JPS6353209A - Softening method for alloy steel for machine structure use - Google Patents

Abstract

PURPOSE:To soften an alloy steel having high hardenability by the annealing for short time by promoting furthermore pearlite transformation at high temp. by adding B in the steel having the specific composition and also after that specifying the annealing condition at A1 point or lower. CONSTITUTION:The steel having by wt% of 0.15-0.65 C, <0.1 Si, 0.2-0.5 Mn, 0.0003-0.01 B, >0.7-1.8 Cr, 0.01-0.1 Al, <0.02 P and S and the remaining part of Fe is provided. After hot-rolling this steel, it is slowly cooled at <=15 deg.C/min cooling velocity in the temp. range from at least 750 deg.C to the completion of pearlite transformation, and then it is kept at 690-740 deg.C for 30min-5hr. Further, in the above steel, one or more elements of <=1% Ni, <=0.3% Mo, <=1% Cu and/or one or more elements of 0.002-0.05% Ti, 0.005-0.05% Nb, 0.005-0.2% V may be contained.

Description

[Detailed Description of the Invention] [Industrial Field of Application] The present invention relates to a method for softening alloy steel for machine structures, and particularly for softening alloy steel for machine structures that is processed into bolts, nuts, etc. by cold forging. This relates to a softening method.

[Prior Art] Conventional 1 machine structural alloy steels have been developed in order to reduce deformation resistance and improve cold forgeability during cold forging, as seen in, for example, Japanese Patent Laid-Open No. 59-136421. Generally, cementite is subjected to spheroidizing annealing. However, since this spheroidizing annealing process usually requires an extremely long time, more than ten hours, it would be industrially effective if it could be replaced with softening annealing, which maintains the temperature below the A□ point for a short period of time. In the described alloy steel for machine structures, the softened annealed material is not far superior to the spheroidized annealed material in terms of softness.

The present inventors analyzed the effect of rolled material strength on the strength of materials annealed at temperatures below the A1 point, and found that if the rolled material is sufficiently softened, it will be possible to form a spherical shape by annealing for a short time at a temperature below the A1 point. It was discovered that it was possible to achieve a softening comparable to that of chemically annealed materials, and a proposal was previously made in Japanese Patent Application No. 13892/1983.

In other words, when annealing at a temperature below the A1 point, 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. Furthermore, as a result of various analyzes of the factors governing the strength of rolled material of machine structural steel, we found that it is effective to coarsen the spacing between pearlite and cementite in order to reduce the strength of rolled material. 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 and rolling cooling conditions for softening by pearlite transformation at as high a temperature as possible after rolling, and the subsequent A0 point or lower. The knowledge was obtained that it is sufficient to select appropriate annealing conditions. By the way, although this manufacturing method is extremely superior in terms of softening low alloy steel with low hardenability, S C
r.

There was still room for improvement in terms of softening alloy steels with high hardenability such as SCMfi.

[Problems to be Solved by the Invention] The present invention has been made in view of the above-mentioned circumstances, and an object of the present invention is to provide a method for softening highly hardenable alloy steel by short-time annealing. It is something to do.

[Means for Solving the Problems] That is, the present inventors have further improved the technique proposed previously, and by adding B, pearlite at high temperatures, which is the most important point in softening the rolled material, has been improved. Further promotes metamorphosis and subsequent A
The present invention was made based on the completely new knowledge that even alloy steels with high hardenability can be softened in a short time if appropriate annealing conditions of 4 points or less are selected.

The present invention has been made based on the above findings, and its gist is that C: 0°15-0.6 in weight%
5%, Si: less than 0.1%, Mn: 0°2-0.5%
, B: 0.0003-0.01%.

Cr: more than 0.7 to 1.8%, AQ: 0.01 to
Contains 0.1%, less than 0.02% P, and 0.02% S
(A)Ni: 1% as necessary.
Hereinafter, Mo: 0.3% or less, Cu: 1% or less of 1 type or more, (B) Ti: 0.002 to 0°05%,
Nb: 0.005~0.05%, V:O, O05~0
．． 2% of one or more types, (7) (A).

