JP2004270029A - Galvanized steel sheet excellent in zinc volatility resistance - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a galvanized steel sheet excellent in surface characteristic (corrosion resistance) and zinc volatile resistance with which the volatilization of zinc can be prevented, even in the case of performing a hot-working (hot press) and quenching after working. <P>SOLUTION: The galvanized steel sheet is obtained by forming the plating layer containing ≥ 60 mass% Zn and ≥ 40g/m<SP>2</SP>Zn quantity on the surface of the steel sheet containing, by mass%, 0.01-0.30% C, 0.005-1.0% Si, 0.01-3.0% Mn, 0.005-0.10% P, ≤ 0.02% S, 0.010-0.500% Al and 0.001-0.01% N. In this galvanized layer, a standard generating free energy variation (ΔG<SP>O</SP>) of oxide is smaller than a standard generating free energy variation (ΔG<SP>O</SP><SB>Zn</SB>) of ZnO at the same temperature at ≤ 1,000°C, and the total element quantity except alkali metal is contained at ≥0.25 mol/m<SP>2</SP>. In this way, a film of these elements is formed on the galvanized surface during heating and thus, the volatilization of the zinc can be restrained. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

  The present invention relates to a galvanized steel sheet having excellent zinc volatility for the purpose of increasing strength and improving corrosion resistance. In this specification, the term "steel plate" includes a steel pipe manufactured from a steel plate.

  In recent years, for the purpose of reducing the weight and improving safety of automobiles, the strength of automobile parts and the materials used for them has been increased, and steel sheets (including steel pipes), which are representative materials, are also made of high-strength steel sheets. Is increasing. However, since high-strength steel sheets are generally high-strength and hard, there is a limit to processing at room temperature, and there have been problems such as shortening the life of a processing die.

  Improvements from steel materials have been promoted to solve these problems, but in recent years, steel sheets have been heated to 800 ° C or higher with the aim of producing even higher strength parts efficiently and at low cost. It is softened and processed at the same time as it is rapidly cooled and quenched to create a part with extremely high strength. "Pre-processing quenching technology" is used as an industrial technology in which pre-processed steel material is processed at room temperature in advance, then heated to 800 ° C or more, then rapidly cooled and quenched to obtain extremely high strength parts. It has become.

On the other hand, zinc-plated steel sheets, which are excellent in cost, are widely used for industrial machines typified by automobiles because corrosion resistance in a use environment is necessary and sufficient. However, since the vapor pressure of zinc is large at low temperatures of 200 mmHg: 788 ° C, 400 mmHg: 844 ° C, and 760 mmHg: 907 ° C, there is a problem that when hot working zinc-coated steel sheets, zinc is volatilized when heated. . Therefore, as described in Patent Document 1, aluminum-plated steel sheets are used for hot working at present, but there are some which are not satisfactory in terms of corrosion resistance and cost.
JP 2000-38640 A

  The present invention solves the above-mentioned conventional problems, and can prevent the volatilization of zinc even when “hot working (hot pressing)” or “hardening after working” is performed, thereby improving the surface properties (corrosion resistance). The purpose of the present invention is to provide a zinc-coated steel sheet having excellent zinc volatility.

The present invention the assignment was made for solving of, at 1000 ° C. or less, the standard free energy change of the oxides at the same temperature than the standard free energy change of _{^{ZnO (△ G 0 Zn) (}} △ G 0) Is small, and contains one or more elements other than alkali metals in Zn plating, the total content of the elements is 0.25 mol / m ² or more, and Zn in the plating layer is 60% or more by mass%, The Zn content is 40 g / m ² or more, and the composition of the steel sheet is, in mass%, C: 0.01 to 0.30%, Si: 0.005 to 1.0%, Mn: 0.01 to 3.0%, P: 0.005 to 0.10%, S: ≦ 0.02%, Al: 0.010 to 0.500%, N: 0.001 to 0.01%. In addition, it is preferable to contain B: 2 to 100 ppm, Cr: 0.02 to 0.500%, Mo: 0.001 to 0.500%, and Ni: 0.001 to 1.000% by mass% as a component element of the steel sheet. As an element, one or more elements selected from Nb: 0.005 to 0.100%, V: 0.005 to 0.100%, Ti: 0.005 to 0.100%, Zr: 0.005 to 0.100% by mass% are contained. Is preferred.

