JPH0576522B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a spheroidizing annealing method for mechanical structural steel, and in particular, a spheroidizing annealing method for softening medium carbon mechanical structural steel to be subjected to cold forging. It is about improvement. (Prior art and problems) Conventionally, medium carbon steel for mechanical structures has been softened in order to lower its deformation resistance and improve cold forgeability during cold forging. Spheroidizing annealing treatment of cementite is used as a method. This spheroidizing annealing treatment is performed, for example, in JP-A-59-
As seen in Publication No. 163421, after heating to a temperature of ₁ point A or more, continuous cooling is performed at a very low rate of about 10°C/hour, or after heating to a temperature of ₁ point A or more, the temperature is just below the ₁ point of A. This is done by keeping it in place for at least 3 hours. However, this requires a very long processing time, which is industrially disadvantageous. On the other hand, attempts are made to increase the cooling rate of continuous cooling for the purpose of shortening the annealing time, or to shorten the retention time just below point _A , resulting in an increase in strength, which both serve the purpose of softening. Unachievable. By the way, rolled materials of medium carbon steel are generally
As seen in Publication No. 59-136421, it has a pearlite structure or a ferrite-pearlite structure. Therefore, in order to reduce the strength, it is necessary to reduce the strength of pearlite, which occupies most of the structure. Generally, the strength of pearlite is inversely proportional to the cementite spacing, so in order to reduce the strength of spherical pearlite, it is necessary to coarsen the cementite spacing of pearlite. However, the intercementite spacing of spherical pearlite is primarily determined by the temperature at which austenite cara pearlite transforms and forms, and the higher the temperature transforms, the coarser it becomes. However, the higher the temperature, the slower the pearlite transformation progresses, and it takes an extremely long time to complete. ( _Means /effects for solving the problem) The present inventors analyzed various conventional findings and examined the factors governing the strength of medium-carbon mechanical structural steel. As a means of promoting pearlite transformation in a nearby temperature range and softening the steel material through short-time spheroidizing annealing, some of the Mn contained in conventional steel is replaced with Cr, and spheroidizing is performed. The present invention was made based on the completely new knowledge that this can be achieved by selecting predetermined annealing conditions. The present invention was made based on the above findings, and the gist thereof is that C0.32 to 0.65% by weight,
Si less than 0.05%, total amount of Mn and Cr in the range of 0.3 to 1.3%, Mn 0.2 to 0.5%, Cr 0.1 to 0.9%, Al 0.005 to
Contains 0.1% and P. Less than 02%, S 0.02
(A)Ni 1% or less, if necessary.
One or more of Cu1% or less, Mo0.3% or less, or (B) Ti0.002-0.05%, B0.0005-0.02
% of one or both of groups (A) and (B), with the remainder consisting of Fe and unavoidable impurities, after heating at 730 to 850°C for 20 seconds to 3 hours, Cool slowly at a cooling rate of 0.5 to 30℃/min, or
Or after heating to 730-850℃ for 20 seconds to 3 hours,
A spheroidizing annealing method for machine structural steel is characterized in that a spheroidizing process is carried out by holding the temperature at a temperature in the range of 670 to 720°C for 5 minutes to 2 hours and then releasing it. The present invention will be explained in detail below. First of all, in the present invention, spheroidizing annealing means a treatment that reduces the tensile strength to 26+65×C% (Kg/mm ² ) or less. This equation was obtained by regression with the amount of C varied from 0.2 to 0.7%, and 26 is a term that depends on the strength of ferrite and pearlite, and 65 is a term that depends on the amount of C, that is, the amount of pearlite. If the strength exceeds the value determined by the amount of C,
It cannot be said that the material was softened by spheroidizing annealing. Next, the reason for limiting the composition of steel, which is the object of the present invention, will be described. First, C is an essential element in order to impart the required strength to the product in the quenching and tempering treatment after cold casting, but if it is less than 0.32%, the required strength cannot be obtained, while if it exceeds 0.65%. However, the strength after quenching and tempering no longer increases, so it was limited to a range of 0.32 to 0.65%. 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, 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, which is a conventional medium carbon mechanical structural steel, has a carbon content of 0.42 to 0.48% and a Si of 0.15 to 0.35%.
%, Mn0.60~0.90%, but the amount of Mn is reduced and Cr is added instead.
