JP2014118628A - MOLTEN Al PLATED STEEL PLATE FOR HOT STAMP, METHOD OF PRODUCING THE SAME, AND HOT STAMP PRODUCT - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a molten Al plated steel plate and a method of producing the steel plate which can prevent formation of a decarbonized layer or scale on the base steel plate of a hot stamp product and hot stamp product using the steel plate.SOLUTION: A molten Al plated steel plate for hot stamps has an Fe-Al-Si intermetallic compound layer of a thickness of 3-12 μm on the boundary surface between the base steel plate and a plated layer and an Al-Si plating layer of a thickness of 8-50 μm on the intermetallic compound layer. When the thickness of the intermetallic compound layer is 3-7 μm, the crack width, in the direction C, of the intermetallic compound layer is 0.6 μm or smaller, and the thickness of the Al-Si plated layer is 8-50 μm. When the thickness of the intermetallic compound layer is greater than 7 μm and equal to or smaller than 12 μm, the crack width, in the direction C, of the intermetallic compound layer is smaller than 1 μm, and the thickness of the Al-Si plated layer is 14-50 μm. A hot stamp product using the steel plate is also provided.

Description

  TECHNICAL FIELD The present invention relates to a hot stamped molten Al plated steel sheet, a manufacturing method thereof, and a hot stamped product, and more particularly, hot stamped molten Al plated that can suppress the occurrence of a decarburized layer or scale on a base steel plate of a hot stamped product after hot stamping. The present invention relates to a steel plate, a manufacturing method thereof, and a hot stamp product manufactured using the steel plate.

  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 stamping (also referred to as hot pressing, hot stamping, die quenching, press quenching, etc.) has recently attracted attention as being compatible with both high strength and high formability. This hot stamping 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 it is baked by cooling after forming to obtain a desired material. It is.

  Hot stamping is promising as a method for forming ultra-high strength members, but usually has a step of heating a steel plate in the atmosphere, and at this time, oxide (scale) is generated on the surface of the steel plate. 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, a technique for suppressing the generation of scale during heating by using an Al-plated steel sheet as a hot stamping steel sheet has been proposed (for example, see Patent Documents 1 and 2).

  However, due to hot stamping, the aluminum plating layer after local rapid heating has changed to an intermetallic compound up to the surface, and this compound is a very brittle material and is prone to cracking during processing. There is a drawback, and after severe molding, corrosion starts from these cracks, and there is a problem that the corrosion resistance after coating is lowered. In order to avoid this problem, a technique of adding Mn to the intermetallic compound has been proposed (see, for example, Patent Document 3). However, a decarburized layer and a scale are generated on the base steel plate of the hot stamp product. It is not a technology to suppress this.

JP 2003-181549 A JP 2003-49256 A JP 2003-34855 A

  The inventors of the present invention have intensively studied to suppress micro cracks generated in the Al plating layer of hot stamped products. Oxygen and water vapor invades from cracks, and a decarburized layer and scale are formed on the base steel plate, thereby deteriorating the weldability and corrosion resistance of hot stamped products and causing a problem of hardness failure on the outermost surface. I found a new thing.

  Accordingly, the present invention provides a molten Al-plated steel sheet that can suppress the formation of a decarburized layer and scale on the base steel sheet of a hot stamp product, and a method for producing the same, and the generation of a decarburized layer and scale on the base steel sheet It is an object of the present invention to provide a hot stamp product that suppresses the above.

  In order to solve the above-mentioned problems, the present inventors have intensively studied to suppress the formation of a decarburized layer and scale on a base steel plate of a hot stamped product. The thickness of the intermetallic compound layer formed between the plating layer and the base steel sheet and the plating thickness of the Al—Si layer that is not the intermetallic compound layer are limited, and the crack width in the C direction to the intermetallic compound layer is limited. It has been found that when hot stamping is performed using a limited hot-dip Al-plated steel sheet, it is possible to suppress the formation of a decarburized layer and scale on the base steel sheet of the hot stamp product, and the present invention has been completed.

