JP6059570B2 - Manufacturing method of steel with excellent cold workability - Google Patents

Description

  The present invention relates to a method of manufacturing a steel material having excellent cold workability used for manufacturing various parts such as automobile parts, bearings, and construction machine parts.

  High carbon steel is used for various parts such as automobile parts, bearings, and construction machine parts. Usually, hot rolled material is formed into a predetermined shape by cold working, cutting, etc., and then quenched and tempered. The final strength adjustment is performed by performing the process, and the manufacturing is performed. The cold workability and machinability of hot-rolled materials are affected by the shape of carbides precipitated in high-carbon steel. Interworkability and machinability deteriorate. Therefore, when cold-working or cutting the hot-rolled material, a process of spheroidizing the carbide by annealing the hot-rolled material (spheroidizing annealing) is performed.

In general, in spheroidizing annealing, (1) a method of slowly cooling a steel material to a slightly higher temperature (for example, 760 to 780 ° C.) of A _c1 point (about 730 ° C.) for 1 to 2 hours, and (2) A _c1 A method of repeating heating and cooling between 20-30 ° C. above and below the point, (3) a method of heating to a temperature of 680-700 ° C. after cold working, or a method of tempering at a temperature of 680-700 ° C. after quenching, (4) A method of cooling from a temperature of 760 to 780 ° C. immediately above the transformation point to a temperature of 700 ° C. and holding at this temperature for about 3 hours and then air cooling is adopted.

  By the way, as a high carbon steel used for bearing steel etc., SUJ2 steel is typical, and this SUJ2 steel is also used after spheroidizing annealing. SUJ2 steel has a high C content of 0.95 to 1.10% and a high Cr content of 1.30 to 1.60%.

On the other hand, spheroidizing annealing is also applied to steel materials in which the amount of Cr is reduced while the amount of C is high (Patent Documents 1, 2, etc.). Patent Document 1 C: 1.41%, Cr: against 0.64% for steel D, and retention heated to 30 to 120 minutes until the first retention temperature of Ac ₁ -100 ° C. to Ac _1, then Ac A spheroidizing annealing method is disclosed in which the steel is heated to a second holding temperature of ₁ + 5 ° C. or higher and then cooled. In Patent Document 2, C: 0.40 to 0.80% by weight, Cr: 0.80% by weight or less of the steel for bearings, after the treatments (1) to (3) below, (4) to (5) A spheroidizing annealing method is disclosed in which the above treatment is repeated once or more and then cooled.

(1) A Rapid cooling is performed after heating and holding at ₃ points or more.
(2) A ₃ point + (5-30) reheating kept at a temperature in the range of ° C..
(3) A ₁ point-held at a temperature range of (5 to 30) ° C.
(4) Hold in a temperature range of (A ₁ point + 5) to (A ₃ point + 30) ° C.
(5) A ₁ point-held in a temperature range of (5 to 30) ° C.

JP-A-4-333527 JP-A-4-362123

  High Cr alloy steels such as the SUJ2 steel are relatively easy to spheroidize, whereas spheroidizing annealing is difficult for high C low Cr steels with reduced Cr while maintaining a high C content. Even if the tissue is spheroidized, it may not be sufficiently softened. Even in the methods of Patent Documents 1 and 2, it is difficult to reliably spheroidize high C low Cr steel, and it is difficult to achieve sufficient softening.

  The present invention has been made paying attention to the above-mentioned circumstances, and its purpose is that the target steel is a high C low Cr steel when spheroidizing annealing is performed to ensure the cold workability of the steel material. However, not only can spheroidizing annealing be performed reliably, but also a technique for sufficiently softening the steel material after spheroidization.

