JP2005082840A - Method for producing hot-forged non-heattreated part excellent in machinability - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a new forging method capable of improving the machinability of a forged product independently of a machinability improving element. <P>SOLUTION: A steel material, which contains, by mass%, of 0.20-0.60% C, 0.10-2.00% Si, 0.30-2.00% Mn, 0.01-0.20% S, 0.05-2.00% Cr, ≤0.060% Al, 0.01-0.05% V, 0.003-0.020% N and if necessary, one or more kinds of Ti, Pb, Te, Ca, Bi, Mg, Zr and the balance Fe with impurity elements, is heated to either higher temperature of (solidus temperature) × 0.94 or 1,250°C as the lower limit temperature and in the range of ≤(liquidus temperature) × 0.98 as the upper limit temperature, and a hot-forging at the super-high temperature is applied so that ≥85% of the surface of a blank is brought into contact with dies in the range of the temperature to make the hot-forged product having ferrite and pearite structure and ≤No.25 average austenitic crystal grain size number. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention is a part that is manufactured and used by hot forging such as automobile engine parts and construction machine parts, for example, and is easy to finish after forging and has excellent machinability. The present invention relates to a tempered part manufacturing method.

  In automobile engine parts and construction machine parts, many parts are manufactured by hot forging. This is because there are many relatively large parts such as a crankshaft and a connecting rod, and the shape is complicated, so that forming by hot forging is the most advantageous in terms of cost. However, hot forging only produces a rough shape. Naturally, after hot forging, finishing is performed by machining or the like in order to process into a final product shape. Therefore, these parts are required to have excellent machinability at a level that facilitates mass production.

  As steel materials used for hot forging, until the first half of the Showa 50s, quenching and tempering was performed after forging using carbon steel or low alloy steel with a slight amount of Cr, Mo, etc. added to carbon steel (tempering) ) To ensure the necessary strength. However, since this method requires a large amount of energy for heat treatment, a lot of non-tempered steel that can obtain the required strength and can omit heat treatment only by air cooling after forging seems to be used. Many materials have been developed and reached today.

  In the development of this large number of non-tempered steels, it is a matter of course that the strength performance of obtaining the required strength without heat treatment has been emphasized. Therefore, improvement of machinability is also emphasized at the same time, and the upper and lower limits of the addition amount of elements for securing strength are determined in consideration of machinability, and machinability improving elements Non-tempered steel for hot forging having machinability that can be mass-produced shown in Patent Documents 1 to 3, for example, has been developed by effectively utilizing an element such as Pb known as Pb.

Japanese Patent Laid-Open No. 1-165749 Japanese Patent Laid-Open No. 1-176055

However, the steel described in the patent document has the following problems.
As described above, machinability is regarded as the most important factor in the conventional non-heat treated steel for hot forging and has been developed. However, the conventional machinability improving method basically attempts to satisfy requirements such as machinability by optimizing component elements. And a chemical component changes very greatly according to the required mechanical characteristic. Therefore, the level of machinability to be obtained changes depending on the amount of C, Mn, and Cr, which are basic component elements for ensuring strength, and the amount of the machinability improving element to be added changes. Therefore, every time a newly developed steel is developed, a large number of trials for performance confirmation must be performed, and the number of man-hours associated with the development is enormous.

In addition, as the demand for further weight reduction has recently increased, it has been required to achieve the required machinability with a forged product with higher hardness. The machinability improvement method has reached its limit. Therefore, development of a machinability improving method not only by improving the components but also by improving the manufacturing method has been strongly desired.

  The present invention has been made to solve the above-described problems, and its purpose is to propose a new machinability improving method from the viewpoint of the manufacturing method, and this and the conventional component optimum type machinability. A method for manufacturing non-heat treated parts for hot forging that can be developed in a shorter period of time by combining improvement measures, and can reliably improve machinability even for forged products with high hardness. It is to provide.

  The invention according to claim 1, which has been made to solve the above-mentioned problems, is mass%, C: 0.20 to 0.60%, Si: 0.10 to 2.00%, Mn: 0.30 to 2 0.00%, S: 0.01 to 0.20%, Cr: 0.05 to 2.00%, Al: 0.060% or less, V: 0.01 to 0.50%, N: 0.003 The lower limit temperature is set to the higher one of the solidus temperature × 0.94 or 1250 ° C., and the upper limit temperature is set to the liquidus temperature × 0. Heated to a range of 98 or less, and subjected to ultra-high temperature hot forging so that 85% or more of the material surface is in contact with the mold in the temperature range of the range, the structure is ferrite pearlite, and the average austenite Hot forging with excellent machinability characterized by being a hot forged product having a grain size number of 2.5 or less This is a manufacturing method for non-heat treated parts.

