KR20010039875A - Carburizing and carbonitriding steel - Google Patents

Abstract

PURPOSE: To improve the impact strength of steel as well as its pitting fatigue strength for the requirements to the miniaturization and lightening of machine parts and the load of high stress. CONSTITUTION: The steel for carburizing and carbo-nitriding contains, by mass, 0.10 to 0.30% C, 0.40 to 1.00% Si and 0.30 to 1.50% Mn, and moreover, P, S, Ni, Cr, Cu, Mo, Al, Nb, N and O are controlled. Furthermore, one or more kinds among V, Ti and B may be incorporated into the above steel, and moreover, one or more kinds among Pb, Bi, Te, Zr and Ca of fixed contents may be incorporated as the elements which further improve its machinability and do not remarkably impede its fatigue characteristics.

Description

Carburizing and Carburizing Nitriding Steel {CARBURIZING AND CARBONITRIDING STEEL}

The present invention relates to carburizing and carburizing steel applied to a cog wheel or a rotary shaft which requires a high impact strength with a high fitting fatigue strength.

In the prior art, in order to reduce the fatigue fatigue strength, in order to reduce the grain boundary oxide phase which causes fatigue cracking, the elements such as Si, Mn and Cr, which are easier to oxidize than Fe, are reduced, and Fe Ni and Mo, which are more difficult to oxidize, are used to adjust the hardenability and mechanical properties, and the technique of imparting surface compressive residual stress by shot peening and slowing the progression of fatigue cracking. On top of this, an example is reported in which addition of Si or V is examined for the purpose of improving the close-fitting fatigue strength by improving the bending fatigue strength.

In recent years, as a response to light weight and high engine power of automobiles and industrial machinery, further miniaturization, light weight, and high stress load of gears and rotating shafts are required. As a result, there is a need to improve the fitting fatigue strength and impact strength. However, a problem arises that these compatibility is difficult with the prior art.

The problem to be solved by the present invention is to improve the fitting fatigue strength and the impact strength at the same time only by adjusting the chemical composition of the steel in view of the above problems.

1 is an explanatory diagram of a test piece shape for tempering hardness evaluation,

2 is an explanatory diagram of a test piece shape for carburizing impact strength evaluation,

3 is an explanatory diagram of carburizing quenching-tempering treatment conditions;

4 is a graph showing the relationship between the tempering hardness and the Si content,

5 is a graph showing the relationship between carburizing impact strength and Si content;

6 is a graph showing the relationship between carburizing impact strength and tempering hardness,

7 shows an overview of a roller / fitting fatigue tester,

8 is an explanatory view of the shape of a roller / fitting fatigue test piece;

9 is an explanatory view of the shape of the load roller of the roller / fitting fatigue tester,

Fig. 10 is a diagram showing the relationship between fitting fatigue life and carburizing impact strength.

As a means to solve the above problems, by increasing the Si content, the tempering hardness of the steel after carburizing or carburizing and nitriding is improved, and the fracture toughness values of the carburizing layer and the core are added by adding Ni and Mo alone or in combination.破壞 靭性 値) is to improve.

In particular, according to a first aspect of the invention, in weight percent,

C = 0.10 to 0.30%,

Si = 0.40 to 1.00%,

Mn = 0.30 to 1.50%,

P = 0.035% or less,

S = 0.005-0.050%,

Ni = 0.00-1.00%,

Cr = 0.30 to 1.50%,

Cu = 0.01 to 0.50%,

Mo = 0.00-1.00%,

Al = 0.010 to 0.035%,

Nb = 0.001-0.050%,

N = 0.0050-0.0200%, and

O = 0.0015% or less,

It is configured to include, and also, the parameter represented by Mo + Ni is 0.30% to 2.00%, provides a carburizing and carburizing steel, characterized in that consisting of the remaining Fe and inevitable impurity elements.

