JP5444650B2 - Plated steel sheet for hot press and method for producing the same - Google Patents

Description

  TECHNICAL FIELD The present invention relates to a hot-pressed plated steel sheet and a method for producing the same, and more particularly to a hot-pressed plated steel sheet having excellent post-painting corrosion resistance and productivity and a method for producing the same.

  In recent years, in steel plate applications for automobiles (for example, automobile pillars, door impact beams, bumper beams, etc.) and the like, steel plates that have both high strength and high formability have been desired. There is TRIP (Transformation Induced Plasticity) steel using martensitic transformation of retained austenite. With this TRIP steel, it is possible to produce a high-strength steel sheet having excellent formability and a strength of about 1000 MPa, but it is possible to secure formability with ultra-high-strength steel having higher strength, for example, 1500 MPa or more. Have difficulty.

  Under such circumstances, hot press (also referred to as hot pressing, hot stamping, die quenching, press quenching, etc.) has recently been attracting attention as having both high strength and high formability. This hot press improves the formability of a high-strength steel sheet by heating it in an austenite region at 800 ° C. or higher and then forming it hot, and after it is formed, it is baked to obtain a desired material. It is.

  Hot press is promising as a method of forming ultra-high strength members, but usually has a step of heating the steel plate in the atmosphere, and at this time, oxide (scale) is generated on the steel plate surface. The process of removing the scale was necessary. However, such post-processes have problems such as the necessity of countermeasures from the viewpoints of scale removal ability and environmental load.

  As a technique for improving this, there has been proposed a technique for suppressing the generation of scale during heating by using an Al-plated steel sheet as a steel sheet for hot pressing (see, for example, Patent Documents 1 to 4).

JP-A-9-202953 JP 2003-181549 A JP 2003-49256 A JP 2003-27203 A

  However, the techniques described in Patent Documents 1 to 3 presuppose heating conditions such as furnace heating with a slow temperature increase rate. In the case of such heating conditions, in order to ensure the corrosion resistance after coating, it was necessary to make the coating amount high. However, if the adhesion amount is too large, there is a concern that the plating may be peeled off during hot molding, and Al-Fe powder may adhere to the mold, which reduces the productivity of the hot press process itself. There was a problem.

  Further, in the case of furnace heating, the temperature rise rate of the steel sheet is usually about 3 to 5 ° C./second, and the productivity of the steel sheet that can be formed by hot pressing is about 2 to 4 pieces / minute, which is very low in productivity.

  Patent Document 4 shows a relatively fast temperature increase of about 20 ° C./second. In such a case, a problem that molten metal hangs down is shown. In order to solve this problem, the temperature of the plating is lowered at a temperature lower than the melting point, and alloying (the phenomenon in which the plating and the steel plate react to change to an intermetallic compound) is advanced during this time, thereby reducing the melting point of the plating. It has been shown to rise. However, also in this case, for example, a 30 μm-thick plating layer requires gentle heating for 60 seconds, and the total heating time is 100 seconds. Therefore, there is still room for improvement from the viewpoint of improving productivity.

  In order to improve the productivity of the hot press, it is effective to perform rapid heating by a heating method using electricity such as electric heating or induction heating. However, when heated rapidly, there is a problem that the plating thickness becomes non-uniform, for example, the sagging described in Patent Document 4 occurs and the plating thickness locally increases. The essential cause of sagging is that the plating melts before alloying during the heating process. That is, when the alloy is formed, such a phenomenon does not occur because the melting point rises. However, when the temperature is rapidly raised, the plating dissolves at a melting point of Al of 660 ° C. or higher, and a phenomenon of moving by gravity or electromagnetic force is observed. . Such a plated steel sheet with a non-uniform plating thickness bites into or adheres to the mold at the time of pressing, which greatly impedes productivity. That is, productivity can be improved by overcoming this sagging phenomenon.

  Therefore, the present invention has been made in view of such problems, and in the plated steel sheet for hot press and the manufacturing method thereof, the object is to ensure excellent corrosion resistance even with a low plating adhesion amount and to improve productivity. And

  As a result of intensive studies to solve the above problems, the present inventors have found that the Al plating layer before the heating step immediately before the hot pressing step, and the Fe—Al existing between the plating layer and the steel plate. By appropriately controlling the thickness of the alloy layer, it has been found that excellent corrosion resistance can be secured even with a low plating adhesion amount, and productivity of hot press can be improved, and the present invention has been completed based on such knowledge. It came to do.