  What should be noted in the present invention is that by performing ultra-high temperature forging which is heated and forged to a higher temperature than normal temperature, a forged product having coarser crystal grains of 2.5 or less is manufactured, It is the point that it was found that the machinability can be improved and finishing can be facilitated by a method different from the addition of the machinability improving element, which is the effect of coarsening.

  As mentioned above, a number of patents have already been filed for non-heat treated steel for hot forging, which is characterized by optimization of the components, thereby ensuring the necessary strength without heat treatment after hot forging. Steels that ensure the necessary machinability have already been proposed.

  On the other hand, ultra-high temperature forging is described in, for example, Japanese Patent Application Laid-Open No. 5-15935, and has already been known as a technique capable of processing a component having a complicated shape with a smaller deformation resistance.

  However, the latter ultra-high temperature forging is a technology developed completely independent of the improvement in machinability from the manufacturing surface, which is the object of the present invention, and is actively used for improving machinability. There was no idea that. In addition, only by improving the machinability by optimizing the chemical components described above, it becomes difficult to obtain machinability that can be mass-produced, particularly when the hardness is Hv 300 or more. There was a need for improvement by the method.

  As a result of repeating a number of experiments and repeated investigations and studies, the inventors of the present invention usually coarsened the crystal grain size by ultra-high temperature forging, and obtained a forged product having an austenite crystal grain size of 2.5 or less. As a result, it has been found that the machinability can be improved without changing the components as compared with Nos. 3-6.

More specifically, when the hardness is relatively low, such as about Hv 200 or less, ferrite is easily deformed, so that the carbide is broken and deformed during machining, and is necessary for generating chips. The energy does not change greatly depending on the crystal grain size, and although it is effective, it is not so large. However, when the hardness is increased to about Hv240 or more, the larger the crystal grains, the smaller the energy associated with the breaking and deformation of the carbide during machining, the less energy generated during machining, and the tool life is improved. It became clear. Machining actually becomes a problem when the hardness of the latter increases, and the machinability can be greatly improved by applying the present invention.

  Next, the reason for limiting the component range of the steel material used in the method for producing a hot forged non-heat treated part excellent in machinability according to the present invention will be described.

  Needless to say, the present invention is not an invention characterized by limiting the range of chemical components. Therefore, the component ranges shown below indicate component ranges that can be used in non-tempering after hot forging, and that the machinability improving effect can be reliably obtained by the ultra-high temperature forging of the present invention.

C: 0.20 to 0.60%
C is an interstitial element that is effective in improving the strength by solid solution strengthening. In order to ensure the necessary strength after forging, it is necessary to contain at least 0.20% or more. However, when the content is increased, the hardness is increased, the machinability is lowered, and finishing after forging becomes difficult even when the method of the present invention is applied, so the upper limit was made 0.60%.

Si: 0.10 to 2.00%
Si is an effective element for use as a deoxidizer during the production of steel. Therefore, the content must be at least 0.10%. However, too much addition facilitates decarburization and decreases hot workability and toughness, so the upper limit was made 2.00%. Desirably, it should be less than 0.70.

Mn: 0.30 to 2.00%
Mn is a basic element for enhancing the hardenability of the steel material and ensuring the required strength, and it is necessary to contain 0.30% or more. However, a large amount reduces machinability and makes it difficult to finish after forging, so the upper limit was made 2.00%.

S: 0.01-0.20%
S is an element necessary for improving the machinability and needs to be contained in an amount of 0.01% or more. However, since a large amount reduces toughness, the upper limit was made 0.20%.

Cr: 0.05-2.00%
Cr, like Mn, is a basic element for enhancing the hardenability of the steel material and ensuring the necessary strength, and it is necessary to contain 0.05% or more. However, since a large amount reduces toughness, the upper limit was made 2.00%.

Al: 0.060% or less Al is an element necessary for deoxidation. However, Al exists in the form of alumina in steel, which adversely affects machinability and lowers fatigue properties as a starting point for fatigue failure. Therefore, it is necessary to suppress the addition to the minimum for deoxidation. Yes, the upper limit was made 0.060%. If importance is attached to machinability, it is preferable to suppress the addition amount as much as possible.
The lower limit is preferably 0.002% from the viewpoint of the deoxidation effect.

V: 0.01 to 0.50%
V is an element that precipitates as V carbonitride in steel during cooling after hot forging, improves fatigue strength by precipitation strengthening, and is an indispensable element for enabling use in non-tempered steel . Therefore, the lower limit of the content is set to 0.01%. However, even if contained in a large amount, the effect is saturated and the cost is increased, so the upper limit was made 0.50%.

