JPS6345441B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing steel wire rods by hot rolling, and in particular, by controlling the process of hot rolling steel wire rods, in particular, the spheroidizing heat treatment performed prior to the next cold working is carried out. The present invention relates to a method of manufacturing hot rolled material that can be carried out quickly and effectively. After hot rolling, steel wire rods are subjected to drawing, cutting, cutting,
Most of the materials are subjected to cold working such as forging, but conventional hot rolled materials generally lack the deformability to withstand these cold workings and have high deformation resistance. Hot-rolled materials cannot be subjected to cold working. In other words, the hot-rolled material is drawn by 20% or more, and then subjected to spheroidization heat treatment, which requires over 20 hours of processing time, to improve the performance of the hot-rolled material. Subjected to cold processing. However, performing these additional processing and heat treatments has the problem of wasting a large amount of thermal energy and processing time, and furthermore, scale loss is unavoidable. Various proposals have been made to address these problems. For example, in the method shown in JP-A No. 47-8503, immediately after hot rolling, the material is passed through a transformation region.
and a method for obtaining a hot-rolled material having a fine pearlite, bainite or martensitic structure by cooling at a rate sufficient to suppress the growth of austenite grains and the formation of pro-eutectoid ferrite, and a method for processing alloy steel. This is a method of obtaining a hot-rolled material by performing forced ventilation to promote the formation of martensite following the step of forming fine austenite particles after hot rolling, and then performing a spheroidization treatment. be. In short, these methods simply utilize the austenite grain size after hot rolling, and produce fine pearlite without pro-eutectoid ferrite,
The aim is to obtain a structure that actively transforms at low temperatures, such as a bainite or martensitic structure. However, these methods do not sufficiently reduce the austenite crystal grain size, so they are not effective in shortening the spheroidizing treatment time, and the strength obtained after the spheroidizing treatment is lower than that of the conventional wire drawing process. It has disadvantages such as higher strength than that obtained after spheroidization treatment and high deformation resistance during cold working. In addition, a proposal similar to this one was published in JP-A-Sho.
51-92719. This method targets alloy steel, and after hot rolling it is rapidly cooled to a temperature within the temperature range of 550℃ to the Ms point to obtain a hot rolled material with an intermediate structure, which is then subjected to spheroidization treatment. . However, this method also aims to simply obtain a bainite structure, and it is not only extremely difficult to obtain a single bainite structure under the disclosed conditions, but also it is difficult to obtain a single bainite structure after spheroidization. The strength is also higher than that obtained after wire drawing as in the past. Therefore, the present inventors have solved the drawbacks of the prior art including the above-mentioned various proposals, can significantly promote spheroidization in the spheroidization treatment performed prior to cold working, and can We conducted extensive research to obtain hot-rolled materials with low strength values after treatment, and found that various types of material that can be obtained by effectively utilizing plastic strain during hot finish rolling for transformation behavior during cooling. We focused on the relationship between the structure and the spheroidization process, and as a result of various studies, we found that the structure transformed into pearlite, bainite, and martensite while retaining hot working strain, despite the formation of pro-eutectoid ferrite. First, it has been found that the softening rate is fast and that it is effective for spheroidization, and that it can reduce the strength after spheroidization treatment. In other words, as a result of investigating the effect of rolling temperature and reduction rate in hot rolling, and more specifically, the effect of rolling temperature range and reduction rate on shortening the spheroidization time, we found that into strips
After being rapidly cooled to a temperature above the Ms point and below 850°C, a plastic strain of 20% or more is subsequently applied, and the structure is transformed by cooling while maintaining the plastic strain. They found that it progresses easily and is extremely effective. To explain this point in more detail, first, the austenite grains that have been rolled and refined in the easy recrystallization region are immediately rapidly cooled, and then
After forming supercooled austenite or metastable austenite at 850°C or less, we apply a plastic strain of 20% or more and immediately cool it while retaining the plastic strain to obtain a transformed structure while retaining the plastic strain. be. Based on this knowledge, the present invention has materialized the above-mentioned manufacturing process by taking into account the arrangement of actual rolling equipment, workability, etc., and the gist thereof is to roughen steel wire rods by hot rolling. When finishing rolling after rolling and intermediate rolling to form a strip, there is a step of rapid cooling to a temperature of 500°C or more and 850°C or less after rolling in a first finishing mill, and then a step of quenching to a temperature of 20% or more 80°C in a second finishing mill. %
The following steps are combined with the steps of applying the following plastic strain, cooling and transforming while maintaining the plastic strain, and obtaining a structure in which finely dispersed pro-eutectoid ferrite is mixed with fine pearlite, bainite, or martensite. The present invention is characterized by a method for producing a wire rod that can be quickly spheroidized. Hereinafter, the present invention will be explained in detail using Examples. First, the reason why each condition is regulated as described above in the present invention is as follows. The equipment for carrying out the manufacturing method of the present invention needs to be arranged in such a way that a cooling process is performed before the final finishing mill. By the way, in the case of wire rod rolling, if a conventional rolling mill row consisting of a roughing mill, first and second intermediate rolling mills, and a finishing mill is used, and the cooling process is simply placed before the finishing mill, the intermediate rolling mill Since the amount of rolling up to the rolling mill is inevitably limited, the amount of rolling that must be shared by the finishing rolling mill naturally increases.
