KR100568367B1 - Method for manufacturing high strength galvannealed steel sheets excellent in drawability and resistance of secondary work embrittlement - Google Patents

Abstract

The present invention relates to a method for producing an alloyed hot dip galvanized steel sheet.

In the present invention, by weight%, C: 0.0005 to 0.01%, Si: 0.05 to 0.30%, Mn: 0.3 to 2.0%, P: 0.02 to 0.1%, S: 0.02% or less, N: 0.003% or less, Sol.Al : 0.01 to 0.2%, B: 0.0003 to 0.003%, Ti: 0.005 to 0.05% and Nb: 0.005 to 0.05% of at least one of the remaining Fe and other unavoidable impurities, the B and Si is 0.005 ≤ B / The steel satisfying Si ≦ 0.02 may be hot and cold rolled, annealed for 10 to 120 seconds at a temperature of 760 to 830 ° C., and then alloyed hot dip galvanized at 480 to 540 ° C.

In addition, according to the present invention there is an effect that can provide a high strength alloyed hot-dip galvanized steel sheet excellent in moldability and secondary workability.

Alloyed hot dip galvanized, high strength, secondary processing brittleness, alloy plating layer, formability

Description

Method for manufacturing high strength galvannealed steel sheets excellent in drawability and resistance of secondary work embrittlement}             

1 is a graph showing the change in ductile brittle transition temperature according to the Fe content of the alloy layer.

The present invention relates to a method for manufacturing an alloyed hot-dip galvanized steel sheet, and more particularly, to a method for manufacturing a high strength alloyed hot-dip galvanized steel sheet having excellent moldability and secondary work resistance for use in automobile panels and structural parts requiring corrosion resistance. It is about.

Recently, in order to improve fuel efficiency of automobiles in relation to environmental problems, researches for lightening the vehicle body have been conducted. Accordingly, the use of high strength steel sheets is increasing. At the same time, in order to extend the life of the vehicle, the demand for corrosion resistance of the vehicle body, such as rust prevention and corrosion of holes, is being strengthened. As a countermeasure, the application rate of plated steel sheet is increasing day by day.

Conventional techniques related to high strength steel sheets excellent in formability include Japanese Patent Application Laid-Open Nos. 57-57945, 59-74232, 5-59491, 5-214487, 63-47338, and 5-9698. There is an arc.

Japanese Laid-Open Patent Publication No. 57-57945 is a steel sheet in which P, Si, Mn, and the like are added to so-called IF steel in which carbonitride-forming elements such as Ti or Nb are added to ultra low carbon steel as a solid solution strengthening element. . However, in the prior art, P is a very effective element as a solid solution strengthening element because P has a higher reinforcing capacity than Si and Mn and a small tendency to decrease r value in the presence of solid carbon, but secondary processing brittleness increases due to grain boundary segregation of P. There is a problem.

Japanese Laid-Open Patent Publication No. 59-74232 is a prior art for solving such a problem of secondary processing brittleness. In the above prior art, a large amount of B is added to ultra low carbon steel to which P is added, so that B in solid solution exists at grain boundaries, thereby ensuring high secondary processing brittleness. However, there is a problem that the suppression effect of secondary processing brittleness by the addition of B is significantly reduced as the amount of P addition and Mn addition increases.

Therefore, Japanese Patent Laid-Open Nos. Hei 5-59491 and Hei 5-214487 propose a solid solution strengthening method using Si and Mn instead of solid solution strengthening by P. However, since Si is an element that is easy to oxidize, an oxide film is formed on the surface during hot rolling and annealing, thereby causing problems such as unplating in the hot dip galvanizing process, and in the case of Mn, since Mn has a small reinforcing ability, Mn is added in a large amount. In this case, the secondary processing brittleness is not only deteriorated, but also increases the cost.

In addition, Japanese Laid-Open Patent Publication No. 63-47338 solves the problem of secondary work brittleness by simultaneously securing an optimum amount of B and solid solution carbon using IF steel containing Si and P. There is a problem in that there is no measure for improvement of plating adhesion and alloying speed.