The production method of the present invention that can achieve the above-mentioned object is: C: 0.7 to 1.5% (meaning mass%, hereinafter the same), and Cr: less than 0.9% (not including 0%) ) Is produced by spheroidizing annealing a steel material having excellent cold workability, and the spheroidizing annealing is composed of a first stage heat treatment followed by a second stage heat treatment. And the conditions of these 1st-stage heat processing and 2nd-stage heat processing have the summary in the following points.
(1) First-stage heat treatment Temperature increase rate: less than 400 ° C./hr Heating temperature: A ₁ point + 30 ° C. to A ₁ point + 40 ° C. and less than 780 ° C. Heating holding time: 1-2 hours Cooling rate: A ₁ point ~ A ₁ point-20 ° C temperature range of 3 ° C / hr or less Cooling end temperature: A ₁ point-35 ° C or less (2) Second stage heat treatment Heating rate: less than 400 ° C / hr Heating temperature: A ₁ point +25 _{1 to} 30 ° C. and less than 780 ° C. Heating and holding time: 0.5 to 1 hour Cooling rate: A ₁ point −5 ° C. to A ₁ point −30 ° C. The temperature range is 1 ° C./hr or less

  The steel material may further contain Si: 0.001 to 0.7%, Mn: 0.1 to 2.0%, Al: 0.001 to 0.1%, the balance being iron and inevitable It may be an impurity. The inevitable impurities include, for example, P: 0.05% or less (not including 0%), S: 0.001 to 0.05%, N: 0.015% or less (not including 0%), etc. is there. The steel materials are further Cu: 0.25% or less (not including 0%), Ni: 0.25% or less (not including 0%), Mo: 0.25% or less (not including 0%) And B: may contain at least one selected from the group consisting of 0.01% or less (not including 0%), Ti: 0.2% or less (not including 0%), Nb : May contain at least one selected from the group consisting of 0.2% or less (not including 0%) and V: 0.5% or less (not including 0%).

  According to the present invention, since the steel material is processed by a specific two-stage heat treatment method, even a high C low Cr steel which is difficult to be spheroidized or softened can be reliably spheroidized and can be sized by carbide. Since the distance between carbides can be increased, softening can be sufficiently achieved.

As a result of intensive studies to solve the above-mentioned problems, the present inventors performed heat treatment in two stages so as to sandwich a process of cooling to a temperature sufficiently lower than the A ₁ transformation point. The heat treatment conditions of the first stage heat treatment are strong enough to dissolve all the pearlite, and the pearlite regenerated by the cooling of the first stage heat treatment is moderate in the second stage heat treatment which is a relatively weak heating condition. When it was decomposed, the carbide was not only spheroidized but also the size of the carbide could be increased and the distance between the carbides could be widened, and it was found that the steel material could be sufficiently softened, thus completing the present invention. Hereinafter, after explaining the steel material which this invention method makes object, this invention method is demonstrated.

1. Target Steel The present invention is intended for high C low Cr steel. High C low Cr steel has the problem that spheroidizing annealing is difficult and softening is difficult. Specifically, the high C low Cr steel contains C in an amount of 0.7 to 1.5% (meaning mass%, hereinafter the same) and Cr less than 0.9% (not including 0%).

  The C is an element necessary for ensuring the strength of the steel material (that is, the strength of the final product), and has an important influence on cold workability and machinability. In addition, since carbide is generated, it must be taken into consideration when designing the spheroidizing annealing method. The amount of C is preferably 0.8% or more. However, if C is contained excessively, the strength becomes too high and cold workability and machinability deteriorate, so the upper limit was set. The amount of C is preferably 1.2% or less, more preferably 1.1% or less.

  Cr is an element that affects the difficulty of spheroidization. In the present invention, even when this Cr is reduced to less than 0.9%, a spheroidizing annealing method is designed for the purpose of surely spheroidizing and sufficiently softening, and specification of the Cr amount is essential. . According to the present invention, even when Cr is 0.8% or less, or 0.6% or less, particularly 0.4% or less, and even 0.3% or less, spheroidization can be reliably performed. The amount may be 0.1% or less. Note that Cr is an element that acts to improve the hardenability of the steel material and increase the strength of the final product, and therefore may be contained in a large amount within a range satisfying less than 0.9%. For example, the Cr amount may be 0.01% or more, preferably 0.05% or more, and more preferably 0.1% or more.

  The steel components to be considered in designing the annealing method in the present invention are the above-described C amount and Cr amount, but the steel materials actually used in the annealing method of the present invention usually have Si, Mn, and Al within the following ranges. In addition, it contains. The balance is not particularly limited, but may be iron and inevitable impurities. The addition amount of Si, Mn, and Al and the reason for the addition are as follows.