Also, at 1000 ° C. or less, the standard free energy change of the oxides at the same temperature than the standard free energy change of _{^{ZnO (△ G 0 Zn) (}} △ G 0) is small, and Zn plating elements except for alkali metal It is preferable that the plating content of the element is not more than 15%, and the standard free formation of oxide at the same temperature is not more than 1000 ° C. and the change in standard free energy of formation of ZnO (ΔG ⁰ _Zn ). It is preferable that the energy change (ΔG ⁰ ) is small, and the content of elements other than alkali metal and elements other than Zn in the Zn plating layer is 5% or less by mass%. Further, the surface of the galvanized steel sheet as described above, Ni, Co, a plating containing 80% or more of Fe in mass%, it is possible to perform 1 g / m ² or more 10 g / m ² or less.

  The galvanized steel sheet having excellent zinc volatility resistance of the present invention can prevent the volatilization of zinc even in the case of hot working or quenching after working, so that it is conventionally used in automobiles and industrial machines. With the ease of use and performance of zinc-based plating of rust-proof steel sheets, the strength of these materials can be dramatically increased with high dimensional accuracy of parts. For this reason, it can greatly contribute to promoting weight reduction, safety improvement, and rust prevention of automobiles and industrial machines at an advantageous price, and the industrial contribution is great.

The present inventor first considers the importance of suppressing the volatilization of zinc in hot working, and in order to suppress the volatilization of zinc, it is possible to coat even following a rapid temperature change during heating, In addition, it was considered that a film having a sufficiently high boiling point should be formed on the surface of the zinc-based plating layer. As a result of an intensive study, it was found that the elemental change in the standard free energy of formation (△ G ⁰ ) of the oxide at the same temperature is smaller than the change in the standard free energy of formation (OG ⁰ _Zn ) of ZnO below 1000 ° C. It has been found that this problem can be solved by including in a zinc plating layer and setting the content of the element to 0.25 mol / m ² or more.

In other words, if a film is formed from the beginning on zinc plating, the film will be deficient due to the difference in elongation at the time of heating and zinc will be volatilized, but elements that are more easily oxidized than zinc and have a sufficiently high boiling point of oxide during zinc plating ( If an alkali metal (other than an alkali metal) is contained, these elements are oxidized on the zinc plating surface during heating to form a film. Since this film is formed on the galvanized surface during heating, it can be re-formed even if a defect occurs in the film due to thermal expansion, shrinkage, etc., and the volatilization of zinc can be suppressed. Such an effect is determined by the amount of these easily oxidizable elements per plating surface area, and the effect is sufficient if the total content of the elements is 0.25 mol / m ² or more. When Zn in the plating layer is less than 60% by mass%, it is difficult to form a film having a sufficiently high boiling point on the surface of the plating layer as described later, and when the Zn content is less than 40 g / m ² , the corrosion resistance is insufficient. Which is not desirable in applications such as automobiles. An example of an easily oxidizable element excluding an alkali metal, in which the standard change in free energy of formation (△ G ⁰ ) of an oxide at the same temperature is smaller than that of ZnO (自由 G ⁰ _Zn ) at 1000 ° C. or less Examples include rare earth elements such as Mg, Al, Si, Ca, Ti, Zr, and La and Ce.

Next, steel types will be described.
Since the present invention obtains strength by quenching in hot working in zinc-based plating, the composition of the steel sheet is 0.01 to 0.30%, Si: 0.005 to 1.5%, and Mn: 0.01% by mass. ~ 3.0%, P: 0.005 ~ 0.10%, S: ≤0.02%, Al: 0.010 ~ 0.500%, N: 0.001 ~ 0.01%, for the following reasons.

