefore, there has been a demand for development of a high strength steel sheet for steel structure material excellent in physical properties at the center and a method for producing the steel sheet using a conventional continuously cast slab.

Japanese Laid-Open Patent Publication No. 1995-276020 Japanese Laid-Open Patent Publication No. 1994-106316

One aspect of the present invention is to provide a steel material for high strength steel sheet having excellent core property and a method for manufacturing the same.

An aspect of the present invention is a steel sheet comprising, by weight, 0.02 to 0.12% of C, 0.3 to 2.5% of Mn, 0.01 to 0.6% of Si, 0.02% or less of P, 0.01% or less of S, 0.005 to 0.1% of Ti, 0.005 to 0.10% of Nb, 5 to 40 ppm of B, 15 to 150 ppm of N, the balance Fe and other unavoidable impurities, and the microstructure contains not more than 1% And more particularly to a steel material for high strength steel sheet having excellent core properties.

In another aspect of the present invention, there is provided a method of manufacturing a semiconductor device, comprising the steps of: 0.02 to 0.12% of C, 0.3 to 2.5% of Mn, 0.01 to 0.6% of Si, A steel slab containing 0.05 to 0.05% of Ti, 0.005 to 0.1% of Nb, 0.005 to 0.10% of B, 5 to 40 ppm of B, 15 to 150 ppm of N and balance of Fe and other unavoidable impurities is heated to a temperature range of 1000 to 1250 캜 ; Subjecting the heated steel slab to rough rolling; A primary cooling step of cooling the rough-rolled steel slab; Subjecting the cooled steel slab to finish rolling to obtain a hot-rolled steel sheet; And a secondary cooling step of cooling the hot-rolled steel sheet; The present invention relates to a method of manufacturing a high strength steel material having excellent core properties.

In addition, the solution of the above-mentioned problems does not list all the features of the present invention. The various features of the present invention and the advantages and effects thereof can be understood in more detail with reference to the following specific embodiments.

According to the present invention, it is possible to provide a steel material for a high strength steel sheet having excellent core property and a method for producing the same.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a photograph taken by an optical microscope of the microstructure of A-1 of the invention invention. Fig.
Fig. 2 is a schematic diagram showing that Z-axis tensile section shrinkage ratio (ZRA) is improved by applying a descending force to the center portion during scrap rolling by cooling and transforming the surface portion of the intermediate material by using an accelerator cooler after completion of rough rolling.
3 is a graph showing the fraction of the mixed structure of martensite and austenite and the Ductile-Brittle Transition Temperature (DBTT) according to the cooling finish temperature.

The present inventors have commonly used slabs having a thickness of 250 to 300 mm at the time of manufacturing a superfine steel material of 80 mm or more, but as the thickness of the final product is thicker, the reduction amount that can be applied at the time of rough rolling and finish rolling is reduced, (ZRA) of the Z-axis is lowered due to the problem that the core pore and the segregation zone can not be removed.

As a result, it has been confirmed that the steel composition for a high strength steel sheet having excellent core properties and core properties and the method for producing the same can be provided by suitably controlling the alloy composition and the production conditions, thereby completing the present invention.

Hereinafter, a steel material for high strength steel sheet having excellent core properties according to one aspect of the present invention will be described in detail.

According to one aspect of the present invention, there is provided a steel material for a high strength steel sheet having excellent physical properties at a center, comprising 0.02 to 0.12% of C, 0.3 to 2.5% of Mn, 0.01 to 0.6% of Si, 0.005 to 0.1% of Al, 0.005 to 0.1% of Ti, 0.005 to 0.10% of Nb, 5 to 40 ppm of B, 15 to 150 ppm of N and balance Fe and other unavoidable impurities, And contains at least 99% of bainitic ferrite in an area fraction.