N: 0.003-0.020%
N is a carbonitride that is combined with V and the like in the steel and is precipitated and strengthened by this carbonitride, and is effective in improving the strength. The lower limit of the content is 0.003. %. However, if it is contained in a large amount, blow holes are generated in the slab and the steel ingot and cause cracking during forging, so the upper limit was made 0.020%.

  Moreover, the steel used by this invention may contain P as an unavoidable impurity other than the said element. However, the amount is at most less than 0.04%.

Next, the reasons for limiting the conditions other than the chemical components of the invention of claim 1 will be described.
The reason why the structure is ferrite pearlite is that the machinability improving effect by the ultra-high temperature forging according to the present invention is most effective when the structure is a ferrite pearlite structure.

The average grain size number of 2.5 or less is a forged product consisting of about 3 to 6 crystal grains obtained by processing at a normal forging temperature unless coarse grains of 2.5 or less are used. This is because a clear machinability improving effect cannot be obtained. In addition, the crystal grain size prescribed | regulated by this invention means the austenite crystal grain size which can be measured by the method prescribed | regulated by JISG0551.
The lower limit of the crystal grain size is preferably about 1 so that the toughness is not greatly lowered.

Next, the reasons for limiting the manufacturing conditions in the manufacturing method described in claim 1 will be described.
In normal hot forging, heating and forging are usually performed at a temperature of about 1000 to 1200 ° C. However, it is not possible to produce a forged product having an average grain size number that is stably 2 or less. In order to manufacture a forged product composed of coarse grains, heating and forging can be performed in an ultra-high temperature region where the temperature is higher than usual. Specifically, ultra high temperature forging is performed in which heating and forging are performed at a temperature at which the lower limit temperature is the solidus temperature × 0.94 or 1250 ° C., whichever is higher, and the upper limit temperature is the liquidus temperature × 0.98. Is achieved.

  In addition, the liquidus temperature of the steel material to forge and the below-mentioned solidus line temperature can be calculated | required by performing a unidirectional solidification test using a rod-shaped raw material.

  Here, the lower limit temperature is set to the higher one of the solidus temperature × 0.94 or 1250 ° C., and at a temperature lower than this, it becomes difficult to make the crystal grain size number 2.5 or less. This is because it becomes difficult to obtain a clear difference between the forged part at the temperature and the machinability.

  Further, in order to obtain a forged product with coarser crystal grains, it is necessary to forge in as short a time as possible after extracting the workpiece extracted from the heating furnace. Because the temperature drop is larger on the surface and the machining after forging is naturally processed from the surface, it is necessary to forge quickly after the heating is completed so that the temperature drop on the surface does not become large. Because.

  On the other hand, the upper limit temperature is set to the liquidus temperature × 0.98. Heating and forging at a temperature exceeding this temperature causes many parts to melt, and forgings of the desired shape are finished by forging. This is because it becomes difficult.

  However, in the present invention, in order to obtain a structure in which crystal grains are coarsened, when heated to a higher temperature than usual and heated to a temperature close to the solidus, a part of the forging material starts to melt, the solidus temperature When heated beyond the range, the number of melting sites increases. When such a material is forged, voids remain after forging, making it difficult to ensure the required strength. Therefore, at the time of ultra-high temperature forging, a high hydrostatic pressure such that most of the material surface (85% or more, especially 90% or more when heated to a temperature exceeding the solidus) is in contact with the mold. By forging in a state, it is necessary to devise and forge so that the number of holes is reduced.

Moreover, in the forged part obtained by the manufacturing method of the present invention, as in the invention described in claim 2, in addition to the steel used in the manufacturing method of claim 1, Ti is further added to increase the strength by precipitation strengthening. Improvements can be made. Hereinafter, the reason for limitation will be described.
Ti: 0.003 to 0.05%
Ti produces carbide, is finely dispersed, and is effective for improving the strength of the steel by precipitation strengthening. Therefore, if a small amount is added, it is effective for improving the performance of the non-tempered part, and the lower limit is set to 0.003%. However, while Ti is effective for precipitation strengthening, it has an effect of generating nitrides and preventing crystal grain coarsening during heating. Prevention of grain coarsening is usually effective in improving the performance of steel, but the present invention is intended to improve machinability by coarsening, so it is necessary to keep the amount added to a small amount. The upper limit was made 0.05%.

  Furthermore, in the forged part obtained by the manufacturing method of the present invention, as in the invention described in claim 3, by using steel further added with elements such as Pb, Te, Ca, Bi, Mg, Zr, The excellent machinability obtained by ultra high temperature forging can be further improved. Hereinafter, the reason for limitation will be described.

