TW201837206A - Surface-treated steel sheet suitably serving as a material of a molded article excellent in fatigue characteristics, spot weldability, and corrosion resistance after coating - Google Patents

Abstract

Disclosed is a surface-treated steel sheet, which is provided with a base material and a plating layer that is formed on the surface of the base material, wherein the average composition of the plating layer contains, in mass%, 0.5-2.0% of Mg and satisfies the equations of 60.0 ≤ Zn + Al ≤ 98.0, 0.4 ≤ Zn/Al ≤ 1.5 and Zn/Al×Mg ≤ 1.6.

Description

 Surface treated steel  

The present invention relates to a surface treated steel sheet.

Structural members (molded bodies) used in automobiles and the like are manufactured by hot stamping (hot press molding) in order to improve strength and dimensional accuracy. When the formed body is produced by hot stamping, the steel sheet is heated to Ac ₃ or more, and is pressed by a die and quenched. That is, in this production, press working and hardening are simultaneously performed. According to the hot stamping, the dimensional accuracy is high, and a high-strength molded body can be manufactured.

On the other hand, since the molded body produced by hot stamping is processed at a high temperature, scale is formed on the surface. Therefore, a technique of suppressing the formation of scale by using a coated steel sheet (surface-treated steel sheet) as a steel sheet for hot stamping and further improving corrosion resistance has been proposed (see Patent Documents 1 to 3).

For example, Patent Document 1 discloses a steel sheet for hot stamping (molding) in which a Zn plating layer is formed. Further, Patent Document 2 discloses an aluminum-coated steel sheet for high-strength automotive members in which an Al plating layer is formed. Further, in Patent Document 3, a Zn-based coated steel material for hot stamping of various elements such as Mn is added to a plating layer of a Zn-coated steel sheet.

 [Practical Technical Literature]    [Patent Document]  

[Patent Document 1] Japanese Patent Laid-Open Publication No. 2003-73774

[Patent Document 2] Japanese Patent Laid-Open Publication No. 2003-49256

[Patent Document 3] Japanese Patent Laid-Open Publication No. 2005-113233

In the technique of Patent Document 1, since Zn remains on the surface layer of the steel after hot stamping, a high corrosion resistance can be expected. However, since Zn is processed in a molten state, molten Zn may intrude into the steel sheet, and cracking may occur inside the steel material. This crack is called Liquid Metal Embrittlement (hereinafter also referred to as "LME"). Moreover, LME deteriorates the fatigue characteristics of the molded body.

Further, in the current situation, in order to avoid the occurrence of LME, it is necessary to appropriately control the heating conditions at the time of steel sheet processing. Specifically, a method in which all of the molten Zn is diffused in the steel sheet until it becomes a Fe—Zn solid solution is used. However, for these methods, it takes a long time to heat, and as a result, there is a problem that productivity is lowered.

Further, in the technique of Patent Document 2, since the coating layer is made of Al having a higher melting point than Zn, the molten metal of Patent Document 1 is less likely to intrude into the steel sheet. Therefore, it is predicted that excellent LME resistance can be obtained, and the molded article after hot stamping is excellent in fatigue characteristics. However, in the steel material in which the Al plating layer is formed, it is difficult to form a phosphate film during the phosphate treatment performed before the coating of the automobile member. In other words, depending on the steel material, there is a possibility that the phosphate treatment property cannot be sufficiently obtained, and the corrosion resistance after coating is lowered.

Further, in the technique of Patent Document 3, the outermost layer (oxide film) after hot stamping is modified to improve spot weldability, but depending on the added element, LME may occur, and hot stamping may not be sufficiently obtained. The fatigue properties of steel. Further, depending on the added elements, not only the fatigue characteristics of the steel but also the phosphate treatment property may be lowered.

The present invention has been made in view of the above problems, and an object of the invention is to provide a surface-treated steel sheet having optimum material properties of a molded article having excellent fatigue properties, spot weldability, and corrosion resistance after coating.

The present invention has been made in order to solve the above problems, and the following surface-treated steel sheets are basically produced.

(1) A surface-treated steel sheet comprising a base material and a plating layer formed on a surface of the base material, wherein a mass % of the average composition of the plating layer contains Mg: 0.5 to 2.0%, and satisfies the following (i) to Iii).

75.0≦Zn+Al≦98.5. . . (i)

0.4≦Zn/Al≦1.5. . . (ii)

Zn/Al×Mg≦1.6. . . (iii)

However, the element symbol in the above formula indicates the content (% by mass) of each element contained in the plating layer.

(2) The surface-treated steel sheet according to the above (1), wherein the mass % of the average composition of the plating layer further contains Si: more than 0% and 15.0% or less.

(3) The surface-treated steel sheet according to the above (1) or (2), wherein the average composition of the plating layer further satisfies the following formula (iv).