As a result, the finishing temperature increases. On the other hand, in the case of a wire finishing mill, the permissible rolling load decreases as productivity increases, and under such a low rolling load,
Rolling at low temperatures below 850℃ becomes impossible. Therefore, with the conventional arrangement of rolling mills in which a cooling step is provided before the finishing mill, it is not possible to roll large unit weight coils at high speed according to the present invention. Therefore,
In the present invention, a finishing rolling mill is installed separately at a position following the cooling line installed at the rear of a normal finishing rolling mill, and the former is designated as the first finishing rolling mill and the latter is designated as the second finishing rolling mill. Therefore, easily
This enables high-speed rolling at rolling temperatures above the Ms point and below 850°C. Moreover, in order to create such a rolling mill row, it is sufficient to simply add a second finishing mill separately to the conventional one, so there is an advantage that the investment cost does not increase. The rolling temperature before the second finish rolling, that is, the quenching temperature after rolling in the first finish rolling mill is set to a temperature of 500°C or more and 850°C or less, which corresponds to the Ms point. 850
When the temperature exceeds ℃, the temperature easily rises to over 900℃ by rolling, and the release of strain after rolling progresses rapidly.
It becomes difficult to utilize strain for transformation behavior. Also, 500
If rolling is carried out at a temperature lower than 0.degree. C., the rolling load becomes too large to be practical, and not only is it not very effective in shortening the spheroidization time. The reduction ratio in the second finish rolling shall be 20% or more and 80% or less. Regarding this point, the present inventors conducted the following tests. Made of SCM435H, 600℃,
A second finish rolling was performed at each rolling temperature of 800℃ and 900℃ with the rolling reduction rate varied over a wide range, and after rolling the rolling temperature was 2 to 5℃/s.
A hot-rolled material was obtained by cooling at a cooling rate of , and the time required for spheroidization was investigated by subjecting it to spheroidization treatment. The results are shown in FIG. The spheroidization time is expressed as a ratio of 1 when the rolling temperature is 800℃ and the reduction rate is 20%, and the spheroidization cycle is as shown in the same figure, from 770℃ to air cooling (AC). The time was taken as the spheroidization time. As can be seen from the figure, when the reduction rate is less than 20%, no significant shortening of the spheroidization time is observed.
On the other hand, if the rolling reduction exceeds 80%, the finishing temperature will rise, so the effect of shortening the spheroidization time will not increase, and on the contrary, the effect will decrease. Next, the temperature is increased from 500℃ to 850℃ using the second finishing mill.
