JPH0284237A - Production of hot rolling stock for high carbon steel wire rod having high workability - Google Patents

Abstract

PURPOSE:To reduce growth of center segregation as less as possible and to prevent breaking of a wire at the time of the wire drawing work by continuously giving squeezing work at the time of continuously casting and controlling the ratio of C content at center part of a cast billet to C content in a ladle. CONSTITUTION:Molten steel having component composition of 0.4-1.0wt.% C. 0.1-1.5wt.% Si, 0.3-2.0% Mn and the balance Fe with inevitable impurities is continuously cast. Then, at solidified end part of molten steel in inner part of the cast billet, as the molten steel, which concentration of C progresses, exists near crater end and if it solidifies as it is, the center segregation is grown. In order to prevent the growth of the center segregation, the squeezing work is executed near the crater end, where the molten metal in the inner part of the cast billet completes the solidification. Then, the ratio C/Co of the C content (C) at the center part in the cast billet to the C content (Co) in molten steel in the ladle is controlled to 0.80-1.05. Therefore, C-concentrated molten steel is pushed out upward and concentration of C at center part is not risen so much. In this result, the ductility after the wire drawing work is improved and the breaking of the wire does not occur and the drawing work to high working ratio can be executed.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention provides a high carbon nail with high workability and suitable as a material for high strength, high ductility steel wire such as wire rope, PCi wire, bead wire and tire cord. The present invention relates to a method for producing a hot-rolled material for one wire rod.

(Prior art) Piano wire, tire cord I, etc., which are high carbon hard steel wires, are finished to a predetermined wire diameter by subjecting the hot-rolled material to wire drawing and heat treatment. It has high strength that satisfies standards such as At the same time, it is necessary to maintain the ductility, such as the reduction of area and torsion value, at appropriate values depending on the various uses.

With regard to tire cords, in response to the demand for lighter automobiles, efforts are being made to increase the tensile strength to over 350 kgf/mm2, but in order to do so, it is necessary that there are no defects inside the wire material. This is important.In particular, center segregation found in the center of the wire may lead to wire breakage during wire drawing.
This leads to a decrease in productivity and yield. Moreover, even if the wire breaks, micro cracks will be generated during processing, which will deteriorate the ductility and toughness after processing and impair the quality characteristics of the final product.

In the case of slabs obtained by continuous casting, center segregation, which causes such adverse effects, is caused by solidification shrinkage, especially at the solidified tip, as well as the vacuum absorption force of voids created by bulging of the solidified shell, etc. As a result, concentrated molten steel components such as c, p, and s are sucked into the slab, resulting in positive segregation remaining at the center of the cross section of the slab. It is thought that the wire drawability deteriorates as a result of the precipitation of grain boundary cementite, the formation of micromartensite, and the non-uniformity of the microstructure.

As a measure to prevent such center segregation, attempts have been made to, for example, use electromagnetic stirring in the secondary cooling zone, but this has not been able to reduce even semi-micro segregation, and the effect cannot be said to be sufficient.

In addition, the so-called in-line reduction method (iron and mesh tube 60
(1974) No. 7, pp. 875-884), but with this method, if the reduction in the area of the slab with a large unsolidified layer was insufficient, cracks would occur at the solidification interface, and the opposite effect would occur. If the rolling reduction is sufficient, there are problems such as strong negative segregation occurring at the center of the thickness of the slab.

In addition, JP-A No. 49-121738 discloses a method in which light reduction is performed by a pair of rolls near the tip of a slab to compensate for the amount of solidification shrinkage in this area by reduction. Japanese Patent Publication No. 54623 proposes a method of greatly reducing the area near the point where the separate placement of the slab is completed using a casting mold.

