JP4610272B2 - Method for producing Zn-Al-Mg alloy-plated steel sheet excellent in resistance to molten metal embrittlement cracking - Google Patents

Description

  The present invention makes use of a Zn-Al-Mg alloy-plated steel sheet that utilizes the excellent high corrosion resistance of a Zn-Al-Mg alloy-plated layer, and does not easily cause molten metal embrittlement cracks even when heated during welding, brazing, etc. It relates to a method of manufacturing.

  A typical corrosion resistant material is a plated steel sheet, and a Zn—Al—Mg alloy plated steel sheet exhibits excellent corrosion resistance even in a severe corrosive atmosphere. When assembling various structures using Zn-Al-Mg alloy-plated steel sheets, a method of welding Zn-Al-Mg alloy-plated steel sheets that have been formed into a predetermined shape is often used. Intergranular cracking is likely to occur in the part. When cracks propagate along grain boundaries, they appear as a molten metal embrittlement phenomenon.

  Molten metal embrittlement cracks occur in the heat affected zone even when, for example, steel materials are welded and large structures such as steel towers and bridges are assembled and then hot dip galvanized (dipped soaking) is used to prevent rust. Cheap. In order to prevent molten metal embrittlement cracking, it is effective to make old austenite grain boundaries unclear and to reduce impurity elements in steel such as B. However, the blurring of prior austenite grain boundaries and the reduction of impurity elements in steel are measures to prevent cracks that occur when being immersed in a hot dip galvanizing bath at about 450 ° C. A separate measure is required to prevent cracks occurring in the heat affected zone immediately after being exposed to a high temperature of Baidu.

Specifically, in order to prevent hot dip galvanized cracks that are likely to occur in hot-dip galvanized welded parts, TiN is deposited to suppress the growth of austenite grains in the heat affected zone during welding, and after welding A method of forming a metal structure composed of fine ferrite grains at the grain boundary of the weld heat affected zone during cooling is known (Patent Document 1). However, the metal structure after welding and cooling is only refined, and it cannot be said that it is a sufficient measure for suppressing cracks occurring in the austenite region immediately after welding.
Japanese Patent Laid-Open No. 10-96021

For ultra-low carbon steel, Nb carbide suppresses abnormal growth of austenite grains when heated at high temperature during welding or brazing, and has excellent formability and fatigue strength, and improved resistance to molten metal embrittlement cracking Is known (Patent Document 2). The prevention of coarsening of austenite grains due to Nb carbide is aimed at improving the formability and fatigue characteristics of the steel sheet, and by containing an appropriate amount of Ni corresponding to the Cu content, molten metal embrittlement cracking is suppressed. Yes.
JP-A-10-195597

Molten metal embrittlement cracking that tends to occur in Zn-Al-Mg alloy-plated steel sheet is considered to be caused by a low melting point Mg-containing phase. Even when the same steel material is used for the base steel, it is usually zinc-based, aluminum The tendency to occur is stronger compared to the system plating. Therefore, an alternative to the conventional molten metal embrittlement cracking prevention method is required.
The present invention is based on the knowledge that the surface condition of the base steel has an effect on preventing the occurrence and progress of molten metal embrittlement cracks, and by forming a strong and thin film on the surface of the steel, An object of the present invention is to produce a Zn-Al-Mg alloy-plated steel sheet with improved resistance to hot metal embrittlement cracking by suppressing the penetration of the hot-dip metal into the steel.

The present invention is characterized in that a steel plate on which a strong and thin film is formed by adding Cr, Cu, Ni or the like together with Si is used as a base steel for hot-dip Zn-Al-Mg alloy plating.
The base steel is C: 0.001 to 0.3 mass%, Si: 0.01 to 1.5 mass%, Mn: 0.05 to 2.0 mass%, P: 0.2 mass% or less, S : 0.03 mass% or less, Cu: 0.05-0.5 mass%, Ni: 0.05-0.5 mass%, Cr: 0.05-0.5 mass% Use base steel containing more than seeds.
After the base steel is subjected to reduction annealing at 500 to 850 ° C., it is introduced into a molten Zn—Al—Mg alloy plating bath to produce a Zn—Al—Mg alloy plated steel sheet having excellent resistance to molten metal embrittlement cracking. .

Effects and embodiments of the invention

When a Zn—Al—Mg alloy plated steel sheet is welded by arc welding or the like, cracks are likely to occur near the boundary between the weld metal and the base metal (weld toe). Since cracking occurs after the welding torch has left the welding point, the mechanism of crack generation is assumed as follows.
The hot dip plating layer at the crack generation site becomes molten as the welding torch approaches, and eventually evaporates and depletes due to the very high temperature of the welding heat. However, immediately after the welding torch has left, when the temperature becomes lower than the evaporation temperature of the hot-dip plating layer, the hot-dip hot-dip plating layer flows at a relatively low temperature from the periphery of the welding point toward the weld toe. At this time, at the weld toe where the stress tends to concentrate in shape, the hot-dip plated metal tends to enter the crystal grain boundary on the surface of the steel due to the tensile stress generated inside the steel during the cooling process. Molten metal embrittlement occurs.

