JPS5947012B2 - Pre-strengthening, stress-relieving steel workpiece of carbon steel or low-alloy steel and its manufacturing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to pre-strengthened and stress-relieved elongated steel products, and more particularly to steels with high tensile strength, high yield strength and low levels of residual stress. It is something.

As is well known to those skilled in the art, pre-strengthened cold-finished steel bars have the characteristics of both high strength and excellent machinability.
Therefore, useful parts can be manufactured from steel bars by various mechanical operations without the need for a subsequent heat treatment process.

To date, pre-strengthened cold-finished steel bars or rods have mostly been produced by cold drawing by passing the rod or rod through a die, giving a relatively large reduction in the cross-sectional area of the rod or rod. It was made by

Cold drawing work is then followed by straightening work (removal of curls)
In this operation, the rod or rod is passed through several rollers and bent one after the other in decreasing directions so that the rod or rod becomes straight.

Straightening devices for this purpose are well known to those skilled in the art and include the Lewis straightening machine and the Medart straightening machine.

It has been found that it is better to perform stress relief work after this straightening work, in order to increase the yield strength of the bar or rod to a high level and at the same time reduce residual stress. .

This stress relief is typically carried out in a batch or continuous furnace maintained at a temperature within the range of 500-1200F (260-649C).

Uniformity maximizes yield strength, both in terms of the uniformity of each bar processed in the furnace, and in terms of mutual uniformity from bar to bar. At the same time, it is an important thing to consider in order to minimize residual stress.

It has been found that the uniformity of the resulting bar or rod is directly related to the degree of temperature uniformity during the heating process.

The production furnaces in which stress-relieving steel bars or rods are produced industrially are, due to economic necessity, inserted into several tons of bars or rods at a time in order to achieve the desired uniformity. for at least several hours, usually more than 5 hours,
Must be maintained at an appropriate temperature.

However, even if the rods and rods are left in the furnace for such a long time, it is not easy to obtain ideal uniformity, both in terms of the temperature at which they are maintained and the time they are kept at that temperature, and it is not economical. It is impossible to try to obtain a production rate of

SUMMARY OF THE INVENTION The object of the present invention is to provide a method for manufacturing bars, rods or similar stress-relieved, pre-strengthened corundum crops, which overcomes the drawbacks mentioned above.

Furthermore, a particular object of the present invention is to have high tensile strength, high yield strength, and low residual stress.
The object of the present invention is to provide a method for manufacturing steel that is stress-relieved and pre-strengthened.

It is also an object of the present invention to be able to produce stress-relieved products economically and quickly, and to ensure that both individual workpieces and the workpieces as a whole are capable of being sufficiently heated during heating operations. It is an object of the present invention to provide a method for manufacturing a long and thin corundum crop subjected to stress relief and pre-strengthening so as to obtain good uniformity.

These and other objects and advantages of the present invention will become more fully apparent from the further description below.

BRIEF DESCRIPTION OF THE DRAWINGS Aspects of one embodiment of the invention are illustrated in the accompanying drawings, which are by way of illustration and not limitation.

The concept of the present invention is based on the following discovery.

In other words, a long and slender corundum workpiece is pre-strengthened by cold working, then straightened, and then subjected to stress relief under heating temperature.This stress relief operation quickly brings the workpiece to the desired temperature of 5 degrees Celsius. Specifically, it was the discovery that the tensile and yield strengths of the steel are maximized and the residual stresses are at their lowest levels when this is done by heating, typically within 10 minutes.

It has been found that the elongated corundum crops produced in this way result in products of exceptionally good uniformity, machinability and very low levels of residual stress.

As illustrated in Figure 1, which is a stress versus strain curve, the cold drawing of a corundum crop increases as the strain of the workpiece increases, until the elastic limit of the steel is reached. Stress increases proportionally.

At the elastic limit, the rate of rise of the curve increases as the strain applied to the bar increases, and the increase in stress becomes much more gradual.

However, when the above-mentioned correction operation is carried out, a somewhat different effect appears.

As shown in Figure 1, the stress versus strain relationship for a corundum crop that is first cold drawn and then straightened is similar to the stress versus strain relationship for a corundum crop that is drawn but not straightened. It's a little different.

As shown by the lower curve in Figure 1, the stress versus strain relationship for a cold-drawn and then straightened workpiece is linear until the elastic limit is reached; As strain increases, stress gradually increases.

Therefore, although the cold drawing operation serves to increase the strength of the steel, it also serves to impart residual stresses.

Straightening operations, on the other hand, serve to rearrange or redistribute residual stresses, but tend to reduce the yield strength of the resulting workpiece.

As shown in Figure 1, the yield strength, i.e. the metal is 0.2%
The stress that exerts the strain is σy.