Therefore, in JP-A-5-9698, the oxide balance of the atmosphere gas in the furnace in the process of hot rolling the IF steel containing Si, Mn, and P after hot rolling at 600 ° C or higher and annealing at 800 to 950 ° C. The coating adhesion was improved by lowering the Si concentration to less than 1.5 mg / m 2 up to 300 kPa on the surface of the steel sheet before being deposited in the molten zinc bath. However, in the prior art, since the Si concentration is adjusted by the oxidation balance during heating, precise control is difficult, and when the Si concentration is 1.5 mg / m 2 or more, the plating adhesion is rapidly degraded and the alloying speed is delayed. .

The present invention is to solve the above-mentioned problems of the prior art, in the high strength alloyed hot-dip galvanized steel sheet containing P-containing Ti-B or Ti-Nb-B-added ultra-low carbon steel as a base, It is an object of the present invention to provide a high strength alloyed hot dip galvanized steel sheet excellent in moldability and secondary workability by suppressing the grain boundary diffusion of zinc into the steel sheet surface by appropriately controlling the Fe content.

The present invention for achieving the above object, in weight%, C: 0.0005 ~ 0.01%, Si: 0.05 ~ 0.30%, Mn: 0.3 ~ 2.0%, P: 0.02 ~ 0.1%, S: 0.02% or less, N : 0.003% or less, Sol.Al: 0.01% to 0.2%, B: 0.0003% to 0.003%, Ti: 0.005% to 0.05%, and Nb: 0.005% to 0.05%, and are composed of the remaining Fe and other unavoidable impurities. Including hot and cold rolling of steels with B and Si satisfying 0.005 ≦ B / Si ≦ 0.02, annealing at a temperature of 760-830 ° C. for 10-120 seconds and hot-dip galvanizing at 480-540 ° C. Is done.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

The present invention controls the formation of steel sheet surface oxide by controlling the ratio of Si and B to ultra low carbon steel to which Si, Mn, P, etc. are added, and by appropriately controlling the alloying temperature during alloying hot dip galvanizing. By controlling the Fe content of the alloy plating layer in contact with 13% or less, it is characterized by ensuring excellent moldability and secondary workability.

Hereinafter, the component and manufacturing conditions of this invention are demonstrated in detail.

C: 0.0005 to 0.01 wt% (hereinafter simply referred to as '%')

The C is preferably an element for inhibiting the development of the (111) aggregate structure of the annealing plate to reduce the formability and to precipitate it in the hot rolled plate by adding Ti or Nb. If the content of C is less than 0.0005%, the degassing cost for lowering carbon content is too high, and if it exceeds 0.01%, the amount of Ti or Nb for depositing C is increased to increase the cost, and if carbide is increased, moldability is increased. Since there is a problem that is lowered, the content is preferably limited to 0.0005 ~ 0.01%.

Si: 0.05 ~ 0.30%

Si is a component effective for increasing the strength as a solid solution strengthening element. If the content of Si is less than 0.05%, the effect of the addition is too small, and if it is added in excess of 0.30% by weight, a complex oxide film is formed with Mn and B, which are concentrated on the surface of the steel, so that unplating occurs. The content is preferably limited to 0.05 to 0.30%.

Mn: 0.3 ~ 2.0%

The Mn is a solid solution hardening element and is an effective component for improving the strength. When the content of Mn is less than 0.3%, the effect of addition is too small, and when added in excess of 2.0%, there is a problem in that moldability and plating adhesion are lowered, so the content is preferably limited to 0.3 to 2.0%. .

P: 0.02 ~ 0.1%

P is a component having the highest solid solution strengthening effect. If the content of P is less than 0.02%, the solid solution strengthening effect is too small. If the content of P exceeds 0.1%, excessive FeTiP is formed to increase the annealing recrystallization temperature, thereby degrading the formability and increasing the grain boundary segregation to increase the secondary processing. Since the brittleness and the spot weldability deteriorate, the content thereof is preferably limited to 0.02 to 0.1%.

