JPS582572B2 - Method for manufacturing strong steel bars with little anisotropy - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a medium-carbon strong steel bar containing Ca.

Generally, steel materials are manufactured by rolling in only one direction, so their mechanical properties are in a direction perpendicular to the rolling direction (hereinafter referred to as the C direction).

), and this has recently become a practical obstacle in the use of steel bars for automobiles, especially shaft materials such as crankshafts.

The reason for poor C-direction toughness is non-metallic inclusions in the steel,
In particular, it is thought that this is because MnS inclusions are stretched in the rolling direction by hot rolling.

The purpose of including Ca in the present invention is to reduce the amount of MnS inclusions, prevent MnS from stretching, and improve C-direction toughness by granulating MnS.

Furthermore, the purpose of performing quenching and tempering heat treatment is to improve the absolute values of toughness and tensile properties.

This quenching is usually done by oil cooling or water cooling from 20 to 60°C above 3 Ac points.

Further, tempering is usually carried out at a temperature in the range of 400 to 650°C.

In the prior art, there is no known example in which Ca is contained in a medium carbon tough material subjected to such quenching and tempering in order to improve the C-direction toughness.

However, there are several other known techniques for using Ca.

For example, Special Publication No. 51-6088, Special Publication No. 47-44855
et al., these include inclusions (usually called bellaque) on the cutting edge of turning tools due to the addition of Ca.

) to prevent shortening of tool life due to wear.

However, the S content of these materials is between 0.030 and 0.
．． The content is as high as 13%, and when used as a strong steel bar, it has the disadvantage of considerably lowering the toughness and tensile properties.

In the present invention, the first aim is to improve the C-direction toughness by adding Ca, and in terms of machinability, it is not intended to extend the life of a turning tool, but rather to extend the life of a drilling tool (drill) compared to that of conventional materials. It is enough that it does not deteriorate compared to Ca-free material).

Also, special public service No. 47-24848, special public service No. 46-27950
There is a method in which the grain size is adjusted by adding Ca to improve cold forgeability and machinability, but the present invention
C as a measure to reduce the amount of ductile MnS inclusions and prevent granulation.
As mentioned above, the purpose is to improve the C-direction toughness.

In this way, the present invention improves the C-direction toughness of high-strength tough materials, which has been difficult to achieve in the past, and the gist of the invention is as follows: C: 0.30-0.50%, Mn: 0.40% ~1.80
%, Si: 0.10-0.40%, S: 0.010-0
．． 030%, Cr: 0.50-1.20%, Mo: 0.
10-0.50%, SolAl: 0.015-0.06
0%, Ca: 0.0010 to 0.0100%, with the remainder consisting of Fe and unavoidable impurities. A steel slab or slab is hot rolled at a rolling ratio of 8 or more, and then quenched and tempered. This is a method for producing strong steel bars with less anisotropy in terms of toughness, and by using such a production method, the C-direction toughness was improved by 50 to 60% compared to conventional materials.

Moreover, in this case, the tensile properties and drill life do not deteriorate compared to conventional materials.

Next, the reason for limiting the components in the present invention will be described.

If C is less than 0.30%, hardenability is poor, and toughness decreases because it is necessary to lower the tempering temperature in order to secure a predetermined strength.

Further, if C exceeds 0.50%, toughness and machinability deteriorate, which is undesirable, so the upper limit was set as above.

If the amount of Mn is less than 0.40%, quenching will be insufficient during quenching, making it impossible to ensure strength and toughness.

On the other hand, if Mn exceeds 1.8%, toughness and weldability deteriorate, so this was set as the upper limit.

Si has the effect of deoxidizing and increasing yield strength and tensile properties, and if it is less than 0.1%, the deoxidizing effect is not sufficient, and if it is added in excess of 0.40%, it will impair toughness.
These amounts were defined as upper and lower limits.

If S is less than 0.010%, toughness will be extremely improved, but machinability will be adversely affected, so this is set as the lower limit.
．． If it exceeds 0.030%, mechanical properties such as toughness and tensile properties deteriorate, so this was set as the upper limit.

SolAl refines the grain size and improves toughness and tensile properties, and its effect is 0.015 to 0.0.
Since it is in the range of 0.060%, this was set as the upper and lower limit.

Ca forms Ca-Al-0 during the molten steel stage, which collects MnS and CaS during the solidification process to generate non-plastic composite inclusions and improve C-direction toughness and machinability.

In medium carbon steel and medium sulfur steel such as the steel of the present invention,
If Ca is less than 0.0010%, the effect of improving toughness and conductivity will be reduced, and if it exceeds 0.010%, the cutting tool life and S content accuracy due to S removal effect will decrease, so this should be avoided. The upper limit was set.