First, the alloy composition of the steel material for a high strength steel sheet having excellent core property according to one aspect of the present invention will be described in detail. (Hereinafter, the content of each component means% by weight)

C: 0.02 to 0.12%

In the present invention, since it is the most important element for determining the size and the fraction of the on-state martensite formed by forming the Martensite-Austenite Constituent (MA), it is necessary to be contained in the steel within an appropriate range. However, when the content of C exceeds 0.12%, the low-temperature toughness is lowered and the fraction of martensite on the road surface exceeds 1%. When the content of C is less than 0.02%, the formation of bainitic ferrite is inhibited, The content is preferably 0.02 to 0.12%. In the case of a plate used as a steel structure for welding, it is more preferable to set the range of C to 0.03 to 0.09% for weldability.

Si: 0.01 to 0.6%

Si is used as a deoxidizing agent and it is effective because it has an effect of improving the strength. However, if it exceeds 0.6%, it lowers the low temperature toughness and also deteriorates the weldability. If it is 0.01% or less, the effect of deoxidation becomes insufficient, which is limited to 0.01 to 0.6%. In addition, since Si improves the stability of the martensite on the surface, it can form many morphological martensite even with a small amount of C, which is helpful for the improvement of the strength, but it results in a decrease in toughness, so that the preferable range of Si is 0.1 to 0.4%.

Mn: 0.3 to 2.5%

Mn is a useful element for enhancing the strength by solid solution strengthening, and therefore, 0.3% or more needs to be added. However, the addition of more than 2.5% significantly reduces the toughness of the welded part due to an increase in the hardenability, so that the content of Mn is preferably 0.3 to 2.5%.

P: not more than 0.02%

P is an element favorable for strength improvement and corrosion resistance, but since it is an element that greatly hinders impact toughness, it is advantageous to make it as low as possible, so that the upper limit is preferably 0.02%.

S: not more than 0.01%

Since S is an element that significantly inhibits impact toughness by forming MnS or the like, it is advantageous to make it as low as possible, so that the upper limit is preferably 0.01%.

Al: 0.005-0.5%

Since Al is an element capable of inexpensively deoxidizing molten steel, Al is preferably added in an amount of 0.005% or more. However, addition of 0.5% or more causes clogging of the nozzle during continuous casting, so it is limited to 0.005 to 0.5%. Further, the solidified Al promotes the formation of on-road martensite of the weld heat affected zone, thereby lowering the impact toughness of the welded portion. Therefore, it is more preferable to set the range of Al to 0.01 to 0.05%.

Ti: 0.005 to 0.1%

In order to exhibit the effect, it is necessary to add 0.005% or more. When the addition of Ti exceeds 0.1%, clogging of the performance nozzle or low temperature toughness , It is limited to a range of 0.005 to 0.1%.

Nb: 0.005 to 0.1%

Nb is the most important element in the production of TMCP steel (thermo-mechanical control process steel) and precipitates in the form of NbC or NbCN, which greatly improves the strength of the base material and the weld. In addition, Nb dissolved at the time of reheating at a high temperature has the effect of suppressing recrystallization of austenite and suppressing transformation of ferrite or bainite to make the structure finer. In addition, in the present invention, not only the bainite is formed at a low cooling rate when the slab is cooled after rough rolling, but also the stability of austenite is greatly increased even after the final rolling, thereby forming bainite even at low speed cooling Also. Therefore, it is preferable that Nb is added in an amount of 0.005% or more, but if it is excessively charged, the possibility of brittle cracks in the edge of the steel increases, which is not preferable.

B: 5 to 40 ppm

B is a very low cost additive element and is a beneficial element showing strong curing ability. It is preferable to add 5 ppm or more since the addition of a small amount greatly improves the strength. However, if it is added excessively, Fe23 (CB) 6 is formed to lower the hardenability and lower the low temperature toughness. Therefore, B is limited to 5 to 40 ppm. B in the present invention contributes greatly to the formation of bainite even in low-speed cooling in cooling after rough rolling.