By adding , the ferrite transformation start temperature as well as the pearlite transformation start temperature and end temperature, which is the point of softening, become higher than S45C steel. Therefore, such steel can undergo pearlite transformation at the same temperature even if it is cooled at a faster rate than S45C. In addition, since the pearlite transformation temperature of this steel shifts to the high temperature side, pearlite transformation can be completed in a short time even when the temperature is maintained near point _A1 after rolling. The present inventors have determined that, for example, the pearlite transformation time of S45C steel when held at 700°C immediately after rolling is
While it takes 120 minutes to transform, it has been confirmed that steel with reduced Mn and Cr addition can complete transformation in just 3 minutes. The reason why the amounts of Mn and Cr added and their total amount are limited as described above is as follows. In order to complete the pearlite transformation in the high temperature range in a short time, it is better to replace Mn with Cr as much as possible.
If Mn is less than 0.2%, S in the steel cannot be sufficiently fixed, and hot embrittlement cannot be suppressed.
On the other hand, if Mn exceeds 0.5%, pearlite transformation at high temperatures cannot be completed in a short time even if Cr is added, so the Mn amount was limited to 0.2 to 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 0.9%, the hardenability of the steel increases and the transformation temperature decreases, so it is limited to 0.1 to 0.9%. Furthermore, the total amount of Mn and Cr was limited to 0.3 to 1.3% because if the total amount is less than 0.3%, hardenability during post-forging quenching treatment cannot be guaranteed.
This is because it takes too much time to complete pearlite metamorphosis if it exceeds . Al is added to prevent coarsening of austenite grain size during quenching after forging.
If it is less than 0.005%, there is no effect, while if it exceeds 0.1%, the effect of suppressing austenite grain coarsening is saturated, and if anything, cold forgeability is deteriorated.
Limited to 0.005-0.1%. Both P and S are elements harmful to cold forgeability. In either case, if it exceeds 0.02%, the negative effects become noticeable, so we limited it to less than this. The above are the basic components of the steel targeted by the present invention, but 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, and (B)Ti0.002 to 0.05 to promote pearlite transformation in high temperature range.
%, B0.0005 to 0.02% of one or both of groups (A) and (B). Group (A) Ni is added to improve toughness and hardenability, thereby increasing strength, but if it exceeds 1%, hardenability becomes too large and cold forgeability increases. This is set as the upper limit because it becomes worse. Cu also improves toughness and hardenability, but
If it exceeds 1%, the effect is saturated, so this was set as the upper limit. Further, Mo improves hardenability and has strong temper softening resistance, but even if it exceeds 0.3%, there is no effect commensurate with the amount added, so this was set as the upper limit. On the other hand, Ti and B in group (B) are both added for the purpose of promoting pearlite transformation in a high temperature range. i.e.
It is more effective to add Ti and B in combination, and Ti
B is added to fix N together with Al and to fully exhibit the hardenability effect of B. By adding Ti and B to increase the hardenability during post-forging quenching, 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. It becomes like this. Ti is limited to 0.002 to 0.05% because if it is less than 0.002%, the N fixing effect will be insufficient, and if it exceeds 0.05%, coarse TiN will be produced which is harmful to cold forgeability. If B is less than 0.0005%, it will not have the effect of increasing hardenability, and if it exceeds 0.02%, coarse B compounds will precipitate and the toughness will deteriorate, so B should be 0.0005 to 0.02%.
limited to. Next, in the present invention, as the spheroidizing annealing conditions for softening treatment, after heating at 730 to 850°C for 20 seconds to 3 hours, (a) slow cooling at a cooling rate of 0.5°C/min to 30°C/min. (b) Cooling, or (b) Holding at a temperature in the range of 670 to 720°C for 5 minutes to 2 hours and then allowing it to cool. (b) By either method, the pearlite transformation in the high temperature range can be completed in a short time, and the tensile strength can be reduced to 26 + 65 ×
C% (Kg/mm ² ) or less. First, the heating temperature was limited to 730℃ or higher.
This is because cementite does not fully dissolve into austenite at temperatures lower than 730°C, so it cannot be transformed into spherical pearlite by subsequent cooling, and the target softness cannot be achieved. On the other hand, if the heating temperature exceeds 850°C, the austenite transforms into layered pearlite instead of spherical pearlite, and the target softness cannot be obtained, so 850°C was set as the upper limit. The reason why the heating time was limited to 20 seconds to 3 hours is for the same reason: If it is less than 20 seconds, cementite dissolution is insufficient, and if it exceeds 3 hours, spherical pearlite cannot be transformed. Next, after heating under the above conditions, (a) If slow cooling is performed by continuous cooling, the cooling rate is 30℃/
If it exceeds 10 minutes, the transformation temperature of the spherical pearlite will drop too much and it will not be possible to soften it, so this was set as the upper limit. From the point of view of softening, the lower the cooling rate, the better.
