JPS6023172B2 - Direct heat treatment high tensile strength wire manufacturing method - Google Patents

Description

Detailed Description of the Invention The present invention improves the direct heat treatment wire method, which uses heat during hot rolling to control the cooling rate. The purpose is to enable stable production of high-tensile wire rods.

It is a well-known fact that in recent years, cooling methods for directly heat-treated wire have become widespread, and have contributed to improving the quality of wire, which was previously impossible.

Recently, there has been a trend to immediately commercialize hot rolled wire rods for the purpose of streamlining secondary processing. For example, in the case of P, steel birch, etc., it is customary to process it in the process of hot rolled wire - cold shaping - heat treatment - port removal, but this is then processed in the process of hot shaping wire - straightening - low temperature tempering. Research is being conducted on the drastic rationalization of manufacturing. However, in order to adopt such a process, it is necessary to have a strength of 145 k9 / rib 2 or more in hot rolling, good ductility, and stable quality over the entire length, which inevitably limits the composition system. Cooling methods during direct heat treatment must also be improved. That is, in order to maintain high strength and ductility, a low carbon component system is required, and in order to obtain high strength, the metal structure must be martensitic.

However, it is technically difficult to obtain a stable martensitic structure during cooling after hot rolling. Because the material temperature after finish rolling is over 100,000, the winding medium (400,000
0 to 650q0) cannot be cooled by forced cooling, and even if the winding temperature is 800°C or higher, it is possible to transform into a martensitic structure depending on the component system, but in this case, the time until martensitic transformation starts The reason for this is that it is not possible to obtain a complete martensitic structure with the current equipment, which has a short adjustment cooling conveyor. As a result of various studies, the inventors found that it was possible to roll the material in a temperature range of 400 to 650 qo by lowering the material temperature at the finish rolling outlet, and completed the invention.

This invention is based on CO. Less than 40% of Cr, Mo, B, etc.
This is a cooling method for directly heat-treated wire rods of low-alloy steel containing one or more types of steel.One or more cooling facilities are installed before the finish rolling process, and the entire length of the rolled wire rod is precooled here to maintain the finish rolling entrance temperature. Cooled to a temperature range of 600 to 900 qo, forcedly cooled after finish rolling, rolled up at a temperature range of 40 to 65000 qo, rolled out on a conveyor and rolled by blast for 700 nos.
It is characterized in that transformation is completed at a cooling rate of EC or higher to form a martensitic structure, and then self-tempering is performed by cooling at a cooling rate of 0.5 to 1.00 C/sec.

In this invention, the finish rolling inlet temperature is set at 600 to 900.
The reason for limiting it to 00 is that cooling to below 6003○ is likely to cause roll breakage due to increased rolling load, and above 90000 the temperature is too high, so forced cooling after finish rolling is required to achieve the target of 400-650. This is because it is not possible to stably and rapidly cool down to the winding temperature at 0:00.

The reason for limiting the winding temperature after finish rolling to 400 to 650oC is that it is actually difficult to cool the temperature below 40,000oC, and if the cooling is strengthened to 400oC or less, the yield point of the rolled wire will suddenly rise. This makes winding difficult. 650 C
This is because martensitic transformation cannot be completely completed on the conveyor.

Furthermore, the cooling rate on the conveyor was set to 700/sec.
bainite at 7°C/sec or higher;
This is because ferrite etc. are generated and a martensitic structure cannot be obtained. Further, the reason why the material is gradually woven at a cooling rate of 0.5 to 1.0 oo/sec after transformation is to provide rutting properties. 1
00/sec or more, the effect cannot be obtained sufficiently;
This is because 5q0/sec does not have this effect at all.

FIG. 1 is a block diagram showing an example of direct heat treatment line cooling equipment for carrying out the present invention, in which 1 is an intermediate rolling mill, 2 is a preliminary cooling device, 3 is a finishing rolling mill, and 4 is a forced cooling device. 5 indicates a sheathing cone, and 6 indicates a forced air cooling device.

Next, embodiments of the invention will be described.

Example Product size 7.1 using the steel type with the composition shown in Table 1
It was hot rolled into a 0 wire rod.

At this time, preliminary cooling of the wire rod before finishing rolling was carried out by installing two water-cooled preliminary cooling devices 2 between intermediate rolling mill 1 and finishing rolling mill 3 shown in FIG. The strength of the wire strip coming out of the intermediate rolling mill at this time was about 970°C. Then, the wire rod cooled to about 50 qo by the two preliminary cooling devices and the wire rod cooled to about 800 qo by one of the same cooling devices are finished rolled, and then cooled to 550 qo by the forced cooling device. After being rolled up, it is spread on a conveyor and the average cooling rate is 7.
Complete the transformation by blast cooling at ℃ sec, and continue to 0.
．． It was slowly cooled at a cooling rate of 70/sec. It has been confirmed in advance using an automatic transformation measuring device that the time required for martensitic transformation at 7° C./sec for this steel type when rolled at 550 pm is approximately 6 $eC. Therefore, the cooling time at a cooling rate of 7°C/sec is 6$ec in this example.
And so. In addition, for comparison with this invention method, the same steel type was finish rolled using the conventional method without pre-cooling before finish rolling, forced cooling was performed to 800 d0 after top rolling, and after winding, it was rolled out on a conveyor and subjected to impact rolling. Transformation was completed by wind at a cooling rate of 7°C/sec. A tensile test and a scale property test were conducted on each wire manufactured under these conditions. The results are shown in Table 2. As is clear from the results in Table 2, it can be confirmed that all the products produced by the method of this invention have higher strength and improved uniformity than the conventional method. In particular, this method 1 of the invention significantly improved the mechanical properties. It can also be seen that the amount of scale is significantly reduced in the method of this invention compared to the conventional method. Table 1 Table 2 (n=50)

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing one embodiment of an apparatus for carrying out the method of this invention. In the figure 1... Intermediate rolling mill, 2... Pre-cooling device, 3...
... Finishing rolling mill, 4... Forced cooling device, 5... Laying cone, 6... Forced cooling device, 7... Conveyor.

Claims (1)

[Claims] 1 In the cooling method for directly heat-treated wire rods, low-alloy steel containing less than 0.40% C is forcibly cooled by water cooling after hot finish rolling, and then transformed by air cooling or other cooling medium. One or more cooling equipment is installed in the area, and the entire length of the rolled wire rod is pre-cooled with water or other cooling medium to cool the finish rolling mill inlet temperature to a range of 600 to 900°C, and after finish rolling, forced cooling is performed to a temperature of 400 to 900°C. It is rolled up in a temperature range of 650°C, and after being rolled up, it is rolled out on a conveyor, and the transformation is completed at a cooling rate of 7°C/sec or more by blowing air to form a martensitic structure.
．． A method for producing a directly heat-treated high-tensile wire rod, characterized in that self-tempering is performed by cooling at a cooling rate of 5 to 1.0° C./sec.