JPH04311523A - Production of high carbon steel wire rod having high strength the excellent in wire drawability - Google Patents

Abstract

PURPOSE:To offer high strength wire rod excellent in high strength particularly by continuous casting with regard to the method for manufacturing a high strength steel wire such as a wire rope, a PC steel wire, a spring and a steel code. CONSTITUTION:A cast slab contg. 0.80 to 1.1% C, 0.1 to 1.0% Si, 0.3 to 1.0% Mn, <=0.003$ Al, 0.1 to 0.3% Cr and the balance Fe with inevitable impurities is held to the temp. range of 1250 to 1300 deg.C for 3 to 18hr, is subjected to blooming rolling into a slab, and this slab is thereafter heated in the range of 1050 to 1100 deg.C slab heating temp., is thereafter rolled, is subsequently coiled at 800 to 930 deg.C and is thereafter cooled at 10 to 40 deg.C/sec cooling rate.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention relates to a method for producing high-strength steel wire rods used in the production of high-strength steel wires such as wire ropes, prestressed steel wires, springs, and steel cords. This invention relates to a method for manufacturing high-strength steel wire.

0002

[Prior art] High carbon steel wire rods are subject to various patenting,
That is, after direct patenting using rolling heat, lead patenting, and air patenting, cold working such as wire drawing is performed, and then blueing or quenching and tempering processes are performed to create wire, rope, PC steel wire, spring,
Used for manufacturing high-strength steel wire such as steel cord.

[0003] As a means of strengthening high carbon steel wire rods, the
As disclosed in Japanese Patent Publication No. 5-460 and Japanese Patent Publication No. 55-9044, methods are generally carried out to refine the lamella spacing of pearlite by adding alloying elements, or to solid solution strengthen the ferrite that constitutes pearlite. It is being said. On the other hand, as a means to improve wire drawability, as shown in Japanese Patent Publication No. 47-51684 and Japanese Patent Application Laid-open No. 52-12611, it is possible to improve wire drawability by finely precipitating carbides or nitrides during patenting. Refining austenite grains is widely practiced.

[0004] In recent years, as the manufacturing process for wire rods has changed from the steel ingot method to the continuous casting method, in addition to the above-mentioned microstructural factors, the center segregation of the wire rods has become a dominant influence on the wire drawability of high carbon steel wire rods. It is becoming clear that For this reason, for example, it is necessary to qualitatively evaluate center segregation based on a macro-etched image or sulfur print of the cross section of the wire, or to measure the difference in hardness between the center of the wire and 1/2 of the radius. Alternatively, you can measure the chemical composition difference between the center and 1/4 of the diagonal length of the billet, or evaluate center segregation using either method, and improve the casting conditions based on the results. In the inspection process, high-strength wire rods that can withstand advanced processing were selected.

[0005]

[Problems with the prior art] The problem with the conventional technology is that the method and criteria for evaluating the center segregation of wire rods are ambiguous, resulting in wire breakage during wire drawing due to early destruction of the center segregation area, or wire breakage in the final product. There are many quality accidents such as insufficient ductility, and for this reason,
This not only caused problems in process control and decreased yield, but also made it difficult to develop necessary and sufficient casting technology to improve center segregation.

Furthermore, because the evaluation method and evaluation criteria for center segregation are empirically based on specific manufacturing equipment or manufacturing processes, technology transfer may be difficult if the wire rod product process or manufacturing equipment is different. was difficult, and the technology had not become universal.

[0007]

[Problems to be Solved by the Invention] In view of the above-mentioned circumstances, an object of the present invention is to provide a method for producing high-carbon, high-strength steel wire rods with high yield, with little deterioration in workability caused by center segregation. .