However, in the case of light reduction by rolls, even if a reduction of several mm/m is applied by multiple pairs of rolls, solidification shrinkage and bulging that occur between the roll pitches cannot be sufficiently prevented. However, if the rolling position was not appropriate, there was a disadvantage that center segregation would worsen. On the other hand, the method of applying a large reduction near the solidification completion point of the slab using a forging die is less prone to cracking at the solidification interface than the in-line reduction method, which uses large reductions using rolls, and also avoids negative segregation as much as possible. Although it has been shown that it is possible to improve semi-macro segregation, if the reduction in the area of the slab with a large unsolidified layer is insufficient, cracks will occur at the solidification interface, and conversely, if the reduction is too large, cracks will occur at the solidification interface. The disadvantage is that strong negative segregation occurs in the center of the piece, and furthermore, the effect cannot be obtained even if the small area of the unsealed solid layer is rolled down, so the current situation is to find the optimal rolling conditions. be.

Therefore, it has not yet been possible to dramatically improve the center segregation that occurs in slabs, and the reality is that depending on the steel type and application, diffusion annealing is applied at the slab stage to deal with it.
It also becomes a significant cost amplifier.

(Problems to be Solved by the Invention) The present invention advantageously solves the above problems, and even when using a continuous casting method, the generation of center segregation is reduced as much as possible, thereby making it possible to The purpose of this invention is to propose an advantageous manufacturing method for hot-rolled material for high-carbon steel wire with high workability, which makes it possible to prevent wire breakage during processing.

(Means for Solving the Problems) That is, the present invention provides C: 0.4 to 1. ．． 0wt% (hereinafter simply indicated as %), S
Molten steel containing i: 0.1 to 1.5% and Mn: 0.3 to 2.0%, with the remainder consisting of Fe and unavoidable impurities, is continuously cast, and at this time, the molten steel inside the slab solidifies. Near the crater end where the process is completed, the C content of the ladle medium-temperature steel (
It consists of forging so that the ratio of C content (C) in the axial center of the slab to C0), C/c, is 0.8 to 1.05, and then hot rolling to form a wire rod. Method for producing hot-rolled material for high-carbon steel wire with high workability (first invention)
It is.

Further, in the present invention, the composition of the molten steel includes: C: 0.4 to 1.0%, Si: 0.1 to 1.5%, and Mn: 0.3 to 2.0%, and V: 0. .05 to 1.0%, Cr: 0.05 to 1.0%, and Ni: 0.05 to 1.0%, in a range in which the total amount does not exceed 1.0%. However, the remainder is a composition of Fe and unavoidable impurities, which is a method for producing a hot-rolled material for high carbon steel wire rod with high workability (second invention).

Furthermore, in the present invention, the composition of the molten steel is as follows: C: 0.4 to 1.0%, Si: 0.1 to 1.5%, Mn: 0.3 to 2.0%, Cr: 0.05 to 0.30% and S: 0.0
08% or less, and the remainder is Fe and unavoidable impurities. Method for producing a hot-rolled material for high-carbon steel wire rod with high workability (Part 3)
invention).

(Function) First, the reason why the composition of molten steel is limited to the above range in the present invention will be explained.

C: 0.4-1.0% It is necessary to compensate for the strength after processing for applications such as hard steel wire, PC steel wire, bead wire, and tire cord, and the amount of C is 0.
．． The lower limit was set at 4%. The higher the C content, the higher the strength of the steel wire, but on the other hand, increasing the C content makes the material brittle and increases the frequency of wire breakage during processing. In particular, if it exceeds 1.0%, mesh-like cementite will precipitate at the prior austenite grain boundaries during controlled cooling after wire rolling, which will greatly impede subsequent wire drawability, so the upper limit is set at 1.0%. Ta. In particular, for high-strength, high-workability materials such as tire cord, 0.6
Approximately 5 to 0.85% is suitable.

Si: 0.1-1.5% Si is an element useful not only as a deoxidizing agent but also for strengthening the matrix, and requires at least 0.1%.

Figure 1 shows the tensile strength (T, S,) and torsion value of 0.7%C-0.7%MrHj21 subjected to patenting treatment with various Sil changes. The tensile strength improves as the amount of Si increases.