That is, the gist of the present invention is as follows.
(1) An Al plating layer containing 3 to 15% by mass of Si coated on the surface of the steel plate, and an Fe—Al alloy layer positioned between the Al plating layer and the steel plate, the Al The thickness of the plating layer is 3 μm or more and 10 μm or less, the thickness of the Fe—Al alloy layer is 6 μm or more, and the sum of the thickness of the Al plating layer and the thickness of the Fe—Al alloy layer is 10 μm or more and 30 μm or less. A hot-pressed plated steel sheet, wherein the center line average roughness Ra of the interface between the Al plating layer and the Fe—Al alloy layer is 0.6 μm or more and 3 μm or less.
(2) C: 0.1 to 0.4%, Si: 0.01 to 0.6%, Mn: 0.5 to 3%, P: 0.005 to 0.05% in mass% as steel components , S: 0.002-0.02%, Al: 0.005-0.1%, Ti: 0.01-0.1%, B: 0.0001-0.01% and Cr: 0.01 In an atmosphere containing oxygen: 3% by volume or more in a box annealing furnace containing an Al-plated steel sheet containing 1% and having an Al-plated amount of 30 to 100 g / m ² per side. The temperature is 450 to 600 ° C., the holding time is 1 to 50 hours, and the temperature integration value is 450 to 20000 ° C. · hour. The composition of the Al plating contains 3 to 15% by mass of Si. A method for producing a hot-pressed plated steel sheet.
(3) C: 0.1 to 0.4%, Si: 0.01 to 0.6%, Mn: 0.5 to 3%, P: 0.005 to 0.05% in mass% as steel components , S: 0.002-0.02%, Al: 0.005-0.1%, Ti: 0.01-0.1%, B: 0.0001-0.01% and Cr: 0.01 Heating an Al-plated steel sheet containing -1% and coated with Al so that the adhesion amount is 30 to 100 g / m ² per side in a hot-dip galvanizing line (CGL) to 750-950 ° C. in continuously have line alloying treatment, the the composition of the Al coating, characterized in that it contains Si 3 to 15 wt%, the manufacturing method of the hot press-plated steel sheet.

  According to the present invention, in the plated steel sheet for hot pressing and the manufacturing method thereof, the thickness of the Al plating layer before the heating step immediately before the hot pressing step is set to a predetermined thickness or less, and the Al plating layer and the Fe—Al alloy layer By making the sum of the thicknesses equal to or greater than a predetermined thickness, it is possible to ensure excellent corrosion resistance even with a low plating adhesion amount and improve productivity.

  Exemplary embodiments of the present invention will be described below in detail with reference to the accompanying drawings.

[Outline of Plated Steel Sheet for Hot Press According to the Present Invention]
As described above, the techniques described in Patent Documents 1 to 3 described above are low productivity processes in which heating takes about 200 seconds or more. When rapid heating is performed by energization heating or the like in order to improve hot press productivity, there is also a problem that molten plating sag occurs on the steel sheet surface as described in Patent Document 4. Here, sagging in a heating method using electricity will be described. Both high-frequency heating and energization heating are heating methods that utilize the resistance heat generation of a steel sheet by passing a current through the steel sheet. However, when a current flows through the steel sheet, a magnetic field is generated, and a force is generated by the interaction between the current and the magnetic field. This force moves the molten metal. Since the direction of the current varies depending on the heating method, it cannot be said unconditionally, and the central portion of the steel plate may be thick, or conversely, the end portion of the steel plate may be thick. In addition, when the blank is placed vertically, it may be pulled by gravity and the plating at the lower part of the blank may be thick.

According to the results of the study by the present inventors, it has been found that in order to prevent the plating from sagging, it is sufficient to reduce the amount of plating adhesion. For example, when an Al-plated steel plate is used and the temperature rising rate is 50 ° C./sec or higher and the temperature rising temperature is 900 to 1200 ° C., if the plating adhesion amount is 30 g / m ^{2 on} one side, plating sag occurs. However, when the amount of plating is 60 g / m ^{2 on} one side, a molten plating sag occurs. On the other hand, if the plating adhesion amount is reduced in order to prevent the plating from sagging, sufficient post-coating corrosion resistance cannot be ensured. That is, since there is a trade-off relationship between improving productivity and ensuring corrosion resistance, conventionally, a hot-press plated steel sheet having both excellent corrosion resistance and excellent productivity has not been obtained.

  Therefore, as a result of intensive investigations to obtain a hot-pressed plated steel sheet that has both excellent corrosion resistance and excellent productivity, the sag of plating in the heating process is the same as that of the Al plating layer before the heating process. It was found that the Fe-Al alloy layer was produced by melting the Al plating layer. In other words, the inventors of the present invention have a low melting point because the Al plating layer is mainly composed of Al, and the molten Al-based metal is drawn to the contracted position of the current conduction circuit by rapid heating using current heating or the like. As a result, it is considered that the plating droops along the energizing direction. On the other hand, since the Fe—Al alloy layer has a high melting point unlike the Al plating layer, it is not melted by heating in the heating step before hot pressing, and as a result, it is considered that it does not contribute to plating dripping.

  As a result of the above examination, the thickness of the surface Al plating layer is made as thin as possible in order to prevent the plating from sagging, and the thickness of the Fe-Al alloy layer is made thicker in order to ensure the corrosion resistance after coating. By securing a sufficient thickness as the Al plating layer and the Fe-Al alloy layer as a whole), the knowledge that a hot-pressed plated steel sheet having excellent corrosion resistance and excellent productivity can be obtained is obtained. The present invention has been completed based on the findings.

