JPS5830938B2 - Continuous heat treatment method for high carbon steel wire rod for high processing cold drawing - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for continuous heat treatment of wire rods for high processing and frequent drawing.

The present invention relates to a method for continuous patenting treatment of high carbon steel wire immediately after hot rolling or after reheating, and in particular, it uses a lead bath while advantageously controlling the cooling curve from a temperature exceeding the austenitizing temperature. The present invention relates to a heat treatment method for obtaining a fine pearlite structure that has both the highest level of strength that can be achieved for the steel type and the toughness to withstand high-level cold working.

This invention involves forcedly cooling a high carbon steel wire rod immediately after hot rolling or after reheating in a specific refrigerant from a temperature above the austenitizing temperature, and then allowing it to cool in the atmosphere or further cooling with water and then winding it. The surface temperature of the wire in the refrigerant is in the range of 350 to 550°C, and the required cooling time at that point is the continuous cooling transformation (so-called OCT) of the steel type.
) The heat transfer coefficient is controlled to be within the starting time, and then the heat transfer coefficient is 20 to 100 Kcal/m2 for 10 to 30 seconds. Cooling in refrigerants h and c. One example is cooling in the air, then cooling with water and winding up, or immediately after exiting the specific refrigerant, processing it into a loop shape and keeping it in the air or further at 400 to 500℃. The heat treatment process is extremely simple and allows for uniform hardness within the cross-sectional area of the wire, as well as the highest level of toughness for the steel type, simply by stacking the bundles in a heat-insulated tank. It's a method.

The invention of this invention is to reduce the starting temperature of forced cooling to 350~
By setting the cooling time in a specific refrigerant to a temperature range of 550°C to 3 seconds or less, the same cooling treatment can be performed to achieve a steel composition range (C; 0.40-0.90%, Mn
; 0.20 to 1.00%), it enables continuous heat treatment to achieve the highest level of toughness of the steel type.

High carbon steel wire rods such as hard steel wire rods and piano wire rods are subjected to a so-called patenting treatment and then cold drawn through dies, that is, wire drawn, to impart predetermined strength and toughness before being used.

This patenting can be roughly divided into direct patenting (abbreviated as L)P), which is performed continuously with the hot rolling process, and normal patenting, which is performed by reheating the hot rolled wire rod or drawn wire rod.

Air patenting (AP) and lead patenting (LP) are generally used for the latter, and LP is mostly used for the purpose of obtaining high toughness.

It is well known that the fine pearlite structure obtained through the patenting process has a great influence on the properties of the wire rod after wire drawing.

However, in AP, the refrigerant is usually atmospheric air, so the heat transfer coefficient (
The heat transfer coefficient (also known as the heat transfer coefficient) is small, making it impossible to achieve high strength, and there are many problems such as the formation of pro-eutectoid ferrite that impairs wire drawability. It is only used for specific purposes.

On the other hand, the first characteristic of LP is that it uses molten metal as a refrigerant, so the initial cooling rate is very low when the heat transfer coefficient is low, and it is easy to transform from moderately cooled austenite to fine pearlite. One of the characteristics of this is that the lead bath can be maintained at 450 to 600℃, which is the transformation temperature range that produces fine pearlite (so-called sorbite), which is suitable for imparting toughness. It was mentioned that it is expected that the formation of

However, it is a well-known fact that the cooling rate of a normally cooled object decreases rapidly as it approaches the temperature of the refrigerant, and when transforming at the refrigerant temperature like LP, the transformation generally starts from the ideal bath temperature. It is undeniable that the transformation was difficult, that is, the transformation was not constant temperature, and the transformation occurred in a higher temperature range, resulting in insufficient strength.

Furthermore, since the possible range of the heat transfer coefficient value of molten lead in the 450 to 600' (m degree range) is narrow, the cooling capacity is insufficient for low carbon/low manganese wire rods that start transforming quickly or wire rods with large wire diameters. Therefore, the transformation started at a higher temperature range, and the highest level of toughness that could be achieved was not achieved.

