JP6274018B2 - High strength steel parts and manufacturing method thereof - Google Patents

Description

  The present invention relates to a method for producing a high-strength steel part by hot-pressing an Al-based plated steel sheet having a hot-dip plated layer mainly composed of Al, and a high-strength steel part produced by the method.

  In recent years, in order to protect the environment and suppress global warming, there has been an increasing demand for suppressing the consumption of fossil fuels, and this demand has affected various manufacturing industries. For example, automobiles that are indispensable for daily life and activities as transportation means are required to improve fuel consumption by reducing the weight of the vehicle body. However, in automobiles, simply reducing the weight of the vehicle body is not permitted in terms of product quality, and it is necessary to ensure appropriate safety as well as weight reduction.

  The body and parts of an automobile are made of a steel plate, and reducing the weight of the steel plate is necessary for reducing the weight of the vehicle body. However, as described above, it is not allowed to simply reduce the weight of the steel plate, and it is required to secure the mechanical strength necessary for ensuring safety as well as reducing the weight of the steel plate.

  The demand for such steel sheets is increasing not only in the automobile manufacturing industry but also in various manufacturing industries. Therefore, research and development have been conducted on steel plates that can maintain the mechanical strength even if the mechanical strength of the steel plates is increased and made thinner than previously used steel plates.

  In general, a steel plate having high mechanical strength tends to have low formability and shape freezing property in forming such as bending. Therefore, when processing a steel plate having high mechanical strength into a complicated shape, the processing itself may be difficult. As one of means for solving the problem regarding the formability, there is a so-called hot press method (also referred to as a hot stamp method, a hot press method, a die quench method, or press hardening).

  In the hot pressing method, a steel plate to be formed is once heated to a high temperature (austenite region), pressed into a steel plate softened by heating, and then cooled. According to the hot pressing method, the steel sheet is once heated to a high temperature to be softened, so that the steel sheet can be easily pressed, and the mechanical strength of the steel sheet can be increased by the quenching effect by cooling after forming. it can. Therefore, a molded product having both good shape freezing property and high mechanical strength can be obtained by the hot pressing method.

  However, when the hot pressing method is applied to a steel sheet, when the steel sheet is heated to a high temperature of, for example, 800 ° C. or higher, iron on the surface of the steel sheet is oxidized and scale (oxide) is generated. Therefore, after hot pressing, a step of removing scale (descaling step) is required, and productivity is reduced. Moreover, when corrosion resistance is required, after processing, it is necessary to give a rust prevention process and metal coating to the steel plate surface, but since a surface cleaning process and a surface treatment process are required, productivity falls.

  As a method for suppressing the decrease in productivity, there is a method of coating a steel plate in advance. Usually, an organic material or an inorganic material is used as a covering material for coating a steel plate. Among them, a plated steel sheet obtained by applying a zinc-based plating having a sacrificial anti-corrosive action to a steel sheet is widely used as a steel sheet for automobiles from the viewpoint of corrosion resistance and steel sheet production technology.

  However, the heating temperature in hot pressing (usually 850 to 1000 ° C.) is higher than the decomposition temperature of the organic material and the boiling point of Zn. The layer may evaporate and cause significant deterioration of the surface properties. Therefore, a steel sheet that is heated to a high temperature and subjected to hot pressing is a steel sheet with an Al-based metal coating having a higher melting point than the melting temperature of the organic material coating or the melting point of the zinc-based metal coating, so-called It is desirable to use an Al-based plated steel sheet.

  Al-based plating prevents scale from adhering to the surface of the steel sheet, eliminating the need for a descaling step and improving productivity. In addition, since the Al-based plating has a rust prevention effect, the corrosion resistance after painting is improved.

  Hot pressing is less productive in the pressing process than ordinary cold pressing. This is because it is necessary to heat the steel plate to 850 ° C. or more once, and it takes about 5 minutes when using a normal furnace.

  In order to compensate for the low productivity, various rapid heating techniques have been studied. In particular, the application of electric heating or high frequency induction heating suitable for rapid heating has been studied. However, when an Al-based plated steel sheet is heated by energization heating or high-frequency induction heating, the Al-based plating layer melts at about 600 ° C., electricity flows through the molten Al, a pinch force is generated, and the molten Al partially aggregates. The phenomenon appears. Hereinafter, this phenomenon is sometimes referred to as “plating deviation”.

  When an Al-based plated steel sheet is heated, Al in the plating layer reacts with Fe in the steel sheet, and the plating layer changes to an Al—Fe-based intermetallic compound. This change is usually called alloying. Since the melting point of Al—Fe is 1100 ° C. or higher, “almost plating” is less likely to occur when alloying occurs.

  In the portion where the “plating deviation” occurs and the thickness of the plating becomes thick, it cannot be completely alloyed by heating, and the corrosion resistance of the portion decreases. Moreover, when the plate thickness is partially increased, a phenomenon in which plating adheres to the mold or galling may occur.

