JPH05192744A - Manufacture of bar steel excellent in drawing workability - Google Patents

Abstract

PURPOSE:To provide a method for manufacturing a bar steel applied to a shaft, etc., used for the driving system of various kinds of machines, automoble, etc., by executing drawing work having large reducing ratio of area without applying heat treatment. CONSTITUTION:At the time of continuously casting molten steel composed of, by weight, 0.20-0.8% C, 0.01-1.0% Si, 0.3-2.0% Mn and at least one kind among 0.05-1.0% Cr, 0.05-1.0% Ni, 0.05-0.5% Mo and 0.05-1.0% V and the balance Fe with inevitable impurities, in the vicinity of a crater end, in which the molten steel in the inner part of a cast billet completes the solidification, the preventing treatment for concentrating the components, in which the C/C0 of C content (C) at a core part in the cast billet to the C content (C0) of the molten steel in a ladle becomes 0.8-1.1, is applied and successively, a thicker bar is formed by hot-rolling. Therefore, by forming the thick bar to the bar steel with the drawing work having >=20% the reducing ratio of area per one time without applying the heat treatment, the development of chevron crack is prevented.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing a steel bar for use in various machine parts, tool materials, shafts of various equipment, and the like.

[0002]

2. Description of the Related Art This type of steel bar is obtained by hot-rolling a billet obtained by hot rolling into a steel bar having a diameter of 10 to 100 mm in a mill, and then subjecting the steel bar to a drawing process to obtain a diameter of 8 to 95 mm. Can be obtained at. The total reduction rate of this steel bar drawing process is 15 ~
If it is 20% or more, pull out in two times,
It is customary to subject the steel bar to heat treatment before it is withdrawn.

[0003]

In the drawing process for obtaining a steel bar, the higher the total area reduction rate, the more disadvantageous is the occurrence of so-called chevron cracks or fractures originating from the vicinity of the central axis of the steel bar. Therefore, if the surface reduction rate is high, heat treatment prior to drawing and two times of drawing are indispensable, but if this drawing is not necessary and the drawing can be completed in one pass, the number of steps and energy can be reduced. , Very advantageous in manufacturing on an industrial scale.

The reason why the heat treatment prior to the drawing process and the two times of drawing are required is that chevron cracks and breakages that occur during the drawing process cannot be avoided. These chevron cracks and fractures are mainly due to macro and semi-macro segregation near the center of the slab, which is generated during solidification during continuous casting.

As a measure for preventing such center segregation, for example, 2
Although electromagnetic stirring in the subcooling zone has been attempted, it has not been possible to reduce even semi-macro segregation, and its effect is not sufficient. At the end of solidification of the slab, a so-called in-line reduction method {iron and steel No. 60 (1974) No. 7 pp. 875-884}, in which a large amount of reduction is applied using a pair of rolls, was also tried. However, if the reduction in the cast area of the large unsolidified layer is insufficient, there is a problem that cracks (hereinafter referred to as internal cracks) occur at the solidified interfaces where C, Mn, P and S segregate. It was

In addition, in JP-A-49-121738, a light reduction is applied by a pair of rolls near the solidification tip of the cast slab,
A method of compensating the amount of solidification shrinkage of the portion by reduction is proposed, and Japanese Patent Application Laid-Open No. 52-54623 proposes a method of greatly reducing the vicinity of the solidification completion point of the slab using a casting mold. .

However, in the case of light reduction by rolls, even if a plurality of pairs of rolls are used to reduce the pressure by several mm / m, it is not possible to sufficiently prevent solidification shrinkage and bulging that occur between roll pitches. The effects of reducing segregation and preventing internal cracking are insufficient, and if the rolling position is not appropriate, there is a disadvantage of promoting the occurrence of center segregation.
On the other hand, the method of using a forging die to largely reduce the vicinity of the solidification completion point of the slab is less likely to crack the solidification interface as compared to large reduction with a roll as in the in-line reduction method, and it is possible to avoid negative segregation as much as possible. Although it has been clarified that it is possible and can improve to semi-macro segregation, internal cracking occurs if the reduction in the cast area of the large unsolidified layer is still insufficient, and even if the area of the small unsolidified layer is reduced. Since the effect cannot be obtained, it is the current situation to seek the optimum rolling reduction condition.

Therefore, the center segregation generated in the cast slab has not been dramatically improved, and the target value of the C concentration of the molten steel is lowered to decrease the concentrations of C, P and S in the segregated portion. It is the actual situation that P and S are set to less than 0.005%, and depending on the type of steel and the application, diffusion annealing or the like is performed at the stage of the slab, which is a substantial increase in cost.

