JP2014014834A - Method of manufacturing high strength steel formed member - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stably impart excellent resistance weldability and coating adhesion to a high strength steel formed member obtained by hot press-forming a galvanized steel sheet.SOLUTION: The galvanized steel sheet is heated to a temperature range of a hardenable temperature or more, the press-forming of the heated galvanized steel sheet is started in the temperature range of the hardenable temperature or more and the heated galvanized steel sheet is quickly cooled in a metal mold in parallel to the press forming or immediately after the press forming, thereby obtaining a formed member, and a surface of this formed member is ground off by projecting media having an average particle diameter of 90 μm or less as a shot bullet by a shot projection device. The high strength steel formed member is manufactured in such a manner that the obtained formed member has a zinc oxide layer having a thickness of 15 g/mor less in an upper part of an iron zinc solid solution phase of a member surface, and has surface roughness of 7 μm or less in a roughness index Rp of JIS regulation.

Description

  The present invention relates to a method for manufacturing a high-strength steel molded member, and specifically relates to a method for manufacturing a high-strength steel molded member having both high resistance weldability and good paint adhesion.

  In recent years, the practical application of a technique for producing a high-strength steel molded member (hot press-formed product) by hot-pressing a forming material made of a zinc-based plated steel sheet has been promoted.

  This high-strength steel forming member is often joined to a product by resistance welding, for example, spot welding, and users of high-strength steel forming members now have resistance weldability of the obtained high-strength steel forming members. There is a demand for improvement.

  Patent Document 1 does not intend to improve resistance weldability of a high-strength steel molded member, but has a zinc-based plating layer having a thickness of 1 to 50 μm including an iron-zinc solid solution phase on the surface of this zinc. A hot press-molded product is disclosed that has high strength and excellent post-painting corrosion resistance and paint adhesion, because the average thickness of the zinc oxide layer present on the system plating layer is 2 μm or less.

International Publication No. 2004/094684 Pamphlet

  However, up to now, including Patent Document 1, little study has been made to improve the resistance weldability of hot press-formed products. On the other hand, users have demanded higher performance than ever before, and as one of them, a high-strength steel molded member having both high resistance weldability and good paint adhesion has come to be demanded.

  An object of the present invention is to provide a method for producing a high-strength steel molded member having both high resistance weldability and good paint adhesion.

  As a result of intensive studies to solve the above problems, the present inventors have found that (i) the resistance weldability of a hot press-formed product is greatly affected by the conductivity of its surface, (ii) When the surface of the press-formed product is abraded by shot blasting or the like, the resistance weldability of the hot press-formed product is improved. Furthermore, (iv) depending on the polishing conditions of the hot press-formed product, It was found that the paint adhesion of molded products deteriorated. The present invention is based on these findings (i) to (iv).

The present invention includes a heating step of heating the zinc-based plated steel sheet to a temperature range higher than or equal to the quenchable temperature, and starting the press molding of the heated zinc-based plated steel sheet at a temperature range equal to or higher than the quenchable temperature. In parallel or immediately after press molding, a hot pressing step for rapidly cooling in a mold to obtain a molded member, and a polishing step for cleaning the surface of the molded member obtained in the hot pressing step, The molded member obtained through the polishing process has a zinc oxide layer having a thickness of 15 g / m ² or less on the iron zinc solid solution phase on the surface of the member, and a roughness index Rp defined by JIS is 7 μm or less. It is a manufacturing method of the high strength steel forming member characterized by having the surface roughness which is.

  In the present invention, it is preferable that the surface of the molded member is polished by projecting a medium having an average particle diameter of 90 μm or less as a shot bullet on the surface of the molded member with a shot projection device.

  According to the present invention, excellent resistance weldability and paint adhesion can be stably imparted to a high-strength steel molded member obtained by hot press forming a zinc-based plated steel sheet.

The form for implementing this invention is demonstrated for every process.
(1) Heating process The zinc-based plated steel sheet is heated to a temperature range above the quenchable temperature.

The coating amount of the galvanized steel sheet only needs to be coated to such an extent that iron scale is not formed on the surface by the heating process, and for that purpose, it is 20 g / m ² or more. Furthermore, in recent years, high corrosion resistance of high-strength steel molded members is often required, and for that purpose, it is 40 g / m ² or more, more preferably 60 g / m ² or more. If the amount of plating is too small, the corrosion resistance may be less than the required level depending on the part.

On the other hand, in order to obtain the later-described surface of the molded member after the later-described polishing step, it is necessary to sufficiently advance the alloying reaction of iron zinc in the heating step. From this point of view, the amount of plating adhesion should be small, preferably 100 g / m ² or less, and more preferably 70 g / m ² or less.

