KR100900657B1 - High Strength Hot Dip Coated Steel Sheet Having Good Formability And Manufacturing Method Thereof - Google Patents

Abstract

In the present invention, C: 0.08 to 0.15%, Si: 0.01 to 0.3%, Mn: 1.0 to 2.0%, sol.Al: 0.02 to 0.05%, Sb: 0.005 to 1.0%, and Mo: 0.05 to 0.3% by weight. It relates to a high strength hot-dip galvanized steel sheet, which is characterized by consisting of B: 0.0002 to 0.002% and the balance Fe and other impurities.

The hot-dip galvanized steel sheet according to the present invention can effectively suppress the coarsening of surface concentrates by including Sb, thereby ensuring stable surface quality, and improving tensile strength by adding Mo and B. Economically valuable steel sheet can be provided.

Tensile Strength, Hardening Hardening, Two-Phase Structure, Martensite, Ferrite, Bainite

Description

High Strength Hot Dip Coated Steel Sheet Having Good Formability And Manufacturing Method Thereof}

The present invention relates to a high-strength hot-dip galvanized steel sheet having excellent workability and a method for manufacturing the same, and more particularly, to a hot-dip galvanized steel sheet having a tensile strength of 490 MPa or higher used in structural members such as automobile inner member and the like and its manufacture It's about how

High-strength hot-dip galvanized steel sheet is used by press working in automobiles, home appliances, electrical appliances, and construction, etc., and refers to a steel sheet coated with metal oxide or organic coating on the upper layer for workability and rust prevention. In particular, hot dip galvanized steel is widely used in such hot dip galvanized steel.

Recently, as the impact safety regulations of automobiles have spread, application of precipitation hardening type high strength steel sheets has been widely applied to structural members such as members, beams, and pillars in order to improve impact resistance of a vehicle body.

Precipitation hardening high-strength steel sheet is designed to absorb the collision energy of the vehicle is characterized by a high yield strength (YS), that is, yield ratio (YS / TS) compared to the tensile strength (TS). This is because it is necessary to increase the thickness of the steel sheet in order to improve the impact resistance in terms of absorbing impact energy during the collision of automobiles, as well as a material of high strength steel having a high yield ratio.

In the conventional precipitation-reinforced high-strength steels, carbon, nitride forming elements such as Cu, Nb, Ti, V, etc. are mainly added to increase the strength by causing precipitation strengthening effects. -115210, Unexamined-Japanese-Patent No. 3-140412, and Unexamined-Japanese-Patent No. 11-241119 are mentioned.

Japanese Laid-Open Patent Publication No. 2001-115210 discloses tensile strength by adding a large amount of Ti and Nb to low carbon steel, hot rolling at a temperature above Ar ₃ transformation point, cooling at a cooling rate of 10 ° C / sec or more and reheating to around 400 to 700 ° C. It proposes a technology for producing high strength steel sheet with a strength of 590 MPa or more. However, when a large amount of Ti and Nb is added as in Japanese Laid-Open Patent Publication No. 2001-115210, since the recrystallization temperature increases, the possibility of material variation by the length of the coil after annealing increases, furthermore, in the manufacture of plated steel sheet. There is also a problem that the possibility of alloying failure is also very high.

On the other hand, Japanese Patent Laid-Open Nos. Hei 3-140412 and Japanese Patent Laid-Open No. Hei 11-241119 relate to a method of manufacturing a precipitation-reinforced high strength steel sheet using Cu precipitates, and a technique of manufacturing a steel sheet by adding 0.8 wt% or more of Cu. Presenting. However, the above techniques also have problems of poor alloying of the plated steel sheet by Cu-based precipitates and problems of deterioration of weldability, thereby causing problems in field applications.

In addition, the above-described prior arts have a problem in that a large amount of expensive alloying elements must be added, and the production cost of steel sheet is increased.

The present invention solves the problems of the prior art by simultaneously utilizing the precipitation strengthening and solid solution strengthening element, and at the same time provide a high-strength hot-dip galvanized steel sheet and a method of manufacturing the same with less tensile strength and less variation.

