JP4072129B2 - Hot pressed steel with zinc-based plating - Google Patents

Description

  The present invention relates to a hot press member that has been subjected to zinc-based plating in various members that require high strength, such as undercarriage members and skeleton reinforcing members of automobiles.

  In recent years, demands for reducing the weight of automobile bodies to reduce fuel consumption have increased due to global environmental problems. On the other hand, there is an effort to increase the strength of the car body due to the increase in passenger safety. Increasing the vehicle strength can be achieved by increasing the thickness of the plate or using reinforcing parts, but this leads to an increase in the amount of steel used, which conflicts with reducing the weight of the vehicle body. Thus, attempts have been made to maintain the amount of steel used by using high-strength steel sheets, and application of TRIP steel that maintains press formability while increasing the tensile strength to about 780 MPa has begun. This steel sheet is an abbreviation for Transformation Induced Plasticity, and is a steel sheet utilizing martensitic transformation of retained austenite. However, it is impossible to increase the strength of this steel plate, for example, 1500 MPa class, and there is a strong demand for the development of a steel plate that can secure such high strength and formability.

  Another technique for achieving both high strength and high formability is hot pressing. This technology eliminates the problem of formability of a high-strength steel sheet by forming the high carbon steel in a state of being heated to a high temperature of 800 ° C. or higher, and obtains a desired strength by quench hardening by rapid cooling after forming. . However, since it is accompanied by heating in the atmosphere, oxides are generated on the surface, which may fall off during the press and damage the press die or the steel plate surface. Oxides have poor adhesion, and can cause paint peeling and deterioration of corrosion resistance. Therefore, it is necessary to add a pickling or shot blasting process for removing oxides before coating the processed product. Moreover, since the obtained press member is not rust-proof galvanized, it cannot be said that it has sufficient corrosion resistance even if a coating process is performed. As a method for suppressing surface oxidation and improving corrosion resistance, a technique of applying aluminum plating to a steel sheet is disclosed. For example, there are JP-A 2000-38640, JP-A 2003-34845, and JP-A 2003-193187. These technologies have already been put into practical use because they exhibit excellent oxidation resistance due to the action of aluminum plating and also have good corrosion resistance. However, since aluminum-plated steel sheets are more expensive than bare steel sheets, the application of zinc-based plated steel sheets is also being studied from the viewpoint of cost reduction of steel materials. Zinc is in the liquid phase or gas phase in the temperature range where the melting is 419 ° C. and the boiling point is 907 ° C. and hot pressing is performed. Therefore, transpiration and oxidation of the plating layer occur during hot pressing, and the obtained steel sheet is excessively alloyed with the base steel sheet. For this reason, zinc is welded to the press die, and corrosion resistance and weldability are significantly deteriorated. As methods for avoiding this problem, Japanese Patent Application Laid-Open No. 2003-126920, Japanese Patent Application Laid-Open No. 2003-126921, Japanese Patent Application Laid-Open No. 2003-73774, and Japanese Patent Application Laid-Open No. 2003-147499 are disclosed. These techniques increase the melting point of the plating layer by forming a Zn—Fe alloy by holding in a relatively low temperature range, or form zinc oxide on the surface layer. Among these techniques, Zn-Fe alloying can certainly be expected to increase the melting point, but for example, to achieve a melting point of 900 ° C. or higher, about 70% by mass of Fe must be contained. With such a composition, the corrosion resistance, coating film adhesion, and weldability are greatly deteriorated. Further, as the alloying progresses, the oxidation reaction is also promoted, resulting in problems such as dropping of the oxide layer and poor adhesion after painting. On the other hand, when the Fe content is suppressed to about 20% by mass, the melting point is as low as about 670 ° C., and the liquid layer comes into contact with the mold to cause welding or galling.

JP 2000-038640 A Japanese Patent Laid-Open No. 2003-034845 JP 2003-193187 A JP 2003-126920 A JP 2003-126921 A JP 2003-073774 A JP 2003-147499 A

  The problem of these techniques is that when a Zn—Fe alloy is formed by a normal heating condition or an alloying reaction in the vicinity of 600 ° C., the Fe content increases and the corrosion resistance deteriorates. Furthermore, when the Fe content increases, the formation of Fe oxide becomes remarkable, resulting in poor paint adhesion. On the other hand, simply suppressing the alloying reaction has a low melting point and cannot suppress welding or galling in the mold.

