JPS6024172B2 - Nitriding steel with high core hardness after nitriding without preheat treatment - Google Patents

Description

[Detailed description of the invention] The present invention relates to steel for nitriding, and in particular, the amount of C and V,
By appropriately balancing the correlation between the amount of C and the amount of Cr, as well as the amount of Si, a hard nitrided layer and a deep hardened layer can be generated after the nitriding process, and even after the nitriding process, a hard nitrided layer and a deep hardened layer can be created without any special pretreatment by heat treatment. This invention relates to a nitriding steel that has successfully maintained extremely high core hardness.

Generally, in order to impart wear resistance and fatigue resistance to steel, special steel is hardened and tempered, or roughened steel is carburized and quenched, but these heat treatments Because the quenching treatment is performed after heating to a high temperature above the transformation point of copper, problems such as coarsening of austenite crystal grains during heating and generation of heat treatment distortion during quenching arise.

On the other hand, nitriding treatment is performed at a relatively low temperature below the transformation point V of the steel, so it is characterized by coarsening of austenite grains, without the occurrence of heat treatment distortion, and in addition, extremely high surface hardness can be obtained. Because of this, it is widely used in automobile parts, office parts, etc.

Conventionally, various materials have been applied to this nitriding treatment, such as ordinary steel, sapphire steel, and low-alloy steel. teeth,
Normally, low-alloy steel and nitriding steel are heat-treated in advance to adjust the structure to suit strength and plowability before nitriding treatment, which necessitates an increase in the heat treatment process for conventional steels. In addition, there was a problem that the strength of the material decreased during the nitriding treatment at a temperature of 500 to 600 pm.

Therefore, it has been extremely difficult to obtain a chamber treated product that has a hard nitrided layer and has high core hardness and is plowable. The present invention was made with the main purpose of improving the drawbacks of conventional nitrided steels when such wear resistance, strength, and toughness are required. C
If the correlation amount of r amount and the amount of Si are appropriately balanced, a very hard nitrided layer (Hv650 or more) and a deep hardened layer will be generated after nitriding, and no special pretreatment is required. They discovered that precipitation hardening prevents softening of the material strength during treatment, resulting in a nitriding steel with extremely high core hardness.

That is, the steel for nitriding according to the present invention has C:0.
4% fullness, Si; 0.36-1.50%, Mn; 0.3
~2.0%, Cr; more than 0.71 ~ less than 3.0%, AI
: 0.30% or less, V; more than 0.21 to less than 1.0%,
The remainder: consists of iron and unavoidable impurities during manufacturing, and is characterized by having a material hardness before nitriding in the range of over 20 Hv to over 500 Hv without quenching and firing, and after nitriding without preheating. It has high core hardness. In the steel of the present invention, the strength of the base metal is related to the amount of C and the amount of V, and the surface hardness after nitriding is related to the amount of C and Cr.

The reasons for limiting the range of addition of each element as described above will be described with reference to experimental results regarding these. C affects the strength of the base material and the surface hardness after nitriding.

The surface hardness after nitriding is as shown in Figure 1.
Comparing the amounts, as the amount of C increases, it decreases. Figure 1 shows
Copper with different amounts of C and Cr shown in Table 1 below was 570
After oC × 3 hours of Toughtride treatment (nitriding treatment), the surface hardness (Hv; measurement load was 5k9) was measured, and
To understand the relationship between the amount of C and the amount of Cd that affect this hardness, it is necessary to understand the relationship between the amount of C and the amount of Cd. This is a diagram in which points of the same hardness are arranged and connected by the amount of C and the amount of Cr.

As is clear from the diagram in Table 1 and Figure 1, the amount of C required to obtain a surface hardness of Hv650 or higher with 0.71% Cr additive is 0.4.
It corresponds to %.

Therefore, in the present invention, the upper limit of the amount of C is set to less than 0.4%. Since Cr combines with nitrogen to increase the hardness of the surface nitrided layer, it is also the most important element in hardened steel.

