JPH0247536B2 - REIKANTANZOYOKYOJINKO - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a carbon steel for mechanical structures with improved cold forgeability, and aims to provide a strong steel with excellent cold forgeability. Generally, high-strength steel is used by quenching and tempering high-carbon steel with a wear resistance and rolling fatigue resistance of 0.4% or more during use to increase the surface hardness. However, even when such high carbon materials are subjected to spheroidizing annealing treatment (hereinafter abbreviated as S.A.), their deformability (as indicated by the degree of processing required until cracking occurs during cold forging) is low. It also has a low deformation resistance (indicated by the amount of force required for processing). Therefore, in the past, cold forging was difficult and hot forging was used. However, hot forging has poor dimensional accuracy and requires a subsequent cutting process. This cutting process requires a large number of man-hours and is very complicated, so cold forging is desired since it has good dimensional accuracy and can greatly reduce the cutting process. As a result of various studies in view of these points, the present inventors found that Si, Mn, P, and N have a more negative effect on deformation resistance than C, and that it is the surface layer that inhibits deformability. We identified that the inclusions were Al ₂ O ₃ -based inclusions and MnS-based inclusions, and in order to deal with them, we required low Si, low Mn, low P, and low N, and further reduced Al and O as much as possible. By compensating for the lack of hardenability with Ti, B, C, or even Cr, and deoxidizing with Ca, we were able to develop a steel material with significantly improved deformation resistance and deformability. The present invention was made based on this knowledge, and its gist is that it contains 0.4 to 0.75% of C and 0.1% of Si.
% or less, Mn 0.4% or less, P 0.01% or less, Al 0.01%
Below, we will reduce N to 0.005% or less, O to 0.005% or less, Ti to 0.05% or less, B to 0.003% or less,
A strong steel for cold forging characterized by adding 0.01% or less of Ca or 0.8% or less of Cr to this, with the balance being Fe and inevitable impurities. The present invention will be explained in detail below. First, in the present invention, C is limited to 0.4 to 0.75% because C is an important alloying element that determines quenching hardness.
If it is less than 0.4%, sufficient surface hardness cannot be obtained.
It was set at 0.4% or more. In addition, if it exceeds 0.75%, quench cracking during quenching tends to occur, so the upper limit was set at 0.75%. Next, in the present invention, Si, Mn, P, Al, N,
One of the characteristics is that O is reduced, and the reason is as follows. That is, since Si is necessary as a deoxidizing element during steel manufacturing, Si was conventionally contained in an amount of 0.2 to 0.3%. However, Si
The strength increase during SA is large, and the deformation resistance especially at high deformation is greatly increased. However, Si
Since we have found that the harmful effects are significantly reduced when the content is less than 0.1%, we set the upper limit to 0.1%. Note that the problem of insufficient deoxidation is compensated for by Ca, which will be described later. Next, Mn fixes S as an impurity and is necessary to prevent surface defects during hot rolling, but the amount is
0.4% is sufficient. In conventional steel, 0.7 to 0.8% of C was added as a relatively inexpensive and useful element for ensuring hardenability. However, like Si, Mn is an element that greatly inhibits the reduction of deformation resistance during high deformation, so from this point of view, the upper limit of the Mn content was set at 0.4%. Note that this lack of hardenability is compensated for by Ti, B, and (Cr), which will be described later. P and N are contained as impurities in steel, but they age harden as the temperature rises during cold forging and cause an increase in deformation resistance, so they should be less than 0.01% and 0.005% respectively.
% or less. Moreover, Al is not only effective in deoxidizing, but is also an effective alloying element that fixes N to become AlN and plays the role of refining crystal grains. However, Al ₂ O ₃ generated during deoxidation causes cracks during cold forging,
As a countermeasure, it is essential to reduce Al and O. In particular, when Ca treatment was performed, it was confirmed that Al ₂ O ₃ -based inclusions were no longer observed when the Al content was 0.01% or less, so the upper limit was set at 0.01%. O combines with Al, Si, etc. to form oxides,
The upper limit is set at 0.005 as it may cause cracking during cold forging.
%. In the present invention, in addition to reducing the above-mentioned components, Ti and B are added to compensate for the lack of hardenability, and Ca is actively added for deoxidation. First, Ti fixes N, supplements the hardenability improvement of B, and becomes TiN, which plays the role of grain refinement, and is essential in products with low Al content. However, the amount of 0.05% is sufficient, and if the amount added is too large, it will generate carbides and cause a decrease in toughness.
It was set to 0.05% or less. The desirable range is