  In order to obtain such a comparatively thick galvanized steel sheet, hot dip galvanization is more cost effective than electrogalvanization. Furthermore, the alloyed hot-dip galvanized steel sheet is faster than the hot-dip galvanized steel sheet, and the interdiffusion between the iron and zinc proceeds more rapidly in the heating process, and finally the formed member without the iron-zinc intermetallic compound phase exists. It is preferable because it is easily obtained.

  As described above, galvanized steel sheets (including galvannealed steel sheets) usually contain a small amount of Al in the plating film. This is because, in hot dip galvanizing, in order to suppress the diffusion of iron zinc at the plating-base steel interface and to control the dross amount of the plating bath, it is usually 0.1 to 0.2% in the plating bath. This is because the front and rear Al are added. As described above, when the amount of Al contained in the iron zinc solid solution phase and the oxide film (zinc oxide layer) of the molded member is excessive, the coating adhesion of the high strength steel molded member is deteriorated. For this reason, it is preferable that Al content in the plating film of a hot dip galvanized steel plate or an alloyed hot dip galvanized steel plate is 0.40% or less.

  When an alloyed hot-dip galvanized steel sheet is used as a processing material for a high-strength steel forming member, it is preferable that the Fe content in the plating film is relatively high. This is because, as described above, the intermetallic compound phase is extinguished by heating for a relatively short time to form a surface layer structure composed of only the zinc oxide layer and the iron zinc solid solution phase therebelow. However, it is not practical to excessively increase the Fe content in the plating film because, in some cases, it is necessary to extremely reduce the line speed during the production of the hot dip plated steel sheet. The preferable range of the Fe content of the plating film is 10 to 20%, more preferably 10 to 15%.

  Although it is generally disadvantageous in terms of cost, it is also possible to use an electrogalvanized steel sheet as a processing material for a high-strength steel forming member. Electroplated steel sheets have the advantage that the plating film usually does not contain Al. The electroplated steel sheet may be an ordinary pure galvanized steel sheet, but is preferably a zinc-iron alloy plated steel sheet (the Fe content of the plating film is 20% or less) or a zinc-8-18% nickel alloy plated steel sheet. In addition to the advantage of not containing Al, the zinc-iron alloy electroplated steel sheet, like the alloyed hot-dip galvanized steel sheet, forms a solid solution phase in a relatively short time and does not leave intermetallic compounds. There is an advantage that can be.

  In the heating step, the galvanized steel sheet is heated to a predetermined temperature in a temperature range that can be quenched. Although heating temperature is based also on the steel component of a plated steel plate, it is about 700-1000 degreeC normally, Preferably it is 850-950 degreeC. If the heating temperature of the galvanized steel sheet is too high, the film thickness of the oxide film layer becomes too large, the surface non-uniformity becomes excessive, and the time required for the polishing treatment for obtaining a uniform surface becomes long. On the other hand, if the heating temperature is too low, it becomes difficult to quench, and depending on the material, the softening becomes insufficient, and an excessive press pressure is required during hot press forming, resulting in defects on the surface of the high-strength steel forming member. It may be a cause of molding defects such as generation or material breakage.

  The heating means is not particularly limited. Usually, a zinc-based plated steel sheet as a work material is heated in an oxidizing atmosphere by a gas furnace or an electric furnace. The oxidizing atmosphere may be air, but may also be air and / or a mixed gas of oxygen and another gas (eg, nitrogen, combustion gas, etc.). When heated in such an oxidizing atmosphere, since the zinc oxide layer is formed on the surface of the plating layer, the amount of zinc evaporation during heating can be suppressed. On the other hand, as will be described later, in order to sufficiently advance the alloying reaction, the zinc oxide layer grows thick accordingly and is not preferable for weldability. Therefore, it is particularly effective to have a polishing step described later. .

  Furthermore, in this heating step, the alloying reaction between the plating layer and the base steel is sufficiently advanced. If the alloying reaction is insufficient, zinc in the liquid phase (or zinc-iron alloy) exists in the surface layer portion of the plated steel sheet before the start of hot press forming, and is obtained by pressing in this state. It is considered that molten zinc cracks occur in the formed member. Therefore, in the heating process, an iron-zinc solid solution phase (structure in which Zn is dissolved in α-Fe) is formed on the surface layer portion of the high-strength steel formed member after hot press forming, and the zinc-iron-based structure is formed. The alloying reaction is sufficiently advanced to such an extent that no intermetallic compound phase exists.

