KR20040007718A - High-strength alloyed aluminum-system palted steel sheet and high-strength automotive part excellent in heat resistance and after-painting corrosion resistance - Google Patents

Abstract

The present invention provides a hot press method used to produce a high strength member for a vehicle, in particular a member requiring high strength, such as a chassis part of a vehicle. More specifically, the present invention provides an aluminum plated steel sheet or aluminum-zinc plated steel sheet suitable for hot forming (hot forming) in which high strength after hot forming is obtained, and a method of producing the same. The present invention also provides a molten aluminum plated steel sheet which suppresses the alloy reaction of the plated layer during press molding and has a beautiful appearance, and an aluminum-based steel sheet for hot press having excellent post-paint corrosion resistance. Specifically, the present invention contains, as mass, C: 0.05 to 0.7%, Si: 0.05 to 1%, Mn: 0.5 to 3%, P: 0.1% or less, S: 0.1% or less and Al: 0.2% or less. Further, at least one element (s) selected from Ti: 0.01 to 0.8%, Cr: 3% or less, and Mo: 1% or less satisfies the following Formula 1: Ti + 0.5 x Mn + Cr + 0.5 x Mo> 0.4 A high-strength aluminum-based plated steel sheet having excellent heat resistance and post-painting corrosion resistance, having a Fe-Al-based coating film containing more than 0.1% of Cr and Mn as a whole on the surface of the steel to be contained.

Description

HIGH-STRENGTH ALLOYED ALUMINUM-SYSTEM PALTED STEEL SHEET AND HIGH-STRENGTH AUTOMOTIVE PART EXCELLENT IN HEAT RESISTANCE AND AFTER-PAINTING CORROSION RESISTANCE}

In pursuing vehicle weight reduction due to global environmental issues, automotive steel sheets must have the highest possible strength. In general, however, if the strength of the steel sheet continues to increase, the elongation and r-value are lowered and thus the moldability is deteriorated. Therefore, the steel sheet is required to have a high strength, high formability and shape fixing at the same time.

One solution to this problem is the Transformation Induced Plasticity (TRIP) steel produced using the martensite transformation of retained austenite, and the application of TRIP steel continues to increase in recent years. However, although it is possible to produce 1,000 MPa class high strength steel having excellent formability using TRIP steel, it is difficult to obtain good formability in a high strength steel sheet such as, for example, an ultra high strength steel sheet having a strength of 1,500 MPa or more. In addition, this technique is difficult to solve the problem of shape fixing.

In order to cope with these problems, there is a warm press technique, a technique that has recently attracted attention as another means of harmonizing high strength and high formability. Japanese Unexamined Patent Publication No. 2000-234153 discloses a technique of increasing strength by forming a steel sheet in a warm state and manufacturing using heat generated during molding. This technique aims to increase the strength by appropriately controlling the composition of the steel, heating the steel sheet in the ferrite temperature range and using precipitation strengthening in this temperature range.

Japanese Unexamined Patent Publication No. 2000-87183 proposes a high strength steel sheet in which the yield strength at a certain temperature during molding is much lower than the yield strength at room temperature, and aims at increasing the precision of press molding. However, the strength obtained by this technique can be limited.

As a technique for overcoming this disadvantage, a technique is disclosed in the paper (SAE, 2001-01-0078) that heats a steel sheet to a temperature range in which austenite single phase is formed and cools it during press molding.

This is a technique of obtaining a desired material properties by forming a steel sheet in a state where the steel sheet is heated to a high temperature of 800 ℃ or more, thereby solving the problem of forming a high strength steel sheet, and then quenching it in a cooling unit after molding. However, because this technique involves heating in the atmosphere, oxides are produced on the surface of the steel sheet and the oxides must be removed in the downstream process. Techniques for applying aluminum plating to inhibit oxidation during heating are also disclosed in this paper (SAE, 2001-01-0078).

As another means, after forming the steel sheet at room temperature, a means for obtaining a high strength by rapidly heating a portion of the steel sheet and rapidly cooling it to quench it may be adopted. In this case, by locally heating the steel sheet, it is possible to increase the strength of only the portion where high strength is required. Japanese Unexamined Patent Publication No. 2000-83640 discloses such a technique, and a steel sheet containing 0.15 to 0.5% of carbon is used for this purpose.

Hot forming techniques and techniques for applying an aluminum plated steel sheet during hot forming are disclosed in the above-mentioned prior art. However, in the prior art, when the steel sheet is heated to a high temperature of 800 ° C. or higher, Fe diffuses to the surface for a short time and the aluminum plating layer is changed to an intermetallic compound referred to as an “aluminum based alloy plating layer” hereinafter. Since the aluminum-based alloy plating layer is very hard and brittle, the aluminum-based alloy sometimes generates cracks and peels off in a powder state, thereby deteriorating post-painting corrosion resistance. Even if no crack is generated, the aluminum plated steel sheet is inferior in post-painting corrosion resistance, for example, to other surface treated steel sheets such as hot dip galvanized steel or alloyed hot dip galvanized steel. More specifically, post-painting corrosion resistance is evaluated after molding at high temperatures, and when scratches are applied, such as cutting cross-shaped, the corrosion of the base steel propagates from the depth direction from that portion to be hot dip galvanized or alloyed. It is easier to proceed than for hot dip galvanized sacrificial steel.

In addition, there is also a possibility that the peeled plating layer accumulates on the die and causes defects during the pressing. The reason is that although the plating layer can withstand the hot forming process, there is a problem of the plating layer that it is substantially difficult to process the whole part at high temperature and the processing is delayed and easily peeled off at the temperature drop. Another problem here is that, when the temperature is lowered, the mother steel constitutes a hard structure mainly composed of martensite as a result of quenching, and therefore the stress is likely to surge.

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to an aluminum-based alloy plated steel sheet suitable for a member produced by a hot press and represented as a structural member for a vehicle component requiring strength, and specifically, a high strength is required like a chassis component such as a vehicle. And the steel used to produce such a member.

1 is a graph showing the effect of the total amount of Mn and Cr in the post-painting corrosion resistance of the aluminum-based alloy plating layer.

2 is a graph showing the relationship between the amount of Ni, Cu and Sn added and the net corrosion resistance.

3 is a graph showing the relationship between the addition amount of Ni, Cu and Sn and post-painting corrosion resistance.

4 is a graph showing the relationship between the amount of Cr and Mo added and the net corrosion resistance.

5 is a graph showing the relationship between the addition amount of Cr and Mo and post-painting corrosion resistance.

Figure 6 is a Fe-Al-Si-based three-phase diagram at a temperature of 950 ℃.

7 is a photograph (backside scanning electron image) of an example of a tissue according to the present invention under an optical microscope.

8 is a graph showing the relationship between the thickness of the outer layer and the workability of the plating layer.

9 is a photograph (back scanning electron image) showing an example of the metallographic structure according to the present invention taken under a microscope.

10 is a view showing a shape obtained by molding the steel sheet according to the present invention.

11 is a graph showing the relationship between heating time and heating temperature according to the present invention.

The present inventors conducted a basic study to solve the above problems, and as a result, after the high-temperature molding, the Fe-Al-based coating containing more than 0.1% of Cr and Mn on the surface of the steel sheet as a whole has excellent post-painting corrosion resistance It was found that plated steel sheets could be produced. The aluminum based plating layer after hot forming and locally rapidly heated is transformed into an aluminum based alloy plating layer to its surface, and this intermetallic compound is very fragile and easily cracks during processing. Aluminum-based plated steel sheet as defined in the present invention means a plated steel sheet having a plating layer composed of Al and Fe content of at least 70%. In addition, since this plating layer has a higher nobler potential than the steel sheet, corrosion of the steel steel starts from cracks and post-paint corrosion resistance tends to deteriorate. It is very effective to add Mn to the aluminum alloy plating layer in order to prevent deterioration of post-painting corrosion resistance. Although the function of Mn is not yet known, Mn serves to bring the potential of the Fe-Al-based plating layer closer to that of the steel sheet by controlling the potential of the alloy layer. In addition to this function, Mn is used in the phosphate treatment, which is a pre-painting treatment. It is also possible to control the shape of phosphate crystals.

Extremely excellent heat resistance, in particular post-process heat resistance, can be obtained by specifying the amounts of C, Si, Mn, P, S and Al as the parent steel components and adding Ti, Cr and Mo such that the addition is effective to satisfy the following formula (1). . These elements accelerate the diffusion between Al and Fe, so that even if cracks are generated in the plating layer, the reaction between Fe and Al proceeds from the vicinity of the cracks, so that the bare steel is hardly exposed.

[Formula 1]

Ti + 0.5 × Mn + Cr + 0.5 × Mo> 0.4

In addition, the present inventors found that by adding N, Nb, V, Ni, Cu, B, Sn and Sb as components of the steel, better post-paint corrosion resistance could be obtained.

On the other hand, it is advantageous that the plating layer after the alloy treatment further contains Si, and further corrosion resistance is obtained by adding Zn, Mg and the like. It is important that Al content of an alloy layer is 35% or less from a viewpoint of suppressing the crack of an alloy layer. Further, preferably, the thickness of the Fe—Al alloy layer should be 3 to 35 μm in view of post-painting corrosion resistance.