  As a result of intensive studies on the means for solving such problems from various angles, the present inventors have prescribed the form and distribution of the intermetallic compound even if the galvanized layer is simply heated to form a Zn-Fe alloy. Unless it was done, it was clarified that mold galling, corrosion resistance, and paint adhesion cannot be controlled to a high degree. That is, as a desirable form of the galvanized layer, the vicinity of the base iron interface is a layered alloy phase composed of a Zn-Fe alloy containing 50 to 80% by mass of Fe inevitably formed, for example, about 60% by mass. The surface layer portion other than is made into an island-like Zn-Fe alloy phase having a spherical form containing 50 to 80% by mass of Fe using a Zn-Fe alloy phase (Γ phase) containing 10 to 30% by mass of Fe as a matrix. Distributing structure suppresses mold galling due to the hard, high melting point spherical alloy phase and achieves both the corrosion resistance and relatively high coating film adhesion due to the relatively small Fe-containing Zn-Fe alloy phase (Γ phase). The present invention was made by obtaining knowledge about the above. Furthermore, a method for producing an alloy phase having such a structure has been successfully found by deep consideration of a phase diagram of a Zn—Fe binary alloy and repeated experiments verified by actually causing a reaction. That is, the growth of the Fe phase (Zn solid solution Fe) at the iron-iron interface is suppressed by raising the temperature in the low temperature region up to 782 ° C. where the Fe phase is formed in the solid phase as much as possible, and the solid phase The solid-phase Fe phase in which Zn is solid-solved is precipitated in a spherical shape by holding it for an appropriate time in the temperature range from 780 ° C., which is the solid-liquid mixing temperature range of Fe and liquid phase, to 900 ° C., which is below the boiling point of zinc. In addition, by cooling to 780 ° C. before starting hot pressing, the remaining liquid phase was precipitated in the Γ phase, and the spherical phase was successfully distributed in islands in the matrix. .

  Therefore, the form of the obtained technology consists of a Zn-Fe alloy layer in which the cross section of the plating layer is layered in the vicinity of the base steel plate interface and contains 50 to 80% by mass of Fe in a bake hardenability steel plate. Zn-Fe alloy phase having a spherical form containing 50 to 80% by mass of Fe in a Zn-Fe alloy phase matrix containing 10 to 30% by mass of Fe is distributed in islands. It is a steel material for hot pressing with excellent corrosion resistance after coating and press workability, which is provided with a molten Zn-Fe alloy layer. Furthermore, it is a steel material for hot press working with excellent corrosion resistance and press workability after coating, in which the size of the spherical alloy phase is 1 to 30 μm in diameter and the area ratio of the cross section is 10 to 80% of the entire plating layer. The hot-pressing steel material having excellent corrosion resistance and press workability after coating, with a plating layer thickness of 5 to 80 μm. Moreover, you may contain 0.001-7% of the sum total of the mass% of 1 type, or 2 or more types of Al, Cr, Mn, Mg, Ti, Ni, Mo, Si in a plating layer. In this method of manufacturing a steel material, when the steel material is heated for hot pressing, the temperature range of the ultimate plate temperature T is set to the value defined by the relational expression (1) or 800 ° C, whichever is higher, as the lower limit. The upper limit is 905 ° C. Furthermore, the temperature increase rate up to 780 ° C. is 5 ° C./second or more, the temperature increase rate from 780 ° C. to the ultimate plate temperature T is 1 ° C./second or more and less than 5 ° C./second, and the holding time in that temperature range is The hot press start temperature is set to 780 ° C. or lower for 5 to 600 seconds.

(Here, [C], [Mn], [Si], and [Ti] are mass% of each element in steel.)
Furthermore, as a component in steel materials, you may contain 1 or more types of B: 0.0003-0.03 mass%, Ti: 0.01-0.3 mass%, Furthermore, Nb: 0.005- Inclusion of 0.5% by mass, V: 0.005 to 0.5% by mass, Mo: 0.005 to 0.5% by mass, improving toughness and grain coarsening by refining the structure of steel Moreover, the hardenability and toughness of steel materials can be improved by containing one or more of Ni: 0.01 to 2.0 mass% and Cu: 0.01 to 2.0 mass%.