Mg+Ca+Ti+Sr+Cr≦2.0. . . (iv)

However, the element symbol in the above formula indicates the content (% by mass) of each element contained in the plating layer.

The surface-treated steel sheet according to any one of the above aspects, wherein the coating layer has an Fe diffusion layer on the side of the base material in the plating layer, and the thickness of the Fe diffusion layer is The ratio of the overall thickness of the coating layer is 15 to 50%.

(5) The surface-treated steel sheet according to the above (4), wherein the mass % of the average composition of the plating layer further contains Fe: 5.0 to 25.0%.

(6) The surface-treated steel sheet according to any one of the above aspects, wherein the mass % of the chemical composition of the base material is C: 0.05 to 0.4%, Si: 0.5% or less, and Mn : 0.5~2.5%.

(7) The surface-treated steel sheet according to any one of the above (1) to (6), which is used for hot stamping.

When the surface-treated steel sheet of the present invention is subjected to hot stamping, a molded body excellent in fatigue characteristics, spot weldability, and corrosion resistance after coating can be obtained.

[Fig. 1] An example of an image in which a cross section of a surface-treated steel sheet according to an embodiment of the present invention is observed by SEM.

The present inventors have reviewed the structure of the surface-treated steel sheet which is excellent in LME resistance at the time of hot stamping, and which is the material of the molded body excellent in spot weldability and corrosion resistance after coating after hot stamping.

First, the present inventors reviewed the method of improving the corrosion resistance of the molded body after coating. As a result, it was found that the corrosion resistance of the molded body after hot stamping can be improved by including Mg in the plating layer of the surface-treated steel sheet. However, it is also known that when hot-stamped a surface-treated steel sheet containing Mg in a plating layer, LME is likely to occur, and fatigue characteristics are deteriorated. In addition, when the Mg content in the plating layer is excessive, the spot weldability of the molded body to be produced is also lowered.

Therefore, the inventors of the present invention have intensively reviewed methods for improving corrosion resistance without deteriorating LME resistance and spot weldability. As a result, it has been found that by appropriately managing the Mg content in the plating layer of the surface-treated steel sheet, it is possible to ensure that all of the above-described characteristics are well balanced.

The present invention is based on the above knowledge. Hereinafter, each requirement of the present invention will be described in detail.

 (A) Overall composition  

A surface-treated steel sheet according to an embodiment of the present invention includes a base material and a plating layer formed on a surface of the base material. For each, as detailed below.

 (B) base metal  

The problem of the present embodiment is that the fatigue characteristics, the spot weldability, and the corrosion resistance after the hot stamping are improved by the configuration of the plating layer of the surface-treated steel sheet. Therefore, the base material of the surface-treated steel sheet of the present embodiment is not particularly limited. However, the component of the base material is in the range described below, and in addition to fatigue properties, spot weldability, and corrosion resistance after coating, a molded article having optimum mechanical properties can be obtained.

The reason for limiting each element is as follows. In addition, in the following description, "%" of the content means "mass%".

 C: 0.05~0.4%  

Carbon (C) is an element which improves the strength of the molded body after hot stamping. If the C content is too small, the above effects cannot be obtained. On the other hand, if the C content is excessive, the toughness of the steel sheet will decrease. Therefore, the C content is set to 0.05 to 0.4%. The C content is preferably 0.10% or more, more preferably 0.13% or more. Further, the C content is preferably 0.35% or less.

 Si: 0.5% or less  

Niobium (Si) is an element that is inevitably contained and has an action of deacidifying steel. However, when the Si content is excessive, Si in the steel is diffused during heating by hot stamping, and an oxide is formed on the surface of the steel sheet to deteriorate the phosphate treatment property. Si, the steel sheet is further increased element Ac ₃ point, then Ac ₃ point rise, the heating temperature during the hot stamping may be beyond evaporation temperature Zn coating. Therefore, the Si content is set to 0.5% or less. The Si content is preferably 0.3% or less, more preferably 0.2% or less. From the viewpoint of the performance of the above-mentioned product, the lower limit of the Si content is not limited, but since it is used for the purpose of deacidification described above, the lower limit is substantially present. According to the desired deacidification level, it is usually 0.05%.

 Mn: 0.5~2.5%  

Manganese (Mn) is an element which improves the hardenability and improves the strength of the molded body after hot stamping. If the Mn content is too small, this effect cannot be obtained. On the other hand, if the Mn content is excessive, the effect is saturated. Therefore, the Mn content is set to 0.5 to 2.5%. The Mn content is preferably 0.6% or more, more preferably 0.7% or more. Further, the Mn content is preferably 2.4% or less, more preferably 2.3% or less.

