JPS59136420A - Preparation of non-quenched-and-tempered steel member having cold processing part - Google Patents

Abstract

PURPOSE:To obtain a steel material having a hot processing part and a cold processing part, by a method wherein a steel material with a specific composition is hot rolled to obtain a steel rod and a part of this steel rod is subjected to hot forging and controlled cooling under a specific condition and, thereafter, the other necessary part of the steel rod is subjected to cold processing. CONSTITUTION:This steel material relates to the tension rod of an automobile and a steel material containing, on a wt. ratio, 0.35-0.50% C, 0.15-0.35% Si, 0.50-1.5% Mn, 0.05-0.20% V, 0.006-0.02% Ni, according to necessity, about 1.0% or less Cr, about 0.15% or less S and about 0.35% or less Pb and comprising the remainder of Fe is hot rolled to make a steel rod. A part of this steel rod is heated to 1,000-1,250 deg.C to perform hot forging and, thereafter, the hot forging steel material is cooled at a cooling speed of 10-100 deg.C/min in a temp. range of 700-500 deg.C by controlled cooling and, subsequently, cold processing is applied to the necessary part other than the hot forging part.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method of manufacturing a non-tempered steel member having a cold-worked part.

For example, tension rods and bolts in automobiles require strength (tensile strength 7.
0 to 90-/-hardness HV220 to 280) is required, and at the same time, good rolling properties are required for threaded portions and bolt threads.

Conventionally, steel members having this type of cold-worked part, such as tension rods, have been manufactured according to JIS 535C as shown in Fig. 1.
, 55Qc or the like (hardness Hv 160 to 220) is partially heated by high frequency (1100 to
1200°C), upsetting, forging or coining, then quenching the entire steel rod (indicated by crosshatching) (water cooling at 880°C for 15-30 minutes) and tempering to 550°C.
~600° C. for 1 and a half hours), and then the outer diameter of the portion corresponding to the threaded portion 1b is cut, and the tension rod 1 is manufactured by cold thread rolling and drilling.

However, in order to quench and temper the entire steel rod l, although the lower arm mounting seat surface 1a of the tension rod l'l has the required hardness (Hv220-280), the threaded part 1b also has the same hardness or higher. This causes problems such as cracking during cold thread rolling and poor durability of the die.

For this reason, heat treatment is sometimes performed after cold working, but the surface layer of the cold worked part (threaded part 1b) oxidizes (decarburizes) during heating, resulting in low fatigue strength, and furthermore, heat treatment reduces dimensional accuracy. A problem arises in which the amount of energy decreases.

The present invention was made in view of the above-mentioned problems, and the composition of the steel and the heat treatment conditions are specified, and the cold-worked portion is not heat-treated (the necessary strength is obtained by the working hardness), thereby achieving good workability. The basic objective is to provide a new non-tempered steel member that can ensure the required strength by applying heat treatment to other parts.

Therefore, the present invention has a weight ratio of c:Q of 35 to 0°5.
0%-Si: 0.15~o, a 5%, Mn
: 0.50~1.5%・V:o, os~0.20%,
A steel material having a composition of N: o, oos ~ 0.02% with the balance Fe is hot rolled to form a steel bar, and a part of the steel bar is heated to 1000 to 1250°C to hot-roll. After forging, the cooling rate is 10 from 700°C to 500°C.
It is characterized by controlled cooling at a rate of ~100°C/min, and then cold-working the necessary parts other than the hot-forged parts.

The basic components of the steel cable material used in the method of the present invention are summarized in Table 1, and one supplement can be added as necessary.

The critical significance of the components of the steel material used in the method of the present invention is as follows.

[Basic ingredients]

C: 0.35 to 0.50% C is a basic component for ensuring strength (hardness) and controls the amount of pearlite that affects the hardness of the base. However, 0
．． If it is less than 35%, the strength will be insufficient, and if it is more than 0.50%, the toughness will be decreased.

Si forms a solid solution with Fe to increase the strength (hardness) of steel. However, if it is less than 0.15%, the effect is insufficient.
If it exceeds 0.35%, machinability deteriorates.

Mn: 0.50 to 1.50% Mn increases the strength and toughness of steel, and compensates for the effect of C without reducing toughness while improving strength.

However, if it is less than 0.50%, the strength will be insufficient;
In the above case (hardenability becomes higher than necessary), bainite is formed in the structure during the cooling process after hot rolling, and machinability deteriorates.

■(1) forms a solid solution with Fe at a temperature of 1000°C or higher,
During the subsequent cooling process, it turns into carbonitrides and crystallizes into ferrite, strengthening the steel. However, if it is less than 0.05%, the effect is insufficient, and if it is more than 0.20%, the effect is saturated, impairing economic efficiency. 0.0.0 because it increases the softening resistance against beating heat and reduces the softened region that occurs at the heating boundary during partial heating.
Effective at 0.5% or higher.

N improves toughness by making crystal grains finer. Also, the combination with ■ causes precipitation hardening. Therefore, it is effective in improving the hardness of the core. However, this effect is observed when the content exceeds 0.006%, and when it exceeds 0.020%, the machinability deteriorates significantly, which is not preferable.

[Effective ingredient] Cr improves the hardenability of steel and contributes to hardening in each cooling process after hot rolling and hot forging. However, 1.0%
If it is more than that, the variation in hardness due to variations in cooling rate may become large, and bainite may be generated in the structure, which is not preferable.

S: 0.15% or less S is effective in improving machinability and has no effect during the cooling process after hot rolling. However, if it exceeds 0.15%, strength and toughness will be significantly impaired.