On the other hand, Si has the effect of increasing the activity of C, especially 1.5
If the content exceeds %, the formation of a decarburized layer becomes noticeable, leading to a rapid decrease in the twist value. For this reason, the Si content was set in the range of 0.1 to 1.5%.

By increasing the strength of the patenting, it is possible to obtain high strength at the final wire diameter without increasing the degree of wire drawing.
Decrease in ductility is reduced. Therefore, in order to ensure high strength and sufficient ductility, it is particularly preferable to set the content to about 0.6 to 1.0%.

Mn: 0.3-2.0% Like Si, Mn not only acts as a deoxidizing agent, but also fixes S, which causes embrittlement of steel, and also improves hardenability, increasing strength and Although it is a useful element for increasing ductility, if the content is less than 0.3%, its addition effect will be poor, while if it exceeds 2.0%, it will not only be expensive but also require controlled cooling after hot rolling or Since it promotes the formation of micromartensite in the heat treatment step during processing and impairs wire drawability, it is added in a range of 0.3 to 2.0%.

V: 0.05-1.0% (2) is an element that improves hardenability and at the same time is a carbonitride-forming element, and effectively contributes to improving strength.

To be effective, at least 0.05%
Requires the addition of However, on the other hand, V has the effect of significantly improving hardenability, so if it is added in too large a quantity, a pearlite structure will not be obtained during patenting during wire drawing, and a martinsite or haynite structure will occur, resulting in the formation of a martinite or haynite structure during subsequent processing. Therefore, the upper limit was set at 1.0%.

Figure 2 shows 5WRH72A with different vl, 5.5mm.
φ indicates the strength of rolled wire.

As is clear from the figure, good improvements in T and S can be seen in the range where vl is 0.05% or more.

However, since the melting temperature of carbonitride is as low as about 950°C, when intermediate patenting is performed during wire drawing, the precipitation strengthening effect cannot be expected for long. Therefore, it is particularly effective for improving the strength of directly drawn materials that do not require heat treatment.

Cr: 0.05-1.0% Cr lowers the transformation point somewhat and reduces the pearlite lamella spacing, so it increases the strength after hot rolling or heat treatment, and also increases work hardening, resulting in low Since high strength can be obtained with a high degree of processing, there is an advantage that the decrease in ductility is small, and in order to make the most of this effect,
Requires addition of at least 0.05%.

However, like Cr, Cr is an element that significantly improves hardenability, so if it is added in too large a quantity, a pearlite structure will not be obtained during patenting during the wire drawing process, and martensite and haynite structures will occur. Therefore, the upper limit was set at 1.0% (preferably 0.3%).

Ni: 0.05 to 1.0% In the case of the steel of the present invention, central segregation is significantly improved, so it is easy to improve the strength by increasing the amount of C, for example, but in this case there is a concern about aging embrittlement. , Ni is added for the purpose of preventing ductility deterioration of the matrix.

Figure 3 shows the strength (T, S,) and reduction of area (R, A,
) shows the results of an investigation into the effect of Ni on

As shown in the figure, this is effective in improving not only strength but also aperture.

Therefore, in the present invention, 0.05% or more of Ni is added. However, since adding too much increases manufacturing costs, which is undesirable, the upper limit was set at 1.0%.

By the way, the total amount of V, Cr and old mentioned above is 1.0
If the total amount of these elements exceeds 1.0%, the hardenability will improve too much, making it difficult to obtain a pearlite structure, making it difficult to process after rolling or heat treatment, and also being expensive. It is important to add it within a range that is not exceeded.

S: 0.008% or less S is known as a harmful element that deteriorates the ductility of steel. In particular, when the steel is processed into ultra-fine wires such as queuer cord to achieve high tensile strength, it is essential to improve the ductility, so consideration must also be given to impurity elements in the steel.

Figure 4 shows the torsion values after patenting with 3 cylinders φ and wire drawing to 0.4 mmφ using 5WR872A with varying S content and various MnS amounts in the steel. Showing the survey results.

As shown in the figure, the torsion value improves as the amount of MnS decreases, and this effect is particularly significant when the area ratio of MnS is 0.10% or less.

