GB1566128A - Heat treating of hot-rolled steel rod - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO HEAT TREATING OF HOT-ROLLED STEEL ROD (71) We, ASHLOW STEEL & ENGINEER ING COMPANY LIMITED, a British Company of Alsing Road, Sheffield S9 1HL, formerly of Charlotte Road, Sheffield, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to the heat treating of hot-rolled steel rod, particularly but not exclusively medium to high carbon steel rod, to impart to it a microscopic structure most suitable for subsequent cold drawing into wire. Such rod leaves the final pass of a rod mill at a high temperature, usually in the range 1000 C to 11000C; at such temperatures iron exists in a phase known as austenite, and steel is a solid solution of carbon in austenite. The rod is subsequently cooled to handling temperature; at some stage in this process, when the rod temperature has fallen to a certain value between about 700do and 900"C depending on its composition, austenite begins to transform to ferrite, which has a much lower capacity for holding carbon in solution.

If the cooling rate is not too rapid, carbon is rejected from solution during the transformation, and combines with iron to form a carbide, Fe3C, known as cementite. The resulting steel consists of regions of perlite embedded in a matrix of ferrite or cementite respectively as the carbon content is less than or greater than the eutectoid value of 0.80%.

Perlite consists of alternating sheets of ferrite and cementite.

If the cooling rate is very rapid the carbon is not rejected from solution upon transformation, and the resulting material is known as martensite. This is hard and brittle and of little use without further heat treatment; it is generally avoided in the rodrolling industry.

A third possibility is bainite, which is another arrangement of ferrite and a carbide.

Simple cooling processes do not produce pure bainite, although intermediate cooling rates can produce structures containing some bainite. These partially bainite structures are also avoided in the industry.

Cooling rate is not the only factor to influence the outsome of the allotropic transformation described above; the austenite grain size within the rod material as its cooling commences is also of major significance.

This is shown in Figure 1 of the drawings accompanying the Provisional Specification.

and which is an isothermal transformation diagram for plain carbon steel containing 0.80% wt. carbon: it gives for each temperature the time required for the transformation to be 50% complete, and also the product of such transformation. Each curve corresponds to a fixed austenite grain size, specified by an ASTM number in a way which is well known to the industry; larger numbers correspond to smaller grains. Although the actual transformation does not take place isothermally, this diagram serves to illustrate its general features. It is clear that fine-grained austenite transforms in a much shorter time than coarse-grained material, which phenomenon is an important factor in the process to be described here.

Transformation times are also influenced by composition, being shorter for lower carbon contents.

For wire drawing purposes it is known that the cold rod should contain fine pearlite, which is produced by transformation at the lower end of the pearlite forming tempera- ture range, such as 600"C, rather than coarse pearlite, formed at higher temperatures, such as 7000 C. Moreover, the colonies of this fine pearlite must be small, corresponding to transformation from fme-graided austenite.

In a process known as 'lead patenting', not an in-line process, which produces what is generally regarded as the best rod in these respects, cold rod from a mill is re-heated and maintained at a suitable temperature above that at which re-transformation to austenite commences. The grain size estab lished in such a transformation depends -upon the austenizing temperature, as shown in Figure 2 of the drawings accompanying the Provisional Specification. A low austeniz ing temperature, such as 900"C, is used, to generate fine-grained austenite, in the ASTM No. range 8 to 5. The rod is then quickly cooled to a suitable pearlite form ing temperature by plunging into a bath of molten lead, typically at 550"C, from which it is removed when the transformation is complete. Typical cooling paths are shown in Figure 1. In principle lead patenting could be used in an in-line manner, which would obviate the expense and inconvenience of re-heating; however, engineering problems have so far prevented such use.

Austenitic grain sizes in the range ASTM Nos. 8 to 1 are generally regarded as normal, in that austenite maintained at any tem perature above the point of allotropic transformation -of steel will assume such a grain size. However, much finer grain sizes, in the super-fine range ASTM Nos. 12 to 10, exist temporarily in austenite which has been hot-worked, as in hot rolling. Grain growth begins immediately after the last rolling pass, and proceeds very rapidly at the elevated temperatures characteristic of rolling. The phenomenon is still a subject of active re search; it is known that growth from the super-fine state to a fine state occurs in a matter of seconds, although a really coarse state probably takes several minutes to de velop. Figure 2 of the diawings accompanying the Provisional Specification shows typical austenitic grain-growth curves of a 'coarse grained' steel (not de-oxydised with aluminium) and a 'fine-grained' steel (de oxydised with aluminium).

It is known to coil hot steel rod after rolling and allow the coil to cool by natural air convection. The consequent slow cooling rate allows substantial grain growth above the point of allotropic transformation, and allows the subsequent transformation to pearlite to take place at too high a tem perature. The resulting microstructure con sists of large colonies of coarse pearlite, which is most unsuitable for wire drawing.

Different portions of the coil cool at dif ferent rates, giving substantial variations in microstructure within the coil, again an undesirable feature.

It is known to cool hot steel rod, after rolling and cooling to a point just above transformation using cold water sprays to retard grain growth, into a series of flat non-concentric rings on a moving conveyor.

