JPS60238424A - Manufacture of partially hardened steel plate - Google Patents

Abstract

PURPOSE:To obtain an advantageous partially hardened steel plate having a soft part and a hardened part on the steel plate by heating the desired place on the surface of a steel plate with a laser beam to produce the steel plate having different local hardness. CONSTITUTION:The desired place on the surface of a recrystallized soft steel plate is rapidly heated with a laser beam. In this case, the place is heated to a temp. higher than an A1 transformation point (about 720 deg.C) at which an austenitic layer emerges, and quenched to 500 deg.C to obtain a hard transformed structure of martensite, fine pearlite, etc. The hardened part is thus produced. And the quenching is carried out by blowing air, gaseous N2, water, etc. against the heated place. In addition, CO2, N2, Ar, H2, etc. which are weakly oxidative are used as the gas for laser, and also preferably used for cooling. When the partially hardened steel plate is used as the floor of an automobile, formability is ensured by the soft part, and the rigidity in use can be remarkably enhanced by the hardened part. Consequently, further gage-down can be made possible.

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to a method for manufacturing partially hardened steel sheets.

[Prior art]

Hot-rolled steel sheets, cold-rolled steel sheets, and their plated steel sheets, etc.
C) It is used in large quantities in automobiles, home appliances, building materials, etc.

In recent years, there has been a strong tendency to build products with thinner gauges and the same characteristics as conventional products, following the trend of making products lighter, thinner, shorter, and smaller. However, if the gauge is lowered, the tensile rigidity and dentability of the molded product will decrease.

Generally, these properties are proportional to the square of the plate thickness and the yield point or tensile strength. If a steel plate with higher tensile strength is used to compensate for the lower gauge, formability will deteriorate.

The present invention provides a method for producing partially hardened steel sheets for use in such applications that are difficult to achieve.
Japanese Patent Publication No. 48-30525 discloses ``Method for manufacturing cold-rolled steel sheet having non-uniform material distribution''. In this method, a spacer or a decarburization inhibitor is applied to a part of the copper strip during decarburization annealing to prevent contact with the annealing atmosphere gas, thereby producing a partially hardened steel sheet.

However, with this technology, the work efficiency of attaching spacers during the manufacturing of oven coils before decarburization annealing is inefficient, and the effect of this is only a few HRB points on the difference in hardness between the decarburized and non-decarburized parts. Unfavorable from the point of view,
It has not yet been implemented.

Therefore, the reality is that no manufacturing technology for partially hardened steel sheets that is advantageous for practical use has been established.

[Purpose of the invention]

SUMMARY OF THE INVENTION An object of the present invention is to provide an extremely advantageous method for manufacturing a partially hardened steel plate having a soft portion and a hardened portion.

For example, when using a steel plate for a car door, if you press a partially hardened steel plate around the door handle, which has the greatest problem with densities, the entire molding will be supported by the soft parts, and the densities around the door will be reduced. Partially hardened steel sheets can be used in a variety of applications, such as by being able to obtain the desired properties such as having a hardened part, which can be used in panels for refrigerators, washing machines, lockers, etc. to increase tension and rigidity. be. It is an object of the present invention to provide a method for manufacturing a partially hardened steel sheet suitable for such uses.6 [Structure and operation of the invention] In order to create partial hardening, which is a requirement of the invention, recrystallized soft steel sheet I learned that it is best to use transformation hardening by partially heating the steel plate rapidly.

In this case, the faster the heating rate, the better so that the heat does not spread to the surroundings. A suitable heating method for this purpose is heating with a laser.

The heating rate is set above the A1 transformation point (approximately 720°C) at which the austenite layer appears, and rapid cooling to below approximately 500°C converts bainite and martensite.

A hardened part is created by obtaining a hard metamorphosed structure such as fine pearlite.

For cooling, although heat diffuses into the surroundings and cools down at a certain rate, it is preferable to cool it forcibly. Methods include blowing air, nitrogen gas, water, etc. onto the heated part,
Immersion in β medium, contact with metal rolls after heating, etc. are adopted.

