JPS6056212B2 - High-strength, high-toughness direct heat-treated steel wire with excellent wire drawability and descalability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is a so-called direct heat treated wire rod in which the wire rod coming out of a hot rolling mill is heat treated by controlled cooling, and the wire rod has the characteristics of high strength and high toughness without being reheated before cold working. The present invention relates to a hard steel wire rod that can be manufactured into a finished wire product having the following characteristics and has excellent descaling properties before cold working.

Conventional hot-rolled hard steel wire rods are generally reheated, subjected to air patenting (hereinafter referred to as AP) or lead patenting (hereinafter referred to as LP), and subjected to cold working such as wire drawing.

Therefore, a large amount of cost and man-hours are required for reheating, and since a thick layer of tight scale with poor releasability is generated, descaling takes a long time and yield decreases due to scale loss.

Recently, direct heat treatment (hereinafter referred to as DP) has been performed on wire rods using the heat retained during hot rolling.

However, various D
P-treated materials have been recognized to have improved descaling properties, but in terms of material quality, they can obtain strength and toughness equivalent to oil-treated materials, but they have not yet reached the point where they are equivalent to LP-treated materials. The DP wire rods used are
There is a wire rod described in Japanese Patent No. 6-30937.

Since this wire rod is controlled and cooled mainly by blast cooling after hot rolling, the cooling rate during pearlite transformation is slower than that in LP treatment, so it does not have a structure comparable to that of LP treatment material. Since it is manufactured by heating the conventional LP wire in the austenite region and immersing it in a lead bath as a single wire and rapidly cooling it, the wire has a uniform fine sorbite structure over its entire length and cross section.

Further, the lead bath temperature is empirically set in the range of 520 to 570'C depending on the required strength, but the transformation is completed at this temperature and there is no possibility that martensitic structure will occur.
Therefore, the LP treated material is reliably stable and has excellent quality. When such heat treatment is performed by DP treatment, it is very difficult to stably obtain the same heat treatment pattern as conventional LP treatment because the wire runs at high speed before winding and the wire density is high after winding. It is difficult to

The DP wire rod disclosed in Japanese Patent Publication No. 46-30937 is regulated and cooled by supplying a cooling medium such as air to the wire rod in a loose coil state piled up in a non-concentric ring shape after winding, so that it can achieve a cooling rate comparable to that of LP processing. If this is applied, the wire will be supercooled to below the MS point and become brittle due to the generation of a martensitic structure, so the cooling rate will inevitably have to be slowed down, making it impossible to obtain a material equivalent to the LP treated material. The object of the present invention is to provide a hard steel wire rod that has excellent cold workability comparable to conventional LP treated wire rods and excellent descaling properties similar to conventional DP treated wire rods.

The wire rod of the present invention is a hard steel wire rod with a C content of 0.40% to 0.85%, and its metal structure is a so-called sorbite structure consisting mainly of fine pearlite particles with fine pro-eutectoid ferrite dispersed therein. Yes, it does not contain supercooled structures such as martensite or bainite. If it contains a coarse layered pearlite structure, cold workability, especially wire drawability, will deteriorate.
This content is minimized to 5% or less using the point count method.

If supercooled structures such as martensite and bainite are included, the toughness will deteriorate, so they are not included at all. The surface oxidation scale of the wire rod of the present invention has an upper layer of Fe3O4 and a lower layer of FeO.
The Fe3O4 layer is thin, and the FeO layer does not contain Fe3O4.

Fe3O is difficult to dissolve in acid, and if this layer becomes thick, the pickling descaling properties deteriorate, so it is desirable to make the layer as thin as possible, but it may be as thin as a conventional DP treated material. Furthermore, for the same reason, the FeO layer must not contain Fe3O4 produced by transformation of FeO.

Furthermore, Fe2O3 further deteriorates descaling properties. It must not have an Fe2O3 layer. The metal structure and scale state of the wire of the present invention are uniform in the length direction of the wire. The degree of uniformity is superior to conventional DP treated wire. The manufacturing method for obtaining the wire rod of the present invention will be explained in detail below.

The wire rod of the present invention is produced by forced cooling after hot rolling, holding the wire rod at a constant temperature just above the Arl point for a period longer than necessary to make the surface and center portions of the wire uniform in the longitudinal direction, and then rapidly cooling it by controlled cooling. do.