 P: 0.03% or less  

Phosphorus (P) is an impurity contained in steel. P segregates at the grain boundary to lower the toughness of the steel and to reduce the resistance to delay. Therefore, the P content is set to 0.03% or less. The P content is preferably as small as possible.

 S: 0.01% or less  

Sulfur (S) is an impurity contained in steel. S will form sulfides to lower the toughness of the steel and degrade the resistance to delay. Therefore, the S content is set to 0.01% or less. The S content is preferably as small as possible.

 sol.Al: 0.1% or less  

Aluminum (Al) is generally used in the deacidification of steel and is inevitably contained. However, when the Al content is excessive, the deacidification proceeds sufficiently, but the Ac ₃ point of the steel sheet rises, and the heating temperature during hot stamping may exceed the evaporation temperature of the Zn plating film. Therefore, the Al content is set to 0.1% or less. The Al content is preferably 0.05% or less. In order to obtain the above effects, the Al content is preferably 0.01% or more. In addition, in the present specification, the Al content is a content of sol. Al (acid-soluble Al).

 N: 0.01% or less  

Nitrogen (N) is an impurity that is inevitably contained in steel. N will form nitrides to reduce the toughness of the steel. N is a case where B is further contained in the steel, and B is combined with B to reduce the amount of solid solution B, and further, the hardenability is lowered. Therefore, the N content is set to 0.01% or less. The N content is preferably reduced as much as possible.

 B: 0~0.005%  

Boron (B) has an effect of improving the hardenability of steel and improving the strength of the molded body after hot stamping, and therefore may be contained as needed. However, if the B content is excessive, the effect will be saturated. Therefore, the B content is set to 0.005% or less. In order to obtain the above effects, the B content is preferably 0.0001% or more.

 Ti: 0~0.1%  

Titanium (Ti) combines with N to form a nitride. When Ti and N are combined as described above, the combination of B and N is suppressed, and the decrease in the hardenability due to the formation of BN can be suppressed. Therefore, it is also possible to contain Ti as needed. However, if the Ti content is excessive, the above effect is saturated, and further, the Ti nitride is excessively precipitated, and the toughness of the steel is lowered. Therefore, the Ti content is set to be 0.1% or less. Further, Ti is made of a pin fixing effect, and the austenite grain size at the time of hot stamping heating is made fine, thereby improving the toughness and the like of the molded body. In order to obtain the above effects, the Ti content is preferably 0.01% or more.

 Cr: 0~0.5%  

Since chromium (Cr) has an effect of improving the hardenability of steel, it may be contained as needed. However, if the Cr content is excessive, Cr carbide is formed. This Cr carbide is difficult to dissolve during hot stamping, so that austenitization is difficult and the hardenability is lowered. Therefore, the Cr content is set to 0.5% or less. In order to obtain the above effects, the Cr content is preferably 0.1% or more.

 Mo: 0~0.5%  

Since molybdenum (Mo) has an effect of improving the hardenability of steel, it may be contained as needed. However, if the Mo content is excessive, the above effects are saturated. Therefore, the Mo content is set to 0.5% or less. In order to obtain the above effects, the Mo content is preferably 0.05% or more.

 Nb: 0~0.1%  

Niobium (Nb) has an effect of refining crystal grains during hot stamping to improve the toughness of steel, and may be contained as necessary. However, if the Nb content is excessive, the above effects are not only saturated, but the hardenability is also lowered. Therefore, the Nb content is set to 0.1% or less. In order to obtain the above effects, the Nb content is preferably 0.02% or more.

 Ni: 0~1.0%  

Nickel (Ni) has an effect of improving the toughness of steel. Ni is further suppressing embrittlement caused by the presence of molten Zn during heating in hot stamping. Therefore, it is also possible to contain Ni as needed. However, if the Ni content is excessive, the effects of these will be saturated. Therefore, the Ni content is set to 1.0% or less. In order to obtain the above effects, the Ni content is preferably 0.1% or more.

In the chemical composition of the base material constituting the surface-treated steel sheet of the present embodiment, the residual portion is Fe and impurities. Here, the term "impurity" refers to a component that is contained in an ore or a waste material as a raw material, or a component that is mixed in a manufacturing environment, etc., which is not intentionally added. meaning.

 (C) coating layer  

In the plating layer in the present invention, Zn and Al are mainly used. That is, the average composition of the plating layer satisfies the following formula (i). The coating layer of the surface-treated steel sheet can improve the fatigue characteristics, spot weldability, and corrosion resistance after coating of the molded body after hot stamping by satisfying the following conditions.

75.0≦Zn+Al≦98.5. . . (i)

However, the element symbol in the above formula indicates the content (% by mass) of each element contained in the plating layer.

