JP4695459B2 - Hot pressed steel with zinc-based plating with excellent corrosion resistance after painting - 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 vehicle body due to the increasing interest 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. Therefore, attempts have been made to maintain the amount of steel used by using high-strength steel sheets, and application of TRIP steel sheets that maintain press formability while increasing the tensile strength to about 780 MPa has begun. The TRIP steel sheet is a steel sheet using martensitic transformation of retained austenite, and TRIP is an abbreviation for Transformation Induced Plasticity. However, this steel plate cannot be further increased in strength, for example, in the 1500 MPa class, and development of a steel plate capable of ensuring formability with such high strength is strongly demanded.

  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. In addition, when alloying progresses, the oxidation reaction of iron in the plating layer is also promoted, resulting in problems such as falling off of the oxide layer and poor coating adhesion after coating. 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. The steel material after hot working is generally used after being painted. However, the steel material that has been rapidly cooled after being pressed at such a high temperature has an oxide film made of Fe or a plating layer component formed on the surface. Therefore, the phosphating film which is the coating ground treatment is not uniformly formed. As a result, depending on the type of paint, there is a problem that the adhesion between the coating film and the base steel sheet is inferior and the corrosion resistance is inferior to that of a general steel material.

JP 2000-38640 A JP 2003-34845 A JP 2003-193187 A JP 2003-126920 A JP 2003-126921 A JP 2003-73774 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. For these problems, merely suppressing the alloying reaction cannot suppress welding and galling in the mold due to a low melting point. Furthermore, poor coating adhesion is due to the non-uniformity of the phosphating film of the coating surface treatment, due to the lack of reactivity of the phosphate bath, and further improvement of the bath is difficult. .

  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 iron-iron interface is a layered alloy phase made of a Zn-Fe alloy containing about 60% by mass of Fe inevitably formed, and the other surface layer portion is Fe The Zn—Fe alloy phase containing 10 to 30% by mass of Zn—Fe alloy phase (Γ phase) as a matrix and having a spherical shape containing 50 to 80% by mass of Fe is distributed in an island shape. Acquired knowledge about the structure that achieves both the anticorrosion by the spherical alloy phase with high melting point and the performance of ensuring corrosion resistance and coating film adhesion by the Zn-Fe alloy phase (Γ phase) with relatively small Fe content. Made. In addition, the inventors have succeeded in finding a method for producing an alloy phase having such a structure through repeated examination of a phase diagram of a Zn-Fe binary alloy and experiments that have been verified by actually causing a reaction. That is, the low temperature region up to 782 ° C. in which the Fe phase is formed in the solid phase is heated in as short a time as possible to suppress the growth of the Fe phase (Zn solid solution Fe) at the base iron interface. 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 technique is quench hardening property containing C: 0.1 to 0.5%, Si: 0.05 to 0.5%, Mn: 0.5 to 3% by mass%. The cross section of the plating layer is made of a Zn—Fe alloy layer containing 50 to 80% by mass of Fe in the vicinity of the base steel sheet interface, and the other part is 10 to 30% by mass of Fe. A Zn-Fe alloy phase matrix containing a Zn-Fe alloy phase having a spherical form containing 50 to 80% by mass of Fe in a Zn-Fe alloy phase matrix containing% by mass is distributed, and the plating layer surface layer Corrosion resistance and press workability after coating with a coating of one or more metal oxides and / or metal hydroxides of Zr, Ti, Si containing 10 to 100 nm thick and containing F element It is an excellent hot-pressed steel material.

  Preferably, it is a hot pressed steel material excellent in corrosion resistance after coating and press workability, 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, Moreover, it is the hot press steel material excellent in the corrosion resistance after coating and press workability whose thickness of the plating layer is 5-80 micrometers. In addition, the plating layer may further contain 0.001 to 7% in total by mass of one or more of Al, Cr, Mn, Mg, Ti, Ni, Mo, and Si.

