JP3931859B2 - Galvanized steel for hot forming and hot forming method - Google Patents

Description

The present invention relates to a hot-dip galvanized steel material for hot forming, and more particularly to a hot-dip galvanized steel sheet for hot press and a hot forming method suitable for use in the manufacture of suspensions, chassis, reinforcing parts and the like for automobiles.

  In recent years, in order to reduce the weight of automobiles, efforts have been made to increase the strength of steel sheets and reduce the thickness of steel sheets used. As a technique for press-forming difficult-to-form materials such as high-strength steel plates, hot forming (hot pressing) techniques are being adopted in which the material to be formed is pre-heated and formed.

  Furthermore, it has been proposed to use a steel sheet coated with zinc-based or aluminum-based plating as a hot forming material for the purpose of suppressing oxidation of the base steel surface and / or improving the corrosion resistance of the press-formed product. Examples of using a galvanized steel sheet for hot forming include JP-A-2001-353548, JP-A-2003-73774, and JP-A-2003-147499.

JP 2001-353548 A JP 2003-73774 A JP 2003-147499 A

  When hot-pressing galvanized steel sheets, the steel is usually heated to a high temperature of 700 ° C or higher before forming. Therefore, during this heating, zinc diffuses into the base steel and forms alloy layers and / or solid solutions. Form a phase. At this time, since the alloy layer (more specifically, the intermetallic compound layer) is relatively brittle and easily falls off during forming, it is preferable that substantially only the Fe—Zn solid solution phase is formed as much as possible.

  Further, when an alloy layer and / or a solid solution phase is formed, an oxide film mainly composed of zinc (hereinafter also referred to as a zinc oxide layer) is simultaneously formed on the surface. This oxide film serves to assist lubrication during forming when the steel sheet is in a high temperature state, but may have poor adhesion after cooling. If the adhesion of the oxide film is inferior, as with non-plated steel plates, the oxide film will fall off during pressing and adhere to the mold, reducing the productivity, or the oxidation falling off on the product after pressing. There is a problem that the film remains and the appearance of the product becomes poor. Moreover, when such an oxide film remains, the coating film adhesiveness of a steel plate will be inferior when it coats at the next process.

  As a countermeasure against the surface oxidation problem during hot forming, it is theoretically effective to make both the atmosphere during heating and the entire atmosphere of pressing work non-oxidizing atmosphere, but the cost of equipment and operation is greatly increased. To be high.

  Under such circumstances, in hot forming of galvanized steel sheets, further improvement for preventing exfoliation of an oxide film (zinc oxide layer) during forming is required in practice.

  The present inventors examined the means for solving such problems from various angles.

  First, hot-formed hot-dip galvanized steel sheets with various steel compositions were investigated, and hot press-formed products in which the adhesion of the zinc oxide layer was greatly reduced were investigated.The interface between the base steel sheet and the zinc oxide layer (i.e., A large amount of Si was concentrated in the interface between the Fe—Zn solid solution phase formed by diffusing zinc in the base steel sheet and the zinc oxide layer. It is considered that Si in the steel preferentially oxidizes over other elements and concentrates at the interface, making it easier for the zinc oxide layer to peel off. As a means for suppressing this Si concentration, the method of reducing the Si concentration in steel was the most effective.

  Next, it was found that if the zinc oxide layer formed during press molding is too thick, peeling is promoted. In order to suppress such excessive formation of the zinc oxide layer, the reduction of the P concentration in the steel and / or the increase of the Ti concentration may exert a remarkable effect in combination with the reduction of the Si concentration in the steel. found.

  In addition, in the alloying process of hot dip galvanized steel sheet performed at around 500 ° C, there is a report that zinc diffusion is delayed due to an increase in the amount of P in the steel or diffusion is promoted by an increase in Ti concentration [CAMP-ISIJ vol.3 (1990) 663]. However, the diffusion behavior at a high temperature of 700 ° C. or higher and the influence of the components in the steel, in which zinc oxide is rapidly formed, as in the case of heating during hot forming, were unclear.

In the present invention, the C concentration in the steel is 0 . 15 to 3.0%, a Si concentration is less than 0.02%, preferably in the surface of the steel plate P concentration is less than and / or Ti concentration 0.025% is 0.02% or more, the adhesion amount Has a zinc-based plating film in which the Zn concentration is 30 to 90 g / m ² per side as Zn and the Al concentration in the film is 0.4% or less, and hot forming is performed after heating to 700 ° C. or higher. use galvanized steel plate and its hot forming process. This zinc-based plating film is preferably an alloyed hot-dip galvanizing film having an Fe content of 8 to 15% by mass.