Adding a large amount of Ca causes the above-mentioned drawbacks, so if the S amount is on the high side (0.030%) within the range of the present invention, Ca alone may cause insufficient spheroidization of MnS inclusions. In this case, other spheroidizing elements, such as T
Combination use with i, Zr, REM, etc. is also effective.

Cr is an element that improves hardenability and increases strength without impairing toughness, but if it is less than 0.50% it is less effective and if it exceeds 1.20% it will deteriorate machinability. was set as the upper limit.

Mo also has the same effect as Cr, but a minimum of 0.10% is required; adding too much will impair toughness and machinability, so the upper limit was set at 0.50%.

Next, the rolling ratio was set to 8 or more because the lower the rolling ratio, the less the sulfide is stretched and the anisotropy of toughness is reduced, but if it is less than 8, porosity remains in the axial core, resulting in poor mechanical properties. This is because it may deteriorate.

Furthermore, the reason why heat treatments such as quenching and tempering are performed is to use the product in the high strength and high toughness range as possible within the above-mentioned component range.

Next, examples of the present invention will be described.

Table 1 shows the chemical compositions of a known Cr-Mo tough steel (hereinafter referred to as basic steel) produced using a 300 kg atmospheric melting furnace and a steel in which Ca is added to the basic steel (hereinafter referred to as invention steel). Ta.

FIG. 1 shows the shape of inclusions formed by hot forging this steel ingot into a 70φ billet.

(The forging ratio in this case is 8.2.

) In the basic steel A, there are a large number of extensible MnS, but in the steel mouth of the present invention, these have changed to spherical inclusions.

Next, quenching and tempering are performed to improve the tensile and toughness properties. Figure 2 shows the case of heating at 840°C for 90 minutes, oil quenching, and tempering at 600°C. The impact value in the C direction at the center of the test piece is shown.

Figure 3 is an explanatory diagram showing the sampling position of the C-direction impact value test piece;
(Test piece: JIS No. 3 U notch, test temperature: 20°C).

The fully quenched material is a 70φ x 20mm thick test material that has been oil quenched to give it a completely quenched structure (martensitic structure), and the incompletely quenched material is a 70φ x 100mm thick steel ingot that has been oil quenched. The impact toughness of the steel of the present invention is 50 to 50% higher than that of the basic steel in any quenched structure.
It has improved by 60%.

FIG. 4 shows the tensile properties in the L direction when a 70φ billet was heated at 840°C for 90 minutes, oil quenched, and tempered at 600°C.

The figure shows that the tensile properties of the steel of the present invention are at a superior level compared to that of the basic steel.

FIG. 5 is an explanatory diagram of the sampling position of a test piece for tensile properties (test piece: JIS No. 4).

Figure 6 shows the tool in the same 70φ billet state after heating at 840°C for 90 minutes, oil quenching, and tempering at 600°C.
SKH9, 10φ, feed: 0.1mm/reV: hole depth: 30mm, cutting oil: #60 spindle (2l/min
) are the drill life test results when drilling and cutting are performed under the following cutting conditions.

Due to the addition of Ca, the drill hole clarity, which is one of the important properties for a shaft steel bar material, does not deteriorate, and in fact is slightly better.

As described above, the present invention is a method for producing a high-strength steel bar with medium carbon and medium sulfur levels, which has significantly improved C-direction toughness and is manufactured by adding Ca and heat-treating it through quenching and tempering. It is possible to supply a highly practical steel material with tensile properties and drill machinability that are almost equivalent to basic steel, and is used for shaft materials such as crankshafts and bulldozer links that require C-direction toughness. It is highly practical and can be widely applied to automobile and industrial machine parts.

[Brief explanation of the drawing]

Figure 1 A and B are the basic steel (conventional steel) A and the invention B. It is a micrograph showing the shape of inclusions after forging, quenching, and tempering heat treatment of Ca-added steel. FIG. 2 is an explanatory diagram showing the C-direction impact values of both steels after quenching and tempering heat treatment. FIG. 3 is an explanatory diagram showing the location of sampling of C-direction impact value test pieces. FIG. 4 is an explanatory diagram showing the tensile properties of quenched and tempered materials. FIG. 5 is an explanatory diagram showing the sampling positions of tensile property test pieces of the present invention steel and basic steel. FIG. 6 is an explanatory diagram showing the drill life test results.

Claims (1)

[Claims] 1C: 0.30-0.50%, Mn: 0.40-1.
80%, Si: 0.10-0.40%, S: 0.010
~0.030%, Cr:0.50~1.20%, Mo:
0.10-0.50%, SolAl: 0.015-0.
060%, Ca: 0.0010 to 0.0100%, with the remainder consisting of Fe and unavoidable impurities, is hot rolled at a rolling ratio of 8 or more, and then quenched and quenched. A method for producing a strong steel bar with little anisotropy, which is characterized by carrying out a return process.