JPH0425343B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention is a non-tempered steel bar with excellent toughness and a method for producing the same. More specifically, regarding the manufacturing method of raw steel bars used for manufacturing automobile bolts, robots, etc., it is possible to process them by cold working such as cold drawing or cold extrusion, or by applying conventional quenching and tempering heat treatment to the steel bars as they are hot rolled. The present invention relates to a non-tempered steel bar having high strength and toughness equivalent to or higher than tempered steel bars, and a method for manufacturing the same. The present invention also relates to a method for producing raw steel bars used in the production of shafts and pin parts for automobiles and construction machinery. The present invention relates to a non-tempered steel bar having high strength and toughness, and a method for producing the same. [Prior Art] Machine structural parts such as bolts and rods for automobiles are required to have high strength and high toughness. Conventionally, these parts have been manufactured by using medium carbon steel or neutral carbon low alloy steel bars as raw materials and toughening them by performing quenching and tempering heat treatment after forming by cold or hot working. If it were possible to omit this heat treatment and produce a high-strength, high-toughness non-tempered steel bar with cold working or hot rolling, the benefits would be extremely large in terms of energy saving and cost. In addition, mechanical structural parts such as shafts and pin parts for automobiles and construction machinery are required to have high strength of 70 kgf/mm ² or more and high toughness. Conventionally, these parts have been manufactured by using medium carbon steel or medium carbon low alloy steel bars as raw materials, cutting them, forming them, and then subjecting them to quenching and tempering heat treatment. This heat treatment is omitted,
If it were possible to produce high-strength, high-toughness non-tempered steel bars as hot-rolled, the benefits would be extremely large in terms of energy savings and costs. The development of non-tempered steel that has high strength and high toughness even when hot-rolled is being actively carried out not only in the field of steel bars but also in the fields of thin steel plates and thick steel plates. For example, in the field of steel sheets, as in Japanese Patent Publication No. 58-2570, Nb and V are added to low carbon steel with C content below 0.25% to prevent the precipitation of pearlite, which is harmful to toughness, and to form a single phase of ferrite. In order to strengthen the ferrite, toughness has been achieved by adding carbonitrides to the carbonitrides to precipitate them, and by rolling the ferrite at temperatures just above the Ar ₃ point or in the two-phase region to make the ferrite grains finer. However, in the manufacture of machine structural parts such as shafts made of steel bars, it is essential to perform surface hardening treatment by induction hardening in the final process to improve the wear resistance. There is no choice but to use medium carbon steel with a content of 0.30 to 0.55% as the material. In the case of medium carbon non-tempered steel, the structure is mainly pearlite, which has high strength and low toughness, so it is difficult to obtain high toughness, which is relatively easy to obtain. For example, as seen in JP-A-59-9122 and JP-A-59-170222, the impact value (uE ₂₀ ) of grade ² non-thermal steel with a strength of 70 to 100 Kgf/mm is just 10 Kgf・m/cm. As low as ² ,
It barely reaches the lower limit of the reference value for S45C quenched and tempered material (8 kgf·m/cm ² ) listed in JIS G 4051. This problem of low toughness is an obstacle to the practical application of medium carbon non-thermal treated steel. Strength can also be added by cold working, but the toughness deteriorates the more cold working is applied, so materials with low toughness cannot be used. Furthermore, if the material strength exceeds 70 Kgf/ ^mm2 , there is a problem that cold workability (tool life and machining load) deteriorates, so cold working with a material strength of 70 to 90 Kgf/ ^mm2 is recommended for cold working. There was a demand for a steel bar material with good properties and toughness. [Problems to be Solved by the Invention] The present invention provides a medium carbon non-heat treated medium carbon steel that has high strength and high toughness as hot rolled and does not have low toughness even after cold working, and a method for producing the same. This is to disclose. [Means for solving the problem] The present inventors used medium carbon steel to achieve a strength of 70 kg.
The results of research aimed at producing non-tempered steel with f/mm ² or higher and excellent toughness (a) The strength of as-rolled material is determined by the carbon equivalent Ceq (=%C+
It can be controlled by adjusting %Si/7+%Mn/5+%Cr/9+%V). (b) In addition to the conventionally known effect of improving toughness by refining ferrite and pearlite crystal grains by lowering the rolling finishing temperature, lowering the temperature below a certain level promotes the transformation of fine ferrite, resulting in the production of ferrite with high toughness. An improvement in toughness can be expected by a rapid increase in the fraction of . Furthermore, when the rolling temperature is lowered and rolling is performed in a two-phase region, the ferrite precipitated during rolling is processed, resulting in a decrease in toughness. (c) However, in medium carbon ordinary steel, even if the fraction of fine ferrite with high toughness is increased by finish rolling in the above-mentioned specific temperature range, as the ferrite fraction increases, C becomes excessive, and the C in pearlite increases. As the amount increases, pearlite becomes brittle,
The improvement in toughness of the steel as a whole is small. (d) When finish rolling of steel in which V is a solid solution is performed in the above specified temperature range, V reacts with excess C generated as the fraction of fine ferrite increases, forming fine particles that increase strength without impairing toughness. By promoting the precipitation of carbides, it is possible to prevent a decrease in strength and embrittlement of pearlite, and improve toughness to the same level as low carbon steel while maintaining high strength. We found the following knowledge. As a result, the strength is 70Kgf/mm ² or more, and the Charpy impact value (uE ₂₀ ) at room temperature is 18Kgf・m/cm ² or more, which is dramatically higher than conventional medium carbon non-heat treated steel, and the medium carbon heat treatment The inventors have invented a medium-carbon non-tempered steel that has strength equal to or higher than that of steel, and a method for producing the same. In the present invention, reasons for limiting the scope of claims will be described below. First, regarding the steel composition, C is 0.30% to improve strength and wear resistance through induction hardening and surface hardening.