N: 15 to 150 ppm

It is necessary to limit the content of N to 150 ppm or less because it increases the strength but greatly reduces the toughness. However, since the N content control of 15 ppm or less increases the steelmaking load, the lower limit of the N content is set at 15 ppm.

The remainder of the present invention is iron (Fe). However, in the ordinary manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably incorporated, so that it can not be excluded. These impurities are not specifically mentioned in this specification, as they are known to any person skilled in the art of manufacturing.

The advantageous effects of the present invention can be attained only by including the alloying elements in the above-mentioned content range, but the characteristics such as the strength and toughness of the steel material, the toughness of the weld heat affected zone, It is preferable to add the following alloying elements within an appropriate range. The following alloying elements may be added singly or in combination of two or more.

Cr: 0.05 to 1.0%

Cr has a great effect on increasing the hardenability by increasing the hardenability. Therefore, in order to obtain the effect, it is necessary to add 0.05% or more, and addition of 1.0% or more greatly reduces the weldability. Further, in order to obtain stable on-state martensite even at a relatively low cooling rate, it is more preferable to add it in the range of 0.2 to 0.5%.

Mo: 0.01 to 1.0%

The addition of a small amount of Mo greatly improves the hardenability and suppresses the formation of ferrite. Therefore, it is necessary to add Mo in an amount of 0.01% or more, but the addition of 1.0% or more increases the hardness of the weld It is advantageous to add 1.0% or less. In the present invention, it is more preferable to limit the range of 0.02 to 0.2% in order to form the ground martensite in an appropriate range in order to secure the tensile strength.

Ni: 0.01 to 2.0%

Ni is the only element capable of simultaneously improving the strength and toughness of the base material. In order to exhibit the effect, 0.01% or more should be added. Ni is an expensive element, do. Therefore, the content of Ni is preferably 0.01 to 2.0%.

Cu: 0.01 to 1.0%

Cu is an element capable of minimizing toughness deterioration of a base material and simultaneously increasing its strength. In order to exhibit the effect, 0.01% or more should be added, and excessive addition is required to limit the product surface quality to 1.0% or less Do.

V: 0.005 to 0.3%

V is low in temperature to be employed as compared with other fine alloys and has an effect of preventing precipitation in the weld heat affected portion and preventing the decrease in strength. It is preferable to add 0.005% or more, and if it is over 0.3% The V content is preferably 0.005 to 0.3%.

Hereinafter, the microstructure of the steel material for high strength steel sheet having excellent core property according to one aspect of the present invention will be described in detail. (Hereinafter, the unit of microstructure means area%).

The microstructure of the steel according to the present invention contains not more than 1% of the martensite structure of the present invention.

As shown in FIG. 3, when the cooling finish temperature is lower than the bainite finish temperature (Bf), the ductile-martensitic structure becomes 1% or less and the Ductile-Brittransition Temperature (DBTT) Can be produced. Therefore, the microstructure of the steel of the present invention controls the martensite structure of the present invention to 1% or less.

Further, as shown in Fig. 1, it is preferable that 1% or less of the island martensite structure is contained, and the remainder has the structure of bainitic ferrite. The bainitic ferrite structure is preferably a structure having a high tensile strength and an excellent impact toughness, and it is preferable that the bainitic ferrite structure contains more than 99% in order to secure a tensile strength of 600 MPa or more and ensure impact toughness.

By satisfying the alloy composition and the microstructure described above, it is possible to provide a steel material having a high tensile strength and excellent tensile strength of not less than 600 MPa and a Z axis tensile cross-sectional shrinkage ratio (ZRA) of not less than 50%.

Hereinafter, a steel material for a high strength steel sheet having excellent core properties according to another aspect of the present invention will be described in detail.