  The gist of the present invention is as follows.

  (1) Hot stamping having an Fe—Al—Si intermetallic compound layer having a thickness of 3 to 12 μm on the interface between the base steel plate and the plating layer and an Al—Si plating layer having an thickness of 8 to 50 μm on the intermetallic compound layer When the thickness of the intermetallic compound layer is 3 to 7 μm, the crack width in the C direction of the intermetallic compound layer is 0.6 μm or less, and the thickness of the Al—Si plating layer is When the intermetallic compound layer thickness is more than 7 to 12 μm, the crack width in the C direction of the intermetallic compound layer is less than 1 μm, and the thickness of the Al—Si plating layer is 14 to 50 μm. A hot-stamped hot-dip galvanized steel sheet characterized by the following.

  (2) In hot dip Al plating of a steel sheet, a bath temperature of 620 to 675 ° C. is immersed in the bath for 5 to 10 seconds, and the penetration plate temperature is bath temperature ± 20 ° C. The hot-stamped molten Al-plated steel sheet according to the above (1), which is subjected to skin pass rolling with a rolling reduction of less than 0.5% after hot-dip aluminum plating.

  (3) A hot stamped product hot stamped using the hot stamped molten Al-plated steel sheet according to (1), wherein the thickness of the decarburized layer formed on the base steel sheet surface layer of the hot stamped product is less than 1 μm. A hot stamping product characterized by being.

  According to the present invention, since the formation of a decarburized layer or scale on the base steel plate for hot stamp products is suppressed, a hot stamp product excellent in weldability and corrosion resistance and having excellent surface properties can be obtained.

It is a microscope picture which shows a decarburization layer and a scale being formed in the base material steel plate of a hot stamp product. It is a figure which shows the relationship between the alloy layer (intermetallic compound layer) thickness (micrometer) of Al hot-stamped steel plate for hot stamping, the Al-Si plating thickness (micrometer), and the scale score of a hot stamp product.

  The present invention will be described in detail below.

  The plated steel sheet for hot stamping according to the present invention is an Al-plated steel sheet having a surface of the steel sheet subjected to hot-dip Al plating, and the hot-aluminum plated layer formed on the surface of the steel material has a composition of mass%, Al: 80 An Fe-Al-Si layer, which is an intermetallic compound layer (hereinafter referred to as an alloy layer), is formed at the boundary surface between the Al plating layer and the steel plate, which is composed of -95% and Si: 5-15%. The composition of the Fe—Al—Si alloy layer is usually Al: 35 to 65%, Si: 5 to 15%, and the balance consists of Fe and inevitable impurities.

  The composition of the molten Al plating and the composition of the alloy layer are known. Since this alloy layer is generally extremely hard and brittle, when it is processed, a crack is generated and becomes the starting point of fracture, and the crack of the Al plating layer is caused from this starting point. In severe cases, some may crack when subjected to skin pass rolling or leveler after plating. Also, heating before hot stamping may cause scale in the base steel sheet, or cracks in the alloy layer during hot stamping after heating, and the cracking of the Al plating layer may be caused from this crack. is there.

  And when such a crack penetrates to the base material steel plate, the base material steel plate exposed by the crack acts as a cathode at the time of corrosion, so it acts as a corrosion starting point, and corrosion propagates along the plating crack. Therefore, the corrosion resistance is significantly deteriorated. Furthermore, the present inventors have introduced oxygen and water vapor in the atmosphere from cracks by heating before hot stamping, and a decarburized layer and scale are formed on the base steel plate. For example, as shown in the micrograph of the cross-sectional structure when the decarburized layer (decarburized layer hardness: Vickers hardness 260) in FIG. 1 is generated, a decarburized layer and a scale are formed on the base steel plate, and the weldability of the hot stamping product It has been newly found that there is a problem that the corrosion resistance is deteriorated and the hardness is poor on the outermost surface.