Also according to the invention, the C concentration in the steel in mass% is 0.15-0.23%, the Si concentration is less than 0.02 % and the P concentration is less than 0.025% and / or the Ti concentration. There the surfaces of the steel plate is 0.02% or more, the adhesion amount is per side 30~90g / ^{m 2} as Zn, and the film in Fe concentration 8% to 15%, coating the Al concentration is 0.4% A zinc-based plated steel sheet for hot forming, which has the following zinc-based plating film and performs hot forming after heating to 700 ° C. or higher, and a hot forming method thereof are also provided.

  In conventional hot press molding, in order to remove the oxide film on the surface of the molded product, shot blasting that projects a fine steel ball onto the surface of the molded product at high speed and a liquid containing an abrasive material are sprayed onto the steel surface at a high pressure. The surface oxide film has been removed by liquid honing, surface grinding with a grinding brush, sandpaper application, and the like. When hot forming is performed using the steel material of the present invention, since the adhesion of the formed oxide film is good, the oxide peeling process on the surface of the molded product becomes simple and may be unnecessary in some cases.

  Next, an embodiment of the present invention will be described mainly for steel plates. In the present specification, “%” defining the steel composition and the plating composition is “mass%”.

1. Steel composition
1) Concentration of C in steel: 0.15% or more, preferably 3.0% or less Hot forming is one of the features of softening the material by heating to make it easy to form. The steel is quenched by quenching with, for example, a higher strength molded product. Since the strength of steel after quenching depends mainly on the amount of carbon (C) contained, the C concentration is set according to the required strength. If a high-strength molded product is required, the C concentration should be 0.15% or more. However, when C is excessively contained, the toughness of the molded product may be lowered, so that the content is preferably 3.0% or less.

  Thus, when C is added for the purpose of quenching, it generally becomes hard and molding at room temperature becomes difficult. In the case of such a material having difficulty in formability, the use of hot forming is advantageous as described above, and the usefulness of the present invention is increased. In other words, even if the C concentration at which the strength can be sufficiently increased by quenching is 0.15% or more, if it is hot forming, severe press forming can be performed without hindrance.

2) Si concentration in steel: less than 0.02% As described above, Si in steel is concentrated at the interface between the zinc oxide layer formed by heating and the steel sheet during heating before hot forming, The adhesion of the zinc oxide layer is greatly reduced. In order to ensure adhesion that can withstand severe hot forming and differential thermal expansion during rapid cooling, the Si concentration in the steel is limited to less than 0.02% .

3 ) P concentration: less than 0.025%, Ti concentration: 0.02% or more Another factor that decreases the adhesion of the zinc oxide layer is an increase in the thickness of the zinc oxide layer. When this thickness is formed more than necessary, internal stress is generated in the film due to a difference in expansion coefficient from the steel sheet, which promotes peeling. In order not to increase the thickness of the zinc oxide layer more than necessary during heating before hot forming, it is necessary to incorporate Zn, which is the original plating component, into the surface layer of the steel sheet as a Fe-Zn solid solution phase to suppress oxidation. It is valid.

  In order to facilitate the incorporation of Zn into the steel sheet surface layer, the P concentration in the steel is preferably 0.025% or less, more preferably 0.020% or less, after the Si concentration in the steel is within the above range. For the same reason, the Ti concentration in the steel is preferably 0.02% or more, more preferably 0.03% or more.

2. Plating film
1) Amount of plating adhesion The steel sheet for hot forming according to the present invention is a zinc-based plated steel sheet in which the surface of the steel is coated with zinc-based plating. The adhesion amount of the zinc-based plating is preferably 90 g / m ² or less as Zn per side (the same applies hereinafter).

If the amount of zinc-based plating is too large, Zn in the plating film cannot be sufficiently taken into the base steel plate as a solid solution phase during heating before hot forming, and an excessive zinc oxide layer is formed. As a result, the adhesion decreases. The lower limit of the amount of zinc-based plating is not particularly limited, but if it is too thin, the required corrosion resistance cannot be secured after press forming, or the zinc oxide layer necessary to suppress oxidation of the steel sheet during heating is formed. Usually, it is about 30 g / m ² or more because it becomes impossible. In the case of more severe heating such as a higher heating temperature, the performance is desirably good in the range of 40 to 80 g / m ² .