In order to partially alloy an Al plating material having an adhesion amount of 60 g / m ² or more on one side, it is necessary to raise the temperature to 750 ° C. or higher, or to heat at 600 ° C. or lower for several tens of hours. No unusual phenomenon is observed when the temperature is raised to 750 ° C. or higher. However, it is difficult to raise the temperature to such a high temperature after Al plating on a normal continuous Al plating line, and it is easier to box anneal the coil. However, there are some problems when the coil is subjected to box annealing. One is that Al-10% Si has a melting point of about 600 ° C., so there is a concern that softened Al is fused in the vicinity of the melting point. This concern disappears when the temperature is lowered. Furthermore, a phenomenon was observed that when heating was performed in a nitrogen atmosphere or a mixed atmosphere of nitrogen and hydrogen, alloying stopped halfway. This is a phenomenon in which voids (holes) accumulate at the interface of the Al plating-alloy layer when alloying progresses partway. This void is considered to be a so-called Kirkendle void, that is, a void generated by a gap between the diffusion rates of Al and Fe. When this phenomenon occurs, alloying no longer proceeds, and as a result of the accumulation of voids, the unalloyed Al part is peeled off. In order to prevent this phenomenon, it is important to contain 3% by volume or more of oxygen in the annealing atmosphere. In such an atmosphere, alloying proceeds to the surface. The reason why the atmosphere affects the state of void accumulation and alloying is unknown at this stage.For example, when the void density is increased due to the increase in local lattice defect density due to the diffusion rate gap, a new A surface needs to be formed, and the atmosphere may affect this surface energy.

[Configuration of plated steel sheet for hot press according to the present invention]
That is, according to the present invention, it has an Al plating layer coated on the steel plate surface, and an Fe-Al alloy layer positioned between the Al plating layer and the steel plate, and the thickness of the Al plating layer is 3 μm or more and 10 μm. The thickness of the Fe—Al alloy layer is 6 μm or more, the sum of the thickness of the Al plating layer and the thickness of the Fe—Al alloy layer is 10 μm or more and 30 μm or less, and the Al plating layer and the Fe—Al alloy A plated steel sheet for hot press having a center line average roughness Ra of 0.6 μm or more and 3 μm or less at the interface with the layer can be obtained.

(General alloy layer structure)
Subsequently, before explaining the configuration of the hot-pressed plated steel sheet according to the present invention, as a premise, the structure of a general alloy layer obtained by heating the Al-plated steel sheet will be described with reference to FIG. To do. FIG. 1 is an optical micrograph showing a general example of the structure of a cross-sectional structure after heat-alloying an Al-plated steel sheet.

  The plated layer of the Al-plated steel sheet before hot pressing is composed of an Al—Si layer and a FeAlSi alloy layer from the surface layer. When this plating layer is heated to about 900 ° C. in a hot pressing process, interdiffusion between Al—Si and Fe in the steel sheet occurs, and the whole changes to an Al—Fe compound. At this time, a phase partially containing Si is also generated in the Al—Fe compound.

  Here, as shown in FIG. 1, the Fe—Al alloy layer after heat-alloying an Al-plated steel sheet generally has a five-layer structure in many cases. In FIG. 1, these five layers are represented by 1 to 5 layers in order from the surface of the plated steel sheet. The Al concentration in the first and third layers is about 50% by mass, the Al concentration in the second layer is about 30% by mass, and the Al concentrations in the fourth and fifth layers are 15 to 30% by mass, respectively. The composition has a width of 1 to 15% by mass. The balance is Fe and Si. Void formation may be observed near the interface between the fourth layer and the fifth layer. The corrosion resistance of such an alloy layer substantially depends on the Al content, and the higher the Al content, the better the corrosion resistance. Therefore, the first layer and the third layer are most excellent in corrosion resistance. In addition, the structure below the fifth layer is a steel base, which is a hardened structure mainly composed of martensite.

FIG. 2 shows a binary phase diagram of Fe—Al. Referring to FIG. 2, it can be determined that the first layer and the third layer are mainly composed of Fe ₂ Al ₅ and FeAl ₂ , and the fourth layer and the fifth layer correspond to FeAl and αFe, respectively. In addition, the second layer is a layer containing Si that cannot be explained from the Fe—Al binary phase diagram, and its detailed composition is not clear, but the present inventors have found that the FeAl ₂ and Fe—Al—Si compounds are fine. It is presumed that it was mixed.

(About the structure of the alloy layer obtained by heating the hot-pressed plated steel sheet of the present invention)
Next, the structure of an alloy layer (hereinafter referred to as “coating layer”) obtained by heating the plated steel sheet for hot press according to the present invention under predetermined conditions will be described. This coating layer contains at least two types of intermetallic compounds selected from the group consisting of FeAl ₂ , Fe ₂ Al ₅ , FeAl ₃ , FeAl, and FeAlSi compounds, and the Al concentration is a predetermined concentration (for example, 40% by mass). ) There may be a single-layer structure in which a region having an Al concentration of a predetermined concentration (for example, 40% by mass) or less is dispersed in a super region.