Regarding cooling after rolling, from the viewpoint of effectively utilizing plastic strain for transformation behavior,
Immediate cooling is preferred. Even if the amount of strain is partially recovered after applying strain, if most of the strain remains,
The effects of effective use will not be destroyed. Also,
Strain that can effectively affect transformation behavior has the same degree of effect on transformation behavior, whether it is a strain immediately introduced by plastic working or a strain that remains after partial recovery. On the other hand, even if the rolling temperature is between 500°C and 850°C, dynamic recrystallization will occur as the rolling reduction increases, and is particularly likely to occur on the high temperature side of the above temperature range.Additionally, if excessive strain is applied, the temperature will rise. By the time the adjustment cooling starts after applying the strain, the material temperature has increased, so the strain release rate increases. Therefore, during cooling, it is preferable to take such phenomena into account and appropriately determine the conditions for adjusted cooling. In addition, since the required amount of strain used for transformation behavior and the rate of strain release after applying plastic strain depend on the composition of the material, the amount of reduction applied in the temperature range of 500°C to 850°C in the second finish rolling. (Reduction ratio) is preferably adjusted to an appropriate amount depending on the component system, taking into account the above-mentioned phenomenon. The structure of the hot-rolled material obtained in the present invention is fundamentally different from that obtained by conventional manufacturing methods. In other words, it is generally known that the faster the cooling rate and the finer the structure, the shorter the spheroidization time. However, if the cooling is too rapid, the structure becomes a single or composite structure of bainite or martensite. The disadvantage is that the strength after spheroidization increases. On the other hand, the product obtained by rolling and cooling at a temperature of 500°C or higher and 850°C or lower, as in the present invention,
The introduced plastic strain promotes the formation of pro-eutectoid ferrite, so that the fine pro-eutectoid ferrite is uniformly dispersed, and the generation of pro-eutectoid ferrite concentrates C etc., while maintaining the plastic strain. The retained austenite transforms to form a structure in which fine pearlite and bainite or martensite are partially mixed, that is, a structure in which finely dispersed pro-eutectoid ferrite is mixed with fine pearlite, bainite, or martensite. Such an organization is
Even if pro-eutectoid ferrite is formed, it is finely dispersed, and furthermore, the transformed structure of fine pearlite, bainite, or martensite is affected by processing strain before transformation, so it becomes spherical in the spheroidization process. This makes it possible to shorten the spheroidizing time, and at the same time, the strength after spheroidizing can be reduced to the same strength as a conventional material that has been spheroidized after wire drawing. The steel wire rods targeted by the present invention include those usually called bar-in-coils, and the composition of the steel wire rods is not particularly limited, such as low carbon steel, high carbon steel, alloy steel,
It can be applied to various component systems used as ordinary wire rod products such as boron steel. However, the essential ingredients for steel wire rods and the ingredients added as necessary are suitable for various uses,
It is necessary to adjust according to the product characteristics. The following is a rough guideline for this purpose. C is a necessary component to obtain strength and is 0.10
% or more is desirable. On the other hand, if it exceeds 0.70%, it is difficult to generate ferrite, which is the main focus of the present invention, which is promoted by low-temperature rolling in the second finishing mill, and it is difficult to maintain a certain level of ferrite when carrying out the present invention. There are some difficulties. Si is used as a deoxidizing agent to produce sound killed steel, and is also an effective component for increasing strength, so it is desirable to contain it at 0.10% or more. 0.10%
If it is less than that, deoxidation may be insufficient and bubbles and nonmetallic inclusions may be easily generated. On the other hand, adding too much Si may deteriorate weldability and be harmful to fatigue resistance.
Since this leads to the formation of SiO ₂ -based inclusions, it is desirable to set the upper limit of addition to 1.50%. Mn is a necessary component when considering hardenability, and is preferably added in a range of 0.30 to 1.80%. If it is less than 0.30%, hardenability may be insufficient and the material may become unstable, and if it exceeds 1.80%, quench cracking is likely to occur. As mentioned above, C, Si, and Mn are necessary components from the viewpoint of steel manufacturing and product properties, but in addition, Cr,
It may be desirable to add one or more of Ni, Mo, and B as necessary to improve product characteristics. The guidelines at that time are as follows. Cr is a component that improves hardenability, and is preferably added when the necessary properties cannot be obtained due to insufficient hardenability due to the amount of added Mn, etc., or when you want to further improve ductility and toughness. . When added, it is preferably in the range of 0.10 to 1.50%; if it is less than 0.10%, it is difficult to achieve the above objective;
If it exceeds , manufacturing costs will increase. Ni is an effective component in contributing to hardenability and improving ductility and toughness, and is also effective in improving hot workability, so if the necessary ductility and toughness cannot be obtained by adding Mn etc. It is desirable to add it to When added, it is preferably in the range of 0.20 to 1.50%; if it is less than 0.20%, it will be difficult to sufficiently achieve the above objective, and if it exceeds 1.50%, the manufacturing cost will increase. Mo is a component that significantly improves hardenability.
