JP2910288B2 - Extremely low carbon steel rods and wires with excellent cold forgeability and magnetic properties - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ultra-low carbon steel rod and a wire rod used for electric parts for automobiles and the like, and particularly to cold forgeability (hereinafter referred to as cold forgeability) required for electric parts and the like for automobiles. ) And ultra low carbon steel rods and wires with improved magnetic properties.

[0002]

2. Description of the Related Art Soft magnetic materials having characteristics that are easily adapted to a magnetic field are used for members constituting a magnetic circuit such as electric parts for automobiles. In other words, steel materials used for automobile electrical components and the like are required to have magnetic properties such that they are easily magnetized by a small external magnetic field and have a small coercive force.
By the way, low carbon steel having a carbon content of about 0.01 to 0.1% is used as the above steel material. After hot rolling this steel slab, the wire is subjected to annealing, lubrication and wire drawing, and the bar is In general, cold forging, cutting, magnetic annealing, and the like are performed after performing cutting, annealing, and lubrication. However, in recent years, from the viewpoint of energy saving and process saving, a demand for a steel material that can be cold forged without annealing is increasing. Under such circumstances, the quality of the forging tool life is regarded as a problem.

Although there are various factors affecting the life of a forging tool, the deformation resistance value of the workpiece is considered to have a great influence on the life of the tool from the viewpoint of the workpiece. In particular, under high-speed forging conditions in which productivity is valued, the temperature of the workpiece increases significantly due to adiabatic deformation, and reaches several hundred degrees (° C.) in the latter stage of processing even if the initial processing temperature is about room temperature. Accordingly, in addition to the work hardening introduced by the working, strain aging hardening is added to the work material, so that the deformation resistance is significantly increased and the tool life is significantly reduced. When the tool life is shortened, the operating rate of the forging machine is reduced, and the production efficiency is significantly impaired.

[0004] Strain age hardening is considered to be caused by the interaction of free C and N in steel, dislocation density increased by working, and temperature. Annealing has an effect of suppressing strain aging by reducing free C in steel in addition to an effect of reducing work hardening of steel, and these two effects reduce the deformation resistance value of a workpiece at several hundred degrees (° C.). It is significantly reduced. For this reason, in order to prolong the tool life under conditions where annealing is not performed, it is necessary to reduce work hardening and suppress strain aging hardening by some means instead of annealing, and thereby reduce deformation resistance. It is.

[0005]

As a technique for reducing the deformation resistance by reducing the work hardening and suppressing the strain aging in the rolled steel material, for example, JP-A-54-7
Nos. 6417, 59-215463, 60-181
No. 233, No. 63-157817, JP-B-57-60
No. 416 has been proposed. Each of these techniques aims to reduce C and N and also to reduce Al, Zr, Cr, T
Special elements such as i and B are added alone or in combination. However, C is 0.04 ~
Since about 0.15 is contained, even though suppression of strain age hardening can be achieved to some extent, it is still insufficient from the viewpoint of reducing work hardening, and it cannot be said that deformation resistance can be necessarily reduced to the limit. . Also, the magnetic properties of the product after the magnetic annealing were not shown or were insufficient even if they were indicated (JP-A-59-215463).

The present invention has been made in view of such circumstances, and it is an object of the present invention to obtain good cold forgeability as it is rolled and to obtain excellent magnetic properties in a product stage subjected to magnetic annealing. The object of the present invention is to provide a steel rod and a wire rod as shown below.

[0007]

According to the present invention, which has achieved the above objects, C ≦ 0.0015%, Si ≦ 0.01%, Mn: 0.02 to
0.1%, P ≦ 0.01%, S ≦ 0.01%, Al: 0.025 to 0.05
%, N ≦ 0.002%, and O ≦ 0.003%, respectively, and are ultra-low carbon steel rods and wires having a gist in the balance of iron and unavoidable impurities. Z: 0.03-0.06% or C
It is also effective to contain any of r: 0.15 to 0.3%, which contributes to suppression of strain aging and prevention of aging of magnetic properties.