In fact, the fact that the strength of the wire heat treated by LP is dependent on q and wire diameter (that is, the wire diameter tends to be lower as the wire diameter increases) has been established and accepted in actual operations.

In addition, when implementing LP, it is particularly necessary to prevent harm to the human body due to high-temperature lead gas and lead oxide, and it has become an unfavorable technology in terms of resource conservation.

Currently, various types of DP methods have been devised and some of them have been put into practical use, but the biggest problem is LP, J:
It is also a much lower intensity upgrade to the LP level.

The cause of low strength in DP is transformation in the high temperature range due to insufficient cooling capacity.

In other words, in the typical cooling pattern of straight jet water cooling followed by forced air cooling in a loop below 700°C, the main transformation occurs during forced air cooling, so the temperature is 450 to 600'C, such as LP. The transformation takes place in a much higher temperature range than the above range, and high strength cannot be achieved.

Since the transformation takes place during cooling in the form of a loop, one of the issues to be solved is that the difference in cooling rate depending on the circumferential position results in a difference in strength. This is important because the more you expect it, the bigger it becomes.

Although it is possible to achieve high strength even if the cooling period is delayed by adding elements such as Cr that significantly delay the start of transformation, this has the opposite effect on the trend toward reducing the amount of added elements, and also reduces the cooling time required. This is not the best method as increasing the length of the process leads to a decrease in workability.

This invention addresses the above-mentioned problems in the prior art regarding heat treatment of high carbon steel wire rods. The aim is to completely solve both the problems of insufficient strength and variations in strength by using the same method while reducing costs.

The method of this invention stipulates that as a condition for cooling from a temperature higher than the austenitizing temperature, the wire surface temperature must first be lowered in a range of 350 to 550°C within the CCT start time in a specific refrigerant; ~100Kcal/m2
An example of a refrigerant with a heat transfer coefficient of hC, the basic content is to achieve the highest level of toughness that the steel type should achieve by undergoing quasi-isothermal pearlite transformation while cooling in the air. 350~550℃
By specifying the cooling time required in temperature range 1 to 3 seconds or less, a single universal cooling system that can be applied to almost all wire steel types to achieve the highest level of toughness. Patterns and vines.

After cooling to a temperature range of 350 to 550℃ in a specific refrigerant,
Immediately after processing the wire rod into a loop shape, the wire is stacked in bundles in an insulated tank in an atmospheric atmosphere and stored. 3. The equipment can be made more compact by means of stirring, and this method is extremely effective for fast linear speeds.

In this invention, the pearlite transformation is carried out in the air, which has an extremely small heat transfer coefficient, after cooling to a temperature range of 350 to 550°C, or in an insulated tank in an atmospheric atmosphere. This means that the uniform distribution of hardness or strength can be changed.

The DP of this invention does not require any reheating after cooling or maintenance in a constant temperature atmosphere, so it is extremely economical and simple.

In the method of this invention, an aqueous solution of an arbitrary solute and an arbitrary temperature including pure water is used as a specific refrigerant, and by employing a method that has been generally considered completely impossible, that is, continuous immersion cooling of the wire in a complete aqueous solution. Not only can the use of lead such as LP be avoided, but the change in flow velocity can be skillfully used to easily and quickly change the heat transfer coefficient of the refrigerant to any level, simplifying work and reducing costs. This had an extremely large effect on the development of society.

Wire surface temperature range achieved in specific refrigerant: 350-550℃
The lower limit was set at 350°C because the lower the temperature, the higher the tensile strength. However, when the temperature is lower than 350°C, the heat-treated structure becomes bainite-like, resulting in a noticeable decrease in toughness, and the subsequent high-level cold treatment This is because processing (i.e., wire drawing) is completely impossible, and the reason why the upper limit was set at 550°C is that although there is almost no decrease in toughness even when the temperature exceeds 550°C, the tensile strength is lower than that of normal LP processing. This is because the level will drop significantly.

The highest level of strength can be obtained by appropriately selecting the wire surface temperature within the range of 350 to 550°C depending on the steel type and wire diameter, but in general, low C-low Mn material content and large diameter wires are on the lower temperature side. Should.

