JPH0759739B2 - Non-heat treated steel bar for high toughness hot forging - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention relates to a so-called non-hot forging type, which is a part of a steel bar machined into a machine part such as an automobile or an industrial machine, which is a hot forged part. The present invention relates to a material steel bar for a heat-treated part, and particularly to a material steel bar having high toughness as hot forged.

[Conventional technology]

Most of steel parts for automobiles and industrial machines are formed by hot forging. By the way, hot forging is carried out by heating to a high temperature of 1200 ° C. or higher, so that the structure becomes coarse and the strength and toughness are remarkably deteriorated. Therefore, after hot forging, it is usual to carry out quenching and tempering treatment (tempering treatment) to make the structure coarsened by high temperature heating during hot forging into a fine structure.

However, such heat treatment is naturally costly and increases the price of parts, so in recent years, even if the heat treatment is omitted, hot forging has a certain strength and toughness. Steel, non-heat treated steel for hot forging under the jurisdiction is now required.

If the only mechanical property required for a heat-forged non-heat treated part is strength, the target strength can be easily obtained only by utilizing the precipitation strengthening of V. This method is disclosed, for example, in JP-A-59-91.
No. 22 gazette. However, this method does not improve the toughness because the structure is still coarse. In order to improve both strength and toughness at the same time, it is indispensable to refine the structure by some method.

In order to refine the structure, one method is to refine the austenite structure during forging heating, and the inventors of the present invention refine the structure by Ti (Japanese Patent Laid-Open No. 63-162813),
Also, it has been proposed that the structure be made finer by adopting a specific component system (Japanese Patent Laid-Open No. 63-57742). According to these inventions, high strength and high toughness can be achieved in the as-hot-forged state, and it is becoming possible to apply the same to the foot parts for automobiles which require high toughness.

[Problems to be Solved by the Invention]

Due to the inventions of the inventors of the present application described above, the toughness of the non-heat treated steel for hot forging has been increased, but the requirements for weight reduction of automobile parts and the like are becoming more and more severe, and the strength and toughness are superior. Non-heat treated steel for hot forging is required.

In order to meet this demand, the present invention is low alloy, low cost,
The present invention is intended to provide a non-heat treated bar steel for hot forging which is more excellent in strength and toughness.

[Means for Solving the Problems]

As mentioned above, it is important to refine the austenite structure during forging heating in order to obtain a steel part excellent in strength and toughness as it is in hot forging, but as a result of detailed experiments, The present inventors have obtained the following two findings, and have been able to invent a non-heat treated bar steel for hot forging which is superior in strength and toughness.

(A) In the hot forging temperature range, the effect of refining the austenite structure by the MnS particles is large, and particularly when the size of the MnS particles is within a specific range, a larger effect can be obtained.

(B) It is known that the addition of Ti prevents coarsening of the austenite structure, but the austenite coarsening preventing effect of MnS and Ti is synergistically due to the combination of Ti and MnS. By improving, and particularly controlling the size of Ti precipitates within a limited range, a stable microstructure can be obtained even in the hot forging temperature range.

That is, the gist of the present invention is: (1) C: 0.18-0.60 Si: 0.10-2.00 Mn: 0.50-2.00 S: 0.01-0.10 Cr: 0.05-1.00 V: 0.03-0.20 N: 0.006 -0.0200 Al: includes 0.02-0.06, balance in steel bar remains in the hot-rolled with a component of Fe, area 0.1 [mu] m ² or more to be observed on the rolling direction and perpendicular bars cross, less than 12.6Myuemu ² The high average toughness non-heat treated steel bar for hot forging is characterized in that the total average area of MnS particles in the range is 4.9 μm ² or less.

(2) In addition, the component further contains Ti: 0.005-0.050 by weight%, and the total average of the maximum diameters of the respective Ti carbonitrides formed by the inclusion of Ti is 0.15 μm or less. It is a high toughness non-heat treated steel bar for hot forging.

The present inventors completed the present invention by conducting the following experiments.

The steels having the components shown in Table 1 were forged into various ingots of 10 kg to 2 tons, and further hot-rolled into steel bars with a diameter of 25 mm to 55 mm to obtain experimental material steel bars with various MnS particle sizes. .