And the ratio of Zn and Al also becomes important. Therefore, the average composition of the plating layer of the present invention satisfies the following formula (ii). When the value of Zn/Al is less than 0.4, the phosphate treatment property cannot be ensured, and the corrosion resistance after coating is deteriorated. Further, when the value of Zn/Al is more than 1.5, LME cannot be suppressed, and fatigue characteristics are deteriorated. The value of Zn/Al is preferably 1.2 or less, more preferably 1.0 or less, and still more preferably 0.8 or less.

0.4≦Zn/Al≦1.5. . . (ii)

Further in the present invention, the mass % of the average composition of the plating layer contains Mg: 0.5 to 2.0%. When the Mg content in the plating layer is less than 0.5%, the effect of improving the corrosion resistance of the molded body after hot stamping is insufficient. On the other hand, if the Mg content is more than 2.0%, the risk of occurrence of LME during hot stamping increases. Further, since Mg is easily oxidized, the surface layer of the formed body after hot stamping is concentrated as an oxide. When the oxide of Mg is high in resistance, if it is excessively concentrated, the weldability of the molded body is deteriorated. The Mg content in the plating layer is preferably 0.6% or more, more preferably 0.8% or more. Further, the Mg content is preferably 1.8% or less, more preferably 1.5% or less.

Further, the Mg content in the plating layer is also required to be adjusted in relation to the content of Zn and Al. Specifically, it is necessary to satisfy the following formula (iii). When the value of Zn/Al×Mg exceeds 1.6, LME cannot be suppressed, and fatigue characteristics are deteriorated. The value of Zn/Al×Mg is preferably 1.4 or less, more preferably 1.2 or less, and still more preferably 1.0 or less.

Zn/Al×Mg≦1.6. . . (iii)

The mass % of the average composition of the plating layer may further contain Si: more than 0% and 15.0% or less. By including Si in the plating layer, the adhesion between the base material and the plating layer can be improved. On the other hand, when the Si content in the plating layer is more than 15.0%, the properties such as corrosion resistance and weldability of the molded body after hot stamping may not be secured. The Si content is preferably 0.1% or more, more preferably 0.3% or more.

When the Si content in the plating layer is increased, the formation of the Fe diffusion layer to be described later is suppressed. Therefore, in the case where the formation of the Fe diffusion layer is to be promoted, the Si content is preferably 10.0% or less, more preferably 5.0% or less.

Further, Cr, Ca, Sr, Ti, or the like may be contained in the plating layer. However, since these elements are easily oxidized as in the case of Mg, the surface layer of the molded body after hot stamping is concentrated as an oxide. Since these oxides are also high in electrical resistance, if the concentration is excessively concentrated, the weldability of the molded body is deteriorated. Therefore, when these elements are contained in the plating layer, the average composition of the plating layer is in the relationship of the Mg content, and it is preferable to satisfy the following formula (iv).

Mg+Ca+Ti+Sr+Cr≦2.0. . . (iv)

Here, in the present invention, the average composition of the plating layer is obtained by the following method. First, the surface-treated steel sheet containing the plating layer was dissolved in a 10% aqueous HCl solution. At this time, in order to dissolve only the plating layer, an inhibitor that suppresses dissolution of Fe in the base material is added to hydrochloric acid. Further, each element to be contained in the solution was measured by induced-integrated plasma luminescence spectrometry (ICP-OES).

The plating layer in the present invention preferably has an Fe diffusion layer on the side of the base material in the plating layer. The Fe diffusion layer is composed of a structure mainly composed of an Fe-Al-Zn phase. The Fe-Al-Zn phase is a main component, and means that the total area ratio of the Fe-Al-Zn phase is 90% or more. The total area ratio of the Fe-Al-Zn phase is preferably 95% or more, and more preferably 99% or more. The Fe-Al-Zn phase of the present invention is a general term for Fe(Al, Zn) ₂ , Fe ₂ (Al, Zn) ₅ or Fe (Al, Zn) ₃ . In particular, the Fe content in the Fe diffusion layer is in the range of 20 to 55% by mass. Further, in the above Fe-Al-Zn phase, Si may be contained.

In the case where the surface-treated steel sheet is cold worked, the Fe diffusion layer may become a starting point of cracking if it exists. Therefore, in general, the Fe diffusion layer is preferably not formed to be preferable. However, when the surface-treated steel sheet is hot-pressed, if the Fe-diffusion layer mainly composed of the Fe-Al-Zn phase in the coating layer exists, the alloying of Zn and Al in the coating layer during hot stamping is promoted. The Fe-Al alloy is rapidly formed. Since the formation of the Fe-Al alloy is promoted particularly in the vicinity of the interface with the base material, the effect of the LME is suppressed. Further, in the present invention, the Fe-Al alloy is a general term for α Fe, Fe ₃ Al, and FeAl.