  In order to suppress “proximity of plating”, attempts have been made to improve the heating method. Patent Document 1 discloses a technique for preventing “deposition of plating” by controlling the amount of plating and energization conditions. Patent Document 2 discloses a technique for preventing “plating misalignment” by controlling the flow of current to a steel sheet during energization. Patent Document 3 discloses that, in an Al-plated steel sheet having a coating containing ZnO, the coating containing ZnO is effective in suppressing “shift of plating” during energization heating.

JP 2010-70800 A JP 2012-115864 A International Publication No. 2009/131233

  However, in the technique disclosed in Patent Document 1, it is necessary to reduce the current density when energizing and heating an Al-plated steel sheet having a large coating amount. Lowering the current density means lowering the heating rate. Patent Document 2 does not disclose the limit of the amount of plating that can suppress “plating deviation”, and the effect based on the control of how the current flows is unclear.

  Patent Document 3 discloses that a coating containing ZnO has an effect of suppressing “close to plating”, but does not disclose current heating conditions. The association with suppression is unclear.

  In view of the problems of the above-mentioned conventional technology, the present invention is a steel that requires an appropriate amount of coating and a coating amount containing ZnO, particularly a rust prevention performance, when applying electric heating to a molten Al-based plated steel sheet. A steel component that solves the problem and a method for manufacturing the steel component, with the object of clarifying an appropriate plating adhesion amount and a coating amount containing ZnO, which can be applied when the component is manufactured by hot pressing. The purpose is to provide.

  In order to solve the above problems, the present inventors evaluated the surface roughness, post-coating corrosion resistance, and spot weldability of steel parts manufactured by hot pressing, and the coating amount of the molten Al-based plated steel sheet, The amount of the coating containing ZnO and the appropriate range of the energization heating conditions were intensively investigated. As a result, the proper range mutually relevant of the coating adhesion amount of a hot-dip Al system plating steel plate, the quantity of the film | membrane containing ZnO, and an energization heating condition was found.

  The present invention has been made based on the above findings, and the gist thereof is as follows.

(1) In a high-strength steel part having an Al—Fe-based intermetallic compound layer on the surface of a high-strength steel part, and further having a surface coating layer on the surface of the Al—Fe-based intermetallic compound layer,
(I) The Al—Fe-based intermetallic compound layer has (i-1) a difference in thickness in the short side direction of the high-strength steel part of 70 μm or less, (i-2) a thin portion having a thickness of 10 to 20 μm, (I-3) The thickness of the diffusion layer on the steel plate side is 5 μm or less,
(Ii) The surface coating layer has (ii-1) surface roughness Ra of 0.5 to 1.5 μm, (ii-2) thickness of 0.1 to 1.0 μm, and ZnO is 0 in terms of Zn. .3~1.5g / ^{m 2} containing
High strength steel parts characterized by that.

(2) The high-strength steel part according to (1), wherein the high-strength steel part is an automotive steel part.

(3) A manufacturing method for manufacturing the high-strength steel part according to (1) or (2),
(I) ZnO is converted to Zn in an amount of 0.3 to 1.5 g in terms of Zn on the surface of a steel sheet having a molten Al-based plating layer having a plating adhesion amount of 30 to 55 g / m ² and an alloy layer thickness of 5 μm or less. / M ² containing surface coating layer,
(Ii) The steel sheet having the surface coating layer is heated to 850 to 1050 ° C. at a heating rate of 20 ° C./second or more, and then
(Iii) Rapid cooling at the same time as molding
A method for producing a high-strength steel part characterized by the above.

(4) The method for producing a high-strength steel part according to (3), wherein the heating is performed by energization heating or induction heating.

(5) The method for producing a high-strength steel part according to (3) or (4), wherein the forming process and rapid cooling are performed using a mold.

  According to the present invention, high-strength steel parts having good post-coating corrosion resistance and spot weldability can be manufactured with high productivity by performing hot press working after rapidly heating a molten Al-plated steel sheet.

It is a figure which shows the cross-sectional structure | tissue of the Al-Fe intermetallic compound layer produced | generated by rapid heating.

  The present invention will be described.

The method for producing a high-strength steel part of the present invention (hereinafter sometimes referred to as “the present invention production method”)
(I) On the surface of a steel sheet having a molten Al-based plating layer having a plating adhesion amount of 30 to 55 g / m ² and an alloy layer thickness of 5 μm or less, 0.3 to 1.5 g ZnO in terms of Zn. / M ² containing surface coating layer,
(Ii) The steel sheet having the surface coating layer is heated to 850 to 1050 ° C. at a heating rate of 20 ° C./second or more, and then
(Iii) It is characterized by rapid cooling simultaneously with the molding process.

The high-strength steel part of the present invention (hereinafter sometimes referred to as “the steel part of the present invention”) has an Al—Fe-based intermetallic compound layer on the surface of the high-strength steel part . In high-strength steel parts having a surface coating layer on the surface of the intermetallic compound layer,
(I) The Al—Fe-based intermetallic compound layer has (i-1) a difference in thickness in the short side direction of the high-strength steel part of 70 μm or less, (i-2) a thin portion having a thickness of 10 to 20 μm, (I-3) The thickness of the diffusion layer on the steel plate side is 5 μm or less,
(Ii) The surface coating layer has (ii-1) surface roughness Ra of 0.5 to 1.5 μm, (ii-2) thickness of 0.1 to 1.0 μm, and ZnO is 0 in terms of Zn. .3~1.5g / ^{m 2} containing
It is characterized by that.