The yield strength σy for a cold-drawn workpiece is:
It is larger than the yield strength σy′ for a workpiece that has been straightened after cold drawing.

The purpose of the stress relief operations described above is to increase yield strength.

FIG. 2 depicts a series of curves representing the effect of thermal stress relief on tensile strength and yield strength properties as a function of the logarithm of the time over which the heat treatment is performed.

Two curves are shown for each property, one for temperature Ill, C approximately 6001" (approximately 316 degrees Celsius)] and the other for temperature T2 [approximately 900 below (approximately 482 degrees Celsius)]. It is for.

As is clear from these curves, both the tensile strength and yield strength of the corundum crop decrease as the stress is relieved by heating when it is straightened after cold drawing, and its tensile strength and yield strength decrease. The rate of reduction increases with heat treatment time.

However, in the case of residual stress, the rate of reduction of the residual stress level is much faster.

In accordance with the practice of the present invention, the use of a rapid uniform heating step during stress relief operations generally reduces residual stress levels in the same way by limiting the time that the workpiece is subjected to heating temperatures. In other words, the use of a rapid uniform heating process achieves stress relief without rapid loss of tensile and yield strength. .

In accordance with the practice of the present invention, cold working, particularly preferably cold drawing, of an elongated corundum workpiece in the shape of a rod or bar, or other shape of uniform cross section, significantly increases the strength of the workpiece. The cross-sectional area can be reduced as much as possible.

This cold drawing operation is performed by conventional drawing techniques, such as those shown in FIG. 3, in which an elongated workpiece 10 is simply passed through an extrusion die 12 to form a pre-strengthened workpiece 14.

As used herein, the term "cold drawing" refers to a drawing operation performed at a temperature below the lower transformation temperature of the steel.

After this cold drawing operation, the workpiece was straightened by a straightening operation, i.e., by bending the bent parts of the workpiece from side to side, and in particular by progressively bending the workpiece so that the deflection gradually decreased. It is used to create workpieces.

For this purpose it is possible to use a conventional Lewis straightening machine, which is schematically shown in FIG. 4, or a Medart straightening machine, which is shown diagrammatically in FIGS. 5 and 6.

These straightening machines are themselves conventionally used, and function to straighten a workpiece into a straight workpiece while gradually bending the workpiece less and less.

In accordance with the practice of the invention, rapid heating of the workpiece for stress relief operations is preferably accomplished by heating the workpiece itself as a resistor.

That is, when a current is passed through the workpiece, the electrical resistance of the workpiece to the current causes rapid heating of the workpiece.

The workpiece is preferably provided with connections at both ends of the rod to connect it to a power source so that the entire rod is fully energized.

Since the current flows uniformly through the rod, the temperature of the rod increases uniformly not only in its axial direction but also in its radial direction.

In this way, both the inside and outside of the workpiece are heated simultaneously, and no thermal deformation occurs.

In a preferred implementation of the invention, low frequency current or preferably direct current can be used.

Therefore, a sufficiently high electric power can be used to heat a large workpiece within a short time.

An example of a suitable means for electrically resistively heating the workpiece 14 is shown diagrammatically in FIG.

As shown, the workpiece 14 is provided with electrical contacts 16 and 18 adjacent thereto, and the electrical current between the contacts 16 and 18 passes along the entire length of the workpiece 14.

Generally, during heating operations, it is preferable to tension the workpiece 14 to accommodate the thermal expansion of the workpiece under heating and to avoid bending or warping.

The slight tension applied to the workpiece during the stress relief operation is thus
Keep the workpiece straight and free from plastic deformation.

Stress relief operations are also somewhat time and temperature dependent.

It has been found that stress-relieving workpieces using electrical resistance heating have a slightly higher temperature of 50 to 200" F (10 to about 93 C) higher than typical furnace heating. Temperature is required to obtain the same mechanical properties since the rate of change of these mechanical properties under a given temperature is relatively slow.

However, because of this, and because the reduction in residual stress occurs much more quickly, a substantially lower level of residual stress is achieved.

It is important in the practice of this invention that the heating operation must be uniform and rapid in order to produce a workpiece with high tensile strength, high yield strength, and low residual stress.

Generally, a heating time of 10 minutes or less, preferably 15 to 120 seconds, is sufficient to achieve the desired temperature.

It may also be desirable to hold the workpiece at the desired temperature for 1 second to 5 minutes.

The temperature at which the workpiece is heated is typically 500'F (260
°C) to the lower transformation point of the steel, preferably from 500 to 1300'l' (260 to about 704 °C)
It is.

The use of slightly higher heating temperatures according to the invention is due to the fact that the entire heating operation is carried out quickly and uniformly, compared to several hours of heating in conventional furnaces.