In order to obtain the above effects, the ratio of the thickness of the Fe diffusion layer of the present invention to the overall thickness of the plating layer is preferably 15 to 50%. When the above ratio is less than 15%, the effect of suppressing LME cannot be sufficiently obtained. On the other hand, if the above ratio is more than 50%, cracking may occur when the steel sheet is spirally wound up. The ratio of the thickness of the Fe diffusion layer to the overall thickness of the plating layer is preferably 20% or more, more preferably 25% or more. Further, the ratio of the thickness of the Fe diffusion layer is preferably 45% or less, more preferably 40% or less.

Fig. 1 is an example of an image in which a cross section of a surface-treated steel sheet according to an embodiment of the present invention is observed by SEM. Moreover, the first figure (a) is an example in which the Fe diffusion layer is actively formed and used under the conditions of the plating treatment. On the other hand, Fig. 1(b) shows an example in which the plating treatment is performed under normal conditions. As can be understood from Fig. 1, the boundary between the Fe diffusion layer in the plating layer and the layers other than the coating layer can be clearly observed.

In addition, as a result of the EPMA analysis of the coating layer, it was confirmed that the Fe content of the Fe diffusion layer was 20% or more, and the Fe-Al-Zn phase in the range of 20 to 55% by mass was mainly composed. Moreover, in the other layers, it is less than 20%. Therefore, in the present invention, the overall thickness of the plating layer and the thickness of the Fe diffusion layer are measured from the results of EPMA analysis and SEM observation. Further, in the present invention, the coating film is observed from the cross section by SEM, and the entire thickness of the plating layer and the thickness of the Fe diffusion layer are measured at arbitrary 12 points, and the average value of the measured values at 10 points except the maximum and minimum is taken as Each thickness is used.

Further, the overall thickness of the plating layer of the present invention is not particularly limited, and may be, for example, 5 to 40 μm. The overall thickness of the plating layer is preferably 10 μm or more, and more preferably 30 μm or less. Further, the thickness of the Fe diffusion layer is not particularly limited, and when it is desired to obtain an effect of suppressing LME, 3 μm or more is preferable. On the other hand, when the thickness of the steel sheet is excessively wound, the steel sheet may be wound in a spiral shape, and cracking may occur, so that it is preferably 10 μm or less.

Further, when the Fe diffusion layer is sufficiently formed and the effect of suppressing LME is desired, the mass % of the average composition of the plating layer further preferably contains Fe: 5.0 to 25.0%.

 (D) Manufacturing method  

In the process of manufacturing the surface-treated steel sheet of the present embodiment, a process of producing a base material and a process of forming a plating layer on the surface of the base material are included. The following is a detailed description of each process.

 [base metal manufacturing process]  

In the production of the base material, the base material of the surface-treated steel sheet is produced. For example, a molten steel having the above chemical composition is used, and the molten steel is used to manufacture a thick plate by a casting method or a block by a block forming method. Next, the base material (hot-rolled sheet) of the surface-treated steel sheet can be obtained by calendering the thick plate or the block by heat. Further, the cold-rolled sheet obtained by subjecting the heat-expanding sheet to pickling treatment and cold-rolling the heat-expandable sheet after the pickling treatment may be used as a base material of the surface-treated steel sheet.

 [coating process]  

In the coating treatment process, an Al-Zn-Mg plating layer is formed on the surface of the above-mentioned base material, and a surface-treated steel sheet is produced. The method of forming the plating layer may be any treatment such as a melt coating treatment, a spray coating treatment, or a vapor deposition coating treatment. In order to improve the adhesion between the base material and the plating layer, it is preferable to contain Si in the plating layer.

For example, the formation example of the Al-Zn-Mg plating layer produced by the melt plating treatment is as follows. That is, the base material is immersed in a molten plating bath composed of Al, Zn, Mg, and impurities, and a plating layer is adhered to the surface of the base material. Next, the base material to which the plating layer was attached was pulled up from the plating bath.

In this process, the thickness of the coating layer can be adjusted by appropriately adjusting the pulling speed of the steel sheet from the coating bath and the flow rate of the scrubbed gas. As described above, the overall thickness of the plating layer is preferably adjusted to be 5 to 40 μm.

Further, in the case where the above-described Fe diffusion layer is to be formed in the plating layer, it is important to control the Si content, the immersion time, and the cooling rate after immersion in the plating bath during the plating treatment. Specifically, in order to promote the formation of the Fe diffusion layer, as described above, the Si content in the plating bath is necessarily low.

Further, the coating bath is immersed for 5 sec or more, and further, after being pulled up from the coating bath, heat is applied or heated, and the average cooling rate is suppressed to 30 ° C / s or less to sufficiently diffuse Fe. However, if the thickness of the Fe diffusion layer is excessive, the immersion time in the coating bath is 15 s or less, and the average cooling rate after immersion is 5 ° C/s or more. good.

