JPS58126922A - Production of low carbon boron steel parts - Google Patents

Abstract

PURPOSE:To prevent formation of coarse crystal grains and to obtain low carbon boron steel parts having excellent toughness surely by cold working a blank material of low carbon boron steel material of specific compsns. at limited working rates and subjecting the same to hardening and tempering. CONSTITUTION:The steel consisting of 0.15-0.30wt% C, 0.15-0.35% Si, 0.60- 1.60% Mn, <=1.20% Cr, 0.0005-0.0030% B, 0.01-0.04% Ti, 0.01-0.04% Al, and the balance Fe and unavoidable impurities is used as a blank material. Such blank material is beforehand formed to a size larger than the shape of products and is subjected to cold working such as cold forging at >=13% working rates. Thereafter, the material is applied with hardening and tempering treatments. By such method, the formation of coarse crystal grains is prevented, and the fine grains of which the grain sizes of the old austenite crystal after the hardening is >=5 ASTM grain size number are obtained surely. This method is used for production of parts for structural purposes such as bolts.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing structural parts such as bolts and similar parts from low carbon Zolon steel.

As is well known, if a small amount of boron (B) is added to low carbon steel, the hardenability can be significantly improved at low cost without using expensive special elements such as Cr or Me. has attracted attention as a steel material suitable for automobile structural parts, such as bolts, and has recently been studied and put into practical use in various ways.

However, since the coarsening temperature of austenite crystal grains in Ron steel is lower than that of other conventional steel types, such as ordinary carbon steel, chromium steel, chromium molybdenum steel, etc., the crystal grain size tends to coarsen during quenching heating. As a result, toughness may be reduced. That is, as shown in Table 1, the present inventors applied water quenching to a low carbon steel and a chromium steel with the composition shown in Table 1 at various temperatures of 850 to 1200°C for 1 hour and then water quenching to reduce the prior austenite grain size. As a result of the measurement, a grain coarsening curve shown in FIG. 1 was obtained.

From Figure 1, in the case of chromium steel, austenite crystal grains are fine in the temperature range of 850 to 900°C, and 950°C
It is clear that the grains become coarser rapidly from about 850°C onwards, whereas in the case of carbon steel, the grains suddenly become coarser from 850°C onwards.

As mentioned above, the reason why the crystal grains of boron steel become coarse at relatively low temperatures is thought to be as follows. In other words, in the case of boron steel, the phoneme in the steel is fixed as TIN etc. by adding TI etc. in order to have the effect of improving hardenability by allowing the B added in a small amount to be solid dissolved in the steel without precipitating as BN. This is thought to be due to the fact that the precipitation of A4N, which is effective in preventing crystal grain coarsening in the normal quenching heating temperature range, is not sufficient.

As is clear from the above, the method of refining the austenite grains of products made from boron steel with the characteristics described above is to lower the quenching heating temperature and coarsen the grains during quenching. Prevention There are ways to excel.

However, when performing cold processing such as cold forging without annealing the material, it is necessary to use low carbon steel with a carbon content of 30% or less as the material; Therefore, the AI transformation point is high, so the material is heated to 880℃.
If the quenching is carried out at a lower temperature than the quenching temperature, quenching defects will occur, so it is practically difficult to set the quenching heating temperature to such a low temperature.

On the other hand, it has also been considered and seen that coarsening of crystal grains can be prevented by shortening the quenching heating time. However, when the present inventors conducted a quenching experiment with a boron steel having the composition shown in Table 1 at a quenching heating temperature of 880°C and various heating times, it was found that Even when heated for a short time, the particles became coarse grains with an A8TM particle size number of 5 or less, and a tendency to mix grains was also observed. Therefore, it is difficult to realize a method of preventing crystal grain coarsening by shortening the heating time using ordinary furnace heating.

