JP5482003B2 - Hot forged non-tempered steel - Google Patents

Description

The present invention relates to a hot forged non-tempered steel that can ensure high yield strength at low cost.

A relatively large part such as an engine part of an automobile such as a crankshaft and a connecting rod is often manufactured by forging in a hot state where deformation resistance can be reduced because a required molding load becomes high. Quenching and tempering, that is, tempering, is the best way to obtain hot forged products with excellent strength. Non-tempered steel for hot forging that can secure high strength and can omit quenching and tempering is often used.

The most frequently used non-heat-treated steel for hot forging is to add V to precipitate carbides in the steel, increase the strength by precipitation strengthening, and ensure the required strength. V It is a ferritic pearlite-type non-heat treated steel, and has been used since the Showa 50s. Many improvements have been made up to the present.

However, in recent years, crude oil prices have changed drastically, gasoline prices have soared frequently, and because of the need to reduce CO ₂ emissions in order to prevent global warming, in order to improve automobile fuel efficiency, engines There is an extremely strong demand for new proposals for strength improvement measures for reducing the weight of parts, while the demand for costs is becoming stricter.

Under such circumstances, development of a new non-tempered steel for hot forging that can greatly improve the strength of the conventional steel without greatly increasing the cost is strongly desired.

Conventionally, the element that is the most important point for increasing the strength of non-heat treated steel for hot forging is considered to be V, and increasing the amount of V is the easiest way to increase the strength. It was thought to be. However, increasing the amount of V leads to an increase in cost, and the price of alloyed iron, which has become indispensable for the production of special steel, has recently become very frequent. In order to provide materials with stable costs, V Currently, it is difficult to adopt strength improvement measures that depend on the increase in quantity.

Under such circumstances, attempts have been recently made to increase the strength by reducing the amount of N contained in steel as an impurity, although it does not significantly affect the cost (Patent Document). 1). That is, V, which is a precipitation strengthening element, is an element that is easily associated with N as well as C. Therefore, when N is present in steel, a part of V is associated with N instead of C, so that the strength is originally improved. It is because the strength improvement effect of added V will fall.

Therefore, in the invention described in Patent Document 1, in order to increase the amount of precipitation of V carbide that can contribute to the strength improvement without greatly changing the V content, In addition to limiting the upper limit of the content rate, Ti, Nb, and Zr, which are elements that are easily bonded to N as well as V, are added to fix N and contribute to improving the strength of all V contained in the steel. The invention of the content which is going to raise the ratio of V to do is proposed.

JP 2008-240130 A

However, the above prior art has the following problems.
In the above document, although the upper limit of N is limited to 80 ppm, not only that, but addition of Ti, Nb, Zr, which is an element that easily binds to N, fixes N and increases the precipitation amount of V carbide. I am trying. However, while limiting the upper limit of N and suppressing the formation of VN, we have proposed an advantageous measure in terms of cost by enhancing the effect of improving the strength of V carbide, but on the other hand, Ti, Zr leading to an increase in cost , Nb must be added. Therefore, there is a problem that it is difficult to manufacture steel with excellent cost.

Ti, Zr, and Nb are elements that are easily bonded to N and at the same time are also bonded to C. Therefore, a part of Ti, Zr, and Nb that should have been added for fixing N is combined with C to form carbides. As a result, contrary to the aim, the cause of reducing the formation of V carbides. Therefore, there is a problem that the desired effect cannot be obtained. Further, when Nb is added, bainite is easily generated, and when bainite is generated in a part of the ferritic pearlite-type non-heat treated steel, there is a problem that the yield strength is lowered.

The present invention has been made in view of such conventional problems, and does not use an N-fixing element such as Ti, Nb, or Zr, and intentionally reduces N as much as possible, thereby reducing the same level as conventional. The object is to make it possible to provide hot forged non-tempered steel that can significantly improve the yield strength compared to non-tempered steel with V content, and can ensure high yield strength at a relatively low cost.

