JPH0533283B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to softening of mechanical structural steel, and particularly to a method of softening medium carbon mechanical structural steel used for cold forging. (Prior Art and Problems) Conventionally, medium-carbon mechanical structural steels have been developed using techniques such as Japanese Patent Application Laid-Open No. 136421/1983, etc., in order to lower deformation resistance and improve cold forging during cold forging. Generally, cementite is subjected to spheroidizing annealing as shown in . However, the spheroidizing annealing of the lever is a very long process, usually over ten hours or more, so if it could be replaced with softening annealing that only maintains the A point below ₁ , it would have a great industrial effect, but it is not specified in JIS. In the case of steel for machine structural use, the softened annealed material is far less soft than the spheroidized annealed material. The present inventors analyzed the effect of rolled material strength on the strength of materials annealed at temperatures below point _A1 , and found that if the rolled material is sufficiently softened, it can be annealed for a short time at temperatures below point _A1 . It has been found that softening comparable to that of spheroidized annealed materials can be achieved. In other words, when annealing at a temperature below point _A , the tensile strength decreases approximately in proportion to the annealing time, but if the strength of the rolled material is sufficiently lowered in advance, it can be reduced to the same strength as the spheroidized annealed material in a short time. I found out what I could get. Therefore, the present inventors analyzed various factors that control the strength of rolled medium carbon steel materials, and found that it is effective to coarsen the cementite spacing of pearlite in order to reduce the strength of rolled materials. It is sufficient to carry out pearlite transformation at as high a temperature as possible in the cooling process after hot rolling, and from this point of view, the steel composition, rolling cooling conditions, and subsequent _A1 point below annealing are required to cause pearlite transformation at as high a temperature as possible after rolling to soften it. The present invention was made based on the completely new knowledge that it is only necessary to select appropriate conditions. (Means and effects for solving the problems) That is, the present invention has been made based on the above knowledge, and its gist is that
C0.32-0.65%, Si less than 0.05%, Mn0.2-0.5%, Cr0.1-0.7 with the total amount of Mn and Cr ranging from 0.3 to 1.1%.
%, contains Al0.005-0.1%, and limits P to less than 0.02%, S to less than 0.02%, and other settings as necessary.
(A) One or more of Ni 1% or less, Cu 1% or less, Mo 0.3% or less, or (B) Ti 0.002 to 0.05%,
B0.0005~0.02%, Nb0.005~0.05%, V0.005~
For steel containing 0.1% of one or more of the groups (A) and (B), the remainder being Fe and unavoidable impurities, the temperature is 5 to 60°C after hot rolling.
After slow cooling at a cooling rate of 680 to 720
This is a method for softening steel for mechanical structural use, which is characterized by holding the temperature at a temperature in the range of 15 minutes to 4 hours. The present invention will be explained in detail below. First of all, in the present invention, softening means that the tensile strength of the rolled material is equal to or less than 28+65×C% (Kg/mm ² ), which is defined by the carbon content (C%). . This equation was obtained by regression with the C content varied from 0.2 to 0.7%, and 28 is a term that depends on the strength of ferrite and pearlite, and 65 is a term that depends on the C content, that is, the pearlite content. If the strength exceeds the value of the same formula determined by the amount of C, it cannot be said that the material has been softened. Next, the reason for limiting the composition of steel, which is the object of the present invention, will be described. First, C is quenched after cold forging. It is an essential element to impart the required strength to the product during tempering treatment, but if it is less than 0.32%, the required strength cannot be obtained, while if it exceeds 0.65%, the strength after quenching and tempering will no longer increase. Therefore, it was limited to the range of 0.32 to 0.65%. Furthermore, since Si increases the strength of the rolled material through its solid solution hardening effect, the content was limited to less than 0.05%, which makes the effect of solid solution hardening negligible.
Furthermore, even if Si is lowered in this way, the hardenability required during hardening treatment does not decrease. Next, regarding Mn and Cr, the most important point of the present invention is that Mn and Cr are added in combination and their contents are determined as described above. In other words, S45C steel, a conventional mechanical structural steel,
C0.42~0.48%, Si0.15~0.35%, Mn0.60~0.90%
However, by reducing the amount of Mn and adding Cr instead, S45C
Compared to steel, the ferrite transformation start temperature and the pearlite transformation start and end temperatures, which are the points of softening, are higher. Therefore, in order to achieve the target softening in a short time, it is better to replace Mn with Cr as much as possible, but if Mn is less than 0.2%, S in the steel cannot be sufficiently fixed, and hot Unable to contain fragility. On the other hand, when Mn exceeds 0.5%
Even if Cr is added, the pearlite transformation at high temperatures cannot be completed in a short time, so Mn
The amount was limited to 0.2-0.5%. Cr is an essential element for promoting pearlite transformation at high temperatures, but if the amount added is less than 0.1%, sufficient effect will not be exhibited. On the other hand, if it exceeds 0.7%, the hardenability of the steel will increase and the transformation temperature will drop, so the content of Cr was limited to 0.1 to 0.7%. Furthermore, the total amount of Mn and Cr was limited to 0.3 to 1.1% because if the total amount is less than 0.3%, hardenability during post-forging quenching treatment cannot be guaranteed.