The present invention advantageously solves the above-mentioned problems. Even when a continuous casting method is used, the formation of center segregation is reduced as much as possible, so that a drawing process with a large surface reduction rate is performed by heat treatment or The purpose is to propose a method that can be realized without the need for twice drawing.

[0010]

That is, this invention is
C: 0.20 to 0.8 wt% (hereinafter simply indicated as%), Si: 0.01 to
1.0% and Mn: 0.3-2.0%, Cr: 0.05-
1.0%, Ni: 0.05 to 1.0%, Mo: 0.1 to 0.5%, V: 0.0
5 to 1.0%, B: 0.0002 to 0.0030% and Ti: 0.002 to 0.0
Molten steel containing at least one of 50% and the balance of Fe and inevitable impurities is continuously cast. At that time, in the ladle near the crater end where the molten steel inside the slab completes solidification. The steel is subjected to a component thickening prevention treatment such that the ratio C / C ₀ of the C content (C) in the shaft portion of the cast slab to the C content (C ₀ ) of the molten steel is 0.8 to 1.1, and then hot-rolled into a steel bar. Then, the steel bar is subjected to a drawing process with a surface reduction ratio of 20% or more without heat treatment, which is a method for manufacturing a steel bar having excellent drawability.

[0011]

First, the reason why the composition of molten steel is limited to the above range in the present invention will be explained. C: 0.20-0.8% The C content is mainly determined by the product strength after drawing or the required strength (surface hardness, effective hardening depth for quenching) of the product after drawing and quenching, but the higher the C concentration, the more The drawability deteriorates, and if C exceeds 0.8%, the impact value decreases, which is not practical, so the upper limit is 0.8%. On the other hand, if C is less than 0.20%, sufficient workability can be secured, so 0.20% was made the lower limit.

Si: 0.01 to 1.0% Si requires at least 0.01% as a deoxidizing agent.
On the other hand, Si has the effect of increasing the activity of C, and especially when it exceeds 1.0%, the formation of a decarburized layer becomes prominent, which causes deterioration in hardenability and fatigue strength, so the upper limit was made 1.0%.

Mn: 0.3-2.0% Mn, like Si, acts not only as a deoxidizing agent but also fixes S which causes embrittlement of steel, and further improves hardenability to improve strength and ductility. It is also a useful ingredient for increasing the amount of ingredients, but if the content is less than 0.3%, its effect is poor.
On the other hand, if it exceeds 2.0%, not only the cost becomes high, but also the formation of micro martensite is promoted in the controlled cooling after hot rolling or the heat treatment process during processing, and the workability especially in cold is impaired. It was supposed to be contained in.

Cr: 0.05 to 1.0% Cr is an effective component for improving the hardenability when used after quenching, and does not lower the drawability, so the content of Cr should be 0.05% or more. To do. On the other hand, if it exceeds 1.0%, the hardness of the steel bar increases and the drawability also deteriorates, so 1.0% is made the upper limit.

Mo: 0.05 to 0.5% Mo is an effective component for improving the hardenability and does not increase the deformation resistance during cold working. Therefore, Mo is contained in an amount of 0.05% or more. However, it is an expensive component, and if it exceeds 0.5%, the deformation resistance also increases, so 0.5% is the upper limit.

Ni: 0.05 to 1.0% Generally, if the strength after quenching is increased, the ductility is reduced.
Is a component that contributes to the improvement of the strength after quenching and is effective in preventing the decrease in ductility, that is, if the content is 0.05% or more, the drawing is improved. On the other hand, if it exceeds 1.0%, the above effect is saturated, so 1.0% is made the upper limit.

V: 0.05 to 1.0% V improves the hardenability and forms carbonitrides to contribute to the strength improvement after drawing. 0.05 for this effect
%, It is necessary to contain 1.0% or more. On the other hand, if it exceeds 1.0%, the strength of the steel bar increases and impairs the drawability.
Is the upper limit.

B: 0.0002 to 0.0030% B is a component useful for improving the hardenability, but the content of B is 0.002.
If it is less than 0002%, its effect is poor, while 0.0030
Even if it exceeds%, the effect is saturated, and no further effect can be expected, so the content was made 0.0002 to 0.0030%.

Ti: 0.002 to 0.050% Ti is a strong deoxidizer, and at the same time, makes the crystal grain size fine,
It has the function of controlling hardenability. However, the content is 0.
If it is less than 002%, its effect is poor, while 0.050
If it exceeds%, the effect reaches saturation, so 0.002-0.
The content was set to be 050%. In addition to the above-mentioned components, Nb and Al may be added in necessary amounts for the purpose of increasing strength, improving hardenability, or refining crystal grains. In addition, steel bars that are generally drawn or quenched to give the final product a specified strength include other quality requirements (such as toughness and impact resistance during product use).
It is preferable to appropriately determine the addition amount within the above-mentioned component composition range.