  According to the hot-pressed steel material of the present invention, it is possible to provide a hot-pressed steel material that has a high strength of 1500 MPa class, has both high press workability and also has excellent post-painting corrosion resistance. .

The reason why the present invention is limited as described above will be described in detail. The base steel material must have good plating wettability during hot dip galvanizing, plating adhesion after plating, and alloy reactivity with the plating layer, and also a component for expressing quenching curability after hot pressing There are provisions. Since the strength after quenching is mainly determined by the amount of contained carbon (C), when high strength is required, the C content is preferably 0.1% by mass or more and 0.5% by mass or less. Even if the upper limit is exceeded at this time, the strength is saturated, and it is easy to cause weld cracking. If Si is too low, fatigue characteristics are deteriorated, so addition of 0.05% by mass or more is desirable. However, since Si forms a stable oxide film on the surface of the steel sheet during recrystallization annealing and inhibits hot dip galvanizing properties, particularly plating wettability, the upper limit is made 0.5 mass%. Mn is known as an element that enhances the hardenability of the steel sheet, and is also an element effective in preventing hot brittleness caused by sulfur (S) inevitably mixed. Therefore, addition of 0.5% by mass or more is necessary. However, if the addition exceeds 3% by mass, the impact characteristics after quenching deteriorate, so it is desirable to make this the upper limit. Ti is an element that improves the oxidation resistance after galvanization. In order to achieve the effect, at least 0.01% by mass is necessary. However, Ti increases the Ac ₃ point and, at the same time, forms C and TiC to reduce the amount of C contributing to the strength. Moreover, since the wettability of plating will be inhibited like Si, when the addition amount increases, the upper limit was made 0.3 mass%. B is added to improve hardenability. In order to exhibit the effect, at least 0.0003 mass% is necessary, and even if it exceeds 0.03% by mass, the effect is saturated. In the component range of the present invention, the Ac ₃ point is in the range of 800 ° C. to 910 ° C., and it is possible to achieve both good hardenability and various performances in the plating layer structure and heating conditions described below. .

When one or more of Nb, V, and Mo are further added to the steel material, it is possible to prevent toughness improvement and grain coarsening due to refinement of the steel material structure. If the content is less than 0.005% by mass, the effect is insufficient, and addition exceeding 0.5% by mass is not economical because the effect is saturated.
Furthermore, when one or more of Ni and Cu are added, the hardenability and toughness are improved. If it is less than 0.01% by mass, the effect is insufficient, and if it exceeds 2.0% by mass, the effect is saturated and it is not economical.

  Next, the Zn—Fe plating layer of the present invention will be described. The corrosion resistance after coating of the Zn-Fe alloy-plated steel sheet varies depending on the Fe content in the plating layer, and shows almost constant good corrosion resistance if Fe is in the range of 5% by mass to 50% by mass. When pure zinc or Fe is contained in an amount of 60% by mass or more, it is abruptly deteriorated, and in the case of 80% by mass or more, it becomes the same level as a bare steel material. The galvanized layer undergoes an alloying reaction with the base iron from a relatively low temperature of about 300 ° C., and the alloying rate increases as the temperature increases. The alloying reaction in the temperature range up to 782 ° C. is solidified if the liquid phase contains about 10% by mass of Fe, and thereafter Γ, δ1, and ξ phases are generated in descending order of the Fe content. The plating layer of the present invention suppresses these alloy layers by increasing the temperature to 782 ° C. as much as possible, and reduces the thickness of the Zn—Fe alloy layer formed near the base iron interface. Spherical crystals are distributed in the form of islands by precipitating a Zn—Fe alloy containing 50 to 80% by mass of Fe in the liquid phase by heating in the temperature range of 800 ° C. or more and 905 ° C. or less. Then, before contact with the press mold starts, the solidification is completed by cooling to 780 ° C., and at the same time, a Zn—Fe alloy layer containing 10 to 30% by mass of Fe as a matrix is formed. When hot-pressing the plating layer having such a structure, the spherical melting point and hardness of the spherical Fe-rich phase suppresses damage to the plating layer due to contact with the mold, and the corrosion resistance after coating is The presence of a plating phase having a high Zn content as a matrix exhibits good corrosion resistance.