  Si is an element that is preferably contained as a deoxidizing element and to increase the strength of the final product by solid solution hardening. The Si content is preferably 0.001% or more, more preferably 0.05% or more, still more preferably 0.1% or more, and particularly preferably 0.2% or more. However, if the amount of Si exceeds 0.7%, the strength is excessively increased and cold workability may be deteriorated. Therefore, the Si content is preferably 0.7% or less, more preferably 0.6% or less, and still more preferably 0.5% or less.

  Mn is an element that effectively acts to improve hardenability and increase the strength of the final product. In order to effectively exhibit such an effect, the content is preferably 0.1% or more, more preferably 0.2% or more, and still more preferably 0.3% or more. However, when it contains excessively, intensity | strength rises excessively and cold workability may deteriorate. Accordingly, the Mn content is preferably 2.0% or less, more preferably 1.5% or less, still more preferably 1.2% or less, and particularly preferably 1.0% or less.

Al is an element that acts as a deoxidizing element, fixes solid solution N present in the steel as AlN, and improves cold workability. In order to effectively exert such effects, the Al content is preferably 0.001% or more, more preferably 0.005% or more, and still more preferably 0.01% or more. However, when the amount of Al is excessive, Al ₂ O ₃ is excessively generated in the steel material, and cold workability may be deteriorated. Accordingly, the Al content is preferably 0.1% or less, more preferably 0.08% or less, and still more preferably 0.05% or less.

Examples of the inevitable impurities include P, S, and N. The reasons for these limitations and the allowable contents thereof are as follows, for example.
P is an element inevitably contained in the steel material, and when grain boundary segregation occurs, it causes ductile deterioration. Therefore, the P content is preferably 0.05% or less (excluding 0%), more preferably 0.04% or less, and still more preferably 0.03% or less.

  S is an element inevitably contained in the steel material, and is present as MnS in the steel material, which may deteriorate ductility and cold workability. Accordingly, the S amount is preferably 0.05% or less, more preferably 0.04% or less, and still more preferably 0.03% or less. On the other hand, S also has the effect | action which improves the machinability of steel materials. Therefore, the S amount is preferably 0.001% or more, more preferably 0.002% or more, and further preferably 0.003% or more.

  N is an element inevitably contained in the steel material. When N is present as solid solution N in the steel material, the hardness increases due to strain aging and the ductility decreases, and cold workability may be deteriorated. Accordingly, the N content is preferably 0.015% or less (excluding 0%), more preferably 0.013% or less, and still more preferably 0.010% or less.

The steel material may further include (1) Cu, Ni, Mo, and B, and / or (2) Ti, Nb, V, and the like.
In the case of (1), Cu: 0.25% or less (not including 0%), Ni: 0.25% or less (not including 0%), Mo: 0.25% or less (not including 0%) And B: at least one selected from the group consisting of 0.01% or less (not including 0%) is contained. These elements are elements that effectively act to improve the hardenability of the steel material and increase the strength of the final product, and may be contained alone or in combination of two or more. From the viewpoint of surely exhibiting the above-mentioned action, it is preferable to contain Cu 0.01% or more, Ni 0.01% or more, Mo 0.01% or more, and B 0.001% or more, more preferably Cu is 0.03% or more, Ni is 0.03% or more, Mo is 0.03% or more, and B is 0.0015% or more. However, when it contains excessively, intensity | strength will become high too much and cold workability may deteriorate. Therefore, the upper limit may be limited more than the above range. For example, Cu is 0.15% or less, Ni is 0.15% or less, Mo is 0.2% or less, and B is 0.008% or less. Is preferred.

  In the case of (2), Ti: 0.2% or less (not including 0%), Nb: 0.2% or less (not including 0%), and V: 0.5% or less (not including 0%) And at least one selected from the group consisting of: These elements are all elements that combine with N present in the steel material to form nitrides and reduce solute N, thereby reducing deformation resistance and improving cold workability. These elements are preferably used alone or in combination of two or more. In order to effectively exert such an effect, it is preferable that Ti is 0.02% or more, Nb is 0.02% or more, and V is 0.05% or more, more preferably, Ti is 0.04% or more. , Nb is 0.05% or more, and V is 0.08% or more. However, when it contains excessively, the nitride formed may increase deformation resistance and may deteriorate cold workability. Therefore, the upper limit may be limited more than the above range. For example, it is preferable that Ti is 0.1% or less, Nb is 0.1% or less, and V is 0.25% or less.