S: 0.02% or less

When the content of S exceeds 0.02%, it forms a harmful inclusion MnS and lowers the hot workability, so the content is preferably limited to 0.02% or less.

N: 0.003% or less

N is a component that forms a precipitate by combining with Ti or Nb in the steel. When the content of N exceeds 0.003%, the drawing property is lowered, so it is preferable to limit the content to 0.003% or less.

Sol.Al: 0.01 ~ 0.2%

Sol.Al is an effective component to deoxidize molten steel to lower oxygen in the steel and to segregate at grain boundaries in high temperature regions to refine hot-rolled sheet grains and carbides. If the content of Sol.Al is less than 0.01%, deoxidation is insufficient. If the content of Sol.Al exceeds 0.2%, not only the alloy cost is excessively increased but also the moldability is degraded, the content is preferably limited to 0.2% or less.

B: 0.0003-0.003%

The B is to increase the grain strength to improve the secondary work embrittlement resistance, and in the case of hot-dip galvanized steel sheet to suppress the phenomenon that the molten zinc diffuses to the grain boundary of the steel sheet to form a Zn-Fe intermetallic compound to weaken the grain boundary It is a valid ingredient. In addition, since the oxide film is formed on the surface of the steel sheet together with Si and Mn, the melting point of the oxide film is lowered and the surface tension is increased, so that the oxide film does not exist before the hot dip galvanizing in the steel sheet containing Si and Mn. Providing a surface has the effect of improving the adhesion between the molten zinc and the steel sheet. If the content of B is less than 0.0003%, the effect of the addition may not be obtained. If the content of B is more than 0.003%, the recrystallization temperature is excessively increased to lower moldability, and therefore the content is preferably limited to 0.0003 to 0.003%. .

At least one of Ti: 0.005 to 0.05% and Nb: 0.005 to 0.05%

Ti is an effective component for fixing formability by fixing nitrogen, sulfur and carbon in steel as precipitates. If the content of Ti is less than 0.005%, it is insufficient to bond with the elements. If the content of Ti is more than 0.05%, it promotes alloying of the plating layer and increases the Fe content of the alloy plating layer adjacent to the steel sheet, thereby inhibiting secondary workability. The content is preferably limited to 0.005 to 0.05%.

The Nb is an effective component to fix the carbon in the steel to improve the formability and to reduce the anisotropy of the r value and elongation. If the content of Nb is less than 0.005%, the effect of the addition is less, and if it exceeds 0.05%, the elongation is lowered, the content is preferably limited to 0.005 ~ 0.05%.

The remainder other than the above components is composed of Fe and other unavoidable impurities.

In the present invention, in order to maximize the effect of the addition of B, it is required to satisfy 0.005 ≦ B / Si ≦ 0.02. In other words, the effect of B is affected by the B / Si ratio, and if the B / Si ratio is less than 0.005, a local boron thickening layer is formed on the surface of the steel sheet to generate a local unplated, if exceeding 0.02 the surface of the boron Since the amount of thickening is increased to increase the fluidity of the composite oxide film to form a coarse oxide film, unplating is increased, so the B / Si ratio is preferably limited to 0.005 to 0.02.

The steel formed as described above is hot rolled and cold rolled, and then annealed at 760-830 ° C. for 10-120 seconds. That is, the cold rolled steel sheet is subjected to annealing for a predetermined time at a predetermined temperature before hot dip galvanizing to ensure formability. If the annealing temperature is less than 760 ℃ or annealing time is less than 10 seconds, the softening of the steel sheet is sufficient by annealing. It does not occur so that moldability deteriorates. In addition, when the annealing temperature exceeds 830 ° C. or when the annealing time exceeds 120 seconds, the amount of thickening of Si, Mn, and B is excessively increased, and the coating area of the oxide film is increased. do.