N is contained in the atmosphere at a ratio of about 78%, and when dissolved in the atmosphere, its inclusion is an element well known as an element that cannot be avoided in production. In particular, special steels such as hot forged non-tempered steel that are the present invention are often manufactured in an electric furnace using scrap rather than being manufactured using a blast furnace or converter, and in the case of melting in an electric furnace. N tends to have a higher content as compared with production in a blast furnace and converter. And conventionally, except for the above-mentioned a few prior applications, N is also an element that forms nitrides and carbonitrides that are precipitates like C, and contributes to improving the strength of non-tempered steel. Despite being recognized and an element that is unavoidably contained in production, no attempt has been made to reduce the content particularly, and the N content described in the prior application Most of the range of the rate was specified as a range completely including the 100 ppm level, which is a level normally contained in a normal manufacturing process by electric furnace melting.

In order to examine whether the strength, particularly the proof stress, can be improved without greatly increasing the amount of the carbide-forming element such as V, the present inventors are in what state the carbide-forming element such as V is. We investigated in detail whether the yield strength could be improved most efficiently when it was present in steel. As a result, as in the previous application, carbide forming elements such as V are combined with N to suppress the existence of nitrides as much as possible, and are preferentially combined with only C instead of N, so that a large amount of carbide is formed. It came to the conclusion that it was necessary to deposit.

However, in the above-mentioned prior application, in order to preferentially bond V with C, in addition to limiting the upper limit of N to 80 ppm, in order to fix N as nitride, it is an element having a large affinity with N It is essential to add one or more of Ti, Nb, and Zr, and the addition of these elements may increase the cost. Therefore, as a result of further investigation, when the upper limit of N is further reduced to 60 ppm, N itself, which can be a nitride, has decreased more than usual, so that N-fixing elements such as Ti, Nb, and Zr are reversed. It is better not to add it, that is, when Ti, Nb, Zr is added in a state where the N content is reduced to 60 ppm or less, since N is already sufficiently small, some of these elements are not N Since it combines with C to form carbide and consumes C as a result, the effect of improving the strength due to precipitation of V carbide is reduced, and when Nb is added, the hardenability is increased and bainite is added. The present invention has been completed by newly finding that a structure is formed and the yield strength is greatly reduced.

The non-heat treated steel for hot forging of the present invention obtained as a result of the above examination is in mass%, C: 0.30 to 0.50%, Si: 0.05 to 1.00%, Mn: 0 50 to 1.50%, S: 0.040 to 0.100%, Cr: 0.50% or less, Mo: 0.05% or less, V: 0.20 to 0.40%, Al: 0. 050% or less, N: 0.0060% or less, Nb: less than 0.02% (except when V (%) + 30N (%) ≦ 0.375%), with the balance being Fe and It is an inevitable impurity, Ceq = C + Si / 7 + Mn / 5 + Cr / 9 + V is 0.9 or more, hardness after hot forging and air cooling is Hv340 or more, and the structure is ferrite pearlite having a bainite area ratio of 10% or less. It is characterized by that.

The hot forged non-tempered steel according to the present invention is characterized in that the upper limit of N, which is an element that cannot be avoided in production, is strictly controlled and is 0.0060% or less. Thereby, the added V is preferentially present as a carbide rather than as a nitride, and by increasing the abundance ratio as a carbide to the total amount of added V, the yield strength is increased without increasing the V addition amount. Is greatly improved. In addition, by reducing the upper limit of N to 0.0060%, the addition of Ti, Nb and Zr, which was added for N fixation in the previous application, becomes unnecessary, and it has a high yield strength with a relatively low cost material. It becomes possible to provide non-heat treated steel for hot forging.

The graph which shows the relationship between N content rate and 0.2% yield strength in the test material whose Ceq is about 0.9. The graph which shows the influence on 0.2% yield strength at the time of containing Ti and Zr. The graph which shows the influence on the bainite area rate at the time of containing Nb. The graph which shows the influence on 0.2% yield strength when bainite arises.