The above amount is necessary, but if it exceeds 0.50%, toughness and cold workability will be impaired, so it is set at 0.30 to 0.50%. Si is an effective element for promoting deoxidation and increasing strength, but since it impairs toughness, the upper limit was set at 0.30%. Mn is an element effective in increasing strength and making the structure finer to increase toughness, and if it is less than 0.60%, the effect is small. However, if it exceeds 2.0%, the toughness will suddenly deteriorate, so it is set at 0.60 to 2.00%. As mentioned above, V is an important additive element for the present invention and is 0.06
If it is less than 0.3%, the effect is small, and if it is more than 0.30%, the effect is saturated, so it was set at 0.06 to 0.30%. Al has the effect of forming nitrides and refining crystal grains, and 0.020% or more is required for sol-Al, but 0.060% or more is required for sol-Al.
If it exceeds 0.020 to 0.060, machinability will be impaired.
%. N is an element necessary for forming Al nitride and V carbonitride, and the lower limit is 0.0020%, but 0.0070%.
If it exceeds 0.0020%, the toughness will be impaired.
It was set as 0.0070%. Note that the same effect can be obtained by adding 0.01 to 0.20% Nb instead of V, but when producing non-tempered steel with the same mechanical properties, Nb-added steel is better than V-added steel. However, there are disadvantages such as high cost due to the components and the need to increase the heating temperature. In the present invention, the strength of the as-rolled material is Ceq = %C
There is a correlation with the carbon equivalent shown by +%Si/7+%Mn/5+%Cr/9+%V, 70-90Kgf/mm ²
In order to achieve this, the carbon equivalent must be between 0.60 and less than 0.90. As can be seen from the formula for carbon equivalent, Cr is also an effective element for improving strength, and if it is less than 0.01%, it has no effect, and if it exceeds 0.50%, the strength improvement effect is almost saturated, and it is not worth it in terms of cost. Because it will disappear,
Cr should be added in the range of 0.01-0.50%. As mentioned above, the major feature of the present invention is that it not only improves toughness by making the conventionally known ferrite crystal grains fine, but also significantly improves toughness by increasing the fraction of ferrite. If steel with the above composition range is not subjected to controlled rolling, the ferrite fraction will be 0.5 if the carbon content is (%C).
-0.63 x (%C) or less, but if controlled rolling is performed in a specific temperature range, it will exceed 0.5 - 0.63 x (%C), and the toughness will significantly improve, so the lower limit of the ferrite fraction was set at 0.5 −0.63×(%C). At this time, the ferrite particle size was refined from No. 7 to 9 to about No. 10 or more and No. 12, which contributed to improving the toughness, and the lower limit of the ferrite particle size was set to No. 10. Furthermore, since the occurrence of bainite or martensite, which are hard structures, deteriorates toughness and cold workability, it was decided to have a ferrite-pearlite structure. Here, the ferrite particle size is the particle size number measured according to the JIS G 0552 comparative method, and the ferrite fraction is measured using an image analysis device equipped with an optical microscope function (product name: Nippon Regulator Co., Ltd.).
LUZEX500) to measure and express the amount of ferrite structure mixed with pearlite structure. The rolling and cooling conditions will be described below. The heating temperature is not particularly limited, but it is important to dissolve V in the steel to produce the desired effect, and it should be at least 1000°C, which is generally applied for V-added steel, and preferably at 1050°C or more. be. The final finish rolling temperature was set at 650 to 800℃ because
In the specific temperature range in which the ferrite fraction increases, the steel with the composition of the present invention undergoes rolling in the two-phase region at temperatures below 650°C, resulting in a decrease in toughness;
This is because the effect of increasing the ferrite fraction above °C suddenly decreases. Furthermore, regarding the cooling rate after rolling, as mentioned above, it is important that the steel of the present invention has a ferrite-pearlite structure, and the precipitation of bainite or martensite deteriorates the toughness.
The temperature was set at 2.0°C/sec or less to prevent precipitation of bainite or martensite. [Example] The present invention will be explained below using an example. Table 1 shows the components of the sample materials. All test materials were melted in a converter, RH treated, and then cast by continuous casting, and rolled from 120 mm square steel pieces to 30 mm round steel bars.
The cooling rate after rolling was 0.8 to 1.0° C./sec in all cases. The test materials were evaluated by a tensile test and a sharpie U-notch impact test at room temperature either as rolled or after being heat treated. Table 2 shows these results. Examples with a circle attached to the number are examples of the present invention, and the others are comparative examples. The table also shows target values and actual measured values for the ferrite fraction depending on the ferrite particle size and carbon content.
Comparative Example 11 is JIS S45C standard steel, which is generally used as tempered steel, and the heat treatment method was JIS G.
4051 was used as a reference. The embodiments of the present invention, , , , all have a strength of 70 to 90 Kgf/mm ² and a strength of 18
It has achieved an impact value of Kgf・m/cm ² or more. Comparative Example 5 has rolling conditions for conventional non-tempered steel, and the finishing temperature is high, so the ferrite crystal grains are large, the ferrite fraction is small, and the toughness is degraded.
Since Comparative Example 8 does not contain V, it is also similar to Comparative Example 9.
The toughness deteriorated due to excessive N content. Comparative example 10
has too much carbon equivalent, so its strength is too high and it is not suitable for cold working. Table 2 shows examples of the present invention.
, the strength and toughness of the cold-worked material when cold drawing is performed with an area reduction rate of 15% are shown, both of which have a strength of 90Kgf/mm2 or ^more and a strength of 12Kgf・m/cm2 ^or more. The impact value is ensured.
Comparing this with S45C tempered steel, which is Comparative Example 11,
It can be seen that the steels of the present invention have the same strength and high toughness, and that it is possible to non-thermalize mechanical structural parts that have conventionally been manufactured with heat-treated steel. FIG. 1 shows a micrograph showing the structure of an as-rolled material of an example as a representative example of the steel of the present invention, and shows its ferrite grain size and ferrite fraction. FIG. 2 is a diagram showing the relationship between the final finishing temperature of rolling, ferrite particle size, ferrite fraction, tensile strength and impact value of the as-rolled material when the above-mentioned sample material symbol C is used. In the range of the final finishing temperature of 650 to 800°C, which is the range of the present invention, the ferrite fraction increases rapidly and the ferrite grains become fine, so that the impact value rapidly increases, but there is almost no decrease in strength. Target strength of 70~90Kg
f/mm ² and impact value of 18 Kgf·m/cm ² or more can be obtained as rolled.