  In this steel material manufacturing method, when the steel material is heated for hot pressing, the temperature range of the ultimate plate temperature T is the value defined by the following relational expression (1) or 800 ° C, whichever is higher. The lower limit is set, and 905 ° C. is set as the upper limit. Furthermore, the rate of temperature increase to 780 ° C. is 5 ° C./second or more, the rate of temperature increase from 780 ° C. to the ultimate plate temperature T is 1 ° C./second to less than 5 ° C./second, and the rate from 780 ° C. to the ultimate plate temperature is reached. The holding time in the temperature range is 5 to 600 seconds, and the hot press start temperature is 780 ° C. or lower.

(Here, [C], [Mn], [Si] and [Ti] are the mass% of each element in steel)

  The hot-pressed steel material of the present invention further contains at least one of B: 0.0003 to 0.03% by mass and Ti: 0.01 to 0.3% by mass in the respective ranges as components in the steel material. Moreover, Nb: 0.005 to 0.5% by mass, V: 0.005 to 0.5% by mass, Mo: 0.005 to 0.5% by mass in each range is contained. Can suppress toughness improvement and grain coarsening due to refinement of the structure of the steel material, and Ni: 0.01 to 2.0% by mass and Cu: 0.01 to 2.0% by mass in the respective ranges. Alternatively, the hardenability and toughness of the steel material can be improved by adding two kinds.

  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 further components to develop quenching hardenability 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. .

  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. 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. Further, the alloying reaction in the temperature range up to 780 ° C. solidifies when about 10% by mass of Fe is contained in the liquid phase, and thereafter Γ, δ1, and ξ phases are generated in descending order of Fe content. The plating layer of the present invention suppresses these alloy phases by increasing the temperature up to 780 ° C. as much as possible, reduces the thickness of the Zn—Fe alloy layer formed in the vicinity of the base iron interface, and thereafter 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 Fe-rich phase with a high melting point and hardness 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 an island shape 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 the plating layer of the present invention can be determined by conducting an elemental analysis with an electron beam probe microanalysis (EPMA) after embedding and polishing a cross-section of a plated steel sheet in a resin.

  The granular Zn-Fe alloy phase containing 50 to 80% by mass of Fe grows with an increase in the 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. % Is necessary and the 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 Al, Cr, Mn, Mg, Ti, Ni, Mo, and Si may be added singly or in combination. 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 temperature reached, 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 upper photograph in FIG. 1 is a cross-sectional photograph of the plated layer of the hot-pressed zinc-based plated steel material of the present invention. The middle and lower photographs are photographs showing the distribution of iron and zinc elements in the upper structure photograph. 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 upper photograph in FIG. 2 is a cross-sectional photograph of a plated layer of hot-pressed zinc-based plated steel produced by a method other than the present invention. The middle and lower photographs are photographs showing the distribution of iron and zinc elements in the upper structure photograph. 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.

  Pretreatment coating film by forming a film of one or more metal oxides and / or hydroxides of Zr, Ti, Si containing F element on the surface layer of the hot-pressed steel plating layer obtained As a result, the adhesiveness and the corrosion resistance are excellent. This coating pretreatment film contains metal ions and complex ions containing fluorine ions at a molar ratio of 4 times or more with respect to the metal ions and / or containing metal ions and fluorine at least 4 times of the metal ions. It is formed on the surface of the plating layer by immersing the metal material in the aqueous treatment solution containing pH 2 to 7. This film formation reaction is active by the action of fluorine ions contained in the aqueous solution, and is active even if some oxide film formed by heat treatment is present. However, in the case of the plating layer of FIG. 2 in which alloying has proceeded excessively, the oxide film on the surface layer is also thick, and a uniform pretreatment film with high adhesion is not formed. It can be said that having a plating layer structure like the material of the present invention is a condition for obtaining good film formation. If the thickness of the film is less than 10 nm, the adhesion of the coating is insufficient, and if it exceeds 100 nm, the effect of adhesion is saturated and the film formation time is excessive, which is not economical. In some cases, cohesive failure occurs, and the adhesiveness may decrease.