As described above, the hot forged non-tempered steel according to the present invention is, in mass%, C: 0.30 to 0.50%, Si: 0.05 to 1.00%, Mn: 0.50. 1.50%, S: 0.040-0.100%, Cr: 0.50% or less, Mo: 0.05% or less, V: 0.20-0.40%, Al: 0.050% or less , N: 0.0060% or less, Nb: less than 0.02% (except when V (%) + 30N (%) ≦ 0.375%) , the balance being Fe and inevitable impurities Ceq = C + Si / 7 + Mn / 5 + Cr / 9 + V is 0.9 or more, the hardness after hot forging and air cooling is Hv340 or more, and the structure is a ferrite pearlite having a bainite area ratio of 10% or less. And
Hereinafter, the reason why the upper and lower limits of each requirement of the present invention such as components are limited will be described.

C: 0.30 to 0.50%
C is an element that combines with V during cooling after hot forging to form carbides and precipitates in ferrite, and improves strength (particularly yield strength) by precipitation strengthening. To achieve the high yield strength of the present invention It is an indispensable element. In order to acquire this effect, it is necessary to contain 0.30% or more. However, as the amount of C increases, the machinability decreases, and machining to a predetermined shape after forging becomes difficult, so the upper limit was made 0.50%.

Si: 0.05-1.00%
Si is an element necessary for deoxidation at the time of refining after melting, and it is necessary to contain at least 0.05% or more. However, when the amount of Si increases, the surface is easily decarburized in the heating and subsequent cooling process during hot forging, which causes a decrease in fatigue strength. Therefore, the upper limit was made 1.00%. Desirably, it is good to set it as 0.30% or less.

Mn: 0.50 to 1.50%
Mn is an element added to adjust deoxidation during steelmaking and balance of strength and toughness of the steel, and it is necessary to contain 0.50% or more. However, when the amount of Mn is increased, a bainite structure is likely to be formed, and when the bainite structure is increased, the yield strength, which is the most important characteristic of the present invention, is reduced. Therefore, the upper limit was made 1.50%.

S: 0.04 to 0.10%
Since it is impossible to process to the final part shape only by hot forging, machining is always required after forging and cooling. S is known as an element effective in improving machinability, and is an element indispensable for securing the machinability necessary when machining the forged product obtained in the present invention into a final shape. Therefore, the content of 0.04% or more is necessary. However, if contained in a large amount, the machinability is improved, but cracking is likely to occur during forging, so the upper limit was made 0.10%.

Cr: 0.50% or less Cr is an effective element for adjusting the balance of strength and toughness of steel in the same manner as Mn. However, when the addition amount is increased, a bainite structure is likely to be formed as in the case of Mn, which causes a decrease in yield strength. Therefore, the upper limit was made 0.50%.

Mo: 0.05% or less Mo is an undesirable element for obtaining a high yield strength because it produces a bainite structure with a very small content as compared with Mn and Cr. Although Mo is contained as an impurity in the present invention, since the influence of the inclusion is large even in a small amount, the upper limit is clearly shown as 0.05%.

V: 0.20-0.40%
V is an element that becomes carbide during cooling after hot forging and precipitates in ferrite and improves strength (particularly yield strength) by precipitation strengthening, and is the most important element for the present invention. However, V is an element that greatly affects the cost, and as the added amount is increased, the effect of improving the strength decreases, and the effect of improving the mechanical properties commensurate with the increase in cost cannot be obtained. %.

Al: 0.050% or less Al is an element that is effective when performing deoxidation treatment, and is an element that is necessary when deoxidation is performed using Al. However, when the addition amount is increased, hard alumina inclusions increase, and the machinability tends to decrease. Therefore, since a large amount is not desirable, the upper limit was made 0.050%.

N: 0.0060% or less N is an element that is contained most in the atmosphere, and is an element that cannot be avoided as an impurity in production when dissolved in the atmosphere. And when it manufactures without especially considering the quantity, it is normal to contain about 0.008 to 0.012% in steel in electric furnace melting. When this amount of N is contained, V is consumed because a part of V is combined with N to form a nitride, so that the strength improvement effect due to the formation of V carbide is reduced.

Therefore, in the present invention, in order to make maximum use of the strength improvement effect due to precipitation of V carbide and to achieve high strength without greatly increasing the amount of V added, the upper limit of N is usually 0.0060% or less. It is a further reduction from the level. In order to stably manufacture a steel having an N content of 0.0060% or less, it is essential to manufacture the steel while checking the content at the time of refining immediately after melting. Note that an N content of 0.0060% or less can be achieved by processing with a vacuum degassing apparatus (RH) or the like.