According to a second aspect of the invention, in the carburizing and carburizing steel, in weight percent

V = 0.01 to 0.50%,

Ti = 0.005-0.050%, and

B = 0.0005 to 0.0050%,

Carburizing and carburizing steel containing at least one element selected from the group consisting of:

According to the third aspect of the present invention, an element for improving the machinability of steel without significantly impairing fatigue characteristics, which is wt%,

Pb = 0.01 to 0.09%,

Bi = 0.04-0.20%,

Te = 0.002-0.050%,

Zr = 0.01-0.20%, and

Ca = 0.0001 to 0.0100%

Carburizing and carburizing steel specified in the said 1st and 2nd aspect containing the 1 or more element chosen from the group which consists of these.

The inventors have conducted extensive studies on the fitting fatigue strength and the impact strength, and as a result, by adjusting the chemical composition of the steel, the strength of the chemical composition can be assumed as the steel for carburizing and carburizing and nitriding. I found a way to improve it.

The main point of the adjustment of the chemical component is the improvement of the tempering hardness by the increase of the Si content, and the fracture toughness values of the carburized layer and the core by the addition of Ni and Mo alone or in combination.

Until this knowledge, many experiments have been repeated. An example thereof is shown below. Based on the research results thus far, it was known that the most important factors governing the fitting fatigue and impact strength of the carburizing gear were the tempering hardness and the carburizing impact strength, respectively. Table 1 shows the chemical components of the inventive steels and the comparative steels used for evaluating these factors.

TABLE 1

Here, "first invention steel" is an invention steel corresponding to claim 1, "second invention steel" is an invention steel corresponding to claim 2, and "third invention steel" is invention steel corresponding to claim 3. These steels are melted by a high frequency vacuum melting furnace. The molten steel mass was shortened to a diameter of 30 mm after heating to 1250 ° C, and further annealed to 925 ° C. Machining from these materials produced one test piece of the shape shown in FIG. 1 to evaluate the tempering hardness, respectively, and three test pieces of the shape shown in FIG. 2 to evaluate the impact strength. Carburizing quenching-tempering treatment was performed on the conditions shown in FIG.

Then, the test piece for tempering hardness shown in FIG. 1 assumed air generation | occurrence | production of frictional heat during cogwheel rotation, and it was air-cooled by maintaining it in the electric furnace heated at 250 degreeC for 8 hours. Then, the test pieces after air cooling were cut perpendicular to the longitudinal direction, and the hardness from the surface to the position of 50 µm was measured every 10 µm with a micro-biccus hardness meter at two locations every arbitrary 90 °, and the average amount thereof was measured. Obtained. These results are shown in Table 1 above as the hardness after tempering. On the other hand, the carburizing impact test piece shown in FIG. 2 performed the Charpy impact test of each steel type, and calculated | required the average value of Charpy impact value. These results are shown in Table 1 above as carburizing impact strength. Below, these data are explained in full detail.

In FIG. 4, the relationship between tempering hardness and Si content is shown. Thereby, as a range whose Si content is 0.40 weight% or more, tempering hardness is 700 HV or more, and it turns out that tempering hardness is higher than steel of less than Si content. This is because the surface hardness is maintained at a high level even after tempering assuming frictional heat during gear wheel rotation by increasing the Si content that increases the tempering / softening resistance as pointed out in the related art. 5 shows the relationship between carburizing impact strength and Si content. First, when the total value of Ni and Mo content is 0.30 weight% or less, it turned out that carburizing impact strength falls as Si content becomes high. This is presumably because the fracture toughness values of the carburized layer and the core portion decreased due to the increase in the Si content. On the other hand, if the total value of Ni and Mo content is less than 0.30% by weight, the carburizing impact strength decreases slightly as the Si content increases, but the degree is much smaller than that of steel having less than 0.30% by weight. It can be seen that. This is presumably because the fracture toughness values of the carburized layer and the core are kept high by the addition of Ni or Mo even if the Si content is increased. Therefore, in the range of the invention steel whose Si content is 0.40% by weight or more and the Ni + Mo parameter represented by the total value of Ni and Mo content is 0.30% by weight or more, the tempering hardness is high and the carburizing impact strength is high. I knew that.

In FIG. 6, in order to demonstrate these relationships easily, the relationship of tempering hardness and carburizing impact strength is shown. It can be seen that the invention steel is much more advantageous than the comparative steel with respect to the improvement of the tempering hardness and the improvement of the carburizing impact strength.