[0008]

The present inventors have studied steel bars and wires having reduced magnetic resistance and excellent magnetic properties from various angles. As a result, elements such as C, Si, Mn, and P, which increase work hardening, are reduced to the utmost, and N involved in strain age hardening is reduced, and then Al (and Zr or Cr as necessary) is further reduced. It has been found that not only good cold forgeability in the as-rolled state, but also excellent magnetic properties at the product stage subjected to magnetic annealing can be achieved if the strain age hardening is suppressed by the addition of Was completed. The reasons for limiting the range of each component in the ultra-low carbon steel rod and wire of the present invention are as follows.

C ≦ 0.0015% C is a basic element that governs the balance between the strength and ductility of a steel material. The strength decreases and the ductility improves as the amount of addition is reduced. Further, since C is dissolved in steel and causes strain age hardening, extremely low C is desirable, and also from the viewpoint of magnetic properties, extremely low C is desirable. The upper limit of the C content is set to 0.0015% from the viewpoint of an allowable limit of work hardening.

Si ≦ 0.01% Since Si is an element that increases the strength and work hardening of the steel material and lowers the ductility, it is preferable that Si is as small as possible, but the upper limit is set to 0.01% as an allowable upper limit.

Mn: 0.02-0.1% Since Mn increases the strength and work hardening of steel, it is desirable for Mn to be extremely low for the purpose of the present invention. However, Mn has the effect of fixing S causing hot embrittlement as MnS and rendering it harmless. From these viewpoints, Mn
Was added at 0.02 to 0.1%.

P ≦ 0.01% Since P increases the strength and work hardening of steel like Mn, it is desirable that P is extremely low for the purpose of the present invention. No, 0.01
% Or less is sufficient.

S ≦ 0.01% S forms sulfide inclusions and lowers ductility. Further, since the magnetic properties are deteriorated, an extremely low S is desirable from the point of the present invention, but 0.01% or less is sufficient.

Al: 0.025 to 0.05% In addition to the role of a deoxidizing agent in steel melting, Al binds to N and precipitates as AlN to fix free N and suppress strain age hardening. In order to exert its effect, it is necessary to add 0.025% or more. However, if the amount is too large, the above effect is saturated and, rather, the increase of the B type inclusion causes cold forging cracking. A
The amount of 1 added was 0.025 to 0.05%.

N ≦ 0.002% Since N causes strain age hardening due to solid solution, it is desirable that N be as small as possible. Therefore, the upper limit is 0.00
2%.

O ≦ 0.003% O is an element that is harmful to the cleaning of steel and also reduces the magnetic properties. Therefore, the upper limit was made 0.003%.

In addition to the above, it is also effective to add Zr or Cr to the steel rod and wire of the present invention, if necessary. The effects of use and the reasons for limiting the range are as follows.

Zr: 0.03 to 0.06% Zr combines with C and N which form a solid solution in the steel, and precipitates a carbon nitride having a sulfide as a nucleus of about 0.1 μm. As a result, free C and N in the steel are reduced, and the strain age hardening is suppressed, so that the deformation resistance value at several hundred degrees (° C.) is reduced. Further, the reduction of free C and N has an effect of preventing the secular change of the magnetic characteristics of the product. In order to exert the above effects, the amount of addition must be 0.03% or more. On the other hand, the amount added is 0.
If it exceeds 06%, the effect of reducing deformation resistance is recognized, but Z
Since the critical upsetting ratio is reduced due to the r-based inclusions, cold forging cracks are likely to occur in a product having a complicated shape. Therefore Z
When adding r, the addition amount needs to be 0.03 to 0.06%.

Cr: 0.15 to 0.3% Cr is a useful element that combines with C and N which form a solid solution in steel to precipitate carbonitrides. It can be said that the effect of Cr for reducing the deformation resistance and preventing the secular change of the magnetic properties of the product is similar to that of Zr. In order to exert its effect, it is necessary to add 0.15% or more, but if it exceeds 0.3%, the above effect is saturated.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the following Examples are not intended to limit the present invention. It is included in the technical scope.