Next, the present invention will be described in more detail based on examples.
Plating was performed from a hot-dip Zn bath having various plating compositions using a cold-rolled steel plate (thickness 1.8 mm) having the steel components shown in Table 1 manufactured through normal hot rolling and cold rolling steps as a base steel plate. The bath composition is Zn-0.2% Al, Zn-11% Al, Zn-11% Al-3% Mg, Zn-7% Al-3% Mg, Zn-11% Al-3% Mg-0.2% Si is there. The hot dip plating was performed by immersing a steel plate in a plating bath using a cleaning-non-oxidation furnace-reduction furnace type manufacturing facility, and after cooling, controlling the coating amount by nitrogen gas wiping and then cooling. You may make it the appearance without a spangle by spraying an air-water mist in the process of cooling and making it cool rapidly. The adhesion amount of the plating was changed at 30 to 300 g / m ² per side. In addition to the hot dipping method, plating layers of other components were prepared by electroplating and vapor deposition. Zn-11% Ni, Zn-5% Cr, Zn-5% Mn, Zn-1% Ni-1% Mo, Zn-2% Ti, Zn-2% Mg and the like. The plating method is not particularly limited, and can be appropriately selected in order to obtain a target plating composition.

  The obtained plated 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 hot press start 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 after being held in a mold until the temperature of the sample reached 50 ° C. and then taken out. The plating layer of the obtained press sample has undergone an alloying reaction with the ground iron by heat treatment, and the plating adhesion amount and the plating composition are significantly different from those of the plating layer before the heat treatment. In general, the amount of adhesion increases and the content of alloy elements decreases due to the mixing of iron.

  The obtained press sample was treated with a metal oxide or metal hydroxide layer. In this treatment, a Zr, Si, Ti-based oxide or hydroxide film layer is applied, and an aqueous solution of 5 g / L ammonium hexafluorozirconate at room temperature, 5 g / L ammonium hexafluorotitanate, or hexafluorosilicate A steel material was immersed in a solution of one or more of 50 g / L aqueous ammonium acid solution, taken out after 2 minutes, washed with water and dried. Separation of oxide and hydroxide was carried out at the pH of the solution. When PH was raised, hydroxide was obtained.

  Tables 2 to 10 collectively show the steel types, heating conditions, plating layer conditions, metal oxide and / or metal hydroxide layer conditions used in the samples of each example.

  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 degree of mold galling with the mold was visually evaluated at the wall of the sample. In the case where no galling was observed, ◯, galling was observed but the plating layer remained, Δ, galling was observed and the plating layer was severely defective, and ◯ and △ were acceptable.

  Corrosion resistance was evaluated using a coating material. For coating, the wall portion of the press sample was cut out and subjected to electrodeposition coating. 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. For some of the comparative samples that did not have a metal oxide or metal hydroxide layer, the phosphate conversion treatment was performed before electrodeposition coating. For this chemical conversion treatment, PB-3080 manufactured by Nippon Parkerizing Co., Ltd. was used. 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 salt spray of JIS Z 2371, 4 hours of drying at 60 ° C. and 30% humidity, and 2 hours of wetting at 50 ° C. and 98% humidity. In the evaluation, the maximum blister width on one side of the cross-cut portion was 2 mm or less, Δ was 4 mm or less, Δ was over 4 mm, 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.

  Further, as an evaluation of coating film adhesion in a harsh environment, a coating film secondary adhesion test was conducted. In this test, the sample was immersed in an aqueous NaCl solution at 53 ° C. and 5% by mass for 240 hours, and the bulge width of the portion where the scratch reaching the substrate was introduced with a cutter was measured after the tape peeling test. The maximum bulge width on one side is 1 mm or less, ◯ is 2 mm or less, Δ is over 2 mm, and ○ and Δ are acceptable.