  First, the manufacturing method of the present invention will be described.

<Fused Al-based plated steel sheet>
A hot-dip Al-based steel sheet is a steel sheet in which a hot-plated layer mainly composed of Al is formed on one or both surfaces of the steel sheet with a plating adhesion amount of 30 to 55 g / m ² per side, and the alloy layer thickness is 5 μm or less. is there. In the production method of the present invention, a surface film layer containing ZnO in an amount of 0.3 to 1.5 g / m ^{2 in} terms of Zn is formed on the surface of the molten Al-based plated steel sheet. This surface film layer will be described later.

(steel sheet)
Since the final product is a high-strength steel part, the steel sheet to which hot-dip aluminum plating is applied has formability that can withstand hot pressing even after plating, and after molding, it has the mechanical strength required for steel parts for automobiles. Any steel plate having a steel structure that develops may be used. As long as this is the case, the steel plate is not limited to a steel plate having a specific component composition, but the following steel plate is shown as an example.

  In mass%, C: 0.10 to 0.40%, Si: 0.01 to 0.60%, Mn: 0.5 to 3.0%, Ti: 0.01 to 0.10%, B: A steel plate containing 0.0001 to 0.10% and the balance being Fe and inevitable impurities (P, S, Al, N, Cr, etc.).

  The reason for limiting the component composition of the steel sheet will be described. Hereinafter, “%” relating to the component composition means “% by mass”.

  C is an element necessary for ensuring required mechanical strength. If it is less than 0.10%, the required mechanical strength cannot be obtained, so 0.10% or more is preferable. More preferably, it is 0.15% or more. On the other hand, if it exceeds 0.40%, melt cracking tends to occur, so 0.40% or less is preferable. More preferably, it is 0.35% or less.

  Si is an element that contributes to improvement of mechanical strength. If it is less than 0.01%, the strength improvement effect cannot be obtained sufficiently, so 0.01% or more is preferable. More preferably, it is 0.05% or more. On the other hand, if it exceeds 0.60%, the wettability is lowered and non-plating occurs in the molten Al-based plating, so 0.60% or less is preferable. More preferably, it is 0.40% or less.

  Mn is an element that increases the hardenability of steel and contributes to the improvement of mechanical strength, forms MnS, and suppresses hot brittleness due to S. If it is less than 0.5%, the effect of addition is not sufficiently exhibited, so 0.5% or more is preferable. More preferably, it is 0.8% or more. On the other hand, if it exceeds 3.0%, the residual γ phase becomes excessive and the strength decreases, so 3.0% or less is preferable. More preferably, it is 2.0% or less.

  Ti is an element that contributes to improvement in mechanical strength and heat resistance of the molten Al-based plating layer. If it is less than 0.01%, the effect of addition is not sufficiently exhibited, so 0.01% or more is preferable. More preferably, it is 0.03% or more. On the other hand, if it exceeds 0.10%, carbides and nitrides are generated and the structure becomes soft, and the required mechanical strength cannot be obtained. More preferably, it is 0.08% or less.

  B is an element that enhances the hardenability of steel and contributes to the improvement of mechanical strength. If it is less than 0.0001%, the effect of addition is not sufficiently exhibited, so 0.0001% or more is preferable. More preferably, it is 0.001% or more. On the other hand, if it exceeds 0.10%, inclusions are generated and become brittle, and the fatigue strength is lowered, so 0.10% or less is preferable. More preferably, it is 0.007% or less.

  The steel plate to be subjected to hot-dip Al plating is Cr: 0.01 to 0.50%, Al: 0.01 to 0.10 as long as the elements other than the above do not impair the properties of the steel plate after hot-dip Al plating. %, N: 0.001 to 0.02%, P: 0.001 to 0.05%, and S: 0.001 to 0.05%.

  For example, Cr, like Mn, is an element that enhances the hardenability of steel and may be contained in an amount of 0.01 to 0.50%. Since Al is an element that functions as a deoxidizer, it may be contained in an amount of 0.01 to 0.10%. Steel sheet to be subjected to hot-dip Al-based plating contains impurities inevitably mixed in the raw material and / or manufacturing process, for example, Ni, Cu, Mo, O, etc., as long as the properties of the steel sheet after hot-dip Al-based plating are not impaired. May be.

  A steel sheet having the above component composition having a molten Al-based plating layer can have a mechanical strength of 1500 MPa or more by heating it to a predetermined temperature and then performing rapid cooling simultaneously with forming by hot pressing. Even if the steel sheet is thin for weight reduction, the required mechanical strength as a steel part can be maintained.

(Molded Al plating layer)
The molten Al-based plating layer is formed on one side or both sides of the steel plate. The molten Al-based plating layer is formed by, for example, a hot dipping method, but the plating method is not limited to the hot dipping method.