  The coating layer is generated by heating an Al-plated steel sheet having an Al-plated steel plate surface (however, the heating conditions are completely different), as in the general alloy layer described above. That is, although the said coating layer is Al plating before a heating process, it is produced | generated when Fe diffuses to the surface and changes to an intermetallic compound in a heating process. In this case, metal Al does not exist in the coating layer on the surface of the steel sheet, but this can be easily confirmed, for example, by detecting an Al peak by X-ray diffraction from the surface. ΑFe is a ferrite phase in which Al is dissolved, and is not an intermetallic compound in the strict sense, but since the boundary with other phases is not clear, in the present invention, this phase is also an intermetallic compound. Called.

  The “single layer structure” is different from the five layer structure as shown in FIG. 1, and a typical structure of the structure is shown in FIG. As can be seen from the comparison between FIG. 1 and FIG. 3, the structure of FIG. 3 is changed from a layered structure in which both the second layer and the fourth layer in FIG. 1 are divided and five layers are laminated to a single layer structure. It has changed. Here, from the viewpoint of ensuring the corrosion resistance after painting, the coating layer is not a structure in which the second layer and the fourth layer in FIG. 1 are simply divided, but the second layer and the fourth layer in FIG. The division progresses (and the volume fraction also decreases), and a sea-like region having a high Al concentration corresponding to the first layer, the third layer, and the fifth layer in FIG. 1 (hereinafter referred to as “high Al region”). It is necessary to have a sea-island structure in which island-like regions having a low Al concentration (hereinafter referred to as “low Al regions”) corresponding to the second layer and the fourth layer in FIG. 1 are dispersed. The high Al region is a region where the Al concentration exceeds 40% by mass, and the low Al region is a region where the Al concentration is 40% by mass or less. Moreover, as a composition in a high Al area | region, Al concentration is 45-55 mass%, and as a composition in a low Al area | region, Al concentration is 15-35 mass% in many cases. The composition and crystal structure of such a coating layer are analyzed using an electron beam microanalyzer (EPMA), a scanning electron microscope-energy dispersive X-ray analyzer (SEM-EDS), a transmission electron microscope (TEM), or the like. Can be specified.

  In order to obtain such a structure of the coating layer, in the Al-plated steel sheet according to the present invention, a temperature from 600 ° C. that regulates the structure of the alloy layer to a temperature that is 10 ° C. lower than the maximum plate temperature (for example, about 850 ° C.). By raising the temperature at a predetermined speed or higher in the region, it is possible to have a sea-island structure in which island-like low Al regions are dispersed in a sea-like high Al region. There is no particular limitation on the heating method, but in order to produce a coating layer having the above-mentioned sea-island structure, it is necessary to perform rapid heating at a temperature rising rate of, for example, 50 ° C./sec. It is preferable to use a heating method using electricity such as heating. Unlike the conventional near-infrared heating that uses radiant heat or furnace heating, the present inventors generate heat from the inside of a steel sheet in current heating or high-frequency dielectric heating. It is presumed that the sea-island structure as described above is likely to be affected.

  Hereinafter, the configuration of the Al-plated steel sheet according to the present invention used for the production of the hot-press plated steel sheet having the coating layer as described above will be described in detail.

(About steel plate)
Since hot pressing is performed simultaneously with pressing with a mold and quenching, the plated steel sheet for hot pressing according to the present invention needs to be a component that is easily quenched. Specifically, as steel components in the steel sheet, in mass%, C: 0.1 to 0.4%, Si: 0.01 to 0.6%, Mn: 0.5 to 3%, Ti: 0 .01-0.1%, B: 0.0001-0.01%, Cr: 0.01-1%, P: 0.005-0.05%, S: 0.002-0.02%, Al: 0.005 to 0.1% is preferable. The amount of C is preferably 0.1% or more from the viewpoint of improving hardenability, and if the amount of C is too much, the toughness of the steel sheet is remarkably lowered, so that it is 0.4% by mass or less. It is preferable. Further, if Si is added in excess of 0.6%, the Al plating property is lowered, and if it is less than 0.01%, fatigue characteristics are inferior, which is not preferable. Mn is an element that contributes to hardenability, and it is effective to add 0.5% or more, but it is not preferable to exceed 3% from the viewpoint of lowering toughness after quenching. Ti is an element that improves the heat resistance after aluminum plating, and it is effective to add 0.01% or more. However, if it is added excessively, it reacts with C and N to lower the steel sheet strength. It is not preferable to exceed 1%. B is an element that contributes to hardenability, and it is effective to add 0.0001% or more. However, since there is a concern of hot cracking, it is not preferable to exceed 0.01%. Cr is a strengthening element and is effective in improving hardenability. However, if it is less than 0.01%, it is difficult to obtain these effects. On the other hand, if the content exceeds 1%, the manufacturability during production and hot rolling is adversely affected. If P is added excessively, it causes brittleness of the steel sheet, so 0.05% or less is preferable. S becomes an inclusion in the steel as MnS, and if MnS is large, it becomes a starting point of fracture, and in order to inhibit ductility and toughness, 0.05% or less is preferable. Since Al is a plating-inhibiting element, 0.1% or less is preferable. Both P and Al are preferably made the lower limit concentration 0.005% from the economical refining limit. Moreover, it is preferable that S is also made the lower limit concentration 0.002% from the economical refining limit. In addition, N, Mo, Nb, Ni, Cu, V, Sn, Sb, and the like can be contained as other components in the steel sheet. Usually, the mass percentage is N: 0.01% or less, Ni: 0.05% or less, and Cu: 0.05% or less.