According to one aspect of the present invention, there is provided a steel material for a high strength steel sheet having excellent physical properties at a center, comprising 0.02 to 0.12% of C, 0.3 to 2.5% of Mn, 0.01 to 0.6% of Si, 0.005 to 0.1% of Al, 0.005 to 0.1% of Ti, 0.005 to 0.10% of Nb, 5 to 40 ppm of B, 15 to 150 ppm of N and balance Fe and other unavoidable impurities, And contains at least 99% of bainitic ferrite in an area fraction.

Hereinafter, a method of manufacturing a high-strength steel material excellent in core properties, which is another aspect of the present invention, will be described in detail.

A method of manufacturing a high strength steel material excellent in core property, which is another aspect of the present invention, is characterized in that it comprises 0.02 to 0.12% of C, 0.3 to 2.5% of Mn, 0.01 to 0.6% of Si, A steel slab containing 0.01 to 0.01% of Al, 0.005 to 0.05% of Al, 0.005 to 0.1% of Ti, 0.005 to 0.10% of Nb, 5 to 40 ppm of B, 15 to 150 ppm of N and balance Fe and other unavoidable impurities Lt; 0 > C to about 1250 < 0 >C; Subjecting the heated steel slab to rough rolling; The rough-rolled steel slab is firstly cooled from the surface of the steel slab to a depth corresponding to 10 to 20% of the thickness of the steel slab at a cooling rate of 5 ° C / s or higher so as to be lower than the bainite transformation start temperature (Bs) Car cooling step; Subjecting the cooled steel slab to finish rolling to obtain a hot-rolled steel sheet; And a secondary cooling step of cooling the hot-rolled steel sheet; .

The steel material manufacturing process of the present invention is composed of the processes of "steel slab heating → rough rolling → first cooling → finish rolling → second cooling", and detailed conditions of each process are as follows.

Slab heating temperature: 1000 ~ 1250 ℃

The steel slab satisfying the above alloy composition is heated to a temperature range of 1000 to 1250 캜. In order to solidify the carbonitride of Ti and / or Nb formed during casting, it is preferable to heat it to 1000 DEG C or more. Further, in order to sufficiently solidify the carbonitride of Ti and / or Nb, it is more preferable to heat to 1050 占 폚 or more. However, when heating is carried out at an excessively high temperature, the austenite may be coarsened, so that the heating temperature is preferably 1250 DEG C or lower.

Rough rolling  Temperature: Tnr ~ 1250 ℃

The heated steel slab is rough-rolled to adjust its shape.

In order to reduce the effect of destroying the cast structure such as dendrites formed during casting by the rolling and to reduce the size of austenite, it is preferable that the rough rolling temperature is not lower than the temperature (Tnr) at which the austenite recrystallization stops. On the other hand, when the rough rolling temperature exceeds 1250 DEG C, the initial austenite grain size becomes large and toughness tends to deteriorate. Therefore, the rough rolling temperature is preferably Tnr to 1250 ° C.

Primary cooling

The rough-rolled steel slab is first cooled from the surface of the steel slab at a cooling rate of 5 ° C / s or higher so as to be lower than the bainite transformation start temperature (Bs) to a depth corresponding to 10-20% of the thickness of the steel slab.

When the depth is less than 10% of the steel slab thickness, that is, when the bainite structure is formed only from the surface of the steel slab to a depth less than 10% of the steel slab thickness, the rolling transmitted to the center of the plate, The power becomes insufficient.

On the other hand, if the depth is greater than 20% of the steel slab thickness, i.e. if the bainite structure is formed from the surface of the steel slab to a depth greater than 20% of the steel slab thickness, have.

As described above, after the completion of the rough rolling, the surface portion is first cooled before the finish rolling to form bainite, which is harder than the austenite, on the surface, thereby reducing the pore or segregation in the center portion Z-axis tensile section shrinkage ratio (ZRA) can be secured by more than 50%.

Finish rolling  Temperature: Bs ~ Tnr

The primary-cooled steel slab is hot-rolled to obtain a hot-rolled steel sheet.

At this time, it is preferable that the finishing rolling is performed in a temperature range of Bs to Tnr.