  First, if C is less than 0.01%, sufficient strength required after heat treatment cannot be secured. On the other hand, if it exceeds 0.30%, the hardness becomes too high and the press workability deteriorates. If the content of Si is less than 0.005%, sufficient strength required after the heat treatment cannot be secured, and the cost in steelmaking increases. Further, when the content is set to 1.0% or less, if it exceeds this, the plating wettability of zinc plating is reduced, and the plating appearance is reduced.

  The reason why Mn is set to 0.01% or more is that if it is less than 0.01%, sufficient strength required after heat treatment cannot be secured. The reason for setting the content to 3.0% or less is that if it exceeds this value, Mn is a strengthening element of steel, and the strength is increased and workability is impaired. If the content of P is less than 0.005%, the cost of steelmaking is increased, and a sufficient strength required after the heat treatment cannot be secured. The content of 0.10% or less is because even a small amount of P is a strengthening element for steel and increases the strength and impairs the workability. In addition, P is concentrated at the crystal grain boundaries and the grain boundaries become brittle. Is an element that is liable to cause cracking, and if added in excess of this, workability is impaired.

  The reason why S is set to 0.02% or less is that S is an impurity, and if it is too much, it not only causes cracking at the time of hot rolling, but also deteriorates the average Rankford value, so it should be reduced as much as possible. Considering the cost, the allowable range is 0.02% or less. Al is an element useful as a deoxidizer for molten steel. To obtain this effect, a content of at least 0.010% is required. However, if the content exceeds 0.500%, the cleanliness of the steel is impaired and surface flaws are likely to occur, so the upper limit of the content is 0.500%. If N is set to less than 0.001%, the cost for steelmaking increases. The content of 0.01% or less means that if added in excess of that, aging due to N will occur and the delayed aging will deteriorate.

  Further, one or more of the steel components B, Cr, Mo, and Ni are expressed in mass%, B: 2 to 100 ppm, Cr: 0.02 to 0.500%, Mo: 0.001 to 0.500%, Ni: 0.001 to 1.000. In the range of%, even if gradual cooling is performed at the time of cooling after heating, the hardenability is particularly improved, and the strength can be greatly increased by quenching after hot working.

  B exhibits an effect of increasing the strength by quenching at 2 ppm or more, and saturates the effect of quenching at 100 ppm or more, resulting in an increase in cost. If Cr is less than 0.02%, sufficient strength required after heat treatment cannot be secured. Further, the upper limit is set to 0.500%, because if it exceeds that, Cr is a strengthening element of steel, and the strength becomes too high, impairing the workability, and the strength increasing performance after heat treatment is saturated. This is because the cost is high and it is economically disadvantageous.

  Mo: If less than 0.001%, sufficient strength required after heat treatment cannot be secured. Further, the upper limit is set to 0.500% because Mo is a strengthening element of steel when it exceeds this, because the strength becomes too high and the workability is impaired, and it is expensive and economically disadvantageous. . If Ni is less than 0.001%, sufficient strength required after heat treatment cannot be secured. Further, the upper limit is set to 1.000%, because if it exceeds this, the strength becomes too high and the workability is impaired, and the strength increasing performance after the heat treatment is saturated. Because it is disadvantageous to

  Elements such as Nb, V, Ti, and Zr all form stable carbonitrides, suppress crystal grain coarsening during heating at 800 ° C or higher during quenching, and prevent deterioration of toughness. It has an effective action. To obtain such an effect, a content of 0.005% or more is required. If the content exceeds 0.100%, coarse carbonitrides are formed, and on the contrary, the toughness is reduced. Therefore, the upper limit of each content is set to 0.100%.

  It has been found that these elements which are not included in the oxidation form an alloy with Zn, but have a property of easily forming an alloy with Fe diffused from the base iron at the time of heating. In order to more effectively exhibit the corrosion resistance of the galvanized steel sheet, the iron concentration after heating is desirably 50% by mass or less. If the content of these easily oxidizable elements exceeds 15% by mass, the diffusion of iron becomes large, and a large thickness of Zn plating is required to secure corrosion resistance. Therefore, it is desirable that the mass% be 15% or less.

  Alloys with elements other than Zn and oxidizable elements hinder the diffusion of oxidizable elements to the surface, so the effect of suppressing the volatilization of zinc by forming an oxide film of oxidizable elements can be obtained with less oxidizable elements. Is preferably 5% by mass or less.

