Quench System

Abstract

Low-carbon unalloyed steel strip is heated to a temperature at least above the A.sub.1 point, preferably above the A.sub.3 point, and is quenched to transform substantially all the austenite to martensite. Quenching is accomplished by passing the heated strip through an elongated restricted quench channel having high-velocity quench liquid flowing through the channel, either concurrently or countercurrently. The temperature and flow velocity of the quench liquid are regulated in the channel so as to provide an initial high rate of heat withdrawal from the strip and a subsequent lower rate of heat withdrawal during the time the strip is quenched through the temperature range of martensite formation, thereby effecting tempering of the martensite.

Description

This invention relates to a novel method for continuously quenching metal strip and more particularly for quenching low-carbon unalloyed steel strip to obtain a microstructure which is at least partially martensitic and preferably fully martensitic.

The principal constituents of steel which determine its properties are ferrite and cementite. At a relatively high temperature, which is dependent upon the carbon content, steel exists in the form known as austenite which is a solid solution of carbon or cementite in ferrite. When steel is cooled slowly from a high temperature at which austenite is stable, the ferrite and cementite precipitate together in a characteristic lamellar structure known as pearlite. However, dependent upon the rate of quenching and other factors, the transformation from austenite to pearlite proceeds through a series of different microstructures. The low-temperature transformation product in the transformation of austenite upon cooling is martensite which is a body-centered tetragonal structure in which the carbon atoms are thoroughly dispersed. Martensitic steels are characterized by high tensile and yield strengths.

Plain carbon steels of relatively high carbon content and certain alloy steels, particularly those containing hardenability agents such as boron, are more easily quenched to a martensitic structure, but the plain carbon steels of relatively low carbon content are considerably more difficult to quench to martensite. As will be understood from the customary isothermal transformation diagrams, plain carbon steels of low carbon content (0.03-0.25 weight percent) require extremely rapid quenching in order to transform substantially all the austenite to martensite. In general, it may be stated that low carbon steel strip must be quenched from the austenitizing temperature to below the temperature for the start of martensite formation in about 0.1 to about 0.8 seconds. In addition, the quenching must be accomplished uniformly so as to obtain a uniform microstructure and so as to avoid excessive warpage or distortion of the strip.

In my prior U.S. Pat. No. 3,410,734, granted Nov. 12, 1968, I have described and claimed a channel quench system which is particularly adapted for the continuous quenching of low-carbon steel strip to obtain a fully or partially martensitic microstructure. In such system, it is found that in most cases a certain amount of in situ tempering or self-tempering of the martensite can occur during the quenching of the strip because the temperature range of martensite formation is relatively high for plain carbon steels of low-carbon content (0.03 to 0.25 weight percent). Consequently, in spite of the rapidity of the quench provided by such system, a significant degree of tempering can take place during the short time required to cool the strip from the martensite transformation temperature range to ambient temperature.

However, it is desirable to be able to regulate the extent of tempering of the martensite rather than to be dependent solely upon the uncontrolled self-tempering effect which occurs as described above. By modifying the operation of a channel quench system, such as the one disclosed in my aforementioned patent, it is possible to provide selective control over the heat withdrawal rate in different portions of the quench system and thereby achieve a controlled degree of tempering. Thus, within limits, it is possible to tailor the quench action to the chemistry of the steel and other process variables so as to obtain varying combinations of physical properties in the end product.

Accordingly, the broad object of the invention is to provide a novel and improved quench system for making low-carbon martensitic steel strip.

A further object of the invention is to provide a novel and improved method for quenching low-carbon steel strip in a channel quench system so as to obtain at least a partially martensitic microstructure with a desired degree of tempering of the martensite.

Another object of the invention is to provide a novel and improved quench method for making low-carbon martensitic steel strip characterized by the provision of different heat withdrawal rates in different portions of the quench system so as to control the extent of tempering of the martensite.

An additional object of the invention is to provide a novel and improved method for increasing the extend of tempering of martensite during the quenching of low-carbon steel strip to form martensitic steel strip.