[0008]

[Means for solving the problem] That is, in terms of weight ratio, C:
0.80-1.1%, Si: 0.1-1.0%, Mn:
0.3 to 0.8%, Al: 0.003% or less, Cr: 0
．． Using a slab containing 1 to 0.3% Fe and the remainder consisting of Fe and unavoidable impurities, the slab is kept at a temperature range of 1250 to 1300°C for 3 to 18 hours and then processed into a steel slab by blooming rolling. After heating the steel billet at a temperature of 1050 to 1100°C, the rolling temperature after rolling was set to 800°C to 93°C.
By cooling at 0°C and then cooling at a cooling rate of 10 to 40°C/sec, the wire rod has high strength and excellent wire drawability, with no formation of pro-eutectoid cementite in the rolled state. This is a method for producing high carbon steel wire rods.

In order to eliminate the ambiguity of the conventional center segregation evaluation criteria, the present inventors abandoned qualitative criteria as much as possible and made efforts to quantify the degree of center segregation, and as a result, the center segregation part of the wire rod As a result of performing EPMA analysis on and studying the maximum height of the segregation peak profile and the width of the segregation zone, it was found that the segregation morphology appears in response to changes in various manufacturing processes. For example, electromagnetic stirring inside the mold,
It is now possible to clearly determine the relationship between manufacturing factors that basically have some influence on center segregation, such as the light reduction of slabs and the influence of heating furnace temperature.

[0010] Based on these basic study results, in order to prevent the formation of pro-eutectoid cementite in the central segregation area, which has been a major obstacle in the production of high carbon steel wire rods, in hot-rolled wire rods. is the C of the center segregation area that forms in the slab.
It has been found that it is effective to reduce the concentration to 1.1% or less. Further, in the case of high carbon containing a large amount of C as in the case of the present invention, the degree of segregation is usually 1.3 to 1.6 times the average composition. Therefore, as mentioned above, the C concentration is 1.1%.
It can be seen that the larger the amount of C, the more difficult it becomes to suppress the amount below. Based on the current casting technology, the present inventors concluded that it is essential to use C diffusion by heating the slab in order to reduce the center segregation at the slab stage to below this target value. . The reasons for the limitations of the present invention will be described below.

[0011]

[Operation] The reasons for the limitations of the present invention will be explained below. C
is set at 0.8% or more to increase the strength of steel, but 1.1
%, grain boundary cementite will occur and wire drawability will drop sharply, so the upper limit was set at 1.1%. Si is added in an amount of 0.1% or more to deoxidize the steel and soften nonmetallic inclusions. In addition, since Si has a strong solid solution strengthening effect on ferrite that constitutes pearlite, it is preferable to add a large amount of Si.
%, surface decarburization increases, so the upper limit was set at 1%. Mn acts as a deoxidizing element for steel, and also has the effect of improving hardenability and producing uniform pearlite within the cross section of the wire. Therefore, Mn is added in an amount of 0.3% or more, but if it is added in excess, it will cause micromarten in the center segregation area. It will be easier to generate a site,
The upper limit was set at 0.8%.

[0012] Al is normally used as a deoxidizing element, but in the case of the present invention, it is not necessary because deoxidation is carried out with Si and Mn. Rather, hard nonmetallic inclusions generated by Al tend to cause wire breakage during wire drawing. This tendency is particularly noticeable when the diameter is small. Therefore, 0.003%
The following was made. In order to satisfy this requirement, it is necessary to eliminate entry routes from refractories, slag, etc. during the manufacturing stage.

Next, Cr will be described. When the amount of C is large as in the present invention, problems such as pro-eutectoid cementite generated at austenite grain boundaries cause wire breakage during cold working stages such as wire drawing. Cr has the effect of suppressing the appearance of such abnormal parts of cementite and further making pearlite finer. However, addition of a large amount increases the dislocation density in the ferrite after heat treatment such as patenting, which significantly impairs the ductility of the wire after wire drawing. Therefore, the lower limit of Cr at which the above effect appears is 0.1%, and the upper limit is 0.1% from the viewpoint of wire ductility.
It was set at 3%.