Containing one or both of group (B), the remainder.

After hot rolling, steel consisting of Fe and unavoidable impurities is slowly cooled at a cooling rate of 15°C/min or less over a temperature range from at least 750°C to the end of pearlite transformation, and then reheated at a temperature range of 690 to 740°C for 30°C. This is a method for softening alloy steel for machine structures, which is characterized by holding for 5 minutes to 5 hours.

The present invention will be explained in detail below.

First of all, in the present invention, softening refers to the tensile strength of the annealed material based on carbon equivalent Ceq (Ceq=C+Si/2
4+Mn/6+Cr15+Cu/13+M.

/4+Ni/40) = 22
+63 Ceq (Kg/mm2) or less. In this strength formula, the Ceq amount is determined within the range of 0°3 to 1.3, and 22 is a term that depends on the strength of ferrite and pearlite, and 63 is a term that depends on the Ceq amount, that is, the pearlite amount. If the strength exceeds the same value determined by the amount of Ceq, it cannot be said to be softening.

Next, the reasons for limiting the composition of steel, which is the object of the present invention, will be discussed.

First, C is an essential element in order to impart the required strength to the product in the quenching and tempering treatment after cold TxI manufacturing, but if it is less than 0.15%, the required strength cannot be obtained;
Since the strength after quenching and tempering will no longer increase even if it exceeds 65%, it is limited to a range of 0.15 to 0.65%.

In addition, since Si increases the strength of the rolled material through its solid hardening effect, the content was limited to less than 0.1% so that the influence of solid hardening can be ignored. Further, even if the Si content is lowered in this way, the hardenability required during hardening treatment does not decrease.

Next, regarding Mn and 8, the most important point of the present invention is that the contents are determined as described above.

That is, for example, SC: r430t, which is a typical conventional chrome steel for machine structures! l is C: 0.28-0.33%,
Si: 0.15-0.35%, Mn: 0.60-0.
85%, Cr: 0.90 to 1.20%, but by reducing the amount of Mn, Cr: 0.90 to 1.20%.
The end temperature of pearlite transformation, which is the point of softening, is higher than that of Cr430 steel.

Although the hardenability decreases as the amount of Mn decreases, this is compensated for by adding B. Furthermore, B does not suppress pearlite transformation in the slow cooling region after rolling, but on the contrary, solid solution B precipitates as a B compound, thereby having the effect of promoting pearlite transformation. Therefore, if B-added steel is slowly cooled after hot rolling, the pearlite transformation at high temperatures can be completed in a short time. Generally, B is used as an element to improve hardenability, but in the present invention, B is used both to promote pearlite transformation after hot rolling and to improve hardenability during heat treatment after cold forging. It is something to be used. Moreover, even if such m is cooled at a higher speed than S Cr430 steel, it undergoes pearlite transformation at the same temperature.

Here, the reason why the amounts of Mn and B added 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 reduce Mn as much as possible.

When MnfJ<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%, the pearlite transformation at high temperature cannot be completed in a short time even if B is added, so the Mn amount was limited to 0.2 to 0.5%.

B is effective in promoting pearlite transformation in the slow cooling region after rolling and significantly increasing the hardenability in heat treatment after cold forging and improving strength, but if it is less than 0.0003%, the effect will not increase. On the other hand, if it exceeds 0.01%, cold forming properties deteriorate, so it is limited to 0.0003 to 0.01%.

In addition, Cr is an essential element to enhance hardenability during quenching treatment after cold forging, and to improve strength and toughness.
．． If it is less than 7%, the effect is insufficient and the steel with high hardenability, which is the objective of the present invention, cannot be obtained.
%, the hardenability of the steel increases too much and the end temperature of pearlite transformation after hot rolling decreases, making it impossible to obtain a soft rolled material.