In addition, there is a tendency for finer grains to be obtained by decreasing the heating rate to the quenching heating temperature, but this effect is not significant, and therefore, the method of reducing the heating rate is effective in making grains finer. It is difficult to obtain a practically sufficient effect for the purpose.

As mentioned above, in boron steel, especially in low carbon boron steel with C0.30% or less, it is possible to reliably prevent grain coarsening and reduce toughness during quenching heating by changing the heat treatment conditions. It was difficult.

This invention was made in view of the above circumstances.
When applying low carbon boron steel of 0.30% or less to parts such as melts that are quenched and tempered after cold working such as cold forging, coarsening of crystal grains is prevented and the quenched edges are improved. The purpose is to reliably obtain fine grains with a prior austenite crystal grain size of 5 or more in ASTM grain size number.

As a result of repeated experiments and studies to achieve the above-mentioned purpose, the present inventors have found that a material made of low carbon boron steel with a C0.30% or less is cold forged to a processing rate (cross-sectional area ratio) of 13 inches or more. Quenching and tempering after cold working of the cape
Therefore, it was discovered that fine particles with an ASTM particle size number of 5 or more could be obtained, and this invention was made. That is, the manufacturing method of the present invention includes C0.15 to 0.30%, SlO
, 15-0.35%, Mn 0.60-1.60%,
C1, 20% or less, BO, 0,0005 to 0.0030
%, TiO, 01-0.04%, A70.01
Cold processing such as cold forging at a processing rate of 13 inches or more is made of steel containing up to 0.04% and the remainder F and unavoidable impurities, and the material is larger than the product shape. It is characterized in that it is then subjected to quenching and tempering.

The method of the present invention will be explained in more detail below.

First, the reason for limiting the composition of steel to be used in the method of this invention will be explained.

C must be contained at least 0.15% in order to ensure strength after quenching and tempering. Also, as the C content increases, the A3 transformation point decreases, so it is possible to achieve even finer grains by lowering the quenching heating temperature, but it is possible to achieve even finer grains by lowering the quenching heating temperature. It becomes difficult to perform cold working without annealing, and addition of B improves hardenability.Although Si is an effective element for deoxidizing molten steel,
If added in excess, the amount of inclusions will increase and the mechanical properties of the product will deteriorate, so the appropriate range is o, 15 to 0.3.
It was set at 5%.

Mn is effective in improving hardenability, but if it is less than 0.60, the effect is not sufficient, while if Mn is contained in excess, cold workability is impaired. In this invention, since it is necessary to perform cold working at a processing rate of 13 inches or more, the content is set at 0.60 to 1.60% as a range that does not cause problems in cold workability.

Cr is also an element that improves hardenability, but if excessively contained, it impairs cold workability, so the upper limit was set at 1.20 inches.

Ti, A1. 5. Both fix N and 0 in the steel. B
The addition of B has the effect of making it a solid solution in the steel and exerting a remarkable hardenability improvement effect, but each 0.01
If the value is less than '-, the effect is not sufficient. Also, TI is 0
．． If it exceeds 0.04%, large amounts of TIN will precipitate, which will adversely affect the mechanical properties of the product, so the amount of TI added is set at 0.01 to 0.04%. On the other hand, Nun is also 0.04-
If it exceeds At20. Since large amounts of coarse inclusions such as the like increase and impair mechanical properties, the content is regulated within the range of 0.01 to 0.04%.

As mentioned above, B is dissolved in steel and has the effect of significantly improving hardenability, but the effect is not sufficient when it is less than 0.0030%. However, the hardenability will not improve any further, and it will cause embrittlement and red heat embrittlement due to F・2B, so o,ooos~O)OO30-
And so.

Next, the relationship between the cold working rate and the austenite grain size when cold working is applied to steel having the above-mentioned composition range will be explained.