The MnS size of the raw steel bar was measured on a cross section perpendicular to the longitudinal direction of the steel bar. The MnS size was measured by the backscattered electron method, which utilizes the fact that the intensity of the backscattered electrons when the material is irradiated with an electron beam depends on the density of the material.

In order to reproduce the size of the austenite structure during hot forging, these steel bars were heated to 1250 ° C in the hot forging temperature range for 30 minutes and water-quenched, and the size of the austenite structure was observed in a cross section perpendicular to the longitudinal direction of the steel bar. Was measured.

In addition, in order to investigate the impact fracture surface transition temperature in the as-hot-forged state, all steel bars were machined to a diameter of 25 mm, heated to 1250 ° C for 30 minutes, and then air-cooled to make JIS No. 3 U-notch Charpy test pieces. An impact test was conducted.

FIG. 1 shows the average of MnS circle equivalent diameters representing the size of MnS on the horizontal axis (diameters of circles having the same area as individual MnS areas observed on a cross section perpendicular to the rolling direction of the steel bar). 1250 ° C on the vertical axis
2 is a drawing showing the austenite crystal grain radius at the time of reheating. In the figure, the MnS circle equivalent diameter is the measured total MnS.
Is calculated for.

Also in FIG. 2, the horizontal axis shows the MnS circle equivalent diameter and the vertical axis shows 1250.
The average of the austenite crystal grain radii during reheating at ℃, the MnS circle equivalent diameter is 0.36 μm or more, MnS having a circle equivalent diameter of less than 4.0 μm, the area is 0.1 μm ² or more,
It is calculated for MnS of less than 12.6 μm ² .

It can be seen that in FIG. 1, the correlation between the diameter of the MnS circle equivalent and the austenite crystal grain size is very small, whereas in FIG. 2, there is a good correlation. That is, MnS having a circle equivalent diameter of 0.36 μm or more and less than 4.0 μm when measured in a cross section perpendicular to the rolling direction of the steel bar controls the size of the austenite crystal grains at the time of reheating. Is the most important point that constitutes.

In Fig. 2, the size of MnS is 0.36μm in equivalent circle diameter.
As for the reason why the above is less than 4.0 μm,
It is as follows.

That is, in all the steel bars, the coexistence of two distributions was observed in the frequency distribution of appearance frequencies of MnS circle equivalent diameters.
One is a distribution having a circle equivalent diameter of 0.36 μm or more and less than about 4.0 μm, and the other is a distribution having a circle equivalent diameter of less than 0.36 μm.

MnS in these two distributions, when heated at high temperature,
Since the effects on the size of the austenite structure were considered to be different, the present inventors set the MnS size to the first level.
The average of all MnS as shown in the figure and the average equivalent circle diameter of 0.36 μm or more and less than 4.0 μm as shown in FIG. 2 are shown. As a result, as shown in Fig. 2, the equivalent circle diameter was 0.36.
It has been found that the average grain size of μm or more and less than 4.0 can accurately represent the austenite grain size during reheating.

In addition, FIG. 3 shows the relationship between the austenite crystal grain size and the impact fracture surface transition temperature (temperature at which a 50% brittle fracture surface occurs) upon reheating at 1250 ° C. Generally, -40 ° C for the legs of automobiles
JIS No. 3 U-notch Charpy impact value of 2kg f ・ m / cm ² or more is required, but experience shows that 2kg f ・ m at -40 ℃
To obtain an impact value of / cm ² or more, the impact fracture surface transition temperature is
It must be below 30 ° C. Therefore, from FIG.
The radius of the austenite crystal grains is desired to be 35 μm or less.

From FIGS. 2 and 3, in order to obtain a fine-grained state in which the austenite grain radius is 35 μm or less at the hot forging temperature of 1250 ° C., the average equivalent circle diameter of MnS having a circle equivalent diameter of 0.36 μm or more and less than 4.0 μm is 2.
It must be 5 μm or less (area 4.9 μm ² or less).

Furthermore, the inventors of the present invention conducted the following tests on Ti carbonitride, Mn
We have found a synergistic effect on the prevention of coarsening of the austenite structure by the combined use of S.