Other objects and advantages of the invention will become apparent from the subsequent detailed description taken in conjunction with the accompanying drawing, wherein:

FIG. 1 is a schematic vertical sectional view of one embodiment of a quench apparatus for the production of martensitic steel strip in accordance with the present invention; and

FIG. 2 is a similar view of another embodiment of the quench apparatus.

The quench system of the present invention is illustrated in the drawing as embodied in a continuous heat-treating and quenching line for making martensitic steel strip. The resulting martensitic steel strip may be used as such or it may be tin plated, galvanized, or aluminum coated.

The steel strip starting material is plain carbon steel having the following composition range (weight percent): carbon 0.03-0.25, manganese 0.20-0.60, phosphorus 0.05 max., sulfur 0.03 max., and the balance iron with residual elements in the usual amounts. Preferably, the steel strip starting material is in work-hardened or as-cold reduced condition, and although the gauge of the strip will usually and preferably be within the range of from about 0.002 to about 0.050 inch, the invention in its broadest aspect is also applicable to steel strip having a thickness as low as about 0.0002 inch and as high as about 0.100 inch.

As seen in FIG. 1, steel strip 10 is fed downwardly through a furnace, shown fragmentarily at 15, after first passing through conventional cleaning and rinsing steps (not shown) in which the residual rolling oil is removed. In the furnace 15 the steel strip is heated to a uniform temperature above the A.sub.1 critical point so that the steel is at least partially austenitized. This temperature may range from about 1,330.degree. F. to as high as about 2,100.degree. F., dependent upon the carbon content, but from a practical standpoint effective results may be obtained within the range of from about 1,330.degree. F. to about 1,750.degree. F. In order to obtain a fully martensitic product the steel strip must be heated above the A.sub.3 critical point, i.e. to a temperature within the range of from about 1,525.degree. F. to about 2,100.degree. F. and particularly within the range of from about 1,525.degree. F. to about 1,750.degree. F. Immediately upon leaving the furnace 15 the heated strip passes into the quench system, designated generally at 20, where the strip is rapidly quenched to ambient or room temperature so as to obtain at least a partially martensitic microstructure. Preferably, the strip is quenched at a rate in excess of the critical cooling rate so that substantially all of the austenite is transformed to martensite.

The quench system 20 comprises a tank 30 provided with a strip exit chute 31 and containing a sinker roll 32. Extending upwardly from the top wall of the tank 30 is an elongated conduit 34 of rectangular cross section which provides a restricted quench channel 35. A trough 36 having a rectangular cross section surrounds the upper end of the conduit 34. A rectangular baffle 37 depends from an inwardly extending peripheral flange 33 on the trough 36, the baffle 37 surrounding the conduit 34 and terminating above the bottom of the trough 36. Extending downwardly from the outlet end of the furnace 15 is a tubular connecting or seal section 38 the lower end of which extends into the trough 36 below the upper end of the conduit 34 and disposed between the conduit 34 and the baffle 37. Water (or other quench liquid) is supplied to the trough 36 by inlet pipes 39, the water flowing downwardly and thence upwardly around the baffle 37 and then overflowing the open upper end of the conduit 34, as indicated by the arrows. The quench water and the strip 10 pass concurrently downwardly through the channel 35 into the tank 30. The strip 10 passes around the sinker roll 32 and then outwardly through the chute 31. The quench water flows from the tank 30 upwardly through the chute 31 and is discharged through a side outlet 39' .

Since the level of the water overflowing the upper end of the conduit 34 is above the lower end of the seal section 38, it will be seen that the section 38 is sealed by the water in the trough 36 so as to prevent infiltration of air into the furnace 15. If desired, a reducing or other nonoxidizing gas may be supplied to the section 38 (by means not shown) for passage upwardly through the furnace 15, thereby preventing oxidation of the strip. A plurality of view ports 40 are provided in the tubular section 38 and the conduit 34 to permit observation of the quench action.

Submerged jet or spray units 41 are provided in the opposite walls of the conduit 34 somewhat below the upper end of the conduit for directing streams of liquid toward opposite sides of the strip 10 across the entire width thereof. In the embodiment shown in FIG. 1, two superimposed units 41 are mounted at each side of the strip 10, but any desired number of such units may be used. Each unit 41 is supplied with water or other quench liquid by means of a supply pipe 42.