  In order to sufficiently advance the alloying reaction, in addition to the heating temperature described above, the heating time, the heating rate, and the like are also affected. If the heating time is extremely short, for example, several seconds, it is difficult to sufficiently form a film in which the alloying reaction does not proceed easily. When heating a steel plate at room temperature in an electric furnace or a gas furnace, the preferred heating time is 4 to 6 minutes, although it depends on the heating temperature and the dimensions of the plated steel plate.

(2) Hot pressing process Press-molding of the zinc-based plated steel sheet heated in the heating process is started in a temperature range above the quenchable temperature, and is rapidly cooled in the mold in parallel with or immediately after press molding. By doing so, a high strength steel molded member is obtained.

  The conditions for hot press molding may be the same as those for known hot press molding. The galvanized steel sheet heated in the heating process is quickly conveyed to a press machine and set, and press-molded. At this time, quenching is achieved by cooling the molded member with a press die. It is effective to incorporate a water cooling mechanism into the press die, and either a direct water cooling method in which water is directly applied to the molding member or an indirect water cooling method in which the inside of the die is water cooled may be used.

  Since such a hot pressing process is well known to those skilled in the art, further explanation is omitted.

(3) Abrasion process The thickness of the zinc oxide layer on the surface of the high-strength steel molded member obtained in the hot pressing process is reduced in the abrading process, or the zinc oxide layer is removed. As a result of the polishing process, the thickness of the zinc oxide on the surface of the high-strength steel molded member is 15 g / m ² or less, and the surface roughness of the high-strength steel molded member is the maximum of the roughness curve of JIS B 0601 (2001). The peak height Rp is 7 μm or less.

  In order to obtain such a surface, a shot blast treatment is preferable as the polishing step. Specifically, shot blasting is performed for several seconds to 60 seconds with an impeller type or air type projection device using a medium having an average particle diameter of about 30 to 300 μm as a shot bullet.

When the zinc oxide layer of the high-strength steel molded member is too thick, the surface conductivity is hindered and the resistance weldability is adversely affected. Further, if the surface roughness becomes too large as a result of the blast treatment, the coating adhesion is adversely affected. In order to stably obtain a high-strength steel molded member having both resistance weldability and paint adhesion, as described above, the thickness of the zinc oxide on the surface after the polishing step is 15 g / m ² or less. Desirably, it is 10 g / m ² or less, and the surface roughness is 7 μm or less in terms of the maximum peak height Rp of the roughness curve of JIS B 0601 (2001).

  As long as the desired surface can be obtained, in addition to shot blasting, sandpaper is used for the blasting, or, for example, a cut wire material or a steel ball obtained by cutting a steel wire as a media for the blasting treatment is used. You may use the grid which crushes and has a sharp protrusion. There is also a method that uses sandpaper that is fine to some degree.

  However, with these means, the surface of the high-strength steel molded member is likely to be rough, and the removal amount of the iron-zinc solid solution phase may increase to adversely affect the corrosion resistance, so that it is difficult to set appropriate conditions. In this regard, it is possible to use a shot blasting process in which the above-mentioned media having a particle size (such as steel and alumina are used as the material. The shape is not particularly limited unless the shape is extremely square, for example). Easy. Further, it is preferable that the particle diameter of the media is as small as possible because it is easy to uniformly treat (high coverage) and a surface with low roughness is easily obtained. In particular, when the average particle diameter of the media is 90 μm or less, it is also preferable from the viewpoint of coating adhesion.

  In this way, excellent resistance weldability and paint adhesion can be stably imparted to a high-strength steel molded member produced by hot press-molding a zinc-based plated steel sheet.

The effects of the present invention will be described more specifically with reference to examples.
(A) Manufacture of high-strength steel molded member (hot pressed member) C: 0.2%, Si: 0.3%, Mn: 1.3%, P: 0.01% of plate thickness containing 0.01% An alloyed hot-dip galvanized steel sheet (symbol: GA) with a hardened cold-rolled steel sheet as the base steel sheet and a plating coating amount of 65 g / m ² per side and a Fe content of 15% in the plating film is used as the processing material. Using.

Further, an electrogalvanized nickel-plated steel sheet (symbol: ZN, plating adhesion amount: 50, 70 g / m ² per side, Ni content in the plating film), using a cold-rolled steel sheet having the same chemical composition and thickness as the base steel sheet Amount: 8 to 18%), hot-dip galvanized steel sheet (symbol GI, plating adhesion amount: 80 g / m ² per side) was also used as a processing material.

(B) Heating process The said plated steel plate was heated at 900 degreeC for 5 minute (s) using the electric furnace of an atmospheric condition. However, the GI material was heated for 7 minutes.