The adhesion amount of the molten Al-based plating is 30 to 55 g / m ² per side. “Plating deviation” is likely to occur due to rapid heating by energization. A magnetic field is generated by the current flowing in the molten Al heated to 600 ° C. or more, and a cohesive force (pinch force) acts on the molten Al according to Fleming's left-hand rule. As the plating adhesion amount increases, the amount of molten Al that agglomerates due to the cohesive force (pinch force) increases.

If the adhesion amount of the molten Al-based plating exceeds 55 g / m ² per side, “plating deviation” appears under the above mechanism. Therefore, the adhesion amount of the molten Al-based plating is 55 g / m ² per side. The following. Preferably, 50 g / m ² per side is the following.

The adhesion amount of the molten Al-based plating affects the corrosion resistance of the steel part of the present invention. If the adhesion amount of the molten Al plating is less than 30 g / m ² per side, the required corrosion resistance cannot be ensured. Therefore, the adhesion amount of the molten Al plating is 30 g / m ² or more per side. Preferably it is 35 g / m ² or more.

The surface coating layer (described later) containing 0.3 to 1.5 g / m ^{2 of} ZnO in terms of Zn is formed on the surface of the molten Al-based plating layer. It has the effect of increasing the corrosion resistance of steel parts after pressing. Therefore, when a surface coating layer containing ZnO is not formed, the adhesion amount of the molten Al-based plating needs to be 40 g / m ² or more per side in order to ensure the corrosion resistance of the steel parts after hot pressing. In order to avoid “plating deviation”, it is necessary to make it 50 g / m ² or less per side.

This means that when the surface coating layer containing ZnO is not formed, the plating adhesion amount is controlled within the range of 40 to 50 g / m ^{2 on} the entire length, full width and front and back of the steel plate, that is, the plating adhesion amount This means that the fluctuation is controlled to 10 g / m ² or less, but this control is extremely difficult with the current ability to control the adhesion amount.

In the production method of the present invention, the lower limit of the adhesion amount of the molten Al-based plating is formed by forming a surface film layer containing 0.3 to 1.5 g / m ^{2 of} ZnO in terms of Zn on the surface of the molten Al-based plating layer. Is 30g / m ² and the upper limit is 55g / m ² , which increases the allowable range of adhesion amount of molten Al-based plating, avoids “delay of plating”, and ensures corrosion resistance of steel parts after hot pressing Are compatible.

  When hot-dip aluminum plating is applied to a steel plate, an alloy layer, that is, an Al—Fe (—Si) -based intermetallic compound layer is formed at the interface between the steel plate and the hot-dip aluminum plating layer. If this alloy layer is too thick, cracks will occur in the alloy layer during heating prior to hot pressing, and scale (Fe oxide) will form in the Al-Fe (-Si) intermetallic compound layer, Corrosion resistance of steel parts after hot pressing decreases. Therefore, the thickness of the alloy layer is 5 μm or less. Preferably it is 4 micrometers or less.

  The lower limit of the thickness of the alloy layer is not particularly limited, but usually it is formed with a thickness of about 3 μm and it is difficult to make it 1.5 μm or less, so the lower limit is substantially 1.5 μm.

  The molten Al-based plating layer contains Al as a main component, but may contain Si in addition. When Si is contained, the thickness of the Al—Fe-based alloy layer formed at the interface between the molten Al plating layer and the steel sheet can be controlled by the reaction of Al in the plating bath and the Fe of the steel sheet during the hot dipping.

  If Si is less than 3%, the Al—Fe-based alloy layer grows thick, promotes cracking of the plating layer during processing, and inhibits corrosion resistance. Therefore, Si is preferably 3% or more. On the other hand, if Si exceeds 15%, the workability and corrosion resistance of the plating layer deteriorate, so 15% or less is preferable.

  The molten Al-based plating layer may contain 1 to 4% of Fe eluted from the equipment or steel strip in the bath as an element other than Si. Furthermore, you may contain about 0.01 to 1% of 1 type, or 2 or more types of Mg, Ca, Sr, Li in Al plating bath.

  The molten Al plating layer formed on one side or both sides of the steel plate suppresses corrosion of the steel plate and prevents the steel plate surface from oxidizing and generating scale (iron oxide) when hot pressing the steel plate. Note that the presence of the molten Al-based plating layer eliminates the scale removal process, the surface cleaning process, the surface treatment process, and the like, thereby improving productivity.

  Moreover, since the melting Al-based plating layer has a higher melting point than plating coating with organic materials or plating coating with other metal-based materials (for example, Zn-based), the hot-pressed molten Al-based plated steel sheet is heated at a high temperature. It becomes possible to process.

<Surface film layer>
A surface film layer containing 0.3 to 1.5 g / m ^{2 of} ZnO in terms of Zn is formed on the surface of the molten Al plating layer. By forming a surface coating layer containing ZnO in the above-mentioned conversion amount on the surface of the molten Al plating layer, the lubricity during hot pressing and the reactivity during chemical conversion treatment are improved, and adhesion of molten Al plating is performed. the appropriate range of amount, can be extended from 40 to 50 g / m ² (when there is no surface coating layer) on 35~55g / m ² (when there is a surface coating layer).