As more fully described in my aforementioned prior patent, each spray unit 41 comprises an elongated boxlike structure having the liquid supply pipe 42 extending into an opening in its rear wall and having an elongated discharge orifice 43 in its front wall extending substantially the entire length of the unit 41 and across the entire width of the strip 10. The interior of the enclosure is provided with a pair of elongated oppositely disposed baffles 44 extending between the ends of the enclosure and dividing the same into parallel interconnected chambers. The quench liquid in passing from the inlet 42 to the orifice 43 follows a tortuous path in which the direction of flow is reversed several times, thereby insuring a uniform distribution of liquid across the entire length of the orifice opening 43.

As will be evident from the drawing, the submerged spray units 41 are mounted in suitable openings in the walls of the conduit 34 so that thin sheets or curtains of water are directed from the orifices 43 substantially perpendicularly toward opposite sides of the strip 10. In fluid kinematics terminology, the unitary sheet or curtain of water from an elongated rectangular orifice, such as 43, may be characterized as having two-dimensional flow, i.e. the flow is identical in parallel planes so as to extend uniformly across the width of the strip 10.

As the heated steel strip 10 moves downwardly from the furnace 15 it passes through the seal section 38 and enters the upper end of the water-filled quench channel 35 where it is immediately immersed in the downwardly flowing stream of water. In addition, the submerged spray units 41 direct water streams against the strip in a direction generally perpendicular to the path of movement of the strip, thereby creating a high degree of turbulence in the uppermost portion or strip entry end of the quench channel. The dimensions of the conduit 34 are restricted so as to provide in the channel 35 a high velocity of water flow relative to the strip 10 while at the same time allowing sufficient clearance to permit passage of the strip 10 without scraping the walls of the conduit.

As previously mentioned, uniformity of quenching is essential not only for the sake of obtaining a strip having uniform microstructure and uniform physical properties but also to avoid warpage and distortion of the strip. Irregular vaporization of the water or other quenching medium in contact with the strip can result in substantial differentials in heat transfer rates between portions of the strip surface in contact with liquid water and other portions in contact with water vapor. These differentials cause different rates of contraction in the steel strip and result in quenching stresses and deformation.

However, in the quench system of the present invention the desired uniformity of quenching is realized as a result of several cooperating factors. The provision of the restricted quench channel 35 results in a high water velocity relative to the strip 10 which, by way of example, may be on the order of 1 to 10 feet/second in a direction generally parallel to the strip 10. Furthermore, the submerged sprays 41 are designed so as to provide water streams in a direction generally perpendicular to the strip 10 at relatively low pressures and relatively high flow rates so as to create substantial turbulence within the channel adjacent the strip entry end thereof. For example, the water pressure in the sprays 41 may be on the order of 20 to 30 p.s.i. at the inlets 42 and on the order of 5 to 10 p.s.i. at the discharge orifices 43. Although the submerged jets 41 are designed to create internal turbulence in the quench channel, nevertheless, the surface of the liquid in the channel 35 where the strip 10 first contacts the quench liquid is maintained substantially smooth, nonsplashing, and nonturbulent so that every point across the width of the strip makes initial contact with the quench liquid at substantially the same time. The low pressure of the high flow rate submerged jets 41 makes it possible to provide the desired surface smoothness while at the same time providing the required internal turbulence below the liquid surface. Thus, a quenched strip of at least partially martensitic structure is obtained which is either flat enough for its intended use or can easily be rolled to flatness.

Any suitable quenching liquid may be used including water, brine or other aqueous salt solution, oil, liquid nitrogen, etc. However, for quenching a heated steel strip to convert austenite to martensite, the preferred quenching media are water and aqueous brine or other aqueous salt solutions. For the latter purpose, the volume rate of flow of the quench liquid must be high enough to achieve the cooling rate required to transform the austenite to martensite, and the turbulence of the quench liquid relative to the strip, particularly at the strip entry end of the quench channel, must be great enough to prevent the accumulation of vapor film which would lead to nonuniformity of quenching and consequent distortion of the strip.

