JPS5933190B2 - Heat treatment method for low carbon boron steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to non-oxidation hardening and tempering of parts made of low carbon boron steel.

Non-oxidation quenching and tempering treatments for parts made of steel are
Conventionally, this is done by matching the carbon content of the material and the carbon potential of the atmospheric gas. However, when low carbon boron steel is treated in this way, it has the disadvantage that the surface hardness is lower than the internal hardness. This is because boron B in the steel is oxidized by a small amount of oxygen in the atmospheric gas during quenching and heating, so the B concentration at the surface of the part decreases, resulting in a depth of about 0.1 to 0.2 mwL below the surface. This is because the hardenability of the surface area decreases. This reduction in surface hardness lowers the fatigue strength of the parts, so it has been problematic to use low carbon 10 boron steel in parts where fatigue strength is always an issue, such as automobile parts. Currently, low-carbon boron steel is widely used mainly as a material for high-strength bolts for construction, where fatigue strength is of little concern. However, in the case of this construction bolt, the delayed fracture strength is emphasized as the characteristic required in Requirement 15, and on the other hand, water quenching is used in the heat treatment process during manufacturing, and the generated water vapor causes carbon buildup in the furnace. Since decarburization is unavoidable due to a decrease in potential, commercially available construction bolts are allowed to decarburize to a degree of about 20%.

Therefore, the decrease in surface hardness caused by deboronization, which is slight compared to the degree of decarburization, has not been considered a problem in the past. However, in parts such as automobile parts that are irregular and subject to high external stress, fatigue strength is always an issue, so decarburization is strictly controlled throughout the material stage and heat treatment process. Furthermore, when using low carbon boron steel instead of conventional steel as a material for automobile parts, mainly for economic reasons, the problem of reduced surface hardness due to deboronization cannot be ignored30. , I needed to solve this. In this way, from the point of view of fatigue strength, it is necessary to compensate for the decrease in surface hardness due to deboronization, but on the other hand, in the case of parts used under average stress, such as high-strength bolts, Similarly to architectural bolts, delayed fracture strength is a problem, so it is desirable to have a low surface hardness. In order to satisfy these two contradictory characteristics, it was necessary to match the internal hardness and surface hardness. The present invention aims to ensure the strength of parts by matching the internal hardness and surface hardness after quenching and tempering of parts made of low carbon boron steel. The present invention provides a method for heat treating low carbon boron steel intended for use as a high strength bolt surface material.

The method for heat treatment of low carbon boron steel according to the present invention includes CO. l
5-0.35% J crystal 0.60-1.30 Gourd CrO. 5O
% or less, TlO. O4% or less, AlO. O4% or less, B
When quenching parts made of low-carbon boron steel with residual cooling and unavoidable impurities in a protective atmosphere, the carbon potential of the atmospheric gas should be set to 0.04 to 0.04 to less than the carbon content of the material. By setting the hardness at a high value of 0.25 and slightly carburizing the part surface, it compensates for the decrease in hardenability of the surface part due to deboronization, and makes the surface hardness and internal hardness of the part match after treatment.

In the present invention, the reason why the composition range of the low carbon boron steel and the setting range of the carbon potential of the atmospheric gas are limited as described above is as follows. The carbon potential of the atmospheric gas required to prevent the surface hardness from decreasing due to deboronization and to match the surface hardness with the internal hardness varies depending on the chemical composition of the material and the quenching cooling rate.

However, in the case of the second aspect of the present invention, oil quenching is assumed because it is necessary to strictly control the atmospheric gas. Based on this premise, the inventors investigated the influence of the chemical components of materials and found that CO. 35% or more, Mnl3OOl
) Above, CrO. Content of 5O(f) or more, or MO
The conditions for adding elements that improve firing properties such as and Ni are as follows:
It has become clear that when these elements alone or in combination are satisfactory, the surface hardening properties of the parts are ensured by these elements, so that the surface hardness does not decrease significantly due to deboronization. The amounts of each component were limited to below the above amounts. Furthermore, since increasing the amount of B added does not have the effect of compensating for the decrease in surface hardness due to deboronization, it was set to 5PF or more simply from the viewpoint of improving hardenability. In addition, in order to ensure component strength, the lower limit of the components is set to CO. l5(f), MnO. 6O(
! ). Furthermore, considering the need for Ti and Al to play a role in fixing nitrogen and oxygen in the steel and dissolving B in the steel, as well as in terms of mechanical strength and machinability, each
．． 04(f) or less. Next, for steel with the above composition range, we used round bar test pieces with two different diameters to determine the optimal carbon potential of the atmospheric gas, that is, the surface area after quenching and tempering (by dropping an indenter directly onto the test piece shaft surface). Depending on the method of measurement) hardness (Hv) and internal sound V! ．． The carbon potential X of the atmospheric gas having the same hardness as that of 1) and the difference Y between this X and the carbon content of the material were investigated, and the results are shown in Table 1 below.