  Pb, Te, Ca, Bi, Mg, and Zr are elements that have an effect of improving machinability. The forged product obtained by the present invention has excellent machinability by performing ultra-high temperature forging as described above as compared with the case of forging at normal temperature. However, the forged product according to the present invention can also improve the machinability obtained by ultra-high temperature forging by adding a machinability improving element, as in the case of conventional steel. It was supposed to be possible. However, even if the addition amount is too large, the hot workability is lowered, so the upper limit is set to 0.30% for Pb, Te, Bi, and 0.01% for Ca, Mg, Zr.

  Thus, the machinability is further improved by using the steel used in the invention of claim 1 or 2 to which a machinability improving element such as Pb, Te, Ca, Bi, Mg, Zr is added. can do. The ultra-high temperature forging method, the structure of the non-tempered part to be obtained, the range of the crystal grain size number, and the reason for the limitation are exactly the same as in the first aspect.

  Further, regarding the point of containing P as an impurity, the steel used in claims 2 and 3 is exactly the same as in the case of claim 1.

  Next, examples of the present invention will be described. Table 1 shows the chemical composition of the steel used as the test material.

  Table 1 also shows the solidus temperature and the liquidus temperature. This is a rod-shaped material of φ15 × 250 mm prepared by further forging and machining a φ75 forged round bar described later. Is a temperature measured by conducting a unidirectional solidification test.

  The test material was prepared by melting steel composed of the components shown in Table 1 in a VIM melting furnace, forging the manufactured steel ingot into a round bar having a diameter of 75 mm, and air cooling. Using the obtained material, ultra-high temperature forging described later was performed, and machinability was evaluated.

In the ultra high temperature forging, a cylindrical test piece having a diameter of 70 mm and a length of 90 mm is prepared from the forged material, and this is placed horizontally, and the height is set under the conditions of heating temperature and pre-forging temperature shown in Table 3 to be described later. Was forged to 35 mm. In addition, forging is performed by adjusting the shape of the portion of the mold that comes into contact with the test material so that the side surface of the test piece comes into contact with the progress of processing, and when the processing is performed until the height reaches 35 mm, 91% The conditions were such that the surface was in contact with the mold. After forging, in order to accurately grasp the degree of coarsening, the crystal grain size was investigated by a method based on JISG0551. Forging is usually performed as a comparison in order to clarify the effects of ultra high temperature forging under the two levels of temperature conditions (within both the heating temperature and the temperature before forging both within the specified range) as shown in Table 2 below. Forging was also performed at the same time.

  However, when ultra-high temperature forging according to the present invention is performed, the crystal grains naturally become coarser than forging at normal temperature, so the number of proeutectoid ferrite formation sites decreases, the ferrite rate decreases, and the hardness increases. The tendency to do. If the hardness increases, the machinability decreases as a matter of course. Therefore, a fair evaluation cannot be made unless the decrease is taken into consideration. Therefore, an experiment was conducted by preparing a material in which the carbon equivalent was adjusted to be slightly lower in advance so that the hardness after forging was equivalent to that in normal forging when ultra-high temperature forging was performed.

  Steel No. shown in Table 1 1 ', 2', 3 'and 4' are materials prepared so as to have the same hardness during ultra high temperature forging. Moreover, since machinability obtained naturally changes when the component changes, it is not a fair comparison to compare steel materials having different components under the same conditions. Therefore, Steel No. The life by the machinability test (test method will be described later) during normal forging 1 to 4 is assumed to be 100, and the life ratio is shown. That is, the numerical value of the life of steel 1 'shown in Table 3 to be described later means the ratio with the life obtained in the machinability test in the case of normal forging of steel 1. 2 to 4 and Steel No. It is a value that has no relation to the lifetime values of 2 'to 4'. Therefore, the difference between the steel life number shown in Table 3 and 100 means the machinability improving effect by the method of the present invention.

  Next, the machinability evaluation test method will be described.

  The machinability was evaluated by performing a peripheral turning test and a drilling test. The test was carried out by producing a φ32 round bar test piece from the forged test piece described above. Specific test conditions are shown in Table 2, and test results are shown in Table 3.

  As is clear from Table 3, when compared to the case of normal forging and the case of ultra-high temperature forging, the hardness increases due to ultra-high temperature forging when compared with the same steel material, and therefore the machinability reduction due to it However, it seems that the effect of machinability improvement due to grain coarsening is offset and a particularly large effect is not obtained. However, in actual manufacturing, hardness is an important management item, and it is adjusted including the components so that the desired hardness can be obtained. Therefore, it is necessary to compare the machinability improving effect with the same hardness.

