JPH07238339A - Non-heat treated steel for high performance hot forging - Google Patents

Abstract

(57) [Summary] [Object] The present invention provides a high performance non-heat treated steel for hot forging which has extremely high toughness even as hot forged. [Composition] C: 0.15 to 0.60% by weight%, S
i: 0.1 to 3.0%, Mn: 0.5 to 2.0%, C
r: 1.0% or less, V: 0.03 to 0.30%, S:
0.02-0.10%, N: 0.003-0.020
%, Al: In the non-heat treated steel for hot forging whose basic component is 0.01 to 0.10%, Mg is added to T. 0.0 as Mg
The oxide and sulfide content of 007 to 0.0280% satisfies the following formula in terms of number ratio. (MgO + MgO.Al ₂ O ₃ ) number / total oxide number ≧ 0.80 (1) MnS number attached to oxide / total MnS number ≧ 0.30 (2) [Effect] The steel of the present invention is MnS is finely dispersed on the converted oxide, and the crystal grains can be refined to a high level. As a result, a mechanical structural component having extremely high toughness can be obtained as hot forged.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel having extremely high toughness as hot forged, that is, a high toughness non-heat treated steel for hot forging.

[0002]

2. Description of the Related Art Generally, mechanical structural parts manufactured by hot forging, such as automobile crankshafts and connecting rods, are hardened and tempered in order to obtain predetermined strength and toughness after molding by hot forging. (Tempering process) is applied. If this tempering process that requires a large amount of heat energy can be omitted and appropriate strength and toughness can be obtained, a large reduction in manufacturing costs will be possible, and in recent years, such non-tempered steel has been required. Is coming. As a conventional non-heat treated steel for hot forging, for example, one using V precipitation strengthening is widely known. However, although this non-heat treated steel is relatively easy to obtain the desired strength, it has the drawback of having a low toughness value. Therefore, JP-A-61-19761
As disclosed in Japanese Patent Laid-Open No. 61-139646 and Japanese Patent Laid-Open No. 61-139646, it is attempted to prevent coarsening of austenite crystal grains and to make the crystal grains finer to compensate for the low toughness value.
It is a non-heat treated steel to which a trace amount of carbonitride forming elements such as 1 and Ti are added.

[0003]

It is well known that one of the means for improving the strength and toughness is to refine the structure of steel, and the carbonitride forming element mentioned above also has a coarse austenite crystal grain. It is used for the purpose of preventing grain formation and, as a result, refining the ferrite-pearlite structure after transformation. However, the normal hot forging temperature range exceeds the melting temperature of these carbonitrides themselves, and depending on the forging conditions, coarsening of austenite crystal grains cannot be prevented. As a result, the structure of the steel forged at a normal hot forging temperature of 1100 to 1300 ° C. is coarse, and the toughness, especially the low temperature toughness, is usually extremely low. The present invention solves such problems and provides a steel having extremely high toughness even in the as-hot-forged state, and a high-toughness non-heat treated steel for hot-forging. That is,
The present inventors have already proposed JP-A-63-57742 and JP-A-1-198450, and have added Mg to the steel in which the components in the steel other than Mg have been optimized, and further include the oxidation. By properly controlling the number of alloys and sulfides, it has become possible to provide steel with dramatically improved toughness.

[0004]

The gist of the present invention is as follows. % By weight, C: 0.15 to 0.60% Si: 0.1 to 3.0% Mn: 0.5 to 2.0% Cr: 1.0% or less V: 0.03 to 0.30 % S: 0.02 to 0.10% N: 0.003 to 0.020% Al: 0.01 to 0.10% Alternatively, in addition to the above composition, Ti: 0.005 to 0.025% Nb: Steel containing 0.005 to 0.100% contains TotalMg: 0.0007 to 0.0280%, the balance being Fe and unavoidable impurities, and the oxides and sulfides contained as a number ratio. The present invention provides a non-heat treated steel for high performance hot forging characterized by satisfying the following formula. (MgO + MgO.Al ₂ O ₃ ) number / total oxide number ≧ 0.80 ... (1) MnS number attached to oxide / total MnS number ≧ 0.30 (2)

[0005]

The greatest technical point of the steel of the present invention is that M
g is included. As a result, the size of oxide-based inclusions (mainly alumina) is miniaturized, and MnS adheres to the fine oxides during the solidification process to achieve fine dispersion of MnS. Adhesion is induced. By this action, ferrite transformation is promoted, crystal grains are dramatically refined, and toughness is significantly improved.