In addition, in order to overcome the above-described problems, the present inventors studied in detail the factors affecting the workability of the aluminum-based steel sheet after the alloy treatment, and learned the following facts. That is, since both the aluminum-based alloy plating layer and the mother steel are hard, the internal stress increases, but since the soft layer is present between the aluminum-based alloy plating layer and the mother steel, the stress is reduced and the peeling resistance is remarkably improved. The soft layer is composed of a ferrite phase, and due to the fact that the thickness of the soft layer is 1 μm or more, it is evaluated to prevent cracking of the alloy layer from propagating to the parent steel.

In addition, in order to overcome the above problems, the present inventors studied in detail the factors affecting the workability of the aluminum-based steel sheet after the alloy treatment, it was found that the following. That is, the plating layer can prevent peeling by optimizing heating conditions. The reason is not clear, but it is evaluated that the plating layer is alloyed according to the heating conditions, the phase structure is complicatedly changed, and there is an appropriate phase structure for obtaining workability. The gist of the present invention is as follows.

(1) A high-strength aluminum-based alloy plated steel sheet having excellent post-painting corrosion resistance, having a Fe—Al-based coating containing more than 0.1% Cr and Mn on the surface of the steel sheet as a whole.

(2) As mass%, C: 0.05 to 0.7%, Si: 0.05 to 1%, Mn: 0.5 to 3%, P: 0.1% or less, S: 0.1% or less and Al: 0.2% or less, and further Cr and Mn having a mass of more than 0.1% as a whole on the surface of the steel containing at least one selected element (s) selected from Ti: 0.01 to 0.8%, Cr: 3% or less, and Mo: 1% or less A high-strength aluminum-based alloy plated steel sheet excellent in heat resistance and post-painting corrosion resistance, having a Fe-Al-based coating containing.

[Formula 1]

Ti + 0.5 × Mn + Cr + 0.5 × Mo> 0.4

(3) In the item (2), the steel is, as mass%, N: 0.1% or less, Nb: 0.1% or less, V: 0.1% or less, Ni: 1% or less, Cu: 1% or less, B: 0.0003 to 0.03 A high strength aluminum-based alloy plated steel sheet excellent in heat resistance and post-painting corrosion resistance, further comprising at least one element selected from%, Sn: 0.1% or less, and Sb: 0.1% or less.

(4) In item (1), the steel is, as a mass%, C: 0.15 to 0.55%, Si: 0.5% or less, Mn: 0.2 to 3%, P: 0.1% or less, S: 0.04% or less, Al: 0.01 to 0.1% and N: 0.01% or less, B: 0.0002 to 0.0050%, Ti: 0.01 to 0.8%, Cr: 2% or less, Mo: 1% or less, Ni: 1% or less, Cu: 0.5% A high strength aluminum-based alloy plated steel sheet excellent in heat resistance and post-painting corrosion resistance, characterized by further containing at least one element (s) selected from below and Sn: 0.2% or less.

(5) The high-strength aluminum-based alloy having excellent heat resistance and post-painting corrosion resistance according to any one of items (1) to (4), wherein the Fe-Al coating further contains 1 to 20% of Si. Alloy plated steel plate.

(6) The high strength aluminum-based alloy plated steel sheet according to any one of items (1) to (5), wherein the Al concentration of the Fe-Al-based coating layer is 35% by weight or less.

(7) The item according to any one of items (1) to (6), wherein the Fe-Al coating further contains any one or both of Zn: 1 to 50% and Mg: 0.1 to 10%. High strength aluminum-based alloy plated steel sheet excellent in heat resistance and post-painting corrosion resistance.

(8) The high strength aluminum-based alloy plated steel sheet according to any one of items (1) to (7), wherein the Fe-Al-based coating layer has a thickness of 3 to 35 µm.

(9) having a coating layer mainly composed of Fe-Al on the surface of the steel sheet, a ferrite layer having a thickness of 2 µm or more at the bottom of the coating layer up to 1/10 or less of the steel sheet thickness, and a base steel mainly composed of martensite at the bottom of the ferrite layer An aluminum-based alloy plated steel sheet for a high strength vehicle member, characterized in that.

(10) The aluminum-based alloy plated steel sheet for high-strength vehicle member according to item (9), wherein the ferrite layer at the bottom of the aluminum-based alloy plating layer and the intermetallic compound layer on the steel sheet surface contains Si.

(11) The aluminum-based alloy plated steel sheet for high strength vehicle members according to item (9) or item (10), wherein the hardness of the ferrite is 200 or less.

(12) A high-strength vehicle component, characterized in that at least a portion thereof is formed by press molding consisting of the steel according to any one of items (1) to (11).

(13) The high-strength vehicle component according to item (12), which has a film having a thickness of 1 to 200 m on at least a part of the surface.

(14) When the vehicle member is produced by hot forming using a steel sheet produced by plating a steel containing 0.05 wt% or more of C as a steel component with a metal mainly composed of Al, the member is produced in the following areas: A, B And molded by press molding after being heated under heating conditions longer than C and D, and at least a part of the member is cooled at a cooling rate of 10 ° C./sec or more.

A (800 ° C, 13 minutes), B (900 ° C, 6 minutes), C (1,050 ° C, 1.5 minutes), D (1,200 ° C, 0.3 minutes)

Example 1

Hereinafter will be described the chemical composition of the steel sheet used in the present invention.

C: The steel sheet according to the present invention has a strength of 1,000 MPa or less after molding, and is obtained by rapidly cooling the steel sheet after hot pressing to transform the structure into a structure mainly composed of martensite. At this time, the content of C is preferably 0.05% or more, and more preferably 0.1% or more in order to stably obtain high strength. On the other hand, even if the content of C is increased to 0.7% or more, the strength is saturated and welding cracks are likely to occur, so the upper limit is set to 0.7%.

When the Si: Si content is too low, the fatigue deteriorates, so the amount of addition of 0.05% or more is preferable. However, Si is also an element which forms a stable oxide film on the surface of the steel sheet during recrystallization annealing and hinders aluminum plating properties. For this reason, the upper limit of Si is set to 1%.

Mn: Mn is well known as an element for improving the hardenability of a steel sheet. In addition, Mn is an element necessary to prevent high temperature brittleness caused by S, which is an unavoidable impurity. For this reason, the addition amount of 0.5% or more is favorable. Mn also improves heat resistance after aluminum plating. However, since the impact resistance after quenching may deteriorate when Mn addition amount exceeds 3%, this value is determined as an upper limit.

Ti: Ti has the most influence on the heat resistance of an aluminum plating layer. When the steel sheet is used in a temperature range such as exceeding 900 ° C as in the present invention, the Ti addition is good from the viewpoint of heat resistance, and in order to achieve the effect, an amount of 0.01% or more is preferable. However, since Ti forms C and TiC and reduces the amount of C contributing to the strength, it is necessary to increase the amount of C in correspondence with the amount of Ti added. In addition, since the effect of Ti is saturated in the quantity of about 0.1%, this value is determined as an upper limit.

Cr: Cr, like Mn and Ti, contributes to improved heat resistance after aluminum plating. However, as in Si, Mn deteriorates aluminum plating property by forming a stable oxide film on the surface of the steel sheet. In addition, Cr is a relatively expensive element, and therefore the upper limit at the time of addition is set to 3%.

Mo: Mo is preferably added to contribute to the improvement of heat resistance after aluminum plating, such as Mn, Ti and Cr. However, Mo is a relatively expensive element and its effect is saturated, so the upper limit thereof is set to 1%.

In the present invention, it is preferable to add an element that improves heat resistance after aluminum plating such as Ti, Mn, Cr, and Mo to satisfy the following formula. In particular, Ti and Cr have an effect of inhibiting the peeling of the plating layer during heating. In addition, Mn can be added in a relatively abundant manner and contribute greatly to heat resistance.

[Formula 1]

Ti + 0.5 × Mn + Cr + 0.5 × Mo> 0.4

The present invention defines that P, S and Al are additionally added to the steel. However, since P and Al deteriorate the ductility and fatigue strength of the steel and S deteriorate the toughness of the steel, the upper limit is determined for each of them. In some cases, at least one of Ni, Nb, V, N, Cu, B, Sn, and Sb may be contained in the steel. Ni and Cu contribute to the corrosion resistance of the steel and B improves the hardenability.

In many cases, steel sheets are used after the aluminum plating layer is alloyed and subsequently painted. In this case, post-painting corrosion resistance is affected by the amount of Mn and Cr in the aluminum-based alloy plating layer. Good post-coating corrosion resistance is obtained when the amount of these elements is 0.1% or more. As a method of adding Mn and Cr, there are a method of diffusing a steel component and a method of adding these components to a plating bath, and any method can achieve the effect as long as the addition amount is 0.1% or more. As is known, one to five phases may coexist in the aluminum-based alloy plating layer depending on the deposition amount of the plating, the steel composition, the heating conditions and the like. Since the influence of the phases near the surface is particularly large, when analyzing the phases, it is preferable to measure them in about 5 parts on the cross section of the region having a depth of 5 μm from the surface using means such as EPMA quantitative analysis and the like. It judges by calculating the average. As described later, in the case of hot-dip aluminum plating, it is common to add Si, in which case Si is further contained in the aluminum-based alloy plating layer. The amount is about 1 to 20% and the amount of Si in one to five phases of the aluminum-based alloy plating layer can vary considerably. Although the Fe-Al coatings of one to five phases may have various compositions such as Fe ₂ Al ₅ , FeAl ₃ , Fe ₃ Al, FeAl, and Al dissolving ferrite, in any composition, it is stable as long as the total amount of Mn and Cr is 0.1% or more. Post-painting corrosion resistance can be obtained. Regarding which phase to analyze when multiple phases are present, Mn and Cr are known to contribute to the improvement of post-coating corrosion resistance and because the corrosion phenomenon is macroscopic, the present inventors are random even if multiple phases are present. We think that by analyzing them in about five parts, general information can be obtained and judged by doing so. Furthermore, in order to observe the cross-sectional structure after heating, it is prescribed to etch the cross section with 2 to 3% of a nitral. This is because the interface between the base metal and the Al dissolving ferrite formed, especially when the heating time is longer, cannot be observed without etching.