  Considering this point, for example, steel No. in normal forging. In consideration of the machinability evaluation result of No. 2 and the increase in hardness due to ultra-high temperature forging, No. 2 Specimen No. 2 with a slightly reduced carbon equivalent compared to 2. Comparing the machinability evaluation results of the 2 'ultra high temperature forged product, it can be seen that the machinability is greatly improved although the hardness is equal. Other steel No. By performing the same comparison for 1, 3, and 4, the magnitude of the machinability improving effect according to the present invention can be grasped.

However, as is clear from the results in Table 3, the machinability improving effect obtained is a steel No. about Hv200 having a relatively low hardness. Steel No. 4 having a relatively high hardness compared to No. 4. 2 is more effective. However, the machinability actually becomes a problem when the hardness is high, and the effect of the present invention is a significant effect on the improvement of the tool life for a forged product that is expected to have a relatively high hardness. Can be obtained.

  As described above, since it was confirmed that a large effect can be obtained by the evaluation of the test piece level, the test material No. An effect confirmation experiment was conducted using a steel material corresponding to 2 '. The experiment was carried out under the conditions of a heating temperature of 1380 ° C. and a pre-forging temperature of 1340 ° C. for ultra-high temperature forged products. In this forging, in order to prevent the generation of voids in the forged product, the area ratio where the surface of the work material during forging was in contact with the die was 91%, and the hydrostatic pressure was increased during forging. . For comparison, the above-mentioned specimen No. Forging was performed at a normal temperature (heating temperature 1240 ° C., pre-forging temperature 1200 ° C.) using a steel material corresponding to 2.

  Then, the site | part where machining is actually performed was cut out from the obtained forged product, the round bar test piece was produced, and the machinability evaluation test similar to the above-mentioned test was implemented. As a result, the hardness of the normal forged product was almost the same as that of the HV283 for the ultra-high temperature forged product, but the machinability was 166 for the tool life in the turning test and 172 for the drill life (both A value obtained when the life of a normal forged product is 100) was obtained.

  Therefore, as a result of measuring the crystal grain size of this test piece, the ultra-high temperature forged product was extremely coarse as 2.0, compared to 4.8 for the normal forged product. The presence or absence of vacancies was also confirmed at the time of the grain size investigation, but the vacancies were sufficiently crushed and good. Therefore, it was confirmed that the coarsening of grains by ultra-high temperature forging was effective in improving machinability, and the contact area between the die and work material during forging was increased, resulting in high hydrostatic pressure. It was found that a good forged product with few voids can be obtained by performing forging in a state.

  As described above, the ultra-high temperature forging in the present invention is a technique developed to enable forging load reduction and production of complex shaped products. Conventionally, mainly for the purpose of improving machinability, It was never applied. However, as described above, it became clear that the machinability can be greatly improved without relying on the machinability improving element by manufacturing forged parts made of coarse grains using ultra-high temperature forging. .

Therefore, it can greatly contribute to the improvement of the machinability of the forged product without depending on the addition of S, Pb, etc., and the effect is extremely large.

Claims (3)

  In mass%, C: 0.20 to 0.60%, Si: 0.10 to 2.00%, Mn: 0.30 to 2.00%, S: 0.01 to 0.20%, Cr: Steel containing 0.05 to 2.00%, Al: 0.060% or less, V: 0.01 to 0.50%, N: 0.003 to 0.020%, and the balance being Fe and impurity elements The lower limit temperature is set to the higher one of the solidus temperature × 0.94 or 1250 ° C., and the upper limit temperature is heated to a range where the liquidus temperature × 0.98 or less. Ultra-high temperature hot forging is performed so that 85% or more of the surface is in contact with the mold, and a hot forged product having a microstructure of ferrite pearlite and an average austenite grain size number of 2.5 or less is obtained. A method for producing a hot forged non-tempered part having excellent machinability.   In addition to the steel used in the method for producing a hot-forged non-tempered part according to claim 1, the method according to claim 1 is applied to a steel containing Ti: 0.003-0.05% by mass. A method for producing a hot forged non-tempered part having excellent machinability.   In addition to the steel material used in the method for producing a hot forged non-tempered part according to claim 1 or 2, Pb: 0.30% or less, Te: 0.30% or less, Ca: 0. The method according to claim 1 is applied to a steel material containing one or more of 01% or less, Bi: 0.30% or less, Mg: 0.01% or less, and Zr: 0.01% or less. A method for producing hot forged non-tempered parts with excellent machinability.