As the nonuniform strain band is introduced into the austenite, the bainitic ferrite which is transformed becomes finer and the impact toughness is improved. Therefore, the austenitic structure of the rough-rolled steel sheet is preferably heated at a temperature lower than the recrystallization temperature (Tnr) Finish rolling.

In addition, since the finish temperature of the finish rolling finishes at the bainite start temperature (Bs) or higher, control cooling can be applied in the accelerating cooler, so that the finish rolling temperature is preferably Bs or more.

Further, it is more preferable that the finish rolling finishes at a temperature of Bs + 50 degrees when a temperature drop is considered before being fed to the accelerating cooler.

Secondary cooling

The hot-rolled steel sheet is cooled to a bainite transformation completion temperature (Bf) or less at a cooling rate of 5 to 50 ° C / s and then subjected to secondary cooling.

When the cooling is terminated at a temperature exceeding Bf, as shown in FIG. 3, the road surface martensite fraction exceeds 1%, and the impact transition temperature (DBTT) is increased to make it difficult to secure low temperature impact toughness.

When the cooling rate is less than 5 占 폚 / s, bainitic ferrite is not formed and granular bainite is formed at a high temperature, which makes it difficult to secure strength.

In summary, in the steel material manufacturing method of the present invention, the steel slab having the composition described above is heated to a temperature range of 1000 to 1250 ° C., and after the completion of the rough rolling, the steel slab is first cooled to transform the surface portion into bainite, After completion of the rolling at a kneading transformation starting temperature Bs or more, the steel sheet is cooled at a cooling rate of 5 to 50 DEG C / s at a bainite finish temperature (Bf) or lower, By forming the site structure, a steel material having a tensile strength of 600 MPa or more and excellent low temperature toughness can be produced.

Hereinafter, the present invention will be described more specifically by way of examples. It should be noted, however, that the following examples are intended to illustrate the invention in more detail and not to limit the scope of the invention. The scope of the present invention is determined by the matters set forth in the claims and the matters reasonably inferred therefrom.

( Example )

Steel slabs having the composition shown in the following Table 1 were produced under the respective production conditions shown in Table 2 below. However, in the embodiment shown in Table 2 below, the first-round cooling is applied so that the coarsely rolled steel slab is brought from the surface of the steel slab to a depth corresponding to 10 to 20% of the thickness of the steel slab, Means cooling at a cooling rate of 5 DEG C / s or more.

The yield strength, the tensile strength, the fraction of the martensite and the impact transition temperature (DBTT) were measured for each of the manufactured structural steels, and the results are shown in Table 3 below.

In Table 1, bainite transformation start temperature Bs, bainite finish temperature Bf and recrystallization temperature Tnr for each steel are described.

River number C Si Mn P S Al Ti Nb B ^* N ^* Additional element Bs Bf Tnr Inventive Steel A 0.04 0.2 1.8 0.013 0.002 0.025 0.013 0.035 7 37 615 495 960 Invention steel B 0.07 0.5 1.6 0.013 0.005 0.032 0.014 0.02 15 53 562 442 803 Inventive Steel C 0.09 0.2 1.5 0.012 0.002 0.013 0.02 0.05 35 40 568 446 1058 Inventive Steel D 0.06 0.3 1.7 0.013 0.002 0.013 0.02 0.04 8 38 Ni: 0.21 548 428 959 Invention steel E 0.07 0.2 1.8 0.013 0.002 0.013 0.02 0.03 15 39 Cu: 0.23 544 424 952 Invention river F 0.05 0.3 2.1 0.013 0.003 0.013 0.02 0.06 22 40 Cr: 0.21 508 388 1055 Invention river G 0.09 0.4 1.6 0.013 0.002 0.013 0.02 0.04 17 45 Mo: 0.11 548 428 937 Invention steel H 0.071 0.3 1.9 0.013 0.002 0.013 0.02 0.01 14 43 V: 0.02 534 415 818 Comparative River A 0.01 0.2 1.5 0.014 0.003 0.034 0.012 0.03 23 37 587 467 925 Comparative Steel B 0.17 0.3 0.8 0.013 0.001 0.038 0.013 0.04 25 30 607 487 1013 Comparative Steel C 0.09 0.4 1.2 0.013 0.005 0.025 0.01 0 14 25 698 578 804 Comparative Steel D 0.079 0.2 1.4 0.015 0.009 0.043 0.009 0.04 2 45 578 458 1005