The deposition amount of aluminum plating affects corrosion resistance, weldability, workability, and the like. If the deposition amount is too small, the post-painting corrosion resistance is insufficient. If the deposition amount is too large, the weldability and workability deteriorate. Regarding the workability, workability deterioration is because the brittle aluminum-based alloy plating layer is likely to peel off during press after heating.

The aluminum plating method is not particularly limited, and a hot dip plating method, an electrolytic plating method, a vacuum deposition method, a clad method, or the like can be used. At present, the most widely used method in the industry is a hot-dip plating method, in which Al-10% Si is mainly used in the plating bath, and Fe is contained therein as an unavoidable impurity. In addition, it has already been described that the addition of Cr and Mn improves post-coating corrosion resistance. As other additive elements, Mg, Ti, Zn, Sb, Sn, Cu, Ni, Co, In, Bi, misch metal and the like are named, and these elements may be applied as long as the plating layer is mainly composed of Al. .

The present invention does not particularly limit the pretreatment and posttreatment of aluminum plating. As the plating pretreatment, pre-plating such as Ni, Cu, Cr or Fe is named and any of these may be adopted. As the post-plating treatment, chromate treatment, resin coating treatment and the like for main rust prevention and lubrication are named, but the organic resin is not good because it disappears when heated. Regarding the chromate treatment, considering the recent regulations for hexavalent chromium, a treatment film containing trivalent chromium such as electrolyte chromate is preferable. Post-treatments other than inorganic chromate treatments may be applied. For the purpose of lubrication, it is also possible to apply treatments such as aluminum oxide, silicon oxide, and MoS _{2 in} advance.

Example 2

Hereinafter, the reason for regulation of the steel component in Example 2 according to the present invention will be described.

N is inevitably included and is not particularly specified when B is not added, but the amount of Ti should be increased when B is added and when the amount is excessively large. As a result, there is a problem that the amount of finally produced TiN increases, high temperature cracking occurs, and costs increase. Therefore, the upper limit is set to 0.1%.

B is added to increase the hardenability during cooling during press molding or after press molding, and at least 0.0002% of addition is necessary to achieve the effect. However, when the addition amount is excessive, there is a problem of high temperature cracking and the effect is saturated. Therefore, an upper limit is set to 0.03%.

Ni, Cu and Sn have the effect of changing the surface cracking state during press molding after high temperature heating by changing the state of alloying the aluminum plating layer during high temperature heating, and since these elements improve the post-paint corrosion resistance of the molded product It is important. With respect to these elements, from the results shown in Figs. 2 and 3 obtained by measuring the addition amounts of Ni, Cu and Sn and the net and post-paint corrosion resistance to the samples after high temperature molding in laboratory tests, the inventors described above. It has been found that Ni, Cu and Sn must be added to satisfy the formula (3) to achieve one effect. At this time, the net corrosion resistance and post-painting corrosion resistance were evaluated by the method tested under the conditions shown in the examples using a sample taken from the part to which the post-hot forming processing was applied.

[Formula 3]

(Ni + 0.5 × Cu + 3 × Sn) ≥ 0.012

With respect to each of Ni, Cu, and Sn, the addition of excessive Ni saturates the effect, increases the cost, and Cu and Sn cause surface cracking, so the upper limit of each element is 1.0%, 1%, and 0.2%, respectively. Is set to.

The production conditions of the steel sheet according to the present invention are not specifically defined, but the following describes the good production conditions.

The steel with the above-mentioned components was cast, and the hot slabs obtained were directly or after heating or once cooled and the reheated slabs were hot rolled. At this time, the difference in the properties of the steel between the direct rolling of the hot slab and the rolling after reheating is hardly recognized. In addition, although the reheating temperature is not specified, it is preferable to regulate the temperature in the range of 1,000 to 1,300 ° C in consideration of productivity.

With regard to hot rolling, either a continuous hot rolling process or a normal hot rolling process in which the slabs are connected to each other and rolled in the final rolling can be adopted. Preferably, the finishing temperature at the time of hot rolling should be controlled above the Ar3 transformation temperature in consideration of the productivity and the precision of the steel sheet thickness.

Cooling after hot rolling is carried out in the usual way. In this case, preferably, the coiling temperature must be controlled to 550 ° C. or higher from the viewpoint of productivity. On the other hand, if the coiling temperature is too high, the pickling capability deteriorates, and therefore, the coiling temperature should preferably be controlled to 750 ° C or lower.

A general method may be used for the pickling process or a cold rolling process, and a general method may also be used for the subsequent aluminum plating process or the subsequent aluminum-zinc plating process. In other words, a Si concentration of 5 to 12% in the plating bath is appropriate for the aluminum plating process, and a Zn concentration of 40 to 50% in the plating bath is appropriate for the aluminum-zinc plating process.

At this time, in relation to the atmosphere of the plating process, the plating may be performed using either a continuous plating facility having a non-oxidation furnace or a continuous plating facility without a non-oxidation furnace as long as general conditions are maintained. Thus, no particular control is required in producing the product according to the invention and thus productivity is not impeded.

In the above production conditions, metal pre-plating is not applied to the surface of the steel sheet before plating, but no special problem occurs even if Ni pre-plating, Fe pre-plating or other metal pre-plating to increase the plating property is used.

Further, even if Mg and Zn are included in the aluminum plating layer or Mg is included in the aluminum zinc plating layer, no special problem occurs and steel sheets having similar properties can be produced.

Example 3

First, the reason for regulation of the steel component in Example 3 according to the present invention will be described.

C is an element added to form a structure after cooling made of martensite and obtain material properties, and an amount of addition of 0.15% or more is required to obtain strength of 1,200 MPa or more. On the other hand, when the addition amount is excessive, since the strength against impact deformation is hardly obtained, the upper limit is set to 0.55%.

Si is a solid solution strengthening element and can increase the strength of the steel sheet at a relatively low cost. However, when excessively added, plating property deteriorates, so the upper limit is set to 0.5%.

Mn is added to allow strength to be obtained after cooling at a wide range of cooling rates.

If the amount of C is large but the amount of Mn added is small, the martensitic structure cannot be obtained at a general range of cooling rates, which is a range that can be obtained during press molding, and thus strength is hardly obtained. The cooling rate range mentioned here is less than 500 ° C./sec for 1.4 mm thick steel sheets. In order to exhibit such a function, the addition amount of 0.2% or more is required. On the other hand, if the amount of Mn is too large, not only the cost increases but the effect is saturated. Therefore, an upper limit is set to 3%.

S is inevitably included and is an element that causes deterioration in processability. Therefore, the amount should be reduced as much as possible. The workability problem is eliminated by reducing the amount to 0.04% or less, so the range is determined to 0.04% or less.

P is a solid solution strengthening element and increases the strength of the steel sheet at a relatively low cost. However, if the addition amount is excessively increased, cracks due to embrittlement during hot rolling or cold rolling are generated, so the upper limit is set to 0.1%.

Al is used as a deoxidizer and an Al content of at least 0.005% is required in the steel to be effective. On the other hand, the addition amount exceeding 0.1% increases the oxide-based medium and deteriorates the surface, so the upper limit is set to 0.10%.

Cr and Mo have the effect of changing the state of surface cracking during press molding after high temperature heating by changing the state of alloying the aluminum plating layer during high temperature heating, and these elements are important because they increase the post-paint corrosion resistance of the molded product. Regarding these elements, from the results shown in Figs. 2 and 3 obtained by measuring the addition amount of Cr and Mo in the lab test, the net corrosion resistance of the sample after hot forming and the post-painting corrosion resistance, the inventors found that Cr and Mo are It was found that it should be added. At this time, the net corrosion resistance and post-painting corrosion resistance were evaluated by the method tested under the conditions shown in the examples using a sample taken from the part to which the post-hot forming processing was applied.

With respect to Cr and Mo, respectively, the addition of excessive Cr causes problems of plating property and cost increase, and Mo saturates the effect and raises the cost. Therefore, the upper limit of each element is set to 2.0% and 1.0%, respectively.

Other ingredients are not specifically defined. Ni is inevitably included and the amount is in the general range and no problems arise.

Although the production conditions of the steel sheet according to the present invention are not particularly defined, the following will describe the good production conditions.

The steel with the above-mentioned components was cast, and the hot slabs obtained were directly or after heating or once cooled and the reheated slabs were hot rolled. At this time, the difference in the properties of the steel between the direct rolling of the hot slab and the rolling after reheating is hardly recognized. In addition, although the reheating temperature is not specified, it is preferable to regulate the temperature in the range of 1,000 to 1,300 ° C in consideration of productivity.

With regard to hot rolling, either a continuous hot rolling process or a normal hot rolling process in which the slabs are connected to each other and rolled in the final rolling can be adopted. Preferably, the finishing temperature at the time of hot rolling should be controlled above the Ar3 transformation temperature in consideration of the productivity and the precision of the steel sheet thickness.