Subsequently, the annealed steel sheet is immersed in a molten zinc bath and then heated again to alloy the zinc plated layer to form various Zn-Fe compounds. In the case of an alloy plating layer of a conventional alloyed hot-dip galvanized steel sheet, compounds such as Γ phase (FeZn ₁₀ ), ζ phase (FeZn ₁₃ ), δ phase (FeZn ₇ ), etc. are formed from the interface with the steel sheet. %, 6.7 ~ 7.2%, 8.5 ~ 13% Fe content. In the present invention, the secondary work embrittlement resistance of the alloyed hot-dip galvanized steel sheet is lowered when a Γ phase having a large amount of Fe is formed on the surface of the steel sheet. By setting it as the following ζ phase and δ phase, the secondary workability is improved.

To this end, in the present invention, the alloying temperature is controlled to 480 ~ 540 ℃ during the alloying treatment. If the alloying temperature is less than 480 ℃ Zn-Fe alloy phase is not formed, if it exceeds 540 ℃ to develop the Γ phase by over-alloying to lower the secondary workability, the alloying temperature is limited to 480 ~ 540 ℃ It is desirable to.

When the alloying treatment is performed at the alloying treatment temperature as described above, the Fe content of the alloy plating layer adjacent to the steel sheet is controlled to 13% or less, thereby obtaining secondary work brittleness of -50 ° C or less.

The high strength alloyed hot dip galvanized steel sheet prepared according to the present invention has not only excellent moldability and secondary workability, but also tensile strength of 340 to 440 MPa.

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

EXAMPLE

The steel having the components shown in Table 1 was hot rolled to 3.2 mm with a hot finish rolling temperature of 920 ° C., wound up to a winding temperature of 560 ° C., and cold rolled to 0.8 mm after pickling.

Then, annealing and alloying hot dip galvanizing was performed under the conditions shown in Table 2 below. The mechanical properties of the specimens thus prepared, the surface inspection and powdering properties of the alloyed hot-dip galvanized steel sheet were investigated, and the results are shown in Table 2 below.

The powdering properties were evaluated by tape peeling width (W: mm) after 60 degree V-bending again.

division Ingredient Content (wt%) B / Si C Si Mn P S N Al B Ti Nb Inventive Steel A 0.0024 0.06 0.55 0.025 0.010 0.0016 0.11 0.0005 0.031 - 0.008 Inventive Steel B 0.0021 0.06 0.52 0.026 0.010 0.0024 0.13 0.0006 - 0.045 0.010 Invention Steel C 0.0018 0.06 0.59 0.030 0.010 0.0023 0.11 0.0006 0.028 0.009 0.010 Inventive Steel D 0.0032 0.14 0.91 0.051 0.011 0.0025 0.06 0.0011 0.021 0.015 0.008 Inventive Steel E 0.0041 0.16 1.21 0.086 0.011 0.0023 0.06 0.0015 0.020 0.047 0.009 Comparative Steel A 0.0120 0.14 0.90 0.071 0.012 0.0020 0.15 0.0010 0.032 0.011 0.007 Comparative Steel B 0.0035 0.50 0.91 0.052 0.011 0.0025 0.06 0.0011 0.021 0.015 0.002 Comparative Steel C 0.0033 0.21 2.25 0.042 0.010 0.0020 0.05 0.0008 0.025 0.030 0.004 Comparative Steel D 0.0035 0.13 0.93 0.123 0.011 0.0022 0.13 0.0012 0.029 0.011 0.009 Comparative Steel E 0.0030 0.13 0.90 0.076 0.012 0.0022 0.13 0.0002 0.032 0.013 0.0015 Comparative Steel F 0.0031 0.12 0.92 0.077 0.012 0.0022 0.13 0.0038 0.032 0.013 0.032 Comparative Steel G 0.0035 0.13 0.96 0.069 0.011 0.0025 0.10 0.0012 0.065 0.012 0.009 Comparative Steel H 0.0037 0.14 0.95 0.070 0.013 0.0015 0.12 0.0011 0.030 0.065 0.008