It is 0.01% to 0.02%. B becomes necessary to compensate for the lack of hardenability due to low Si and low Mn. If B is absent, the quench hardening depth cannot be obtained and the properties of strong steel cannot be obtained. B can ensure sufficient hardenability with a trace amount of 0.003% or less, does not affect the increase in strength during SA, and is a very effective alloying element for cold forging strong steel. Furthermore, if this amount is too large, it will precipitate to grain boundaries and cause grain boundary embrittlement, so the upper limit was set at 0.003%. Note that the desirable range is 0.001 to 0.002%. Ca is a strong deoxidizing element, especially for low Si and low Al
In this material, Ca is essential. Furthermore, it combines with the impurity S to eliminate the harm caused by S, and also prevents the harm caused by flaws during hot rolling with low Mn. However, if the amount is too large, it may absorb even O in the air during agglomeration or reduce the refractory.
Since it becomes CaO and causes a decrease in rolling contact fatigue properties, it is set at 0.01% or less. Note that the desirable range is 0.002% to 0.004%. Next, regarding Cr, when hardenability is still insufficient with Ti and B alone, Cr has high hardenability and does not significantly increase the strength during SA, so it is excellent because it has low Si, low Mn, and low P. , it is desirable to add Cr while keeping the N low. In this case, 0.8% is sufficient to compensate for the decrease in hardenability due to low Si, low Mn, low P, and low N. If the amount is large, the strength during SA will increase, so the upper limit should be set.
It was set at 0.8%. The strong steel referred to here refers to steel that has been quenched and tempered, and has a surface hardness of 500 on the Vikas scale.
In addition, the absorbed energy of U-notch yalpy (JIS2202, No. 3 test piece) at room temperature is 5.
It is a steel material weighing more than Kg・m. Next, the effects of the present invention will be explained in more detail based on Examples. Using steel materials with the chemical composition shown in Table 1, we performed spheroidizing annealing treatment, investigated the load and cracking conditions at that time, and conducted induction hardening and tempering tests on the surfaces of 30 test materials. The results were also shown. Nos. 1 to 7 in Table 1 are conventional steels, and Nos. 8 to 13 are steels of the present invention. First, No. 1 has a C content of 0.44%, the load during the compression test was 69 tons, and the hardness after induction hardening and tempering was 620, but the cracking rate during the compression test was 10.8%. . No. 2 is a lower N version of No. 1, and the load during the compression test is reduced to 67 tons, but the cracking rate is 11.2%.
There is also. In No. 3, when Cr is added with low Si and Mn, the compressive load is further reduced to 59 tons, but the cracking rate is high at 13.1%. No. 4 has a C content of 0.56%, and the hardness after quenching and tempering is as high as 690, but the compressive load is 76t.
Therefore, the cracking rate is 32.6%, which is about 1/3. No. 5 is a case where Ti and B are added to low Si, and although the quenching and tempering hardness increases to 710, the compressive load decreases to 74t, which is better than No. 4. However, the cracking rate remains high. No. 6 is a case where C is 0.64%, and the hardness after quenching and tempering is 730, but the compressive load is 83t, and the cracking rate is 67.8%, which is about 2/3rd. . No. 7 is a case where Ca is added alone, and although the splitting rate decreases somewhat, about 60% of the samples still crack. In comparison, No. 8, which is the steel of the present invention, has C.
Although the hardness reaches 610 when quenched and tempered at 0.44%, the compressive load is as low as 50t, and cracking is unlikely. No. 9 had a hardness of 630 after quenching and tempering when Cr was added, and similarly, no cracks were observed. No. 10 has a hardness of 690 after quenching and tempering when the C content is 0.5%, and the compressive load increases somewhat to 59t, but the cracking rate is very low at 0.5%. No. 11 shows the same excellent properties when it has even lower Mn and Cr is added. For No. 12, the hardness after quenching and tempering reaches 720 when C is 0.63%, but the compressive load further increases to 64t, and the cracking rate increases slightly to 1.2%.
It exhibits far superior properties compared to conventional steel. No. 13 was obtained by making No. 12 even lower in Mn and adding a small amount of Cr, but the same characteristics as No. 12 were obtained. As described above, the steel of the present invention provides a steel material that can withstand cold forging while satisfying sufficient strength after quenching and tempering, and is very useful industrially.

【table】

【table】

Claims (1)

[Claims] 1 Contains 0.4 to 0.75% C, 0.1% or less Si, 0.4 Mn
% or less, P0.01% or less, Al0.01% or less, N0.005%
Below, we will reduce O to 0.005% or less, and
A strong steel for cold forging, characterized by the addition of 0.05% or less of Ti, 0.003% or less of B, and 0.01% or less of Ca, with the balance being Fe and unavoidable impurities. 2 Contains 0.4 to 0.75% of C, Si, 0.1% or less,
Mn 0.4% or less, P 0.01% or less, Al 0.01% or less,
N0.005% or less, O00.005% or less, Cr0.8% or less, Ti0.05% or less, B0.003
% or less, Ca0.01% or less is added, and the balance is Fe and unavoidable impurities.