If ZnO is less than 0.3 g / m ^{2 in} terms of Zn in the surface coating layer, the “surface of plating” will appear during energization heating, and the chemical conversion processability will decrease, and the corrosion resistance after coating will decrease. ZnO in the layer is 0.3 g / m ² or more in terms of Zn. Preferably it is 0.5 g / m ² or more.

On the other hand, if ZnO exceeds 1.5 g / m ^{2 in} terms of Zn in the surface coating layer, the spot weldability of the steel part is reduced, so that ZnO in the surface coating layer is 1.5 g / m ^{2 in} terms of Zn. The following. Preferably it is 1.2 g / m ² or less.

  The surface coating layer is formed by applying a suspension of ZnO fine particles to the surface of the molten Al-based plating layer using, for example, a roll coater, baking, and drying. A suspension of ZnO fine particles may be mixed with an organic binder and applied to the surface of the molten Al-based plating layer. Alternatively, a suspension of ZnO fine particles or a mixed solution of ZnO fine particles and an organic binder may be applied by a powder coating method.

Examples of the organic binder include water-soluble resins such as polyurethane resins, polyester resins, acrylic resins, and silane coupling agents. The coating liquid for forming a surface coating layer, as components other than ZnO and organic binder, for example, oxides such as _{SiO 2, TiO 2, Al 2} O 3, and / or, Mg, Ca, Ba, Zr, P, You may contain compounds, such as B, V, and Si.

  It is not necessary to use an organic binder, but if it is not used, the adhesion of the coating solution to the molten Al-based plating layer is slightly low, and there is a concern that it may be partially peeled when rubbed with a strong force. It is preferable to do.

  Baking and drying after application are performed using, for example, a hot air furnace, an induction heating furnace, or a near infrared furnace as appropriate. At this time, depending on the type of the organic binder, instead of baking / drying after coating, for example, curing treatment may be performed with ultraviolet rays or electron beams. The baking temperature is preferably 200 to 600 ° C. In addition, the formation method of a surface coating layer is not limited to the said method illustrated, Various formation methods can be employ | adopted.

  The particle diameter of ZnO in the surface coating layer is not particularly limited, but a diameter of about 50 to 1000 nm is preferable. The particle size of ZnO is the particle size after hot pressing. Specifically, a molten Al-based plated steel sheet having a surface coating layer is placed in a furnace at 900 ° C. for 5 to 6 minutes, and then rapidly cooled with a mold. Observe and measure the particle size of ZnO.

  During the retention, the organic matter such as an organic binder and the oxide and / or compound are decomposed and disappeared, and only ZnO remains in the surface film layer, so that the particle size of ZnO can be easily measured. .

<Manufacture of steel parts>
In the production method of the present invention, a hot-dip Al-based plated steel sheet (hereinafter sometimes referred to as “the present plated steel sheet”) on which a surface coating layer containing 0.3 to 1.5 g / m ^{2 of} ZnO in terms of Zn is formed. (Ii) Heat to 850 to 1050 ° C. at a heating rate of 20 ° C./second or more, and then (iii) rapidly cool simultaneously with the forming process to produce a steel part.

(heating)
The plated steel sheet of the present invention is heated to 850 to 1050 ° C. at a heating rate of 20 ° C./second or more. The heating means is not particularly limited, but current heating or high frequency induction heating is preferable in that a heating rate of 20 ° C./second or more can be easily achieved. Preferably, it is 30 ° C./second or more. In order to realize high productivity, it is preferable that the heating rate is fast, and the upper limit is not set. However, in order to avoid “delay of plating”, 200 ° C./second or less is preferable.

  When the plated steel sheet of the present invention is heated to 850 to 1050 ° C., the molten Al-based plating is melted, and an inter-diffusion with Fe causes an alloy layer mainly composed of an Al—Fe compound or an Al—Fe—Si compound (Al— Fe (-Si) alloy layer) is formed. Since the melting point of the Al—Fe (—Si) alloy layer is high and is about 1150 ° C., when the molten Al-based plating layer changes to the Al—Fe (—Si) alloy layer, “plating deviation” does not occur.

  There are a plurality of Al-Fe compounds and Al-Fe-Si compounds, which change to a compound having a high Fe concentration when heated at high temperature or for a long time. As a result, the alloy layer exhibits a complex structure in which a plurality of Al—Fe compounds and / or Al—Fe—Si compounds are complicated.

  When the plated steel sheet of the present invention is furnace-heated, the alloy layer shows a structure in which five layers are laminated, but when rapidly heated by current heating or induction heating, the alloy layer does not show a clear layered structure. In the production method of the present invention, the structure of the alloy layer is not defined, but the alloy layer is heated by rapid heating, so that a clear layered structure is not obtained.