2. 2. Manufacturing Method 2.1 Spheroidizing Annealing The present invention reliably spheroidizes and sufficiently spheroidizes even the high C low Cr steel as described above, which is difficult to spheroidize and difficult to soften sufficiently. It is an invention that softens to improve its cold workability. The spheroidizing annealing method employed in the production method of the present invention is characterized in that the heat treatment is performed in two stages, and the heat treatment conditions of the first stage heat treatment are strong enough to dissolve all the pearlite. Adopted, and the pearlite regenerated by the cooling of the first stage heat treatment is appropriately decomposed by the second stage heat treatment which becomes a relatively weak heating condition. A more detailed description of the first stage heat treatment is as follows.

2.1.1 First-stage heat treatment In the first-stage heat treatment, first, the steel material is heated at a heating rate of less than 400 ° C./hr. This heating is performed up to a temperature of A ₁ point +30 or more and A ₁ point + 40 ° C. or less (however, less than 780 ° C.) and held at this temperature for 1 to 2 hours. Next, cooling is started, and in this cooling, a temperature range of A ₁ point or lower and A ₁ point −20 ° C. or higher is gradually cooled at a rate of 3 ° C./hr or lower. Cooling is carried out to A ₁ point -35 ° C. or lower. In this first heat treatment, all of the pearlite is once dissolved, and then regenerated as pearlite with a wide lamellar spacing in the cooling process. Since the lamella spacing is widened, the carbide size can be increased and the distance between carbides can be increased when the spheroidization is performed by the subsequent second stage heat treatment, and the steel can be sufficiently softened. The point A ₁ is the transformation temperature from the austenite + cementite phase to the austenite + cementite + ferrite phase, which can be purchased from comprehensive thermodynamic calculation software (Thermo-Calc. CRC Research Institute, database is TTLFE). The value obtained by inputting the chemical composition of the steel material.

  The reason why the rate of temperature rise of the steel material is set in the above range in the first heat treatment is that if the temperature is exceeded, the temperature tends to overshoot and the heating temperature and time are affected. The temperature rising rate is preferably 300 ° C./hr or less, more preferably 200 ° C./hr or less. If the temperature raising rate is too slow, the production efficiency is lowered. Therefore, the rate of temperature rise is, for example, 5 ° C./hr or more, preferably 10 ° C./hr or more, more preferably 20 ° C./hr or more.

The reason why the heating temperature and the heating time in the first stage are set in the above-described range is to prevent the undissolved pearlite from remaining completely by this heating. Further, by increasing the heating temperature at the first stage, it is possible to remove the processing distortion of the steel material and contribute to softening of the steel material. If the heating temperature is too low or the heating time is too short, undissolved pearlite will remain, resulting in an insufficiently spheroidized part, or even if the second stage heat treatment succeeds in spheroidizing. The carbide size is reduced, and the distance between carbides is reduced. Even if the heating temperature is further increased or the heating time is further increased, the effect is saturated. The heating temperature is preferably A ₁ point + 31 ° C. to A ₁ point + 39 ° C., more preferably A ₁ point + 32 ° C. to A ₁ point + 38 ° C. Moreover, this heating temperature becomes like this. Preferably it is 775 degrees C or less, More preferably, it is 760 degrees C or less. The heating time is preferably 1.3 to 2 hours, more preferably 1.5 to 2 hours.

  The reason why the cooling rate of the first stage is set in the above range is to suppress the generation of fine reproduction pearlite and to make reproduction pearlite with a wide lamella interval. When the lamella spacing is increased, the carbide spacing can be increased by the second heat treatment. When fine regenerated pearlite is generated, it is melted too much during the heating of the second stage heat treatment, and pearlite is easily regenerated by subsequent cooling, and the steel material cannot be sufficiently softened. The cooling rate is preferably 2.5 ° C./hr or less, more preferably 2.0 ° C./hr or less. The lower limit of the cooling rate can be set as appropriate, but may be, for example, 0.1 ° C./hr or more, preferably 0.5 ° C./hr or more, or 1 ° C./hr or more.