Even if the cooling rate was lowered further than 0.5° C./min, the strength hardly decreased, so this was set as the lower limit. Within this range, particularly, a heating temperature of 730 to 780°C and a cooling rate of 1 to 8°C/min are desirable conditions for achieving both softening and productivity. On the other hand, (b) when performing retention, the upper limit of the retention temperature is
The temperature was limited to 720°C because if the temperature exceeds 720°C, it would take more than 2 hours to complete the spherical pearlite transformation, which is not practical. However, below 670°C, the cementite spacing becomes so small that the target softness cannot be achieved. Further, if the retention time is less than 5 minutes, the pearlite transformation will not be completed and the target softening will not be achieved, so the retention time of 5 minutes was set as the lower limit. On the other hand, even if it was held for more than 2 hours, the softening would no longer proceed, so 2 hours was set as the upper limit. After the retention, the material is left to cool. This is because the pearlite transformation is completed by the retention, and there is no uniformity in the subsequent gradual cooling. Hereinafter, the effects of the present invention will be explained in more detail with reference to Examples. Examples Table 1 shows the chemical composition of the sample material and the annealing conditions for the material which was finished to 11φmm by normal hot rolling and then allowed to cool. In the same table, test numbers No. 1, 4, 7,
Examples 8, 11, 14-17, and 27-29 are examples of the present invention, and the others are comparative examples. Using these materials, tensile tests were conducted using JIS No. 14A test pieces, and cold forging properties were evaluated using 11φmm x 21mm.
The test piece was subjected to a compression test with a true strain of 2 to determine the presence or absence of cracking, and the mark ◯ indicates that no cracking occurred, and the mark x indicates that cracking occurred. Also,
The toughness value after quenching and tempering is after heating to 900℃ for 0 minutes.
The material is oil quenched and then tempered at 600℃ for 1 hour.
This was determined by conducting an impact test at 20°C using a JIS No. 3 test piece. These test results are also listed in Table 1. The products of the present invention are all 26+65×C% (Kg/
mm ² ), which is well below the target strength. On the other hand, in Comparative Example No. 2, the cooling rate after heating was too high, so the cementite spacing became fine, and in No. 3, the heating temperature was too high, resulting in the formation of layered pearlite. None of them were softened. Comparative examples Nos. 5, 9, and 12 all had inappropriate retention conditions; No. 5 had too short a retention time, No. 9 had too high a retention temperature, and No. 12 had too high a retention temperature. In both cases, it was not possible to lower the intensity below the target intensity because it was too low. On the other hand, comparative examples No. 6, 10, and 13 are all cases where the heating conditions are inappropriate; No. 6 is too short, No. 10 is too long, and No. 13 is too short. Because the temperature was too low, none of them were softened. Comparative examples Nos. 18 to 26 have inappropriate steel compositions. Nos. 18, 19, and 20 all contain too much Mn, and no. 18 and 19 do not contain Cr. Not softened. Also, No. 20 had poor cold forgeability due to too much Al. No. 21 has too much Cr and is not softened. No. 22 achieved sufficient softening, but the hardenability was insufficient due to the small total amount of Mn and Cr, and the necessary strength could not be secured during the quenching and tempering treatment after forging. . No.23 is Si
This is an example where the amount was too large to soften. No.24 is
This is an example in which the total amount of Mn and Cr is too large and the material is not softened. Nos. 25 and 26 are examples showing that the contents of P and S are too high, resulting in poor cold forgeability and poor toughness after production.