Moreover, since the upper limit of N is suppressed to 0.0060% in the present invention, the strength of V carbides can be obtained without adding N elements having affinity with N such as Ti as in the prior application. The improvement effect can be sufficiently obtained. In addition, since N-fixing elements such as Ti are elements that easily bind to C, there is no consumption of C due to the addition of N-fixing elements, and as a result, the amount of precipitated V carbide increases without increasing the amount of V addition. The strength that can be achieved with the same V content can be increased.

Nb: less than 0.02% Nb is known as an element that fixes N, and is known as an important element in the above-mentioned prior application. However, Nb is an element that causes hardenability to increase when dissolved and causes a bainite structure even with a small content. In that case, as it is repeatedly described, it causes a decrease in yield strength. Therefore, it is an undesirable element in the present invention, and since it is necessary to clarify the content of the allowable upper limit, the upper limit is less than 0.02%. did.

For Ti and Zr known as N-fixing elements other than Nb, since there is little influence on the bainite structure formation by containing a small amount compared to Nb, there is no particular upper limit, but positive addition is not preferable as described above, so Ti Desirably, it is less than 0.010% and Zr is less than 0.020%.

Further, Ceq (carbon equivalent) = C + Si / 7 + Mn / 5 + Cr / 9 + V is 0.9 or more. In the above formula, C, Si, Mn, Cr, and V are the contents (mass%) in steel of each component.

The purpose of the present invention is to achieve a high strength of 850 MPa with a 0.2% proof stress. To achieve this, the components are adjusted so that the carbon equivalent defined by the above formula is 0.9 or more. Therefore, it is necessary to enhance precipitation strengthening of the V carbide already described.

In addition, although the upper limit is not shown, since the machinability tends to decrease if the carbon equivalent becomes too high, it is desirable that the upper limit is about 1.2 or less.

Next, in the hot forged non-heat treated steel of the present invention, it is specified as Hv340 or more. If importance is attached to the machinability, it is desirable that the hardness be Hv300 or less, as is the case with many conventional non-heat treated steels. However, the present invention aims to achieve a 0.2% proof stress of 850 MPa or more, and for that purpose, the hardness is required to be Hv340 or more. By implementing the measures as described above, such as setting the hardness to Hv 340 or more and further making the structure a ferrite pearlite structure with a bainite area ratio of 10% or less, the target excellent proof stress can be achieved. it can.

Moreover, the structure after hot forging of the hot forged non-heat treated steel in the present invention is a ferrite pearlite structure having a bainite area ratio of 10% or less.

As already explained many times, the present invention is basically based on a ferrite pearlite structure, and it is desirable that the bainite structure causing a decrease in yield strength is as small as possible. Furthermore, when the ratio of a bainite structure becomes high, even if it compares by the same hardness, there exists a problem that machinability falls. However, if the presence of the bainite structure is small, the adverse effect is relatively small. Therefore, in order to clarify the maximum allowable value, the upper limit is specified as 10%.

In addition, even if each component content rate and Ceq satisfy all the above-mentioned conditions, when all the components are a combination of conditions where bainite is likely to occur (for example, Mn, Cr, Mo, Nb are all close to the upper limit value) Depending on the cooling conditions after forging, the bainite area ratio may be 10% or more. In this case, it is necessary to adjust the components so that the bainite area ratio is within the range of 10%, and only the hot forged non-heat treated steel after the bainite area ratio is adjusted to be 10% or less is within the scope of the present invention. And the desired strength can be ensured.

Next, the effects of the present invention will be described with reference to examples.
Table 1 shows chemical components of the test materials used as examples. Among these, steels 1 to 9 are examples that satisfy the conditions of the present invention, and steels 10 to 32 are test materials corresponding to comparative examples in which some of the conditions do not satisfy the conditions defined in the present invention.