The present invention has been made from the above research results. Next, the reason for limitation is demonstrated about the chemical component of this invention. In addition, it is conceivable that the chemical composition of the steel for the cogwheel can be adjusted to various ranges in consideration of its use environment, that is, the cogwheel size, load strength and carburizing or carburizing conditions. In the present invention, it was confirmed that the effects of the invention can be obtained in any range of chemical components that can be assumed, and the claims of component claims were made.

C: 0.10 to 0.30 wt%

C needs to add at least 0.10% by weight or more in order to secure the core hardness required for the cog wheels. However, the excessive addition raises the hardness of the core too much, and deteriorates the toughness of the core. In order to avoid this, the upper limit should be limited to 0.30% by weight.

Therefore, the amount of C added was in the range of 0.10 to 0.30% by weight.

Si: 0.40 to 1.00 wt%

Si is the most important element in the steel of the present invention. In particular, Si is an element which makes softening in the temperature range of 200-250 degreeC considered that gear | wheel etc. reach | attain in rotation. In order to exert these effects, at least 0.40% by weight or more of addition is required. However, the excessive addition not only lowers the toughness of the carburized layer and the core portion, but also impairs the carburizing property or deteriorates the cold-breakability (coldness) and machinability because the steel material before carburizing becomes too hard. In order to avoid this, it is necessary to limit an upper limit to 1.00 weight%.

Therefore, the addition amount of Si was made into the range of 0.40 to 1.00 weight%.

Mn: 0.30 to 1.50 wt%

Mn is an element necessary for securing hardenability, and at least 0.30% by weight or more of addition is required. However, the excessive addition causes the steel material before carburization to become too hard, resulting in deterioration of cold resistance and machinability. In order to avoid this, it is necessary to limit an upper limit to 1.50 weight%.

Therefore, the amount of Mn added was in the range of 0.30 to 1.50% by weight.

P: 0.035% by weight or less

P is an element that segregates at the austenite grain boundary and weakens the grain boundary, thereby decreasing toughness and fatigue strength. Inclusion in excess of 0.035% by weight makes this detrimental.

Therefore, content of P was limited to 0.035 weight% or less.

Ni: 0.00-1.00 weight%

Ni is an important element in the present invention steel, such as Mo, which will be described later in Si. In particular, Ni is an element that improves fracture toughness of the carburized layer and the core like Mo.

Therefore, it is an element which should be added unless Mo is added. However, since Ni is an expensive element, its excessive addition is undesirable from an economical point of view, and furthermore, the surface hardness decreases by promoting the formation of residual austenite, and the steel material before carburizing becomes too hard, resulting in cold insulation. And deterioration of machinability. In order to avoid this, the upper limit should be limited to 1.00 weight%.

Therefore, the addition amount of Ni was made into 0.00 to 1.00 weight% of range.

Cr: 0.30 to 1.50% by weight

Cr is an element necessary for securing hardenability, and at least 0.30% by weight or more of addition is required. However, the excessive addition causes the steel material before carburization to become too hard, resulting in deterioration of cold resistance and machinability. In order to avoid this, it is necessary to limit an upper limit to 1.50 weight%.

Therefore, the addition amount of Cr was made into the range of 0.30 to 1.50 weight%.

Mo: 0.00-1.00 weight% or less

Mo is an important element in the present invention steel, such as Ni, which is followed by Si. In particular, Mo is an element that improves fracture toughness values of carburized layers and cores, such as Ni. Therefore, when Ni mentioned above is not added, it is an element which should be added. However, since Mo is an expensive element, its excessive addition is not preferable from an economical point of view, and the steel material before carburization becomes too hard, thereby deteriorating cold-breaking property and machinability. In order to avoid this, it is necessary to limit an upper limit to 1.00 weight%. And when Ni mentioned above is added, it may not need to add it.

Therefore, Mo addition amount was made into 0.00 to 1.00 weight% of range.