As shown in FIG. 1, if the Cr content is the same, the surface hardness increases as the Cr content increases. 0.4
The amount of Cr that can provide a surface hardness of Hv 650 to 1000 with a C amount of less than 0.7% is more than 0.71 and less than 3.0%, so the Cr amount is limited to this range.

In the present invention, one of the goals is to obtain a steel for nitriding with a surface hardness of Hv650 or more after nitriding treatment,
If the Cd content is 0.71 or less, this target cannot be achieved due to the relationship with the C content. V precipitates vanadium carbonitride during the nitriding process and prevents the strength of the material from softening.

Figure 2 shows steels with different amounts of V in Table 2 below, ranging from 800 to 85.
Hot rolling at a temperature of 0qo (cooling rate after rolling is 80oC)
/min) and measured the base material hardness, and the results are shown in Table 2. This is a diagram obtained by connecting points of the same hardness in relation to the amount of C and the amount of V.

Precipitation hardening due to V varies greatly depending on the amount of C in the steel, but in the present invention, the base metal has the strength (
One of our goals is to achieve a hardness of Hv200 to less than 500, which is the same level as the core hardness), and the V amount that can achieve a hardness of Hv200 to less than 50 degrees when the C amount is 0.4%. is 0.21% from the results in Figure 2.
Since the amount of V in the steel of the present invention is more than 1.0% and less than 1.0%, the V content of the steel of the present invention is 0.
．． More than 21% and less than 1.0%. Si significantly improves the rice grain properties of the base material after chambering treatment. Figure 3 shows steels with different amounts of Si shown in Table 3 below, which are treated with 570 oo
The results of measuring the base material impact value after heat treatment for ×4 hours are shown in relation to the amount of Si. As is clear from FIG. 3, when the amount of Si is 0.35% or less, the impact value decreases rapidly. Therefore, the amount of Si is set to 0.36% or more. However, even if more than 1.50% of Si is added, the effect is small as is clear from FIG. 3, so the Si amount range is set to 0.36 to 1.50%. Table 3 Mn is added as a deoxidizing agent and also has the effect of increasing competitiveness, so its range is set at 0.3 to 2.0%.

A and Cr combine with nitrogen to increase the hardness of the nitrided layer on the surface, but when the amount of A increases, the product becomes more sandy and surface rough, reducing the product value, so it is less than 0.3%. shall be.

Examples of the steel of the present invention will be given below, and the reason for limiting the component content and effects of the steel of the present invention will be specifically explained in comparison with a comparative example.

Example 1 Table 4 shows the chemical components (% by weight) of the copper tested in this example.

Table 4 Each steel of sample number A to day in Table 4 was heated to a finishing temperature of 800qo.
As described above, the material was rolled under normal rolling conditions with a winding temperature of 500 to 700 oo, and the hardness Hv of the hot-rolled material after this rolling (measured load 5k9) was measured.

Each value is shown in Table 4. As is clear from these Hv values, the steels B to F of the present invention have Hv values exceeding 200 and 50.
Comparative steel A, in which V is lower than the range of the present invention, cannot obtain a sufficient Hv value.

That is, it is clear that when the C content is low, the V content is required to be 0.21% or more in order to obtain a hardness of Hv200 or more. Also, when the C content is 0.4%, the V content is 1.0%.
If you try to obtain Hv500 (G steel), and if you try to obtain a value of Hv200 to 50 degrees, like the base metal strength of conventional carburized materials, the V amount will exceed 0.21% and 1. This indicates that it is necessary to add less than 0%. Next, sample number D
, E and F copper, 1250qC x 1 hin
The effect of changing the cooling rate on hardness was investigated, and the results shown in Figure 4 were obtained.