【table】

Claims (1)

[Claims] 1% by weight: C0.32-0.65%, Si less than 0.05%, Mn0.2-0.5%, Cr0.1-0.9% when the total amount of Mn and Cr is in the range of 0.3-1.3%. , containing 0.005 to 0.1% Al, and limiting P to less than 0.02% and S to less than 0.02%, with the balance consisting of Fe and unavoidable impurities, which was heated to 730 to 850°C for 20 seconds to 3 hours. A spheroidizing annealing method for mechanical structural steel, which is then subjected to a spheroidizing process of slow cooling at a cooling rate of 0.5 to 30°C/min. 2 Weight%: C0.32-0.65%, Si less than 0.05%, Mn0.2-0.5%, Cr0.1-0.9%, Al0.005-0.1 when the total amount of Mn and Cr is in the range of 0.3-1.3%. %, and limits P to less than 0.02%, S to less than 0.02%, one or more of Ni 1% or less, Cu 1% or less, Mo 0.3% or less, the remainder consisting of Fe and unavoidable impurities. A spheroidizing annealing method for steel for mechanical structural use, which comprises heating the steel to 730 to 850°C for 20 seconds to 3 hours, and then subjecting the steel to a spheroidizing treatment in which the steel is slowly cooled at a cooling rate of 0.5 to 30°C/min. 3 By weight: C0.32-0.65%, Si less than 0.05%, Mn0.2-0.5%, Cr0.1-0.9%, Al0.005-0.1 when the total amount of Mn and Cr is in the range of 0.3-1.3%. %, P is limited to less than 0.02%, S is limited to less than 0.02%, and contains one or two of Ti0.002-0.05% and B0.0005-0.02%, and the remainder is Fe and unavoidable impurities. Spheroidizing annealing of steel for machine structural use, characterized in that the steel is heated to 730 to 850°C for 20 seconds to 3 hours and then slowly cooled at a cooling rate of 0.5 to 30°C/minute to form a spheroid. Law. 4 In terms of weight%, C0.32-0.65%, Si less than 0.05%, Mn0.2-0.5%, Cr0.1-0.9%, Al0.005-0.1 when the total amount of Mn and Cr is in the range of 0.3-1.3%. %, and limit P to less than 0.02% and S to less than 0.02%, (A) Ni 1% or less, Cu 1% or less, Mo 0.3% or less.
(B) Contains one or two of the groups (A) and (B) containing 0.002 to 0.05% of Ti and 0.0005 to 0.02% of B, with the remainder being Fe and unavoidable impurities. For steel, heated to 730~850℃ for 20 seconds~
A spheroidizing annealing method for mechanical structural steel, which comprises heating for 3 hours and then gradually cooling at a cooling rate of 0.5 to 30° C./min. 5 In terms of weight%, C0.32-0.65%, Si less than 0.05%, Mn0.2-0.5%, Cr0.1-0.9%, Al0.005-0.1 when the total amount of Mn and Cr is in the range of 0.3-1.3%. %, and P is limited to less than 0.02%, S is limited to less than 0.02%, and the balance is Fe and unavoidable impurities. After heating to 730-850°C for 20 seconds to 3 hours, it is heated to 670-720°C. 1. A spheroidizing annealing method for mechanical structural steel, which comprises performing a spheroidizing process by maintaining the temperature at a temperature in the range of 5 minutes to 2 hours and then allowing it to cool. 6 In terms of weight%, C0.32-0.65%, Si less than 0.05%, Mn0.2-0.5%, Cr0.1-0.9%, Al0.005-0.1 when the total amount of Mn and Cr is in the range of 0.3-1.3%. %, and limits P to less than 0.02%, S to less than 0.02%, one or more of Ni 1% or less, Cu 1% or less, Mo 0.3% or less, the remainder consisting of Fe and unavoidable impurities. For use in mechanical structures, steel is subjected to a spheroidizing treatment in which the steel is heated to 730 to 850°C for 20 seconds to 3 hours, held at a temperature in the range of 670 to 720°C for 5 minutes to 2 hours, and then released. Spheroidizing annealing method for steel. 7 In terms of weight%, C0.32-0.65%, Si less than 0.05%, Mn0.2-0.5%, Cr0.1-0.9%, Al0.005-0.1 when the total amount of Mn and Cr is in the range of 0.3-1.3%. %, P is limited to less than 0.02%, S is limited to less than 0.02%, and contains one or two of Ti0.002-0.05% and B0.0005-0.02%, and the remainder is Fe and unavoidable impurities. A machine that performs a spheroidizing process on steel made of steel by heating it to 730 to 850°C for 20 seconds to 3 hours, holding it at a temperature in the range of 670 to 720°C for 5 minutes to 2 hours, and then allowing it to cool. Spheroidizing annealing method for structural steel. 8 Weight%: C0.32-0.65%, Si less than 0.05%, Mn0.2-0.5%, Cr0.1-0.9%, Al0.005-0.1 when the total amount of Mn and Cr is in the range of 0.3-1.3%. %, and limit P to less than 0.02% and S to less than 0.02%, (A) Ni 1% or less, Cu 1% or less, Mo 0.3% or less.
(B) Contains one or two of the groups (A) and (B) containing 0.002 to 0.05% of Ti and 0.0005 to 0.02% of B, with the remainder being Fe and unavoidable impurities. For steel, heated to 730~850℃ for 20 seconds~
After heating for 3 hours, heat to a temperature in the range of 670-720℃ for 5 hours.
A spheroidizing annealing method for machine structural steel, which is characterized by performing a spheroidizing treatment of holding for minutes to 2 hours and then allowing it to cool.