  Given the occurrence mechanism of molten metal embrittlement cracking, it can be said that it is effective to prevent molten metal embrittlement cracking by preventing the molten plated metal that has flowed into the weld toe from entering the crystal grain boundaries on the steel surface. . Therefore, in the present invention, by using a base steel on which a strong and thin film in which Cr, Cu, Ni, etc. are concentrated together with Si is formed, it is possible that hot-dipped metal penetrates into crystal grain boundaries on the surface of the steel material. Suppressed.

  A film in which Cr, Cu, Ni, etc. are concentrated together with Si has a high environmental barrier, and acts as a barrier against hot-dip plated metal that is generated when a Zn-Al-Mg alloy-plated steel sheet is heated at high temperatures during welding and brazing. . Therefore, molten metal embrittlement due to the penetration of the hot-dip metal into the grain boundary on the surface of the steel sheet is suppressed, and the resistance to hot metal embrittlement cracking of the Zn—Al—Mg alloy-plated steel sheet is improved. And since it is a thin film containing Cr, Cu, Ni, etc., there is little fall of plating property and a healthy molten Zn-Al-Mg alloy plating layer is formed in the steel plate surface.

Hereinafter, alloy components, contents, and the like included in the base steel will be described.
C: 0.001 to 0.3% by mass
It is an effective component for securing the material strength, and the C content is set to 0.001% by mass or more in order to obtain the required strength. However, since the ductility of the steel sheet is lowered by the solid solution in the ferrite phase and the formation of carbides, the upper limit is regulated to 0.3% by mass.

-Si: 0.01-1.5 mass%
It is a solid solution strengthening component that raises the material strength, and forms a film on the surface of the steel sheet due to high heat during welding, and exhibits the action of preventing molten metal embrittlement. Such an effect is observed when Si is added in an amount of 0.01% by mass or more. The Si-containing film incorporates Cr, Cu, Ni, etc. that are easily concentrated on the steel sheet surface, and is reformed into a stronger and denser film that is effective for suppressing molten metal embrittlement. However, when an excessive amount of Si exceeding 1.5% by mass is contained, the ductility of the steel sheet is lowered, and a Si concentrated layer that is harmful to plating adhesion tends to be formed on the steel sheet surface. Preferably, the Si content is selected in the range of 0.01 to 0.5% by mass.

Mn: 0.05 to 2.0 mass%
It is an element that prevents embrittlement due to S and is effective in improving the strength, and an effect of adding Mn is seen at 0.05% by mass or more. However, an excessive amount of Mn exceeding 2.0% by mass deteriorates workability and weldability, and forms a Mn-concentrated layer on the steel sheet surface that is harmful to plating adhesion. Preferably, the Mn content is selected in the range of 0.1 to 1.5 mass%.

-P: 0.2 mass% or less Since it is a component which has a bad influence on ductility, it is so preferable that P content is low in the use where high workability is requested | required. On the other hand, since it is an effective component for improving the strength, it does not adversely affect the workability and plating adhesion for high strength applications (specifically, 0.2% by mass or less, preferably 0.15% by mass). In the following, P can be added.
S: 0.03 mass% or less Since it is a component that causes hot embrittlement and is harmful to workability and corrosion resistance, it is preferably reduced as much as possible. In the present component system, the upper limit of the S content is set to 0.03 mass% (preferably 0.015 mass%) in consideration of manufacturing costs.

Cr, Ni, Cu: 0.05-0.5 mass%
Both are components that are easily concentrated on the surface of the steel sheet, and modify the Si-containing film into a strong and dense film. Si, Mn, etc. are known as elements that form a film on the surface of steel. However, only the effect of forming a film by adding Si or Mn alone or by adding Si or Mn alone is a molten metal embrittlement that tends to occur during welding. The cracks cannot be prevented sufficiently. On the other hand, when one or two or more of Cr, Cu, and Ni are added together with Si, a much stronger and denser film is formed on the surface of the steel material than when adding Si alone. The coating exhibits a sufficient barrier function with respect to the molten plated metal and improves the resistance to molten metal embrittlement cracking. Such an effect can be obtained by adding 0.05% by mass or more of Cr, Cu and / or Ni. However, if Cr, Cu, Ni or the like is added excessively, the workability of the steel material is lowered, so the upper limit of the addition amount was set to 0.5 mass%.