The plated layer of the present invention must be obtained by heating and cooling cycle conditions that ensure the hardenability of the base steel by hot pressing. For this reason, the base steel needs to be hardened by quenching by heating to a temperature of Ac ₃ or higher to form an austenite single phase, followed by hot pressing and then quenching. The Ac ₃ point can be calculated as the steel composition formula (1) with reference to the Andrews formula (J. Iron Steel Inst., 203: 721 (1965)). Therefore, the lower limit of the ultimate plate temperature is set to the higher value of the minimum temperature 800 ° C. and Ac ₃ point for obtaining the plated layer alloy layer structure of the present invention. The upper limit of the ultimate plate temperature was 905 ° C. not exceeding the boiling point of zinc. In order to distribute the granular crystals in an island shape, the heating rate was defined in order to shorten the residence time in the low temperature region up to 780 ° C., and the residence time in the precipitation temperature region up to 800-910 ° C. was further defined. If this condition is not met, it is impossible to obtain a structure in which the granular Zn-Fe alloy is distributed in islands in the plated layer, and the Zn-Fe alloy phase generated at the base iron interface is present throughout the plated layer. It will occur.

  The structure of such a plating layer can be determined by elemental analysis using an electron beam probe microanalysis (EPMA) after embedding and polishing a cross-section of a plated steel sheet in a resin.

  A Zn—Fe alloy phase containing 50 to 80% by mass of Fe grows with an increase in alloying time of 800 to 905 ° C., but its abundance is at least 10% as an area ratio determined by observation in the cross-sectional direction. The required maximum is 80%. If it is 10% or less, the galling resistance is inferior, and if it exceeds 80%, the corrosion resistance and paint adhesion are inferior. 20 to 50% is desirable. The size of this phase is 1 to 30 μm in average diameter.

  The thickness of the plating layer is 5 to 80 μm. If it is thinner than 5 μm, the corrosion resistance becomes insufficient, and even if it is thicker than 80 μm, the allowance for improving the corrosion resistance is saturated, the unevenness of the plating appearance becomes severe, and the appearance after coating is impaired.

  The plating layer is based on a Zn—Fe alloy, but one, two or more of Al, Cr, Mn, Mg, Ti, Ni, Mo, and Si may be added. Al suppresses the alloying reaction of Zn and Fe and at the same time has an effect of improving corrosion resistance. All other additive metals improve the wettability of the plating bath to the steel plate substrate, the corrosion resistance after coating, and the oxidation resistance. If the total amount of added metal is less than 0.001% by mass, the effect is not obtained, and if it exceeds 7% by mass, the effect is saturated and the plated layer becomes brittle.

The heating temperature of the steel material needs to be the austenite single-phase temperature range of the steel material as the maximum attained temperature, and the lower limit is 800 ° C. or the Ac ₃ point estimated from the Andrews formula in order to make the Fe-liquid coexistence temperature range. It is higher than the higher one, and the upper limit is set to a temperature range of 905 ° C. or lower which is lower than the boiling point of zinc. The temperature range up to 780 ° C. is preferably heated quickly, and at least 5 ° C./second. On the other hand, the temperature range from 780 ° C. to the maximum plate temperature is an indispensable temperature range for obtaining the plating layer structure of the present invention, and may be held for 5 to 600 seconds. The temperature at which hot pressing is started is preferably 780 ° C. or lower. If it exceeds 780 ° C., there is a possibility that a part of the plating layer is dissolved, and there is a possibility of mold galling or welding. On the other hand, at 650 ° C. or lower, quenching is insufficient and the strength is insufficient.

  The photograph of Fig.1 (a) is a cross-sectional photograph of the plating layer of the hot press galvanized steel material of the present invention. The photographs in FIG. 1B and FIG. 1C are photographs showing the distribution of iron element and zinc element in the structure photograph in FIG. The Zn—Fe layer containing about 60% by mass of Fe in the vicinity of the base iron interface, and the other parts are spherical Zn containing about 60% by mass of Fe in a Zn—Fe alloy matrix layer containing about 20% by mass of Fe. -The Fe alloy layer is distributed in an island shape.