  And in order to suppress the decarburization layer and scale which arise in the base material steel plate of a hot stamp product, the present inventors are the Al-Si layer and the Fe-Al-Si alloy of the plating layer of the hot-dip hot-dip galvanized steel plate. If the hot stamping is performed using a hot-dip Al-plated steel sheet manufactured by skin pass rolling with the layer thickness limited to a predetermined thickness or less and a skin pass rolling of a predetermined reduction ratio or less, it is formed on the base steel sheet of the manufactured hot stamp product. The present invention has been made by finding that the decarburized layer and the scale can be suppressed.

  FIG. 2 is a diagram showing the relationship between the alloy layer thickness (μm) of the hot stamped molten Al-plated steel sheet, the thickness of the Al—Si layer excluding the alloy layer (μm), and the scale score of the hot stamp product.

As shown in Table 1 below, the scale score was determined according to the state of occurrence of the scale in the plating layer by observing the microscopic structure 100 times. As shown in Table 1, scale scores 0 and 1 are good, but scale scores 2 and 3 are in a state where scale generation is wide and decarburized layers are also generated.

  As shown in FIG. 2, the Al-Si layer thickness is not 8 μm or more when the alloy layer thickness of the hot stamped molten Al-plated steel sheet is 3 to 7 μm, preferably 3 to 6.5 μm, and the crack width of the alloy layer is 0.6 μm or less. The scale and decarburized layer of grade 2 (◇) are generated in hot stamped products, which is not preferable. Further, when the alloy layer thickness is more than 7 to 12 μm and the crack width of the alloy layer is less than 1 μm, the scale of grade 2 (◇) and the occurrence of a decarburized layer may occur unless the Al—Si layer thickness is 14 μm or more. In FIG. 2, a score 0 (no scale in the plating layer) is indicated by ◯, a score 1 is indicated by □, a score 2 is indicated by ◇, and a score 3 is indicated by Δ.

  The upper limit of the thickness of the Al—Si layer present on the upper part of the alloy layer is not particularly limited. However, if it is too thick, a long time is required for alloying when the hot stamp is heated. The upper limit of the thickness of the Si layer was 50 μm. The alloy layer thickness and the Al—Si layer thickness can be measured from the cross section of the plated steel sheet using an optical microscope, a scanning electron microscope, or the like.

  Therefore, in the present invention, in order to be able to suppress the formation of a decarburized layer and scale on the base material steel plate of the hot stamp product, the alloy layer thickness of the hot stamped molten Al plated steel plate is 3 to 7 μm, The thickness of the Al—Si layer that is not an alloy layer is 8 to 50 μm, the thickness of the alloy layer is more than 7 to 12 μm, and the thickness of the Al—Si layer that is not an alloy layer is 14 to 50 μm.

  In the present invention, the steel plate component of the hot stamped steel plate is not particularly limited, but in a hot stamp product, a mold is used to simultaneously press and quench to obtain a high strength of 780 MPa or more. It is preferable to use a steel plate having a composition rich in hardenability, and the amount of C in the steel is 0.15 to 0.4% by mass. When the C content is less than 0.15%, sufficient quenching strength cannot be obtained, and when the C content exceeds 0.4 mass%, the toughness of the steel sheet is significantly reduced. Therefore, in the present invention, any high-strength steel sheet having a C content in the steel of 0.15 to 0.4 mass% can be used.

  As such a steel plate, for example, in mass%, C: 0.15 to 0.4%, Si: 0.3% or less, Mn: 0.8 to 1.55%, Cr: 0.1 to 0 A steel plate having a composition comprising 0.5%, B: 0.0015 to 0.005%, Al: 0.04% or less, and the balance of Fe and inevitable impurities can be exemplified. In addition, if necessary, Ti, Nb, Mo, V, Ni, Cu, and W may be contained alone or in combination of two or more, each 0.05% or less.

