JP2008284599A - Method for manufacturing high strength steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing high strength steel by which hardness of a press worked steel part can be kept high, blanking of the steel subjected to press working is facilitated, thereby reduction in the manufacturing yield is prevented. <P>SOLUTION: The method for manufacturing high strength steel comprises a first step of applying a decarburization preventive agent D for preventing decarburization of carbon in the steel during the hot press forming onto a part not to be blanked of the steel (a workpiece W), a second step of performing the hot press forming of the workpiece W, and a third step of blanking a part Wa not coated with the decarburization preventive agent. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a method for producing a high-strength steel material, and in particular, a method for producing a high-strength steel material that can increase the hardness of a press-worked part and can be easily punched when processing a high-tensile steel plate (high-tensile). It is about.

Recently, improvements in automobile fuel efficiency have been sought to reduce CO ₂ emissions, and for that reason, further weight reduction of automobile parts has been researched and developed. For example, a body part formed by press-molding a steel plate is a main part constituting an automobile. Therefore, it is no exaggeration to say that the weight of an automobile can be reduced by reducing its weight.

  In order to reduce the weight of steel plates, so-called High Tensile Strength Steel Sheets, which are high strength steel plates (high strength steel plates), have been applied to body parts of automobiles. By using this high tension, the thickness of the member can be reduced, and the weight of the member can be reduced.

  On the other hand, collision safety performance is an important factor for automobiles, and the demand for improvement is increasing day by day. Therefore, in order to improve the collision safety performance, it is necessary to further increase the strength of the high-strength steel plate such as the high tension described above.

  One of the approaches from the processing technology side that aims to further increase the strength of the high-strength steel sheet is hot pressing. This is to increase the strength of the steel sheet by heating the steel sheet to about 900 ° C. and then immediately inserting it into a mold (mold) and performing rapid cooling by press molding and the mold (quenching). For example, the tensile strength can be increased to about 1500 MPa. By increasing the material strength, the material can be thinned, leading to a reduction in the weight of the member.

  By the hot pressing described above, a high-strength member having a Vickers hardness exceeding 350 Hv can be processed. However, on the other hand, if you try to perform punching (outside punching, etc.) after hot pressing, the cutting edge of a general punching blade (tool) will be worn or damaged immediately, so it will be frequently punched. It becomes necessary to replace the blade, and the manufacturing yield is greatly reduced. Therefore, there is a method of performing other punching processes such as laser processing, but in this method, the manufacturing efficiency is extremely lowered as compared with the punching process using a punching blade.

  Here, in Patent Document 1, a decarburized layer is formed on the surface of a steel material to improve the workability by reducing the amount of surface carbon, and after being processed into a predetermined shape, a carburizing process and a tempering process are performed. Thus, a method for producing a high-strength member that recovers the strength of a steel material whose strength has been reduced by the initial decarburization is disclosed.

JP-A-5-320748

  According to the manufacturing method of the high strength member of patent document 1, the workability can be improved, and as a result, the carbon amount of the steel material can be recovered to prevent the strength of the steel material from being reduced. However, the processing method that once reduces the carbon content and performs the carburizing process and the tempering process after the desired processing increases the number of steps, and the manufacturing yield is inevitably lowered. In addition, by performing decarburization treatment from the steel material surface, even if carburization treatment is performed thereafter, the material properties of the steel material change, and it is very unclear whether the strength always recovers reliably.

  The present invention has been made in view of the above-mentioned problems, and can maintain the hardness of a pressed steel part high, and can be easily punched after pressing, thereby reducing the production yield. An object of the present invention is to provide a method for producing a high-strength steel material that is not allowed to occur.

  In order to achieve the above object, the method for producing a high-strength steel material according to the present invention provides a decarburization inhibitor for preventing the carbon in the steel material from decarburizing during hot press forming at a location where the steel material is not punched. A first step of applying, a second step of hot press forming the steel material, and a third step of punching a portion where the decarburization inhibitor is not applied. Is.