  The photograph of Fig.2 (a) is a cross-sectional photograph of the plating layer of the hot press galvanized steel produced by a method other than the present invention. The photographs in FIG. 2B and FIG. 2C are photographs showing the distribution of iron element and zinc element in the structure photograph in FIG. The plated layer has no spherical crystals and the entire plated layer is a substantially uniform Zn-Fe alloy, but an oxide layer of Zn and Fe is formed on the surface layer portion.

Next, the present invention will be described in more detail based on examples.
From a molten Zn bath containing 0.2% by mass of Al in the bath, using a cold-rolled steel plate (plate thickness 1.8 mm) of the steel components shown in Table 1 manufactured through normal hot rolling and cold rolling steps as a base steel plate Plating was applied. In the hot dip plating, a steel plate was immersed in a plating bath using a cleaning-non-oxidation furnace-reduction furnace type manufacturing facility, and after cooling, the plating adhesion amount was controlled by nitrogen gas wiping and then cooled. In the process of cooling, air-water mist was sprayed to quench the appearance so that there was no spangle. Table 2 shows the plating composition of the produced sample and the amount of plating attached. The hardened property, the galling property with the mold, the corrosion resistance after electrodeposition coating, and the coating adhesion were evaluated for the plated steel sheet thus manufactured. The evaluation method is described below.

  The steel sheet was heated to a predetermined temperature in an electric furnace in an air atmosphere. Two electric furnaces up to 780 ° C. and electric furnaces in a temperature range higher than 780 ° C. were arranged in parallel, and the temperature increase rate in each temperature range was controlled by moving the steel plate. After completion of the predetermined heat treatment, the sheet was taken out, set in a hat-bending-shaped press die, allowed to cool to a predetermined temperature, and then pressed. The steel sheet was quenched by contact with the die by press working, but the speed was 30 ° C./second. Thereafter, the sample was held with a mold until it reached 50 ° C. and then removed. Table 2 summarizes the heating conditions for each sample.

  The obtained press sample was visually evaluated on the wall of the sample for the degree of mold galling with the mold. The case where there was no galling was evaluated as ◯. Evaluation of corrosion resistance cut out the wall part of the press sample, and performed chemical conversion and electrodeposition coating. The chemical conversion treatment used PB-3080 manufactured by Nippon Parkerizing Co., Ltd. Electrodeposition coating was carried out by adjusting the electrodeposition voltage using a cationic type U-80 manufactured by Nippon Paint Co., Ltd. to a film thickness of 20 μm. A crosscut was introduced into the surface of the obtained coating sample with a cutter and subjected to a corrosion test. The corrosion test was evaluated with 100 cycles of JASO M609 composed of SST, drying and wetting processes. One cycle of this corrosion test consists of 2 hours of spraying salt water of JIS Z 2371, 4 hours of drying at 60 ° C. and 30% humidity, and 2 hours of wetting at 50 ° C. and humidity of 98%. In the evaluation, when the maximum bulge width on one side of the cross-cut portion was 2 mm or less, ◯, 4 mm or less was Δ, 4 mm was over, and ○ and Δ were passed. For coating adhesion, the coating sample was immersed in pure water at 50 ° C. for 240 hours, taken out, dried and left for 24 hours, then peeled off when the tape was peeled off with a grid cut (10 × 10) at 2 mm intervals. Evaluated by percentage. The number of peels was zero, ◯, 5 or less, Δ, more than 5 x, and ○ passed. Moreover, the Vickers hardness of the steel plate was evaluated with a load of 100 g. The alloy layer structure in the cross section of the plating layer was evaluated by embedding in a resin and polishing the surface of EPMA for the elements containing the plating layer. The results obtained are summarized in Table 3.