In addition, the required cooling time in the 350-550℃ temperature range 1 is 3.
The reason why we were able to create a cooling pattern that can be applied to all types of steel by making it less than 1 second is because the CCT start time of the lowest C-low Mn hard wire material is approximately 3 seconds. It is something that

In order to enable rapid cooling to a temperature range of 350 to 550°C, the temperature of the forced refrigerant is limited to 300°C or less.

The heat transfer coefficient of the refrigerant in the temperature range of 350 to 550℃ is 20
The reason why the temperature was limited to -100 Kcal/m2h°C is as follows.

In the case where transformation begins, the wire temperature decrease should be as small as possible, and for this purpose the smaller the heat transfer coefficient of the refrigerant, the better; however, on the contrary, after the transformation begins, it is necessary to remove the heat of transformation generated in order to accelerate the transformation. A certain value of the heat transfer coefficient must be ensured.

For this reason, the heat transfer coefficient of the refrigerant during transformation is set at 20 to 100 Kcal/in relation to the temperature range of 350 to 550°C.
It was defined as rrFh'C.

By the way, the heat transfer coefficient when cooled in the atmosphere is 30 to 50 K.
c ai/m2h 'C, and is one of the appropriate refrigerants for the above purpose.

The high carbon steel wire rod to which the above heat treatment method is applied generally contains 0.40 to 0.90% of C, and contains Si2MnP, S, At, etc. as normal components other than C in an appropriate range. It is a wire rod, and further contains Ni and Cr as additional elements.
, Mo, V, B, Ti, Nb, REM, etc., are also included.

The present invention will be explained in detail below.

Figure 1 shows an example of a cooling curve when a wire rod is heated above the austenitizing temperature (AC 3 points) and subjected to AP and LP.

In the figure, each upper line corresponds to a large-diameter material, and the lower line corresponds to a small-diameter material.

Curve (2) has a high transformation temperature range, so it produces pro-eutectoid ferrite and becomes coarse layered pearlite, resulting in low strength and insufficient wire drawability.

In the case of curve (2), a fine layered pearlite (namely, nrubite) phase is exhibited, and the strength is dramatically higher than in the case of curve (2), and it also has sufficient wire drawability.

The lower limit line of curve ■ indicates that transformation starts near the lead bath temperature;
It approaches so-called isothermal transformation, but the thicker the wire, the closer it gets to the upper line and the transformation starts at a temperature higher than the lead bath temperature, resulting in a decrease in strength compared to small diameter materials.

In fact, in the past, it has been accepted that the strength of large-diameter materials decreases, but in order to avoid such a decrease in strength due to the large diameter, which is a natural consequence of cooling theory, it is generally recommended to use Cr or other materials. By adding these new alloying elements, we aim to delay the start of transformation so that it starts near the lead bath temperature, and we have seen success in this, but the addition of such new alloying elements increases costs and reduces resource savings. Since this goes against the main idea, it cannot necessarily be said to be an essential improvement measure, and it would be more desirable to first try to solve the problem by devising heat treatment as in the present invention.

The running speed of the enclosed wire is extremely high (for example, 5.5 rranφ
For P, it is clear that the shorter the cooling time from the heating temperature, which includes the time required for transformation, the better.In fact, it is intended to shorten the transformation start time. It can be said to be the physical strength of the body.

Even in the case of LP, if it becomes possible to shorten the heating time, it is natural that the running speed of the line will be increased to increase work efficiency, and one way to avoid the increase in size of the equipment that arises is to reduce the CCT start time. can be shortened.

The shorter the CCT start time, the more the work efficiency can be improved, but in normal LP, the transformation starts from a higher temperature range, so the degree of strength decrease increases, and in the end, this invention is not new. Technology will become essential.

Curve 2 in FIG. 1 is a typical cooling curve based on the Q invention, where the solid line means the specific refrigerant and the broken line means the atmospheric refrigerant.

The upper line is suitable for small diameter materials or high C-highland materials, and the lower line is suitable for large diameter materials.