In Table 1, B * and N * are expressed in units of ppm, the units of the remaining elements are% by weight, and the units of Bs, Bf, and Tnr are ° C.

No. Rough rolling condition Primary cooling
Applicability Finish rolling condition Secondary cooling Remarks Steel grade number Slab
thickness Reheating
Extraction temperature Rough rolling
Termination temperature Rolling start
Temperature Rolling finish
Temperature Cooling rate Cooling
Termination temperature Inventive Steel A A-1 300 1065 985 O 910 870 7 415 Honor A-2 250 1080 1000 O 890 850 8 430 Honor A-3 300 1120 1040 O 880 840 9 420 Honor A-4 250 1110 1030 X 820 780 6 410 Comparative Example A-5 300 1050 970 X 850 810 4 570 Comparative Example Invention steel B B-1 300 1070 990 O 748 708 7 420 Honor B-2 250 1075 995 O 733 693 8 415 Honor B-3 300 1110 1030 O 723 683 9 405 Honor B-4 250 1105 1025 X 755 715 6 410 Comparative Example B-5 300 1060 980 X 745 705 7 555 Comparative Example Inventive Steel C C-1 300 1085 1005 O 1013 973 8 430 Honor C-2 250 1075 995 O 988 948 10 417 Honor C-3 300 1125 1045 O 978 938 8 395 Honor C-4 250 1105 1025 X 820 780 7 357 Comparative Example C-5 300 1060 980 X 955 915 6 560 Comparative Example Inventive Steel D D-1 300 1075 995 O 914 874 7 433 Honor D-2 250 1065 985 O 889 849 9 422 Honor D-3 300 1115 1035 O 879 839 10 400 Honor D-4 250 1095 1015 X 820 780 7 362 Comparative Example D-5 300 1150 1100 X 840 800 5 565 Comparative Example Invention steel E E-1 300 1060 980 O 907 867 6 415 Honor E-2 250 1050 970 O 882 842 9 427 Honor E-3 300 1100 1020 O 872 832 11 415 Honor E-4 250 1080 1000 X 820 780 8 400 Comparative Example E-5 300 1035 1015 X 850 810 7 545 Comparative Example Invention river F F-1 300 1080 1000 O 1010 970 6.5 415 Honor F-2 250 1070 990 O 955 915 9.5 405 Honor F-3 300 1120 1040 O 975 935 11.5 385 Honor F-4 250 1100 1020 X 820 780 8.5 359 Comparative Example F-5 300 1055 1035 X 950 910 7.5 580 Comparative Example Invention river G G-1 300 1095 1015 O 887 847 5.5 419 Honor G-2 250 1085 1005 O 867 827 7.5 409 Honor G-3 300 1135 1055 O 857 817 10.5 399 Honor G-4 250 1115 1035 X 820 780 9.5 367 Comparative Example G-5 300 1130 1070 X 900 860 8 577 Comparative Example Invention steel H H-1 300 1073 993 O 768 728 6.7 400 Honor H-2 250 1064 984 O 748 708 8.9 411 Honor H-3 300 1113 1033 O 738 697 11.5 389 Honor H-4 250 1096 1016 X 820 780 10.5 377 Comparative Example H-5 300 1053 973 X 918 877 9.5 567 Comparative Example Comparative River A A-1 300 1077 910 O 875 835 8 387 Honor A-2 250 1067 905 O 855 815 9 396 Honor A-3 300 1125 900 O 845 805 10 380 Honor Comparative Steel B B-1 300 1090 1010 O 963 923 9 375 Comparative Example B-2 250 1075 995 O 943 903 10 389 Comparative Example B-3 300 1110 1000 O 933 893 11 395 Comparative Example Comparative Steel C C-1 300 1087 795 O 753 714 12 410 Comparative Example C-2 250 1077 790 O 733 694 15 390 Comparative Example C-3 300 1119 787 O 723 684 12 400 Comparative Example Comparative Steel D D-1 300 1099 950 O 955 915 8 395 Comparative Example D-2 250 1088 970 O 935 895 9 380 Comparative Example D-3 300 1067 980 O 925 885 7 399 Comparative Example