However, it is an expensive ingredient. Therefore, it is preferable to add Mn etc. only when the necessary product characteristics cannot be obtained by adding Mn etc.
A range of 0.10 to 0.50% is desirable. If it is less than 0.10%, it will be difficult to fully achieve the above purpose;
%, manufacturing costs will increase. B is a component that significantly improves hardenability even in a small amount, and is an inexpensive hardenability improving element. When added, it is preferably in the range of 0.0003 to 0.005%. If it is less than 0.0003%, hardenability cannot be sufficiently improved, and if it exceeds 0.005%, the austenite crystal grains will become coarser, and carbides and nitrides will precipitate at the austenite grain boundaries, making it difficult to harden. In addition to reducing properties, ductility and toughness also deteriorate. Naturally, in order to further exhibit the effect of B on hardenability, it is desirable to add Al or Ti to fix N. Example Steel with the composition shown in Table 1 was melted in a 200-ton converter, subjected to RH degassing treatment, and then continuously cast.
After forming into a bloom of 300 x 350 mm and billet processing, it was rolled into a wire rod with a diameter of 7 to 120 mm under the rolling conditions shown in Table 1. In addition to examining the rolled structure of the obtained wire rod, a strength test was conducted on the wire rod after the spheroidization treatment, and the spheroidization rate was also examined. The results are also shown in Table 1.

【table】

[Table] As is clear from the table, in the case of the conventional method in which wire drawing is performed before spheroidizing treatment (A), it takes a long time to spheroidize, and When the inter-rolled material is directly subjected to short-time spheroidization treatment (B), not only is the spheroidization insufficient, but also the strength is increased. In contrast, in the case of the method of the present invention, the hot-rolled material has a structure in which fine pro-eutectoid ferrite is mixed with fine pearlite, bainite, or martensite, and this is shortened without wire drawing. Just by applying time spheroidization processing,
The spheroidization rate is the same as that of the conventional method, and the strength after spheroidization is also the same. In this example, medium-carbon mechanical structural carbon steel and alloy steel are shown as representative steel types, but as mentioned above, the present method is applicable to steels with a C content higher than that of eutectoid steel in which pro-eutectoid ferrite does not form. It goes without saying that the present invention can be applied to wire rod steel types other than those of the present invention, and the same effects as those of this embodiment can be achieved. As is clear from the above description, according to the present invention, the hot rolled material can be subjected to the spheroidization treatment without being subjected to wire drawing as in the conventional method.
Moreover, the processing time in the spheroidization process can be significantly shortened, and it is possible to obtain a steel wire rod with a strength as low as the strength after spheroidization obtained in the conventional process. Significant effects can be expected, such as the scale loss associated with wire drawing being extremely reduced, the spheroidization process being able to be carried out in a short time, and the productivity of the next process of cold working being improved. be.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the relationship between the rolling reduction in the second finishing mill and the spheroidizing treatment time.

Claims (1)

[Scope of Claims] 1. When finish rolling a steel wire rod after hot rough rolling and intermediate rolling into a strip, the temperature is 500°C or more and 850°C after rolling in the first finishing mill.
A stage of rapid cooling to the following temperature, followed by applying a plastic strain of 20% to 80% in a second finishing mill, and cooling and transforming while maintaining the plastic strain to form finely dispersed pro-eutectoid ferrite. A method for producing a wire rod capable of rapid spheroidization, characterized by combining the steps of obtaining a structure containing a mixture of fine pearlite, bentnite, or martensite. 2 Steel wire rod contains C0.10~0.70%, Si0.10~1.50%,
The method according to claim 1, which has a composition comprising 0.30 to 1.80% of Mn, with the balance consisting of Fe and unavoidable impurities. 3 Steel wire rod contains C0.10~0.70%, Si0.10~1.50%,
Contains Mn0.30~1.80%, further Cr0.10~1.50%,
Ni0.20~1.50%, Mo0.10~0.50% and B0.0003~
The method according to claim 1, wherein the method has a composition comprising one or more of 0.005% and the remainder consisting of Fe and unavoidable impurities.