In addition, zinc-based plating has a low melting point, and it is necessary to suppress fusion by transferring a steel sheet in a heating furnace.However, this method can be applied to equipment such as an in-furnace grinding device. By applying a plating containing 80% or more by mass of Co, Co and Fe in an amount of 1 g / m ² or more and 10 g / m ² or less, it alloys from the plating layer surface during heating, raises the melting point of Zn, and Handling can be made more advantageous. The above-mentioned plating amount depends on the amount of zinc plating of the lower layer, but the effect is from 1 g / m ² or more, and if it exceeds 10 g / m ² , the effect of suppressing fusion is saturated, and the film forming effect by the easily oxidizable element is rather hindered. And promotes the volatilization of Zn.

  Further, the surface of the zinc-based plated steel sheet of the present invention may be treated with silicates, phosphates, and chromium oxides, as necessary, to serve as an auxiliary means for suppressing the volatilization of zinc in the plating, and to provide corrosion resistance and paint adhesion. Attempts to improve the formability, press formability, weldability, etc. do not depart from the scope of the present invention. Still further, the steel sheet applied in the present invention is a cold-rolled steel sheet and a hot-rolled steel sheet manufactured by a normal manufacturing method, and the effect thereof does not largely change depending on a manufacturing method such as roughness.

  The galvanized steel sheet of the present invention can prevent the volatilization of zinc even when hot worked, and can also prevent the volatilization of zinc when quenching is performed after cold working. Examples of the cold working include press (bending, drawing), hydroforming (bending, expanding, contracting, and flattening). After that, quenching is performed at 880-70 ° C, but zinc does not evaporate. Next, examples will be described.

Tables 1 to 15 show comparative examples along with examples of steel components of hot-rolled steel sheets and cold-rolled steel sheets prepared by a normal production method and zinc-based plating configurations and performances thereof. In these tables, three tables of serial numbers are one set, and the steel type, plating configuration, and performance of the same embodiment are shown. Since it is difficult to add the oxidizable element to the plating layer by the electroplating method, each oxidizable element was added to a plating bath in which Zn was melted, and was prepared by a normal hot-dip Zn plating method. The symbols in the “plating” column in Table 2 indicate G: hot-dip galvanizing, GA: alloyed hot-dip galvanizing, E: electro-galvanizing, and EA: electro-alloy galvanizing. Incidentally, Si> 0.2%, Mn> 1.5%, the steels B> 15 ppm, since the plating wettability is insufficient, the base plating the Fe plating at 5 g / m ² electroplating was performed molten Zn plating . The upper plating was prepared by electroplating using an existing plating bath as described below. % In Tables 1, 2, and 3 is% by mass. Blank columns indicate no addition.

Electric Fe plating: Ferrous sulfate plating bath Electric Fe-Zn alloy plating: Ferrous sulfate-zinc sulfate plating bath Plating composition: Fe85% Zn15%
Fe-P plating: Iron sulfate-hypophosphorous acid plating bath Plating composition: P0.1% Iron remaining
Fe-Ni plating: iron sulfate-nickel sulfate plating bath Plating composition: Ni60% Fe40%
Electric Ni plating: Watt bath Electric Ni-Zn plating: Nickel sulfate-zinc sulfate bath Plating composition: Ni95% Zn5%
Ni-P plating: plating bath with hypophosphorous acid added to watt bath Plating composition: P8% Ni remaining
Electric Co plating: Cobalt sulfate plating bath

In the hot treatment, the steel sheet was held at 930 ° C. for 3 minutes in an air atmosphere, taken out, and then cooled with water.
The plating amount and composition were used in combination with the fluorescent X-ray method and the dissolution method of dissolving with 5% hydrochloric acid and measuring the ICP. The Fe concentration in the plating after the hot working and the weight loss of the plating were determined by dissolving the hot-treated material with 5% hydrochloric acid, measuring the ICP, calculating the basis weight, and evaluating the difference from the original plating amount.