【table】

【table】

【table】

〔Effect of the invention〕

As explained above, the present invention solves the problem of low toughness seen in conventional medium carbon non-tempered steel bars by limiting the steel material composition, metallographic structure, and rolling method, and improves cold workability. This is a medium-carbon non-tempered steel bar with a strength of 70 Kgf/mm ² or more and an impact value of 18 Kgf/cm ² or more, and a method for producing the same, which is extremely valuable industrially.

[Brief explanation of drawings]

FIG. 1 is a micrograph showing the metallographic structure of steel in Example (A) of the present invention. FIG. 2 is a diagram showing the relationship between final finishing temperature of rolling, ferrite particle size, ferrite fraction, tensile strength of as-rolled material, and impact value.

Claims (1)

[Claims] 1 C 0.30-0.50%, Si 0.30% or less, Mn 0.60-2.00%, Cr 0.01-0.50%, V 0.06-0.30%, sol-Al 0.020-0.060%, N 0.0020-0.0070%, However, %C+%Si/7+%Mn/5+%Cr/
9+%V value is contained as less than 0.60 to 0.90, the remainder is a component consisting of Fe and unavoidable impurities,
Moreover, the ferrite particle size is No. 10 or more and the ferrite fraction is 0.5-0.63× depending on the carbon content (%C).
As-rolled steel bar with a ferrite-pearlite structure of (%C) or more, with a tensile strength of 70 to 90 kg
f/mm ² and a Charpy impact value of 18Kg at room temperature.
A non-tempered steel bar with excellent toughness characterized by a toughness of f・m/cm ² or more. 2 C 0.30-0.50%, Si 0.30% or less, Cr 0.01-0.50%, Mn 0.60-2.00%, V 0.06-0.30%, sol-Al 0.020-0.060%, N 0.0020-0.0070%, however, %C + %Si /7+%Mn/5+%Cr/
A steel billet containing a 9+% V value of less than 0.60 to 0.90, with the remainder consisting of Fe and unavoidable impurities is heated, followed by hot rolling at a final finishing temperature of 650 to 800°C, and after rolling. A method for producing a non-tempered steel bar with excellent toughness, characterized by cooling up to 500°C at a cooling rate of 2.0°C/sec or less.