(About Al plating)
The plated steel sheet for hot press according to the present invention is manufactured by alloying a part of the Al plated layer in an Al plated steel sheet having Al plated on the surface of the steel sheet. The plating method is not particularly limited, and electroplating, vacuum deposition, cladding, and the like including hot-dip plating are possible. Currently, the most widely used industrially is the hot dipping method. Usually, a plating bath containing 3% by mass to 15% by mass of Si can be used as an inevitable impurity. Fe and the like are mixed. Other additive elements may be Mn, Cr, Mg, Ti, Zn, Sb, Sn, Cu, Ni, Co, In, Bi, Misch metal, etc., but as long as the plating layer is mainly Al. Is possible. Addition of Zn and Mg is effective in terms of making red rust unlikely to occur, but excessive addition of these elements having a high vapor pressure causes generation of fumes of Zn and Mg, Zn on the surface, and powdery substances derived from Mg The addition of Zn: 60% by mass or more and Mg: 10% by mass or more is not preferable.

  Moreover, in this invention, it is preferable to contain 3-15 mass% of Si as a composition of Al plating. Si has a function of suppressing alloy layer growth during Al plating. In hot press applications, the necessity to suppress alloy layer growth is small, but in hot-dip plating, products for various uses are manufactured in one bath, so in applications that require the workability of Al plating. It is necessary to suppress alloy layer growth. When the amount of Si is less than 3% by mass, the alloy layer grows, so that the workability as an Al-plated steel sheet is lowered. On the other hand, if the amount of Si is too large, it will crystallize out as coarse crystals in the plating layer, impairing corrosion resistance and plating workability. For this reason, it is preferable that Si amount is 15 mass% or less.

Moreover, in this invention, it does not specifically limit about the plating pre-processing and post-processing of Al plating. Ni, Cu, Cr, Fe pre-plating and the like may be used as the plating pretreatment, but this is also applicable. Further, as a post-plating treatment, chromate treatment, resin coating treatment, or the like may be performed for the purpose of primary rust prevention and lubricity. However, with regard to the chromate treatment, a trivalent treatment film such as electrolytic chromate is preferable in consideration of recent hexavalent chromium regulations. In addition, post-treatment other than inorganic chromate is also applicable. In order to impart lubricity, it is also possible to perform surface treatment in advance using alumina, silica, MoS ₂ or the like.

  The Al-plated steel sheet according to the present invention has an Al plated layer located on the surface layer and an Fe—Al alloy layer located between the Al plated layer and the steel sheet (base material). At this time, it is preferable that the roughness of the interface between the Al plating layer and the Fe—Al alloy layer is 0.6 μm or more and 3 μm or less as the centerline average roughness Ra of the interface. By setting the roughness of the interface between the Al plating layer and the Fe—Al alloy layer within this range, in the heating step, dripping of the molten plating is prevented, and the thickness of the alloy layer (coating layer) becomes non-uniform. This can be prevented. The reason for this is that when the temperature is 600 ° C. or higher, Al—Si begins to melt, but the Fe—Al alloy layer does not melt at this temperature, and the interface roughness between the Fe—Al alloy layer and the Al plating layer is large. This is presumed to be due to physically hindering the movement of the molten metal.

The interface roughness between the Fe—Al alloy layer and the Al plating layer can be measured by the following method. The Al-plated steel sheet with _{3% NaOH + 1% AlCl 3} · 6H 2 O solution by electrolytic stripping at a current density of 20 mA / cm ² about the counter electrode as stainless steel can be melted only Al plating. After peeling off the Al plating layer, the roughness of the Fe—Al alloy layer surface can be measured using a normal roughness meter or the like.

  Next, a method for adjusting the interface roughness between the Al plating layer and the Fe—Al alloy layer will be described. In the hot-dip Al plating method, since the Fe—Al alloy layer is generated in the Al plating bath, the plating conditions naturally have a great influence. Specifically, since the kind of the alloy layer generated varies depending on the amount of Si in the bath and the bath temperature, the interface roughness can be adjusted by adjusting this condition. For example, when the Si amount in the bath is increased to 10 to 12% and the bath temperature is set to about 640 to 650 ° C., the interface roughness increases, whereas the Si amount is set to 6 to 8% and the bath temperature is set to 650 to 650 ° C. By setting the temperature to about 670 ° C., the interface roughness becomes small. This is because when the Si content in the bath is high, a rod-like alloy layer is easily formed.