・ Reduction annealing before hot dipping In a continuous hot dipping plating line, the steel strip is fed into a reduction annealing furnace to activate the steel plate surface, and then introduced into the hot dipping bath and adheres to the surface of the steel strip pulled up from the hot dipping bath. The hot dip plated steel sheet on which the hot dip plated layer is formed with a predetermined adhesion amount is manufactured by removing the excess hot dip plated metal by gas wiping or the like. When the steel strip is subjected to reduction annealing at a heating temperature of 500 to 850 ° C. in the reduction annealing process prior to introduction into the hot dipping bath, an appropriate amount of Cr, Cu, Ni, etc. is concentrated on the steel strip surface layer, resulting in a strong and dense film It is formed on the belt surface. If the reduction annealing temperature is too high, the concentration of Cr, Cu, Ni, etc. proceeds remarkably, and plating defects such as non-plating tend to occur. On the other hand, when the reduction annealing temperature is too low, heating is required for a long time to concentrate an appropriate amount of Cr, Cu, Ni, etc. on the steel strip surface layer.
Specifically, when Cr, Cu, Ni or the like is concentrated to 5 atomic% or more by reduction annealing, a strong film that is not destroyed by high heat during welding is generated. Since the film strengthening action of Cr, Cu, Ni, etc. is affected by the amount of Si and Mn, it is preferably restricted to Si: 25 atomic% or less and Mn: 15 atomic% or less. Further, if there is too much Cr, Cu, Ni, etc. in addition to Mn, Si, non-plating is likely to occur during hot dipping, so that excessive addition of Cr, Cu, Ni, etc. is avoided as described above.

  The molten Zn—Al—Mg alloy plating bath contains Mg: 0.05 to 10% by mass, Al: 4 to 22% by mass, Ti: 0.001 to 0.1% by mass, B A hot dipping bath containing 0.0005 to 0.045% by mass and at least one easily oxidizable element such as rare earth elements, Y, Zr, and Si: 0.005 to 2.0% by mass is used. The composition of the hot dip plating bath is almost equally reflected in the composition of the hot dip Zn—Al—Mg alloy plating layer.

  Mg forms a Zn-based corrosion product containing Mg on the outermost layer of the plating layer, and the corrosion product acts as an inhibitor to impart high corrosion resistance. A part of the corrosion product also flows into the weld bead part and the cut end face, and corrosion of the bead part and the cut end face is suppressed. Mg is an effective component for forming a Zn—Mg-based intermetallic compound in the plating layer to harden the plating layer. In order to exert such effects, the Mg content is adjusted to a range of 0.05 to 10% by mass (preferably 1 to 4% by mass).

While Zn and Mg in the plating layer form a Mg-containing Zn-based corrosion product, Al forms a Zn—Al-based corrosion product with extremely strong adhesion, contributing to improvement in corrosion resistance. Moreover, Zn / Al / Zn ₂ Mg ternary eutectic appears in the solidified structure of the plating layer due to the Al content. The Zn / Al / Zn ₂ Mg ternary eutectic structure is finer than the Zn / Zn ₂ Mg binary eutectic structure and is effective in improving corrosion resistance and hardening the plating layer. In order to form a Zn—Al-based corrosion product having strong adhesion and to form a Zn / Al / Zn ₂ Mg ternary eutectic structure, an Al content of 4% by mass or more is required. However, as the Al content increases, the melting point of the plating metal rises, and it is necessary to keep the plating bath at a high temperature, and the productivity of the material also deteriorates, so the upper limit of the Al content is 22% by mass. It is preferable.

When Ti and B, which are optional components, are added, the formation of a Zn ₁₁ Mg ₂ phase that impairs the surface appearance is suppressed, and the Zn—Mg-based intermetallic compound that crystallizes in the plating layer is substantially only Zn ₂ Mg. . Specifically, Ti: 0.001% by mass or more (preferably 0.002% by mass or more) can effectively suppress the formation of the Zn ₁₁ Mg ₂ phase. However, when an excessive amount of Ti exceeding 0.1% by mass is contained, Ti—Al-based precipitates grow in the plating layer, and the plating layer is uneven, and the appearance is impaired.

Suppression of the formation of the Zn ₁₁ Mg ₂ phase can also be achieved by containing 0.0005% by mass or more (preferably 0.001% by mass or more) of B. However, when an excessive amount of B exceeds 0.045% by mass, Ti—B-based precipitates and Al—B-based precipitates grow in the plating layer, and the plating layer is uneven, resulting in a loss of appearance. It becomes like this. Furthermore, the surface gloss deterioration phenomenon can be suppressed by adding 0.005% by mass or more of at least one of rare earth elements, Y, Zr, and Si, which are easily oxidizable elements. However, since an improvement effect commensurate with the increase cannot be obtained even if excessive addition is made, the upper limit of the addition amount of rare earth elements, Y, Zr, Si, etc. is made 2.0 mass%.