The present invention contains, in weight percent, C: 0.08 to 0.15%, Si: 0.01 to 0.3%, Mn: 1.0 to 2.0%, sol.Al: 0.02 to 0.1%, and Sb: 0.005 to 1.0%, and Mo: 0.05 -0.3%, B: 0.0002 ~ 0.002% or a mixture thereof, the balance relates to a high strength hot-dip galvanized steel sheet with excellent processability, characterized in consisting of Fe and other impurities.

Furthermore, the present invention, after heating the steel slab of the above-described component, 1150 ~ 1250 ℃, hot finishing rolling at 880 ~ 920 ℃; Manufacturing a hot rolled steel sheet by winding in a temperature range of 580 to 640 ° C; After pickling the hot rolled steel sheet, cold rolling at a reduction ratio of 55 to 90%; Recrystallization annealing the cold rolled steel sheet by the cold rolling at 750 ~ 830 ℃; And after the annealing the cold-rolled cold-rolled steel sheet at 450 ~ 600 ℃, cooling to 3 ~ 20 ℃ / sec and tempered rolling at 0.3 ~ 0.7% reduction rate, the production of high strength hot-dip galvanized steel sheet excellent in workability It is about a method.

The present inventors conducted research and experiments to improve the problems of the prior art, as a result of controlling the fraction of the microstructure of the steel by appropriately adjusting the component-based conditions, rolling conditions and heat treatment conditions as described in the present invention, high-strength hot-melt plating excellent in workability It has been found that steel sheet can be manufactured economically.

Hereinafter, the component conditions of the present invention will be described in detail. However, the content of the alloying element is weight%.

C plays an important role as a precipitate forming element. If the content of C is less than 0.08%, the formation of cementite or pearlite is accelerated, thus making it difficult to secure tensile strength. On the other hand, if the C content exceeds 0.15%, there is a high possibility of crack cracking in the steelmaking process, the rolling load increases during cold rolling, and the martensite structure is formed upon cooling after annealing. Because of its shape, mechanical properties may be degraded. Therefore, the content of C is limited to 0.08 to 0.15%.

Si should be added at least 0.01% to improve the strength by deoxidation and solid solution strengthening, but if it exceeds 0.3%, the oxide may be concentrated on the surface of the steel sheet during continuous annealing, causing surface defects. It is desirable to limit it to 0.3%.

Mn is a stabilizing element of austenite and is added at least 1.0% to prevent the transformation of austenite into martensite until it is deposited in the plating bath. However, if the added amount exceeds 2.0%, the desired strength is secured, but the elongation decreases rapidly. Therefore, the annealing of the Mn oxide to the steel sheet surface during annealing is severe, and the surface cleanliness and oxidation resistance are lowered. Therefore, the content of Mn is limited to 1.0 to 2.0%.

P, S and N are elements that are inevitably added as impurities in the steel, so it is desirable to manage them as low as possible.

Al is an element added for deoxidation. If the content is less than 0.02%, the effect of addition cannot be sufficiently obtained and it is a factor of deterioration of weldability. Therefore, Al is added more than 0.02%. In addition to the possibility of defects on the surface of the steel sheet due to the excessive formation of inclusions during the production operation, there is a problem that the manufacturing cost increases, the content is limited to 0.02 ~ 0.1%.

Sb is a very important component in the present invention is added 0.005 ~ 1.0%, Sb inhibits the surface thickening of MnO, SiO ₂ , Al ₂ O ₃ and the like, and coarsening of the surface thickening according to the temperature rise and hot rolling process changes Add because it has an excellent effect on suppression. However, when the content of Sb is less than 0.005%, the above-described effects are hardly obtained. On the other hand, when the content of Sb is more than 1.0%, a better effect can not be expected as well as the manufacturing cost and workability are lowered. Limit the content of.

Mo and B are generally known as elements to increase hardenability, inhibiting bainite transformation during cooling, and after cooling to room temperature, martensite and retained austenite in ferrite To facilitate mixing and metallization, Mo and / or B are added at least 0.05% or at least 0.0002%, respectively. However, if the addition amount of Mo and / or B exceeds 0.3% or 0.002%, respectively, since the sufficient ferrite volume fraction does not grow, rather the martensite volume fraction increases, resulting in sufficient tensile strength. Since it cannot be obtained, 0.05 to 0.3% of Mo and / or 0.0002 to 0.002% of B are added.

