JP2011032498A - Surface-treated steel sheet for hot pressing and method for manufacturing hot-pressed member using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a surface-treated steel sheet for hot pressing, capable of yielding a hot-pressed member excellent in pitting corrosion resistance, even when exposed to a corrosive environment, and a method for manufacturing the hot-pressed member using the same. <P>SOLUTION: The surface-treated steel sheet for hot pressing has a Zn-Al alloy plating layer containing >50 mass% but ≤60 mass% Al and an inorganic compound having a melting point of 500-1,000°C deposited in this order on the surface of the steel sheet. The amounts of the Zn-Al alloy plating layer and the inorganic compound deposited are 20-100 g/m<SP>2</SP>and 1-5,000 mg/m<SP>2</SP>, respectively, per one side of the steel sheet. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention is for a hot press suitable for a hot press for processing a surface-treated steel plate suitable for an automobile undercarriage member or a vehicle body structural member, particularly a steel plate heated using a die composed of a die and a punch. The present invention relates to a surface-treated steel sheet and a method for producing a hot press member using the same.

  2. Description of the Related Art Conventionally, many undercarriage members and body structure members of automobiles are manufactured by pressing a steel plate having a predetermined strength. In recent years, from the viewpoint of the preservation of the global environment, there has been a strong desire to reduce the weight of automobile bodies, and efforts have been made to increase the strength of steel sheets to be used and to reduce their thickness. However, as the strength of the steel plate increases, the press workability decreases, and it is often difficult to process the steel plate into a desired member shape.

  Therefore, Patent Document 1 describes a process called a hot press that enables both easy processing and high strength by simultaneously processing a heated steel sheet using a die and punch die and simultaneously cooling it. Technology has been proposed. However, in this hot press, the steel sheet is heated to a high temperature of around 950 ° C before hot pressing, so scale (Fe oxide) is generated on the steel sheet surface and the scale peels off during hot pressing. There is a problem that the mold is damaged or the surface of the member after hot pressing is damaged. In addition, the scale remaining on the surface of the member may cause poor appearance, reduced coating film adhesion (paintability), and reduced corrosion resistance. For this reason, the scale on the surface of the member is usually removed by processing such as pickling or shot blasting, but this complicates the manufacturing process and causes a decrease in productivity.

  Therefore, there is a demand for a steel sheet for hot pressing that can suppress the formation of scale during heating before hot pressing and improve the paintability and corrosion resistance of the member after hot pressing, and a plating layer on the surface. Steel sheets provided with a coating such as have been proposed. For example, Patent Document 2 discloses a coated steel sheet coated with Al or an Al alloy. In this coated steel sheet, it is stated that decarburization and oxidation are prevented during heating before hot pressing, and extremely high strength and high corrosion resistance can be obtained after hot pressing. Patent Document 3 describes a Zn-Fe base that prevents corrosion and decarburization and has a lubrication function when hot pressing a steel sheet coated with Zn or a Zn base alloy during heating before hot pressing. A hot pressing method is disclosed in which an alloy compound such as the above compound or a Zn-Fe-Al-based compound is formed on the surface of a steel sheet. It has been shown that a member produced by this hot pressing method, particularly a member using a steel plate coated with Zn-50 to 55 mass% Al, can provide an excellent corrosion prevention effect. Further, Patent Document 4 discloses a hot pressed member in which a plated layer containing a Fe—Zn solid solution phase is formed on the surface of a member after hot pressing using an alloyed hot-dip Zn plated steel sheet. It has been shown that this hot press member can provide excellent hot press workability (coating layer adhesion), corrosion resistance, and weldability.

GB 1490535 Japanese Patent No. 3931251 Japanese Patent No. 3663145 Japanese Patent No. 4039548

  However, in the coated steel sheet coated with Al or Al alloy described in Patent Document 2, the plated layer of Al or Al alloy is damaged at the time of hot pressing, and the thickness of the hot-pressed member when it is exposed to a corrosive environment. There is a problem that corrosion tends to proceed in the direction and the perforated corrosion resistance is lowered. In addition, even in steel sheets coated with Zn or Zn alloy described in Patent Documents 3 and 4, Zn is oxidized during heating before hot pressing, or the plated layer of Zn or Zn alloy is damaged during hot pressing, When these hot press members are also exposed to a corrosive environment, there is a problem that the corrosion resistance perforated decreases.

  It is an object of the present invention to provide a surface-treated steel sheet for hot pressing that can provide excellent perforated corrosion resistance even when the hot pressed member is exposed to a corrosive environment, and a method for producing a hot pressed member using the same. And

  As a result of intensive studies on the surface-treated steel sheet for hot pressing that can provide excellent perforated corrosion resistance even when the hot pressed member is exposed to a corrosive environment, the inventors have obtained the following knowledge.

  i) If an inorganic compound having a melting point of 500 to 1000 ° C. is present on the plating layer and this film is melted during heating before hot pressing, the slidability during hot pressing is improved and the plating layer is damaged. Can be prevented.

  ii) In order to reliably obtain excellent perforated corrosion resistance, in addition to the above inorganic compounds, Zn having a sacrificial anticorrosive effect as a plating layer and a dense corrosion product in a corrosive environment to form water and oxygen And a plating layer containing Al with a barrier effect that suppresses the penetration of corrosion factors such as salt into the plating layer, that is, Zn- containing Al in excess of 50 mass% and 60 mass% or less It is effective to use an Al alloy plating layer.

The present invention has been made on the basis of such knowledge, and in order, on the surface of the steel sheet, a Zn-Al alloy plating layer containing Al of more than 50% by mass and 60% by mass or less and an inorganic compound having a melting point of 500 to 1000 ° C. The Zn-Al alloy plating layer has an adhesion amount per side of 20 to 100 g / m ² , and the inorganic compound has an adhesion amount per side of 1 to 5000 mg / m ^2. A surface-treated steel sheet for hot pressing is provided.

  The present invention also uses the surface-treated steel sheet for hot pressing of the present invention having an inorganic compound having a melting point lower than the heating temperature before hot pressing, and is heated to the heating temperature before hot pressing to perform hot pressing. The manufacturing method of the hot press member which performs is provided.

  According to the present invention, it is possible to provide a surface-treated steel sheet for hot pressing that provides excellent perforated corrosion resistance even when the hot pressed member is exposed to a corrosive environment. The surface-treated steel sheet for hot pressing according to the present invention is suitable for an automobile undercarriage member and a vehicle body structural member manufactured by hot pressing, particularly for a member in which perforation due to corrosion becomes a problem.

1) About the Zn-Al alloy plating layer As described above, when an inorganic compound having a melting point in a specific temperature range is present on the Zn-Al alloy plating layer and this inorganic compound is melted during heating before hot pressing, The slidability during hot pressing is improved, the plating layer is hardly damaged, and the perforated corrosion resistance can be prevented from being lowered.

  However, even if the slidability during hot pressing is improved, depending on the Al content and adhesion amount of the Zn-Al alloy plating layer, the perforated corrosion resistance may be reduced. There is a need to.

  When the Al content of the Zn-Al alloy plating layer is 50% by mass or less, the ratio of Al on the plating layer surface becomes too low, and the coverage of dense corrosion products containing Al decreases, so corrosion in corrosive environments The barrier effect against the permeation of factors is reduced, and the perforated corrosion resistance is reduced. On the other hand, if the Al content exceeds 60% by mass, the Zn ratio on the surface of the plating layer becomes too small, and the sacrificial anticorrosive effect due to Zn decreases, so that the perforated corrosion resistance decreases. Therefore, the Al content of the Zn—Al alloy plating layer is more than 50% by mass and not more than 60% by mass, preferably 52 to 58% by mass. Here, the Al content of the Zn—Al alloy plating layer can be determined by, for example, the following wet analysis method. That is, the entire plating layer having a known adhesion area is dissolved in an aqueous solution obtained by adding 1 g / l of hexamethylenetetramine as an inhibitor to a 6% by mass hydrochloric acid aqueous solution, and the plating adhesion amount is determined from the weight loss at this time. In addition, the Al content is obtained by subjecting the solution collected at this time to atomic absorption analysis or ICP analysis, and the ratio of Al content to the amount of plating deposition (Al content) is calculated.

  The balance other than Al in the plating layer is Zn and inevitable impurities. As unavoidable impurities, Mg: less than 1.0 mass%, Si: less than 1.0 mass%, and Ni: less than 0.1 mass% are acceptable. In this way, Mg: less than 1.0 mass%, Si: less than 1.0 mass%, the adhesion of dross is reduced, the occurrence of cracks in the plating layer during pressing is reduced, and the workability is excellent. is there. Further, Ni: less than 0.1% by mass has an advantage of excellent blackening resistance.

If the adhesion amount per side of the Zn—Al alloy plating layer is less than 20 g / m ² , Zn does not have sufficient sacrificial anticorrosive effect, so sufficient perforated corrosion resistance cannot be obtained. On the other hand, if the adhesion amount per side exceeds 100 g / m ² , a large amount of cracks are generated in the plating layer during pressing, and the perforated corrosion resistance decreases. Therefore, the adhesion amount per one side of the Zn—Al alloy plating layer is 20 to 100 g / m ² , preferably 30 to 90 g / m ² . Here, the adhesion amount of the Zn—Al alloy plating layer can be measured by, for example, the wet analysis method described above.

  A known plating method can be applied to the formation method of the Zn—Al alloy plating layer in the present invention. Especially, the hot dipping method which forms a plating layer on the steel plate surface by immersing the steel plate in the molten Zn-Al plating bath is preferable from the viewpoint of productivity and economy. At this time, the Zn—Al plating bath composition may be adjusted so that the Al content of the Zn—Al alloy plating layer is more than 50 mass% and 60 mass% or less. Further, the amount of deposition of the Zn—Al alloy plating layer can be adjusted by changing the conveying speed of the steel sheet and the flow rate of the wiping gas, as is usually done. In the present invention, the plating layer may be alloyed, but it is preferable not to perform the alloying treatment because the cost increases.

  Examples of the plating method include an electroplating method and a vapor deposition method in addition to such a hot dipping method.

2) Inorganic Compound In the present invention, an inorganic compound having a melting point in a specific temperature range is present on the Zn—Al alloy plating layer in order to improve perforated corrosion resistance. This means that if an inorganic compound having a melting point lower than the temperature at the time of heating before hot pressing is present and this film is melted at the time of heating, the slidability in the hot state can be significantly improved due to the fluid lubrication effect. As a result, damage to the plating layer is reduced, and perforated corrosion resistance is improved. Further, since the slidability is remarkably improved, the plating layer can be prevented from being damaged, so that there is an advantage that the contamination of the mold can be suppressed and the productivity can be improved. In the present invention, the heating temperature before hot pressing is the temperature of the steel plate during heating before hot pressing.

  However, if the melting point of the inorganic compound is less than 500 ° C., the inorganic compound melts at an early stage of the heating process during heating before hot pressing, and the molten inorganic compound tends to adhere to the heating furnace. The amount of the inorganic compound is reduced, the hot slidability is lowered, and the perforated corrosion resistance is lowered. On the other hand, since the temperature during heating before hot pressing is generally around 700-1200 ° C, if the melting point of the inorganic compound exceeds 1000 ° C, the inorganic compound may not melt or not melt sufficiently during heating. The slidability in the hot state is lowered and the perforated corrosion resistance is lowered. Therefore, the melting point of the inorganic compound is 500 to 1000 ° C, preferably 500 to 900 ° C. As the inorganic compound, a compound that melts at that temperature may be appropriately selected according to the temperature at the time of heating before hot pressing. Normally, the steel sheet temperature decreases after the heating and before hot pressing. Therefore, the inorganic compound that has been melted at the time of heating may not be in a molten state when it is actually pressed and slid. Even in this case, the inorganic compound is once melted and is in a softened state, and thus has an effect of improving slidability. However, it is more preferable that the heating temperature before hot pressing be higher by 50 ° C. or higher than the melting point of the inorganic compound because the inorganic compound can be reliably maintained in a molten state during hot pressing.

On the other hand, if the amount of the inorganic compound deposited on one side is less than 1 mg / m ² , the amount of the inorganic compound in the molten state is small, the hot slidability is insufficient, and the perforated corrosion resistance is reduced. On the other hand, if the adhesion amount per side of the inorganic compound exceeds 5000 mg / m ² , not only is it uneconomical because the effect of improving the slidability is saturated, but also many inorganic compounds remain after hot pressing, Inhomogeneous chemical conversion and coating treatment, and perforated corrosion resistance is reduced. Therefore, the adhesion amount per one side of the inorganic compound is 1 to 5000 mg / m ² . Here, the adhesion amount of the inorganic compound can be measured by, for example, the following method. That is, a method of measuring from a weight change before and after applying and drying an inorganic compound, a method of measuring and measuring a weight change by dissolving and removing the inorganic compound with a solution capable of dissolving only the inorganic compound, and the plating layer on which the inorganic compound is adhered to the acid. For example, the dissolved solution is analyzed by atomic absorption analysis or ICP analysis, and the amount of an element serving as a marker is quantified and converted to an inorganic compound amount.

  In addition, when the boiling point and decomposition temperature exist in an inorganic compound, the compound in which those temperature exceeds 1200 degreeC is desirable. This is because if the boiling point or decomposition temperature of the inorganic compound is 1200 ° C. or less, the inorganic compound disappears in the form of gas during heating before hot pressing, or decomposes and does not melt. As an inorganic compound satisfying these conditions, for example, sodium tetraborate decahydrate (borax) having a melting point of 741 ° C. and a boiling point of 1575 ° C. is preferable. However, depending on the heating conditions before hot pressing, this sodium tetraborate decahydrate may change to an anhydride at 350 to 400 ° C and melt at 878 ° C if the temperature is further increased. Therefore, the use of this sodium tetraborate decahydrate is suitable when the heating temperature before hot pressing is 878 ° C. or higher. In addition, examples of inorganic compounds applicable to the present invention include ammonium sulfate, strontium nitrate, antimony (III) oxide, calcium hydroxide, sodium carbonate, and sodium fluoride.

  Furthermore, in order to fix an inorganic compound on a Zn-Al alloy plating layer, it is also possible to coexist an organic compound with an inorganic compound. However, since it is an inorganic compound that has the effect of improving hot slidability, when the inorganic compound and the organic compound coexist, the ratio of the inorganic compound is preferably 50% by mass or more. Even if an organic compound coexists, since heating before hot pressing is performed at 700 to 1200 ° C., the organic compound reacts with oxygen in the atmosphere and disappears as oxygen dioxide and water. There is no effect on the plating layer.

  In order for these inorganic compounds to be present on the plating layer, known methods can be applied. For example, there is a method in which a solution such as an aqueous solution containing an inorganic compound is applied on the Zn—Al alloy plating layer and dried to evaporate moisture and solvent so that the inorganic compound exists on the plating layer. Examples of the coating method include a bar coater method, a brush coating method, a roll coater method, a dipping method, and a spray method.

  In order to hot-press the surface-treated steel sheet for hot pressing according to the present invention, it is necessary to heat under normal conditions before pressing, that is, to 700 to 1200 ° C. As a heating method, an electric furnace or a gas furnace For example, heating by heating, flame heating, energization heating, high frequency heating, induction heating, etc. can be applied.

  In addition, as the steel plate of the material for the surface-treated steel sheet for hot pressing according to the present invention, it is intended to be an automobile suspension member or a vehicle body structural member as a member after hot pressing, so that it is 780 MPa after hot pressing. A steel plate with high hardenability obtained with the above strength, for example, in mass%, C: 0.19 to 0.24%, Si: 0.12% or less, Mn: 1.35 to 1.55%, Al: 0.04% or less, Cr: 0.4 to 0.5% B: A hot-rolled steel plate or a cold-rolled steel plate having a composition containing 0.0015 to 0.0025% and the balance of Fe and inevitable impurities can be used. If necessary, at least one of Ti, Nb, Mo, V, Ni, Cu, and W can be contained.

  As the steel plate of the material, in mass%, C: 0.23%, Si: 0.12%, Mn: 1.5%, Al: 0.03%, Cr: 0.4%, B: 0.0022%, the balance consists of Fe and inevitable impurities A cold-rolled steel sheet having a composition and a thickness of 1.6 mm was used. First, this cold-rolled steel sheet was immersed in a Zn—Al plating bath having a different Al content, and the amount of adhesion was adjusted by gas wiping to form a Zn—Al alloy plating layer on the steel sheet surface. Table 1 shows the Al content in the obtained Zn-Al alloy plating layer and the adhesion amount per one side, and this Al content was the same as the Al content in the Zn-Al plating bath. Next, on the Zn-Al alloy plating layer, after applying an aqueous solution of an inorganic compound having a melting point as shown in Table 1 with a bar coater, drying is performed at 120 ° C. for 10 minutes to make the inorganic compound exist, and surface treatment is performed. Steel plates No. 1 to 35 were produced. Table 1 shows the amount of inorganic compound deposited on one side of the produced surface-treated steel sheet. In addition, the inorganic compound made to exist here does not contain an organic compound but consists only of an inorganic compound.

Cut out a 50mm x 300mm sample from the prepared surface-treated steel sheets No. 1 to 35, heat it at 950 ° C (steel plate temperature) for 10 minutes in an electric furnace in an air atmosphere, then take the sample out of the furnace and immediately use a flat mold The load was adjusted so that the surface pressure was 10 MPa, the sample was pulled out at a drawing speed of 100 mm / sec, and sliding was performed at a high temperature assuming a hot press. At this time, the temperature of the steel plate during sliding was 900 ° C. In addition, about the surface treatment steel plates No. 34 and 35, the heating temperature was changed as follows and hot pressing was performed. That is, for No. 34, the heating temperature before hot pressing was 770 ° C., and for No. 35, the heating temperature before hot pressing was 1045 ° C. The steel plate temperatures during sliding at this time were 720 ° C. and 995 ° C., respectively. Then, a 50 mm × 100 mm sample was cut out from the sample after sliding and subjected to chemical conversion treatment. At this time, the chemical conversion treatment was performed using a chemical conversion treatment solution PB-L3020 manufactured by Nihon Parkerizing Co., Ltd. under conditions adjusted so that the amount of adhesion on the cold-rolled steel sheet was 2.0 to 2.5 g / m ² . . After the chemical conversion treatment sample thus obtained was sealed on the untreated surface and end, a salt spray test in accordance with JIS Z2371-2000 was conducted, and rust was removed after 480 hours. The maximum reduction in thickness caused by the drilling of corrosion was measured. The perforated corrosion resistance was evaluated according to the following criteria, and it was determined that the object of the present invention was achieved if it was ○ or Δ.
○: Maximum thickness reduction ≤ 0.3mm
Δ: 0.3 mm <maximum thickness reduction amount ≦ 0.5 mm
×: 0.5 mm <maximum thickness reduction amount The results are shown in Table 1. It can be seen that the surface-treated steel sheets No. 1 to 17, 34 and 35 for hot pressing according to the present invention are excellent in perforated corrosion resistance.

Claims (2)

On the surface of the steel sheet, in order, having a Zn-Al alloy plating layer containing Al of more than 50% by mass and 60% by mass or less and an inorganic compound having a melting point of 500 to 1000 ° C, per one surface of the Zn-Al alloy plating layer A surface-treated steel sheet for hot pressing, wherein the adhesion amount is 20 to 100 g / m ² and the adhesion amount of the inorganic compound per one side is 1 to 5000 mg / m ² .   The surface-treated steel sheet for hot pressing according to claim 1, which has an inorganic compound having a melting point lower than the heating temperature before hot pressing, and performing hot pressing by heating to the heating temperature before hot pressing The manufacturing method of the hot press member characterized by these.