In addition, although P, Cu, Ni, Ti, and Zr among the components described in Table 1 are not described in the claims, the contents contained as impurities in the test material are shown. Is. In addition, Ti and Zr are allowed to be contained only as impurities in the present invention. However, in order to clarify the difference from the case of positive addition, some of the test materials are positively added. The content rate is shown. Here,-in the table indicates that the content is less than 0.01% for Nb and less than 0.009% for Ti and Zr. In Table 1, steel No. 1 with Ti of 0.009% is used. 6, steel No. with Zr of 0.018%. 7 means that since a scrap containing Ti and Zr as impurities was used, a specimen containing a small amount of Ti and Zr was obtained.

And after melting the electric furnace which consists of a component of Table 1, and hot-rolling the steel bar, and manufacturing a 35-mm round bar, it is 1200 ° C which is the standard temperature in actual hot forging. After heating and holding for 30 minutes, the fan was cooled with air to cool to room temperature, and the hardness, bainite area ratio, and 0.2% yield strength were measured as shown below.

The hardness and the bainite area ratio were measured on the D / 4 portion of the T section after cutting the test material. Hardness was evaluated by measuring Vickers hardness (based on JISZ2244 (2003)). Further, the bainite area ratio was measured by corroding with nital and observing with an optical microscope.

The strength was measured by measuring 0.2% proof stress by a tensile test. The tensile test was carried out with a test piece cut out from D / 4 part in the same manner as the place where the hardness and bainite area ratio were measured.
Table 2 summarizes the measurement results of hardness, bainite area ratio, and 0.2% proof stress.

The evaluation of the experimental results will be described based on the results shown in Table 2 with reference to FIGS.

First, in order to show the test results that are the most characteristic of the present invention, FIG.

FIG. 1 shows that the component was adjusted to about 0.9 so that the change in Ceq does not affect the yield strength, and the N content was changed from 59 ppm to 200 ppm when the V content was about 0.2%. Tensile test for specimens (steel Nos. 2, 10, and 11) and specimens (steel Nos. 1, 12, and 13) in which the V content was about 0.3% and the N content was changed from 47 ppm to 227 ppm The results of 0.2% yield strength obtained by the above are shown.

From FIG. 1, the 0.2% proof stress greatly changes with the change in the N content, and even if the V content is not so high as 0.2%, the N content should be 60 ppm or less. It can be seen that a high strength of 850 MPa or more can be obtained. Further, when the V content is 0.3%, the yield strength can be increased to a higher level of about 900 MPa. The effect of obtaining this high proof stress is difficult when no measures are taken to reduce the N content as in the prior art, and as is apparent from FIG. Thus, it can be obtained only when hot forging is performed using a base material that is intentionally manufactured so that the N content is 60 ppm or less.

Next, FIG. 2 is a plot of the results of Table 2 in order to grasp the influence on the 0.2% proof stress due to the contents of Ti and Zr added for N fixation in the previous application. The test material used for the result of FIG. 2 is a test material (steel No. 1, 2, 6 to Ceq) of 0.90 to 0.94 among the examples corresponding to the present invention shown in Table 2. 8), a comparative example (steel Nos. 19 and 25) containing about 150 ppm without reducing N and positively adding Ti or Zr as an N-fixing element, and N to 60 ppm or less as in the present invention Although it is reduced, it is a test material (steel Nos. 14 to 25) in which an N-fixing element is positively added unlike the present invention. As in the case of FIG. 1, in consideration of the influence on the proof stress due to the change in Ceq, a test material adjusted so that Ceq was about 0.9 except for a part was used. However, some of the comparative examples include a test material having a Ceq of about 1.0.

As is clear from the results of FIG. 2, in the present invention, the test materials (steel Nos. 19 and 25) that showed a yield strength of 850 MPa or more for all the test materials and did not reduce N among the comparative examples. Similarly, the yield strength was 850 MPa or more. Steel No. In Comparative Examples Nos. 19 and 25, the V No. content is 0.20 to 0.21% and the Ceq is 0.90%. 2 is almost the same as the test material of the present invention described above, it can be determined that the difference between the V content and Ceq is slightly lower than the yield strength of the test material of the present invention. It can be said that the proof stress equivalent to that of the invention is exhibited. Therefore, when the N content is high, it can be said that the addition of Ti and Zr, which are N-fixing elements, shows a certain effect.