  A desirable surface state for a steel part is a state in which the surface is alloyed to the surface and the Fe concentration in the alloy layer is not high. When unalloyed Al remains in the alloy layer, the Al remaining portion is rapidly corroded and the coating film is liable to swell. On the other hand, if the Fe concentration in the alloy layer becomes too high, the corrosion resistance of the alloy layer itself is lowered and the coating film is liable to swell. In either case, the coating film swells easily, and the corrosion resistance after painting of the steel parts decreases.

  This is because the corrosion resistance of the alloy layer depends on the Al concentration in the alloy layer. Therefore, in order to ensure excellent post-painting corrosion resistance in steel parts, the alloy layer needs to be in an appropriate alloyed state. An appropriate alloying state is determined by the amount of plating and heating conditions.

  In the production method of the present invention, the plated steel sheet of the present invention is heated to 850 to 1050 ° C. in order to make the molten Al-based plating layer completely alloyed. If the heating temperature is less than 850 ° C., alloying is insufficient, so the temperature is set to 850 ° C. or higher. Preferably it is 900 degreeC or more. On the other hand, if the heating temperature exceeds 1050 ° C., the diffusion layer in the alloyed layer grows too much and the spot weldability decreases, so the temperature is set to 1050 ° C. or less. Preferably it is 1000 degrees C or less.

(Molding / Cooling)
The plated steel sheet of the present invention heated to 850 to 1050 ° C. is rapidly cooled simultaneously with the forming process. At the same time that steel parts are manufactured by forming, the required mechanical strength is imparted to the steel parts by the action of rapid cooling. For example, high mechanical strength of 1500 MPa or more can be imparted to the steel part.

<Steel parts>
In the steel part of the present invention ,
(I) The Al—Fe-based intermetallic compound layer has (i-1) a difference in thickness in the short side direction of the high-strength steel part of 70 μm or less, (i-2) a thin portion having a thickness of 10 to 20 μm, (I-3) The thickness of the diffusion layer on the steel plate side is 5 μm or less,
(Ii) The surface coating layer has (ii-1) surface roughness Ra of 0.5 to 1.5 μm, (ii-2) thickness of 0.1 to 1.0 μm, and ZnO is 0 in terms of Zn. .3~1.5g / ^{m 2} containing.

(Al-Fe intermetallic compound layer)
The steel parts of the present invention are applied to bumper beams, door impact beams, center pillars and the like that are relatively elongated in shape. In this case, the elongated steel plate is heated, but the short sides on both sides of the elongated steel plate are sandwiched between the electrodes and energized in the long side direction. Then, the “pitch of plating” occurs parallel to the energization direction due to the pinch force generated by energization, and the “plating shift” occurs near the center of the long side.

  When the difference in thickness in the short side direction of the Al—Fe intermetallic compound layer formed on the surface of the steel part exceeds 70 μm due to this “proximity of plating”, in the portion of the plating thickened by “proximity of plating” In some cases, the alloy cannot be completely alloyed by heating, the corrosion resistance of the part is lowered, and the phenomenon that the plating adheres to the mold or galling occurs. Therefore, in the steel part of the present invention, the difference in thickness in the short side direction of the Al—Fe intermetallic compound layer formed on the surface of the steel part is set to 70 μm or less. Preferably it is 50 micrometers or less.

  Moreover, the thickness of the thin site | part of an Al-Fe-type intermetallic compound layer shall be 10-20 micrometers. If the thickness of the thin part is less than 10 μm, sufficient corrosion resistance cannot be obtained. Preferably it is 13 micrometers or more. On the other hand, if the thickness of the thin portion exceeds 20 μm, it is difficult to suppress “plating deviation”, and therefore the thickness is set to 20 μm or less. Preferably it is 17 micrometers or less.

  Furthermore, the thickness of the diffusion layer on the steel sheet side in the Al—Fe-based intermetallic compound layer is set to 5 μm or less. FIG. 1 shows a cross-sectional structure of an Al—Fe-based intermetallic compound layer generated by rapid heating. After the cross section of the steel plate was polished, it was etched with a nital solution and observed with an optical microscope. In the diffusion layer having the cross-sectional structure shown in FIG. 1, the thin layer is a diffusion layer on the steel sheet side. Note that a layer having a thin color tone and a layer having a high color tone on the layer are combined to form a diffusion layer. The outer layer of the diffusion layer is not layered but has a complicated structure.

  In the steel part of the present invention, the thickness of the diffusion layer on the steel sheet side is set to 5 μm or less. The spot weldability depends on the thickness of the diffusion layer. If the diffusion layer exceeds 5 μm, dust is likely to be generated, and the appropriate range of welding current is narrowed, so that it is 5 μm or less. Preferably it is 4 micrometers or less. Note that there are various factors in the occurrence of spot welding dust, and it is necessary to optimize them, which will be described later.

(Surface film layer)
Since the steel parts of the present invention are steel parts for automobiles, spot weldability is important. The present inventors investigated the spot weldability of a surface coating layer (hereinafter sometimes referred to as “ZnO-containing coating layer”) containing 0.3 to 1.5 g / m ^{2 of} ZnO in terms of Zn. As a result, it was found that spot weldability differs greatly when AC and DC are used as the welding power source.