  There is no particular limitation on the heating method using the hot stamp in the present invention. Although it is possible to use a normal furnace heating or an infrared heating method using radiant heat, electrical heating such as energization heating or high-frequency induction heating, which can perform rapid heating at a heating rate of 50 ° C./second or more. It is also possible to use a heating method using. However, at this time, there is a possibility that a phenomenon in which the molten Al locally rises due to electromagnetic interaction (shift) occurs. The upper limit of the rate of temperature rise is not particularly specified, but when using the heating method such as the above-mentioned 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.

  In the present invention, the thickness of the decarburized layer after hot stamping is limited to less than 1 μm. This is because when the thickness of the decarburized layer after hot stamping is 1 μm or more, there is a concern that a hardness failure is caused on the outermost surface and the fatigue strength is lowered.

  The decarburized layer as used in the present invention is defined as a layer having a hardness (Hv) of 350 or less and formed on the base steel sheet surface layer (in the steel sheet). And a decarburized layer is 3 vol. It can be observed after% nital etching. In the present invention, three fields of view were observed at a magnification of 100, and after determining the presence or absence of a scale and a decarburized layer, those having a decarburized layer were magnified 1000 times and the thickness of the decarburized layer was measured.

In order to make the thickness of the decarburized layer formed on the surface layer of the base steel plate of the hot stamp product 1 μm or less, it is preferable to control the crack width in the C direction of the alloy layer of the hot-dip Al-plated steel plate. That is, when the alloy layer thickness is 3 to 7 μm, the crack width in the C direction of the alloy layer is 0.6 μm or less, and when the alloy layer thickness is thicker than 7 to 12 μm, the allowable crack width is larger. It is preferable that the thickness is less than 1 μm. As described above, since the alloy layer is an intermetallic compound and has poor ductility, when the base material is processed such as a skin pass, cracks are generated in the alloy layer. Once a crack has been generated, a new crack is also generated, but the already generated crack often expands its width. When the crack width is large, a decarburized layer and scale generated in the base steel plate starting from the crack at the time of hot stamping are easily formed.
Therefore, in the present invention, the width of the crack in the C direction generated on the base steel sheet surface layer (inside the steel sheet) is set to 0.6 μm or less or less than 1 μm depending on the thickness of the alloy layer. The crack width is the maximum value of the width.

The crack width is measured by removing the Al—Si plating layer of the molten Al-plated steel sheet by a method such as electrolytic peeling and observing the surface of the alloy layer with an electron microscope. The width of the crack specified in the observation field is counted. In the observation image, the crack is observed linearly, and the largest crack width in the observation visual field is measured. Electrolytic peeling can be performed by, for example, constant current electrolysis at a current density of 20 mA / cm ² using 3% NaOH and 1% AlCl ₃ .6H ₂ O solution.

  Next, the manufacturing method of the hot stamped hot-dip galvanized steel sheet according to the present invention will be described.

The hot rolling and cold rolling conditions of the steel sheet are not particularly limited, and are performed under normal manufacturing conditions.
The manufacturing method of the hot-dip Al-plated steel sheet according to the present invention is such that the bath temperature is 620 to 675 ° C. and the immersion time in the bath is 2 to 10 seconds, and the intrusion plate temperature is bath temperature ± 20 ° C. After hot-dip Al plating under the conditions, it is manufactured by performing skin pass rolling with a rolling reduction of less than 0.5%.

  As the molten Al plating bath, a bath composition containing 3% to 15% by mass of Si in Al can be used, and inevitable impurities such as Fe are mixed therein. Other additive elements may be Mn, Cr, Mg, Ti, Zn, Sb, Sn, Cu, Ni, Co, In, Bi, Misch metal, etc., as long as the plating layer is mainly Al. Additive elements are also applicable. Si in the bath has a function of suppressing the growth of the alloy layer at the time of Al plating, and 3% by mass or more of Si is required to make the alloy layer thickness 12 μm or less. On the other hand, if the amount of Si is too large, it will crystallize out as coarse crystals in the plating layer and inhibit the corrosion resistance and the workability of plating, so the amount of Si is preferably 15% by mass or less.