Cooling is then effected by forcing air through these non-concentric moving rings which are assembled into a full coil when their temperature has fallen to handling value. The consequent overall cooling rate is more rapid and more uniform than that given by natural air convection acting on a fully formed coil, and a much better product results. The disadvantages of this method are ones of space and expense, and of environmental degradation. The two cooling stages involved constitute a total cooling line length which is typically 250 ft., and powerful fans needed to produce the cooling air are expensive to provide and to operate, as well as being excessively noisy.

The object of the invention is to exploit fully the super-fine grain structure which exists in hot rod during and immediately after rolling, by initiating a single stage rapid cooling process as soon as practicable after the last roll stand. One advantage of capturing the super-fine grains in this way is to minimize the time required for austenite to transform to pearlite (as shown in Figure 1 of the drawings accompanying the Provisional Specification), which enables the rapid cooling rate to be maintained throughout the transformation, and hence minimizes the physical extent of the device. A related advantage is that the resulting structure consists of super-small eolonies of fine pearlite, and has excellent cold-drawing properties. A further advantage is that the rapid cooling rate so allowed enables cheaply available liquid quenching agents to be used, particularly hot water, which in more normal sircumstances would produce martensite.

According to the present invention, means for effecting a single stage rapid cooling process on hot-rolled steel rod comprises a laying head immediately adjacent to the last roll stand of a hot-rolling mill train, the laying head in use laying the hot rod into a series of substantially flat non-concentric loops on a roller conveyor immersed in - a bath of hot liquid - quenching agent, the conveyor being 2inclined upwardly - from adjacent the laying head and being driven so as to carry the loops clear of the liquid.

The conveyor is driven at such a speed that the loops emerge clear of the liquid when the transformation from austenite to pearlite is complete, and the temperature of the rod may be above the boiling point of the liquid quenching agent at the stage of leaving the liquid, as the ductility of the rod, particularly in the case of low carbon rod, is improved by allowing the last stages of cooling, below about 400"C, to take place at a slow rate. The temperature of the liquid in the bath may be maintained substantially at boiling point by the hot rod being continuously immersed in it, and a condensing hood is preferably provided over the bath to cause vapour generated by heat from the rod to condense and fall back into the bath.

The loops - of- - rod may be assembled into coils after leaving the bath.

An embodiment of the invention will now be described, by way of example only, with reference to Figures 3 and 4 of the drawings accompanying the Provisional Specification and in which Figure 3 is a diagrammatic side view of means for effecting a single stage rapid cooling process on hot-rolled steel rod. Figure 4 is a diagrammatic section on the line 44 of Figure 3.

In Figures 3 and 4 a standard laying head 1 is situated as close as possible to the last roll stand 2 of a hot-rolling mill train, and the laying head lays the hot rod 3 into a series of substantially flat non-concentric loops on a conveyor consisting of rollers 4 immersed in a hot water bath 5 in a tank 6, which is typically 60 ft.

long (i.e. substantially shorter than any air cooling line) and the conveyor slopes upwards away from the laying head and the rollers are driven through sprockets 7 at such a speed that the loops emerge clear of water when transformation from austenite to pearlite is complete. A hood 8 with condensing tubes 9 containing circulating cold water condenses steam generated by heat from the rod so that the condensed water falls back into the tank 6. The loops of rod 3 are conveniently assembled into coils (not shown) after leaving the bath 5.

In this way have been produced high carbon rods up to 0.85% in the diameter range 5.5 mm to 12.5 mm with a consistently good cold-drawing pearlitic structure. In contrast, pieces of identical rod in the lower half of this diameter range heated in a furnace to rolling temperatures have given martensite on quenching into boiling water.

Figures 5 and 6 of the drawings accompanying the Provisional Specification are examples of the microstructure (x500) produced by this process, in 8 mm diameter rod containing 0.86% wt. carbon and 0.73% wt. manganese, at the surface and centre respectively.

WHAT WE CLAIM IS:- 1. Means for effecting a single stage rapid cooling process on hot-rolled steel rod comprising a laying head immediately adjacent to the last roll stand of a hot-rolling mill train, the laying head in use laying the hot rod into a series of substantially flat nonconcentric loops on a roller conveyor immersed in a bath of hot liquid quenching agent, the conveyor being inclined upwardly from adjacent the laying head and being driven so as to carry the loop clear of the liquid.

2. Means as in Claim 1, wherein a condensing hood is provided over the bath to cause vapour generated by heat from the rod to condense and fall back into the bath.

3. A method of using the means of Claim 1, wherein the conveyor is driven at such a speed that the loops emerge clear of the liquid when the transformation from austenite to pearlite is complete and the temperature of the rod is above the boiling point of the liquid quenching agent at the stage of leaving the liquid.

4. A method of using the means of Claim 1 or Claim 2, wherein the temperature of the liquid in the bath is maintained substantially at boiling point by the hot rod being continuously immersed in it.

5. A method as in Claim 3 or Claim 4, wherein the loops of rod are assembled into coils after leaving the bath.

6. Means for effecting a single stage rapid cooling process on hot-rolled steel rod substantially as hereinbefore described with reference to Figures 3 and 4 of the drawing accompanying the Provisional Specification.

7. A method of effecting a single stage rapid cooling process on hot-rolled steel rod substantially as hereinbefore described with reference to Figures 3 and 4 of the drawings accompanying the Provisional Specification.

8. Hot-rolled steel rod cooled by the means or method of any one of the pre

Claims (1)

1. ceding Claims.