In Table 2, the slab thickness is in mm, the cooling rate is in ° C / sec, and the remaining temperature units are in ° C.

Steel grade number Product thickness
(mm) YS
(Mpa) TS
(Mpa) ZRA
(%) Martensite DBTT
(° C) Inventive Steel A A-1 80 576 689 75 0.8 -63 A-2 85 574 693 65 0.9 -60 A-3 80 580 695 60 0.8 -62 A-4 90 558 685 35 0.7 -64 A-5 85 512 691 30 3.5 -27 Invention steel B B-1 80 534 661 69 0.8 -62 B-2 85 538 663 75 0.8 -63 B-3 80 544 666 70 0.7 -65 B-4 90 533 657 34 0.7 -64 B-5 85 500 671 29 3.1 -31 Inventive Steel C C-1 80 587 686 77 0.9 -60 C-2 85 596 691 65 0.8 -62 C-3 80 590 683 60 0.6 -67 C-4 90 570 676 34 0.4 -74 C-5 85 534 689 30 3.2 -29 Inventive Steel D D-1 80 581 718 65 0.9 -59 D-2 85 590 723 66 0.8 -61 D-3 80 599 724 70 0.6 -66 D-4 90 584 711 34 0.4 -73 D-5 85 526 721 25 3.3 -28 Invention steel E E-1 80 602 802 57 0.8 -63 E-2 85 611 811 78 0.9 -60 E-3 80 623 816 77 0.8 -63 E-4 90 603 806 32 0.6 -66 E-5 85 566 814 30 2.9 -33 Invention river F F-1 80 691 904 75 0.8 -63 F-2 85 700 912 80 0.7 -65 F-3 80 719 916 67 0.5 -69 F-4 90 686 904 34 0.4 -74 F-5 85 645 919 33 3.7 -24 Invention river G G-1 80 598 723 67 0.8 -62 G-2 85 608 728 78 0.7 -64 G-3 80 624 736 79 0.6 -66 G-4 90 621 730 34 0.5 -72 G-5 85 573 742 28 3.6 -25 Invention steel H H-1 80 608 808 66 0.6 -66 H-2 85 613 815 79 0.7 -63 H-3 80 631 821 85 0.6 -68 H-4 90 641 818 36 0.5 -70 H-5 85 603 831 33 3.4 -27 Comparative River A A-1 85 504 514 65 0.6 -68 A-2 90 504 517 60 0.6 -66 A-3 80 511 519 70 0.5 -70 Comparative Steel B B-1 85 499 503 75 0.5 -71 B-2 90 498 507 65 0.6 -68 B-3 80 500 511 70 0.6 -67 Comparative Steel C C-1 85 491 529 68 0.7 -64 C-2 90 509 536 66 0.6 -68 C-3 80 489 528 65 0.6 -66 Comparative Steel D D-1 85 534 577 65 0.6 -67 D-2 90 540 578 75 0.5 -70 D-3 80 523 574 60 0.6 -66

As shown in Tables 1 to 3, the inventions manufactured according to the production method of the present invention using the inventive steels A to H satisfying the alloy composition of the present invention are characterized in that the warp martensite is 1% or less by area and the tensile strength (ZRA) of not less than 50% and the impact transition temperature (DBTT) of not more than -50 ° C, the strength is high and the core property is excellent.