Al ₂ O ₃ as an oxide inclusion in molten steel
When Mg is added in the presence of the above, the composition of the oxide is modified from Al ₂ O ₃ to MgO.Al ₂ O ₃ or MgO. At this time, MgO.Al ₂ O ₃ or Mg
Compared with Al ₂ O ₃ , O has a smaller interfacial energy with molten steel, so that it is less likely to aggregate and coalesce, and miniaturization is achieved.
As the size of the oxide inclusions in the steel material is larger, stress is more likely to be concentrated on that portion, which easily causes fatigue fracture. Therefore, by reducing the size of the oxide-based inclusions, this problem is greatly improved. Even in non-heat treated steel, the fatigue strength is greatly improved.

As a result of detailed examination of the steel in which the oxides have been refined in this way, MnS is crystallized with the oxides as nuclei in the solidification process, so that fine dispersion of MnS is achieved.
It has been clarified that a nitride such as VN is easily attached to the fine MnS. As a result, ferrite transformation is promoted, crystal grains are dramatically refined, and toughness is significantly improved. The reason why MnS tends to adhere to the oxide is that Mg is combined with MnS and the oxide. That is, MnS
Is present as (Mn.Mg) S, and the oxide is MgO.Al ₂ O ₃ or Mg as described above.
Since it is O, both are attached using Mg as a medium. Further, the reason why nitride such as VN is easily attached to MnS is also due to the same reason. Since Mg forms a nitride of Mg ₃ N ₂ , (V · Mg) N is also present.

The optimum Mg content considering the above is Tot
Displayed as alMg (= T.Mg), 0.0007-
It becomes 0.0280% by weight. T. If the Mg content is less than 0.0007% by weight, the fineness of the alumina is insufficient, so that the fine dispersion of MnS and the VN adhesion are insufficient, and the grain refinement effect does not appear. T. When Mg exceeds 0.0280% by weight, the grain refining effect is saturated, and no further effect can be expected.

Next, the reasons for defining the number of oxides will be described. Oxides other than MgO and MgO.Al ₂ O ₃ exist outside the scope of the present invention due to unavoidable mixing in the steel refining process. By setting this amount to be less than 20% of the total number, the fine dispersion of oxide-based inclusions was stabilized at a higher level, and a further material improvement effect was observed.
+ MgO.Al ₂ O ₃ ) number / total oxide number ≧ 0.80
Stipulated. The oxide is a number ratio existing in a certain area, and its size is 1.0 μ or more in equivalent circle diameter. Quantitative analysis of oxide composition was performed by an X-ray microanalyzer.

Next, the reasons for defining the number of oxides will be described. As described above, MnS crystallized with the oxide as the nucleus promotes the ferrite transformation, and enables the grain refinement and the steel material toughness to be significantly improved. Therefore, as a result of investigating an appropriate range for accelerating the ferrite transformation, it was revealed that the ferrite transformation is highly stabilized when the number of MnS attached to the oxide is 30% or more of the total number of MnS. Number / total MnS number ≧ 0.30. Here, the number of MnS attached to the oxide / total M
The specific method for satisfying the number of nS ≧ 0.30 is not specified, but the number of oxides condition is adjusted within the range of the present invention.
Further, it can be achieved by appropriately controlling the solidification conditions of steel. Like MnS, MnS is also a number ratio existing in a certain area, and its size is 0.2 μm or more in equivalent circle diameter. In terms of composition, MnS that is a composite of Mg
Also targeted.

Next, a method for producing the steel of the present invention will be described. The method for producing the steel of the present invention is not particularly limited.
That is, melting of the mother molten steel may be performed by either the blast furnace-converter method or the electric furnace method. Also, the method of adding Mg to the molten steel is not specified, and the method of adding the Mg source by free fall, the method of blowing with an inert gas, the method of supplying the iron wire filled with the Mg source into the molten steel, etc. are adopted. You can do it. As for the addition place, a molten steel ladle, a continuous casting tundish, a continuous casting mold, etc. may be freely selected. Metal Mg as a Mg source,
Mg-Coke, Mg-Al alloy, Mg-Si alloy, M
Granular products such as g-Si-Mn alloys can be used and these and F
It is also possible to mix and use granular products of e, Al, and Fe-Si alloy. Further, the method for producing a steel ingot or a slab from the molten steel and rolling it is not limited.

Next, the reasons for defining the components other than Mg will be described. C is added as an indispensable element for securing the strength as a machine structural part, but if its content is less than 0.15% by weight, the amount of alloying elements becomes large in order to obtain a predetermined strength and hardness. At the same time as being economical, excessive addition of alloying elements undesirably increases the hardenability more than necessary, so the lower limit was made 0.15% by weight. Further, when the C content exceeds 0.60% by weight, the toughness is remarkably lowered, which is not preferable, so the upper limit was made 0.60% by weight.