However, in the comparative examples (steel Nos. 14 to 25) in which Ti and Zr, which are N-fixing elements, were positively added while reducing N, the V content was partially 0.35% or more, and Ceq was In spite of the fact that 1.0% high test material is included and it is an advantageous condition for obtaining high hardness and proof stress, the proof stress is less than 850 MPa for all the test materials and the hardness However, there was a decrease in Hv340 or less. This is because when N is reduced to 60 ppm or less as in the present invention, the addition of the N-fixing element does not contribute to the increase in V carbide, but conversely consumes a part of C that may become V carbide. Thus, it is estimated that the proof stress has decreased.

Next, FIG. 3 shows a sample material in which the components except Nb and N satisfy the conditions of the present invention among the comparative examples (steel Nos. 26 to 29 and components other than Nb satisfy the conditions of the present invention. For the test material (steel No. 30), by comparing with the results of the test material (steel No. 1 to 9) of the present invention which does not contain Nb or whose content is less than 0.01%, Fig. 3 is a graph plotting the influence of the Nb content on the bainite area ratio, as clearly shown in Fig. 3. Regardless of whether or not N is reduced, bainite is produced when Nb is contained. As is apparent from the results of FIG. 4, it can be seen that the proof stress decreases.

Here, the bainite area ratio does not increase as the Nb content increases, but reaches a maximum when a small amount of Nb is added, and when the Nb content is increased, the bainite area ratio gradually decreases. The reason for this is that only a small amount of Nb is solid-dissolved, and as a result of increasing the addition amount, Nb that does not dissolve solidly precipitates as carbides and nitrides, and as a result of obtaining a grain refinement effect, there is an effect that hardenability is reduced. It is estimated that it was reflected in the results. Moreover, it is lower than the test material of the present invention whose hardness is almost the same as Hv340 or less in some steels, as in the case of addition of Ti and Zr, the effect of V carbide reduction. It is estimated that

Next, FIG. 4 shows a sample material (steel Nos. 26 to 32) in which bainite was generated in a part of the structure by containing Nb or Mo in the comparative example, and the present invention in which bainite was not generated. The 0.2% proof stress is compared for the test materials. Here, Steel No. Considering that Ceq of 26 to 32 is 0.90 to 0.98, in order to eliminate the influence of Ceq, for the test material of the present invention, the test material with Ceq of 1.0 or more was excluded. And plotted. As is clear from this result, if the bainite area ratio is 10% or less, the influence on the proof stress is small, and a high 0.2% proof stress of 850 MPa or more can be secured, whereas the inclusion of Nb and Mo Thus, it can be seen that the yield strength of the test material in which bainite was generated at an area ratio of 10% or more is greatly reduced.

As is clear from the results described above, in the present invention, by reducing the N content to 60 ppm or less, the consumption of V due to the formation of nitride is suppressed, and the effect of improving the strength by precipitation strengthening of the V carbide can be obtained more greatly. By doing so, it is possible to manufacture hot forged non-tempered steel that can obtain higher strength even with the same V addition amount. Furthermore, by setting the N content to 60 ppm or less, it is not necessary to add an N-fixing element, and it becomes possible to produce a non-heat treated steel that is very excellent in terms of cost, and the effect is very great.

Claims (1)

In mass%, C: 0.30 to 0.50%, Si: 0.05 to 1.00%, Mn: 0.50 to 1.50%, S: 0.040 to 0.100%, Cr: 0.50% or less, Mo: 0.05% or less, V: 0.20 to 0.40%, Al: 0.050% or less, N: 0.0060% or less, Nb: less than 0.02% (However, the case of V (%) + 30N (%) ≦ 0.375% is excluded), and the balance is made of Fe and inevitable impurities, and Ceq = C + Si / 7 + Mn / 5 + Cr / 9 + V is 0.9 or more. A hot forged non-tempered steel, characterized in that the hardness after hot forging and air cooling is Hv 340 or more and the structure is ferrite pearlite having a bainite area ratio of 10% or less.

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