  The steel materials shown in Table 1 were melted in a vacuum melting furnace and cast into an ingot, and then a hot-rolled steel strip having a thickness of 5 mm was manufactured through forging and hot rolling.

Each hot-rolled steel strip was surface-ground to remove scales, and then sent to a reduction annealing furnace. In a reduction annealing furnace, the steel strip surface is activated by heating in an inert atmosphere of H ₂ : 50% by volume and N ₂ : 50% by volume, and Si, Cr, Cu, Ni, etc. are applied to the steel strip surface layer. Concentrated. The steel strip subjected to reduction annealing was sampled, and the influence of the reduction annealing conditions on the concentration of Si, Cr, Cu, Ni, etc. and the physical properties of the coating formed on the steel strip surface was investigated. In addition, as a steel strip surface layer, the area | region from the steel strip surface to 1 micrometer depth was selected.

  As can be seen from the investigation results in Table 2, in the steel strip to which Cr, Cu, Ni or the like was added, concentration of Si, Cr, Cu, Ni or the like on the steel strip surface layer occurred. On the other hand, in the steel strip containing no Cr, Cu, or Ni, the coating formed on the surface of the steel strip is relatively porous, and the amount of Mn that adversely affects the plating properties has been increased.

The steel strip after the reduction annealing was immersed in a molten Zn—Al—Mg alloy plating bath (Al: 6% by mass, Mg: 3% by mass, Zn: balance) at a bath temperature of 400 ° C. After the steel strip was pulled up from the hot dipping bath, the amount of plating adhered was adjusted to 90 g / m ² by gas wiping.
A test piece was cut out from the obtained plated steel sheet and subjected to a high temperature tensile test.

  In the high-temperature tensile test, the specimen was austenitized by heating from room temperature to 1000 ° C. at a rate of temperature increase of 100 ° C./second, and then cooled to a set temperature of 800 ° C. and 550 ° C. at a cooling rate of 50 ° C./second. The test piece kept at the set temperature was pulled until it broke. While observing the fracture surface of the fractured test piece, the fracture elongation ratio was calculated as the ratio of the fracture elongation of the plated material to the fracture elongation of the non-plated material.

  As seen in the investigation results in Table 3, a Zn-Al-Mg alloy-plated steel sheet using a group A steel band in which Cr, Cu, Ni and the like are concentrated in the steel band surface layer together with Si is used as the base steel. The breaking elongation ratio was large, and there was no significant variation in breaking properties compared to the unplated material. That is, it can be confirmed that the molten metal embrittlement cracking resistance is improved by concentrating Si, Cr, Cu, Ni, etc. on the steel strip surface layer and forming a dense and strong film on the steel strip surface.

  On the other hand, a plated steel sheet in which a molten Zn-Al-Mg alloy plating layer is formed on the steel strip B1 without addition of Cr, Cu, Ni, is remarkably broken by molten metal due to the molten plated metal when exposed to a high temperature atmosphere. The cross section was brittle fractured. Even if Cr, Cu, Ni, etc. are added, the steel strip B2 that has been subjected to reduction annealing at a high temperature of 860 ° C. is melted with defects such as non-plating due to excessive concentration of Cr, Cu, Ni, etc. on the steel strip surface. A plating layer was formed, and it was difficult to stably produce a normal hot dip plated steel sheet.

  As explained above, by carrying out reduction annealing of steel strips and steel plates to which Cr, Cu, Ni, etc. are added at a heating temperature of 500 to 850 ° C., the inclusion of Cr, Cu, Ni, etc. together with Si is more robust. A dense film is formed on the surface of the steel sheet. This coating has a high environmental barrier ability and prevents the plated metal melted by high-temperature heating from entering the crystal grain boundary of the steel sheet surface layer. Therefore, even if the Zn-Al-Mg alloy-plated steel sheet is welded, there is no molten metal embrittlement that tends to occur at the weld toe, and various structural materials utilizing the high corrosion resistance of the molten Zn-Al-Mg alloy plating bath A suitable hot dip plated steel sheet is obtained.

Claims (1)

C: 0.001 to 0.3 mass%, Si: 0.01 to 1.5 mass%, Mn: 0.05 to 2.0 mass%, P: 0.2 mass% or less, S: 0.03 Including one or more of Cu: 0.05-0.5% by mass, Ni: 0.05-0.5% by mass, Cr: 0.05-0.5% by mass The base steel having a composition consisting of Fe and unavoidable impurities in the balance is subjected to reduction annealing at 500 to 850 ° C. and then introduced into a molten Zn—Al—Mg alloy plating bath. Method for producing a Zn-Al-Mg alloy-plated steel sheet having excellent resistance.