Fusing property of plating, two sheets of plated steel are stacked, 5g / cm ² weight is placed on them, and after holding at 930 ° C for 3 minutes as well, it is left on the heat insulating material and gradually cooled down to room temperature. The steel plate was tilted to determine the presence or absence of a drop. The strength was measured by holding the steel plate at 930 ° C for 3 minutes in the air atmosphere, removing it, cooling it with a gas jet in order to strictly evaluate the strength after burning, and then evaluating the JIS No. 5 tensile test piece in the L direction, Those exceeding 800 MPa were regarded as good.

  The chemical conversion treatment is as follows: For hot treatment, hold the steel sheet at 930 ° C for 3 minutes in the air atmosphere, take out, water-cooled steel sheet, degrease, and use Palbond LA35 (manufactured by Nippon Parker Rising Co., Ltd.) as specified by the manufacturer A chemical conversion treatment was performed, and observation was performed at 1000 times the SEM to observe the presence or absence of invisibility of the chemical conversion treatment film. Corrosion resistance was determined by applying a 15 μm cationic electrodeposition coating (Powernics 110: manufactured by Nippon Paint Co., Ltd.) to the test piece subjected to the chemical conversion treatment and performing a cross-cut, followed by the SAE-J2334 American Automobile Manufacturers Association standard corrosion test conditions. The width (one side) of the coating film from the cross cut portion after 180 cycles was measured.

  As described above, the present invention dramatically improves the strength of these materials by improving the dimensional accuracy of parts with the ease of use and performance of zinc-based plating of rust-proof steel sheets conventionally used in automobiles and industrial machines. Therefore, it is thought that it is possible to greatly contribute to promoting weight reduction, improvement of safety, and improvement of rust prevention of automobiles and industrial machines at an advantageous price, and the industrial contribution is great.

Claims (6)

At 1000 ° C. or less, the standard free energy change of ZnO standard free energy change of the oxides from the same temperature _{^{(△ G 0 Zn) (△}} G 0) is small, and one of the elements except for alkali metal or 2 contained in the seed or Zn plating, in a content of the element total 0.25 mol / m ² or more, Zn plating layer on a mass% of 60% or more, Zn weight of 40 g / m ² or more, the composition of the steel sheet % By mass, C: 0.01 to 0.30%, Si: 0.005 to 1.0%, Mn: 0.01 to 3.0%, P: 0.005 to 0.10%, S: ≦ 0.02%, Al: 0.010 to 0.500%, N: 0.001 A zinc-based plated steel sheet having excellent zinc volatility, characterized by containing about 0.01%.   2. The steel sheet according to claim 1, further comprising, by mass%, B: 2 to 100 ppm, Cr: 0.02 to 0.500%, Mo: 0.001 to 0.500%, and Ni: 0.001 to 1.000% as mass elements. A zinc-coated steel sheet having excellent zinc volatility described.   Further, as a component element of the steel sheet, in mass%, one or two selected from Nb: 0.005 to 0.100%, V: 0.005 to 0.100%, Ti: 0.005 to 0.100%, Zr: 0.005 to 0.100% The galvanized steel sheet having excellent zinc volatility resistance according to claim 1 or 2, which contains at least one kind. At 1000 ° C. or less, the standard free energy change of the oxides at the same temperature than the standard free energy change of _{^{ZnO (△ G 0 Zn) (}} △ G 0) is small, and the elements except for alkali metal in Zn plating The galvanized steel sheet having excellent zinc volatility resistance according to any one of claims 1 to 3, wherein the galvanized steel sheet contains a plating content of the element of 15% or less by mass%. At 1000 ° C. or less, the standard free energy change of the oxides at the same temperature than the standard free energy change of _{^{ZnO (△ G 0 Zn) (}} △ G 0) is small, elements and, other than Zn and excluding alkali metal The galvanized steel sheet according to any one of claims 1 to 4, wherein a content of the element in the Zn plating layer is 5% or less by mass%. On the surface of the galvanized steel sheet according to any one of claims 1 to 5, Ni, resistant zinc Co, a plating containing 80% or more of Fe in mass%, was applied 1 g / m ² or more 10 g / m ² or less Galvanized steel sheet with excellent volatility.