Also, the more you use the lower edge line in each case, the higher the strength can be achieved.

Curve ■ is 350 to 5 from the heating temperature according to the method of this invention.
A cooling curve is shown when the time required for cooling to a temperature range of 50°C is 3 seconds.

Depending on the CCT start line shown, 5WRH62A
Wire U is cooled to 400°C and left to cool in the air for 1.5 seconds before transformation begins, while 5WRH82B wire is cooled to the same temperature of 400°C and left to cool in the air for about 12 seconds before transformation begins. .

In order to achieve the highest level of toughness for each steel type, it is necessary to select an appropriate cooling temperature within the quenching temperature range of 350 to 550°C depending on the wire diameter and composition (JIS composition: C and Mn). If the cooling time is set to 3 seconds or less, there is no need to provide this forced cooling time separately depending on the wire diameter and composition, so this is an extremely effective technique.

Figure 2 shows the conventional LP processing process, in which the wire rods 5, 900 to 105, are sent out from the supply stand 1.
While traveling inside the heating furnace 2 heated to 0'C, it is heated to the austenitizing temperature (A c 3 points) or higher, and 4
Lead bath 3 maintained at a constant temperature within the range of 50 to 600°C
The pearlite transformation is almost completed while passing through, and thereafter it is wound up on the winding drum 4.

On the other hand, in the heat treatment method according to the present invention, for example, when a complete immersion cooling method of the wire is applied using an aqueous solution as a specific refrigerant that performs forced cooling from the heating temperature, the third
As shown in the continuous heat treatment process shown in the figure, the arrangement of the supply stand 1, heating furnace 22, and take-up drum 4 is almost the same as in the case of LP, but the position corresponding to the lead bath 3 (aqueous solution ρ
A cooling water tank 6 is installed that enables circulation and is equipped with a set of accessory devices for automatically controlling the liquid temperature, concentration, and flow rate.

In the second type of heat treatment method according to the present invention, after forced cooling in the tubular cooling tank 8 from the heating temperature to a predetermined temperature range of 350 to 550°C, the wire 5 is immediately processed into a loop shape and placed in the heat insulating tank 10. The process of stacking and storing bundles is shown in FIG. 4 as an example of direct patenting (DP).

The ultra-high speed wire 5 that came out of the final rolling roll 7 passed through a tubular cooling tank 8, was immediately processed into a loop shape by a laying cone 9, and was then vertically lowered to the atmosphere or further heat-retained for 400 to 500'CVc. The bundles are stacked in a heat insulating tank 10 and held in that state at least until the pearlite transformation at the final end is completed.

Next, the material of the wire rod obtained by the heat treatment method according to the present invention will be explained with reference to examples.

Example Table 1 shows the dimensions and main chemical components of the test wire.

One of the rolling ones! 1 or the wire rod of the wire drawing box, 840-9
The sample was heated and held at 60°C and forcedly cooled by an immersion cooling method using an aqueous solution of anionic polysoap (quenching agent) as a coolant, and then allowed to cool in the atmosphere.

Table 2 shows the cooling conditions and the materials of the heat-treated materials.

For steel B, which has a slow start of transformation, the cooling time in the atmosphere in the CCT start box after forced cooling was divided into three stages, long and short, 12.9.0 seconds, but in each case the time was 1401'-q/r.
It was possible to achieve a heat treatment strength of LP level or higher (guaranteed drawing RA > 45%), which exceeds RAN.

Furthermore, when the forced cooling time is less than 6 seconds, it is clear that steel A and B, which have very different CCT start times, can be worked at exactly the same wire speed and cooling tank length, which improves work efficiency and improves work efficiency. It is extremely effective.

Figure 5 shows the wire surface temperature θ at the end of forced cooling for steels A and B.

and the strength δB′i of the heat-treated material? The relationship with RA is shown.

The forced cooling time at three heating temperatures of 840 and 900960'C is 6 seconds for both A and B steel types.

Both steels A and B have almost no dependence of hardenability on heating temperature in the temperature range of 840 to 960°C.