Cr, Ni, and Ca are fine-grained elements, and when one or two or more of them are added at 0.05% or more, the workability deterioration can be suppressed even when the volume fraction of martensite and retained austenite is large. . However, when the amount of one or two or more of Cr, Ni, and Ca exceeds 0.6%, the effect does not increase anymore, so add 0.05 to 0.6%.

Hereinafter, the manufacturing conditions of the present invention will be described in detail.

The cold rolled steel sheet of the present invention is manufactured through a hot rolling process, a cold rolling process and a continuous annealing process for the steel slab formed as described above.

First, the steel slab having the above-described components is heated to 1150 to 1250 ° C. and hot finished rolled at 880 to 920 ° C. for complete solid solution of Nb and reduction of load during hot rolling. If the finish rolling temperature exceeds 920 ℃ yield strength may be reduced because the grain size during hot finish rolling increases.

Then, it is wound in a temperature range of 580 ~ 640 ℃ to produce a hot rolled steel sheet. If the coiling temperature is less than 580 ° C, the solid solution C may not be completely precipitated, thereby lowering the precipitation strengthening effect and may cause a problem of poor plate shape during winding. On the other hand, if the coiling temperature exceeds 640 ℃ precipitates can be coarsened and also the strength of precipitation strengthening is not so great yield strength may be lowered.

The hot rolled steel sheet manufactured as described above is cold rolled after pickling treatment. Here, the cold rolling reduction rate is 55 to 90% or more. If the cold rolling reduction is less than 55%, sufficient crystals are not formed due to the reduction of the critical nucleation site for precipitate nucleation. While there is a problem in that the material deviation is large, in the case of exceeding 90%, the cold rolling load is generated so as to limit the reduction ratio as described above.

Next, the cold rolled steel sheet is subjected to annealing at a recrystallization temperature or higher in a continuous annealing line. It is preferable to perform recrystallization annealing at 750-830 degreeC. If the annealing temperature is less than 750 ℃, that is, the unrecrystallized region, the material variation in the coil is very high, which causes a problem in practical use of the steel sheet. Annealing temperature conditions are limited to 750 ~ 830 ℃ because the yield strength may not meet the standard.

And hot-dip galvanizing is carried out at a common temperature of 450 ~ 600 ℃, plated, cooled to 3 ~ 20 ℃ / sec, and then tempered rolling at 0.3 ~ 0.7% reduction rate to the final production of high strength hot-dip galvanized steel sheet with excellent workability do.

The tissue fraction of the present invention is characterized by consisting of ferrite, martensite and bainite, it is preferable that the martensite fraction has a 30 to 45% tissue. If the martensite fraction is less than 30%, the high tensile strength targeted by the present invention cannot be secured, and if the martensite fraction is less than 45%, the elongation is insufficient and the effect excellent in workability cannot be expected. In addition, the bainat fraction is a structure capable of securing elongation without significantly inhibiting the effect of improving the tensile strength of martensite.

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

(Example)

Hot-rolled process, cold-rolled in the steel slab formed as shown in the following Table 1 (target value of the steel composition of the invention steel and comparative steel: unit wt%) After performing the process, the hot-dip plating process and the temper rolling process, the tissue fraction and tensile strength were measured and the results are shown in Table 2 below.

Processes other than the conditions in Table 2 were performed according to conventional methods and the mechanical property values are for specimens in which the tensile test specimen was parallel to the rolling direction.

division C Si Mn Al Sb Mo B Cr Ni Ca division A 0.095 0.07 1.15 0.08 0.12 0.06 0.0005 0.13 0.14 0.17 Foot river B 0.105 0.14 1.35 0.07 0.23 0.07 0.0002 - - - C 0.115 0.21 1.67 0.03 0.76 0.28 0.0004 - - - D 0.148 0.22 1.97 0.02 0.94 0.09 0.0015 0.35 0.33 0.39 E 0.132 0.11 1.73 0.09 0.27 0.08 0.0007 0.21 0.44 0.31 F 0.127 0.16 1.30 0.08 0.45 0.16 0.0008 0.44 0.40 0.33 G 0.111 0.07 1.10 0.09 0.33 0.18 0.0010 - 0.22 - H 0.120 0.17 1.71 0.07 0.97 0.11 0.0012 - 0.41 - I 0.096 0.15 1.66 0.07 0.85 0.21 0.0007 0.25 - - J 0.085 0.10 1.21 0.03 0.75 0.19 0.0006 0.45 - - K 0.103 0.03 1.45 0.02 0.41 0.10 0.0012 - 0.27 L 0.112 0.05 1.12 0.05 0.44 0.08 0.0060 - 0.44 M 0.052 0.12 2.24 0.040 - 0.03 - - - - Comparison river N 0.080 0.04 1.83 0.035 - 0.02 - - - - O 0.087 0.17 2.19 0.05 - 0.03 - - - - P 0.096 0.13 2.37 0.023 - 0.03 - - - -

