Companhia Siderúrgica Paulista - COSIPA, Brazil
The determination of the rolling stock thickness during the holding step of controlled rolling is of utmost importance. Metallurgically speaking, any alteration in this parameter implies in a change in the total strain distribution between the roughing and finishing steps. When this intermediate thickness is increased, strain applied during the roughing step is decreased, with a corresponding increase of strain applied in the finishing step.
A greater strain degree applied in the non-recrystallization range of austenite, that is, during the finishing step, will obviously reflect in a more strain hardened structure after rolling. So, this final austenitic structure will be more refined, as it will have a greater amount of grain/sub-grain boundaries available for the posterior transformation of austenite to ferrite. This structure will have a minimised grain size if the strain degree applied during the roughing step is equal or greater than 60%. In this case, microstructure after the roughing step will be constituted of recrystallized austenite with minimum grain size.
The effect of a greater intermediate rolling stock thickness over mechanical properties is not so clear. In principle, the more refined microstructure would lead to a simultaneous increase in mechanical resistance and toughness. However, other microstructural effects, like alterations in the morphology of the constituents and decrease in the precipitation hardening potential in ferrite can lead to unexpected results. So, any alteration of this process parameter must be cautiously done: a previous analysis about its possible effects for each kind of microalloyed steel is strongly recommended.
From the point of view of productivity, the increase of intermediate thickness means shorter rolling stocks during the delay between roughing and finishing steps. Hence, a greater number of rolling stocks can be simultaneously processed using tandem rolling. However, cooling time necessary for these heavier rolling stocks to reach the ideal start temperature for the finishing step can be excessive, as its roughing step ends earlier (i.e., temperature at the end of the roughing step is higher) and, as they are thicker, their cooling rates are significantly lower than thinner intermediate rolling stocks. So, there is an optimum intermediate stock thickness, which balances these contradictory tendencies and allows maximum productivity during controlled rolling.
The quest for a more efficient controlled rolling process with consistent quality products motivated the development of a research project to determine the effect of intermediate rolling stock thickness over the mechanical properties of the product and the productivity of controlled rolling process. A Nb-Ti microalloyed steel, suitable for the production of shipbuilding plates according to the DH-32 standard, was chosen for these experiments. Plates with final thickness (tf) of 12.7, 25.4 and 32.0 mm were submitted to controlled rolling using different intermediate rolling stock thickness (ti), as shown in Table I. Slab thickness was the same for all plates studied. For the sake of simplicity, the intermediate rolling stock thickness values were multiples of the final product thickness values.
| Final Thickness tf [mm] | Intermediate Rolling Stock Thicknesses ti [mm] | |||
|---|---|---|---|---|
| 2 tf | 3 tf | 4 tf | 5 tf | |
| 12.7 | 25.4 | 38.1 | 50.8 | 63.5 |
| 25.4 | 50.8 | 77.1 | 101.6 | - |
| 32.0 | 64.0 | 96.0 | - | -
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A statistical analysis between the mechanical properties determined from plates produced using different values of intermediate rolling stock thickness can be seen in Table II. This comparison was carried out using Variance Analysis (ANOVA) for the 12.7 and 25.4 mm, while the Student Test was used for the 32.0 mm plate, as plates with this thickness were tested using only two values of intermediate rolling stock thickness.
The results of Table II show that the variation in the intermediate rolling stock thickness did not affect the 12.7 mm plate. However, as final product thickness increases, the effect of that rolling parameter must not be neglected. For the 25.4 mm plate, yield strength was somewhat influenced by intermediate rolling stock thickness, increasing from 380 to 395 MPa as ti was changed from 50.8 to 77.1 mm (2 to 3 tf). In the case of the 32.0 mm plate, all properties but total elongation were affected by the alteration on the intermediate rolling stock thickness from 64 to 96 mm (2 to 3 tf), as can be seen in Table III. These findings show that the effect of the intermediate rolling stock thickness is more significant for heavy plates. Considering mechanical strength, this effect was more explicit for yield than for tensile strength, as can be seen by the evolution in the yield ratio values.
| - | Yield Strength | Tensile Strength | Yield Ratio | Total Elongation | Charpy Energy at -20oC |
|---|---|---|---|---|---|
| ANOVA tf = 12.7 mm | EQUAL 58.7% | EQUAL 47.5% | EQUAL 60.1% | EQUAL 38.8% | EQUAL 12.4% |
| ANOVA tf = 25.4 mm | DIFFERENT 98.3% | EQUAL 92.4% | EQUAL 50.2% | EQUAL 71.2% | EQUAL 22.2% |
| STUDENT tf = 32.0 mm | DIFFERENT 99.4% | DIFFERENT 99.9% | EQUAL 98.9% | EQUAL 83.6% | DIFFERENT 98.9% |
These results lead to the conclusion that, as total deformation (slab to plate) increases, the effect of the intermediate rolling stock thickness is reduced. That is, the distribution of strain between the roughing and finishing steps does not influence the product properties above a certain grade of total deformation. This conclusion is confirmed by the decreasing values of confidence degree produced by the Student and ANOVA tests as the thickness of the final plate is reduced.
So, at least for plates with thickness equal or less than 25.4 mm, the alteration in the intermediate rolling stock thickness does not lead to significant alterations in the mechanical properties of final plate. So, the value of this parameter can be chosen only regarding the most favourable condition to maximise plate mill productivity through the use of a optimised tandem rolling scheme. In the case of heavier plates, the use of a thicker intermediate rolling stock must be done with care; in this case, C and Mn contents of the steel were judiciously reduced to balance mechanical properties. These reduction in the plate alloy content leads to additional benefits, as a slight economy in the consumption of ferro-alloys and some improvement in the product weldability.
| - | Yield Strength [MPa] | Tensile Strength [MPa] | Yield Ratio [%] | Total Elongation [%] | Charpy Energy at -20oC [J] |
|---|---|---|---|---|---|
| ti = 64.0 mm (2 tf) | 360 | 495 | 73 | 30 | 110 |
| ti = 96.0 mm (3 tf) | 400 | 505 | 79 | 29 | 150
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This study made feasible the use of optimised values of intermediate rolling stock thickness according to the specific dimensions (thickness and width) of final plate, leading to a 21% increase of the overall productivity of the plate mill during the controlled rolling process.
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Last Update: 14 August 1997 | |
| © Antonio Augusto Gorni |