θ. and δ8 have a completely one-to-one correspondence.

For A steel, θ.

<Exceeds LP level strength at 450℃, and B steel has θc less than 5
Exceeds I and P level intensity at 50°C.

θ.

75 & θ tends to increase as δ8 becomes smaller.

It is clear from the decrease in RA that below a certain value, a bainite structure is formed and the toughness is significantly reduced.

That is, as shown in Fig. 5, steel A and steel B are both θ.

350℃, B steel θ. RA drawable at <400℃
Divide the lower limit of 40%.

θ that is drawable (RA > 40%) and capable of scaling intensities above the highest level of LP.

The range of A steel is 350 x θ.

θc〈55o℃ was obtained for B steel at 350℃.

This high toughness makes it possible to scale θ.

It has been recognized in LP that the range depends on the material, especially the C content.

In other words, in the case of LP, Am 450-500°C, Bdl
They have shown the highest level of toughness for LP treatment at a lead bath temperature of 500 to 550°C.

Therefore, as a conclusion of the heat treatment method based on this invention, a temperature range of 350° C. to 550° C. was determined in order to obtain the highest level of toughness that should be achieved overall for 40-90% C wire.

FIG. 6 shows the results of drawing the heat-treated wire showing the highest strength of each steel A and B.

The drawing limits for both A and Bd are equivalent to but higher than LP, and the ultimate strength exceeds that of the ULP treated material.

As explained based on the above embodiments, the continuous heat treatment method for a wire rod containing 40 to 90% C according to the present invention is performed by heating the wire rod to a temperature above the austenitization temperature (AC 3 points) at a diameter wire surface temperature of 3.50 to 550. Forced cooling in the appropriately cooled austenite state in the °C range, then a heat transfer coefficient of 20 to 10
When running in a refrigerant with a temperature of 0Kcal/m2h℃, fine pearlite transformation occurs when the bundles are piled up in a loop shape.
Due to its extremely simple controlled heat treatment process, it can be applied as a DP on hot rolled wire rods or as a patenting method that can replace conventional LP, both of which provide the highest level of toughness for LP. It is possible to easily exceed the maximum toughness that the steel type should achieve, and for the former, it is possible to equalize the strength, and for the latter, it is possible to increase the linear speed C and make it non-polluting. The problems of the prior art can be solved advantageously.

The present invention has the following advantages over the prior art.

■ Superior toughness compared to the LP method.

■ There are no pollution problems like the LP law.

■ Less high-temperature oxidation scale compared to conventional direct patenting methods.

[Brief explanation of the drawing]

Fig. 1 is a graph showing the continuous cooling transformation start line of high carbon steel material, air patenting of wire rod, lead patenting diagram, and cooling curve in the case of patenting based on this invention. Patenting process,
Further, Fig. 3 is an explanatory diagram showing an example of the patenting process by complete immersion cooling of the wire rod using an aqueous solution as the forced cooling medium of the present invention, and Fig. 4 is an explanatory diagram showing an example of the patenting processing process by completely immersion cooling of the wire rod according to the present invention. An explanatory diagram showing an example of a direct patenting process using a method of stacking and storing bundles in a heat insulating tank, and Figures 5 and 6 are charts showing material characteristics of wire rods after heat treatment according to an embodiment of the present invention. be. 1... Supply stand, 2... Heating furnace, 4...
Winding machine, 5... wire rod, 6... cooling water tank, 7... rolling roll, 8... tubular forced cooling tank, 9... laying cone, 10... insulation tank.

Claims (1)

[Claims] 1 High carbon steel wire rod immediately after hot rolling or after reheating,
When patenting is performed through forced cooling during the running process of passing through a refrigerant maintained at a temperature from above the austenitizing temperature to below 300°C, this forced cooling lowers the wire surface temperature to a range of 350 to 550°C. The time necessary for
0Kcal/m2. h. A method for continuous heat treatment of high carbon steel wire rods for high workability cold drawing, which comprises completing the transformation while running in a C refrigerant.