As a result, the wire has a scale consisting of a thin iron oxide layer rich in FeO, and the metal structure is a so-called sorbite structure consisting mainly of fine pearlite with fine pro-eutectoid ferrite dispersed therein, and martensite. and high strength equivalent to LP treated material that does not contain supercooled structures such as bainite,
The material has high toughness, has excellent wire drawability, and has descaling properties that are comparable to those of conventional DP-treated materials.

In order to obtain a material equivalent to the LP treated material, it must have a uniform sorbite structure. For this purpose, cooling control below Arl is important, and below Arl, it is necessary to keep the cooling rate between the surface layer portion and the center portion of the wire as close to constant as possible. In order to obtain the strength and baggability equivalent to LP treated material through DP treatment, the temperature after hot rolling must be 1000℃ or higher, and the temperature must be 10 to 100℃/Sec depending on the steel type and wire size.
must be cooled to an appropriate temperature at an appropriate cooling rate.

However, if such cooling is simply performed continuously, it is extremely difficult to rapidly cool the surface layer and the center of the wire at a uniform speed in the longitudinal direction, resulting in differences in structure. The material equivalent to the material cannot be obtained.

As a result of experimental studies, the present inventors have found that in the cooling process after finish rolling, the S curve is By rapidly cooling down to the nose at a cooling rate almost similar to that of LP-treated materials, and further maintaining the temperature at a constant temperature between 520 and 58C, at which untransformed austenite transforms in a short period of time, a material equivalent to that of LP-treated materials can be obtained. It has been shown that it can be obtained stably.

In addition, in order to obtain excellent descaling properties, Fe3O4
The thinner the FeO layer is, the more it exists only in the surface layer, and the less Fe, O, is generated, the better the releasability is.A thinner FeO layer is better for chemical descaling, and a thinner FeO layer is better for mechanical descaling. A thicker one is preferable.

To stably secure such a good scale structure,
After finish rolling and until pinch rolls, the steel is forced to cool to below 800°C for chemical descaling and to about 85°C for mechanical descaling, and immediately forced to cool to a temperature just above Arl. Fe3O, which is hardly soluble, is thin and exists only in the surface layer, and the FeO layer is thin for chemical descaling, and thick for mechanical descaling.

, /Vrl takes about a few seconds, and the FeO layer does not become thicker because it is subsequently rapidly cooled.

FeO becomes unstable at temperatures below 570°C, and
Below C, transformation of FeO occurs, and Fe3O4 and Fe eutectoid. Therefore, it is desirable to rapidly cool from just above the Arl to the S curve nose, then hold the temperature at 520°C to 580°C, and then complete the cooling in as short a time as possible to prevent eutectoid transformation. CCT and T
The completion time of the transformation is determined from the Tr curve, and immediately after the completion of the transformation, cooling with hot water or water is performed to bring the line temperature to 200°C or less.

It has been revealed that by such cooling treatment, not only can a material equivalent to that of LP treated material be obtained, but also that descaling properties comparable to those of conventional DP treated materials can be stably obtained.
In order to clarify the present invention, an example of manufacturing the wire according to the present invention will be described.

Figure 1 shows the regulated cooling equipment.

A rolled wire rod 2 sent from a wire rod finishing mill 1 at about 1000° C. is introduced into a laying device 5 after passing through a cooling guide pipe 3 and a pinch roll 4 .

Cooling water is introduced into this cooling guide tube, and the wire is
Forced cooling to 50°C. The laying device 5 is provided at a position where the wire can be guided onto the conveyor 6, and the wire is formed into a loop shape, and while being conveyed in a series of loops on the conveyor, the wire is placed directly above Ar″1 (Arl~Arl+50°C).
The wire passes through a heat retention zone 7 set at a temperature of Ru.

This treatment makes it possible to rapidly cool the wire at the required speed within the wire cross section and in the longitudinal direction in subsequent quenching.

The equalization time in the heat retention zone is determined according to the wire diameter,
Requires 3 seconds or more.