Next, the heating temperature of the slab will be described. In order to systematically understand the effect of improving center segregation due to C diffusion in the center segregation area of the slab, the present inventors conducted EPMA analysis of the center segregation of the as-cast slab, and determined that the maximum segregation degree (C concentration in the enriched area/ The average C concentration) was found to be approximately 1.6. In the present invention, in order to reduce such concentrated C by diffusion, it is necessary to effectively use both heat sources of slab heating and wire heating. In recent years, with the advancement of CC, a method of directly connecting slabs to rolling is being adopted, but in the case of high carbon steel with an increased amount of C as in the present invention, it is difficult to prevent the formation of pro-eutectoid cementite with this method. It has been found. According to the present inventors, it has been found that it is effective to reduce the C concentration in the center segregation part to at least 1.1% at the slab stage, and then reduce it to 1.05% by heating the wire thereafter. In any case, in order to satisfy these requirements, CC-DR (a process that directly connects continuous casting and rolling) cannot satisfy the above target values.

[0015] The lower limit of the heating temperature was set at 1250°C because
This is because below this temperature, the time required for diffusion becomes enormous. In the case of the present invention, it is desirable that the heating temperature is higher;
Temperatures exceeding 300°C will reduce the lifespan of the refractories and cause significant welding and decarburization on the surface of the slab, making it difficult to manufacture from the viewpoints of economy and surface quality, so the upper limit was set at 1300°C.

The reason for limiting the heating time varies depending on the grain size of the target central segregation, but 3 hours is the minimum time necessary for the C diffusion effect to satisfy the target value. From the viewpoint of C diffusion, the longer the time, the more advantageous it is, but heating for more than 18 hours saturates the effect from the viewpoint of preventing pro-eutectoid cementite, and leads to waste of thermal energy.

The lower limit of the heating temperature of the steel billet is set at 1050° C. because below this temperature, the time required for diffusion becomes longer, which significantly impairs productivity. The upper limit was set at 1100°C because surface decarburization would be large. The winding temperature after rolling is 8
The temperature was set at 00°C because mechanical descaling properties would be significantly impaired at temperatures below this temperature. The reason why the upper limit was set at 930℃ is that at temperatures higher than this, the thickness of the scale that forms increases, so during the cooling stage, the difference in thermal contraction between the base steel and the scale causes cracks in the scale, and oxygen is supplied through these cracks. As a result, thin magnetite is formed at the interface between the base iron and the scale. This is because magnetite remains on the surface of the steel wire after mechanical descaling, resulting in poor lubrication during wire drawing. The reason why the lower limit of the cooling rate after wire rod rolling is set to 10° C./sec is that if the cooling rate is lower than this, pro-eutectoid cementite will be formed after wire rod rolling. The reason why the upper limit is set to 40° C./sec is that if the temperature exceeds this value, the formation of martensite becomes noticeable, leading to wire breakage during wire drawing.

[0018]

[Example] Table 1 shows examples. Levels 1 to 4 shown in Table 1
is the method of the present invention, and levels 5 to 8 indicate those in which the slab and wire heating method is a conventional process. Further, level 9 indicates a method in which the amount of C exceeds that of the method of the present invention, and level 10 indicates a comparative method in which the slab heating time is shorter than that of level 2.

[0019] In the method of the present invention, all the products are melted in a 250-ton converter, and after being continuously cast into slabs of 300 x 500 mm, the slabs are charged into a soaking furnace and subjected to C diffusion treatment. Segregation was reduced and the steel billet was heated to obtain a 5.5 mm wire rod. On the other hand, in the conventional method, after passing through a recuperation furnace after continuous casting, wire rod rolling was performed to obtain a 5.5 mm wire rod. The evaluation method is briefly described below.