  The manufacturing method of the high strength steel material according to the present invention is to decarburize a portion that is punched at the time of punching (external punching) after press forming when manufacturing a high strength steel workpiece with a predetermined shape by hot press forming. By applying a decarburization inhibitor to the parts that are not hot-pressed and not punched without applying (or spraying) the inhibitor, from the parts that are not punched during hot-pressing By actively removing the carbon component in the steel material, the increase in strength is suppressed, thereby increasing the strength of the steel material workpiece and increasing the efficiency of the punching process. Here, the hardness of the punched portion is a Vickers hardness capable of cold punching: about 250 to 300 Hv, and the hot-pressed portion other than the punched portion has a Vickers hardness of 350 Hv or more and a tensile strength of about 1500 MPa. Product can be manufactured.

  The hot press passes steel in a furnace at a high temperature of 800 to 950 ° C. and an atmospheric pressure atmosphere, holds the steel in a high temperature state in a die (die), and rapidly dissipates the heat of the steel from the die. By quenching and press forming into a predetermined shape with a punch, a high-strength steel workpiece with a desired shape is produced. In this hot press processing, carbon in the steel material is decarburized, and therefore it is necessary to perform a decarburization treatment to obtain a predetermined hardness and strength after the press processing. Before passing the steel workpiece into the furnace, apply a decarburization inhibitor to a predetermined part of the steel material and do not apply a decarburization inhibitor to the part to be externally drawn in the end region of the steel material. It promotes decarburization in a high-temperature furnace.

  Here, the decarburization inhibitor used is composed of a mixed material containing, for example, semi-molten boron oxide as a main component and silica or resin in addition, and Condorsal 0090 (registered trademark) (Parker Heat Treatment Industrial Co., Ltd.) Can be used.

  In a hot-pressed steel material, a part that is not coated with a decarburizing inhibitor is punched into a high-strength workpiece of a predetermined shape, for example, it is integrated with other steel materials by spot welding, etc. The composite work (plywood) to be completed is completed.

  Moreover, in a preferred embodiment of the method for producing a high-strength steel material according to the present invention, the steel material is a steel material having a carbon content in the range of 0.15 wt% to 0.3 wt%, and the decarburization. The inhibitor is characterized in that the coating thickness on the steel material is set in the range of 0.1 mm or more and 1.2 mm or less.

  It can be said that the hardness of the steel material is determined by its carbon content, and if the carbon content in the steel material is less than 0.15% by weight, sufficient height and strength cannot be obtained even by hot press molding. Further, if the carbon content in the steel material exceeds 0.3% by weight, the steel material is not sufficiently decarburized in the high-temperature furnace at the punching location, so that the hardness at the location is too hard, Blade wear and damage is immediately noticed. Therefore, the carbon content in the steel material is in the range of 0.15 wt% to 0.3 wt%.

  Furthermore, regarding the application thickness of the decarburization inhibitor to the steel material, if the application thickness is less than 0.1 mm, even if the decarburization inhibitor is applied, a predetermined hardness and Strength cannot be obtained. On the other hand, if the coating thickness exceeds 0.3 mm, the lubrication action during press working becomes too high, and there is a problem that wrinkles are generated in the pressed part. Therefore, the coating thickness of the decarburization inhibitor on the steel material is set in the range of 0.1 mm to 1.2 mm.

  Furthermore, the thickness (plate thickness) of the steel material used is preferably 3.2 mm or less. If the plate thickness is larger than 3.2 mm, the decarburization of the carbon component inside the steel material is not promoted even in the punched portion where the decarburizing inhibitor is not applied, and as a result, there is a high possibility that efficient punching is hindered. It is.

  As can be understood from the above description, according to the method for producing a high-strength steel material of the present invention, a steel material having a predetermined shape with high hardness and high strength can be produced, and punching after hot press working (outline punching) Can be performed efficiently, leading to an improvement in manufacturing yield.

  Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram illustrating a state in which a decarburization inhibitor is applied to a predetermined portion of the workpiece, and FIG. 2 is a diagram illustrating a situation in which the workpiece to which the decarburization inhibitor is applied passes through the high temperature furnace. FIG. 3 is a diagram for explaining a situation before the workpiece that has passed through the high-temperature furnace is held and fixed to the upper and lower dies and press working is performed with a punch. FIG. 4 is a perspective view of a hot-pressed work, and FIG. 5 is a perspective view of a composite work composed of the work of FIG. 4 and another flat work. The shape of the steel plate workpiece to be hot pressed and the shape of the plywood are not limited to the illustrated embodiment.

  FIG. 1 illustrates a workpiece W made of a steel plate. More specifically, a decarburization inhibitor D (for example, Condorsal 0090 (manufactured by Parker Heat Treatment Co., Ltd.)) is applied to the front and back surfaces of the workpiece W. The both end portions show the work which is defined as an uncoated area Wa of the decarburization inhibitor. As will be described later, the uncoated area Wa is an area that is punched out after hot pressing, and facilitates punching by promoting decarburization of contained carbon during hot pressing. Therefore, this is a region where no decarburization inhibitor is applied.

  The thickness of the workpiece W is 3.2 mm or less, and the carbon content is set in the range of 0.15 wt% or more and 0.3 wt% or less. Furthermore, the coating thickness of the decarburization inhibitor applied to the surface of the workpiece W is set in the range of 0.1 mm to 1.2 mm.

  As shown in FIG. 1, a work W having a predetermined region coated with a decarburizing inhibitor is passed through a high temperature furnace F shown in FIG. 2. The high-temperature furnace F is adjusted in the furnace to about 900 ° C. by an internal heater H, and the pressure in the furnace is set to about atmospheric pressure. As the workpiece W placed on the belt conveyor C passes through the high temperature furnace F over a predetermined time, the high temperature workpiece W is transferred to the press booth outside the high temperature furnace F.

  FIG. 3 shows an upper die D1 and a lower die D2 for press working, and a punch P that moves up and down in a through hole D2 'in the lower die D2 and a fitting hole D1' in the upper die D1.

  The workpiece W in a high temperature state is held and fixed by the upper die D1 and the lower die D2, and the widened portion P1 at the upper part of the punch P bends the workpiece W from below and is rapidly cooled by the dies D1 and D2. Thus, the workpiece W1 (high strength steel material) after hot pressing as shown in FIG. 4 is formed.

  The decarburization preventive agent is not applied to the both end portions (punching region Wb) of the workpiece W1 shown in FIG. 4 before hot press processing. The hardness is low. For example, the hardness of the punching region Wb is about 250 Hv, and the hardness of the other hot-pressed regions is 350 Hv or more.

  The punching region Wb is a region whose outer shape is punched into a predetermined shape, and as shown in FIG. 5, this region is punched to produce a high-strength workpiece W1 ′, and then another flat steel plate workpiece W2. By performing spot welding S at a plurality of locations, a synthetic work W3 having excellent three-dimensional tensile strength, bending strength, etc. can be manufactured.

[Experiments and results on the hardness and punching performance after hot pressing when the thickness of the steel sheet, the amount of carbon contained, the application range of the decarburization inhibitor and the thickness of the application are changed]
As shown in Tables 1 and 2 below, the present inventors prepare a total of 10 steel material test pieces of Examples 1 to 3 and Comparative Examples 1 to 7, and the carbon content in the steel material for each test piece, The steel material thickness, the decarburization inhibitor application range, and the decarburization inhibitor application thickness were each changed to verify the carbon content and hardness in the steel material after hot pressing, and further the outer shape punchability.

  In the experiment, the mold (die and punch) shown in FIG. 3 was used, and the decarburization inhibitor used was a condensal 0090 whose main component is boron oxide, whose viscosity was adjusted with xylene. Spray coating was applied to the thickness set for each test piece, followed by drying at room temperature for 30 minutes, and then forced drying at 80 ° C. for 1 hour. The coating thickness after drying was measured with a micrometer.

