JP3396282B2 - Press hardening method - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a press hardening method for preventing or reducing deformation due to hardening treatment.

[0002]

2. Description of the Related Art When a steel product is quenched, it is unavoidable that deformation occurs due to a temperature difference during heating, a temperature difference during cooling, a phase transformation time difference, and the like at the product site. Conventionally, as a method of removing the generated deformation, a method of correcting by plastic deformation using a straightening machine according to the product after quenching or quenching and tempering is generally adopted. For example, in shafts used for automobile transmissions, a quenched shaft is supported at two points, and a central portion of the shaft is straightened by applying pressure with a pressing machine.

In addition, there is an example in which a press quenching method in which stress is applied during quenching to cause plastic deformation during transformation is used for the purpose of reducing strain in carburized gears, disks, leaf springs and the like. In this case, a so-called Gleason Quenching Press is often used for the treatment, and oil is applied only to the portion to be quenched. The cooling control in this method is performed by a combination of the amount of oil and time.

On the other hand, there is an example in which a press quenching method using immersion cooling is applied to quenching a leaf spring. This method is a method of dipping and cooling together with a pressed die and then taking out after cooling, and the accuracy of deformation is a low level such that the flatness is in the range of 1 mm to 2 mm in total length. Moreover, the press-quenching method has a problem that the processing cost is high, and thus it has not been adopted so much recently.

[0005]

Recently, there are increasing demands for high-precision quality even for products having a long length. In this case, in the conventional straightening machine, it takes time to correct the deformation such as bending, and the efficiency is remarkably reduced, and when the correction amount is large, it may be broken. For example, a certain type of rail requires straightness (straightness) and further higher hardness in order to fully exhibit its performance. Further, corrosion resistance and the like depending on the use environment are also required. Therefore, the materials for this application are usually medium and high carbon steels, containing appropriate amounts of alloying elements, hard and high strength. Therefore, in order to correct the deformation of the steel material due to the quenching treatment, a large force is required for the plastic deformation, and the size of the apparatus is inevitably increased. Further, when the deformation is large, breakage occurs in straightening, and an economical loss such as discarding an expensive product as a defective product is also large.

Further, even if the generated deformation is corrected by straightening, some deformation remains and large residual stress exists on the surface. Most products are machined after quenching, but at this time, it is necessary to increase the amount of processing in order to remove the deformation. In addition, residual stress on the surface is released and new deformation occurs, which is a serious problem in precision machinery and the like.

The present invention is intended to solve the above-mentioned problems of the prior art, and prevents deformation and bending due to the quenching process regardless of whether it is a high hardness steel material or a long product. Alternatively, it is an object of the present invention to provide a press quenching method that can be significantly reduced, can be efficiently and reliably quenched, and can be easily implemented with a simple apparatus.

[0008]

Means for Solving the Problems As a result of various studies to solve the above problems, the present inventor has found that during press hardening, quenching of a steel material heated to an austenitic structure is
Cooling immediately above the martensitic transformation start temperature (hereinafter referred to as Ms temperature) and with a small temperature difference depending on the part causes less deformation after the completion of martensitic transformation, and prevents the shape of the press die from deforming. The present invention has been found to have an effect on the present invention and has reached the present invention.

That is, according to the present invention, in the press quenching method for press quenching a steel material, the temperature of the quenching cooling liquid is controlled to be not higher than the martensite transformation start temperature and not lower than the martensite transformation end temperature (hereinafter referred to as Mf temperature). Every
For a long length of hardened steel that has been heated under appropriate conditions, one or more sets of press dies with opposing die surfaces having flat or curved die surfaces, or opposing die surfaces Fixed by pressing with one or more sets of press dies having a mold surface consisting of a flat surface with unevenness surface or a curved surface with unevenness surface, or a combination of these. Hold as it is , quenching for a long length
After reducing the temperature difference between the steel material and the press die , in that state, dip in the cooling liquid together with the press die , cool and transform to martensite , and hold by the press die device.
In this state, the press hardening method is characterized in that it is pulled up from the cooling liquid and allowed to cool .

In the case where one set or a plurality of sets of press dies are press dies having a die surface composed of a surface having unevenness on a flat surface or a surface having unevenness on a curved surface, a pair of opposing molds is used. It is desirable to make the shapes of the irregularities of the press die asymmetrical to each other. Then, the quenching steel material held in the press die is immersed in the cooling liquid in the first cooling step in which it is first immersed in the cooling liquid, and after that, it is pulled up from the cooling liquid as it is, and after being held for a predetermined time, it is again immersed in the cooling liquid. It is desirable to include the second cooling step.