The present inventors are the first steel with the above-mentioned composition range! ! Prepare a steel with the composition shown as melon steel in , cold forge the steel at various working rates, and then heat treat it under conditions that are considered to be the most likely to cause coarsening within the range of commonly used heat treatment conditions, i.e. 900 A soaking treatment was carried out for 1 hour at ℃ to obtain old austenite which had been quenched. As is clear from Fig. 3, as the cold working rate increases, the grains become finer, and in particular, if the cold working rate is 13 or more, fine grains with an A8TM grain size number of 5 or more can be obtained. Ta. Even if the quenching heating conditions are such that crystal grain coarsening is likely to occur, by performing cold working such as cold forging with a processing rate of 13% or more in advance, fine grains with an ASTM grain size number of 5 or more can be produced. Therefore, there is no need to particularly lower the quenching heating temperature, but in order to further refine the grains, it is desirable to set the temperature as low as possible, above the transformation point A. Note that the tempering treatment after quenching may be performed at a temperature below the A1 transformation point according to a conventional method.

The manufacturing method of the present invention is most suitably applied to gelt or similar products, but when producing gelt by cold forging, upsetting forging is usually adopted, so the cold working rate is 11. Usually, it is highest at the bottom and lowest at the root shaft. Therefore, the cold working rate of the root details becomes 13- or more, and as a result, the prior austenite crystal grain size of the entire product can be refined to ASTM grain size number 5 or more. In this way, in order to make the cold working rate (cross-sectional area ratio) of the gel shaft part 13 inches or more, K
For this, it is sufficient to use a material that is 1.07 times or more the maximum diameter of the gel shaft part of the product.

Examples of this invention will be described below.

Example CO, 28 yen, 810.2356. Mn 1.06%
, Cr011・5%, Bo, 012%, ? 10.023
Low carbon consisting of %%AL0.03, balance F and unavoidable impurities? A bar with an outer diameter of 12IEII (conventional method) made of steel with a nominal diameter of 12 IEII and a bar with an outer diameter of 13 m made of steel with the same low carbon content (inventive method) were used.
The gelt (2) was prepared by cold forging, soaked at 900°C for 1 hour, quenched, and then tempered at 500°C. When the prior austenite crystal grain size of the gel shaft after quenching was investigated, the results shown in Table 2 were obtained.

As is clear from Table 2, the prior austenite grain size after quenching becomes significantly larger when the shaft processing rate is 17- compared to when the shaft processing rate is ~〇-.
It was confirmed that the particles were fine particles with a particle size number of 6 or more.

As is clear from the above explanation, according to the manufacturing method of the present invention, when manufacturing parts such as bolts using low carbon boron steel, cold forging with a processing rate of 1311 or more is performed before quenching and heating. By performing processing, fine grains with A8TM grain size number 5 or higher can be reliably and easily obtained as prior austenite crystal grains after quenching, and therefore products such as gelt with excellent toughness can be reliably and easily obtained. Obtainable.

[Brief explanation of drawings]

Figure 1 is a diagram showing austenite grain coarsening curves in conventional normal Geron steel and chromium steel, and Figure 2 is a line ratio showing the relationship between quenching heating time and austenite grain size in conventional Geron steel. FIG. 2 is a diagram showing the relationship between austenite grain size and austenite grain size. Applicant Toyota Motor Corporation Representative Patent Attorney Yutaka 1) Hisashi Take (and 1 other person) 1 (8taka, LL Sakiseki Hitoshiki) 0 880・C1=b ηHitteiζ, Kaken (Tonight's 3rd Zuyumon〃Ω2san% ヰ

Claims (1)

[Claims] C0.15-0.30% (weight%, same below), S10
．． 15-0.35%, Mn 0.60-1.60
%, Cr1.20 or less, Bo, 0005-0.003
0chi, Ti O, 01~0.04chi, kl 0.01~
The material is made of steel containing 0.04fi and the remainder F and unavoidable impurities, and the shape of the material is larger than the product shape, and the material is subjected to cold working at a processing rate of 13- or more. A method for manufacturing low carbon Zolon steel parts, characterized by quenching and tempering after death.