Therefore, when the Fe diffusion layer is formed in the plating layer, and the ratio of the thickness of the Fe diffusion layer to the overall thickness of the plating layer is to be adjusted to a range of 15 to 50%, the immersion time to the coating bath is 5~. For 15 s, the average cooling rate after immersion is preferably 5 to 30 ° C / s or less.

 (E) Hot stamping conditions  

By applying hot stamping to the surface-treated steel sheet of the present invention, a molded body excellent in fatigue characteristics, spot weldability, and corrosion resistance after coating can be obtained. By performing hot stamping under the conditions described below, it is possible to obtain a molded article excellent in the above characteristics more reliably. Further, before the hot stamping, the rust preventive oil film forming treatment and the masking processing may be performed as needed.

 [hot stamping process]  

The usual hot stamping is carried out by thermally heating the steel sheet to a stamping temperature range (hot working temperature range), followed by hot working and further cooling. According to the usual hot stamping technique, in order to shorten the manufacturing time, the heating rate of the steel sheet is increased as much as possible. In addition, when the steel sheet is heated to the hot stamping temperature range, since the alloying of the plating layer is sufficiently performed, the normal hot stamping technique does not pay attention to the control of the heating conditions of the steel sheet.

However, in order to obtain the molded article excellent in the above characteristics more satisfactorily, it is preferable to carry out the alloying heat treatment for maintaining the predetermined temperature range for a certain period of time when the surface-treated steel sheet is heated up to the hot stamping temperature. Then, after the alloying heat treatment is applied, the heat is applied to the hot stamping temperature (hardening heating temperature), and hot working and cooling are performed.

Specifically, first, the surface-treated steel sheet is placed in a heating furnace (a gas furnace, an electric furnace, an infrared furnace, or the like). In the heating furnace, the surface-treated steel sheet is heated to a temperature range of 500 to 750 ° C, and alloying heat treatment is maintained for 10 to 450 s in this temperature range. By performing the alloying heat treatment, Fe in the base material is diffused in the plating layer, and alloying is performed. By this alloying, it is possible to suppress LME. Further, the alloying heating temperature is not necessarily constant, and it may be varied within the range of 500 to 750 °C.

After the alloying heat treatment is completed, the surface-treated steel sheet is heated to a temperature range of Ac ₃ to 950 ° C, followed by hot working. At this time, the temperature of the surface-treated steel sheet in the temperature range of Ac ₃ to 950 ° C (oxidation temperature range) is limited to 60 s or less. When the temperature of the surface-treated steel sheet is within the oxidation temperature range, the oxide film of the surface layer of the plating layer grows. When the temperature of the surface-treated steel sheet in the oxidation temperature range exceeds 60 s, the oxide film may be too long, and there is a concern that the weldability of the molded body is lowered. On the other hand, since the rate of formation of the oxide film is extremely fast, the lower limit of the time in which the temperature of the surface-treated steel sheet is within the oxidation temperature range is more than 0 s. However, when the surface-treated steel sheet is heated in a non-oxidizing atmosphere such as a 100% nitrogen atmosphere, since the oxide film is not formed, the heating is performed in an oxidizing atmosphere such as an air atmosphere.

When the temperature of the surface-treated steel sheet in the oxidation temperature range is 60 s or less, the conditions such as the heating rate and the maximum heating temperature are not particularly limited, and various conditions for hot stamping can be selected.

Next, the surface-treated steel sheet taken out from the heating furnace was press-formed using a mold. In this process, the steel sheet is hardened by a mold at the same time as this press forming. A cooling medium (for example, water) is circulated in the mold, and the heat of the surface-treated steel sheet is promoted by the mold to be hardened. By the above process, a molded body can be produced.

Further, an example of a method of heating the surface-treated steel sheet using a heating furnace has been described, but it may be heated by electric heating. In this case, the steel sheet is also heated for a predetermined period of time by energization heating, and the steel sheet is press-formed using a mold.

 [Anti-rust oil film formation process]  

The rust-preventing oil film formation process may be carried out arbitrarily in the production method after the coating process, and before the hot-pressing process, the rust-preventing oil is applied to the surface of the surface-treated steel sheet to form an rust-preventing oil film. When the time from the surface-treated steel sheet to the hot stamping is long, the surface of the surface-treated steel sheet may be oxidized. However, since the surface of the surface-treated steel sheet in which the rust-preventing oil film is formed by the rust-preventing oil film formation process is difficult to oxidize, the rust-preventing oil film is formed, and the formation of the scale of the molded body can be suppressed. Further, the method for forming the rust preventive oil film may be any known technique.