Typical line speeds may range from about 100 feet/minute to about 2,000 feet/minute dependent upon the gauge of the strip and the carbon content. The temperature of the quench water is regulated, as described below, and the steel strip will normally be cooled from its austenitizing temperature range of 1,330.degree.-2,100.degree. F. (1,525-2,100.degree. F. in the case of a fully martensitic product) to approximately the water temperature before leaving the quench tank. If desired, the water or other quench medium may be recirculated through a heat exchanger for temperature control.

Assuming that the quenching operation has been carried out under optimum conditions, as discussed above, so as to achieve uniformity of quenching across the full width of the strip, the final quenched product will have acceptable flatness for many end uses, as mentioned above. However, the quenched strip can readily be rolled, as on a temper mill, to provide adequate commercial flatness for any desired end use. For example, successful flattening is usually obtained by a single pass through a twin-stand four-high temper mill, each stand having two work rolls and two backup rolls. Because of the unusual hardness of a fully martensitic strip, the work rolls may have a high degree of roughness without impairing the surface of the strip, thereby providing adequate flattening in a single pass.

Other means may also be employed for creating the necessary turbulence in the quench channel 35 by causing high-velocity transverse flow of coolant in addition to longitudinal flow of coolant through the channel. For example, the quench channel may be provided with transverse baffles extending inwardly from the outer walls of the channel toward the strip so as to direct a portion of the quench medium from its generally longitudinal path parallel with the strip to a transverse path generally perpendicular to the strip.

The currents or streams of water directed transversely or generally perpendicularly against the strip by the submerged sprays 41, in cooperation with the high relative velocity of the main flow of water through the restricted quench channel 35 parallel to the strip, cause sufficient turbulence in the quench medium to eliminate steam pockets which would otherwise tend to form or accumulate at the strip surface. Such vapor formation causes a vapor barrier which results in nonuniform quenching, nonuniform transformation to martensite, and consequent warping and distortion of the strip.

In the quenching action which is accomplished in the channel 35, the quench period may be considered as having three portions. First, the temperature of the heated strip is lowered rapidly to the temperature range of martensite formation, i.e. to the martensite start temperature (M.sub.s). Next, the strip is cooled through the temperature range of martensite formation, i.e. from the martensite start temperature (M.sub.s) to the martensite finish temperature (M.sub.f). Finally, the strip is cooled to ambient temperature or the temperature of the quench liquid. Since drastic quenching is required to effect transformation of austenite to martensite in low-carbon steel, it has generally been thought that the strip should be quenched as rapidly as possible from its elevated temperature through the temperature range of martensite formation. As heretofore indicated, however, a certain amount of in situ or self-tempering of the martensite takes place in spite of rapid quenching because of the fact that the martensite transformation temperature is quite high for low-carbon steels. For example, for plain carbon steel containing 0.03 weight percent carbon and 0.40 weight percent manganese, the martensite start temperature is estimated to be about 990.degree. F. and the martensite finish temperature is estimated as about 610.degree. F. For plain carbon steel of 0.25 weight percent carbon and 0.40 weight percent manganese the respective martensite start and finish temperatures are estimated as about 805.degree. F. and about 470.degree. F.

In accordance with the present invention, highly advantageous results are obtained by selective control of the rate of quenching or heat withdrawal in different portions of the quench period. Thus, in the initial portion of the quench period the temperature of the strip is lowered as rapidly as possible from its elevated austenitizing temperature to the temperature range of martensite formation, i.e. to the martensite start temperature. Thereafter, however, the rate of quenching or heat withdrawal is substantially diminished during quenching through the temperature range of martensite formation, i.e. from the martensite start temperature to the martensite finish temperature. As a result of this selective variation or control, an increased tempering of the martensite is obtained, as compared with a quench system relaying wholly on in situ or self-tempering, so as to obtain a final product having improved ductility for a given tensile strength level. For example, increasing the quench time through the temperature range of martensite formation from 0.3 second to 0.5 second can result in a significant increase in the tempering effect on the martensitic structure.