If it exceeds %, the hardenability of the steel material will increase too much and the rolled material will not become soft and will not fall below the target strength.
Limited to 0.3-1.1%. Furthermore, Al is added to prevent coarsening of the austenite grain size during the quenching process after forging, and if it is less than 0.005%, it has no effect;
If it exceeds 0.005% to 0.1%, the effect of suppressing austenite grain coarsening will be saturated and will actually deteriorate cold forgeability, so it is limited to 0.005 to 0.1%. Both P and S are elements harmful to cold forgeability. In either case, if it exceeds 0.02%, the negative effects become noticeable, so the content was limited to less than this. The above are the basic components of the steel targeted by the present invention, but in the present invention, in order to improve the strength and toughness of the steel, (A) Ni 1% or less, Cu 1% or less,
One or more types of Mo0.3% or less, or (B)Ti0.002 to promote pearlite transformation in high temperature range
0.05%, B0.0005~0.02%, Nb0.005~0.05%,
(A), (B) of one or more types of V0.005-0.1%
It is also possible to contain one or both of the groups. First, Ni, Cu, and Mo in group (A) are all added to improve the strength and toughness of products, but Ni1
%, Cu1%, and Mo0.3%, the effects are saturated, so these were set as upper limits. On the other hand, Ti, B, Nb, and V in group (B) are all added for the purpose of promoting pearlite transformation in a high temperature range. That is, it is more effective to add Ti and B in combination, and Ti is added in order to fix N together with Al and fully exhibit the hardenability effect of B. Ti
By increasing the hardenability during post-forging quenching treatment by adding B and B, it becomes possible to reduce the total amount of Mn and Cr, and the pearlite transformation at high temperatures is completed in a shorter time. Become. Ti is 0.002%
Below 0.05%, the N fixed effect is insufficient;
If it exceeds 0.2%, coarse TiN will be generated which is harmful to cold forgeability, so it is limited to 0.002% to 0.05%. B is
If it is less than 0.0005%, the effect of increasing hardenability will not be exhibited.
If it exceeds 0.02%, coarse B compounds will precipitate and the toughness will deteriorate, so it is limited to 0.0005 to 0.02%. Both Nb and V are added for the purpose of refining the austenite grain size after rolling and promoting transformation, but if each is less than 0.005%, no refining effect can be expected, while Nb is 0.05% and V is 0.1%. %, coarse carbonitrides of Nb and V will precipitate and deteriorate toughness and cold forgeability.
Nb was limited to 0.005-0.05%, and V was limited to 0.005-0.1%. Next, the softening treatment conditions in the present invention will be described. First, the reason why we limited the cooling rate after hot rolling to 60°C/min or less for steel with the above chemical composition is that if it is cooled faster than 60°C/min, it will undergo pearlite transformation at low temperatures, which will strengthen the rolled material. This is because the target softening cannot be achieved even in the subsequent annealing at 680 to 720°C. On the other hand, although it is advantageous to have a lower cooling rate, 5°C/min was set as the lower limit because the strength after annealing does not change significantly even if the cooling rate is lower than 5°C/min. In addition, the reason why the annealing temperature after rolling was limited to 680-720℃ is because the target softening cannot be achieved at temperatures lower than 680℃, while elements such as Mn segregate within the steel when the temperature exceeds 720℃. This is because the parts become austenitized. The retention time was limited to 15 minutes to 4 hours, but this is because softening does not proceed to the target value in less than 15 minutes, while it becomes sufficiently softened within 4 hours and there is no need to retain it any longer. Note that hot rolling conditions are not particularly limited, but a hot rolling start temperature of about 1050° C. or lower is desirable, and a low-temperature finish of 900° C. or lower is particularly desirable from the viewpoint of austenite grain refinement, which is advantageous for promoting transformation. Hereinafter, the effects of the present invention will be explained in more detail with reference to Examples. (Example) Table 1 shows the chemical composition of the sample material, as well as the cooling rate and softening treatment conditions after finishing it to 11φmm by normal hot rolling. Test numbers 1, 3, 5, 7 in the same table,
9, 11-14, and 24-27 are examples of the present invention, and the others are comparative examples. Using these materials, tensile tests are conducted according to JIS
Conducted with No. 14A test piece, cold forgeability evaluation was 11φmm×
It was determined by the presence or absence of cracks when a 21 mm test piece was subjected to a compression test with a true strain of 2. An ○ mark indicates that no crack occurred, and an x mark indicates that a crack occurred.
In addition, the toughness value after quenching and tempering was determined by heating the material to 900℃ for 30 minutes, oil quenching, then tempering it to 600℃ for 1 hour, and then performing an impact test at 20℃ using a JIS No. 3 test piece. It is something that These test results are also listed in Table 1.

【table】

Claims (1)

[Claims] 1. C0.32-0.65% by weight, Si less than 0.05%, Mn0.2-0.5%, Cr0.1-0.7% in the range of the total amount of Mn and Cr from 0.3 to 1.1%. , Al 0.005 to 0.1%, P is limited to less than 0.02%, S is limited to less than 0.02%, and as necessary, (A) Ni 1% or less, Cu 1% or less, Mo 0.3% or less.
species or two or more species, or (B) Ti0.002-0.05%, B0.0005-0.02%,
Contains one or more of Nb0.005-0.05%, V0.005-0.1%, one or both of groups (A) and (B), and the remainder is
After hot rolling, the steel containing Fe and unavoidable impurities is gradually cooled at a cooling rate of 5 to 60°C/min.
A method for softening steel for machine structural use, which is further maintained at a temperature in the range of 680 to 720°C for 15 minutes to 4 hours.