 [shadowing process]  

This process is a process in which the surface-treated steel sheet is subjected to shearing and/or punching after the rust-preventing oil film forming process and before the hot stamping process, and the steel sheet is formed into a specific shape. The sheared section of the steel sheet after the masking process is easily oxidized. However, when the rust preventive oil film is formed in advance on the surface of the steel sheet, the rust preventive oil in the above-mentioned sheared section is also enlarged to some extent. Thereby, oxidation of the steel sheet after the masking process can be suppressed.

Although an embodiment of the present invention has been described above, the above embodiment is merely an exemplification of the present invention. Therefore, the present invention is not limited to the above-described embodiments, and modifications may be appropriately made without departing from the spirit and scope of the invention.

Hereinafter, the present invention will be more specifically illustrated by the examples, but the present invention is not limited to the examples.

 [Example 1]  

First, prepare the base metal. That is, a thick plate was produced by a continuous casting method using the molten steel of the chemical composition shown in Table 1. Next, the hot-rolled steel sheet was rolled by hot rolling, and the hot-rolled steel sheet was further pickled, and then cold-rolled to produce a cold-rolled steel sheet. Further, this cold rolled steel sheet was used as a base material (thickness: 1.4 mm) of the surface-treated steel sheet.

Then, the base material thus produced was subjected to a plating treatment according to the conditions shown in Table 2, and the surface-treated steel sheets of the respective test examples were produced.

The measurement of the average composition of the coating layer of the obtained surface-treated steel sheet was performed. In the measurement, first, the surface-treated steel sheet containing the plating layer was dissolved in a 10% aqueous HCl solution. At this time, in order to dissolve only the plating layer, an inhibitor that suppresses dissolution of Fe in the base material is added to hydrochloric acid. Further, each element to be contained in the solution was measured by ICP-OES.

Further, the cross section of the surface-treated steel sheet was cut out, and the overall thickness of the plating layer and the thickness of the Fe diffusion layer were measured by SEM observation. The measurement results of these are shown in Table 3.

Then, the surface-treated steel sheets of the respective test examples were subjected to a hot-bend V-bend test, a spot weldability evaluation test, and a post-coating corrosion resistance evaluation test as shown below.

 [Hot V bending test]  

The surface-treated steel sheets of the respective test examples were subjected to alloying heat treatment at 700 ° C for 120 s, and then heated at 900 ° C for 30 s, and immediately subjected to hot-bending V-bending processing to form a molded body using three types of manual presses. Further, the shape of the mold is a shape in which the outer portion of the bending radius generated by the V-bending process is bent, and each has a shape extending by 10%, 15%, and 20%.

Then, the presence of the LME was observed by observing the reflected electron image using the SEM and the reflected electron sensor in the thickness direction cross section of the V-bending portion of the molded body. Further, the case where the cracking progresses to the base material (where the Fe concentration is 98% or more) occurs as LME. In the evaluation of the LME resistance by the hot-bend V-bend test, the crack-free one was evaluated as excellent (1) when 20% extended, and cracked when 20% extended, but the crack-free one was evaluated as good when 15% extended. 2), rupture occurred at 15% extension, but no crack was evaluated when 10% extension was (3), and rupture occurred at 10% extension (4).

Further, when the end position of the crack is determined to be difficult in the above observation, energy dispersive X-ray analysis (EDS) is performed on the surrounding area of the crack end position by using an energy dispersive X-ray microanalyzer. To determine whether the crack is extended until the base material. In this case, the field in which the total content of Al and Zn is more than 0.5% is used as a plating layer, and the field inside the steel material is considered as a base material.

 [Spot weldability evaluation test]  

The surface-treated steel sheets of the respective test examples were subjected to alloying heat treatment at 700 ° C for 120 s, and then heated at 900 ° C for 30 s. Immediately, the steel sheets were poured into a flat mold having a water-cooling box to produce a plate-shaped molded body. In addition, the portion where the cooling rate at the time of hot stamping is slow is also hardened to a cooling rate of 50 ° C / s or more until the tempering of the transition point of the granulated iron in the field (410 ° C).

For these molded bodies, spot welding was performed using a DC power source and a pressing force of 350 kgf. The test is performed by various welding currents, and the value of the fusing diameter of the welded portion exceeding 4.7 mm is set as the lower limit value, and the value of the welding current is preferably increased, and the value of dust generation during welding is set as the upper limit. Further, the value between the upper limit value and the lower limit value is appropriately set to the current range, and the difference between the upper limit value and the lower limit value is used as an index of spot weldability. In the evaluation of spot weldability, the value of 1.5A or more was evaluated as excellent (1), and those of 1.0A or more and less than 1.5A were evaluated as good (2), and those of 0.5A or more and less than 1.0A were evaluated. The evaluation was (3), and those less than 0.5 A were evaluated as not (4).