  In Table 3, within the scope of the present invention, the degree of mold galling is small, the corrosion resistance after coating and the coating adhesion are good, and the Vickers hardness is 500 and sufficient hardness. Since the strength of the steel material can be approximated by about three times the hardness, in the case of 500, the strength is 1500 MPa. On the other hand, out of the scope of the present invention, mold galling occurs, the corrosion resistance after painting is poor, the paint adhesion is poor, and it does not have the performance as a steel material. Moreover, since hardness is not securable with the steel materials of Comparative Examples 1-7, it turns out that the hardenability which is the main objective which performs a hot press is bad.

  From the above, the hot-pressed zinc-based plated steel sheet obtained in the present invention has high workability because it is processed at a high temperature while having a very high strength of 1500 MPa, and the corrosion resistance after coating. Since the paint adhesion is also good, it can be applied to structural members and reinforcing members of automobile bodies. If the material of the present invention is applied to a vehicle body, it becomes possible to simultaneously achieve a high strength and light weight of the vehicle body, leading to protection of irreplaceable human life and protection of the natural environment by reducing fuel consumption.

It is a cross-sectional-structure observation microscope picture of the hot press Zn type plating material produced by this invention. The Zn—Fe layer containing about 60% by weight of Fe in the vicinity of the base iron interface, and the other parts are spherical Zn containing about 60% by weight of Fe in a Zn—Fe alloy matrix layer containing about 20% by weight of Fe. -The Fe alloy phase is distributed in islands. It is a cross-sectional structure | tissue observation microscope picture of the hot press Zn type plating material produced by methods other than this invention. The plating layer has no spherical crystals. Moreover, although the whole plating layer is a substantially uniform Zn—Fe alloy, an oxide layer of Zn and Fe is formed on the surface layer portion.

Claims (8)

  A steel material containing C: 0.1 to 0.5% by mass, Si: 0.05 to 0.5%, Mn: 0.5 to 3%, the plating layer cross section, and the vicinity of the base steel sheet interface is layered The Zn—Fe alloy layer containing 50 to 80% by mass of Fe and the other part is Fe in mass% in a Zn—Fe alloy phase matrix containing 10 to 30% by mass of Fe. A hot-pressed steel material excellent in corrosion resistance and press workability after coating, having a Zn-Fe alloy plating layer in which a Zn-Fe alloy phase having a spherical shape containing 50 to 80% is distributed in an island shape.   The corrosion resistance and press workability after coating according to claim 1, wherein the Zn-Fe alloy phase having a spherical shape has a diameter of 1 to 30 µm and a cross-sectional area ratio of 10 to 80% of the entire plating layer. Excellent hot-pressed steel.   The hot-pressed steel material excellent in corrosion resistance and press workability after coating according to claim 1 or 2, wherein the thickness of the plating layer is 5 to 80 µm.   The plating layer further contains 0.001 to 7% in total by mass% of one or more of Al, Cr, Mn, Mg, Ti, Ni, Mo, and Si. Hot pressed steel with excellent corrosion resistance and press workability after coating. When the steel material is heated for hot pressing, the ultimate plate temperature T is set to the value defined by the formula (1) or 800 ° C., whichever is higher, 905 ° C. or less, and the rate of temperature increase up to 780 ° C. Is 5 ° C / second or higher, the rate of temperature increase from 780 ° C to the ultimate plate temperature is 1 ° C / second or higher and lower than 5 ° C / second, the holding time in that temperature range is 5 to 600 seconds, and further hot press The hot-pressed steel material excellent in corrosion resistance and press workability after coating according to any one of claims 1 to 4, wherein the starting temperature is 780 ° C or lower.

(Here, [C], [Mn], [Si], and [Ti] are mass% of each element in steel.)   The steel material further contains at least one of B: 0.0003 to 0.03% by mass and Ti: 0.01 to 0.3% by mass, according to any one of claims 1 to 5. Hot pressed steel with excellent corrosion resistance and press workability after coating.   The steel material contains at least one of Nb: 0.005 to 0.5 mass%, V: 0.005 to 0.5 mass%, and Mo: 0.005 to 0.5 mass%. A hot-pressed steel material excellent in corrosion resistance and press workability after coating according to any one of claims 1 to 6.   The coating material according to any one of claims 1 to 7, wherein the steel material contains at least one of Ni: 0.01 to 2.0 mass% and Cu: 0.01 to 2.0 mass%. Hot pressed steel with excellent later corrosion resistance and press workability.

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