As the test material, steel of the composition shown in Table 2 was cast into 300 kg ingots, 50 kg and 20 kg ingots, and the diameter was 30 mm during hot working.
It is a rolled steel bar. These steels from 1100 ° C
After heating at 1300 ° C for 30 minutes, water quenching was performed, and the prior austenite grain size in a cross section perpendicular to the rolling direction was examined.

The results are shown in FIG. In the case of No. 1 in which the amount of S was small and Ti was added, the austenite crystal grains were extremely fine at 1200 ° C. or less, but were rapidly coarsened at a temperature exceeding 1200 ° C. Further, in No. 2 in which sufficient S is added but Ti is not present, the crystal grain size is stable, but it is coarse compared to the addition of Ti. On the other hand, in Nos. 3, 4 and 5 in which Ti and S are added, it is found that the crystal grain size is extremely fine even at high temperatures. That is, it was found that the effects of Ti carbonitride and MnS for preventing austenite crystal grain coarsening are synergistic.

As a result of taking an extraction replica from the cross section of the raw steel bar and observing it with an electron microscope, the average maximum diameter of Ti carbonitride No. 3 is
0.23 μm, No. 4 was 0.15 μm, and No. 5 was 0.06 μm.

From this result, the synergistic γ structure coarsening prevention effect is
Is especially remarkable when the average size of is less than about 0.15 μm,
It can be seen that in the temperature range of 1250 or higher, the grain size becomes finer grains of size 5 or higher. Here, the average of the maximum diameters of Ti carbonitrides is the diagonal length of the surface in the case of cubic Ti carbonitrides, and the maximum diameter in the case of spherical Ti carbonitrides.

[Work]

 The reasons for limiting the constituent features of the present invention will be described below.

Regarding C, C is an essential element for strengthening steel. 0.18% or more is required to secure the strength required for mechanical structures. However, addition of 0.60% or more deteriorates toughness, so the upper limit was made 0.60%.

Regarding Si, Si is added as a deoxidizing agent, but in addition to this, it functions as solid solution Si to strengthen steel. If it is less than 0.10%, it is insufficient as a deoxidizing agent, and if it is more than 2.00%, the steel becomes unnecessarily hard and the toughness is significantly deteriorated.

Regarding Mn, Mn acts not only as a deoxidizing agent, but also as a hardening element, which has the function of refining the structure and acts as a strengthening element. If it is less than 0.50%, the strength is insufficient, but if it exceeds 2.00%, the toughness is deteriorated.

Regarding S, which is the most important element of the present invention, formation of MnS prevents coarsening of the austenite structure in the hot forging temperature range. This effect is expected at 0.01% or more,
When it exceeds 0.10%, the anisotropy of impact value becomes large. Also,
S improves machinability.

Regarding Cr, Cr has the effect of increasing strength and toughness through refinement of the structure. This effect cannot be expected below 0.10%. If added in a large amount, it decreases toughness, so the upper limit is 1.20.
%.

Regarding V: V significantly strengthens steel by precipitating as carbides during cooling. 0.03% is necessary for exerting a strengthening effect, but if added over 0.20%, it becomes too hard and deteriorates toughness.

Regarding Al, Al acts as a deoxidizer and also refines the structure as carbonitride. If less than 0.005%, the deoxidizing effect cannot be expected, but
Even if added over 0.05%, the micronization effect of the structure is saturated.

Regarding N, N is a Ti nitride that does not dissolve Ti at high temperature, and is added to refine the structure. In order to deposit a sufficient amount of Ti nitride, N needs to be 0.006% or more. However, even if added in a large amount, the amount of solute N increases and only causes deterioration of toughness, so N is made 0.020% or less.

Regarding Ti, it is Ti that is indispensable in the second aspect of the claim, and it is finely dispersed as carbonitride in the steel to refine the austenite structure during reheating. 0.005% or more is required to make the structure finer, but the effect is saturated when it exceeds 0.050%.

The size of MnS particles is a major factor that determines the size of the austenite structure during forging heating, and is an essential requirement for the present invention. MnS is refined by so-called pinning the austenite grain boundaries. AGS during heating, the size when measured on the rolling direction and the cross section perpendicular bars are 0.1 [mu] m ² or more, as determined by the average size of MnS of less than 12.6μm ^2. To obtain a microstructure, the 0.1 [mu] m ² or more, the average of MnS of less than 12.6Myuemu ² needs be 4.9 [mu] m ² or less.