The reason why the first stage cooling end temperature is set in the above range is to prevent the second stage heating from entering while the austenite remains. If the second stage of heating is entered with austenite remaining, the cementite will melt too much, and pearlite will be easily generated by subsequent cooling, resulting in insufficient spheroidization and insufficient softening of the steel. . The cooling end temperature may be A ₁ point −37 ° C. or lower, or A ₁ point −40 ° C. or lower. The lower limit of the cooling end temperature is not particularly set, but may be, for example, A ₁ point −100 ° C. or higher, preferably A ₁ point −50 ° C. or higher, from the viewpoint of production efficiency.

2.1.2 Second-stage heat treatment In the second-stage heat treatment, after completion of the first-stage cooling, the steel material is heated at a heating rate of less than 400 ° C./hr. This heating is performed up to a temperature of A ₁ point + 25 ° C. or higher and A ₁ point + 30 ° C. or lower (however, less than 780 ° C.) and held at this temperature for 0.5 to 1 hour. Next, cooling is started, and in this cooling, a temperature range of A ₁ point −5 ° C. or lower and A ₁ point −30 ° C. or higher is gradually cooled at a rate of 1 ° C./hr or lower. After widening the pearlite lamella spacing at the first stage and then spheroidizing it by heat treatment at the second stage, the carbide size can be increased and the distance between carbides can be increased, making the steel sufficiently soft. it can.

  The reason why the temperature increase rate of the steel material is set in the above range in the second heat treatment is that if the temperature is exceeded, the temperature tends to overshoot and the heating temperature and time are affected. The temperature rising rate is preferably 300 ° C./hr or less, more preferably 200 ° C./hr or less. If the temperature raising rate is too slow, the production efficiency is lowered. Therefore, the rate of temperature rise is, for example, 5 ° C./hr or more, preferably 10 ° C./hr or more, more preferably 20 ° C./hr or more.

The reason why the heating temperature and the heating time of the second stage are set within the above ranges is to appropriately decompose the regenerated pearlite generated by the heat treatment of the first stage. If the heating temperature is too low or the heating time is too short, regenerated pearlite remains and the steel material becomes insufficiently softened. On the other hand, if the heating temperature is too high or the heating time is too long, the cementite will be dissolved too much, and pearlite will be easily generated by the subsequent cooling, and the steel material will not be softened sufficiently. The heating temperature is preferably A ₁ point + 26 ° C. to A ₁ point + 29 ° C. Moreover, this heating temperature becomes like this. Preferably it is 775 degrees C or less, More preferably, it is 770 degrees C or less. The heating time is preferably 0.6 to 0.9 hours.

  The reason why the cooling rate of the second stage is set in the above range is to suppress the generation of regenerated pearlite. When regenerated perlite is generated, the spheroidization becomes insufficient. The cooling rate is preferably 0.9 ° C./hr or less, more preferably 0.8 ° C./hr or less. The lower limit of the cooling rate can be set as appropriate, but may be, for example, 0.05 ° C./hr or higher, preferably 0.1 ° C./hr or higher.

2.2 Step before spheroidizing annealing In manufacturing the steel material of the present invention, the step before spheroidizing annealing is not particularly limited, and for example, a normal method for manufacturing a steel material may be followed. For example, a steel material that has been adjusted to have a predetermined component is cast, and if necessary, the steel material that is hot-rolled after being subjected to split rolling can be used. Moreover, even if it is the steel material obtained by the other method, as long as it is the steel material in which the spheroidization of the carbide | carbonized_material is not made, any can be used.

  The steel material spheroidized and annealed as described above is excellent in cold workability (particularly cold forgeability). Therefore, it can be easily formed into a predetermined shape. This formed body is subjected to final strength adjustment such as quenching and tempering, and becomes a steel part. The present invention is extremely useful for manufacturing various parts such as automobile parts such as bearings and construction machine parts.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

Experimental Example 21 steel products (1 ton) of φ15 mm were produced per steel type by hot rolling steel materials (steel types A to H) having the composition shown in Table 1 below. 7 wire coils per layer (21 in total for 3 layers) are inserted into a 3 layer (stage) stack type batch furnace, and the first stage (first time) heat treatment is performed under the conditions shown in Table 2 below. Then, the second stage (second time) heat treatment was performed.