Cooling after hot rolling is carried out in the usual way. In this case, preferably, the coiling temperature must be controlled to 550 ° C. or higher from the viewpoint of productivity. On the other hand, if the coiling temperature is too high, the pickling capability deteriorates, and therefore, the coiling temperature should be controlled to 750 ° C or less.

A general method can be used for the pickling process or a cold rolling process, and a general method can also be used for the subsequent aluminum plating process or the subsequent aluminum-zinc plating process. In other words, a Si concentration of 5 to 12% in the plating bath is appropriate for the aluminum plating process, and a Zn concentration of 40 to 50% in the plating bath is appropriate for the aluminum-zinc plating process.

At this time, in relation to the atmosphere of the plating process, the plating may be performed using either a continuous plating facility having a non-oxidation furnace or a continuous plating facility without a non-oxidation furnace as long as general conditions are maintained. Thus, no specific control is required in producing the product according to the invention so productivity is not impeded.

In the above production conditions, metal pre-plating is not applied to the surface of the steel sheet before plating, but no special problem occurs even if Ni pre-plating, Fe pre-plating or other metal pre-plating to improve the plating property is used. Further, even if Mg and Zn are included in the aluminum plating layer or Mg is included in the aluminum zinc plating layer, no special problem occurs and steel sheets having similar properties can be produced.

Example 4

By heating the aluminum alloy layer and converting it to an alloy layer, i.e., a Fe-Al-based coating layer, and controlling the amount of Al to 35% or less, the Fe-Al-based coating layer is converted into Fe + Fe ₃ Al in which Al and Si are dissolved. This results in much better processability and corrosion resistance in the processed part than in the plating layer containing a phase other than Fe + Fe ₃ Al.

C: The steel sheet according to the present invention has a high strength of 1,000 MPa or more after molding, and is obtained by rapidly cooling it after hot pressing to transform the structure into a structure mainly composed of martensite. To do this, a C content of at least 0.05% is essential. On the other hand, even if the C content is increased to 0.7% or more, the strength is saturated and welding cracks are likely to occur, so the upper limit is set to 0.7%.

Steel components other than C are not specifically defined, but elements such as Si, Mn, P, Al, N, Cr, Mo, Ti, Nb, B, Ni, Cu, V, Sn, Sb, etc. are added or inevitable impurity elements. It is named as. These elements can be added if necessary. Examples are as follows. That is, Mn and B are effective for hardenability, Cr, Ti, and Mo are effective for heat resistance of the aluminum plating layer, and Ni and Cu improve corrosion resistance. Preferred addition ranges are as follows. That is, Mn: 0.5 to 3%, Si: 1% or less, P: 0.1% or less, S: 0.1% or less, Al: 0.1% or less, N: 0.01% or less, Cr: 2% or less, Mo: 0.5% or less , Ti: 0.5% or less, Nb: 0.1% or less, B: 0.05% or less, Ni: 1% or less, Cu: 1% or less, V: 0.1% or less, Sn: 0.1% or less and Sb: 0.1% or less.

As described above, the smaller the deposition amount of the aluminum plating, the easier the Fe + Fe ₃ Al region is formed. In this regard, the deposition amount is preferably 120 g / m 2 or less per side.

The aluminum plating method is not particularly limited, and a hot dip plating method, an electrolytic plating method, a vacuum deposition plating method, a clad method, or the like may be used. Currently, the most widely used method in the industry is a hot-dip plating method. In general, Al-10% Si is mainly used in the plating bath, and Fe is included as an unavoidable impurity. In this case, Si enters the alloy layer after heating and the amount of Si may vary depending on the phase structure. However, since the amount of Si is 15% or less in the Fe + Fe ₃ Al region, this value is determined as the upper limit of Si. As other additive elements, Mn, Cr, Mg, Ti, Zn, Sb, Sn, Cu, Ni, Co, In, B, micrometals, etc. are named, and these elements may be applied as long as the plating layer mainly consists of Al. . The addition of Zn and Mg is effective to hardly cause red rust. However, excessive addition of these elements with high vapor pressure causes generation of steam containing Zn and Mg, generation of powdery substances on the surface by Zn and Mg, and the like. Therefore, it is not preferable to add Zn at 60% or more and Mg at 10% or more.

The present invention does not particularly limit the pretreatment and posttreatment of aluminum plating. As the plating pretreatment, preplating such as Ni, Cu, Cr or Fe is designated, and any of these may be adopted. As the post-plating treatment, chromate treatment, resin coating treatment, etc. are named for the purpose of main rust prevention and lubrication, but the organic resin is not good because it disappears when heated. Regarding the chromate treatment, considering the recent restrictions on hexavalent chromium, a treatment film containing trivalent chromium such as electrolyte chromate is preferable. Post-treatments other than inorganic chromate treatments may be applied. For the purpose of lubrication, it is also possible to apply treatments such as aluminum oxide, silicon oxide, and MoS _{2 in} advance.

Example 5

In Example 5, the limitation on the composition of the aluminum-based plating layer will be described. Herein, an aluminum-based plating layer means a plating layer that has not yet been heated and thus has not yet been alloyed.

Si: It is effective to reduce the thickness of an alloy layer of aluminum-based plating (a layer composed of an intermetallic compound formed at the interface between the plating layer and the steel sheet). Si is also effective in forming Mg ₂ Si and increasing corrosion resistance by adding Mg in combination. In order to acquire an effect, it is effective that addition amount is 1% or more. Excessive addition of Si increases the melting point, and when the Si single phase crystallizes in the plating layer, the crystallized Si single phase causes deterioration of corrosion resistance. Therefore, an upper limit is set to 15%.

Mg: Mg is known to be effective in improving corrosion resistance, particularly in salt damaged environments. By forming Mg ₂ Si in particular, the exhaust efficiency to the environment is increased and the corrosion resistance is improved. In order to obtain corrosion resistance, 0.5% or more of Mg is added. Since excessive addition causes severe oxidation on the surface of the plating bath and increases the melting point of the plating bath, the addition amount is determined to be 10% or less. In order to suppress oxidation of the plating bath surface, the addition of a small amount (0.5% or less) of Ca is effective and approximately this amount of Ca is added.

Zn: has a function of suppressing corrosion of the basic steel and the mother steel than the potential of Al or steel. The effect starts to appear when the amount of Zn added is 1% or more, and the best properties are obtained when the amount of Zn is 20 to 60%. However, when an amount exceeding 60% is added, the corrosion resistance is further deteriorated. Therefore, the lower limit and the upper limit of Zn are set to 1% and 60%, respectively.

When the part is substantially produced using the aluminum-based plated steel sheet according to the present invention, the quenching is carried out by rapidly cooling the part using a die after press, so that the tissue is mainly composed of martensite. In order to obtain sufficient strength as a member, the content of martensite is determined to be 60 vol% or less. At this time, the content of martensite is calculated by grinding and etching the specimen, after which the specimen is observed using an optical microscope and subjected to image analysis.

Example 6

In Example 6, the reason why the components and the like of the steel sheet are regulated will be described.

The steel sheet according to the present invention has a soft ferrite layer having a thickness of 2 μm to 1/10 or less of the thickness of the steel sheet at the interface between the intermetallic compound layer formed by alloying the aluminum plating layer and the base steel mainly composed of martensite. Generally, about 1-10% Al is present in the ferrite layer. The ferrite phase may also contain Si in addition to Al. The thickness is limited to 1 μm or more. The reason is that sufficient peeling resistance of the plating layer is not obtained if the thickness of the soft ferrite layer is not larger than the thickness. On the other hand, this layer is soft and, if too thick, the strength of the entire steel sheet is deteriorated. For this reason, the upper limit should be determined to be 1/10 or less of the steel sheet thickness. In general, aluminum plated steel sheets are sometimes produced by a hot dip plating method, in which case an aluminum-based alloy plated layer (referred to as "alloy layer") is likely to grow at the interface between the steel plate and the plated layer. If this layer grows excessively, the workability of the steel sheet is deteriorated, and therefore the aluminum plated steel sheet is produced in a plating bath to which about 10% Si is added. In the present invention, if the aluminum plated steel sheet is processed at high temperature after the entire plating layer is alloyed by heating, Si does not necessarily need to be added to the plating bath, but there is no particular problem even when Si is added.

In the present invention, it is preferable that the hardness of the ferrite layer is not higher than 200 in the beaker hardness. As mentioned above, the ferrite layer serves to prevent the hard alloy layer from peeling off during processing. For this reason, it is preferable that the ferrite layer is soft. However, since the layer contains a solid solution strengthening element such as Al, Si or the like, it is harder than a general ferrite layer.

Next, other components related to the present invention will be described. First, with respect to the steel component, since the present invention aims at improving the strength due to high strength region and quenching, which cannot be obtained with regular TRIP steel or the like, the amount of C is preferably 0.05% or more, preferably 0.1% or more. In connection with other elements of steel, elements such as Si, Mn, Ti, B, Cr, Mo, Al, P, S, N and the like are generally used. Si is effective for fatigue and Mn and B contribute to improvement of hardenability. Ti, Si, Cr, Mo, and Al are elements that improve heat resistance after aluminum plating.