  In addition to this, the present invention performs heating for growing the Fe—Al alloy layer after Al plating, and the interface roughness can be adjusted also by this heating condition. For example, when the heating is performed for about 20 hours or more at a low temperature of about 400 ° C., the interface roughness becomes small, while the interface roughness increases under the heating condition of 500 ° C. or more for 1 to 10 hours. In the case of box annealing, the upper limit is 600 ° C. because there is a concern that the coils are fused to each other near the melting point.

  As one embodiment of the present invention, there are a form in which heating is continuously performed in a CGL (hot dip galvanizing line) after Al plating, and a form in which box annealing is performed after winding the coil once. While the conditions need to be adjusted, the latter can be adjusted only by the conditions of box annealing regardless of the Al plating conditions.

  Further, the sum of the thickness of the Al plating layer and the thickness of the Fe—Al alloy layer is preferably 10 μm or more and 30 μm or less. If the sum of the thickness of the Al plating layer and the thickness of the Fe—Al alloy layer is 10 μm or more, it is preferable since sufficient post-coating corrosion resistance can be secured as a hot press member after the heating step. The larger the thickness, the better the corrosion resistance, but on the other hand, the larger the sum of the thickness of the Al plating layer and the thickness of the Fe-Al alloy layer, the easier it is for the coating layer generated by the heating process to be lost during processing. The thickness of the coating layer is preferably 30 μm or less.

  The thickness of the Al plating layer is preferably 3 μm or more and 10 μm or less. According to the examination results of the present inventors, as described above, the main cause of dripping of the molten plating during the heating process is not the Fe—Al alloy layer but mainly the surface Al plating layer. I think that is due to this. Therefore, if the thickness of the surface Al plating layer is reduced, it is considered that the dripping of the plating can be prevented even if the thickness of the Fe—Al alloy layer therebelow is increased. From such a viewpoint, it is preferable that the thickness of the Al plating layer is 10 μm or less. However, if the Al plating layer is too thin, the Al content in the plating layer is reduced, and the coating layer after the heating step may not have the above-described sea-island structure, so the thickness of the Al plating layer is 3 μm or more. It is preferable to do.

  Furthermore, the thickness of the Fe—Al alloy layer is preferably 6 μm or more. As described above, the interface roughness between the Fe—Al alloy layer and the Al plating layer is controlled. The operating conditions for increasing the interface roughness are consistent with the conditions for increasing the thickness of the Fe—Al alloy layer. Actually, when the interface roughness is 0.6 to 3 μm, the Fe—Al alloy layer becomes 6 μm or more. Therefore, this condition cannot be controlled independently, but as a result of controlling other factors.

[Method for Producing Plated Steel Sheet for Hot Press According to the Present Invention]
The configuration of the hot-pressed plated steel sheet according to the present invention has been described in detail above. Subsequently, the method for producing a hot-pressed plated steel sheet according to the present invention having such a configuration will be described in detail.

As described above, the plated steel sheet for hot pressing according to the present invention has C: 0.1 to 0.4%, Si: 0.01 to 0.6%, Mn: 0.5 as a steel component, as described above. -3%, Ti: 0.01-0.1%, B: 0.0001-0.01%, Cr: 0.01-1%, so that the adhesion amount is 90 g / m ² or less An Al-plated steel sheet coated with Al is heated at a temperature increase rate of 50 ° C./hour to 50 ° C./second to 500 ° C. to 1000 ° C. and then cooled at a cooling rate of 50 ° C./hour to 50 ° C./second. It is manufactured by performing heat treatment. By this heat treatment, a part of the Al plating layer after Al plating is alloyed with Fe in the base material, and the thickness of the Fe—Al alloy layer is increased.

  After raising the temperature from 450 ° C. to 950 ° C. at a temperature raising rate of 50 ° C./hour to 50 ° C./second, cooling at a cooling rate of 50 ° C./hour to 50 ° C./second, Fe of As diffusion proceeds, the thickness of the Fe—Al alloy layer can be increased, and the thickness of the Al plating layer can be reduced to 3 μm to 10 μm.

  The heat treatment is not particularly limited as long as a part of the Al plating layer is alloyed after Al plating. For example, box annealing (BAF annealing), induction heating after Al plating in CGL, etc. Can be carried out continuously. When the alloying treatment is performed, the thickness of the Al plating layer can be controlled by adjusting the annealing conditions, that is, various conditions such as the temperature increase rate, the maximum plate temperature, and the cooling rate.

As described above, if equipment can be attached to the current CGL, it is possible to heat the CGL, but if there is no such equipment, BAF annealing is performed. As conditions at this time, it is preferable to set the temperature: 450 to 600 ° C., the holding time: 1 to 50 hours, and the temperature integrated value 450 to 20000 ° C./hour in an atmosphere containing oxygen: 3% by volume or more. Here, the temperature integrated value means the product of temperature (° C.) and retention time (hour). Since this value is 20000 or more and almost alloyed to the surface, it is preferable to make it less than this in the present invention. When the adhesion amount is less than 30 g / m ^2, it is difficult to obtain good post-coating corrosion resistance. When the adhesion amount exceeds 100 g / m ² , the Al—Fe alloy layer is peeled off and the mold is hot-pressed. Adhesion to the surface becomes a problem. Even when heated at a temperature of less than 450 ° C., the growth of the alloy layer is slow, requiring more than 100 hours and difficult to implement industrially. When heated to over 950 ° C., alloying proceeds too much and the sea-island structure alloy layer You won't get. In BAF annealing, if it exceeds 600 ° C., molten Al is fused, which is not preferable. If the holding time is less than 1 hour, sufficient soaking time cannot be obtained by BAF annealing, and the conditions are not stable.