As for other hardening principles, it is also possible to harden by dissolving C9N etc. in the iron lattice by heating below the A1 point, but although stretcher strain is likely to occur, the hardening is not large, so the above method is better. ing.

Since the surface of the steel sheet hardened in this manner has a tendency to oxidize, the oxidized layer is removed by pickling.

However, if the oxide layer develops thickly, it becomes difficult to remove and the chemical conversion properties of the entire surface deteriorate, so the gas used for the laser is carbon dioxide or nitrogen, which have weak oxidizing properties.

Argon, hydrogen, etc. are used, and these gases are also preferably used for cooling.

For example, this partial hardening treatment may be performed on a cold rolled steel plate after recrystallization annealing. In the case of hot rolled steel plate,
Since recrystallization occurs immediately after rolling and softens the coil, it can be applied to the coil after winding.

Kerr et al. said that it is preferable to do a skin pass after treatment in terms of material quality. When hardening is performed after skin-passing, dislocations introduced by skin-passing are pinned by the heat of hardening, which not only makes it easier to form stretcher strains, but also deteriorates workability. In addition, the shape deteriorates due to thermal distortion, so skin pass rolling is required again.

As for the mausoleum, a laser heating device is installed somewhere in the continuous annealing furnace after recrystallization annealing to perform the hardening process. Depending on the type of hardening pattern, one or more laser guns are installed and controlled by a computer. Run the gun to heat the coil. Annealing h in a box type annealing furnace
The steel plate may be hardened, for example, on a shearing line or an unwinding line.

In principle, any type of steel plate may be used as the steel plate used for this hardening treatment. Transformation hardening and solid solution hardening of C! Niyama? -Metal L-C is I, but this condition is almost always satisfied with industrially manufactured steel.

It is thought that there is no solid solution of C in ultra-low carbon steel to which Ti and Nb are added, but when it is actually used, a low-temperature transformed structure is formed and sufficient hardening occurs.

In fact, the hardened parts of this steel are moderately hardened,
It is preferred because of its excellent workability.

Next, an example will be described.

〔Example〕

The steel plates of 0.7mm shown in Table 1 were processed on a continuous annealing line.
Partial hardening treatment was performed. A plurality of 10 kW carbon dioxide laser guns were arranged in the width direction of the coil at intervals of 3 cm, and the distance from the steel plate surface was adjusted so that the surface of the steel plate was heated.

A steel plate recrystallized and annealed at about 750°C was overaged at about 400°C, and when the temperature reached about 200°C, the required heating area was heated to about 900°C using a carbon dioxide laser.

Thereafter, it was immediately cooled by contacting it with a water-cooled roll or by blowing nitrogen gas or water onto it. The cooling rate from point A1 to 500°C at that time is shown in Table 1.

Figure 1 shows the shape corrected by applying a skin pass of 1%.

Table 2 shows the tensile property values of the tensile test pieces cut out from the soft part. In addition to maintaining good properties in the soft part,
It can be seen that the hardened portion is significantly hardened over a width of about 1 cm.

〔Effect of the invention〕

If a car floor is formed using a steel plate with a so-called hardened part as in the example, the soft part will ensure formability, and the hardened part will significantly increase the rigidity during use. This allows you to lower the gauge even further.

Furthermore, when used as panels for lockers or washing machines, high tensile rigidity can be obtained.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram showing the hardness distribution in the width direction of a steel plate manufactured according to the present invention.

Claims (1)

[Scope of Claims] (1) A method for producing a partially hardened steel plate, which comprises heating the surface of the steel plate with laser light to create areas with partially different hardness. (2) The method for producing a partially hardened steel sheet according to claim (1), which comprises heating to an A1 transformation point or higher using a laser beam. (2) The method for manufacturing a partially hardened steel sheet according to claim (2), in which cooling from the A1 transformation point or higher is forcibly cooled by blowing gas or water onto the heated portion. (4) Claim (2) that after heating to the AI transformation point or higher, it is forcibly cooled by contacting the roll surface.
A method for producing a partially hardened steel plate as described in Section 1. (5) The partially hardened steel cap described in claim (2) is immersed in water during cooling from the A+ transformation point or above.