With regard to the configuration of the aluminum plating layer, the aluminum plating layer is mainly composed of Al and may contain Si as described above. As other additive elements, Cr, Mg, Ti, Sb, Sn, Zn and the like are named, and these elements are applicable as long as the plating layer mainly consists of Al. However, Zn has a low boiling point, and when added excessively, Zn generates zinc powder on the surface during heating, which causes wear and tear during press processing. Therefore, the addition amount of 60% or more is not preferable.

FeAl ₃ , Fe ₂ Al ₅ , Fe ₃ Al, Fe ₂ Al ₈ Si, and the like are generated as elements of the aluminum-based alloy plating layer. These phases are easy to form a multi-layered structure in a stacked manner, but the structure of these phases is not particularly restricted. When the composition mainly consists of Al and Fe and Si is added to the aluminum plating bath, about 5 to 10% of Si is contained. These elements are 90% or more of the total composition. In addition, a small amount of Al which is not alloyed may remain, but as long as the amount is small, it does not affect the properties of the aluminum plated steel sheet.

The thickness of the ferrite layer is measured with an optical microscope. The thickness of the ferrite layer is clearly observed by etching the nitrile of about 2% after polishing the cross section. However, there are cases where borders with intermetallic compound layers are rarely observed, in which case EPMA analysis is additionally adopted. By doing so, the ferrite layer can be easily identified from the difference between the irradiation intensity of Fe and the irradiation intensity of Al. The thickness measurement of the ferrite layer is performed by randomly measuring the thickness of about five parts and calculating the average value. In addition, the hair cavity consists of a tissue composed mainly of martensite, which can be observed, for example, by picral etching and using an optical microscope.

7 illustrates an example of a tissue in accordance with the present invention. Two layers are recognized at the interface, and as a result of qualitative analysis using EPMA, the lower layer is a ferrite layer about 5 μm thick. At this time, in FIG. 7, reference numeral 1 denotes a layer composed of Al: 26.85%, Si: 9.83%, Fe: 59.92%, and reference numeral 2 is composed of Al: 49.54%, Si: 3.11%, and Fe: 44.89%. Layer, reference numeral 3 represents a layer composed of Al: 30.75%, Si: 8.88%, Fe: 56.91%, and reference numeral 4 represents a layer composed of Al: 9.59%, Si: 2.92%, Fe: 84.02%. Indicates.

Finally, with regard to the heating and cooling conditions of the plated steel sheet, the heating and cooling methods are not particularly limited. In order to obtain hardened tissue, the cooling rate naturally has a great influence on the hardened tissue, and a cooling rate of about 30 ° C./sec or more is preferred. This depends on the steel component, and for steels with good hardenability, the desired structure, mainly composed of martensite, can be obtained even at a cooling rate of about 10 ° C./sec; As required. Optimum heating conditions are required to obtain a ferrite layer of the desired thickness.

Example 7

In Example 6, the reason why the components and the like of the steel sheet are regulated will be described.

The present invention is a high strength steel sheet having a high strength of about 1,000 PMa or more after molding, and is a steel sheet obtained by transforming the steel sheet into a structure mainly composed of martensite by rapidly cooling the steel sheet after being subjected to a high-temperature press or after rapid heating to quench it. Therefore, the hardness of the hair steel is determined to be 250 or more. Hardness is measured in beaker hardness. In order to obtain a route, the amount of C is preferably 0.05%. With respect to other elements of the steel, no particular limitations have been determined, but elements such as Si, Mn, Ti, B, Cr, Mo, Al, P, S, N and the like are generally used. Si is effective for fatigue, and Mn and B contribute to improvement of hardenability. Ti, Si, Cr, Mo, and Al are elements that improve heat resistance after aluminum plating.

Further, in the present invention, the steel sheet has an aluminum-based alloy plating layer mainly composed of Al and Fe on the surface of the mother steel, and the thickness of the aluminum-based alloy plating layer is limited to 3 to 35 µm, preferably 3 to 18 µm. Restrictions are determined by considering the balance between weldability, post-paint corrosion resistance and heat resistance as described above. That is, if the thickness of the layer is smaller than 3 mu m, sufficient post-painting corrosion resistance and corrosion resistance cannot be obtained. In particular, when Si is not added to the aluminum plating layer, deterioration of heat resistance is obvious. The upper limit of the thickness is determined by spot weldability. The same spot weldability (electrode life) as the galvanized steel sheet can be obtained when the plating thickness is 35 탆 or less and the same spot weldability as the molten alloyed zinc coated steel sheet can be obtained when the plating thickness is 18 탆 or less. FeAl ₃ , Fe ₂ Al ₅ , Fe ₃ Al, Fe ₂ Al ₈ Si, or the like may be formed as the alloy layer. In addition, the formation of an Al-containing ferrite layer is often recognized at the interface between the alloy layer and the parent steel.

In general, in an aluminum plated steel sheet, the Fe—Al-based alloy layer grows easily and deteriorates workability. Therefore, it often happens that Si is added to suppress the growth of the alloy layer and to improve workability. In the case of hot press, the steel sheet does not need to be added specially because the aluminum plating layer is heated and alloyed to the surface and then hot processed. However, Si can of course be added. In the case of rapid heating of a part of the steel sheet, addition of Si is inevitable because processing is applied at room temperature. Cr, Mg, Ti, Sb, Sn, Zn and the like are designated as other additive elements of the aluminum plating layer, and these elements are applicable as long as the plating layer is mainly composed of Al. However, Zn has a low boiling point, and when added excessively, Zn generates zinc powder, which causes wear and tear during press processing. Therefore, the addition amount of 60% or more is not preferable.

The present invention does not particularly limit the pretreatment and posttreatment of the plating, the heating method of the steel sheet during the crimping and the cooling method thereof. As the post-plating treatment, chromate treatment, resin coating treatment and the like are named for the purpose of main rust prevention and lubrication. However, with regard to chromate treatment, considering the recent restrictions on hexavalent chromium, treated films containing trivalent chromium such as electrolyte chromate are good. The resin film treatment is generally effective for improving moldability, and is particularly effective when a part of the steel sheet is rapidly heated after molding. When the steel sheet is heated and molded, the resin film is decomposed and has no effect.

The method for producing aluminum plated steel sheet was not particularly specified. Normal steelmaking conditions and hot rolling conditions may be applied. Aluminum plating is generally performed by a hot dip plating method, but is not limited thereto, and electroplating in a non-aqueous solvent, vapor deposition treatment, and the like can be used. As the plating transition pretreatment, Ni preplating or the like can be named and adopted.

The method of heating and cooling the steel sheet is not particularly limited to any of the above-described methods. Heating methods such as electric heating, furnace heating, high frequency heating, etc. can be adopted. Among these methods, high frequency heating is suitable for rapidly heating a part of the steel sheet.

Example 8

In Example 8, the reason why the components, etc. of the steel sheet are regulated will be described.

As described above, the present invention is a technique of obtaining a desired strength by forming an aluminum plated steel sheet in hot after heating the plated steel sheet and cooling it immediately to quench it, and the components of the steel sheet to allow excellent hardenability to the steel sheet. Required. In order to achieve this condition a C content of at least 0.05%, preferably at least 0.1% is required. In connection with other elements of steel, elements such as Si, Mn, Ti, B, Cr, Mo, Al, P, S, N and the like are generally used. Si is effective for fatigue and Mn and B contribute to improvement of hardenability. Ti, Si, Cr, Mo, and Al are elements that improve heat resistance after aluminum plating.

After heating, elements of an aluminum-based alloy plating layer such as FeAl ₃ , Fe ₂ Al ₅ , Fe ₃ Al, Fe ₂ Al ₈ Si, or the like may be generated on the surface. These phases are easy to form a multilayered structure in a lamination manner, but the structure of these phases is not particularly restricted. When the composition consists mainly of Al and Fe and Si is added to the aluminum plating bath, about 5 to 10% of Si is contained. These elements are 90% or more of the total composition. Further, it is determined that the plated layer is transformed into an aluminum-based alloy plated layer by heating, and then press-processed the aluminum plated steel sheet. This is because if Al remains abundantly on the surface, the weldability and post-paint corrosion resistance deteriorate.

With regard to the heating and cooling conditions of the present invention, the heating method and the cooling method are not particularly limited. Any one of heating in an atmosphere furnace, induction heating, electric heating and the like can be used. At this time, the heating rate is also not limited. The heating rate naturally depends greatly on the thickness and shape of the steel sheet. According to new knowledge, the longer the lag time in the furnace, the better the plating adhesion during subsequent molding. The heating temperature is in the range of approximately 800 to 1,200 ° C., preferably 900 to 1,000 ° C., and it is important to hold the steel sheet in this temperature range for several minutes. However, the retention time depends on the temperature and the retention time is four of A (800 ° C., 13 minutes), B (900 ° C., 6 minutes), C (1,050 ° C., 1.5 minutes), and D (1,200 ° C., 0.3 minutes). It is determined that it must be longer than the point.

However, the long retention time deteriorates the productivity in press molding. Substantially sufficient plating adhesion can be obtained by heating at 950 ° C. for 5-8 minutes whose conditions are determined as a compromise point. Cured tissue is naturally greatly affected by the cooling rate and a cooling rate of 10 ° C. or more is required to obtain the cured structure. This depends on the steel component, and for steel sheets with good hardenability, a desired structure consisting mainly of martensite can be obtained even at a cooling rate of about 10 ° C., in other cases a cooling rate of about 30 ° C. depends on the quality of the steel. As required.