Further, the first cooling step is performed in a first cooling tank in which a first cooling liquid having a quenching cooling liquid temperature of Ms temperature or lower and Mf temperature or higher is stored, and the second cooling process is performed. The cooling step is performed by immersing in a second cooling tank in which a second cooling liquid whose temperature is below the Ms temperature and above the Mf temperature and which is controlled to a temperature lower than that of the first cooling liquid is stored for cooling. It is desirable to continue the martensitic transformation.

[0012]

In the press-quenching according to the present invention, first, the heated steel material to be quenched is pressed and held by the press die to shape it. At this time, heat flows from the hardened steel material to the press die due to the temperature difference between the hardened steel material and the press die. Then, in that state, it is immersed in a cooling liquid for an appropriate time. At this time, the cooling of all the surfaces contacting with the cooling liquid is made uniform, and the temperature difference between the press die and the hardened steel material is reduced. After a certain period of time, the liquid is temporarily taken out of the cooling liquid. At this time, heat is transferred from the inside of the hardened steel material to the outside, but the entire hardened steel material is kept at a temperature higher than the Ms temperature and kept for an appropriate time so as to approach the Ms temperature. Then, it is again immersed in the cooling liquid to start the martensitic transformation. In the first immersion in the cooling liquid, the effect of improving productivity by increasing the cooling efficiency of the steel to be hardened is added.

The purpose of press quenching is to correct the deformation that occurs. A structure having a structure in the middle of martensitic transformation is prone to plastic deformation, but a structure having completed transformation becomes difficult to correct. Therefore, in the present invention, the temperature of the cooling liquid is set to M
By controlling the temperature to be s temperature or lower and Mf temperature or higher, quenching of the steel to be hardened heated to an austenite structure is started at the Mf temperature or higher and is corrected during the martensitic transformation, so that the correction is easy and effective. In this way, the deformation after the completion of martensite transformation is small and effective press hardening can be performed.

Although one cooling tank can sufficiently exhibit the above effect, it is more efficient to provide two cooling tanks. Coolant temperature in both cooling tanks is below Ms temperature and M
By controlling the temperature of the cooling liquid in the second tank to a temperature lower than that in the first tank within the range of temperature f or higher, the martensite transformation is carried out in the first tank halfway, and the martensite in the second tank is further increased. Continue the transformation. According to this method, the martensite transformation progresses further at the end of the quenching treatment, and the deformation at the end of transformation can be reduced as compared with the case of treating in one tank. When the treatment is carried out in one tank, it is practical to carry out the treatment within a range in which the temperature of the cooling liquid is not extremely higher than the Mf temperature or higher.

On the other hand, it has been confirmed that the press surface of the press mold to be used is effective even if it is simply flat.
If there is a difference in the time of contact with the press die depending on the part of the steel to be hardened, if a press die with a flat die surface is used, deformation due to elastic deformation may remain in the hardened steel after quenching. is there. As a method to prevent this deformation,
It has been found that a press die having a die surface having an appropriate curvature is effective.

Further, it has been found that it is more effective to make unevenness on the die surface of the press die in order to improve the pressing effect in the press die, to uniformly cool the die, and to improve the cooling efficiency. . The uneven shape was found to be effective even when it was vertically symmetrical, but there are cases where vertical asymmetry is even more effective.Therefore, find the optimal uneven shape of the press die depending on the shape of the steel product to be quenched. Apply.

[0017]

EXAMPLES In each of the following examples, a press die apparatus as shown in FIG. 1 was used to press and fix a hardened steel material having a rectangular cross section heated to a predetermined temperature in the vertical and horizontal directions. It was performed by immersing the press die apparatus together with the cooling liquid for a predetermined time. Cooling is performed only in the primary cooling step by dipping in the cooling liquid once in Example 1, and in Example 2 the primary and second steps by dipping in the cooling liquid twice.
It was carried out in the next cooling step.

In FIG. 1, reference numeral 1 denotes a receiving die which constitutes one of the pressing dies, and a receiving die surface is formed so as to intersect the upper surface thereof at a right angle, and one side thereof is a receiving die surface in the vertical direction. 2, the other side is a receiving die surface 3 in the left-right direction. Then, the pressing die 4 in the vertical direction, which is the other press die that moves at right angles to face the receiving die surfaces 2 and 3 and presses the hardened steel material 15 received by the receiving die surfaces.
Further, the pressing die 5 in the left-right direction is driven by an appropriate driving device, and the heated quenching steel material 15 is received and held by the die surface and the pressing die surface to adjust the shape.