[0021]

EXAMPLE A slab having a 115 mm square section was produced by melting in a 150 kg vacuum furnace. This was heated in a heating furnace of a wire rolling mill and then rolled into a wire having a diameter of 16 mm. Table 1 shows the chemical composition of the test steel, and Table 2 shows the properties of the wire. Further, since the strain aging characteristics of the wire are governed by the amounts of C and N dissolved in the steel, the dissolved N was measured. Solid solution (C +
The amount of N) was determined by a method for determining a strain aging amount ΔTS described below. That is, a wire having a diameter of 16 mm was drawn to a diameter of 15.66 mm, the wire was subjected to aging treatment at 80 ° C. × 2 hours, and a strain aging amount ΔTS represented by the following equation was obtained. ΔTS (kgf / mm ² ) = (tensile strength after aging treatment)-(tensile strength immediately after drawing)

[0022]

[Table 1]

[0023]

[Table 2] The smaller the amount of strain aging ΔTS, the smaller the amount of solid solution (C + N), and therefore the lower the deformation resistance at several hundred degrees (° C.).
Cylindrical test piece from wire having a diameter of 16mm (14mmφ × 21mm ^h)
Was prepared, and the deformation resistance was determined from an end face constraint compression test at an upsetting rate of 60%. FIG. 1 shows a change in deformation resistance with temperature. The strain rate of the forging machine is a practical processing rate of 2 / sec. These results show that the deformation resistance of the steel of the present invention at 200 to 400 ° C. is much lower than that of the conventional steel.

A cylinder (14 mmφ × 21 mm ^h ) with a depth of 0.8 mm
Using a grooved test piece with a notch with a tip radius of 0.15 mm, the critical upsetting ratio at which cracks occurred at the groove bottom was determined from an end face restraint compression test. The result is shown in FIG. These results show that the steel of the present invention shows excellent ductility.

A cylindrical test piece (14 mmφ × 21 mm ^h ) was placed at a height of 4.
8mm (Upsetting ratio 80%) Compressed at the end face, outer diameter 28.0mm
A ring-shaped test piece having a diameter of 20.0 mm in diameter and a height of 2.95 mm was prepared, and subjected to magnetic annealing at 850 ° C. for 5 hours, and then examined for DC magnetization characteristics. Table 3 shows the results. Table 3 shows that the steel of the present invention has a higher magnetic flux density and a lower coercive force than the conventional steel, and has extremely excellent magnetic properties.

[0026]

[Table 3]

[0027]

The present invention is constituted as described above, has excellent cold forgeability as it is rolled, and has excellent magnetic properties when it is made into a product. Carbon steel rods and wires were obtained.

[Brief description of the drawings]

FIG. 1 is a graph showing a change in deformation resistance of a test steel with temperature.

FIG. 2 is a graph showing a limit upsetting ratio in each test steel.

Claims (2)

(57) [Claims] 1. C ≦ 0.0015% (meaning by weight%, the same applies hereinafter), Si ≦ 0.01%, Mn: 0.02-0.1%, P ≦ 0.01
%, S ≦ 0.01%, Al: 0.025 to 0.05%, N ≦ 0.002
% And O ≦ 0.003%, and the balance consists of iron and unavoidable impurities, and is an ultra-low carbon steel rod and wire excellent in cold forgeability and magnetic properties. 2. C ≦ 0.0015%, Si ≦ 0.01%, Mn: 0.
02 to 0.1%, P ≦ 0.01%, S ≦ 0.01%, Al: 0.025 to
0.05%, N ≦ 0.002%, O ≦ 0.003%, respectively, Zr: 0.03 to 0.06% or Cr: 0.15 to 0.3%, the balance being iron and inevitable impurities. Extremely low carbon steel rods and wires with excellent cold forgeability and magnetic properties.