Although the invention, in its broadest aspect, is not limited to any specific way of obtaining the desired selective control over quench rate in the quench channel, a particularly convenient method comprises regulating the temperature and flow velocity of the quench liquid in the different portions of the quench channel. More specifically, a portion of the quench liquid is introduced at one end of the elongated quench channel and another portion of quench liquid is introduced into the channel at a submerged location within the channel and adjacent to but spaced from the strip entry end of the channel. By maintaining a temperature differential between the two portions of quench liquid, the desired control over quench rate can be obtained.

Referring to the quench system shown in FIG. 1 wherein the strip and the quench liquid move concurrently downwardly through the quench channel, it will be seen that the strip 10 is first contacted by the water which is introduced through the inlets 39 to the trough 36 and overflows the upper end of the conduit 34. Thereafter, the strip 10 is contacted by a mixture of the water introduced through the lines 39 and the additional water introduced through the submerged spray units 41. The desired difference in quench rate is obtained by regulating the temperature of the separate streams of water supplied through the inlets 39 and 42. Since it is necessary to quench the strip initially as rapidly as possible to the martensite start temperature, the water supplied through the inlets 39 should be at a relatively low temperature inasmuch as this water makes the initial contact with the strip 10. Thereafter, in order to slow down the rate of heat withdrawal as the strip is quenched through the temperature range of martensite formation, the water supplied to the spray units 41 through the lines 42 should be heated to a relatively higher temperature. The respective temperatures of the two sources of water and the relative volume flow rates from the two sources are such that the temperature of the combined or commingled water streams is higher than the temperature of the water supplied through the inlets 39, thereby effecting heat withdrawal from the strip at a diminished rate as the strip temperature is lowered through the martensite transformation range.

As previously explained, a plurality of submerged spray units 41 are preferably provided in superimposed relation in the quench conduit 34 at the opposite sides of the strip 10, and the temperature of the water supplied through the spray units at successive elevations may be adjusted as required to insure a diminished rate of heat withdrawal only after the strip temperature has been lowered to the martensite start temperature. For example, in FIG. 1, it might be desirable to have the temperature of the water introduced through the uppermost spray units 41 substantially the same as the temperature of the water introduced through the inlets 39, and higher temperature water would be introduced only through the lowermost spray units 41. Also, instead of having fixed spray units 41, a similar result could be obtained by providing mechanical means for adjusting the elevations of the spray units 41 relative to the quench conduit 34. By either means, however, it is possible to select the proper elevation for effecting a diminished quench rate corresponding to the point at which the martensite transformation begins. As will be understood, the latter location will vary with strip gauge, line speed, and steel chemistry.

By way of example, for low-carbon steel strip having a thickness of 0.024 in., a carbon content of 0.03 to 0.25 weight percent, and a manganese content of 0.20 to 0.60 weight percent, the heat withdrawal rate in the initial portion of the quench period may be from about 1.times.10.sup.6 to about 3.times.10.sup.6 B.t.u./sq. ft./hr., and the diminished heat withdrawal rate when passing through the martensite transformation temperature range may be from about 0.25.times.10.sup.6 to about 0.5.times.10.sup.6 B.t.u./sq. ft./hr. Such heat withdrawal rates may correspond to a time range of from about 0.35 sec. to about 1.33 sec. for the initial portion of the quench period and from about 1.24 sec. to about 3.6 sec. during cooling through the martensite transformation range. Obviously, the exact values will depend on the aforementioned variables. As an example of the temperature differential between the different portions of quench water in the FIG. 1 embodiment, the relatively low-temperature water introduced through the inlet 39 may be from about 35.degree. F. to about 90.degree. F. and the relatively higher temperature water introduced at one or more elevations through the spray units 41 may be heated to a temperature of from about 120.degree. F. to about 200.degree. F.

In the FIG. 2 embodiment of the invention, the apparatus is modified slightly to accommodate countercurrent movement of the quench water relative to the downwardly moving strip. Thus, in FIG. 2 the trough 36 surrounding the upper end of the quench conduit 34 has been modified by the omission of the peripheral flange 33, the depending baffle 37, and the water inlets 39. Instead, an upright baffle or weir 50 extends upwardly from the bottom of the trough 36 in spaced relation between the outer walls of the trough and the seal conduit 38. The upper end of the baffle 50 extends above the lower end of the seal conduit 38. One portion of quench water is introduced to the system through an inlet 51 pipe extending into the tank 30 and passes upwardly through the quench channel 35 from the lower end thereof. As the water approaches the upper end of the quench channel 35 it is commingled with additional quench water introduced through the spray units 41, and the commingled or combined water stream overflows the upper end of the quench conduit 34 into the trough 36. The water then overflows the upper edge of the baffle or weir 50 and is discharged from the trough 36 through an outlet pipe 52.