  The bath temperature of the molten Al plating is closely related to the alloy layer thickness in the plating layer, and the alloy layer becomes thicker as the bath temperature becomes higher. In order to make the alloy layer thickness 12 μm or less, the bath temperature needs to be 675 ° C. or less. And the minimum of bath temperature is decided by melting | fusing point of a bath composition, and 620 degreeC is a minimum. The melting point of the bath is about 600 ° C. with Al-10% Si. If it is less than 620 ° C., the operation cannot be performed because it partially solidifies in consideration of temperature variation in the bath. Therefore, in the present invention, the bath temperature is set to 620 to 675 ° C. Moreover, since the alloy layer thickness is also affected by the immersion time in the plating bath, the immersion time is preferably 3 to 10 seconds in order to make the alloy layer thickness 3 to 12 μm. In addition, if the penetration temperature of the steel sheet is too low, the reactivity with the Al plating bath is impaired, and if it is too high, the alloy layer tends to grow. Therefore, in order to obtain a good plating layer, the bath temperature and the penetration plate temperature are almost the same. In order to form a good plating layer having no plating failure with an immersion time of 3 to 10 seconds, it is necessary to set the intrusion plate temperature to the plating bath to a bath temperature ± 20 ° C. If the penetration plate temperature is too high, the reaction proceeds in the bath and the alloy layer grows thick. If the penetration plate temperature is too low, a temperature difference locally occurs in the bath, and an Al-Fe compound called dross crystallizes on the steel plate. The appearance quality is impaired, and depending on the degree, the steel sheet is pressed. Therefore, the alloy layer thickness and the plating thickness can be adjusted by the bath temperature of the molten Al plating, the immersion time in the plating bath, and the penetration temperature of the steel plate.

  Moreover, after hot-dip Al plating, in order to ensure the shape correction and surface smoothness of the steel sheet, it is desirable to perform the same skin pass rolling (temper rolling) as before. And the rolling reduction in that case shall be less than 0.5%. When the rolling reduction is 0.5% or more, stress concentrates on the crack site once generated, the C-direction crack width of the alloy layer tends to increase beyond 1 μm, the rolling reduction is lower, and the alloy thickness is thinner. C direction crack width can be reduced. When the C-direction crack width of the alloy layer increases, oxygen and water vapor in the atmosphere enter from the site in the subsequent heating step, and a decarburized layer and scale are formed on the base steel plate. Therefore, the skin pass reduction ratio is less than 0.5%, but is preferably 0.4% or less. More preferably, it is 0.3% or less. The lower limit of the rolling reduction is not particularly limited, but is preferably 0.05% or more.

Hereinafter, the present invention will be described in detail based on examples.
In mass%, C: 0.23%, Si: 0.21%, Mn: 1.2%, P: 0.01%, S: 0.003%, Cr: 0.2%, B: 0.0. Hot-dip Al-Si plating using a cold-rolled steel sheet having a thickness of 1.6 mm, containing 0027%, Al: 0.03%, Ti: 0.03%, with the balance being composed of Fe and inevitable impurities gave. The Si concentration was set to 9% by mass, and various alloy layer thicknesses were obtained by changing the bath temperature, the penetration plate temperature, and the immersion time. After lifting from the plating bath, N ₂ gas wiping was performed to adjust the Al-Si plating adhesion amount (Al-Si thickness), and then skin pass rolling was performed to produce a hot stamped molten Al plated steel sheet.

The alloy layer thickness, Al-Si layer thickness (not including the alloy layer thickness), and C-direction crack width of this steel plate were measured. Alloy layer thickness, Al-Si layer thickness, the optical microscope from the cross section, also cracks width alloy layer by constant current electrolytic stripping the Al-Si layer with _{3% NaOH-1% AlCl 3} · 6H 2 O in Was exposed, and the surface was observed at a secondary electron image of 5000 times.

  This steel plate was processed under conditions corresponding to hot stamping. After being heated to 900 ° C by being inserted into an electric furnace in the atmosphere, held for 1 minute, cooled to a temperature of about 700 ° C in the atmosphere, and then rapidly cooled by pressure bonding between molds having a thickness of 50 mm. did.

  After performing the heating-rapid cooling treatment, the cross section of the plating layer was observed. After the cross section polishing, the plating layer-steel plate interface is observed by etching with nital, and the quenched structure of the steel plate can also be observed. At this time, as shown in FIG. 1, a scale, that is, an oxide of Fe, was observed inside the alloyed plating layer. A decarburized layer was also observed on the outermost surface layer of the steel plate where the scale occurred.

Table 2 shows the alloy layer thickness and the Al—Si layer thickness when the bath temperature, the approach plate temperature, the immersion time, and the amount of plating adhesion were changed. It also lists the skin pass reduction rate and the scale generation score. The scale score is the standard shown in Table 1.

  As shown in Table 2, the scale generation state and the decarburization state vary depending on the alloy layer thickness, the Al—Si thickness, and the crack width of the alloy layer. The inventive examples all had good scale scores of 0 and 1.

  On the other hand, when the Al—Si thickness is very thin as 5 μm as shown in the comparative examples of Nos. 9, 14, 19, and 24, or the Al—Si layer is 10 μm as shown in the comparative examples of Nos. 5-7. When the thickness of the alloy layer is less than 14 μm and the thickness of the alloy layer is more than 7 μm in the range of 8.5 to 11.5 μm, the generation of scale cannot be suppressed (the scale scores are 2 and 3).

  In addition, when the thickness of the alloy layer is 13 μm, even if the Al—Si layer is as thick as 10 μm as indicated by No. 8, scale generation cannot be suppressed (scale score 3).

  Scale is also likely to occur when the skin pass reduction rate is high (Nos. 36 and 37) and the penetration plate temperature is high (No. 38).

  Based on the above test results, by properly controlling the conditions of the alloy layer thickness of the hot stamped hot-dip galvanized steel sheet, the crack width in the C direction of the alloy layer, and the thickness of the Al-Si plating layer, It was found that generation of decarburized layers and scales on the base steel plate can be suppressed.

Claims (3)

  Molten Al for hot stamping having an Fe-Al-Si intermetallic compound layer having a thickness of 3 to 12 μm on the interface between the base steel plate and the plating layer, and an Al-Si plating layer having an thickness of 8 to 50 μm on the intermetallic compound layer When the thickness of the intermetallic compound layer is 3 to 7 μm, the crack width in the C direction of the intermetallic compound layer is 0.6 μm or less and the thickness of the Al—Si plating layer is 8 to 50 μm. And when the intermetallic compound layer thickness is more than 7 to 12 μm, the C-direction crack width of the intermetallic compound layer is less than 1 μm, and the thickness of the Al—Si plating layer is 14 to 50 μm. A hot-stamped hot-dip Al-plated steel sheet.   When performing hot-dip aluminum plating on a steel plate, the molten aluminum is immersed in an Al-Si plating bath having a bath temperature of 620 to 675 ° C under a condition that the immersion time in the bath is 3 to 10 seconds and the penetration plate temperature is bath temperature ± 20 ° C. The method for producing a hot stamped hot-dip Al-plated steel sheet according to claim 1, wherein after the plating, skin pass rolling with a rolling reduction of less than 0.5% is performed.   A hot stamped product hot-stamped using the hot stamped molten Al-plated steel sheet according to claim 1, wherein the thickness of the decarburized layer formed on the base steel sheet surface layer of the hot stamped product is less than 1 µm. And hot stamping products.

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