Si acts as a deoxidizer and is used as a solid solution strengthening element. If it is less than 0.1% by weight, the action as a deoxidizing agent is insufficient. If it is 3.0% by weight or more, hardness is increased more than necessary and toughness is deteriorated. Mn is added for adjusting strength and deoxidizing. If it is less than 0.5% by weight, the strength is lowered, and if it exceeds 2.0% by weight, the toughness is lowered and the production becomes difficult.

Cr, like Mn, can be added up to 1% by weight in order to supplement strength, but if it exceeds 1.0% by weight, toughness deteriorates. V improves toughness by solid solution strengthening and precipitation strengthening. To obtain this effect, 0.
It is necessary to add more than 03% by weight. However, 0.30
The effect is saturated even if more than wt% is added.

S is an essential element for improving the machinability of steel and forming MnS. This effect is possible with 0.02% by weight or more. However, if it exceeds 0.10% by weight,
It is not preferable because the toughness decreases. N combines with V to exert a precipitation strengthening action as VN and improves toughness. 0.003
If the amount of N is less than wt%, no sufficient effect can be expected.
When it exceeds 20% by weight, the amount of solid solution N is too much and the toughness decreases. Al is added for deoxidation, but at the same time AlN is formed and the crystal grains are refined. If it exceeds 0.10% by weight, the toughness deteriorates, which is not preferable, and 0.01% by weight
If it is less than the above, the fine grain effect of AlN cannot be expected.

Ti and Nb are added to further refine the crystal grain size. If Ti is less than 0.005% by weight and Nb is 0.005% by weight, the effect is not exhibited. On the other hand, Ti is 0.025% by weight and Nb is 0.
If it exceeds 100% by weight, the grain refining effect is saturated,
Rather, it deteriorates toughness, which is not preferable. In addition, Ni, Mo, and Cu can be added to the steel of the present invention. By adding these elements, the strength of the forged product can be improved. Examples of the present invention will be described below, and effects of the present invention will be described.

[0017]

Example A slab having the chemical composition shown in Table 1 was produced by a blast furnace-converter-continuous casting method. Mg was added by a method of supplying an iron wire filled with a mixture of metallic Mg particles and Fe-Si alloy particles to the molten steel in the ladle discharged from the converter. Next, slab rolling and bar steel rolling were performed to manufacture a round bar having a diameter of 85 mm, which was then heated to 1250 ° C., forged into a front axle for automobiles, and allowed to cool in the atmosphere. Cut out a U-notch Charpy test piece, a tensile test piece, and a hardness test piece from this front axle,
The impact value, strength and Vickers hardness were measured. Table 2 shows the test results of each test material, and it can be seen that the steel of the present invention has high toughness in the as-forged state.

[0018]

[Table 1]

[0019]

[Table 2]

[0020]

As described above in detail, the steel of the present invention has extremely high toughness as hot forged and is useful as a machine structural component, and its industrial ripple effect is extremely remarkable. There is something.

Claims (2)

[Claims] 1. By weight%, C: 0.15 to 0.60% Si: 0.1 to 3.0% Mn: 0.5 to 2.0% Cr: 1.0% or less V: 0.0. 03-0.30% S: 0.02-0.10% N: 0.003-0.020% Al: 0.01-0.10% TotalMg: 0.0007-0.0280%, and the balance. Of Fe and unavoidable impurities, and the oxides and sulfides contained therein satisfy the following expression in terms of number ratio: high-performance hot-forged non-heat treated steel. (MgO + MgO.Al ₂ O ₃ ) number / total oxide number ≧ 0.80 ... (1) MnS number attached to oxide / total MnS number ≧ 0.30 (2) 2. In% by weight, C: 0.15 to 0.60% Si: 0.1 to 3.0% Mn: 0.5 to 2.0% Cr: 1.0% or less V: 0.0. 03-0.30% S: 0.02-0.10% N: 0.003-0.020% Al: 0.01-0.10% Ti: 0.005-0.025% Nb: 0. 005 to 0.100% TotalMg: 0.0007 to 0.0280%, the balance consisting of Fe and inevitable impurities, and the oxides and sulfides contained therein satisfy the following formula as a number ratio. High performance non-heat treated steel for hot forging. (MgO + MgO.Al ₂ O ₃ ) number / total oxide number ≧ 0.80 ... (1) MnS number attached to oxide / total MnS number ≧ 0.30 (2)