It is a well-known technique to reduce the strain in a metal wire by heating it after drawing it.

For example, Young U.S. Pat.
You can see this in issue 32.

However, since Young does not straighten the wire before the strain becomes normal, he does not suffer from the problem of reduced yield strength, as opposed to the present invention.

The present invention is applicable to various carbon steels and low alloy steels, such as carbon steel containing 0.10 to 0.75% by weight of carbon and low alloy steel containing 0.10 to 0.75% by weight of carbon. be able to.

The invention is particularly suitable for the manufacture of rods and bars made of Al5I/SAE grade 1144° 4142 and 4142H steels and 4142 and 4142H grade steels with additives to improve machinability.

Generally, such steel contains 0.10 to 0.75% by weight of carbon,
Manganese 0.50-1.50% by weight, Shin 0.01-0
．． 0.07% by weight, 0.01-0.50% by weight of sulfur, 0-1.50% by weight of chromium, 0-0.50% by weight of molybdenum and 0.10-0.80% by weight of silicon, the balance being iron. and normal impurities.

Having described the basic concept of the invention, the following examples will now be described.

However, these are for the purpose of illustrating the implementation of the present invention and are not intended to be limiting.

Example 1 A group of similarly heated and hot-rolled 1144 steel rods was descaled and then passed through a die into relatively thick 1144 steel rods.
It was drawn using a drawing machine, and then straightened using a rotary Medart straightening machine.

This group of bars is then divided into two subgroups, the first subgroup is processed in a roller hearth furnace for 5 hours, while the second subgroup is heated using a resistance heating method with a heating time of 60 to 90 seconds. Stress relief was performed.

The obtained mechanical strength properties are as follows.

Furnace heating Resistance heating (468℃) (482℃) Tensile strength (K9/ran2) 84.7 8
8.9 Yield strength K9/m 70.7 7
9.1 Elongation - 1210 Drawing (% 3227 Hardness (Rc) 23 23 The bar was then tested for machinability on a 1" run, 6-spindle National Acme automatic threading machine.

The test parts were made through eight hours of testing at the scheduled cutting speed and feed each time.

Tests have shown that the material stress relieved by resistive heating is clearly superior in two observed parameters: perforation properties and partial growth.

For the furnace-heated material, the drill had to be changed three times during the 8-hour test, but for the resistance-heated steel, the same drill could be used for the entire 8-hour test.

Similarly, resistance-heated steel exhibits only 0,020 inches of partial growth (which is related to tool wear).
mm), whereas for the furnace-heated material it was 0.
It was 0,015 inches (0,038 m).

During this test, further interesting observations were made regarding the uniformity of the two states of the material.

As a result of making a wider range of samples in both conditions and conducting hardness tests, we found that the furnace heat treated material has a hardness difference of 6 points in Rc, but the resistance heat treated material has a difference of only 4 points. It turns out there isn't.

Therefore, it can be seen that the uniformity of the resistance heat treated steel is significantly better than that of the furnace heat treated steel.

Example 2 Another group of 1144 steel bars, all with the same heat treatment, were processed as described in Example 1 to obtain the same mechanical properties.

The heating furnace temperature in this example was 850'F (approximately 454°C), and the temperature at which the steel bar was resistance heated was 1030'F (approximately 554°C).
℃).

Cutting tests including very light surface removal cutting using a boat-shaped tool and long center cutting were conducted on the materials in both conditions to find out the maximum cutting limit and to compare them.

This critical parameter measures the lifespan of a boat tool on an automatic thread cutting machine in terms of the number of parts that have been made at the time of failure.

As a result of this test, 1290 parts were made from the material that was stress relieved by resistance heating before the failure of the forming tool occurred, but only 970 parts were made from the material that was treated by furnace heating.

Therefore, it can be concluded that stress relief treatment by resistance heating in this operation increased the life of the forming tool by 33%.

Example 3 A third group of 1144 steel bars, all with the same heat treatment, were again processed as described above to obtain the same mechanical properties, but the furnace heating temperature was below 900C (approx. 482°C). The resistance heating temperature was 1050'F (approximately 566°C).

The materials in these two processing states were assembled to cut a 1''-57 screw thread.
t and Whitney) was tested on a lead thread cutter.

Deformations resulting from residual stresses in pre-strengthened cold-finished steel bars are a major problem for lead screw producers.

A measure of this deformation is the lead error (axial error) over a given length.

0.0003 inches per foot (0.025 mm/
m) Lead errors below are generally considered to be excellent,
Usually obtained by roughing and finishing followed by grinding.

The following lead errors were measured after making a sufficiently deep cut in one pass on both stress-relieved materials.