2 ) Plating composition The composition of the zinc-based plating film is not particularly limited, and even pure zinc plating film contains one or more alloy elements such as Al, Mn, Ni, Cr, Co, Mg, Sn, and Pb. It may be a zinc alloy plating film added in an appropriate amount depending on the purpose. In addition, one or more of Fe, Be, B, Si, P, S, Ti, V, W, Mo, Sb, Cd, Nb, Cu, Sr, etc. that may be inevitably mixed in from raw materials May be contained.

The formation method of the galvanizing is not particularly limited, but the hot dip plating method is advantageous in order to obtain a relatively large coating amount of Zn as 30 g / m ² or more. At this time, the hot-dip galvanized steel sheet manufactured in a continuous hot dip galvanizing line usually has an Fe-Al alloy layer formed at the interface, which is an obstacle to diffusion of zinc into the base steel during hot forming. It can be. Therefore, it is preferable that such an interface alloy layer is not formed thick. Therefore, the Al concentration in the plating film is 0.4% or less, preferably 0.35% or less. On the other hand, in order to suppress the formation of the Fe—Zn alloy layer during plating, the Al concentration of the plating film is preferably 0.15% or more.

  The formation of the interfacial alloy layer is affected by the bath temperature, intrusion material temperature, immersion time, Al concentration in the bath, etc., but since the coating Al concentration is generally higher than the Al concentration in the bath, Al in the bath As a density | concentration, about 0.08 to 0.20% is a suitable range.

  Further, among the hot dip galvanized steel sheets, the alloyed hot dip galvanized steel sheets are excellent because there is much less peeling after hot forming. The alloyed hot-dip galvanized steel sheet has a high melting point of plating and the absence of the above-mentioned interfacial Fe-Al alloy layer, which is advantageous for zinc to diffuse into the base steel and form a solid solution phase. is there. In pure zinc-based plating such as hot-dip galvanized steel sheets, the melting point of the plating is as low as about 420 ° C, so zinc tends to evaporate, and the Fe-Al layer present at the interface hinders Zn diffusion. The main oxide film is easily formed thick.

  In addition, the Fe concentration in the plating film of the galvannealed steel sheet is preferably in the range of 8 to 15%. If the degree of alloying is too low, a pure zinc phase having a low melting point tends to remain on the surface, and a zinc-based oxide film tends to be formed thick.

  On the other hand, the higher the Fe concentration in the plating film (that is, the higher the degree of alloying of the plating), the higher the melting point of the plating, which is advantageous for preventing evaporation during hot forming. From this viewpoint, the preferable Fe concentration in the plating film is 13% or more. However, the Fe concentration of a general galvannealed steel sheet is controlled to be low (8 to 13%) due to powdering problems. For this reason, when trying to increase the Fe concentration for hot forming steel, it is necessary to make galvannealed steel sheets (changing alloying heat treatment conditions), which adversely affects operability and product yield. Effect.

  In the alloyed hot-dip galvanized steel sheet, when the Al concentration in the plating bath is high, micro unevenness such that a part of the plating film after the alloying treatment is missing is easily formed. In this case, a sufficient zinc oxide layer is not locally formed at the time of hot forming, and the zinc oxide layer is easily peeled off starting from the location. Also in this sense, the Al concentration in the bath is set to a low level where the Al concentration in the plating film is 0.4% or less, preferably 0.35% or less.

3. Hot forming In the hot forming of the zinc-based plated steel sheet of the present invention, the material is usually heated to about 700 to 1000 ° C, and then press forming is performed. Depending on the type (composition) of the steel plate or the strength required for the molded product, heating at a slightly lower temperature may be sufficient.

  Examples of the heating method include heating by an electric furnace or a gas furnace, flame heating, energization heating, high-frequency heating, induction heating, and the like. If you want to achieve quenching of the material by heating at this time, after heating to the quenching temperature that achieves the target hardness (usually about 700 to 1000 ° C), hold the temperature for a certain period of time, For example, press molding is performed using a mold through a water-cooled tube, and at that time, the mold is rapidly cooled by contact with the mold. Of course, the product characteristics after hot pressing may be controlled by heating the press mold and changing the quenching temperature or the cooling rate.

  As a form of hot forming of a steel material, press forming of a plate material is common, but there are also bending processing, drawing forming, stretch forming, hole expansion forming, flange forming and the like. Therefore, the form of the steel material may be a bar material, a wire material, a pipe material, etc. in addition to the plate material (steel plate).