division Heating temperature Rolling temperature Winding temperature Annealing Temperature Cooling rate Cold rolling reduction Temper rolling reduction Ferrite Volume Fraction Martensitic volume fraction TS (Mpa) Elongation evaluation division A 1175 887 597 777 4 ° C / sec 56 0.4 50 39 515 Good ○ Foot river B 1215 901 604 797 15 ° C / sec 55 0.7 60 38 657 Good ◎ C 1215 904 610 794 17 ° C / sec 59 0.4 57 40 697 Good ◎ D 1167 905 621 811 17 ° C / sec 75 0.4 57 42 974 Good ◎ E 1203 910 612 805 19 ° C / sec 67 0.5 50 41 992 Good ◎ F 1190 905 611 812 13 ° C / sec 55 0.4 60 39 782 Good ◎ G 1167 911 589 767 19 ° C / sec 67 0.4 54 42 882 Good ◎ H 1234 913 588 812 18 ° C / sec 87 0.7 53 43 792 Good ◎ I 1207 902 601 823 10 ° C / sec 66 0.6 61 35 901 Good ◎ J 1178 899 611 772 11 ° C / sec 64 0.6 55 42 922 Good ◎ K 1198 898 623 783 17 ° C / sec 62 0.5 62 36 791 Good ◎ L 1205 917 614 801 9 ° C / sec 63 0.4 56 41 801 Good ◎ M 1189 921 597 798 10 ° C / sec 87 0.6 50 47 999 Bad x Comparison river N 1235 910 584 767 29 ° C / sec 86 0.8 49 49 978 Bad x O 1256 876 583 794 11 ° C / sec 71 0.4 90 7 442 Good ○ P 1145 905 590 777 4 ° C / sec 65 0.5 63 30 513 Good ○

As described above, by adding Sb to the hot-dip galvanized steel sheet of the present invention, it is possible to effectively suppress the coarsening of the surface thickener, and to ensure stable surface quality and to improve the tensile strength by adding Mo and B. By controlling the martensite and bainite fraction, high tensile strength and elongation can be secured at the same time, and a small amount of expensive alloying elements can be added to provide a steel plate with high industrial and economic value.

Claims (4)

By weight% C: 0.08 ~ 0.15%, Si: 0.01 ~ 0.3%, Mn: 1.0 ~ 2.0%, sol.Al:0.02~0.1%, Sb: 0.005 ~ 1.0%, Mo: 0.05 ~ 0.3%, B: 0.0002 ~ 0.002% and balance Fe and other impurities, The microstructure contains 30 to 45% martensite by volume fraction, is made of residual ferrite, and has high processability, characterized in that it contains 5% or less (excluding 0%) bainite. Plated steel sheet. According to claim 1, wherein the hot-dip galvanized steel sheet is a high-strength melt excellent workability, characterized in that it further comprises 0.05 to 0.6% of one or two or more components selected from the group consisting of Cr, Ni and Ca in weight% Plated steel sheet. delete Heating the steel slab composed of the components of claim 1 or 2 to 1150 to 1250 ° C. and then hot finishing rolling at 880 to 920 ° C .; Manufacturing a hot rolled steel sheet by winding in a temperature range of 580 to 640 ° C; After pickling the hot rolled steel sheet, cold rolling at a reduction ratio of 55 to 90% to produce a cold rolled steel sheet; Recrystallization annealing the cold rolled steel sheet at 750 ~ 830 ℃; And After the annealing the cold-rolled steel sheet at 450 ~ 600 ℃ hot-dip plating, it is cooled to 3 ~ 20 ℃ / sec and tempered rolling at 0.3 ~ 0.7% reduction rate, manufacturing method of high strength hot-dip galvanized steel sheet excellent in workability .