Here, the carbon potential is the measured value of the surface carbon content of each test piece. In addition, each test piece has martensite of 90 (f) or more in the center, and the steel components not shown in the table are T in each test piece.
lO. O3(:f), AlO, O3OI), and the remainder of Fe. As seen from Table 1 above, Y is +0.04 to 0
．． Since 25 is optimal, it is limited to this range. Hereinafter, the present invention will be explained in more detail with reference to Examples. Example 1 C0.23%, Mnl, 1O%, CrO. l2(f),
BO, OOl5(fl), TlO. O3Ol), AlO
．． A bolt with a suction diameter of 12 mm was actually manufactured using low carbon boron steel consisting of O3Ol) and the balance was Fe, and Y was 0.
When this bolt was quenched and tempered in an atmospheric gas of Indicated.

A photograph (X4OO) of the surface structure of this bolt is shown in FIG. Comparative Example 1 The same bolt as in Example 1, Y
In other words, the hardness was quenched and tempered in a normal gas atmosphere in which the carbon potential and the carbon content of the material were matched, and the change in hardness with depth was similarly investigated.
Shown in the figure.

Furthermore, a photograph (X4OO) of the surface structure of the bolt is shown in FIG. First, from FIG. 1, bolt a obtained in Example 1 shows a constant hardness of 350H regardless of whether it is on the surface or inside of the bolt, and like bolt b obtained in Comparative Example 1, It can be seen that there is no decrease in hardness at the surface area.

Furthermore, as is clear from a comparison of Figures 2 and 3, the steel structures on the surface of each bolt A and b are different, and the internal structure of bolt b is similar to bolt a in terms of early toughness. However, the surface structure of sorbite is an incompletely quenched structure in which ferrite precipitates at the prior austenite grain boundaries, and this reduces the hardness of the surface region.

Example 2 The same bolt as in Example 1 was quenched and tempered in the atmosphere gas of various carbon potentiometers, and the hardness at a certain depth of the surface (measured by directly dropping an indenter) was measured. The changes were investigated and the results are shown in a graph in Figure 4.

From this figure, it can be seen that the hardness of the surface portion increases almost in proportion to the increase in the carbon potential of the atmospheric gas.

Further, when the fatigue durability limit of each bolt A and b was investigated, the value was 6.5k9/Mltl5.

It was 8 kg/Md.

From this result, the fatigue durability limit value of bolt a produced by the method of the present invention is improved by about 10% compared to bolt b produced by the conventional method. As described above, according to the heat treatment method of the present invention, by setting the carbon potential of the atmospheric gas higher than the carbon content of the material, it is possible to compensate for the decrease in surface hardenability due to the deboronization phenomenon of low carbon boron steel. . As a result, we were able to solve the problem of decreased surface hardness that had conventionally been a problem when using low-carbon boron steel, and achieved significant improvements in terms of increasing component fatigue strength and preventing dents. In addition, if the carbon potential of the atmospheric gas is set high, it can be processed in the same atmospheric gas as medium-carbon chromium steel and chromium-molybdenum steel, which are frequently used.
Producing these steels in parallel has many advantages such as great convenience.

[Brief explanation of drawings]

Figure 1 shows the hardness (H) versus distance (Mm) from the surface of bolts A and b obtained by heat treatment in Example 1 and Comparative Example 1.
2 is a steel structure diagram near the shaft surface of bolt a, FIG. 3 is a steel structure diagram of the above-mentioned portion of bolt b, and FIG. 4 is a steel structure diagram of the bolt obtained in Example 2. A graph showing changes in hardness (Hv) of the surface portion (measured by dropping an indenter directly onto the surface portion) with respect to the carbon potential of the atmospheric gas.

Claims (1)

[Claims] 1 In weight ratio: C: 0.15-0.35% Mn: 0.60-1.30% Cr: 0.50% or less Ti: 0.04% or less Al: 0.04 % or less B: 5 to 30 ppm Fe: When quenching parts made of low carbon boron steel with the remainder composition in a protective atmosphere, the carbon potential of the atmospheric gas is set 0.04 to 0.25 higher than the carbon content of the material. A method for heat treatment of low carbon boron steel, characterized in that the gas is used to slightly carburize the surface of a component.