JPH0229726B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a rod for a rod mill that can dramatically extend its service life, and a method for manufacturing the same.

Rods for rod mills have a diameter of 60 to 120, as is well known.
A steel rod with a length of 3,000 to 7,000 mm, which is placed in a rotating drum and used to crush rocks, ores, etc. that are thrown into the rotating drum. As the rotating drum rotates, a rod that is locked to the trough of the corrugated lining of the rotating drum and raised to a predetermined height is released and is placed on another rod that is located at the bottom of the rotating drum. This is done by the impact force of falling rocks and ores, and by the rolling of multiple rods at the bottom of a rotating drum that press and crush the rocks and ores caught between them. The rod for the rod mill is required to have impact resistance and wear resistance.

In response to this demand, rods for conventional rod mills are made of low-alloy steel that has relatively high impact resistance and wear resistance, such as manganese steel for mechanical structures or hot-rolled steel bars equivalent to manganese chrome steel. A normalized version of this is used, and its service life is approximately 800 hours.

By the way, the rod for a rod mill is a consumable item, and when the diameter becomes approximately 1/3 of the new one due to wear during use, the above service life is reached and the rod is replaced with a new one. The rod mill equipment must be stopped for a long time to perform replacement work, and the price of the rod for the rod mill as a consumable item and the loss of time for the replacement work account for a large proportion of the manufacturing cost. The sand making industry and the ore crushing industry demand a rod for a rod mill that has a long service life and is inexpensive enough to match the service life.

In order to meet the demands of the mill industry, one of the inventors of this application previously filed a patent application No. 54-37083 (Japanese Patent Publication No. 60
No. 8286), he invented a method for manufacturing a rod for a rod mill and a rod for a rod mill having a heat-treated hardened layer on the surface layer, and provided the same to the above-mentioned industry. The technical idea of the invention is a rod for a rod mill, which is made of a material made of low alloy steel, such as SAE1547, and has an effective hardening layer formed on the surface layer with a thickness in the range of approximately 5 to 15% relative to the outer diameter. The heat-treated hardened layer on the surface layer increases wear resistance, and the core material maintains impact resistance. This invention succeeded in making the service life more than twice that of conventional rods when crushing rocks for sand making, but when crushing iron ore, etc. At the practical stage, it was found that the effect was limited to extending the service life. The difference in the above effects is thought to be caused by the difference in hardness between rock and ore. As a countermeasure to this problem, if we simply make the hardened layer on the surface of the rod harder than the current one, the hardened layer will crack or chip during the impact, which will not only make it impossible to achieve the purpose of crushing, but also cause the rod to break and cause the rod to break. This is not preferable as it may cause damage to the drum lining.

The present invention solves the problems of not only conventional rods that do not undergo quenching treatment, but also the quenched rods of the earlier invention, and reliably extends the service life by almost double when ores are to be crushed. The present invention provides a possible rod for a rod mill and a method for manufacturing the same.

The gist of the first invention of the present application is as follows: (1) A surface layer portion with a thickness corresponding to 10 to 25% of the diameter of the steel rod and the remaining portion up to the center in the entire cross section of a hot rolled large diameter steel bar made of low alloy steel. (2) A rod for a rod mill is characterized in that: (2) an effective hardening layer and a heat-affected zone are formed by heat treatment including rapid heating and rapid cooling;

Therefore, in the invention of the above-mentioned patent application No. 54-41066, wear resistance is added by an effective hardened layer on the surface layer having a thickness in the range of approximately 5 to 15% of the outer diameter of the material, and the central portion is made of the same material as the material. However, the technical concept of the present invention is to apply rapid heating and rapid cooling to the entire cross section of the steel bar, thereby improving the steel bar's impact resistance. An effective hardening layer is formed in the surface layer with a thickness equivalent to 10 to 25% of the rod diameter due to rapid cooling from the predetermined quenching temperature reached by rapid heating, and the remaining part up to the center has a recrystallization temperature. With the above, the coarsening of the crystal grains is prevented by rapid cooling from a temperature close to the above quenching temperature, and the heat-affected zone has become a fine mixed structure in which ferrite/pearlite, which remains as fine crystal grains, and martensite or bainite are mixed. It is on the rod for the rod mill that is formed.
In other words, due to the high toughness maintained by the microstructure of the heat affected zone, the rod, which has a hardened layer approximately twice as thick as that of the prior invention, maintains high impact resistance, and The synergistic effect of the relatively high hardness maintained by the affected area and the hardened layer, which is twice as thick, increases the rod's abrasion resistance...durability, and furthermore, there are no parts of the material that are left intact. Therefore, the risk of peeling of the hardened layer from the boundary due to impact is eliminated, and as a result, the rod has a hardness higher than that of the quenched hardened layer of the prior invention and has improved wear resistance.