Al: 0.010% to 0.035% by weight

Al combines with N to form AlN, thereby miniaturizing austenite grains, thereby improving toughness of the carburized layer and the core. In order to exhibit the effect, at least 0.010% by weight or more of addition is required. However, excessive additions encourage the production of Al ₂ O ₃ inclusions that are harmful to fatigue strength. In order to avoid this, an upper limit must be limited to 0.035 weight%.

Therefore, the addition amount of Al was made into the range of 0.010 to 0.035 weight%.

Nb: 0.001-0.050 weight%

Nb combines with C and N in steel to form carbonitrides, which is effective for reducing the size of austenite grains such as AlN. Even if this particle size is fine, the element improves the toughness of the carburized layer and the core. In order to exert the effect, 0.01% or more of addition is required. However, the excessive addition forms and precipitates a coarse and large carbonitride, and reduces the toughness of a carburized layer. In order to avoid this, it is necessary to limit an upper limit to 0.050 weight%.

Therefore, the amount of Nb added was in the range of 0.001 to 0.050% by weight.

O: 0.0015% by weight or less

In steel, O exists as an oxide type interference | inclusion, and is an element which damages fatigue strength.

Therefore, the upper limit of O was prescribed | regulated to 0.0015 weight% or less.

N: 0.0050-0.0200 weight%

N combines with Al and Nb to form AlN and NbN, refines austenite crystal grains, and improves toughness of the carburized layer and the core. In order to exert the effect, at least 0.0050% by weight or more of addition is required. However, excessive addition causes foaming on the surface of the steel mass during solidification or deteriorates the forging of the steel material. In order to avoid this, the upper limit should be limited to 0.0200% by weight.

Therefore, the addition amount of N was made into 0.0050 to 0.0200 weight% of range.

Parameters shown in Ni + Mo: 0.30% to 2.00%

Ni and Mo are elements which improve the fracture toughness values of the carburized layer and the core portion reduced by an increase in the amount of Si addition, as described in the respective sections. At least 0.30% by weight of the parameter indicated in Ni + Mo is required. However, since Ni and Mo are expensive elements, their excessive addition is not preferable from an economic point of view, and the steel material before carburization becomes too hard, thereby deteriorating cold-breaking property and machinability. For this reason, an upper limit must be limited to 2.00 weight%.

Ch: 0.01 to 0.50 wt%

C is an element in which precipitation hardening can be expected in a relatively high temperature range of 400 to 600 ° C. Therefore, it should be added when it is assumed to be used so severely that the temperature of the tooth surface or the rotating surface is significantly increased, or when used in a high temperature environment near a jet propeller or turbine such as an aircraft material. In order to exert the effect, addition of 0.01% or more of Cu is required. However, excessive addition increases hot brittleness and also inhibits carburization. In order to avoid this, an upper limit must be limited to 0.50 weight%.

Therefore, the addition amount of Cu was made into 0.01 to 0.50 weight% of range.

V: 0.01 to 0.50% by weight

V is an element that forms carbide even at a relatively low temperature near the carburizing temperature, improves the hardness of the carburized layer and improves hardenability. Therefore, in order to exert the effect, 0.01% or more of addition is required. However, the excessive addition deteriorates the toughness of the carburized layer and is not preferable from an economical point of view, since V is an expensive element. In addition, the steel material before carburization becomes too hard, thereby deteriorating cold-breaking properties and machinability. In order to avoid this, it is necessary to limit an upper limit to 0.50 weight%.

Therefore, the amount of V added was in the range of 0.01 to 0.50% by weight.

Ti: 0.005-0.050 weight% or less

Ti is an element added to prevent N in steel from bonding to B described later to form BN and to deteriorate the hardening effect of B. Therefore, in order to achieve the effect, addition of 0.005% or more is required. However, it is necessary to limit the upper limit to 0.050% by weight since a large amount of TiN may form a large TiN and become a starting point for fatigue breakdown.

Therefore, Ti addition amount was made into 0.005 to 0.050 weight% of range.

B: 0.0005 to 0.0050 wt%

B is an element which improves hardenability, without degrading the cold-breakability and machinability of the steel material before carburizing. Therefore, when such an effect is needed, it should be added. In order to exert the effect, addition of 0.0005% by weight or more is required. However, even if it exceeds 0.0050 weight%, the effect is saturated and hot workability deteriorates. Therefore, it is necessary to limit the upper limit to 0.0050% by weight.