The finer the Ti (maximum diameter of Ti precipitate), the finer the austenite structure, but the average is 0.15 μm.
In the following cases, the austenite structure refining effect is synergistic with the effect of MnS, and a stable microstructure can be obtained even at high temperatures.

Note that addition of Pb, Bi, and Ca for the purpose of improving the machinability of the steel of the present invention does not affect the effect of the invention. For these elements, the machinability is improved by adding Pb: 0.02%, Bi: 0.02%, Ca: 0.001% or more. These elements may be added according to the purpose, the amount may be added, but in any case if a large amount is added, anisotropy of mechanical properties increases, and toughness decreases, so the upper limit is Pb: 0.35% or less, Bi: 0.35% or less, Ca: 0.02% or less is desirable.

〔Example〕

Steels having compositions shown in No. 1 to No. 37 in Table 3 were cast into 300 kg ingots and rolled into steel bars having a diameter of 60 mm. No.38 to No.42
Was cast into a 150 kg ingot and then rolled into a steel bar with a diameter of 20 mm to 100 mm. In addition, No. 43 to 46 are 25 kg, 500 kg, 1 respectively
Cast into t, 2t, and 4t steel ingots to make steel bars with a diameter of 60 mm.

These as-rolled steel bars, the MnS size on the cross section perpendicular to the rolling direction is measured by the backscattered electron method, and the Ti precipitate size is observed and measured with an electron microscope by taking an extraction replica from the same position. did. The measurement results are also shown in Table 3.

These steel bars were heated at 1250 ° C for 30 minutes, forged into a propeller shaft for automobiles, and air-cooled. Tensile test pieces and impact test pieces were sampled from this propeller shaft and used for the test. The results are shown in Table 4.

It can be seen that the steel of the present invention has high strength and high toughness as compared with the conventional steel.

[Effect of the invention] As described above, the component system and MnS particle size of the present invention,
If a steel having a Ti carbonitride size is produced, it will be a steel with high strength and high toughness as hot forged, and is suitable as a material for hot forged parts such as automobiles and industrial machines.

[Brief description of drawings]

Figure 1 shows the total measured in the cross section perpendicular to the rolling direction of the raw steel bar.
2 is a drawing showing the relationship between the average circle equivalent diameter of MnS (the diameter of a circle assuming a circle of the same area as MnS) and the austenite grain radius upon reheating at 1250 ° C. Fig. 2 shows Mn measured in a cross section perpendicular to the rolling direction of the raw steel bar.
It is a drawing showing the relationship between the S-circle equivalent diameter and the austenite grain radius when reheating at 1250 ° C. MnS is 0.36 μm or more
MnS with a circle equivalent diameter of less than 4.0 μm, in terms of area is 0.
1 [mu] m ² or more, is obtained by calculating the average target of MnS of less than 12.6μm ^2. FIG. 3 is a drawing showing the relationship between the austenite crystal grain size at the time of heating at 1250 ° C. and the impact fracture surface transition temperature of a sample obtained by heating a 25 mm diameter machined steel bar to 1250 ° C. for 30 minutes and air cooling. FIG. 4 is a drawing showing the austenite grain size when the raw steel bars shown in Table 2 are reheated.

Claims (2)

[Claims] 1. In weight%, C: 0.18-0.60 Si: 0.10-2.00 Mn: 0.50-2.00 S: 0.01-0.10 Cr: 0.05-1.00 V: 0.03-0.20 N: 0.006-0.0200 Al: 0.02-0.06 wherein the balance is in a steel bar remains in the hot-rolled with a component of Fe, rolling direction perpendicular bars cross on the observed area 0.1 [mu] m ² or more in the total average of MnS particles in the range of less than 12.6Myuemu ² A high toughness non-heat treated steel bar for hot forging characterized by having an area of 4.9 μm ² or less. 2. The component further contains Ti: 0.005-0.050 by weight%, and the total average maximum diameter of the Ti carbonitrides formed by the inclusion of Ti is 0.15 μm or less (1). Non-heat treated steel bar for high toughness hot forging described.