Next, the present invention will be described in detail based on examples.

Example 1

Molten aluminum plating was applied to cold rolled steel sheets (thickness 1.2 mm) and pickled steel sheets (thickness 1.8 mm) as materials produced through the normal hot rolling and cold rolling processes with the steel composition shown in Table 1. In Table 1, numbers 1, 3, 5, 7, and 9 are cold rolled steel sheets and the rest are hot rolled steel sheets. In hot-dip aluminum plating, a non-oxidation furnace-reduction line was used, and the deposition amount of the plating was adjusted to 40 g / m2 per side by the gas wiping method after plating, and then the steel sheet was cooled to zero spangle. ) Received the treatment. At this time, the plating bath composition was Al-10% Si-2% Fe. Fe in the bath is a component inevitably supplied by the plating apparatus and the steel sheet in the bath. The plating appearance was satisfactory without any non-plating defects. The hardenability and heat resistance of the molten aluminum plated steel sheet thus produced were evaluated. The evaluation method is presented below.

As a practical alloying process, an aluminum plated steel sheet was inserted between the dies made of steel in a nitrogen atmosphere, heated at 950 ° C. for 30 minutes and then cooled. The cooling rate was 100 ° C / sec. Peeling of the steel plate plating layer was visually evaluated. In addition, the beaker hardness was measured by applying a load of 100 g in the steel plate cross section. The steel plate was then subjected to commonly used chemical treatments for aluminum plating, steel plate and galvanizing, and was subjected to 20 μm using anionic electrolytic precipitation paint (Nippon Paint Co., Ltd., Powernix 110). After being coated in thickness, baked, cut into crosses, a salt spray test (JIS-z2371) was carried out for 20 days, and the depth of corrosion from the cuts was measured. At this time, the depth of the cross cut was about 50 μm. Therefore, the value obtained by subtracting 50 μm from the recorded value is the intrinsic corrosion depth. In addition, in order to obtain the concentrations of Mn and Cr in the Fe-Al intermetallic compound, Mn was polished in a region up to 5 μm from the surface, quenched in five sections of the cross section, and then quantitatively analyzed for specimens using EPMA. And the amount of Cr was measured. The evaluation values are shown in Table 2.

Heat resistance evaluation formula

○: no peeling

(Triangle | delta): Partial peeling from an edge

X: peeling occurrence

Post-painting corrosion resistance evaluation formula

○: corrosion depth of 80 μm or less

X: corrosion depth greater than 80 µm

(Each value includes the depth of the cross cut.)

If the amount of C is too low as in the ninth steel sheet, the strength is lowered. In general, a value close to the material strength MPa is obtained by multiplying the beaker hardness by three, in which case at most only 600 MPa class strength is obtained. If the addition amount of elements effective for quenching, such as Mn and B, is small as in steel sheet No. 10, the quenching effect is not obtained even if the amount of C is large, and the strength tends to decrease to some extent. In addition, when the value of Ti *: Ti + 0.5 x Mn + Cr + 0.5 x Mo is as low as that of steel sheet 11, the plating layer is peeled off after heating and the heat resistance tends to deteriorate to some extent. In the case of the first to eighth steel sheets in which the amount of element addition in steel is properly controlled, heat resistance and post-paint corrosion resistance show good results.

Next, in Table 1, the seventh steel sheet of this example was plated in a plating bath mainly composed of Al-10% Si-2% Fe and to which Mn and Cr were added. The amount of plating deposited was 60 g / m 2 per side and a good plating appearance was obtained. The steel sheet was heated at 900 ° C. for 2 minutes and then quenched in a die. The cooling rate was 100 ° C / sec. Table 3 shows the amount of Mn and Cr in the bath and the evaluation results. If the amount of Mn and Cr in the intermetallic compound is as small as that of the first steel plate, the corrosion resistance is poor after coating. However, the corrosion resistance is improved when the amount of Mn and Cr is increased in the plating bath. The relationship between the amount of Mn and Cr and post-coating corrosion resistance in the intermetallic compound is shown in FIG. 3. Post-painting corrosion resistance is understood to improve with increasing amounts of Mn and Cr.

Example 2

Steels with various chemical compositions shown in Table 4 were cast, reheated to temperatures in the range of 1,050 to 1,250 ° C., and then hot rolled, pickled, cold rolled and annealed, and plated (aluminum plating or aluminum-zinc plating: galvalnium). ( ^"Galvalium?" plating) holds a, then it was treated with a reduction ratio (reduction rate) of 0.8% skin-pass rolling. Further, as the alloy process, these steel sheet be heated to a temperature of 900 to 1,000 ℃, 5 minutes at this temperature After being held for a while, they were press-molded into a die at room temperature, after which there was an investigation of the properties, which was carried out by cutting the specimen from the rapidly cooled portion of the press and applying a tensile test to it. Silver specimens were cut into No. 5 test specimens according to JIS-z2201 and performed according to the test method described in JIS-z2241. There.

The specimens were cut from the parts that were processed during press molding and the net corrosion resistance and post-processing corrosion resistance were evaluated as the surface properties after hot forming. Net corrosion resistance was assessed by applying a moisture tank test (relative humidity: 95%, temperature: 40 ° C.) to the specimen for 3 days, and post-paint corrosion resistance was tested by salt spray test for 30 days after the specimen was cut crosswise. It evaluated by adding JIS-z2134). An anionic electrolytic precipitation paint was adopted as the paint in this case and the coating thickness was 15 mu m. Net corrosion resistance was judged from appearance using the symbols ○ or ×, and the evaluation formula was × for red rust and ○ for no red rust. Similarly, post-painting corrosion resistance was judged from appearance using ○, Δ or ×, and the evaluation formula was ○ for swelling of 2 mm or less, △ for swelling of greater than 2 mm and 4 mm or less, and 4 mm. It was x about larger swelling. The first to seventh steels are steels having components within the range specified in the present invention, and all steel sheets produced under conditions within the range specified in the present invention were able to obtain high strength after high temperature molding, and also have a net corrosion resistance and There was no problem with post-painting corrosion resistance. At this time, in the case of the seventh steel, the results of the steel sheet produced on the condition that the annealing temperature is outside the range specified in the present invention is also shown, in this case, because the strength of the steel sheet is too high, then no evaluation of the properties was performed . In the case of the eighth and ninth steels, the composition of the steel is outside the range specified in the present invention. As a result, the strength after hot forming is low in the eighth steel, which is one of the objectives of the present invention, and in the case of the ninth steel, the net corrosion resistance and the post-painting corrosion resistance were not obtained.

Example 3

Steels with various chemical compositions shown in Table 6 were cast and reheated to a temperature in the range of 1,050 to 1,250 ° C., then hot rolled, pickled, cold rolled and annealed, followed by plating treatment (aluminum plating or aluminum-zinc plating). The temper rolling process was carried out at the reduction ratio of 0.8%. In addition, as a practical alloying process, these steel plates were heated to a temperature of 900 to 1,000 ° C., held for 5 minutes at this temperature, and then press-molded into a die at room temperature, which was then investigated for properties. Investigation of the material properties was carried out by cutting the specimen from the rapidly cooled portion of the press and subjecting it to a tensile test. The test was performed by cutting the specimen into No. 5 test piece according to JIS-z2201 and following the test method described in JIS-z2241. The results of the evaluation are shown in Table 7.

The specimens were cut from the parts that were processed during press molding and the net corrosion resistance and post-processing corrosion resistance were evaluated as the surface properties after hot forming. Net corrosion resistance was assessed by applying a moisture tank test (relative humidity: 95%, temperature: 40 ° C.) to the specimen for 3 days, and post-paint corrosion resistance was tested by salt spray test for 30 days after the specimen was cut crosswise. It evaluated by adding JIS-z2134). An anionic electrolytic precipitation paint was adopted as the paint in this case and the coating thickness was 15 mu m. Net corrosion resistance was judged from appearance using the symbols ○ or ×, and the evaluation formula was × for red rust and ○ for no red rust. Likewise, post-painting corrosion resistance was judged from appearance using ○, △ or ×, and the evaluation formula was ○ for swelling of less than 2 mm and △ for swelling of less than 2 mm and greater than 4 mm. It was x about swelling. Steels Nos. 1 to 9 are steels having components within the ranges specified in the present invention, and all steel sheets produced under conditions within the ranges specified in the present invention were able to obtain high strength after hot forming, and also have net corrosion resistance and coating. There was no problem with corrosion resistance. At this time, in the case of the tenth steel, the composition of the steel is outside the range specified in the present invention, and thus, net corrosion resistance and post-painting corrosion resistance were not obtained.

Example 4

Molten aluminum plating was applied to cold rolled steel sheets (thickness 1.2 mm) and pickled steel sheets (thickness 1.8 mm) as materials produced through the normal hot rolling and cold rolling processes with the steel composition shown in Table 8. In Table 8, numbers 1, 3, 5 and 7 are cold rolled steel sheets, and the rest are hot rolled steel sheets. In hot-dip aluminum plating, a non-oxidation furnace-reduction line was used, and the deposition amount of the plating was adjusted to 80 g / m2 per side by the gas wiping method after plating, and then the steel plate was cooled and subjected to zero sequin treatment. . At this time, the plating bath composition was Al-10% Si-2% Fe. Fe in the bath is a component inevitably supplied by the plating apparatus and the steel sheet in the bath. The plating appearance was satisfactory without any non-plating defects. The properties are shown in Table 9.

The hardenability and workability of the molten aluminum plated steel sheet thus produced were evaluated. Workability is a de facto alloying process in which an aluminum plated steel sheet is heated at 950 ° C. for 10 minutes in the atmosphere, and then inserted between the dies made of steel to cool it (cooling rate is 30 ° C./sec), and to room temperature. After cooling, an impact test was applied thereto. In addition, beaker hardness in the steel plate cross section was measured by applying a load of 100 g.

Evaluation formula for machinability

○: no peeling

△: crack generation

X: powdery peeling occurrence

If the amount of C is rather low as in the eighth steel plate, the strength is rather low. In general, a value close to the tensile strength (MPa) of the material is obtained by multiplying the beaker hardness by three, in which case at most only 600 MPa class strength is obtained. For the first to seventh steels, both strength and workability show good results. In this case, the amount of Al in the Fe-Al film layer is analyzed qualitatively using EMPA, and the value is about 15%. The value is then obtained by analyzing the five parts of the specimen cross section after quenching to a depth of 10 μm from the surface and averaging the analytical data.

Next, in Table 8, the first steel sheet of this example was plated in a plating bath mainly composed of Al-10% Si-2% Fe and added with Mn and Cr, and the deposition amount of the plating was from 60 g / m < 2 > And up to 200 g / m 2. The samples thus obtained were heated at 950 ° C. under different atmospheres with different retention times, and workability was evaluated by an impact test, and the Al content of the Fe—Al coating layer was measured by the method described in the first item of Example 4. As shown in Table 10, the workability depends on the Al amount of the Fe—Al coating layer, and when the Al amount is 35% or less, good workability is obtained. In addition, the amount of Al in the Fe-Al film layer depends on the amount of deposition and the retention time. The smaller the amount of deposition and the longer the retention time, the more the diffusion progresses and the amount of Al in the Fe-Al film layer is smaller.

Example 5

Molten Al-Si-Mg plating was applied to the cold rolled steel sheet (thickness 1.2 mm) and the pickled steel sheet (thickness 1.8 mm) as the material produced by the normal hot rolling and cold rolling processes with the steel composition shown in Table 11. In Table 11, A, C, E and G are cold rolled steel sheets and the rest are hot rolled steel sheets. In hot dip plating, a non-oxidation furnace-reduction furnace line was used, and the deposition amount of the plating was adjusted to 40 g / m 2 per side by the gas wiping method after plating, and the steel sheet was then cooled and subjected to zero sequin treatment. At this time, the plating bath composition was Al-8% Si-6% Mg-1% Fe-0.1% Ca. Fe in the bath is a component inevitably supplied by the plating apparatus and the steel sheet in the bath. The plating appearance was satisfactory without showing a sequin pattern and without any non-plating defects. The production conditions in this case are shown in Table 12.

The hardenability and workability of the molten aluminum plated steel sheet thus produced were evaluated. The evaluation method is mentioned later. The hot dip galvanized steel was subjected to a 5% tensile stress and then the steel was heated at 950 ° C. for 5 minutes, and as a virtual alloying process, it was then cooled while inserted between the steel sheets. The cooling rate was about 30 ° C./sec. Heat resistance was evaluated by visually observing the specimen after cooling. The net corrosion resistance was then evaluated by applying a moisture tank test (relative humidity: 95%, temperature: 40 ° C.) to the specimen for 3 days, and post-paint corrosion resistance was sprayed with salt for 30 days after the specimen was cut crosswise. It was evaluated by applying the test (JIS-z2134). An anionic electrolytic precipitation paint was adopted as the paint in this case and the coating thickness was 15 mu m. In addition, the beaker hardness of the steel plate was measured by applying a load of 100 g.

Heat resistance evaluation formula

○: good

△: occurs on the crack-shaped pattern surface

×: red scale generation

Net Corrosion Resistance Evaluation Formula

○: good

△: red rust occurs

Post-painting corrosion resistance evaluation formula

◎: paint swelling of 1 mm or less

○: paint swelling of 2 mm or less

(Triangle | delta): Paint swelling of 2-4 mm

X: paint swelling larger than 4 mm

If the amount of C is too low as in the seventh steel sheet, sufficient strength is not obtained. Generally, a value close to the material strength MPa is obtained by multiplying the beaker hardness by three, in which case at most only 800 MPa class strength is obtained. In addition, if the amount of elements effective for quenching such as Mn, B, etc. is small as in the eighth steel sheet, the quenching effect is not obtained even if the amount of C is large. In the case of the first to sixth steel sheets in which the addition amount of elements in the steel is appropriately controlled, both the strength and the corrosion resistance are good results for the Mn-B system or for the Mo-Cr-Ni system.

Then, using the steel E of Table 11, the relationship between the plating composition and the properties of the plated steel sheet was investigated by varying the plating composition of the hot dip plating line. The relationship between the composition of the plated layer and its properties after plating is summarized in Table 13. At this time, the beaker hardness was in each case in the range of 470 to 510.

In the case of the 8th steel plate which is Mg-Zn type, corrosion resistance was bad. On the other hand, Si-Mg-based steel sheets 4 to 7 showed excellent corrosion resistance. Similarly, the first to third steel sheets which are Si-Mg-Zn-based showed excellent corrosion resistance.

Example 6

Molten aluminum plating was applied to the cold rolled steel sheet (thickness 1.2 mm) as a material having the steel composition shown in Table 14 and produced through the normal hot rolling and cold rolling processes. In hot-dip aluminum plating, a non-oxidation furnace-reduction line was used, and the deposition amount of the plating was adjusted to 60 g / m2 per side by the gas wiping method after plating, and then the steel plate was cooled and subjected to zero sequin treatment. . At this time, the plating bath composition was Al-10% Si-2% Fe. Fe in the bath is a component inevitably supplied by the plating apparatus and the steel sheet in the bath. The plating appearance was satisfactory without any non-plating defects. The aluminum plated steel sheet thus produced was heated to 950 ° C., and as a practical alloying process, the workability (peel resistance) of the plated layer was evaluated during air cooling.

At this time, the thickness of the ferrite layer was changed by changing the heating time and heating pattern. The processing was applied by impact test and the peeling state was visually judged by varying the processing temperature during cooling, and the workability of the plating layer was evaluated at the lowest temperature where the plating layer was not peeled off. At this time, in the case of this steel, the hardenability was good even at a cooling rate of 10 ° C./sec, and a structure mainly composed of martensite was obtained even in the case of air cooling. The relationship between the intermediate ferrite layer and the lowest temperature at which good workability in this case can be obtained is shown in FIG.

As shown in Fig. 8, it is understood that the peeling resistance of the plating layer is improved when the thickness of the ferrite layer is 2 µm or more, preferably 4 µm or more. When the thickness of the ferrite layer was about 0.5 mu m, powdery peeling of the plating layer was found even when processing at 800 deg. In addition, as a practical alloying process, the workability of the plating layer depends on the deposition amount of the plating. Even if the thickness of the ferrite layer is 2 μm, when the deposition amount is 30 g / m 2 per side, the lowest temperature at which good processing is obtained is about 500 ° C. In addition, the tissue of the hair cavity was observed by optical microscopy and subsequent image differentiation, and at least 80% of the tissue consisted of martensite under all conditions.

Example 7

Molten aluminum plating was applied to cold rolled steel sheets (thickness 1.2 mm) and pickled steel sheets (thickness 1.8 mm) as materials produced through the normal hot rolling and cold rolling processes with the steel composition shown in Table 15. In Table 15, the numbers 1, 3 and 5 are cold rolled steel sheets and the rest are hot rolled steel sheets. In hot-dip aluminum plating, a non-oxidation furnace-reduction line was used, and the deposition amount of the plating was adjusted to 60 g / m2 per side by the gas wiping method after plating, and then the steel plate was cooled and subjected to zero sequin treatment. . At this time, the plating bath composition was Al-10% Si-2% Fe. The plating appearance was satisfactory without any non-plating defects. The aluminum plated steel thus produced was heated to 950 ° C. and was actually a alloying process which was then press formed when the temperature reached about 600 ° C. while cooling by a water cooled die. The peeling state of the plating layer at the portion where the bending process was applied was visually judged and peeling of the plating layer was not observed for all the steel sheets. At this time, the thickness of the ferrite layer was 10 to 20 μm, and the martensite content in all the hair steels was 80% or more. At this time, the cooling rate was about 150 ° C / sec.

Example 8

Molten aluminum plating was applied to the cold rolled steel sheet (thickness 1.2 mm) as a material produced by the normal hot rolling and cold rolling processes having the steel composition shown in Table 16. In molten aluminum plating, a non-oxidation furnace-reduction line was used, and the deposition amount of the plating was adjusted to 30 to 80 g / m2 per side by the gas wiping method after plating, and then the steel sheet was cooled to zero sequin treatment. Received. The plating bath composition at this time was Al-10% Si-2% Fe. Fe in the bath is a component inevitably supplied by the plating apparatus and the steel sheet in the bath. The plating appearance was satisfactory without any non-plating defects. The hardness after quenching of the aluminum plating layer thus produced, the thickness of the intermetallic compound, weldability, heat resistance, and post-painting corrosion resistance were evaluated. Quenching was applied by heating the steel sheet at 950 ° C. for 0.5-20 minutes in the atmosphere, and then as a practical alloying process, dies were used to squeeze the steel sheet in a flat state and cool it. At this time, the cooling rate was about 300 ° C / sec. By doing so, quenched steel sheets having alloy layers alloyed to the surface with different thicknesses by different deposition amounts and heating times were obtained. At this time, the appearance of all steel sheets was almost uniform. Evaluation method and evaluation formula for property are as follows.