  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 obtained results are shown in Tables 8-10. Within the scope of the present invention, the degree of mold galling is small, the post-coating corrosion resistance, the coating adhesion, and the coating film secondary 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, the occurrence of mold galling, poor corrosion resistance after painting, poor paint adhesion, and poor secondary adhesion of the coating film occur, and it does not have the performance to be provided as a steel material for vehicle bodies. Specifically, Comparative Example 1 has poor corrosion resistance because the metal oxide or hydroxide layer is a conventional phosphate film, and Example 2 has heating conditions outside the scope of this patent. In Examples 3 and 4, since the heating conditions were outside the conditions of the present invention, galling and poor corrosion resistance occurred, and Comparative Example 5 had a quenching start temperature of the present invention. Since the condition is not met, the hardness of the steel material is insufficient.

  From the above, the hot-pressed galvanized steel sheet obtained in the present invention has a high workability because it is hot, while the strength is as high as 1500 MPa. 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 | tissue observation photograph of the hot press Zn type plating material produced by this invention. Zn-Fe layer containing about 60% by mass of Fe in the vicinity of the base iron interface, the other part is a spherical Zn containing about 60% by mass of Fe in a Zn-Fe alloy matrix layer containing about 20% by mass of Fe -Fe alloy phase is distributed in islands. It is a cross-sectional structure | tissue observation result of the hot press Zn type plating material produced by methods other than this invention. Spherical crystals do not exist in the plating layer. Further, 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.

Claims (8)

  Steel plate containing C: 0.1-0.5%, Si: 0.05-0.5%, Mn: 0.5-3% by mass%, plating layer cross section, layered near the base steel plate interface and Fe 50-50% by mass It consists of a Zn-Fe alloy layer containing, and the other part has a spherical shape containing 50-80% Fe by mass in a Zn-Fe alloy phase matrix containing 10-30% Fe by mass%. There is a Zn-Fe alloy plating layer in which the Zn-Fe alloy phase is distributed in an island shape, and the surface layer of the plating layer has a thickness of 10 to 100 nm and contains one or two of Zr, Ti, Si containing F element A hot-pressed steel material having a coating of the above metal oxide and / or metal hydroxide and excellent in corrosion resistance after painting.   The hot-pressed steel material having excellent corrosion resistance after coating according to claim 1, wherein the spherical alloy phase has a diameter of 1 to 30 µm and a cross-sectional area ratio of 10 to 80% of the entire plating layer.   The hot-pressed steel material having excellent corrosion resistance after coating according to claim 1 or 2, wherein the plating layer has a thickness of 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 in mass%. Hot pressed steel with excellent corrosion resistance after coating. When the steel material is heated for hot pressing, the ultimate plate temperature T is the value defined by the relational expression (1) or 800 ° C, whichever is higher, 905 ° C or less, and the temperature rises to 780 ° C. The speed is 5 ° C per second or higher, the rate of temperature rise from 780 ° C to the ultimate plate temperature T is 1 ° C per second or higher and less than 5 ° C per second, and the holding time in the temperature range from 780 ° C to the ultimate plate temperature is 5 to 600 seconds The method for producing a hot-pressed steel material having excellent corrosion resistance after coating according to any one of claims 1 to 4, wherein the hot-press start temperature is 780 ° C or lower.
(Here, [C], [Mn], [Si], [Ti] are the mass% of each element in steel)   The steel sheet further contains at least one of B: 0.0003 to 0.03 mass% and Ti: 0.01 to 0.3 mass% in the respective ranges after the coating according to any one of claims 1 to 4. Hot pressed steel with excellent corrosion resistance.   In steel sheet, Nb: 0.005-0.5 mass%, V: 0.005-0.5 mass%, Mo: 0.005-0.5 mass% are contained in one or more types in the respective ranges to improve toughness and grain size by refinement of steel structure. The hot-pressed steel material excellent in corrosion resistance after coating according to any one of claims 1 to 4, wherein the crystallization is suppressed.   The hardenability and toughness of the steel material improved by containing at least one of Ni: 0.01 to 2.0 mass% and Cu: 0.01 to 2.0 mass% in the respective ranges in the steel sheet. Hot pressed steel with excellent corrosion resistance after painting as described in Crab.