  When a DC power source is used in a steel member having a small amount of molten Al-based plating and having a ZnO-containing coating layer, spot weldability is greatly reduced. Specifically, generation of dust is likely to occur. However, when rapid heating is performed on a molten Al-based plated steel sheet having a ZnO-containing coating layer and the molten Al-based plating is alloyed, a decrease in spot weldability is suppressed, and dust is hardly generated.

  When rapid heating is applied and the molten Al-based plating is alloyed, a phenomenon that the surface of the plating layer is smoothed is recognized, and this smoothing is considered to affect generation of dust. Therefore, in the steel part of the present invention, ZnO formed on the surface of the Al—Fe-based intermetallic compound layer adopting arithmetic average roughness: Ra defined in JIS B 0601 ('01) as an index of surface roughness. The roughness of the coating layer is 0.5 to 1.5 μm.

  If Ra exceeds 1.5 μm, dust is likely to be generated during spot welding, so Ra is set to 1.5 μm or less. Preferably it is 1.2 μm or less. There is a limit to smoothing even with rapid heating, and it is impossible to make it less than 0.5 μm, so 0.5 μm is the lower limit. Preferably it is 0.7 micrometer or more.

  The thickness of the ZnO-containing coating layer formed on the surface of the Al—Fe-based intermetallic compound layer is 0.1 to 1.0 μm. If the thickness of the ZnO-containing coating layer is less than 0.1 μm, the corrosion resistance after coating decreases, so the thickness is made 0.1 μm or more. Preferably it is 0.3 micrometer or more. On the other hand, if it exceeds 1.0 μm, spot weldability is lowered, so the thickness is made 1.0 μm or less. Preferably it is 0.7 micrometer or less.

The ZnO-containing coating layer is a coating containing 0.3 to 1.5 g / m ² of ZnO in terms of Zn, and a thickness of 0.1 to 1.0 μm is 0.3 to 1. It corresponds to 5 g / m ² .

  Next, examples of the present invention will be described. The conditions in the examples are one example of conditions used for confirming the feasibility and effects of the present invention, and the present invention is based on this one example of conditions. It is not limited. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

<Example 1>
A cold-rolled steel sheet having a composition shown in Table 1 (thickness 1.2 mm) was subjected to hot-dip Al plating. The annealing temperature (?) At this time was about 800 ° C., and the Al plating bath contained about 2% Fe eluted from the steel sheet in addition to Si: 9%. The adhesion amount after plating was adjusted to 25 to 60 g / m ² on one side by a gas wiping method and cooled.

After cooling, a coating solution in which an acrylic binder is added to a suspension containing about 50 nm of ZnO in an amount of 20% with respect to the amount of ZnO is applied with a roll coater and baked at about 80 ° C. A surface film layer was formed. The adhesion amount was 0.1 to 4.0 g / m ^{2 in} terms of Zn.

  A 100 × 500 mm steel plate having a ZnO-containing coating layer was energized and heated so that a current flowed parallel to the long side of the steel plate. A thermocouple was connected to the steel plate, and measurement was performed until the steel plate temperature reached the maximum temperature, and an average heating rate from 50 ° C. to the attained temperature of −30 ° C. was calculated.

  After reaching the maximum temperature, the steel plate was sandwiched between about 700 ° C. with a flat mold and pressed down, held at the bottom dead center for 10 seconds, cooled and quenched. The hardened steel sheet was evaluated for occurrence of “plating deviation”, spot weldability, and corrosion resistance. The evaluation method is as follows.

(1) Occurrence of “Plating Deviation”, Surface Roughness The plate thickness was measured in the short side (100 mm) direction of the test piece collected from the quenched steel plate, and the difference between the maximum plate thickness and the minimum plate thickness was calculated. Further, a sample was cut out from the vicinity of the center and the end of the short side, and the Al—Fe layer was observed with an optical microscope.

  In the test piece having a large plate thickness difference, “plating deviation” occurred in the central portion, and the plate thickness difference and the Al—Fe layer thickness difference were almost the same. Prior to observation with an optical microscope, the specimen was corroded with 2% nital, and the corroded surface was observed with an optical microscope and imaged. The thickness of the diffusion layer was measured with an optical micrograph. In addition, the surface roughness Ra was measured with a roughness meter at a portion where “plating deviation” in the vicinity of the end of the test piece did not occur.

(2) Spot weldability A 30 × 50 mm sample was cut out from the test piece, and the appropriate current range in spot welding was investigated. Welding conditions were 300 kgf under pressure, DC power source, energization time 15 cycles (60 Hz), chromium copper, DR (tip 6φ: 40R) electrode, the lower limit was 4√t, and the upper limit was dust generation.

◎: Appropriate range over 2 kA ○: Appropriate range over 1.5 kA up to 2 kA △: Appropriate range over 1 kA up to 1.5 kA ×: Appropriate range over 1 kA

(3) Corrosion resistance After subjecting the test piece to a chemical conversion treatment liquid (PB-SX35) manufactured by Nippon Parkerizing Co., Ltd., an electrodeposition paint (Powernics 110) manufactured by Nippon Paint Co., Ltd. is 15 μm. It was painted with aim and baked at 170 ° C.