  On the other hand, in the heating in CGL, it is preferable that the ultimate plate temperature is 750 to 850 ° C. In order to initiate the growth of the alloy layer, it is necessary that the temperature is 750 ° C. or higher, and when the temperature exceeds 850 ° C., alloying proceeds to the surface.

(About the heating process before hot pressing)
In addition, the Al-plated steel sheet obtained as described above is in a temperature range from 600 ° C. considered to mainly govern the structure of the alloy layer in the subsequent hot press process to a temperature 10 ° C. lower than the maximum achieved plate temperature, It can be rapidly heated at a heating rate of 50 ° C./second or more. By performing such heating, the structure of the coating layer having the sea-island structure as described above can be generated. The heating method is not particularly limited, and it is possible to use a normal infrared heating method using a furnace or radiant heat, but rapid heating at a heating rate of 50 ° C./second or more is possible. It is more preferable to use a heating method using electricity such as energization heating or high frequency induction heating. The upper limit of the rate of temperature rise is not particularly specified, but when using the heating method such as the above-described current heating or high frequency induction heating, the upper limit is about 300 ° C./second due to the performance of the apparatus.

  Moreover, in this heating process, it is preferable that the maximum reached plate temperature is 850 ° C. or higher. The reason why the maximum attainable plate temperature is set to this temperature is that the steel plate is heated to the austenite region and the alloying is sufficiently advanced to the surface.

  The steel sheet after hot pressing becomes a final product through welding, chemical conversion treatment, electrodeposition coating, and the like. Usually, cationic electrodeposition coating is often used, and the film thickness is about 1 to 30 μm. After electrodeposition coating, coating such as intermediate coating and top coating may be applied.

  Hereinafter, the present invention will be described more specifically with reference to examples.

Example 1
Using a cold-rolled steel sheet (thickness: 1.2 mm) having a steel component as shown in Table 1 that has undergone a normal hot-rolling process and cold-rolling process, hot-dip Al plating was performed. For the hot-dip Al plating, a non-oxidation furnace-reduction furnace type line was used. After plating, the amount of plating was adjusted to 30 to 100 g / m ² on one side by a gas wiping method, followed by cooling. The plating bath composition at this time was Al-9% Si-2% Fe. Fe in the bath is inevitable supplied from plating equipment or strips in the bath. The plating appearance was good with no unplating.

  Next, this steel plate was heated under BAF conditions and CGL, respectively. BAF heating conditions were 350 to 600 ° C. and 1 to 30 hours. The heating in CGL was cooled without taking a holding time after reaching 800 ° C.

  Next, this steel plate was processed under conditions equivalent to hot pressing. It heated to 900 degreeC or more in air | atmosphere, cooled in air | atmosphere to the temperature of about 700 degreeC, and then rapidly cooled by crimping between 50 mm-thick molds. The cooling rate between the molds at this time was about 150 ° C./second. In order to observe the influence of the heating rate, three types of heating methods, namely, electric heating, near infrared heating, and electric furnace radiation heating were used. The heating rate at this time was about 60 ° C./second for electric heating, about 40 ° C./second for near infrared heating, and about 5 ° C./second for electric furnace radiation heating.

  The corrosion resistance after painting of these samples was evaluated. Moreover, in order to evaluate the nonuniformity of the plating thickness by dripping about the steel plate after a heating, the plate | board thickness change before and behind a heating was measured.

  In the evaluation of the corrosion resistance after coating, chemical conversion treatment was performed with a chemical conversion treatment solution PB-SX35T manufactured by Nippon Parkerizing Co., Ltd., and then cationic electrodeposition paint Powernics 110 manufactured by Nippon Paint Co., Ltd. was applied with a thickness of about 20 μm. After that, a cross cut was put into the coating film with a cutter, a composite corrosion test (JASO-M609) determined by the automobile engineering association was performed for 180 cycles (60 days), and the swollen width from the cross cut (maximum swollen width on one side) was measured. .