  After the heat treatment, six wire coils were selected without deviation from the 21 wire coils, and used as evaluation coils. About one coil, the sample was extract | collected from the edge part and HV hardness was measured in D / 8 position (D is a diameter) of a cross section. It implemented in all six coils, and made the average value HV of the said steel materials. The results are shown in Table 2.

No. In No. 13, the cooling rate of the first stage heat treatment was too fast and fine regenerated pearlite was generated. Further, since the second stage heat treatment temperature was too high and the cooling rate was too fast, fine regenerated pearlite was generated after the second stage heat treatment, and the steel material became hard. No. In No. 14, since the second stage heat treatment temperature was too high and the cooling rate was too fast, fine regenerated perlite was generated and the steel material became hard. No. In No. 15, the pearlite remained undissolved because the first stage heat treatment temperature was too low. In addition, since the heat treatment temperature in the second stage is too low, the re-solid solution of the pearlite melted and regenerated in the first stage is insufficient, and the cooling rate is too fast, so that the pearlite is regenerated during cooling and the steel is hardened. It was. No. In 16 and 17, the heat treatment time in the second stage was too long and the cementite was too melted, and pearlite was generated by subsequent cooling, and the steel material was hardened. No. No. 18 produced fine pearlite because the cooling rate of the first stage heat treatment was too fast. Even if the second heat treatment was appropriately performed, the pearlite was too melted, so that pearlite was easily generated by subsequent cooling, and the steel material was hardened. No. No. 19 entered the second stage heating with austenite remaining because the cooling end temperature after the first stage heat treatment was too high. Even when the heating temperature at the second stage was lowered, the carbides were excessively melted, and the subsequent cooling rate was fast, so that pearlite was regenerated and the steel material became hard. No. Since No. 20 is a conventional steel having a high Cr content and is not a target steel for the main heat treatment, spheroidization was not appropriately performed, and the steel material became hard.
No. In Nos. 1 to 12, since the two-stage heat treatment was carried out under suitable conditions for high C low Cr steel, it was possible to reliably spheroidize, increase the carbide size, and widen the carbide interval. Fully softened.

Claims (5)

C: 0.7-1.5% (meaning mass%; hereinafter the same), and Cr: less than 0.9% (not including 0%) steel is spheroidized and cold workability The spheroidizing annealing is composed of a first-stage heat treatment followed by a second-stage heat treatment, and the conditions for the first-stage heat treatment and the second-stage heat treatment are as follows: A method for producing a steel material having excellent cold workability as described below.
(1) First-stage heat treatment Temperature increase rate: less than 400 ° C./hr Heating temperature: A ₁ point + 30 ° C. to A ₁ point + 40 ° C. and less than 780 ° C. Heating holding time: 1-2 hours Cooling rate: A ₁ point ~ A ₁ point-20 ° C temperature range of 3 ° C / hr or less Cooling end temperature: A ₁ point-35 ° C or less (2) Second stage heat treatment Heating rate: less than 400 ° C / hr Heating temperature: A ₁ point +25 _{1 to} 30 ° C. and less than 780 ° C. Heating and holding time: 0.5 to 1 hour Cooling rate: A ₁ point −5 ° C. to A ₁ point −30 ° C. The temperature range is 1 ° C./hr or less   The steel material further contains Si: 0.001 to 0.7%, Mn: 0.1 to 2.0%, Al: 0.001 to 0.1%, the balance being iron and inevitable impurities The manufacturing method of Claim 1 which uses a thing. The inevitable impurities include P: 0.05% or less (not including 0%), S: 0.001 to 0.05%, N: 0.015% or less (not including 0%). 2. The production method according to 2 . The steel material is further
Cu: 0.25% or less (excluding 0%),
Ni: 0.25% or less (excluding 0%),
4. At least one selected from the group consisting of Mo: 0.25% or less (not including 0%) and B: 0.01% or less (not including 0%). The manufacturing method as described. The steel material is further
Ti: 0.2% or less (excluding 0%),
5. The composition according to claim 1, comprising at least one selected from the group consisting of Nb: 0.2% or less (not including 0%) and V: 0.5% or less (not including 0%). The manufacturing method as described.