In the present invention, in the continuous casting of molten steel having the above-described preferable composition, the ladle is subjected to a component concentration preventing treatment near the crater end where the molten steel inside the slab completes solidification. The ratio C / C ₀ of the C content (C) in the slab axial part to the C content (C ₀ ) of the medium molten steel is 0.
Control to 8 to 1.1. Forging treatment is particularly advantageous here as a component thickening prevention treatment, but the present invention is not limited to this, and the C / C ₀ ratio is 0.8.
Other means may be used as long as it can be controlled to ~ 1.1.

The reason why the C / C ₀ ratio can be controlled by the above forging process will be described below. That is, at the final stage of solidification of the internal molten steel, molten steel with a high concentration of C exists near the crater end, so if it solidifies as it is, central segregation will occur, but if forging processing is performed before solidification, As a result of the C-rich molten steel being extruded upward, the C concentration in the central portion does not rise so much. Therefore, the C content in the slab axial center can be adjusted by adjusting the timing of forging processing according to the degree of C enrichment.

Here, the upper limit of the C / C ₀ ratio is 1.1,
This is because if it exceeds 1.1, chevron cracks and fractures will occur. On the other hand, the lower limit of the C / C ₀ ratio is set to 0.8 because the tensile strength of the product is lowered and the final product is inconvenient to use. This is because the center of the slab has a negative segregation with a C / C ₀ ratio of less than 0.8, and the C and Mn contents necessary for strength assurance cannot be maintained.

Therefore, according to the present invention, C in the slab axial center portion to be controlled by the component concentration preventing treatment such as forging processing.
The / C ₀ ratio is limited to the range of 0.50 to 0.95. In addition,
As a preferred forging processing method, the inventors have previously described in JP-A-60
There is a continuous forging method disclosed in Japanese Patent Publication No. 82257.

[0024]

EXAMPLES Molten steel having the chemical composition shown in Table 1 (symbols A to
C) is continuously cast in a mold of 400 × 560 mm, and the C / C ₀ ratio of the slab axial center is near the crater end where the molten steel inside the slab completes the solidification with respect to the slab being drawn. 0.9-1.
With the goal of 05, forging was continuously applied to produce bloom. Then, it was hot rolled into a 150 × 150 mm billet by a billet mill. 26 and 32 mmφ with a steel bar mill
Hot rolled into steel bars. After that, this steel bar was subjected to 22 (hexagonal bar diameter) and
A 27 mmφ steel bar was used.

[0025]

[Table 1]

Further, as in the conventional process, after continuous casting, a bar steel was similarly processed without forging, and a comparative example was obtained. The molten steel heating degree at the time of tapping is cast in the range of 27 to 30 ° C., and the hot rolling temperature from slab rolling to bar rolling should be the same temperature history in both Examples and Comparative Examples of the present invention. did. The steel bar thus obtained was examined by an ultrasonic flaw detection method for the presence or absence of chevron cracks and the product reduction. As shown in the results of Table 2, no chevron cracks were generated in the steel bar obtained according to the present invention.

[0027]

[Table 2]

[0028]

As described above, according to the present invention, the component concentration preventing treatment is continuously applied during the continuous casting, and C / C _{0 of the} slab axial center portion is obtained.
By controlling the ratio, it is possible to prevent the generation of chevron cracks, and therefore it is possible to realize a drawing process with a large surface reduction rate without requiring heat treatment.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display location C22C 38/54

Claims (1)

[Claims] 1. C: 0.20 to 0.8 wt%, Si: 0.01 to 1.0 wt% and Mn: 0.3 to 2.0 wt%, further Cr: 0.05 to 1.0 wt%, Ni: 0.05 to 1.0 wt%, Mo: At least one of 0.05 to 0.5 wt%, V: 0.05 to 1.0 wt%, B: 0.0002 to 0.0030 wt%, and Ti: 0.002 to 0.050 wt% is contained, and the balance is composed of Fe and inevitable impurities. Continuous casting of molten steel
In the vicinity of the crater end where the molten steel inside the slab completes the solidification, the ratio C / C ₀ of the C content (C) in the axial portion of the slab to the C content (C ₀ ) of the molten steel in the ladle is 0.8 to 1.1. It is characterized in that it is subjected to a component thickening prevention treatment and then hot-rolled into a steel bar, and then this steel bar is subjected to one drawing process with a surface reduction rate of 20% or more without heat treatment. A method for manufacturing a steel bar with excellent drawability.