Therefore, the inventors investigated the S content at which the area ratio of MnS is 0.10% or less, and found that it should be 0.008% or less.

Now, in the present invention, during continuous casting of molten steel having a preferable composition as described above, the C content of the molten steel in the ladle is reduced by performing forging near the crater end where the internal molten steel of the front piece i completes solidification. The ratio C/CO of C-containing ff1(C) at the axial center of the slab to 1(Go) is 0.80~
ff+lJ to 1.05? Wholesale.

The reason why the C/Co ratio can be controlled by forging is as follows.

In other words, at the final stage of solidification of the internal molten steel, molten steel with advanced C concentration exists near the crater end, so if it solidifies as it is, it will become centrally segregated, but if forging is performed before solidification, this will occur. As a result of the C-enriched molten copper being pushed upward, the C concentration in the center does not increase much. Therefore, by adjusting the timing of forging according to the degree of enrichment of C, the C content in the axial center of the slab can be adjusted.

Figure 5 shows the C/CO of the axial center of the slab from the slabs obtained by continuous forging according to the present invention using 5WRII 82B, and from the conventional method without forging. The results of an investigation into the occurrence of wire breakage when steel materials with various ratios were collected, rolled into wire rods, and then drawn under wire drawing conditions (die approach angle -25°) where cuppy wire breakage is likely to occur are shown.

As shown in the figure, when the C/C ratio is less than 0.8 and the negative segregation rate is large, and conversely when the C/CO ratio is more than 1.1 and the positive segregation rate is large, the cuppy A disconnection occurred. The reason for this is thought to be the non-uniformity of the pearlite structure at the center of the cross section of the wire or the precipitation of grain boundary cementite.

Therefore, in the present invention, the C/C ratio at the axial center of the slab to be controlled by forging is limited to a range of 0.80 to 1.05.

In addition, as a preferable forging method, there is a continuous forging method previously disclosed by the present inventors in JP-A-60-82257.

Next, by the forging method according to the method of the present invention described above, C/C
When the slabs with a ratio of 0.80 to 1.05 were hot rolled into wire rods, the maximum hardness in the center 0.5 mm2 of the cross section of these rolled wire rods and the average hardness of the matrix were The ratio was below 1.1.

On the other hand, when wire drawing was performed on wires with various hardness ratios between the center and the matrix, the degree of internal crank generation detected during wire drawing was as shown in Figure 6. It has been found that when it exceeds 1.1, it rapidly decreases and internal cranking occurs early.

(Example) Molten steel (symbols A-L) having the chemical composition shown in Table 1 was
C/C8 ratio of the axial center of the slab: 0.95 near the crater end where the molten steel inside the slab completes solidification for the slab that is continuously cast in a 0x340mm mold and is being drawn.
With the goal of continuous forging processing, the C/C ratio is 0.
．． Bloom was produced by controlling it within the range of 88 to 1.03. Then, 150x150m by blooming and billet mill
It was hot rolled into a fillet size of m. Furthermore, in the wire mill 5
, 5 mmφ or 11 mmφ, the former was rolled up at 850°C, while the latter was rolled up at 900°C, and cooled under control in a Stelmore line.

In Table 1, 0.02% or less of Cr, 0.003% or less of ■, and 0.02% or less of Ni are all mixed as unavoidable impurities.

On the other hand, the comparative material was subjected to the same wire rod rolling without forging after continuous casting according to the conventional process.

For symbols C, K, and L, the heating temperature of the molten steel at the time of tapping was controlled to about 20°C, but all the steels were cast in the range of 27 to 30°C. In addition, consideration was given to the hot rolling temperature from blooming to wire rod rolling so that the temperature history was the same for both the inventive steel and the comparative material.

Table 2 shows the results of an investigation into the properties of these rolled wire rods.