The evaluation of the corrosion resistance after painting was performed by the method prescribed in JASO M609 established by the Automotive Engineers Association. A wrinkle was applied to the coating film with a cutter knife, and only the end face was sealed for a corrosion test. In the corrosion test, the state of corrosion after 180 cycles (60 days) was observed and rated as follows. One side means that the maximum swelling on one side from the heel was measured. As a comparative material, an alloyed hot-dip galvanized steel sheet having a single side of 45 g / m ² was used and evaluated in the same manner. The score was ○.

◎: Swelling width on one side 3 mm or less ○: Swelling width on one side 3 mm or more and 5 mm or less △: Swelling width on one side 5 mm or more and 7 mm or less

Table 2 shows the results. The plating adhesion amount (g / m ² ), ZnO amount (g / m ² ), end Al—Fe layer thickness (μm), and ZnO thickness (μm) are all per one side. The plate thickness difference (mm) is a numerical value influenced by the plating layers on both sides. The ZnO amount (g / m ² ) is an amount converted to Zn, and the ZnO thickness (μm) is the thickness of the entire surface coating layer.

  In Table 2, when the amount of plating is small (level 1), the corrosion resistance decreases, and conversely, when the amount of plating is large (level 6), a difference in plate thickness occurs between the center and the end. ing. The difference in plate thickness corresponds to the occurrence of “plating deviation”.

At level 6, a plate thickness difference of 0.13 mm occurs. This plate thickness difference is 0.065 mm per side, which corresponds to a thickness difference of 65 μm of the Al—Fe layer and is within the scope of the present invention, but the plating adhesion amount (58 g / m ² ) is within the scope of the present invention. Exceeded mold adhesion during hot pressing.

  Level 7 is a case where a surface coating layer containing ZnO is not formed. In this case, “plating deviation” occurs, and a plate thickness difference is likely to occur. At level 7, die adhesion occurred during hot pressing. On the other hand, when the ZnO-containing coating layer is too thick (level 10), spot weldability (DC weldability) when using a DC power supply is lowered.

  In levels 11 to 21, various heating conditions were changed. When the ultimate temperature is too low (level 17), or when the ultimate temperature is too high (level 21), the corrosion resistance and DC weldability deteriorate. When the ultimate temperature is too low, it is presumed that the alloying of the plating layer does not proceed sufficiently and the characteristics are deteriorated. When the ultimate temperature is too high, it is presumed that the Fe concentration in the Al—Fe layer is increased too much, the diffusion layer is too thick, and the characteristics are deteriorated.

  Level 22 is a case where the heating rate is lowered to a level equivalent to that of electric furnace heating. In this case, the direct current weldability is reduced.

  From the results shown in Table 2, it can be seen that, by making various conditions appropriate, excellent corrosion resistance and spot weldability can be secured while suppressing “deposition of plating” that occurs during energization heating.

  As described above, according to the present invention, high-strength steel parts having good post-coating corrosion resistance and spot weldability are manufactured with high productivity by hot-pressing hot-dip Al-plated steel sheets and then hot-pressing them. be able to. Since the high-strength steel parts of the present invention are suitable for machine structural parts that require strength and parts for automobiles, the present invention has high applicability in the automobile manufacturing industry and industrial machine manufacturing industry.

Claims (5)

In a high-strength steel part having an Al-Fe-based intermetallic compound layer on the surface of a high-strength steel part, and further having a surface coating layer on the surface of the Al-Fe-based intermetallic compound layer,
(I) The Al—Fe-based intermetallic compound layer has (i-1) a difference in thickness in the short side direction of the high-strength steel part of 70 μm or less, (i-2) a thin portion having a thickness of 10 to 20 μm, (i-3) thickness of the steel sheet side of the diffusion layer has a 5μm or less,
(Ii) The surface coating layer has (ii-1) surface roughness Ra of 0.5 to 1.5 μm, (ii-2) thickness of 0.1 to 1.0 μm , and ZnO is 0 in terms of Zn. A high-strength steel part containing 3 to 1.5 g / m ² . The high-strength steel part according to claim 1 , wherein the high-strength steel part is an automobile steel part. A manufacturing method for manufacturing the high-strength steel part according to claim 1 or 2,
(I) ZnO is converted to Zn in an amount of 0.3 to 1.5 g in terms of Zn on the surface of a steel sheet having a molten Al-based plating layer having a plating adhesion amount of 30 to 55 g / m ² and an alloy layer thickness of 5 μm or less. / M ² containing surface coating layer,
(Ii) The steel sheet having the surface coating layer is heated to 850 to 1050 ° C. at a heating rate of 20 ° C./second or more, and then
(Iii) A method for producing a high-strength steel part characterized by rapid cooling simultaneously with forming. The method for producing a high-strength steel part according to claim 3 , wherein the heating is performed by energization heating or induction heating. The method for producing a high-strength steel part according to claim 3 or 4 , wherein the forming process and the rapid cooling are performed using a mold.