Furthermore, AQ is a necessary element to prevent coarsening of austenite grain size during quenching after cold forging and to fix N as an AQN compound to ensure the hardenability effect of B. If it is less than 0.01%, there is no effect, while if it exceeds 0.1, the above effect is saturated and the cold forgeability is deteriorated, so it is limited to 0.01 to 0.1%.

Both P and S are elements harmful to cold forgeability. In either case, if the content exceeds 0.02%, the adverse effects become significant, so the content was limited to less than 0.02%.

The above are the basic components of the steel targeted by the present invention. In the present invention, (A) Ni: 1% or less, Mo: 0.3% or less, Cu:
1% or less of one or more types.

(B) Ti: 0.002-0.05%, Nb: 0.
0O5 to 0.05%, V: 0.005 to 0.2%, one or more of the groups (A) and (B) or both may be contained. First, Ni in group (A) is added to improve toughness and increase hardenability to increase strength, but if it exceeds 1%, hardenability increases too much and cold forgeability deteriorates. Therefore, this is set as the upper limit.

MO is an element that improves hardenability and provides strong temper softening resistance, but even if it exceeds 0.3%, an effect commensurate with the amount added cannot be expected, so this is set as the upper limit.

Like Cu+JNi, it improves toughness and hardenability.

If it exceeds 1%, the effect will be saturated, so this was set as the upper limit.

On the other hand, Ti, Nb, and V in group (B) are all added for the purpose of refining austenite crystal grains after hot rolling and promoting pearlite transformation in a high temperature range.

Ti combines with N to form a TiN compound, prevents coarsening of austenite crystal grains after hot rolling, and promotes pearlite transformation in a high temperature range. In addition, it is more effective to add Ti and B in combination; Ti, together with AQ, fixes N and sufficiently promotes the pearlite transformation of B after hot rolling and the hardenability effect after cold forging. It is added to achieve the desired effect.

If Ti is less than 0.002%, the effect of fixing N is insufficient, while if it exceeds 0.05%, coarse TiN or TiC that is harmful to cold forgeability and toughness is generated.
It was limited to 0.002-0.05%.

Both Nb and V are added for the purpose of refining the austenite grain size after rolling and promoting pearlite transformation, but if each is less than 0.005%, no finer effect can be expected; on the other hand, if Nb is 0.05% .

If (2) exceeds 0.2%, coarse carbonitrides of Nb and V will precipitate and deteriorate toughness and cold forgeability.
Nb is 0.005-0.05%, and V is 0.005-0.005%
Each was limited to 0.2%.

Next, the softening treatment conditions in the present invention will be described.

First, the cooling rate after hot rolling of steel with the above chemical composition was limited to 15°C/min or less because cooling faster than 15°C/min produces low-temperature transformed pearlite or bainite, and the rolled material Since the strength of 6 increases, the subsequent 6
This is because the target softening cannot be achieved even by annealing at 90 to 740°C. Although it is advantageous to have a slow cooling rate, in consideration of practical aspects of equipment and productivity, a cooling rate of 3 to b is a preferable cooling rate range that achieves both softening and productivity. Regarding the slow cooling temperature range, slow cooling may be performed immediately after hot rolling, but slow cooling from 750° C. is sufficient for the component system targeted by the present invention. The slow cooling stop temperature was set as the pearlite transformation finish temperature because if slow cooling is stopped before the end of pearlite transformation, high-strength low-temperature pearlite or bainite will be generated and hardened in the subsequent cooling process. The pearlite transformation end temperature varies depending on the steel type and cooling rate, but for steel with the composition targeted by the present invention, it is approximately 65
The temperature is 0 to 680°C.

In addition, the reason why the annealing temperature after rolling was limited to 690-740°C is because the target softening cannot be achieved at temperatures lower than 690°C, while elements such as Mn segregate within the steel when it exceeds 740°C. This is because the parts become austenite. The holding time was limited to 3° minutes to 5 hours, but this is because softening does not progress to the target value if it is less than 30 minutes, while it becomes sufficiently soft within 5 hours and there is no need to hold it any longer. 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 and the results obtained by normal hot rolling.
The cooling conditions etc. after finishing to 3φm111 are shown.