B. Furnace heating stress removal 0.0012"/ft (0
,1rran/m) B, Resistance heating stress removal 0.00005"/ft (
0,004M/m) Both of these two states have the same tensile strength of 125,000 ps.
It is surprising that such results were obtained, considering the fact that they were made with i (87,5K9/man2>), and the only difference is the heating method. .

This result shows that the residual stress level of the steel bar subjected to stress relief by resistance heating is more than one order of magnitude lower than that of the steel bar subjected to furnace heat treatment.

The details of the treatment, operation and use of the invention depart from the spirit and scope of the invention, particularly as limited by the claims.

It will be appreciated that various changes and reforms can be made without escaping.

Next, embodiments of the present invention will be listed.

(1) Steel according to claim 1, wherein the steel contains 0.1 to 0.75% by weight of carbon.

(2) The steel according to claim 1, wherein the steel is Al5I/SAE 1144 steel.

(3) Steel has carbon 0.10 to 0.75% by weight and Mn 0.5
0-1.5% by weight, Po, 01-0.07% by weight, SO
, 01-0.50% by weight, CrO-1.5% by weight, Mo
Steel according to claim 1, comprising 0.50% by weight of O''-0 and 110-0.80% by weight of 5iO, the balance being iron and normal impurities.

(4) The steel according to claim 1, wherein the workpiece is heated by passing an electric current through the workpiece.

(5) The method according to claim 2, wherein the workpiece is heated by passing an electric current through the workpiece.

(6) The workpiece temperature is 500 to 1300'F (260 to about 70'F)
3. A method according to claim 2, wherein the method is heated to a temperature within the range of 4° C.).

(7) The method according to claim 2 (C), wherein the workpiece is heated to a desired temperature within a range of 1 second to 5 minutes.

(8) The method according to claim 2, wherein the workpiece is straightened by passing through a Lewis straightening machine.

(9) The method according to claim 2, wherein the workpiece is straightened by passing through a Medart straightening machine.

(10) The method according to claim 2, wherein the steel is carbon steel or low alloy steel containing 0.10 to 0.75% by weight of carbon.

(1) The method according to claim 2, wherein the steel is Al5I/SAE 1144 steel.

12. The method of claim 2, wherein the workpiece is drawn to reduce its cross-section and increase its tensile strength.

(13) Claim 2 in which the workpiece is subjected to cold drawing
The method described in.

(14) The method according to claim 2, wherein the cold working is a cold drawing operation performed at a temperature below the lower transformation point of the steel.

15. The method of claim 2, wherein the workpiece is heated at a substantially uniform rate over its cross section.

(16) The method according to claim 2, wherein the workpiece is straightened by passing the workpiece between a plurality of rollers, bending the workpiece, and straightening the workpiece.

(17) A workpiece made of carbon steel or low alloy steel is pre-strengthened by cold drawing at a temperature below the lower transformation point of the steel, and then the workpiece is passed between a plurality of rollers. Reverse bend and straighten, and then pass current through the workpiece to heat the workpiece to 500'F (260°C) within 10 minutes.
The workpiece is heated substantially uniformly across its cross-section to a temperature within the lower transformation temperature of the steel, so that mechanical properties are maintained at a high level while residual stresses are at a low level. stress relief, characterized by carrying out stress relief;
A method for manufacturing a pre-strengthened elongated corundum crop.

(18) Pre-strengthening a workpiece formed of carbon steel or low alloy steel by cold working and then processing the workpiece within 10 minutes from 500'F (260°C) to the lower transformation point of the steel. A stress-relieving pre-strengthened workpiece characterized by rapid heating to a temperature within the range of How things are manufactured.

[Brief explanation of drawings]

FIG. 1 is a curve showing the relationship between stress and strain in a corundum crop before and after straightening. FIG. 2 is a curve showing the effect of the time period on which steel is subjected to a stress relieving operation by heating on tensile strength, yield strength, and residual stress level. FIG. 3 is a schematic diagram illustrating a cold drawing operation. FIG. 4 is a schematic diagram showing a straightening operation using a Lewis straightening machine. Figures 5 and 6 illustrate the operation of the Medart type deburring machine. FIG. 7 shows a preferred stress relief operation in accordance with the practice of the present invention.

Claims (1)

[Claims] 1 A workpiece of elongated shape made of carbon steel or low-alloy steel is pre-strengthened by cold working, then straightened by bending the workpiece in the opposite direction, and then the workpiece is heated to 500°C within 10 minutes. By rapidly heating the steel to a temperature ranging from 260°C to the transformation point of the steel, stress is removed so that residual stress is reduced to a low level while maintaining high mechanical properties. A method for manufacturing a pre-strengthened, stress-relieving corundum crop of elongated shape made of carbon steel or low-alloy steel, characterized by: