JPS6357742A - Non-heattreated steel for hot forging having high toughness - Google Patents

Abstract

PURPOSE:To obtain a steel having high toughness in its hot-forged condition, by providing, for a non-heattreated steel for hot forging having high toughness, a composition which consists of specific amounts of C, Si, Mn, Cr, V, S, N, Al, and the balance Fe with impurities and in which carbon equivalent value is regulated to a specific range. CONSTITUTION:The non-heattreated steel for hot forging having high toughness has a composition which consists of, by weight, 0.18-0.28% C, 0.1-1.00% Si, 0.6-2.0% Mn, <=1.0% Cr, 0.03-0.20% V, 0.02-0.10% S, 0.0030-0.0200% N, 0.02-0.06% Al, and the balance Fe with inevitable impurities and in which carbon equivalent represented by an equation is regulated to 0.68-0.96%. A steel of the above composition is refined by the use of a vacuum melting furnace, etc., so as to be formed into a steel bar, which is forged into the desired product. In this way, the steel having high toughness in its hot-forged condition can be obtained and, moreover, it is useful as machine parts.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a steel that has high toughness even after hot forging, a so-called high-toughness non-thermal steel for hot forging.

(Prior art) Generally, mechanical structural parts manufactured by hot forging, such as automobile crankshafts and connecting rods, are quenched and tempered to obtain a predetermined strength and toughness after being formed by hot forging. quality treatment). If moderate strength and toughness can be obtained by omitting this tempering process, which requires a large amount of thermal energy, it will be possible to significantly reduce manufacturing costs, and in recent years, such non-thermal steels have been in demand. It's starting to look like this.

As a conventional non-temperature steel for hot forging, for example, one utilizing 'V precipitation strengthening is known. However, although it is relatively easy to achieve the desired strength with this non-tempered steel, it has a drawback of having a low toughness value.

Therefore, AJ! tried to compensate for the low toughness value by preventing the coarsening of austenite crystal grains and making them finer. , non-thermal refining with trace amounts of carbonitride-forming elements such as Ti added (JP-A-61-19761, JP-A-61-
139646).

By adding Ag, Ti, etc., the toughness value has been greatly improved compared to conventional products, making it possible to apply it to parts around the legs of vehicles that require height and toughness, such as knuckles.

(Problems to be Solved by the Invention) It is well known that one of the ways to improve strength and toughness is to refine the structure of steel, and the carbonitride-forming elements mentioned above also reduce the coarseness of austenite grains. It is used for the purpose of preventing oxidation and, as a result, making the ferrite-pearlite structure after transformation finer.

However, the normal hot forging temperature range exceeds the melting temperature of these macromolecular compounds themselves, and depending on the forging conditions, coarsening of austenite cannot be prevented. As a result, steel forged at a normal hot forging temperature of 1.100-1300C usually has a coarse structure and extremely low toughness, especially low-temperature toughness.

The present invention solves the above problems and provides a steel that has high toughness even after hot forging, and a high toughness non-temperature steel for hot forging.

(Means for Solving the Problems) The present inventors have developed a steel with a carbon content of 0.18% to 0.28%.
・We discovered that it was possible to significantly improve the toughness of hot-forged steel, and as a result of further intensive research, we found that by setting the ingredients in the steel within an appropriate range, we were able to improve the toughness of the conventional hot-forged steel. It has superior toughness to steel [7th grade]. That is, in weight %, C: 0.18-0.28, Si: 0.1
1.00%, Mn: 0.6 2.0%, Cr: 1
．． 0% or less, V: 0.03-0.20chi, S: 0.02
-0.10%, N: o, oO: 3o - 0,0200
%, Ae: 0, O' 2-0.06%, the remainder consists of Fc and inevitable impurities, and the following carbon content If C
It is a non-tempered steel for hot forging with high envelopment and eq of 0.68-0.96.

Ceq-(%)=C+Mn15+Si/7+Cr/9
4-1.54V Next, the reasons for limiting the composition of the high toughness non-tempered M steel for hot dipping according to the present invention will be explained.

C is added as an essential element to ensure the strength of mechanical structural parts, but its content is 0.18
If it is less than 0%, a large amount of alloying element is required to obtain the specified strength and hardness, which is uneconomical, and at the same time, an excessive increase in alloying element will undesirably increase hardenability, so the lower limit is set to 0. .18 staff.

Moreover, when the amount of C exceeds 0.28 inch, the toughness decreases.

Si acts as a deoxidizing agent and is used as a solid solution strengthening element. When it is less than 0.1%, the action of the deoxidizing agent and ~ is insufficient. If the coefficient is 1.0 or more, the hardness increases and the toughness deteriorates.

Mn has a strength adjustment and deoxidizing effect. If it is less than 0.6%, strength decreases, and if it exceeds 2.0%, toughness decreases and manufacturing becomes difficult.

Similar to Mn, C can be added in small amounts to compensate for hardness, but if it exceeds 1.0, the toughness deteriorates.

(2) Improves toughness through solid solution strengthening and precipitation strengthening. To obtain this effect, 0.03% or more is required. But 0
．． Even if 20% or more is added, the improvement in the effect will be small and the toughness will actually decrease, so the upper limit was set at 0.20.

S is an essential element for improving machinability. This effect is expected to be greater than 0.02% (·S).

However, S exceeding 0.10% reduces toughness.

N combines with V, acts as precipitation strengthening as VN, and improves toughness. If the amount of N is less than 0.0030%, a sufficient effect will not be exhibited, but if it exceeds 0.0200%, the toughness will decrease due to solid solution of N, so the upper limit was set at 0.0200%.

M is added for deoxidation, but at the same time forms AIN and refines crystal grains. If AQ exceeds 0.06%, the toughness deteriorates, but if it is less than 0.02%, no fine grain effect of AlN is expected.

The reason for limiting Ccq is to ensure hardness. .

The hardness required for mechanical structural parts is 210 to 280 on the Pinkers hardness, so the required Ce q is 0.
．． Chill at 68-0.96%.

(Example) Steel having the components shown in Table 1 was melted using a 150 ky vacuum melting furnace. 85 by forging 150kP steel
After making it into a φ steel bar, it was heated to 1250C and forged into a front axle for an automobile.

A U-notch Charpy test piece, a tensile test piece, and a hardness test piece were cut out from this front axle, and the impact value, strength, and Pinkers hardness were measured.

Table 2 shows the test results for each test material.

As shown in Table 2, it can be seen that the products of the present invention have high toughness as hot-forged.

(Effects of the Invention) The steel of the present invention has high toughness as a hot-worked steel and is useful as a mechanical structural component.

Claims (1)

[Claims] C: 0.18-0.28%, S1: 0.1-1.00%, Mn: 0.6-2.0%, Cr: 1.0% or less, in weight%. Contains V: 0.03-0.20%, S: 0.02-0.10%, N: 0.0030-0.0200%, Al: 0.02-0.06%, the balance being Fe and unavoidable It consists of impurities and has the following carbon equivalent Ceq. High toughness non-tempered steel for hot forging having a content of 0.68-0.96%. Ceq. (%)=C+Mn/5+Si/7+Cr/9+
1.54V