Therefore, the gist of the second invention for manufacturing the rod for a rod mill according to the first invention is as follows: (1) While feeding a hot rolled large diameter steel bar made of low alloy steel in the axial direction, (2) ) By sequentially passing a plurality of at least two induction heating coils and a plurality of at least two coolers provided at predetermined intervals along the feed passage, (3) the steel bar has the above-mentioned properties. The surface layer from the surface of the steel bar to a predetermined depth is brought to a predetermined quenching temperature, and the remaining portion to the center is heated to a predetermined quenching temperature by the time it passes through multiple induction heating coils and reaches the first cooler of the multiple coolers. The temperature is raised to a temperature close to the above quenching temperature, (4) the first cooler quenches and rapidly cools the surface layer, and (5) the subsequent cooler conducts heat from the center of the steel bar to the surface. (6) An effective hardening layer is formed on the surface layer with a thickness equivalent to 10 to 25% of the diameter of the steel rod, (7) and the remaining portion down to the center is coated with rapid cooling that accelerates the absorption of heat. A method of manufacturing a rod for a rod mill is provided, which comprises forming a heat-affected zone, and then subjecting the steel rod to a tempering treatment for a predetermined period of time.

The invention of the present application will be described in detail below with reference to FIGS. 1 to 4, taking as an example a case in which a rod for a rod mill is manufactured from a hot-rolled steel bar having a diameter of 90 mm.

As the material steel rod, a hot-rolled large-diameter low-alloy steel material such as a material equivalent to manganese steel for machine structures is used as it is in the present invention, and is subjected to the heat treatment process shown in the flowchart of FIG. Ru. In the above heat treatment process, for example, a first induction heating coil, a second induction heating coil, which are provided at a predetermined interval along the feeding path in the process of feeding the steel rod in the axial direction,
The process is roughly divided into a process of performing rapid heat treatment using a rapid heat treatment line consisting of a first cooler and a second cooler, and a process of tempering the steel bar subjected to the rapid heat treatment in an electric furnace. The distance between the first induction heating coil and the second induction heating coil and the distance between the second induction heating coil and the first cooler in the above rapid heat treatment line are determined by the power output and frequency of the induction heating coil, The distance between the first cooler and the second cooler is set according to the correlation of various conditions including the material, the steel rod diameter, the feed rate, etc., and the interval between the first cooler and the second cooler is determined by the material of the steel rod, the steel rod diameter, and the feed rate. It is set in a predetermined manner based on the correlation between the speed and the cooling capacity of each cooler.

For example, 150 rapid heat treatment lines each equipped with induction heating coils connected to a 150KW-1KHz power supply and coolers each with a flow rate of 150/min.
A steel bar that is fed in the axial direction at a feed rate of mm/min is shown in Fig. 2 with the passage of feeding time t min and temperature °C.
As shown in the relationship diagram, it is rapidly heated by the induced magnetic flux of the first induction heating coil during approximately 1 minute of passing through the first induction heating coil, and when it reaches the coil outlet, the surface temperature is approximately 870℃, and the center part Temperature is 100~
Preheat to around 150℃. The surface temperature of the steel rod decreases due to heat conduction toward the center while reaching the second induction heating coil spaced apart by a predetermined distance, while the temperature at the center further increases. The steel rod that has reached the second induction heating coil is rapidly heated again by the induction magnetic flux of the coil, and when it reaches the coil outlet, the surface temperature is approximately 920℃ and the center temperature is approximately 600 to 650℃. After being heated, the surface temperature reaches approximately 760°C by the time it reaches the first cooler separated by a predetermined interval.
However, the temperature at the center has increased to about 730-750°C, which exceeds the Ac ₁ transformation point, due to heat conduction from the surface direction. The steel rod that has reached the first cooler is rapidly cooled by a predetermined flow rate of cooling fluid, and the surface layer over a thickness equivalent to 10 to 25% of the steel rod diameter is hardened by the rapid cooling. This is due to the temperature gradient between the surface, which has dropped by 50 degrees Celsius due to rapid cooling by the first cooler, and the center part, which is still at a temperature of around 650 degrees Celsius. Rapid cooling is performed to promote the absorption of heat returning from the center direction, which moves toward the surface by heat conduction, thereby maintaining the temperature of the surface layer at at least 70 to 80 degrees Celsius, and rapidly reducing the temperature of the center portion. The temperature is reduced to approximately 150℃.
In this way, it sequentially passes through a heat treatment line consisting of rapid heating and rapid cooling in about 8 minutes. The steel rod that has been subjected to the rapid heat treatment is discharged from the heat treatment line, and is then subjected to a tempering treatment step of, for example, 180 to 250° C. for 5 hours in an electric furnace to become a rod product for a rod mill.

By undergoing the above rapid heat treatment process,
A hot-rolled raw steel bar of 90 mmφ with a grain size of about 4 to 6 has the surface layer shown in Figure 3 a and b.
As shown in the crystal grain micrograph (×100) and microstructure micrograph (×400), a martensitic structure with a grain size of 8 is formed, and the remaining part up to the center has a tertiary structure. As shown in the central tissue micrograph (×400) in Figure c,
A heat-affected zone consisting of a mixed structure of fine martensite or bainite, ferrite, and fine pearlite is formed.

FIGS. 4a and 4b are photographs of a full-scale cross-section of a 90 mmφ rod according to the present invention and hardness distribution characteristic curves obtained from hardness measurements at multiple points in the diameter direction of the cross-section.