If the retention temperature becomes below the Arl point, transformation from austenite to ferrite + pearlite progresses during this time, making it impossible to obtain the desired sorbite structure. Furthermore, if the temperature is higher than Ar'1+50'C, the difference in the structure between the surface layer and the center of the wire becomes large during subsequent rapid cooling, and in addition to making it impossible to obtain a material equivalent to the LP treated material, there is a hard to dissolve material on the surface of the wire. A thick scale of the Fe3O4 layer is generated, resulting in poor chemical descaling properties. Next stirred 45
By immersing the wire in a loose form in a tank 8 at 0 to 550°C, or by spraying steam and water onto the wire from above and below the conveyor,
Cool at 1000C/sec. The above cooling rate is determined depending on the steel type and wire size. Salt immersion is more advantageous in terms of temperature unevenness in the cross section and longitudinal direction of the wire, and by performing strong stirring on the high temperature side and weak stirring on the low temperature side, it is possible to maintain a uniform temperature in the bath and at the same time. A cooling rate almost close to that of LP processing can be obtained.

After most of the austenite is transformed into sorbite through this rapid and uniform cooling process, the temperature is further increased to 520-580°C.
It is immersed loosely in a weakly stirred salt tank 9 and kept there until the untransformed austenite is completely transformed into sorbite. After the transformation is completed, it is quickly cooled with hot water or water using the cooling nozzle 10. , the line temperature should be below 200°C.

Finally, it is wound into a coil. As a means of preventing untransformed austenite from becoming martensite or bainite and converting it to solvite, it is immersed in a salt tank heated to a temperature different from the salt tank that achieves a cooling rate almost similar to that of LP treatment, as described above. Alternatively, the salt may be continuously passed through a single salt tank with a temperature gradient so that the outlet side can be controlled at 520 to 580°C.

When immersed in a salt tank with strong stirring at 520 to 550°C, rapid cooling and holding are performed in one tank, and a cooling curve almost similar to that of LP treatment can be obtained.

By such cooling treatment, not only can a material equivalent to LP treated material be obtained, but also a stable descaling property comparable to that of conventional DP treated material can be obtained.

The controlled cooled wire of the present invention has the following features.

(1) It has excellent toughness and high tensile strength, which is on the same level as LP treated material.

(2) Has a sorbite structure.

(3) Uniform properties over the entire length.

(4) Fe3O4 is thin and exists only in the surface layer, and
It has a thin scale layer in which no Fe3O4 is generated in eO.

The features (1), (2), and (3) contribute to improving the drawability of the wire, and the features (3) and (4) contribute to improving the descaling property.

Patenting a hot rolled wire rod makes the mechanical properties of the rolled wire rod uniform over its entire length, thereby preventing wire breakage during wire drawing or processing defects caused by insufficient toughness during subsequent cooling processing. disappears. The wire of the present invention has the uniform structure of the LP-treated wire over its entire length, and its tensile strength is uniform and clearly higher than that of the conventional DP-treated wire.

As is clear from the micrographs described below, the microscopic structure of the wire of the present invention consists of uniformly dispersed fine pearlite, a so-called sorbite structure, and does not contain hardened structures such as martensite and bainite.

In addition, the wire rod of the present invention was forced to cool immediately above the Arl in addition to forced water cooling after finish rolling, and after being held for a few seconds, the temperature was 450 to 55.
Rapid cooling in a salt bath at 0℃, followed by retention in a salt bath at 520 to 580℃, and then cooling with warm water or water to bring the line temperature to below 200℃, but the line temperature is below 20 degrees from the start of cooling. Since the transformation is completed within a short time of less than 6@, eutectoid transformation of the scale is hardly observed by microscopic observation, and Fe3O is thin and exists only in the surface layer, presenting a thin scale layer.

This is because the wire temperature is cooled to 850℃ or less (800℃ or less for chemical descaling) by forced water cooling after finish rolling, and the oxidation of the wire surface is blocked in the salt bath. Contributing.

The wire rod of the present invention requires less time in chemical descaling than conventional DP-treated wire rods, and has superior descaling properties in mechanical descaling. Examples of the wire rod of the present invention will be shown below along with comparative examples of conventional DP-treated materials and LP-treated materials.

