GB2194186A

GB2194186A - Method of rolling steel workpieces

Info

Publication number: GB2194186A
Application number: GB08620380A
Authority: GB
Inventors: Ronald Arthur Varo
Original assignee: ASHLON ENG Ltd
Current assignee: ASHLON ENG Ltd
Priority date: 1986-08-21
Filing date: 1986-08-21
Publication date: 1988-03-02
Also published as: GB2194186B; GB8620380D0

Abstract

A method of hot rolling high strength steel workpieces, such as reinforcing bars, comprises at least two stages of rolling. The last stage of rolling, during which the cross- section of the workpiece is reduced by at least 50%, is effected in two or more passes at a temperature above the AR3 of the material with a time delay of less than 1 second between passes. The method results in workpieces which have high strength, good weldability and cold deformability without resorting to alloying. <IMAGE>

Description

SPECIFICATION Method of rolling steel workpieces This invention relates to a method of hot rolling high strength steel workpieces, such as reinforcing bars in the size range 5-40mm diameter.

Steel workpieces, such as reinforcing bars, have to be produced at an economically low manufacturing cost but must have an adequate tensile yield strength whilst for many grades still displaying good weldability and cold deformability.

Strengthening of steel workpieces by the addition of certain alloying elements, such as V, Ti or Nb, is known but these micro-alloying element are very expensive- and the process is not particularly economical.

Various methods of partial quenching and tempering from the rolling heat of steel workpieces is known and, in such processes, the surface of the workpiece is quenched to below the martensite point and then, in the course of subsequent temperature equalisation, is tempered to a certain balancing temperature by the heat remaining in the core.

Steel treated in this way displays a concentric peripheral zone of quenched and tempered structure and a ferritic-pearlitic core.

The extremely rapid quenching which is required in this method of producing steel workpieces limits the rolling speed which can be attained and it is difficult to coil the workpieces which have been so cooled.

It is also known that a steel workpiece can be strengthened by grain refinement or grain elongation by thermo-mechanical treatment (TMT). Generally, TMT can consist of deformation of austenite during rolling at a temperature just above the AR3 point to ensure full grain refinement before subsequent transformation to ferrite, bainite or martensite. This technique is referred to as high temperature thermo-mechanical treatment (HTMT). For HTMT to be fully effective, it is necessary to suppress austenite grain growth after hot deformation. The extent to which this can be achieved depends on the final rolling temperature and the delay period before quenching.

The lower the finishing temperature, the less grain growth will occur.

It is an object of the present invention to provide a method of:hot rolling steel workpieces in which high strength, good weldability and cold deformability can be obtained without resorting to alloying.

According to a first aspect of the present invention, in a method of hot rolling steel workpieces, the workpieces are rolled in at least two stages and the last stage of rolling, during which the cross-section of the workpiece is reduced by at least 50%, is effected in two or more roll passes at a temperature above the AR3 of the material with a time delay of less than one second between passes.

According to a second aspect of the present invention, in a method of hot rolling steel workpieces, the workpieces are rolled in at least two stages and, immediately prior to the last rolling stage, the workpiece is rapidly cooled to a temperature close to, but above, the AR3 temperature of the material and the last rolling stage, during which the cross-section of the workpiece is reduced by at least 50%, is effected in two or more roll passes at a temperature above the AR3 temperature of the material with a time delay of less than one second between passes.

According to a third aspect of the present invention, in a method of hot rolling steel workpieces, the workpieces are rolled in at least two stages and, immediately prior to the last rolling stage, the workpiece is rapidly cooled to a temperature close to, but above, the AR3 temperature of the material and the last rolling stage, during which the cross-section of the workpiece is reduced by at least 50%, is effected in two or more roll passes at a temperature above the AR3 temperature of the material with a time delay of less than one second between passes and, thereafter, the workpiece is water cooled in less than two seconds to a temperature range between the austenite and the AR, transformation temperature.

Initially, the workpiece will be coarse grained due to a re-heating process before rolling.

Maximum grain refinement will be favoured by a rolling schedule having the following features:- 1. Low austenising temperature and many deformation/recrystallisation cycles, i.e.

many passes in the rolling mill but on a substantially falling temperature scale.

2. The heaviest draft available being used on each pass, particularly the last few passes of the last rolling stage.

3. The deformation distribution with respect to temperature is such that the workpiece is on gauge at a temperature just above the AR3 temperature.

A rolling schedule incorporating these features is shown schematically in Fig. 1 of the accompanying drawings but this mill practice may have shortcomings on some existing rolling mills where the power demand at reduced rolling temperature may be too high, or the rolling stand strength is too low, or with relatively large delays between passes. A more normal practice from the point of view of most existing mills is shown diagrammatically in Fig. 2.

As shown in Fig. 2, the workpiece would be rolled in the roughing and intermediate stages at normal or, preferably, reduced temperatures according to mill capability until the workpiece was at least 50% by cross-sectional area above the final size prior to entry to the last rolling stage. These roll stands will vary in number according to the final roll size but will not be less than two passes and, generally, not more than ten.

A typical analysis of the steel workpiece would include 0.15% C, 1.3% Mn and 0.3% Si.

As the workpiece leaves the last intermediate rolling stand, travelling at around 5.10 metres per second, a surface temperature of about 1050"C would be recorded. The workpiece is then directed into a high pressure water cooling box, typically 5-15 m long containing a series of water injector nozzles and means for removing excess surface water on exit. Means are provided for temperature control such that the workpiece on entry to the finishing stands is at a temperature near to, but above, the AR3 temperature. Not less than 50% total reduction of area takes place at the finishing passes and water cooling means are incorporated between the stands to control the heat generated due to friction and deformation. There is a delay of not more than one second between passes.The workpiece rolled to its final dimensions then enters a second water cooling box without delay and this second cooling box will be typically 5-30 m long housing separately controllable water cooling nozzle injectors and means for removing excess surface water on exit, typically by a water and/or air nozzle injecting in the opposite direction to the direction of travel of the hot rod. The water box operates to reduce the surface temperature of the rod down to not les than 500"C at which point there is no danger of producing surface martensite. On exit from the water box the rod is directed through a pinch roll and loop laying head which delivers the rod in a continuous flat spiral on to a moving conveyor. During this time there is substantial temperature equalisation between the surface and centre of the moving rod to not less than 600 C. This temperature is high enough to avoid martensitic transformations and the hot strength is low enough to permit practical loop formation without cobbling. Thereafter the cooling rate in air is controlled to produce a ferrite/pearlite structure which avoids bainite/martensite transformations.

Claims

1. A method of hot rolling steel workpieces wherein the workpieces are rolled in at least two stages and the last stage of rolling, during which the cross-section of the workpiece is reduced by at least 50%, is effected in two or more roll passes at a temperature above the AR 3 point the workpiece material with a time delay of less than one second between passes, where AR3 is the temperature at which austenite begins to convert to ferrite upon cooling the steer.

2. A method as claimed in claim 1 which comprises the additional step of rapidly cooling the workpiece, immediately prior to the last rolling stage, to a temperature close to, but above, the AR3 temperature of the workpiece material.

3. A method as claimed in claim 1 or claim 2 which comprises the additional step of water cooling the workpiece, after the last roll pass, in less than two seconds to a temperature range between the austenite and the AR, transformation temperature, where AR1 is the temperature at which the conversion of ausenite to ferrite or to ferrite plus cementite is completed upon cooling the steel.

4. A method of hot rolling steel workpieces substantially as hereinbefore described.