  In this example, a cold rolled steel sheet having a composition shown in Table 1 and having a thickness of 1.2 mm was used as a plating base material, and hot dip plating was performed using a vertical hot dip plating simulator. The plating conditions were changed within a range of 25 to 100 g / m as a bath temperature of 460 ° C., an Al concentration in the bath of 0.1 to 0.3%, and a plating adhesion amount per side of Zn: Zn. Some of them were subjected to alloying treatment after hot dipping at 510 ° C. for various treatment times.

  Next, these specimens were heated at 950 ° C. for 5 minutes in a heating furnace in an air atmosphere. In this example, the temperature of the steel plate reached 900 ° C. in approximately 2 minutes. It was taken out from the heating furnace, and the cylindrical drawing was hot-formed at a high temperature as it was. The hot forming conditions were a drawing height of 25 mm, a shoulder radius R5 mm, a blank diameter of 90 mm, a punch diameter of 50 mm, and a die diameter of 53.5 mm.

  The adhesion state of the upper layer film after hot forming was evaluated as follows from the area ratio of the peeled piece adhered to the tape after peeling the side surface after the cylindrical drawing.

A: Peel area ratio is less than 5%,
○: peeling area ratio is 5% or more and less than 20%,
Δ: peeling area ratio is 20% or more and less than 30%,
X: Peeling area ratio is 30% or more.

  In addition, the powdering resistance at room temperature was also evaluated. For the anti-powdering test, a tape is attached to the evaluation surface in advance, and a 60 ° bend with the evaluation surface inside is performed with a jig with a tip of 5 mmR. After bending back, the width of the black part of the peeled tape is measured. And evaluated as follows.

Black width less than 2mm is ◎, 2-4mm is ◯, 4-6mm is △, more than 6mm is X;
These test results are shown in Table 2.

  In Table 2, Nos. 1 to 20 are examples of hot dip galvanized steel sheets (hereinafter referred to as GI), and Nos. 21 to 24 are examples of galvannealed steel sheets (hereinafter referred to as GA).

  First, regarding GI, those within the scope of the present invention had a peel area of less than 30% in the film adhesion after hot forming. Furthermore, steel with less P concentration (No. 4) and steel with higher Ti (No. 5) have better film adhesion, and those satisfying both (No. 6, 7) are even better. Met. On the other hand, the steel with an excessive amount of Si (No. 3), the one with insufficient or excessive plating adhesion (No. 8, 14), and the one with an excessive amount of Al in the plating film (No. 20) The film adhesion was poor.

  Next, when GA was used, compared with GI of the same steel type, film adhesion was remarkably excellent. However, as the degree of alloying increased (that is, the Fe% in the film increased), the powdering resistance tended to deteriorate.

Claims (5)

The C concentration in the steel is 0.15 to 3.0%, the Si concentration is less than 0.02%, and the P concentration is less than 0.025% and / or the Ti concentration is 0.02% or more. on the surface of the steel plate is the coating weight is per side 30~90g / ^{m 2} as Zn, and the film in the Al concentration has a zinc-based plating film is not more than 0.4%, 700 ° C. prior to molding Hot-worked zinc-based plated steel sheet heated above.   The zinc-based plated steel sheet according to claim 1, wherein the zinc-based plated film is an alloyed hot-dip galvanized film having an Fe content of 8 to 15% by mass. In steel, the C concentration in the steel is 0.15-0.23%, the Si concentration is less than 0.02%, the P concentration is less than 0.025%, and / or the Ti concentration is 0.02% or more. on the surface of a steel plate, the adhesion amount is per side 30~90g / ^{m 2} as Zn, and the film in Fe concentration 8% to 15%, zinc-based plated coating film in the Al concentration is 0.4% or less A galvanized steel sheet for hot forming that is heated to 700 ° C. or higher prior to forming. The C concentration in the steel is 0.15 to 3.0%, the Si concentration is less than 0.02%, and the P concentration is less than 0.025% and / or the Ti concentration is 0.02% or more. on the surface of the steel plate is the coating weight is per side 30 to 90 g / ^{m 2} as Zn, and 700 a galvanized steel sheet which film in Al concentration having a zinc-based plating film is not more than 0.4% A method for hot forming of a steel sheet, in which hot forming is performed after heating to a temperature of ℃ or higher.   The method for hot forming a steel sheet according to claim 4, wherein the zinc-based plating film of the galvanized steel sheet is an alloyed hot-dip galvanized film having an Fe content of 8 to 15% by mass.