 [Testing for corrosion resistance after painting]  

The surface-treated steel sheets of the respective test examples were subjected to alloying heat treatment at 700 ° C for 120 s, and then heated at 900 ° C for 30 s. Immediately, the steel sheets were poured into a flat mold having a water-cooling box to produce a plate-shaped molded body. In addition, the portion where the cooling rate at the time of hot stamping is slow is also hardened to a cooling rate of 50° C./s or more until the tempering of the granulated iron at the metamorphic starting point (410° C.).

Further, for each molded body, a surface conditioning treatment agent (trade name: Prepalene X) manufactured by Paccarat Co., Ltd., Japan, was used, and the surface was adjusted to room temperature at 20 s. Next, a phosphate treatment was carried out using a zinc phosphate treatment liquid (trade name: PALBOND3020) manufactured by Paccarat Co., Ltd., Japan. Specifically, the temperature of the treatment liquid was set to 43 ° C, and the molded body was immersed in the treatment liquid for 120 s. Thereby, a phosphate coating is formed on the surface of the steel material.

After the above-mentioned phosphate treatment was carried out, a cationic plating coating material manufactured by Nippon Paint Co., Ltd. was electroplated and applied by a voltage of 160 V for each molded body, and further baked at a baking temperature of 170 ° C for 20 minutes. Baked and painted. The film thickness control of the coating material after electroplating was performed by the surface-treated steel sheet before hot stamping, and the electroplating coating was carried out under conditions of 15 μm.

The composite body after electroplating was cross-cut until it reached the steel material of the material, and a composite corrosion test (JASO M610 cycle) was performed. The corrosion resistance was evaluated from the coating expansion width, and the coating expansion width after the implementation of the 180-cycle composite corrosion test was 2.0 mm or less was evaluated as excellent (1), and those exceeding 2.0 mm and 3.0 mm or less were evaluated as good (2). In the case of more than 3.0 mm and 4.0 mm or less, it was evaluated as (3), and those exceeding 4.0 mm were evaluated as not (4).

 [Evaluation results]  

In the present invention, an object of the present invention is to provide a surface-treated steel sheet which is excellent in material of a molded article which is excellent in all the balances of fatigue properties (LME resistance), spot weldability, and corrosion resistance after coating. Therefore, the evaluation results of these are considered together, and any one of the tests is excellent or good, and the evaluation of any one of them is unacceptable. The test had an unacceptable total evaluation C and was set as unqualified. The results of those are shown in Table 4.

As is apparent from Table 4, it was confirmed that the surface-treated steel sheet of the present invention was used as a material and hot-stamped under suitable conditions to obtain fatigue resistance (LME resistance), spot weldability, and resistance after coating. A well-formed and excellent molded body of all corrosive properties.

 [Industrial Availability]  

When the surface-treated steel sheet of the present invention is subjected to hot stamping, a molded body excellent in fatigue properties, spot weldability, and corrosion resistance after coating can be obtained. Therefore, the molded article of the surface-treated steel sheet of the present invention can be optimally used as a structural member used in automobiles and the like.

Claims (7)

 A surface-treated steel sheet comprising a base material and a plating layer formed on a surface of the base material, wherein the mass % of the average composition of the plating layer contains Mg: 0.5 to 2.0%, and satisfies the following (i) to (iii) ), 75.0 ≦ Zn + Al ≦ 98.5. . . (i) 0.4≦Zn/Al≦1.5. . . (ii) Zn/Al×Mg≦1.6. . . (iii) However, the element symbol in the above formula indicates the content (% by mass) of each element contained in the plating layer.    The surface-treated steel sheet according to claim 1, wherein the mass % of the average composition of the plating layer further contains Si: more than 0% and 15.0% or less.    The surface-treated steel sheet according to claim 1 or 2, wherein the average composition of the coating layer further satisfies the following formula (iv), Mg+Ca+Ti+Sr+Cr≦2.0. . . (iv) However, the element symbol in the above formula indicates the content (% by mass) of each element contained in the plating layer.    The surface-treated steel sheet according to any one of claims 1 to 3, wherein the coating layer has an Fe diffusion layer on a side of the base material in the plating layer, and a thickness of the Fe diffusion layer on the plating layer The ratio of the overall thickness is 15 to 50%.    The surface-treated steel sheet according to claim 4, wherein the mass % of the average composition of the plating layer further contains Fe: 5.0 to 25.0%.    The surface-treated steel sheet according to any one of claims 1 to 5, wherein the mass % of the chemical composition of the base material contains C: 0.05 to 0.4%, Si: 0.5% or less, and Mn: 0.5 to 2.5%. .    The surface-treated steel sheet according to any one of claims 1 to 6, which is used for hot stamping.