(C) Hot pressing process:
In order to simulate press forming immediately after the plated steel plate was taken out of the electric furnace, it was pressed by a flat plate press having a jacket water cooling mechanism to prepare a quenched sample. The water-cooled press mold was held for 30 seconds. Thereby, quenching was achieved simultaneously with press molding.

(D) Abrasion process:
In the projection method shown in Table 1, the sample No. 1 was obtained by changing the cover rate, the thickness of the zinc oxide layer, and the surface roughness. 1-39 were created.

  The polishing was performed using the media type, particle size, abrasive shape, polishing conditions, and projection time shown in Table 1. The contents of the conditions 1 to 5 in the column of “Scouring conditions” in Table 1 are listed below.

Condition 1: Air pressure 0.3 MPa, projection distance 100 mm, nozzle diameter 8 mm
Condition 2: Air pressure 0.5 MPa, projection distance 100 mm, nozzle diameter 8 mm
Condition 3: Air pressure 0.5 MPa, projection distance 100 mm, nozzle diameter 8 mm
Condition 4: Air pressure 0.5 MPa, projection distance 100 mm, nozzle diameter 8 mm
Condition 5: Load 9.8 N, 10 reciprocations Further, shot bullets having a particle diameter of 200 and 300 μm were projected by a direct pressure type shot projector, and other shot bullets were projected by a suction type shot projector.

  Sandpaper grinding was performed by winding several types of sandpaper with different abrasive grains around a 70 × 30 mm iron block, reciprocating the entire surface 10 times with a load of 9.8 N, and polishing.

  The coverage of the blasting process (excluding sandpaper grinding) is a binary value obtained by applying a commercially available pressure-sensitive paper to the quenched sample in advance, scouring it, and changing the proportion of the projected portion discolored by digital image processing. I asked for it.

(E) Sample No. 1 to 39 Analysis and Evaluation (E1) Surface Zinc Oxide Sample No. The surface of 1-39 was dissolved with 5% chromic acid, and the solution was quantified by ICP emission analysis.

(E2) Surface texture (existence of iron zinc solid solution phase)
First, sample no. X-ray diffraction spectra were taken from surfaces 1 to 39. In any of the samples shown in Table 1, no peak derived from the zinc-iron intermetallic compound was observed. Next, sample No. Samples for cross-sectional observation were cut out from 1 to 39, and after resin embedding and mirror polishing, the surface layer portion was observed with a backscattered electron (BSE) image of SEM from the cross-sectional direction. Sample No. in Table 1 The thicknesses of the iron zinc solid solution phases 1 to 39 were about 10 to 30 μm, and the zinc concentration in the solid solution phase was in the range of 25 to 35%.

(E3) Paint adhesion Sample No. 1 to 39, after chemical conversion treatment (treatment liquid: PBL-3080 manufactured by Nihon Parkerizing Co., Ltd., treatment conditions are standard conditions in the treatment liquid), electrodeposition coating [GT10 made by Kansai Paint, film thickness target value 15 μm, voltage and energization pattern: 200 V slope energization (pressurization from 0 V to 200 V in 30 seconds)], baking 160 ° C. × 20 minutes). This electrodeposition coating material was immersed in a 5% sodium chloride aqueous solution at 50 ° C. for 1000 hours, and then the coated surface was subjected to a 2 mm wide cross-cut test 25-mass peel test specified in JIS K 5600, and rated. A rating of 8 or more was indicated as pass (◯), a rating of 10 was indicated as pass (合格), and a rating of 6 or less was indicated as x.

(F) Measurement of chemical composition composition ratio (P / P + H) A chemical conversion treatment layer of about 2.0 to 2.3 g / m ² was formed on the sample surface using a commercially available zinc phosphate treatment solution. The X-ray diffraction method was used to measure the diffraction intensity (cps) of zinc phosphate crystal hoplite: H, zinc iron phosphate crystal phosphophyllate; P, and the ratio was defined as P / (P + H). When P / (P + H) was 0.20 or more, it was judged as acceptable, and the others were regarded as unacceptable.

(G) Resistance weldability:
The electrical resistance of the surfaces of Samples 1 to 39 was measured under the following conditions, and this was evaluated as spot weldability.

  A steel plate was sandwiched between DR-type electrode tips (diameter 6 mm, tip curvature 40 mm), a constant current 2 A was applied for 4 seconds with a pressurizing force of 0.3 MPa, and the resistance value was measured from the voltage value between the electrodes. An electrical resistance of 30 mΩ or less was evaluated as good (◯), and the other resistance was determined as bad (×).

  The above test conditions and test results are summarized in Table 1. Sample No. in Table 1 is shown below. 1 to 39 will be described.

