JPS589816B2 - Manufacturing method of non-thermal rolled steel bar - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a non-thermal rolled steel bar having mechanical properties equivalent to that of a medium carbon steel bar that is tempered as rolled without being subjected to normalizing treatment after rolling. Relating to a manufacturing method.

Medium carbon steel or medium carbon low alloy steel is often used as a mechanical structural steel material for shaft materials and upsetter materials, but these are steel materials with a ferrite + pearlite structure that have been subjected to surface hardening treatment such as induction hardening. There are many things to do.

Therefore, as a pretreatment, it is customary to perform a normalizing treatment after rolling for the purpose of uniforming the structure of the core and induction hardened portion.

Furthermore, normalizing treatment is often performed for the purpose of improving machinability.

The purpose of this invention is to omit the above-mentioned normalizing treatment of medium carbon steel bars, which is usually carried out. and medium carbon steel consisting of unavoidable impurities, or C 0.30% to 0.6% or less, Mn 1.5% or less, SolAl0
．． A steel containing one or both of the following: 0.05% or less, Cr 0.3% or less, V 0.05% or less, and Nb 0.05% or less is subjected to final finishing rolling at a rolling temperature of 850 to 1050°C. It is characterized by rolling at a temperature range of 850 to 950°C with humidity and cooling at a cooling rate of 0.3 to 5.0°C/sec, that is, normal cooling.

The structure of the steel bar produced by this invention method is a non-tempered steel bar consisting of pro-eutectoid ferrite and pearlite colonies of No. 6 or higher grains in the as-rolled state, and the yield ratio, ductility, and toughness are equivalent to or equal to that of tempered steel. It has the same or better characteristics.

Many studies have been conducted on temper rolling for steel sheets mainly composed of ferrite structures with a C content of 0.30% or less. No manufacturing method has been developed to improve the mechanical properties of medium carbon steels with a carbon content of 0.60% or more, mainly by controlling the size of pearlite colonies.

That is, in this invention, in hot rolling, the rolling temperature is set to 850.
Rolling is carried out at a temperature range of ~1050°C with a processing amount of 30% or more of the total working amount, and a final finishing rolling temperature of 850-950°C, and a rolling rate of 0.3-5.0'C/sec. (Normal cooling) By cooling at the cooling rate, rolled materials (steel bars)
The present invention was made based on the findings from experimental results that the microstructure and mechanical properties of the material can be improved to the same or better properties than conventional normalized materials.

The reason for limiting the chemical composition of the steel bar in this invention is as follows.

This invention aims to make the mixed structure of ferrite and pearlite (ferrite colony) finer (no. 6 or higher), so when the C content is 0.30% or less, pro-eutectoid ferrite becomes the main component, Moreover, if it is more than 0.60%, the structure becomes only pearlite and the effect is small, so the C content is set to more than 0.30% to 0.60%.

Mn is a solid solution hardening element and is a necessary element to increase the strength and toughness of steel, but if it exceeds 1.5%, the impact properties tend to deteriorate, so the Mn content is 1.5% or more. %
The following was made.

Cr is a hardenability adjusting element for medium carbon steel, so if it is added as a hardenability adjusting element, if it is added in an amount of 0.30% or more, 0.30% is added to prevent formation of structures such as bainite after rolling.
% or less.

Al, V, and Nb form fine carbides and nitrides, and the addition of trace amounts of these forming elements is effective in reducing the size of pearlite colonies.

However, even if the SolAl content is 0.05% or more, no significant difference is observed in the effect of increasing the content, so it is set to 0.05% or less.

Like Al, V and Nb are effective in reducing pearlite colonies, but a content of both elements of 0.05% or less is sufficient to reduce pearlite colonies, and 0.05% or less content is sufficient to reduce pearlite colonies.
Since no significant difference in the effect was observed even if Nb was added in an amount of 0.05% or more, Nb was set at 0.05% or less, and Nb was also set at 0.05% or less.

In addition, the reason why the rolling temperature was set at 850 to 1050°C is because a normal mixed structure of ferrite and pearlite cannot be obtained at temperatures below 850°C, resulting in a partially acicular ferrite and bainite structure. However, at a rolling temperature of 1050° C. or higher, the fine pearlite structure targeted by the present invention cannot be obtained and pearlite develops significantly.

Furthermore, the final finish rolling temperature was set at 850 to 950°C based on the knowledge from many experimental results that this temperature range is the optimum condition for obtaining fine grain steel with pearlite colony No. 6 or higher.

Also, the reason why the total amount of processing at a rolling temperature of 850 to 1050°C is set to 30% or more is because the total amount of processing within this temperature range is 30% or more.
If it is less than %, the structure tends to become coarse due to the small amount of lattice defects and accumulated energy introduced into the crystal during processing (the desired microstructure cannot be obtained).

This invention has been achieved by rolling the above-mentioned medium carbon steel under the above-mentioned rolling conditions.
By making the rolled material from about #5.5 into fine grains of #6 or higher, it was possible to improve mechanical properties, especially yield ratio, ductility, and toughness, without normalizing as in the past. It is.

Furthermore, in addition to the improvements in structure and mechanical properties described above, the steel bar produced by the method of this invention has favorable properties such as thinner and more uniform scales and an extremely small decarburized layer compared to normalized materials. First, due to improved microstructure and mechanical properties, it has better machinability and cold workability than ordinary rolled materials.

Next, an example of test results will be shown in Examples.

In a 70t converter, steel with the chemical composition shown in Table 1 is melted and 1
After blooming to a diameter of 80 mm, the steel bars were rolled into 28 mm diameter steel bars using the method of the present invention and the comparative example (conventional method) under the rolling conditions shown in Table 2.

Table 3 shows the mechanical properties and microstructure results of the steel bars rolled under the above conditions.

As is clear from the comparison of the steels produced by the method of the present invention shown in the above table and those processed by the conventional method for normalizing after rolling, the mechanical properties of the steels produced by the method of the present invention are significantly improved. It shows properties equivalent to or better than those subjected to conventional normalizing treatment.

Furthermore, as shown in Table 4, the scale properties of the decarburized layer test results show that the amount of scale is reduced to about 1/3, and the decarburized layer is reduced to 1/2 to 1/3.

As described above, according to the method of the present invention, the normalizing step is omitted in the production of steel bars, and it is possible to obtain steel bars that have properties equivalent to or better than those subjected to normalization treatment.

Claims (1)

[Claims] 1 C over 0.30% ~ 0.60% or less, Mn 1.5
Medium carbon steel containing 0% or less, or more than 0.30% C to 0.60% or less, Mn 1. Sol below 50%
Al 0.05% or less, Cr 0.3% or less, V 0.05%
Hereinafter, NbO. Medium carbon low alloy steel containing one or more of 0 5% or less is rolled at a rolling temperature of 850 to 1050°C, and the final finish rolling temperature is 850 to 950°C.
The temperature is controlled to maintain a humidity range of ℃, and the total processing amount is 3.
A method for producing a non-thermal rolled steel bar characterized by rolling with a working amount of 0% or more.