As is clear from the results in Figure 4, 12~240oo/m
Precipitation hardening by vanadium carbides is obtained when cooling at a cooling rate in the range of in. In manufacturing the ball of the present invention,
In consideration of the nitriding treatment (taking into consideration the decrease in material hardness), it is necessary to manufacture the product by cooling at a cooling rate of 24 pi/min or less so as not to cause an over-aging state. Figure 5 shows the relationship between the holding time and hardness when the steels with sample numbers ○, E, and F, manufactured under these cooling conditions, were held at the normal nitriding temperature of 570 qo. It is.

As is clear from the results shown in FIG. 5, no decrease in the hardness of the material was observed if the treatment time was 8 hours or less. Therefore, it is clear that the key of the present invention maintains high core hardness even when subjected to the usual tuftride treatment (570 qo x 3 hours). Example 2 Table 5 shows the chemical composition (% by weight) of the steel tested in this example.

Table 5 Each of the steels with sample numbers 1 to N in Table 5 was rolled under normal rolling conditions and then subjected to nitriding under the same conditions (57000×4H
r), and the impact values of each were examined and the results shown in Table 6 were obtained.

Table 6 also shows the impact values when heat treatment is performed after 15% and 30% cold rolling, taking into consideration cases where nitriding is performed after product processing. displayed. However, the impact test was conducted at room temperature (2300℃) using a 2-rib U-notch impact test piece. From the results in Table 6, it can be seen that Si
It is clear that the impact value significantly improves as the amount of steel increases, and in the case of the steel of the present invention, there is only a slight problem that the impact value decreases due to processing, and that a nitrided product with excellent strength and grade can be obtained.

For example, K steel shows approximately twice the impact value of J steel, and as is clear from the results in Figure 3, sufficient rutting resistance cannot be obtained after nitriding unless the Si content is 0.36% or more. do not have. Moreover, if it is 0.36% or more, even processed products will maintain sufficient quality after nitriding treatment. 76 Example 3 Table 7 shows the chemical components (% by weight) of the copper sampled in this example.

After rolling each steel ○ to S in Table 7 under normal rolling conditions, 57
Tuftride nitriding treatment of 0 qo x fine r was performed.

The cross-sectional hardness distribution of the obtained nitrided product was measured, and the results shown in FIG. 6 were obtained. As is clear from the results in Figure 6, C
As the amount of r increases, the surface hardness increases, and this phenomenon itself is a well-known fact, but in the case where Cr is 0.7% or less (comparative steels ○ and P), it is difficult to achieve the objective of the present invention. It is not possible to obtain a surface hardness of Hv650 or higher.

Therefore, in the case of the steel of the present invention, the Cr content must exceed 0.71%. Table 7

[Brief explanation of drawings]

Figure 1 shows the relationship between the amount of C and the amount of Cr on the surface hardness Hv (measured load 5k9) after nitriding treatment (57000 x fine r tuftride treatment), and Figure 2 shows the relationship between the amount of C and Cr after rolling (normal hot rolling). Figure 3 is a diagram showing the relationship between C content and V content on base material strength (base metal hardness) in
The figure shows the relationship between cooling rate and base metal hardness, showing the influence of cooling rate in the steel manufacturing process on the precipitation hardening behavior of vanadium carbides. Figure 5 shows the holding time and base metal hardness when held at the nitriding temperature. FIG. 6 is a diagram showing the cross-sectional hardness distribution when copper with different amounts of Cr is nitrided. Figure 1 Figure 2 Figure 5 Figure 6 Figure Dimensions Figure 3

Claims (1)

[Claims] 1% by weight, C: less than 0.4%, Si: 0.36-1
．． 50%, Mn; 0.3 to 2.0%, Cr; more than 0.71 to less than 3.0%, Al; 0.30% or less, V; 0.2
More than 1% to less than 1.0%, remainder: consisting of iron and unavoidable impurities during manufacturing, and characterized in that the material hardness before nitriding is in the range of more than 200 Hv and less than 500 Hv without quenching and tempering. Nitriding steel with high core hardness after nitriding without heat treatment.