[0020]

[Table 1]

[0021] The mechanical properties of the wire rod were tested in accordance with the provisions of JIS Z2241. The C concentration in the central segregation area was determined by performing line analysis using EPMA in a direction across the central segregation area of the wire, and measuring the value at the maximum concentration area of the central segregation area. A special die having an approach angle of 30° was used to evaluate the wire drawability. Using this method, a strong tensile stress was applied to the center segregated area, and the harmful effects of pro-eutectoid cementite generated in the center segregated area were promoted and evaluated. The wire drawing limit was determined by breaking the wire during wire drawing using the aforementioned 30° die, and determining the true strain from the wire diameter of the steel wire on the side entering the die at the time the wire breakage occurred. This is defined as the limit strain that allows wire drawing, and is shown in (1).

[0022] Limit strain for wire drawing = 2 ln (d0/d1) ......
(1) d0 = Initial drawing wire diameter d1 = Indicates the diameter of the steel wire on the entry side of the broken die. The larger this value is, the better the wire drawability is. In addition, since the present invention aims to supply a wire rod as hot-rolled without the formation of pro-eutectoid cementite, the lead patenting (LP) treatment before wire drawing is omitted and the wire rod as hot-rolled is pickled. Afterwards, it was subjected to Bonde-Bonderlueube treatment and subjected to wire drawing.

As can be seen from the results in Table 1, in the case of the steel of the present invention, despite the high C content, it can be seen from the microstructural observation that no pro-eutectoid cementite is observed in the segregated area at the center of the wire. On the other hand, the steel used in the present invention treated by the conventional process has levels 5 to 8, but pro-eutectoid cementite is observed at these levels, and the C diffusion effect at the slab stage of the present invention is clearly visible. This also appears in the C concentration measured by EPMA as shown in Table 1, and corresponds to the fact that the C concentration in the center segregation area is lower in the method of the present invention.

Next, wire drawability will be described. It can be seen that the wire drawable limit strains of Levels 1 to 4 of the method of the present invention are higher than those of Levels 5 to 8 of the conventional method, and have good wire drawing performance. On the other hand, level 9 of the comparative method is higher than the amount of C in the method of the present invention, which indicates that the formation of pro-eutectoid cementite cannot be prevented even if C diffusion at the slab stage is carried out for the maximum time stipulated in the method of the present invention. show. In this case, the formation of pro-eutectoid cementite can be prevented by keeping the slab heating temperature for 30 hours or more, but in any case, this is in the category of non-industrial productivity and is not practical. Further, level 10 indicates a case where the slab heating time prescribed in the method of the present invention is short, and in this case as well, the formation of pro-eutectoid cementite cannot be prevented. In level 11, the cooling rate after wire rod rolling was slower than in level 3, so the formation of pro-eutectoid cementite was observed.

As described above, it can be seen that in order to achieve high strength with good wire drawability in a region where the amount of C is high, the manufacturing method according to the present invention must be used.

[0026]

[Effects of the Invention] The method of the present invention makes it possible to manufacture high-carbon steel wire rods with good wire drawability, and industrially it is possible to achieve high tensile strength in wire ropes, springs, steel cords, etc. It has become possible to stably supply materials that greatly contribute to weight reduction and labor saving in the field.

Claims (1)

[Claims] Claim 1: Weight ratio: C: 0.80-1.1%, Si: 0.1-1
．． 0%, Mn: 0.3-0.8%, Al:
Using a slab containing 0.003% or less, Cr: 0.1-0.3%, and the remainder consisting of Fe and inevitable impurities, it was maintained at a temperature range of 1250-1300°C for 3 to 18 hours. After making slabs into steel slabs by blooming rolling,
After heating the steel billet at a temperature of 1050 to 1100°C, the coiling temperature after rolling is 800°C to 930°C, and the subsequent cooling rate is 10 to 40°C/sec, so that the wire rod is in a rolled state. A method for producing a high carbon steel wire rod with high strength and excellent wire drawability, which is characterized by no formation of pro-eutectoid cementite.