  Table 2 summarizes the heating conditions, structure, and property evaluation results. Numbers 1 and 2 are no heating, 3 to 14 are BAF annealing, and 15 is heating in CGL. It can be seen that the corrosion resistance after coating has a strong correlation between the Al layer thickness and the total Fe—Al layer thickness, and that the thickness change before and after heating (presence of dripping) has a large influence on the Al layer thickness. Under conditions where the Al layer is thick like Nos. 2, 5, 7, and 13, it is difficult to suppress sagging by energization heating. However, simply reducing the thickness of the Al phase makes it difficult to ensure corrosion resistance after coating. On the other hand, in Nos. 4, 6, 8, and 9 in which the Fe-Al layer was increased by heating moderately after Al plating, it was possible to achieve both suppression of dripping and ensuring corrosion resistance after coating. In addition, numbers 10 and 11 are examples of near-infrared heating and electric furnace heating. In a heating method with a low heating rate such as electric furnace heating, the corrosion resistance after coating tended to be somewhat inferior. No. 15 had a low temperature, a small temperature integrated value, and the growth of the alloy layer was insufficient. No. 13 could not be evaluated due to plating peeling after heating.

  No. 12 is heated at a relatively low temperature for a long time. When heated under such conditions, the roughness of the interface between the Al layer and the Fe—Al layer decreases. However, with such roughness, the deterrence against the movement of the molten metal is weak, and the drooping is not sufficiently suppressed. Also, the interface roughness was affected by the plating conditions, and the interface roughness was different between numbers 1 and 2.

(Example 2)
Cold-rolled steel sheets (plate thickness 1.2 mm) having various steel components shown in Table 3 were subjected to hot Al plating in the same manner as in Example 1. The amount of plating was 40 g / m ^{2 on} one side. These Al-plated steel sheets were heated at 500 ° C. for 6 hours in an air atmosphere. Next, in order to simulate a hot press, heating was performed by energization heating at a temperature increase rate of 600 to 890 ° C. at 60 ° C./second and an ultimate temperature of 900 ° C., and then the mold was quenched. The results of measuring the hardness after quenching (Vickers hardness, load 10 kg) are also shown in Table 3. However, if the amount of C in the steel is low, the hardness after quenching decreases, so the amount of C is 0.1 mass. % Is preferable.

( Reference Example 3)
A cold-rolled steel sheet (thickness 1.4 m) having the steel components shown in Table 1 was subjected to hot-dip Al plating. The bath composition at this time was Al-18% Si-2% Fe, and Fe was an inevitable impurity. This was BAF annealed at 550 ° C. for 3 hours. As a result, the Al layer was 8 μm, the Fe—Al layer was 8 μm, and the total was 16 μm. This material was heated again under conditions equivalent to hot pressing, and after reaching to 950 ° C. by an electric heating method, it was quenched in a mold and evaluated under the same conditions as in Example 1. As a result, the plate thickness did not change, but the swollen width of the coating film was 9 mm, and the corrosion resistance after coating was poor.

  As mentioned above, although preferred embodiment of this invention was described referring an accompanying drawing, it cannot be overemphasized that this invention is not limited to this example. It will be apparent to those skilled in the art that various changes and modifications can be made within the scope of the claims, and these are naturally within the technical scope of the present invention. Understood.

It is an optical microscope photograph which shows the general example of the structure of a cross-sectional structure | tissue after heat-alloying an Al plating steel plate. It is explanatory drawing which shows the binary system phase diagram of Fe-Al. It is an optical microscope photograph which shows an example of the structure of the cross-sectional structure | tissue of the coating layer which concerns on this invention.

Claims (3)

Al plating layer containing 3 to 15% by mass of Si , coated on the steel plate surface;
An Fe-Al alloy layer positioned between the Al plating layer and the steel sheet;
Have
The thickness of the Al plating layer is 3 μm or more and 10 μm or less,
The thickness of the Fe—Al alloy layer is 6 μm or more,
The sum of the thickness of the Al plating layer and the thickness of the Fe—Al alloy layer is 10 μm or more and 30 μm or less,
A plated steel sheet for hot pressing, wherein a center line average roughness Ra of an interface between the Al plating layer and the Fe—Al alloy layer is 0.6 μm or more and 3 μm or less. As steel components, C: 0.1-0.4%, Si: 0.01-0.6%, Mn: 0.5-3%, P: 0.005-0.05%, S: 0.002-0.02%, Al: 0.005-0.1%, Ti: 0.01-0.1%, B: 0.0001-0.01% and Cr: 0.01-1% In an atmosphere containing oxygen: 3% by volume or more in a box annealing furnace, the temperature of an Al-plated steel sheet that is Al-plated so that the adhesion amount is 30 to 100 g / m ² per side. : 450 to 600 ° C., holding time: 1 to 50 hours, temperature integral value: 450 to 20000 ° C./hour for heating ,
The method for producing a plated steel sheet for hot press, wherein the composition of the Al plating contains 3 to 15% by mass of Si . As steel components, C: 0.1-0.4%, Si: 0.01-0.6%, Mn: 0.5-3%, P: 0.005-0.05%, S: 0.002-0.02%, Al: 0.005-0.1%, Ti: 0.01-0.1%, B: 0.0001-0.01% and Cr: 0.01-1% Is continuously alloyed by heating an Al-plated steel sheet that has been plated with Al so that the adhesion amount is 30 to 100 g / m ² per side in a hot dip galvanizing line. There line processing,
The method for producing a plated steel sheet for hot press, wherein the composition of the Al plating contains 3 to 15% by mass of Si .