Here, the length test piece is shown as the average value of a total of 50 rings that were continuously sampled from three rings taken from both ends and the center of the rolled coil. For other characteristics,
Approximately 200 kg of the tensile specimen collection coil was divided into 10 parts, and the cross section thereof was investigated.The results are shown below. Note that the center spot was expressed as presence or absence regardless of its degree.

As is clear from the same table, in the steel of the present invention, no center spot indicating center segregation was observed, and there was no precipitation of micromartensite or grain boundary cementite.

In contrast, in all comparative materials, center spots were observed, and micromartensite and grain boundary cementite were also detected in some parts.

Such a difference is manifested in the area of area, which is one of the indicators of wire drawability, and the steel of the present invention clearly shows a higher value in any of the steels.

Next, regarding rolled materials with symbols B, C, and J with high C content,
Wire drawability was examined by the Cubbie fracture acceleration method (dice approach angle 25°) using a draw hench.

The results are shown in Table 3.

The results are shown in Table 3, and it can be seen that the wire drawability of the steel of the present invention is clearly superior for all steel types, and that the steel of the present invention can be further drawn even under severe conditions.

FIG. 7 shows the relationship between the degree of wire drawing and the reduction of area for the steel type designated by symbol B, and it is clear that the ductility of the steel of the present invention is higher even after processing.

Next, since the customer draws the wire from the wire to the intermediate wire, 5.
Continuous wire drawing was carried out under normal wire drawing conditions in 9 passes from 5 mm φ to 2.0 mm φ, and the material properties were investigated.

The results are shown in Table 4.

In addition, 30 tensile specimens and 30 torsion specimens were consecutively collected, and the average value is shown.

As is clear from the results shown in Table 4, the ductility of the steel of the present invention is much superior to that of conventional steel, which proves that it has good wire drawability. The effects of Si, Mn and ■ on strength and aperture are also fully demonstrated.

Furthermore, regarding symbols K and L, from 11 valent φ to 4.3 cylinder φ
4.3m by continuous wire drawing under normal wire drawing conditions with 9 buses.
The material of the mφ was similarly investigated.

Since the image quality is high in both C and Mn, and wire breakage or internal cranking is likely to occur, the Cambie fracture surface ratio (the percentage of the fracture surface having a cuppy shape after the tensile test) was also investigated.

Table 5 shows that the cuppy fracture surface ratio of about 5% was observed in the flat conventional steels K and L, which were tested in 50 consecutive tensile tests, and the reduction of area was also low. The reason for this is presumed to be that minute internal cranks were generated during machining. Furthermore, some disconnections were observed in L.

On the other hand, the steel of the present invention, which had improved center segregation, had no Cubby fracture ratio or wire breakage, and had good drawing performance. The effect of Ni on the aperture was also clearly recognized.

Example 2 Molten steel (symbol M-R) having the chemical composition shown in Table 6 was
C/c of the center part of the slab in the vicinity of the crater end where the molten copper inside the slab completes solidification for the slab that is continuously cast in a 0 x 340mm mold and is being drawn.

Ratio: Continuously subjected to forging processing with the aim of achieving a ratio of 0.95,
Bloom was produced by controlling the C/C ratio within a range of 0.88 to 1.03. Then 1 by blooming and billet mill
Hot rolled into a 50 x 150 mm billet. Further, after hot rolling to 5.5mmφ in a wire mill, 850°C
It was wound up and cooled in a controlled manner using a Stelmore line.

On the other hand, the comparative material was subjected to the same wire rod rolling without forging after continuous casting according to the conventional process.

In addition, consideration was given to the hot rolling temperature from blooming to wire rod rolling so that the temperature history was the same for both the inventive steel and the comparative material.

Table 7 shows the results of an investigation of the strength properties of these rolled wire rods.

As is clear from the table, center spots were observed in all comparative materials, and micromartensite and grain boundary cementite were also detected in some parts.
In the steel of the present invention, no center spot indicating center segregation was observed, and no micromartensite or grain boundary cementite was precipitated.

Next, the rolled materials with symbols N and Q were examined using the Cubby fracture acceleration method (dice approach angle 25°) using a draw hench.
) was used to examine wire drawability.