I)b has the same effect as S. However, 0.35%
If it exceeds this level, the strength and FM properties will be significantly impaired.

Next, as shown in FIG. 2 and 3, the heat treatment method for the composition steel of the present invention is as follows.

The hardness Hv of the steel rod 2 is 200 to 260.

The temperature at which the portion corresponding to the lower arm mounting seat surface 2a (indicated by crosshatching) is partially heated by high frequency is as follows:
1,000 to 1,250"C. A temperature of 1,000°C or higher is required because it is necessary to fully dissolve ■ into the austenite. However, even if heated to 1,250°C or higher, the effect does not change, and on the contrary, the crystallization The grains become coarser and the toughness decreases.

Regarding the heating rate, it is desirable to heat and shape the product quickly in order to reduce the softening region of the heating boundary ◎.

The cooling rate after hot forging must be adjusted in the range of 10 to 100°C/min from 700"C to 500'C. If the cooling rate is too fast, bainite will precipitate in the matrix and The machinability decreases, and if it is too slow, the effect of precipitation of the V-monoxide is lost.However, this range is relatively wide for the composition steel of the present invention, and can be sufficiently achieved by cooling in the atmosphere.

After the hot working of upsetting, forging, or coining is completed, the outer diameter of the portion 2b corresponding to the threaded portion of the steel bar 2 is cut. At this time, the hardness Hv of the portion 2a corresponding to the lower arm mounting seat is 220 to 280, and the hardness I4V of the portion 2b corresponding to the threaded portion is 200 to 260.

Next, cold thread rolling and hole drilling are performed.

The hardness of the threaded portion 2b is increased by work hardening due to cold thread rolling (small drawing area (30-40%), which makes it difficult to plastically deform).
will improve by 20. By the way, in conventional carbon steel, the hardness HV improves by 15 because the reduction of area is large (40 to 50%).

Figure 3 shows the tension rod 2 manufactured in this way.
It can be seen that the hardness at the heating boundary area (indicated by crosshatching) is lower.

By the way, the purpose of the present invention is to focus only on the parts of the parts that are subjected to high load stress (in the case of the tension rod 2, the effective cross-sectional area for tensile loads is small and the shape factor is large, so especially a.
The stress in part b is high. ), it is necessary to obtain the required strength through precipitation strengthening and cold work hardening of VS nitrides, so care must be taken to prevent stress concentration from occurring in such low hardness areas ◎. be. Furthermore, it is desirable to minimize the range in which the hardness decreases (particularly the region indicated by ! where the hardness is equal to or less than that of the base material).

For this purpose, it is important to perform local heating quickly, and the steel cable material is desired to have a high resistance to softening against heating.

In this respect, ■ works effectively. In other words, in the conventional steel cable material that does not contain ■, the softening starts gradually at about 300°C, whereas in the steel of the present invention with 0.1% V added, as shown in Fig. 4, It hardly softens up to about 650°C. This resistance to softening increases with ■ amount, so as needed? All you have to do is adjust the V amount.

Next, an example will be explained.

(1) Tension rod z (see Figure 3) ■Component (
to) c: 0.43, Si: o, :25,
V: 0.10Cr: 0, l3-S: 0.
025゜N: 0.01 Mn:Q, 85゜■Rolled material hardness: Hv220 ■Hot forming part hardness: Hv250 (Heating temperature 1200℃) ■Cold forming part hardness: l-1v230 (Cooling rate 700 ~500°C40°C/min) (2) Bolt 3 (3a head, 3b threaded part) (see Figure 5) ■Component @ CiO, 48, St: 0.25-Mn: 0
．． 85Cr:Q, 15*S:o, o2o, V:o,
1°N: 0.01 ■Rolled material hardness 2HV230 ■Hot forming part hardness: Hv270 (Heating temperature 1150°C) ■Cold forming part hardness: Hv240 (Cooling rate 700-500°C 80°C/ As is clear from the above explanation, the present invention specifies the steel composition and heat treatment conditions, heat-treats the hot-worked portion of the steel bar, and does not heat-treat the cold-worked portion. As a result, the hot-worked parts can obtain the required strength through the precipitation of carbonitrides, while the soft cold-worked parts can be easily subjected to cold working such as thread rolling. become able to.

In addition, it is possible to reduce the amount of hardness and mold at the heating boundary between the hot-worked part and the cold-worked part.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing the manufacturing procedure for a conventional steel member having a cold-worked part, and FIG. FIG. 3 is a graph showing the hardness distribution of the tension iron, FIG. 4 is a graph showing the change in hardness due to heating of the tension iron, and FIG. 5 is a side view of the bolt. 2... m4'J, 2... tension rod, 2a.
・・Lower arm mounting seat part (corresponding part), 2b・
... Threaded part (corresponding part), 3... Bolt,
3a...head, 3b...screw part. Patent applicant: Toyo Kogyo Co., Ltd. Agent
Person: Patent Attorneys (2) above Figure 1

Claims (1)

[Claims] (1) Weight ratio: C: o, as ~ 0.50%, Si
: 0.15-0.35%, Mn: 0.50-1
．． 5%. V: o, os~0.20%, N: 0.006~0.
A steel rod is formed by hot rolling an N4 material having a composition of 0.02% and the balance of Fe.
After hot forging by heating to 50'O, 700°C
to 500°C at a cooling rate of 10 to io. 1. A method for manufacturing a non-thermal treated steel member having a cold-worked part, characterized in that controlled cooling is carried out within the range of 'C/min, and then necessary parts other than the hot-forged part are cold-worked.