(H) GA steel ball projection system Sample No. in Table 1 1-4 and 6-14 are examples of the present invention. Sample No. From the projection results of 1-4, 6-14, sample no. It can be seen that 1-4, 6-14 have good resistance weldability and paint adhesion.

  In contrast, sample no. No. 5 is a comparative example in which the amount of zinc oxide exceeds the range of the present invention, and resistance weldability was unsatisfactory.

  Sample No. No. 15 is a comparative example in which the amount of zinc oxide and the surface roughness exceed the range of the present invention, and both resistance weldability and paint adhesion were unsatisfactory.

  Furthermore, sample no. When compared with the present invention examples 1 to 4 and 6 to 14, when the value of P / P + H is 0.40 or more (sample Nos. 9 to 13), the SDT 1000h peeling rating evaluation is 10 (that is, the coating adhesion is ◎). These are obtained when the diameter of the steel ball, which is a medium, is as fine as 50 μm or less, and the zinc phosphate iron crystal phosphophyllite has higher alkali resistance than the phosphate of zinc phosphate crystal. The ratio was increasing.

(I) GA material alumina projection system Sample No. in Table 1 Nos. 16 to 19 are examples of the present invention in which the amount of zinc oxide and the surface roughness satisfy the scope of the present invention, both resistance weldability and coating adhesion were good, and particularly coating adhesion was good.

  Furthermore, sample no. As understood from the results of 16 to 19, the coating adhesion is very good by using a fine alumina particle having a particle size of 40 to 80 μm.

(J) GA material grit projection system Sample No. in Table 1 Nos. 20 to 23 were comparative examples in which the removal of the zinc oxide layer was achieved but the surface roughness exceeded the range of the present invention, and thus the coating adhesion was unsatisfactory. If the surface roughness of the base material is excessive, it is presumed that corrosion from the protruding portion that cannot be covered with the coating film thickness proceeds.

(K) GA material sandpaper grinding system Sample No. in Table 1 Among samples Nos. 24-27, the sample No. 2 in which both the amount of zinc oxide and the surface roughness satisfy the scope of the present invention. Only No. 26 had good resistance weldability and paint adhesion. Thus, it can be seen that grinding with sandpaper is more rough than the shot blasting process and it is difficult to set appropriate conditions.

Sample No. No. 28 is a reference example in which no polishing is performed, the resistance weldability is unsatisfactory because the amount of zinc oxide is as large as 18 g / m ² , and the coating roughness is also low because the surface roughness exceeds the range of the present invention. It was unsatisfactory.

(L) Zn-Ni70 plating material steel ball projection system, Zn-Ni50 plating material alumina projection system Nos. 29 to 32 are Zn—Ni 70 materials, and the amount of zinc oxide and the surface roughness are within the scope of the present invention. Nos. 29 to 31 had good cross-cut peeling ratings after SDT 1000h.

  Sample No. Nos. 33 and 34 are obtained by projecting Zn—Ni50 material with alumina, both the amount of zinc oxide and the surface roughness are within the scope of the present invention, and In particular, No. 33 had a good grid peeling rating after 1000 hours of SDT.

  Sample No. No. 35 is a reference example in which no polishing is performed, the resistance weldability is unsatisfactory because the amount of zinc oxide is larger than the range of the present invention, and the coating adhesion is unsatisfactory because the surface roughness is 10 μm. there were.

(M) GI plated steel ball projection system Sample No. in Table 1 36 to 38 are examples of the present invention in which the amount and roughness of zinc oxide satisfy the scope of the present invention, and both resistance weldability and paint adhesion were good.

Furthermore, sample no. No. 39 is a reference example in which no cleaning is performed, and the coating adhesion is poor because the amount of zinc oxide is as large as 18 g / m ^2, and the corrosion resistance is also poor because the surface roughness is 8 μm. .

Claims (2)

A heating process for heating the galvanized steel sheet to a temperature range above the quenchable temperature;
Hot pressing process in which press forming of the heated galvanized steel sheet is started in a temperature range higher than the quenchable temperature, and is rapidly cooled in the mold in parallel with the press forming or immediately after the press forming. When,
A polishing step of cleaning the surface of the molded member obtained in the hot press molding step,
The molded member obtained through the abrading step has a zinc oxide layer having a thickness of 15 g / m ² or less on the surface of the iron zinc solid solution phase and has a roughness index Rp defined by JIS of 7 μm or less. A method for producing a high-strength steel molded member having a certain surface roughness.   The high-strength steel molded member according to claim 1, wherein the surface of the molded member is polished by projecting a medium having an average particle diameter of 90 µm or less as a shot bullet on the surface of the molded member with a shot projection device. Production method.