In this embodiment of the invention the portion of the quench water introduced through the inlet pipe 51 is heated to a relatively higher temperature, whereas the water introduced through the spray units 41 is at a relatively lower temperature. As a result, the mixture of quench water streams which first contacts the downwardly moving strip 10 has a lower temperature than the temperature of the quench water at a lower point in the channel 35 below the spray units 41, and the desired difference in heat withdrawal rates is maintained in the manner previously described. As an example of the temperature differential, the heated water introduced through the line 51 may have a temperature of from about 120.degree. F. to about 200.degree. F., and the lower temperature water introduced through the lines 42 may be from about 35.degree. F. to about 90.degree. F.

In both systems of FIGS. 1 and 2, the variation in heat withdrawal rate in the different portions of the quench channel is obtained by regulating the temperature and flow velocity of the quench water. In the concurrent flow system of FIG. 1, the maximum flow rate and liquid velocity in the channel 35 is below the spray units 41, and the temperature of the water at this point is greater than the temperature of the water in the channel above the spray units 41 so that the net result is a lower rate of heat withdrawal in the lower portion of the quench channel. In the countercurrent flow system of FIG. 2, the temperature of the water in the channel 35 below the spray units 41 is again higher than the temperature of the water above the spray units 41, but, in this case, the waterflow rate and liquid velocity is greater in the upper portion of the quench channel above the spray units 41. Thus, the net result in the FIG. 2 system is that a higher heat withdrawal rate is obtained in the upper portion of the quench channel.

Although the invention has been described with reference to certain specific embodiments, it should be understood that various modifications and equivalents may be used without departing from the scope of the invention as defined in the appended claims. In addition, although the invention has been described with reference to the continuous quenching of steel strip, it is also within the scope of the invention to utilize separate sheets of steel.

Claims

I claim:

1. In the method of quenching a strip of plain carbon steel having a carbon content of from about 0.03 weight percent to about 0.25 weight percent and a manganese content of from about 0.20 weight percent to about 0.60 weight percent wherein the strip is heated to a temperature above the A.sub.1 critical point so as to at least partially austenitize the steel and thereafter the heated strip is passed through an elongated restricted quench channel and quench liquid is also passed through said channel for quenching the strip at a rate in excess of the critical cooling rate so as to transform substantially all of the austenite to martensite; the improvement which comprises controlling the quench conditions in one portion of said channel adjacent the strip entry end thereof so as to obtain a relatively high heat withdrawal rate during the initial portion of the quench period and thereby rapidly lowering the strip temperature to the range of martensite formation, and controlling the quench conditions in the remaining portion of said channel so as to obtain a relatively lower heat withdrawl rate during quenching through the temperature range of martensite formation, thereby effecting tempering of the martensite.

2. The method of claim 1 further characterized in that said quench conditions are controlled by regulating the temperature and flow velocity of the quench liquid in the respective portions of the quench channel.

3. The method of claim 1 further characterized in that the heat withdrawal rate in said one portion of said channel is from about 1 .times. 10.sup.6 to about 3.times.10.sup.6 B.t.v./sg. ft./hr. and the heat withdrawal rate in said remaining portion of said channel is from about 0.25.times.10.sup.6 to about 0.5.times.10.sup.6 B.t.u./sq. ft./hr.

4. The method of claim 1 further characterized in that the time for lowering the temperature of the heated strip to the range of martensite formation is from about 0.35 sec. to about 1.33 sec., and the time for quenching the strip through the temperature range of martensite formation is from about 1.24 sec. to about 3.6 sec.

5. The method of claim 1 further characterized in that the quench liquid passed through said one portion of said channel is at a lower temperature than the quench liquid passed through said remaining portion of said channel.