The results are shown in Table 8.

The results are shown in Table 8, and it can be seen that the wire drawability of the steel of the present invention is clearly superior for all steel types, and that the steel of the present invention can be further drawn even under severe conditions.

Furthermore, wire drawing and heat treatment are repeated from the hot-rolled material (5.5 mmφ) to reduce the wire diameter to 1.65 mmφ before finishing wire drawing.
The final patenting conditions here are heating temperature: 9
After setting the lead bath temperature at 30°C and 565°C, the treatment was continued. Thereafter, the wire was subjected to a plus plating process and then subjected to final wire drawing to a finished diameter of 0.25 mmφ.

Table 9 shows the results of examining the tensile strength, reduction of area, and torsion value of the steel wire thus obtained.

As is clear from Table 1, the steel of the present invention has higher strength than conventional steel.Although it has high strength, the drawing and twist values are the same or higher, and it can be used as an ultra-fine wire. This sufficiently satisfies the characteristics of Particularly good values have been obtained in cases where a suitable amount of Cr is added and S contamination is suppressed.

(Effects of the Invention) Thus, according to the present invention, by continuously applying forging during continuous casting and controlling the C/C of the axial center of the slab, the ductility, especially after wire drawing, can be significantly improved. , it can be processed to a high degree of processing without causing wire breakage like conventional steel. This also leads to cost reduction by omitting heat treatment,
In addition, it not only becomes possible to eliminate the conventional restrictions on seeds for reducing center segregation, but it also greatly contributes to stabilizing quality.

By adding V, Cr or Ni, the strength and ductility of the steel wire can be increased, and by applying the present invention, it is possible to produce high carbon steel wires etc. that have even better strength and ductility. .

[Brief explanation of the drawing]

Figure 1 is a graph showing the relationship between Si content and strength and twist value of patented material. Figure 2 is a graph showing the relationship between Vl and strength of rolled wire rod. Fig. 4 is a graph showing the relationship between the area ratio of sulfide inclusions and twist value, Fig. Figure 6 is a graph showing the relationship between the C/C ratio and the number of cuppy wire breaks. FIG. 7 is a graph showing the relationship between the degree of processing and the reduction of area after processing under special wire drawing conditions. Fig. 1 Fig. 3 05 f, 0 57 f ((utt%) Fig. 2 Amount of N1 (vt%) Fig. 4 V f (utt7.> 1 Area of chemotactic system intervention g (%) Figure 5: C/Ca of Itsuji, Figure 6: Ratio of l-lix, I-sharaku, in Nakanomi fishing Figure 7: f, 0 15 2.0 Processing forgery: 21nDo/D 2.5

Claims (1)

[Claims] 1. Contains C: 0.4 to 1.0 wt%, Si: 0.1 to 1.5 wt%, and Mn: 0.3 to 2.0 wt%, the remainder being Fe and inevitable impurities. Molten steel with a composition of
It is characterized by forging so that the ratio C/C_0 of the C content (C) at the axial center of the slab to C_0) is 0.8 to 1.05, and then hot rolling to form a wire rod. A method for producing hot-rolled material for high-carbon steel wire with high workability. 2. The composition of the molten steel includes C: 0.4 to 1.0 wt%, Si: 0.1 to 1.5 wt%, and Mn: 0.3 to 2.0 wt%, and V: 0.05 to 1.0 wt%. The total amount of at least one selected from 1.0 wt%, Cr: 0.05 to 1.0 wt%, and Ni: 0.05 to 1.0 wt% is 1.0 wt%.
2. The manufacturing method according to claim 1, wherein the content is within a range not exceeding %, and the remainder is composed of Fe and unavoidable impurities. 3. The composition of the molten steel is: C: 0.4 to 1.0 wt%, Si: 0.1 to 1.5 wt%, Mn: 0.3 to 2.0 wt%, Cr: 0.05 to 0.30 wt% % and S: 0.008 wt % or less, with the remainder being Fe and unavoidable impurities.