The rod of the present invention, shown by curve A in FIG. 4b, is
Bitsker hardness Hv600 from the surface to a depth of approximately 10mm
or more, Hv500 or more up to a depth of approximately 15mm, or approximately 25mm in depth
It maintains Hv400 or more up to mm, indicating that the thickness of the effective hardened layer is extremely thick, and it is clear that a relatively hard heat-affected zone with Hv390 is formed from a depth of approximately 25mm to the center. I have to. Incidentally, in the figure, the shaded area C represents the hardness distribution range of several commercially available rods that have no hardened layer at all, and the curved line B represents the hardness distribution range of only the surface layer according to the prior invention. Diameter 5~
The hardness distribution characteristic curve of a rod in which an effective hardening layer of 15% is formed and the center part is made of the same material is also shown. Comparison with these conventional rods confirms that the method for rapid heat treatment of large diameter steel rod cross sections of the present invention is effective and that the obtained rods of the present invention have extremely high wear resistance.

Next, in order to demonstrate the durability of the rod of the present invention, the present inventor used the rod of the present invention in an actual rod mill for crushing iron ore, and compared it with the rod of the prior invention. For comparison test, 90mmφ/length made of the same material
A 4120 mm hot-rolled material was used, one of which was subjected to the manufacturing method according to the present invention, and the other was subjected to the manufacturing method according to the prior invention, and the material wears out as it is used in a rod mill. The axial center diameter of each rod was measured. FIG. 5 shows the results, and is a trend diagram obtained by plotting the above measured values on coordinates with the horizontal axis representing the service life hr and the vertical axis representing the diameter mm. The rod of the present invention is shown by a solid line A, and the rod of the prior application is shown by a broken line B. Figure 5 shows that the rod of the present invention has almost twice as good wear resistance as the rod of the prior invention, and also has sufficient impact resistance, proving that it is extremely durable. Ta.

In the above embodiment of the method for manufacturing the rod of the present invention,
The explanation was given using two induction heating coils for rapid acceleration to the center of the steel bar, and two coolers for rapid cooling to the center of the steel bar. When the diameter is large and the power output is small, it is necessary to use three or more induction heating coils and three or more coolers depending on the diameter of the steel bar to rapidly cool the steel bar to the center. In addition, in order to manufacture the rod of the present invention, which aims to obtain the effects of rapid heat treatment consisting of rapid heating and rapid cooling on a steel rod having a diameter of at least 60 mm or more, it is necessary to In a heat treatment line equipped with two induction heating coils and a cooler, even if the power output of the induction heating coil is large or the cooler is long, the thick surface layer, which is a feature of the present invention, is effective. Numerous experiments have confirmed that it is extremely difficult to form a heat-affected zone consisting of a hardened layer and a fine structure extending to the center.

Since the rod for a rod mill according to the present invention has high durability, it will be used in the sand making industry and ore crushing industry where it is used, as it can significantly reduce manufacturing costs.

[Brief explanation of drawings]

Fig. 1 is a flowchart showing the heat treatment process which is the manufacturing method of the rod for a rod mill according to the present invention, and Fig. 2 shows the surface and center of the steel bar changing over time during the rapid heating and rapid cooling processes during the heat treatment process. Fig. 3a is a crystal grain micrograph of the surface layer of the rod of the present invention, Fig. 3b and c are micrographs of the structure of the surface layer and center of the rod of the present invention, Fig. 4a 4b is a hardness distribution characteristic curve obtained from the hardness measurements in the diametrical direction in FIG. 4a, and FIG. FIG. 4 is a diagram showing the relationship between the elapse of usage time and the wear of the rod diameter in comparison with the invention rod.

Claims (1)

[Scope of Claims] 1. In the entire cross section of a hot-rolled large-diameter steel bar made of low-alloy steel, the surface layer over a thickness corresponding to 10 to 25% of the diameter of the steel bar and the remaining part up to the center are rapidly A rod for a rod mill, characterized in that it has an effective hardened layer and a heat-affected zone formed by heat treatment including heating and rapid cooling. 2. While feeding a hot-rolled large-diameter steel bar made of low-alloy steel in the axial direction, at least two or more induction heating coils are installed at predetermined intervals along the feeding path, and at least two or more induction heating coils are installed at predetermined intervals along the feeding path. By sequentially passing through a plurality of coolers, the steel rod is heated to a predetermined depth from the surface of the steel rod by the time the steel rod passes through the plurality of induction heating coils and reaches the first cooler of the plurality of coolers. The surface layer is heated to a predetermined quenching temperature, and the remaining portion up to the center is heated to a temperature close to the above quenching temperature.The surface layer is quenched and rapidly cooled in the first cooler, and then the steel rod is heated in a subsequent cooler. Rapid cooling is applied to promote the absorption of return heat that conducts heat from the center to the surface, creating an effective hardening layer on the surface layer with a thickness equivalent to 10 to 25% of the steel rod diameter, and all the way to the center. A method for manufacturing a rod for a rod mill, characterized in that a heat affected zone is formed in the remaining portion of the rod, and then the steel rod is subjected to a tempering treatment for a predetermined period of time.