Test number No. in the same table, 5.7.10-14.17゜18
．． Nos. 20, 21, 27, and 29 are examples of the present invention, and the others are comparative examples. Using these materials, the tensile test was conducted according to JIS1
The evaluation of cold forgeability was performed using a No. 4A test piece, and the evaluation of cold forgeability was determined by the presence or absence of cracks when a compression test was performed at an upsetting rate of 4o% using a 1oφmm x 15mm test piece with a V-notch of Q, 5+nm depth. The test results are also listed in Table 1, where the ○ mark indicates that no cracking occurred, and the x mark indicates that cracking occurred.

As shown in the table, all of the examples of the present invention achieved a target strength of 22 + 63 Ceq (kg/mwt
”), and a satisfactory degree of softening has been obtained. Cold forging properties are also satisfactory.

On the other hand, the comparative example No. 2.15°16.2
4.28 are all examples in which softening was not achieved due to inappropriate cooling conditions or annealing conditions after hot rolling. That is, in No. 2.16, the cooling rate after rolling was too fast, so the strength of the rolled material was too high, and it could not be softened by short-time annealing. - On the other hand, No. 15 was No because the annealing temperature was too high.

On the other hand, 28 was too low, so the target strength was 22+6 in both cases.
It was not possible to reduce the temperature to below 3Ceq (kg/mo+''). In addition, No. 24 is an example in which the annealing time was too short and softening was not achieved.

Comparative examples No. 4 are Mn, No. 6 are Cr.

In No. 9, the Si content was too high, so even if the cooling conditions or annealing conditions after rolling were optimized, the target softening was not achieved.

Moreover, since the content of B, No. 3, and Al1 of Comparative Example No. 001 were too small, softening could not be achieved in any of them.

Furthermore, comparative example No. 008 is B, No. 23 is Ti.
No. 19 has too much Si and Mn content, so it has high strength, and it has too much Nb, so it has poor cold forging properties. It also has poor forging properties. No.

No. 22 is an example in which the targeted softening was achieved, but the cold forgeability was poor because the amount of V was too large.

Moreover, No. 25.26 is an example in which the amounts of p and s were too large, respectively, and there was a problem in cold forgeability.

[Effects of the Invention] As is clear from the above examples, the present invention can produce alloy steel for mechanical structures in a short time annealing by optimally selecting the steel material composition, cooling conditions after hot rolling, and annealing conditions. This makes it possible to soften the material, and its industrial effects are extremely significant.

Claims (2)

[Claims] (1) C: 0.15-0.65%, Si: 0.
less than 1%, Mn: 0.2-0.5%, B: 0.0003-0.01%, Cr: more than 0.7-1.8%, Al: 0.01-0.1%, For steels containing P and S of less than 0.02% and S of less than 0.02%, with the remainder consisting of Fe and unavoidable impurities, the temperature range after hot rolling is from at least 750°C to the end of pearlite transformation. A method for softening alloy steel for mechanical structures, which comprises slow cooling at a cooling rate of 15° C./min or less, and then maintaining the temperature in a temperature range of 690 to 740° C. for 30 minutes to 5 hours. (2) C: 0.15-0.65%, Si: 0.
Contains less than 1%, Mn: 0.2-0.5%, B: 0.0003-0.01%, Cr: more than 0.7-1.8%, Al: 0.01-0.1% However, P is limited to less than 0.02%, S is limited to less than 0.02%, and further (A) Ni: 1% or less, Mo: 0.3% or less, Cu: 1
% or less, (B) Ti: 0.002 to 0.05%, Nb: 0.00
5 to 0.05%, V: 0.005 to 0.2%, one or more of the groups (A) and (B), and the remainder is Fe and unavoidable impurities. After hot rolling, the steel is slowly cooled in a temperature range from at least 750°C to the end of pearlite transformation at a cooling rate of 15°C/min or less,
A method for softening alloy steel for machine structural use, which comprises maintaining the temperature in a temperature range of 690 to 740°C for 30 minutes to 5 hours.