The shapes of the receiving die surfaces 2 and 3 and the pressing die surfaces 7 and 6 are shown in FIG. 2A along the longitudinal direction of the hardened steel material 15, that is, in the direction perpendicular to the paper surface in FIG. When the mold surface is formed flat as shown in FIG.
As shown in (b), it was performed in the case where the semicircular arc-shaped grooves 10 were formed on the surface of the die at a predetermined pitch to form the grooves in an uneven shape.
Then, in order to investigate the influence of the combination of the shapes of the mold surfaces, when both the pressing mold surface and the receiving mold surface are flat as shown in FIG. 2 (a), both sides as shown in (b) are also used. Both are uneven shapes, and the pitches of the unevenness are the same and both are symmetrical, and as shown in (c), the pitch of the grooves 10 is deviated from the opposing mold surface and the uneven shape is asymmetrical, As shown in (d), an experiment was conducted on the case where one side was uneven and the other side was flat. In the examples shown in (b) and (c), the pitch of the unevenness between the pressing die surface and the receiving die surface was formed in the same shape with the same pitch, but as shown in FIG. The groove 10 on the mold surface and the groove 10 'on the other mold surface may be formed in different shapes.

Example 1 Cooling process: Steel for quenching only in primary cooling: SUS440C Shape of steel for quenching: Rectangular section (40 mm x 20 mm) Length 750 mm Heating method: Furnace heating 10 minutes Pressurization: 75 kg / cm ² Number of samples : 10 each

[0021]

[Table 1]

The heated steel material press-quenched as described above was allowed to cool and cooled to room temperature, and then the amount of deformation and hardness were measured. The result is shown in FIG. Also, as a comparative example,
Heated steel material similar to that of Example 1 was heated to 1030 ° C and 1060 ° C.
Each of them was heated 10 times, and without being pressed, it was immersed in a cooling liquid having a liquid temperature of 60 ° C. for 200 seconds and then pulled up. After sufficient cooling, its deformation amount and hardness were measured. The results are also shown in FIG.

Example 2 Cooling process: Primary and secondary cooling Quenching steel material: SUS440C steel Quenching steel shape: Rectangular cross section (48 mm x 20 mm) Length 800 mm Heating method: Furnace heating 10 minutes Pressurizing pressure: 75 kg / cm ² Number of implementations: 10 each

Example 2-1 A hardened steel material heated to 1030 ° C. or 1060 ° C. was
Each of them is fixed and held by a press mold device having a flat mold surface, immersed for 250 seconds in a cooling liquid having a liquid temperature of 30 ° C to perform primary cooling, and then cooled while being held by the press mold device. It was pulled out of the liquid and allowed to cool for 25 seconds, and then again immersed in the cooling liquid until the material temperature of the quenched steel material reached 40 ° C. for secondary cooling, after which it was pulled and allowed to cool.

Example 2-2 A steel to be quenched, which was heated to 1030 ° C. or 1060 ° C.,
Each of them is fixed and held by a press mold device having a flat mold surface, immersed for 250 seconds in a cooling liquid having a liquid temperature of 30 ° C to perform primary cooling, and then cooled while being held by the press mold device. It is pulled out of the liquid and allowed to cool for 25 seconds, after which the temperature of the steel to be quenched is 40 ° C in another cooling liquid with a liquid temperature of 30 ° C.
It was immersed until the temperature reached ℃, and the secondary cooling was performed, and then it was pulled up and allowed to cool.

With respect to the steel materials to be heated in each of the above-mentioned examples, the amounts of deformation and hardness in the vertical direction and the horizontal direction were measured after allowing to cool sufficiently. The results are shown in FIG. 3 together with the above examples and comparative examples. In addition, FIG. 3 shows a steel material heated at 1030 ° C. and a hardened steel material heated at 1060 ° C.
Vertical deformation amount and lateral deformation amount (mm) and hardness (H
rc) is shown on the vertical axis, and the solid line in the vertical axis direction is 1 in each example.
The range of the minimum value and the maximum value of the results obtained for 0 pieces is shown, and the average value is indicated by a mark (upward and downward) or a mark (horizontal direction).

As is apparent from FIG. 3, in the case of the comparative example which is the quenching by the conventional method, the deformation amount is about 3 mm in both the vertical direction and the lateral direction, whereas the press-quenching according to each example of the present invention. It was confirmed that the amount of deformation of the put-in heat-hardened steel material was 0.8 mm or less in both directions, and the amount of deformation was remarkably small as compared with the comparative example. Particularly, as compared with the case of Example 1 in which the cooling is only the primary cooling step, the secondary cooling is performed in which the cooling liquid is pulled up during cooling, left to cool, and further immersed in the cooling liquid for cooling. In the case of Example 2, there is almost no deformation and a remarkable effect is shown.