Therefore, the addition amount of B was made into 0.0005 to 0.0050 weight% of range.

S: 0.005-0.050 weight%

Most of S exists in steel as an emulsion type inclusion, and it is an element effective in improving machinability as a component formed by cutting, such as a cogwheel. To that end, addition of at least 0.005% by weight or more is required. However, excessive addition becomes a factor which causes fatigue strength fall. In order to avoid this, it is necessary to limit an upper limit to 0.050 weight%.

Therefore, the addition amount of S was made into 0.005 to 0.050 weight% of range.

Pb: 0.01% to 0.09% by weight

Pb is an element which improves machinability more in addition to addition of S alone. In order to exert the effect, at least 0.01 wt% or more of addition is required. However, the excessive addition is an element that causes fatigue strength reduction. In addition, as 0.01 weight% or more, in the handling of Pb, a dust collector and a method are legally regulated. In order to avoid this, it is necessary to limit an upper limit to 0.09 weight%.

Therefore, the addition amount of Pb was made into 0.01 to 0.09 weight% of range.

Bi: 0.04-0.20 wt%

Bi is an element which adds by S addition and improves machinability more. Therefore, at least 0.04% by weight or more must be added to achieve the effect. However, excessive addition reduces toughness. In order to avoid this, it is necessary to limit an upper limit to 0.20 weight%.

Therefore, Bi addition amount was taken as 0.04 to 0.20 weight%.

Te: 0.002-0.050 wt%

Te is an element which adds by S addition and improves machinability more. In order to exert the effect, at least 0.002% by weight or more of addition is required. However, excessive addition produces hot brittleness. In order to avoid this, it is necessary to limit an upper limit to 0.050 weight%.

Therefore, the addition amount of Te was made into 0.002 to 0.050 weight% of range.

Zr: 0.01 to 0.20 wt%

Zr is an element which adds by S addition and improves machinability more. In order to exert the effect, at least 0.01 wt% or more of addition is required. However, excessive addition reduces toughness. In order to avoid this, it is necessary to limit an upper limit to 0.20 weight%.

Therefore, the addition amount of Zr was made into the range of 0.01 to 0.20 weight%.

Ca: 0.0001 to 0.01 100% by weight

Ca is an element which adds by S addition and improves machinability more. To do so, at least 0.0001% by weight or more of addition is required. However, excessive addition reduces toughness. In order to avoid this, it is necessary to limit an upper limit to 0.0100 weight%.

Therefore, the addition amount of Ca was made into 0.0001 to 0.0100 weight% of range.

Embodiment of the Invention

Next, the present invention will be described in more detail with reference to specific examples. Table 2 shows the chemical composition of the invention steel which was melted in the fire furnace based on the above knowledge and the comparative steel for comparison with it.

TABLE 2

Invented steel A in the table is boron-free, invented steel B is boron-containing steel. In addition, comparative steel I is SNCM420H of JIS (Japanise Industrial Standard), and comparative steel II is steel which increased Si content based on SCM420H of JIS.

The roller / fitting fatigue test and carburizing impact test were performed on these steels, and their fitting fatigue life and carburizing impact strength were evaluated. 7 shows an outline of a roller / fitting fatigue tester. Where 1 is the test piece, 2 is the load roller, 3 and 4 are the cog wheels, 5 is the bearing, 6 is the coupling, 7 is the transmission belt and 8 is the motor. 8 shows the shape of the roller / fitting fatigue test piece, and FIG. 9 shows the shape of the load roller of the roller / fitting fatigue tester.

First, the invention steel and the comparative steel were hot forged to a diameter of 30 mm, and then annealed. Then, the roller / fitting fatigue test pieces shown in FIG. 8 and the carburizing impact test pieces shown in FIG. 2 were prepared five by one. Next, these test pieces were subjected to a carburizing quenching-tempering treatment under the conditions shown in FIG. 3.