6. The method of claim 5 further characterized in that said lower temperature is from about 35.degree. F. to about 90.degree. F. and the quench liquid passed through said remaining portion of said channel is at a temperature of from about 120.degree. F. to about 200.degree. F.

7. The method of claim 1 further characterized in that said channel is disposed vertically and said strip and said quench liquid are passed downwardly through said channel in concurrent relation.

8. The method of claim 1 further characterized in that said channel is disposed vertically and said strip is passed downwardly through said channel and said quench liquid is passed upwardly through said channel in countercurrent relation.

9. The method of claim 1 further characterized in that one portion of said quench liquid is introduced into one end of said channel and another portion of said quench liquid is introduced into said channel at a submerged location within said channel adjacent to but spaced from the strip entry end of the channel.

10. The method of claim 9 further characterized in that said other portion of said quench liquid is directed in the form of liquid sheets substantially perpendicularly against the opposite surfaces of the strip and extending uniformly across the width of the strip.

11. The method of claim 9 further characterized in that said strip and said quench liquid are passed through said channel in concurrent relation, and said other portion of said quench liquid has a higher temperature than said one portion of said quench liquid.

12. The method of claim 11 further characterized in that said quench liquid comprises an aqueous liquid, said one portion of said quench liquid being at a temperature of from about 35.degree. F. to about 90.degree. F. and said other portion of said quench liquid being at a higher temperature within the range of from about 120.degree. F. to about 200.degree. F.

13. The method of claim 9 further characterized in that said strip and said quench liquid are passed through said channel in countercurrent relation, and said one portion of said quench liquid has a higher temperature than said other portion of said quench liquid.

14. The method of claim 13 further characterized in that said quench liquid comprises an aqueous liquid, said other portion of said quench liquid being at a temperature of from about 35.degree. F. to about 90.degree. F. and said one portion of said quench liquid being at a higher temperature within the range of from about 120.degree. F. to about 200.degree. F.

15. In the method of quenching a strip of plain carbon steel having a carbon content of from about 0.03 weight percent to about 0.25 weight percent and a manganese content of from about 0.20 weight percent to about 0.60 weight percent wherein the strip is heated to a temperature above the A.sub.1 critical point so as to at least partially austenitize the steel and thereafter the heated strip is passed through an elongated restricted quench channel and quench liquid is also passed through said channel for quenching the strip at a rate in excess of the critical cooling rate so as to transform substantially all of the austenite to martensite, one portion of said quench liquid being introduced into one end of said channel and another portion of said quench liquid being introduced into said channel at a submerged location within said channel adjacent to but spaced from the strip entry end of the channel; the improvement which comprises:

regulating the temperature and flow velocity of the quench liquid in one portion of said channel adjacent the strip entry and thereof so as to obtain during the initial portion of the quench period a relatively high heat withdrawal rate which is sufficient to rapidly lower the strip temperature to the martensite start temperature; and

regulating the temperature and flow velocity of the quench liquid in the remaining portion of said channel so as to obtain, during quenching from the martensite start temperature to the martensite finish temperature, a heat withdrawal rate which is less than the heat withdrawal rate in said one portion of said channel, thereby effecting increased tempering of the martensite.

16. The method of claim 15 further characterized in that said strip and said quench liquid are passed through said channel in concurrent relation, and said other portion of said quench liquid has a higher temperature than said one portion of said quench liquid.

17. The method of claim 16 further characterized in that said quench liquid comprises an aqueous liquid, said one portion of said quench liquid being at a temperature of about 35.degree. F. to about 90.degree. F. and said other portion of said quench liquid being at a higher temperature within the range of from about 120.degree. F. to about 200.degree. F.

18. The method of claim 15 further characterized is that said strip and said quench liquid are passed through said channel in countercurrent relation, and said one portion of said quench liquid has a higher temperature than said other portion of said quench liquid.

19. The method of claim 18 further characterized in that said quench liquid comprises an aqueous liquid, said other portion of said quench liquid being at a temperature of from about 35.degree. F. to about 90.degree. F. and said one portion of said quench liquid being at a higher temperature within the range of from about 120.degree. F. to about 200.degree. F.