On the other hand, in spite of using the inventive steels A to H satisfying the alloy composition of the present invention, when the first cooling step is not performed, it is confirmed that the Z axis tensile cross-sectional shrinkage ratio (ZRA) is less than 50% have.

In addition, even though inventive steels A to H satisfying the alloy composition of the present invention are used, when the secondary cooling finishing temperature is out of the control range of the present invention, graphite martensite is formed to exceed 1% (DBTT) is higher than -50 ° C.

Further, when comparative steels A to D which do not satisfy the alloy composition of the present invention were used, even when produced according to the production conditions of the present invention, the tensile strength was less than 600 MPa and high strength could not be obtained.

It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined in the appended claims. It will be possible.

Claims (11)

0.001 to 0.1% of Al, 0.005 to 0.05% of Al, 0.005 to 0.1% of Ti, 0.01 to 0.6% of Cr, 0.02 to 0.12% of C, 0.3 to 2.5% , Nb: 0.005 to 0.10%, B: 5 to 40 ppm, N: 15 to 150 ppm, and the balance Fe and other unavoidable impurities. The microstructure is composed of a high strength pole Steel for roof structure.
The method according to claim 1,
The steel material may further comprise one or more selected from the group consisting of 0.05 to 1.0% of Cr, 0.01 to 1.0% of Mo, 0.01 to 2.0% of Ni, 0.01 to 1.0% of Cu and 0.005 to 0.3% of V Which is excellent in the properties of the center part including the high strength steel.
The method according to claim 1,
A portion of the microstructure of the steel material excluding bare martensite is bainitic ferrite, which is excellent in core property and has high strength.
The method according to claim 1,
The steel material has a thickness of 80 mm or more and is excellent in the properties of the center portion.
The method according to claim 1,
The steel material has a tensile strength of 600 MPa or more and is excellent in core property.
The method according to claim 1,
The steel material has a Z axis tensile cross-sectional shrinkage (ZRA) of 50% or more.
0.001 to 0.1% of Al, 0.005 to 0.05% of Al, 0.005 to 0.1% of Ti, 0.01 to 0.6% of Cr, 0.02 to 0.12% of C, 0.3 to 2.5% Heating the steel slab to a temperature range of 1000 to 1250 캜, the steel slab containing 0.005 to 0.10% of Nb, 5 to 40 ppm of B, 15 to 150 ppm of N, and the balance Fe and other unavoidable impurities;
Subjecting the heated steel slab to rough rolling;
The rough-rolled steel slab is firstly cooled from the surface of the steel slab to a depth corresponding to 10 to 20% of the thickness of the steel slab at a cooling rate of 5 ° C / s or higher so as to be lower than the bainite transformation start temperature (Bs) Car cooling step;
Subjecting the cooled steel slab to finish rolling to obtain a hot-rolled steel sheet; And
A secondary cooling step of cooling the hot-rolled steel sheet to a temperature lower than the bainite transformation completion temperature (Bf) at a cooling rate of 5 to 50 占 폚 / s;
Wherein the core material has an excellent physical property.
8. The method of claim 7,
Wherein the steel slab comprises one or more selected from the group consisting of 0.05 to 1.0% of Cr, 0.01 to 1.0% of Mo, 0.01 to 2.0% of Ni, 0.01 to 1.0% of Cu and 0.005 to 0.3% of V, Wherein the core material has a high mechanical strength.
8. The method of claim 7,
Wherein the steel slab has a thickness of 250 to 300 mm.
8. The method of claim 7,
Wherein the rough rolling is performed in a temperature range of Tnr to 1250 占 폚.
8. The method of claim 7,
Wherein the finishing rolling is performed in a temperature range of Bs to Tnr.

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