SWRH62A (C: 0.62%, Mn: 0
．． 47%) and SWRS8OB (C: 0.80%, Mn
:0.80%) was hot rolled into 8 and 10C @ steel pieces,
After further heating to 1100-1200°C, the diameter was 5.5mm. It was rolled into φ and 13 mm φ wire rods.

Table 1 shows the mechanical properties of the wire of the present invention, the conventional DP-treated material, and the LP-treated material, the layered pearlite ratio observed under a microscope, the results of the wire drawability acceleration test, and the results of chemical and mechanical descaling.

Residual scale ratio The strength and toughness of the wire rod of the present invention are on the same level as the LP-treated material, and the strength is clearly higher than that of the conventional DP-treated material.The toughness is about the same for the small diameter (5.5φSWRH62A), but for the large diameter (13φSWRS8OB) ) is clearly superior.

The microscopic structure of the wire rod of the present invention is composed of a fine pearlite so-called sorbite structure, and the amount of coarse layered pearlite, which is poor in wire drawability, is 5% or less by point counting method.

Figure 2 is 5.5φSWRH62Al Figure 3 is 13φSW
The microscopic structure (×500) of RS8OB is shown.

A is a wire rod of the present invention, B is a conventional DP treated material, and C is an LP treated material. The drawability of the wire rod of the present invention was at the same level as the LP-treated material in an accelerated fracture test, and was superior to the conventional DP-treated material.

The descaling property of the wire rod of the present invention is reduced by a chemical descaling method, although the time is slightly shorter than that of the conventional DP-treated material, and it is much shorter than that of the LP-treated material.

In addition, the mechanical method shows a much better residual scale rate than the LP treated material, and is in the good level range of the conventional DP treated material. As shown in the micrograph of the scale in Figure 4, the surface of the scale of the LP treated material is covered with a layer of Fe3O4, followed by a somewhat thick layer of Fe3O4, and a layer of FeO on the surface of the wire. A large amount of transformation to Fe3O4 has occurred in the wire, and the thickness of the entire scale layer is about twice that of the wire of the present invention.

In the conventional DP treated material, Fe3O4, which is almost the same as the wire of the present invention, is thin and exists only in the surface layer, and Fe3O4 is present in the FeO.
It has a thin scale that does not occur. The wire rod of the present invention is composed of a Fe3O4 layer and a FeO layer.
The layer is thinner than the FeO layer and cracks can be seen in the Fe3O4 layer. That is, it seems that the wire rod of the present invention has somewhat more cracks and thus has a more brittle scale, which is thought to give it a slight advantage in descaling properties. As described above, the wire rod of the present invention is a so-called directly heat-treated wire rod that does not require reheating, has uniform properties over the entire length, and has descaling properties that are as easy to descale as conventional DP-treated wire rods. It is a wire rod with good wire drawability and has high strength and toughness equivalent to LP treated material, which cannot be obtained with DP treated material.

Therefore, there is no need for a large amount of cost and man-hours associated with reheating, and the descaling time is also shortened, resulting in extremely large industrial effects.

[Brief explanation of drawings]

'' Figure 1 is an explanatory diagram of the tempering cooling equipment of the present invention, Figure 2 is a microscopic photograph (x500), and Figure 3 is another microscopic photograph (x500).
500), Figure 4 is a scale micrograph (×500)
shows. 1...Wire rod finishing rolling machine, 2...Rolled wire rod, 3...Cooling guide pipe, 5...Louiing device, 7...Heat insulation Zone, 8, 9...
・Salt tank, 10... Cooling nozzle.

Claims (1)

[Claims] 1 A hard steel wire rod with a C content of 0.40% to 0.85%, the metal structure of the wire rod being a sorbite structure mainly composed of fine pearlite particles and fine pro-eutectoid ferrite dispersed therein, The structure does not contain supercooled structures such as martensite and bainite, and the layered coarse pearlite structure is 5% or less by point counting method, and the surface oxidation scale of the wire is Fe_3O_4 in the upper layer and Fe_3O_4 in the lower layer. Fe
It consists of two layers of O, the Fe_3O_4 layer is thinner than the FeO layer, there is no Fe_3O_4 in the FeO layer, and
A high-strength, high-toughness directly processed steel wire rod with excellent wire drawability and descalability, characterized in that the metallographic structure and scale state are uniform in the length direction of the wire rod.