  In the hot pressing, each test piece was heated in a furnace at 900 ° C. for 20 minutes under atmospheric pressure, and then immediately inserted into a mold and pressed.

  Each carbon content of the molded part (pressed part) and punched part of the obtained test piece after hot pressing was subjected to ICP analysis, and the hardness was measured with a Vickers hardness meter.

  In addition, the number of pieces that could be continuously punched was measured by normal outer shape punching (implemented with a normal steel blade). Here, the reference number of continuous punching was set to 1000, and test pieces satisfying the number were evaluated as acceptable (◯), and test pieces less than that were evaluated as unacceptable (×).

  The outline of each test piece used in the result of this experiment is shown in Table 1, and the result of this experiment is shown in Table 2.

  As shown in Table 2, all of Examples 1 to 3 can be favorably drawn out, and the hardness of the molded part shows a hardness of 350 HV or higher.

  On the other hand, in Comparative Example 1, since the decarburization inhibitor was applied to the punched portion, the punched portion became too hard, and the target number of punches could not be reached.

  In Comparative Example 2, the application amount of the decarburization inhibitor was too large (application thickness was 2.5 mm), and a large number of press wrinkles occurred due to an increase in the lubricating action during press working.

  In Comparative Example 3, the amount of carbon in the steel material is too small (0.12% by weight), and the hardness of the molded part after hot pressing cannot reach a predetermined hardness (350 HV). In Example 4, the amount of carbon in the steel material is too large (0.32% by weight), and since the decarburization at the time of hot pressing is insufficient, the hardness of the punched part is too hard to reach the target punching number. Could not reach.

  In Comparative Example 5, since the decarburization inhibitor was not applied to the molded part, decarburization was promoted from the molded part during hot pressing, and as a result, the predetermined hardness was not obtained in the molded part.

  In Comparative Example 6, although the decarburization inhibitor was applied, the amount of coating (coating thickness) was too small, so that decarburization was promoted from the molded portion, and the predetermined hardness was not obtained.

  Lastly, in Comparative Example 7, although the steel was decarburized from the surface of the punched portion, the steel material was too thick and decarburization from the inside was suppressed. As a result, the target number of punches could not be reached.

  From the above experimental results, a predetermined amount (thickness) of decarburization inhibitor is applied to the hot-pressed portion, and no decarburization inhibitor is applied to the punched portion, and if the steel material is too thick, it is stamped. It was proved that the decarburization from the processed part was suppressed and the punching performance was lowered.

  The embodiment of the present invention has been described in detail with reference to the drawings. However, the specific configuration is not limited to this embodiment, and there are design changes and the like without departing from the gist of the present invention. They are also included in the present invention.

It is the figure explaining the state by which the decarburization inhibitor was apply | coated to the predetermined location of the workpiece | work. It is the figure explaining the condition where the workpiece | work with which the decarburization inhibitor was apply | coated passes the inside of a high temperature furnace. It is a figure explaining the situation before the work which passed the high-temperature furnace is held and fixed to the upper and lower dies, and press working is performed with a punch. It is a perspective view of the hot-pressed work. It is a perspective view of the synthetic | combination workpiece | work which consists of the workpiece | work of FIG. 4, and another flat plate workpiece.

Explanation of symbols

  W, W1, W1 '... work, W3 ... synthetic work, Wa ... decarbonization inhibitor uncoated area, Wb ... punching area, D ... decarburization inhibitor, F ... high temperature furnace, H ... heater, C ... conveyor D1, D2 ... Die, P ... Punch,

Claims (2)

A first step of applying a decarburization inhibitor for preventing carbon in the steel material from decarburizing during hot press forming to a portion of the steel material that is not punched;
A second step of hot press forming the steel material;
A third step of punching a portion where no decarburization inhibitor is applied, and a method for producing a high-strength steel material. The steel material is a steel material having a carbon content of 0.15 wt% or more and 0.3 wt% or less,
The method for producing a high-strength steel material according to claim 1, wherein the decarburization inhibitor has a thickness applied to the steel material in a range of 0.1 mm or more and 1.2 mm or less.

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