The hardness is not so different from that of the comparative example, but the hardness of this example is slightly increased as a whole. That is, in the press quenching by the method of the present invention, the hardness as the quenching effect is the same as or higher than that of the conventional one and the sufficient quenching effect is exhibited, but the deformation amount is remarkably reduced as compared with the conventional one. , Has a remarkable effect.

Further, the influence of the shape of the die surface of the press die apparatus is slightly smaller than that in the case where the die surfaces are mutually flat or when one or both of the die surfaces are processed to have irregularities. In addition, in the case of mold surfaces that were processed to be uneven, the amount of deformation tended to decrease when the unevenness was arranged asymmetrically, and the effect of the shape of the mold surface on the deformation after quenching was recognized. .

The heating temperature of the steel to be quenched is 106
It can be seen that the amount of deformation tends to be smaller and the hardness tends to increase when heated to 0 ° C. and hardened than when heated to 1030 ° C. Further, in the case of Example 1, the influence of the temperature of the cooling liquid is that the deformation amount is smaller in the case of 40 ° C. than in the case of 60 ° C.

[0031]

As is clear from the above description, according to the press-quenching method of the present invention, it is possible to perform the quench-hardening treatment even for medium hardness and high carbon steel with high hardness and long products. The deformation and bending that occur can be greatly reduced, and quenching can be performed reliably. Therefore, the straightening work step after quenching can be eliminated or simplified, and the production efficiency can be greatly improved.

Further, since the straightening work process after quenching can be eliminated or simplified, it is possible to greatly reduce the breakage and defective products generated during the straightening work. further,
Since there is little deformation or bending, the amount of machining can be reduced, and the efficiency and yield can be improved.

[Brief description of drawings]

FIG. 1 is a schematic view of a press die device used in an example of the present invention.

FIG. 2 is an explanatory view of a combination of shapes of die surfaces of a press die device.

FIG. 3 is a diagram showing hardness and deformation amount for each example and each comparative example.

[Explanation of symbols]

1 Receiving die 2, 3 Receiving die die surface 4 Vertical pressing die 5 Horizontal pressing die 6 Vertical pressing die surface 7 Horizontal pressing die surface

─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-1-242714 (JP, A) JP-A-57-164925 (JP, A) JP-A-57-164924 (JP, A) JP-A-54- 66318 (JP, A) Jitsu 18-18768 (JP, Y1) (58) Fields investigated (Int.Cl. ⁷ , DB name) C21D 1/18

Claims (7)

(57) [Claims] 1. A press-quenching method for press-quenching a steel material, wherein the temperature of a quenching cooling liquid is kept below a martensite transformation start temperature and above a martensite transformation end temperature, and heating is carried out for a long length. Steel material
Pressed with one set or multiple sets of press dies and hold them fixed to reduce the temperature difference between the steel to be hardened and the press dies.
After cooled by immersion into the press die each said coolant in this state, is martensite transformation, the pre
Press quenching method, characterized in that it is pulled up from a cooling liquid and allowed to cool while being held by a die device . 2. The press hardening method according to claim 1, wherein the one or more sets of press dies are formed by a combination of parallel press dies having die surfaces each having a flat surface or parallel curved surfaces. 3. One or more sets of press dies are parallel press dies having a die surface composed of planes having concavo-convex surfaces or curved surfaces having concavo-convex surfaces. The press-quenching method according to claim 1, which comprises a combination. 4. The press hardening method according to claim 3, wherein the shapes of the concavities and convexities of the parallel press dies are asymmetric with each other. 5. The press die of one set or a plurality of sets comprises a press die having a flat surface or a curved surface, and a parallel press having a surface having an uneven surface or a surface having an uneven surface. The press hardening method according to claim 1, which is a combination with a die. 6. The quenching steel material held in the press die is immersed in the cooling liquid in a first cooling step in which the quenching steel material is once immersed in the cooling fluid, and the quenching steel material after the first cooling step is pressed in the press die. 6. The press hardening method according to claim 1, comprising a second cooling step of pulling out from the cooling liquid in the state of being held in step 1, holding for a predetermined time, and then immersing in the cooling liquid again. 7. The first cooling step, wherein the first cooling step is a first cooling tank in which a first cooling liquid in which the temperature of the quenching cooling liquid is equal to or lower than the martensite transformation start temperature and equal to or higher than the martensite transformation end temperature is stored. And the second cooling step is performed in the first
7. The second cooling tank in which a second cooling liquid, which is controlled to have a temperature lower than that of the cooling liquid of (1) and lower than or equal to the martensitic transformation start temperature and equal to or higher than the martensitic transformation end temperature, is stored in the second cooling tank. Press hardening method.