division Steel grade Annealing Temperature (℃) Annealing time (seconds) Alloying temperature (℃) Tensile Strength (kg / ㎠) Elongation (%) Drawability r value Powder Ring (W) Plating Invention 1 Inventive Steel A 790 90 500 35.2 41.6 2.02 2 Good Invention 2 Inventive Steel B 790 90 500 36.6 41.0 1.93 2 Good Invention 3 Invention Steel C 790 90 500 36.5 41.3 2.09 2 Good Invention 4 Inventive Steel D 790 90 520 41.6 38.8 1.76 3 Good Invention 5 Inventive Steel E 790 90 520 45.9 36.5 1.62 4 Good Comparative Material 1 Comparative Steel A 790 90 520 44.3 33.6 1.20 4 Good Comparative Material 2 Comparative Steel B 790 90 520 41.5 36.3 1.65 7 Bad Comparative Material 3 Comparative Steel C 790 90 520 46.5 31.8 1.39 9 Bad Comparative Material 4 Comparative Steel D 790 90 520 46.8 32.0 1.46 6 Bad Comparative Material 5 Comparative Steel E 790 90 520 40.8 39.2 1.78 7 Bad Comparative Material 6 Comparative Steel F 790 90 520 42.0 34.3 1.37 8 Bad Comparative Material7 Comparative Steel G 790 90 520 40.9 37.4 1.74 7 Bad Comparative Material 8 Comparative Steel H 790 90 520 43.5 34.7 1.45 4 Good Comparative Material 9 Inventive Steel D 740 90 520 43.8 28.6 1.22 3 Good Comparative Material 10 Inventive Steel D 850 90 520 39.4 37.7 1.67 6 Bad

As shown in Table 2, Inventive Materials 1 to 5 satisfying the scope of the present invention had a drawability of 1.6 or more showing moldability, and exhibited good characteristics of 4 mm or less in powdering property of the alloy plating layer.

On the other hand, Comparative Materials 1 to 10, which do not satisfy the scope of the present invention, were poor in drawing or powdering properties.

FIG. 1 is a drawing of a cylindrical cup with a drawing ratio of 2.0 using Inventive Material 5 and observing the fracture pattern generated along the wall of the cup when the weight is dropped while the cup temperature is changed in a low temperature bath. The relationship between the DBTT (ductility-brittle transition temperature), which is an index indicating secondary work brittleness of the alloyed hot dip galvanized steel sheet according to the Fe content of the alloy plating layer adjacent to the steel sheet, is shown. Table 3 shows the DBTT according to the Fe content of the alloy layer and the adjacent steel sheet.

Fe content of the alloy layer (wt%) DBTT (℃) 6.9 -65 7.3 -60 7.5 -60 7.5 -60 8 -55 8.8 -55 9.6 -55 10 -50 11 -50 14 -40 16 -40 17 -35 25 -25 27 -20 29 -20

As described above, according to the present invention, by using the ultra-low carbon steel containing one or more kinds of Ti or Nb which properly controlled the B / Si ratio, the Fe content of the interface between the steel sheet and the alloy plating layer is reduced to 13% or less, thereby forming There is an effect that can provide a high-strength alloyed hot-dip galvanized steel sheet having excellent properties and secondary processing brittleness of less than -50 ℃.

Claims (2)

By weight%, C: 0.0005 to 0.01%, Si: 0.05 to 0.30%, Mn: 0.3 to 2.0%, P: 0.02 to 0.1%, S: 0.02% or less, N: 0.003% or less, Sol.Al: 0.01 to 0.2%, B: 0.0003% to 0.003%, Ti: 0.005% to 0.05%, and Nb: 0.005% to 0.05%, and are composed of at least one of Fe and other unavoidable impurities, and B and Si are 0.005 ≦ B / Formability and secondary processing, including hot and cold rolling of a steel satisfying Si ≤ 0.02, annealing for 10 to 120 seconds at a temperature of 760 to 830 ° C, and then hot-dip galvanizing at 480 to 540 ° C Method for producing high strength alloyed hot dip galvanized steel sheet having excellent brittleness. The method of claim 1, wherein the Fe content of the alloy plating layer adjacent to the steel sheet after the alloying hot dip galvanizing is 13% or less.

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