[Hardness]

A beaker hardness at the center of the steel plate cross section was measured by applying a load of 100 g.

[Thickness of intermetallic compound]

The thickness of the intermetallic compound was measured by observing the cross section of the steel sheet using a microscope, then etching the steel sheet with 2% nitrile and then analyzing the structure using EPMA. An example of the analysis is shown in FIG. Reference numeral 1 represents a layer composed of Al: 26.85%, Si: 9.83%, Fe: 59.92%, and reference numeral 2 represents a layer composed of Al: 49.54%, Si: 3.11%, Fe: 44.89%. 3 represents a layer composed of Al: 30.75%, Si: 8.88%, Fe: 56.91%, and reference numeral 4 represents a layer composed of Al: 9.59%, Si: 2.92%, Fe: 84.02%.

[Weldability]

Spot weldability was evaluated on the following conditions.

Electrode: Domed electrode made of aluminum oxide dispersed copper and having an electrode tip of 6φ-40R

Pressure: 600 Kgf

Welding current: 10kA

Welding time: 12 cycles (60 kPa)

Evaluation formula

○: continuous spot welding greater than 2,000 cycles

Δ: 1,200 to 2,000 cycles of continuous spot welding

×: continuous spot welding smaller than 1,200 cycles

[Corrosion resistance after painting]

The steel sheet was subjected to chemical treatment for about 2 minutes in a chemical treatment solution commonly used for aluminum plating and galvanizing the steel sheet, and then coated with an anionic electrolytic precipitation paint to a thickness of 20 μm, and 2 minutes at 140 ° C. Was baked for 20 days and then salt spray tested for 20 days, and post-painting corrosion resistance was judged by the depth of corrosion at the cross cut. At this time, the depth of the cross cut part using the cutter was about 50 micrometers. Therefore, the value minus 50 μm is the true corrosion depth.

The evaluation results are summarized in Table 17.

Evaluation formula

○: corrosion depth of 80 μm or less

X: corrosion depth greater than 80 µm

As shown in Table 17, when the thickness of the alloy layer is small, the steel sheet is excellent in spot weldability but poor in post-painting corrosion resistance. If the thickness is too thin like the first steel sheet, the steel sheet is poor in post-painting corrosion resistance, and if the thickness is too thick as the seventh steel sheet, the steel sheet is poor in spot weldability. Also in the case of the fifth and sixth steel sheets, the steel sheets tend to be poor in spot weldability. Therefore, when spot weldability is considered important, it is desirable to make the thickness of the alloy layer thinner.

Example 9

The steel sheet made of the steel shown in Table 18 was plated with a metal mainly composed of Al-10% Si-2% Fe, and a good plating appearance was obtained. After quenching under the same conditions as in item 1 of Example 7, the thickness of the alloy layer was measured and the value was in the range of 8 to 15 μm. With respect to these steel sheets, the same evaluation items as those in item 1 of Example 7 were evaluated. As a result, all steel sheets obtained evaluation results corresponding to the evaluation grade ○ of Example 1, and showed good weldability and good post-painting corrosion resistance.

Example 10

Molten aluminum plating was applied to the cold rolled steel sheet (thickness 1.2 mm) as a material having the steel composition shown in Table 19 and produced through the normal hot rolling and cold rolling processes. In hot-dip aluminum plating, a non-oxidation furnace-reduction line was used, and the deposition amount of the plating was adjusted to 60 g / m2 per side by the gas wiping method after plating, and then the steel plate was cooled and subjected to zero sequin treatment. . At this time, the plating bath composition was Al-10% Si-2% Fe. Fe in the bath is a component inevitably supplied by the plating apparatus and the steel sheet in the bath. The plating appearance was satisfactory without any non-plating defects. The aluminum plated steel sheet thus produced was heated in air, then held at various temperatures and molded into the shape shown in FIG. At this time, the steel sheet was cooled with a water cooled die. The heating rate was about 5-10 ° C./sec and the cooling rate was about 100 ° C./sec at the higher cooling rate and about 20 ° C./sec at the lower cooling rate although it varies depending on the site. At this time, workability (peel resistance) of the plating layer was evaluated. Peeling of the plating layer was formed in stripe shape or spot shape on the compressed surface. The relationship between the heating conditions and the peeling state of the plating layer is shown in Table 20. In addition, the heating conditions according to the invention are shown in FIG.

Evaluation formula of adhesion

○: no peeling of the plating layer

(Triangle | delta): A crack partly generate | occur | produces in a plating layer

X: peeling generation of the plating layer

As shown in Table 20, adhesion is not yet considered perfect even after 20 minutes of heating when the temperature is as low as 800 ° C. as a de facto alloying process. When heating temperature rose, favorable adhesiveness was obtained by the retention time of 10 minutes or less. Adhesiveness is a de facto alloying process that continues to be insufficient when the retention time is 5 minutes at 900 ° C. or 2 minutes at 1,000 ° C.

As mentioned above, the present invention provides a hot press method for forming high strength automobile parts and will greatly contribute to the weight reduction of future automobiles.

Claims (14)

A high-strength aluminum-based alloy plated steel sheet having excellent post-painting corrosion resistance, characterized by having a Fe—Al-based film containing more than 0.1% of Cr and Mn as a whole on the surface of the steel sheet. C: 0.05 to 0.7%, Si: 0.05 to 1%, Mn: 0.5 to 3%, P: 0.1% or less, S: 0.1% or less, and Al: 0.2% or less, and further Ti: 0.01 to Fe- containing more than 0.1% Cr and Mn as a whole on the surface of the steel containing at least one element (s) selected from 0.8%, Cr: 3% or less, Mo: 1% or less to satisfy the following formula (1): A high strength aluminum-based alloy plated steel sheet excellent in heat resistance and post-painting corrosion resistance, characterized by having an Al coating. [Formula 1] Ti + 0.5 × Mn + Cr + 0.5 × Mo> 0.4 The steel according to claim 2, wherein the steel is N: 0.1% or less, Nb: 0.1% or less, V: 0.1% or less, Ni: 1% or less, Cu: 1% or less, B: 0.0003 to 0.03%, Sn: 0.1 A high strength aluminum-based alloy plated steel sheet excellent in heat resistance and post-painting corrosion resistance, further comprising at least one element selected from% or less and Sb: 0.1% or less. The steel according to claim 1, wherein the steel is C: 0.15 to 0.55%, Si: 0.5% or less, Mn: 0.2 to 3%, P: 0.1% or less, S: 0.04% or less, Al: 0.01 to 0.1% and N as a mass. : 0.01% or less, B: 0.0002 to 0.0050%, Ti: 0.01 to 0.8%, Cr: 2% or less, Mo: 1% or less, Ni: 1% or less, Cu: 0.5% or less and Sn: 0.2% A high strength aluminum-based alloy plated steel sheet excellent in heat resistance and post-painting corrosion resistance, further comprising at least one element (s) selected from the following. The high-strength aluminum-based alloy plated steel sheet according to any one of claims 1 to 4, wherein the Fe-Al-based coating further contains 1 to 20% of Si. The high-strength aluminum-based alloy coated steel sheet according to any one of claims 1 to 5, wherein the Al concentration of the Fe-Al-based coating layer is 35% by weight or less. The heat resistance according to any one of claims 1 to 6, wherein the Fe-Al coating further contains any one or both of Zn: 1 to 50% and Mg: 0.1 to 10%. High strength aluminum alloy plated steel plate with excellent corrosion resistance after painting. The high-strength aluminum-based alloy plated steel sheet according to any one of claims 1 to 7, wherein the Fe-Al-based coating layer has a thickness of 3 to 35 µm. A coating layer composed mainly of Fe-Al on the surface of the steel sheet, a ferrite layer having a thickness of 2 µm or more to 1/10 or less of the steel sheet thickness at the bottom of the coating layer, and a hair steel mainly composed of martensite at the bottom of the ferrite layer. An aluminum-based alloy plated steel sheet for high strength vehicle members. 10. The aluminum-based alloy plated steel sheet for a high strength vehicle member according to claim 9, wherein the ferrite layer contains Si at the bottom of the aluminum-based alloy plating layer and the intermetallic compound layer on the steel sheet surface. The aluminum-based alloy plated steel sheet for high-strength vehicle members according to claim 9 or 10, wherein the hardness of the ferrite phase is 200 or less. High-strength vehicle component, at least a part of which is formed by press molding consisting of the steel according to any one of claims 1 to 11. 13. The high-strength vehicle component according to claim 12, having a film having a thickness of 1 to 200 m on at least a part of the surface. When the vehicle member is produced by hot press using a steel sheet produced by plating steel containing at least 0.05% by weight of C as a steel component with a metal mainly composed of Al, the member is produced by the following areas A, B, C and The hot press method for high-strength vehicle member characterized by being heated by the heating conditions of time side longer than D, and shape | molding by press molding, and at least one part is cooled at a cooling rate of 10 degreeC / sec or more. A (800 ° C, 13 minutes), B (900 ° C, 6 minutes), C (1,050 ° C, 1.5 minutes), D (1,200 ° C, 0.3 minutes)