About the roller / fitting fatigue test piece, the roller fitting fatigue test was done on condition shown in Table 3, and the fitting fatigue life was calculated | required. About the carburizing impact test piece, the Charpy impact test was done and the carburizing impact strength was calculated | required. Table 4 shows these results. The roller / fitting fatigue test was carried out up to a power recovery of 20.00 × 10 ⁶ , and the test was terminated when no fitting occurred. 10, the result of Table 4 is put together.

TABLE 3

Suggestion Contents Hertz stress 2940 MPa Slip rate -40% Revolutions 1000 r.p.m lubricant Engine oil (Komatsu pure oil EO-30-cd) Lubricant Temperature 50 ℃ Stand roller / crowning diameter 300 mmR

TABLE 4

Fitting fatigue life (× 10 ⁶ ) Carburizing Impact Strength (J / cm ² ) n = 1 n = 2 n = 3 n = 4 n = 5 n = 1 n = 2 n = 3 n = 4 n = 5 Invented Steel A 20.00 ^* 20.00 ^* 20.00 ^* 20.00 ^* 20.00 ^* 35.3 36.1 32.8 33.1 37.7 Invented Steel B 20.00 ^* 20.00 ^* 20.00 ^* 20.00 ^* 20.00 ^* 30.1 34.2 32.2 31.6 34.0 Comparative Steel I 0.75 0.66 0.51 0.47 0.38 35.5 32.1 32.0 33.0 34.8 Comparative Steel II 15.70 ^* 20.00 ^* 13.30 20.00 ^* 20.00 ^* 8.9 15.2 12.2 11.1 7.8

From this table, it can be seen that the fitting fatigue life of both the invention steel A and the invention steel B is 20.00 × 10 ⁶ or more, and the carburizing impact strength is 30 J / cm ² or more. On the other hand, in Comparative Steel I, the carburized impact strength is 30 J / cm ² or more, but the fitting fatigue life is short. In addition, although comparative steel II has good fitting fatigue life, it turns out that carburizing impact strength is low.

Therefore, it was confirmed that the invention steel had a high fitting fatigue strength and a high impact strength.

As described above, the present invention can only improve the strength of the impact, such as the improvement of the fitting fatigue strength, by simply fixing the chemical composition of the steel, and solve the problem to be solved by the invention.

Therefore, as an effect of the present invention, even in the manufacturing process of developing, it is possible to reduce the size and weight of the carburized cog wheels, and also to increase the output power even with the same shape and dimensions, and to reduce costs in the industry using cog wheels. It contributes widely to the reduction and the improvement of reliability.

Claims (4)

In weight percent, C = 0.10 to 0.30%, Si = 0.40 to 1.00%, Mn = 0.30 to 1.50%, P = 0.035% or less, S = 0.005-0.050%, Ni = 0.00-1.00%, Cr = 0.30 to 1.50%, Cu = 0.01 to 0.50%, Mo = 0.00-1.00%, Al = 0.010 to 0.035%, Nb = 0.001-0.050%, N = 0.0050-0.0200%, and O = 0.0015% or less, Carburizing and carburizing nitrile steel, characterized in that, including, and the parameter represented by Mo + Ni is 0.30% to 2.00%, consisting of the remaining Fe and inevitable impurity elements. The method of claim 1, Weight percent, V = 0.01 to 0.50%, Ti = 0.005-0.050%, and B = 0.0005 to 0.0050%, Carburizing and carburizing steel further containing at least one element selected from the group consisting of: The method of claim 1, In weight percent as an element to improve the machinability of steel, Pb = 0.01 to 0.09%, Bi = 0.04-0.20%, Te = 0.002-0.050%, Zr = 0.01-0.20%, and Ca = 0.0001 to 0.0100% Carburizing and carburizing steel further containing at least one element selected from the group consisting of: The method of claim 2, In weight percent as an element to improve the machinability of steel, Pb = 0.01 to 0.09%, Bi = 0.04-0.20%, Te = 0.002-0.050%, Zr = 0.01-0.20%, and Ca = 0.0001 to 0.0100% Carburizing and carburizing steel further containing at least one element selected from the group consisting of: