ROLLING PROCESS FOR CLAD STEEL

Abstract: A process for rolling a clad steel satisfactorily reduces the adverse influence of the difference of the deformation resistance of the metal layers composing the clad steel and thereby improves the yield of the rolling process. The rolling process for clad steel includes a step pre-form rolling for a length of 20% to 80% of the overall length of the clad steel. After pre-forming rolling, reduction rolling is performed at a draft substantially equal to or greater than the draft of the pre-form rolling. According to the invention, pre-form rolling process and reduction rolling process are performed for at least two passes.

Description: The present invention relates generally to a process for rolling a clad steel which combines iron base plate and stainless steel plate, cupronickel plate, monel metal plate, titanium plate, aluminium plate or the like. More specifically, the present invention relates to a hot rolling process for the clad steel which successfully avoids influence of difference of draft of the different materials of the clad steel. Generally, the different material metal plates composed into the clad steel have different deformation resistance in hot rolling. This causes a difference in drafts which results in difference of rolling reductions. Therefore, the thickness ratio of the different material plates is differentiated at the entrance of a rolling mill and at the exit thereof. Namely, the plate made of the material having smaller deformation resistance relative to the other, is reduced at greater magnitude than that of the other. This natually causes difference of expansion length of composed plates thus lowering the yield of the hot rolling process. This is because the material having the smaller deformation resistance tends to flow toward the material having greater deformation resistance to form single layer longitudinal ends where only the smaller deformation resistance material exists. In order to eliminate influence of the difference in expansion rates between the material metal plates forming the clad steel, Japanese Patent First (unexamined) Publication (Tokkai) Showa 61-232003, published on October 16, 1986, discloses a process for rolling of clad steel, in which pre-form rolling is performed at least at one of longitudinal ends. After pre-forming rolling, reduction rolling is performed for the overall length of the clad steel. The publication further discloses that the preferred length of the end portion of the clad steel, for which pre-forming rolling is to be performed, is equal to the thickness of the clad steel at the maximum. When the shown process is practically applied to a clad slab, the section on which the perform pre-form rolling is performed, becomes less than 10% of the overall length of the slab. Furthermore, the above-mentioned prior proposal suggests the process of pre-form rolling to reduce the thickness of the corresponding to the thickness of the final product. Therefore, in many case, substantial reduction is performed in one pass of pre-form rolling. This conventional process including pre-form rolling for the longitudinal end did not satisfactorily avoid the adverse influence of the expansion rates of the difference deformation resistance of the materials from which the clad steel was formed. Furthermore, when hot rolling was experimentarily performed according to the process proposed in the aforementioned publication, the portion where the thickness ratios of the layers forming the clad metal fluctuates and is not maintained constant. The thickness ratio of the layers will be hereafter referred to as "clad ratio" are formed at the longitudinal ends. The portion where the clad ratio fluctuates, will be hereafter referred to as "uneven clad ratio portion". By the presence of this uneven clad ratio portions, yield of the hot rolling of the clad steel is degraded even though attempt is made for improving yield. Therefore, it is an object of the present invention to provide a process for rolling a clad steel which satisfactorily reduces the influence of the difference between the deformation resistances of the material metals composing the layers in the clad steel and can improve the yield of rolling. In order to accomplish aforementioned and other objects, in a rolling process for a clad steel, according to the present invention, pre-form rolling is performed over 20% to 80% of the overall length of the clad steel. After pre-form rolling, main reduction rolling is performed at a draft substantially equal to or greater than the draft of the pre-forming rolling. According to the invention, pre-form rolling process and reduction rolling process are performed for at least two passes. Accordingly the present invention provides a process for rolling elongate clad steel comprising a first layer of a first material and a second layer of a second material, which second material has a lower malleability than that of the first material, which process comprises the steps of:- (i) pre-form rolling the steel at a first given draft from one end of the clad steel, (ii) reduction rolling the clad steel from the other end of the clad steel, characterised in that the pre-form rolling is effected over a length of from 20% to 80% of the total length of the slab, in that the reduction rolling is effected at a second given draft which is greater than or equal to the first given draft and in that steps (i) and (ii) are repeated as desired. In embodiments of the invention, the reduction rolling is performed on the portion of the clad steel maintained unrolled or, alternatively, over entire length of the clad steel. The present invention will be understood more fully from the detailed description given herebelow and from the accompanying drawings of the preferred embodiment but are for explanation and understanding only. In the drawings: Figs. 1(A), 1(B), 1(C), 1(D), 1(E), 1(F) and 1(G) are illustrations showing the preferred process of rolling for a clad steel, according to the present invention; Figs. 2(A), 2(B), 2(C), 2(D), and 2(E) and Figs. 3(A), 3(B), 3(C) and 3(D) are illustrations showing conventional rolling processes for clad steels, in which Figs. 2(A), 2(B), 2(C), 2(D) and 2(E) illustrate the process disclosed in the aforementioned Tokkai Showa 61-232003 and Figs. 3(A), 3(B), 3(C) and 3(D) illustrate the conventional reverse rolling process; Fig. 4 is a sectional view of the clad steel rolled through the process of Figs. 2(A), 2(B), 2(C), 2(D) and 2(E); Fig. 5 is a graph showing the relationship between pre-form rolling length, uneven clad ratio portion of the longitudinal ends formed due to difference of the expansion of the material metal plates, and clad rate; Fig. 6 is a graph showing relationship between pre-form rolling length, uneven clad raio portion, and rolling temperature; Fig. 7 is a graph showing relationship between pre-form rolling length, uneven clad ratio portion, and draft; Fig. 8 is a graph showing relationship between uneven clad ratio portion and number of passes in rolling process; Fig. 9 is a graph showing relationship between pre-form rolling length and uneven clad ratio portion; Fig. 10 is a graph showing relationship between rolling speed ratio and uneven clad ratio portion; Fig. 11 is a graph showing relationship between pre-form rolling length and uneven clad ratio portion; Fig. 12 is a graph showing relationship between pre-form rolling length and uneven clad ratio portion; and Fig. 13 is a graph showing relationship between number of passes in rolling and uneven clad ratio portion. Referring now to the drawings, Figs. 1(A), 1(B), 1(C), 1(D), 1(E), 1(F) and 1(G) illustrate the preferred rolling process according to the present invention. Rolling is performed by upper and lower rolls 1a and 1b by passing a clad steel 4 in the form of a plate, slab or so forth. The clad steel 4 is composed of a pair of layers of different material metals 2 and 3. A portion of the clad steel 4 is subject to pre-form rolling by being passed through the clearance between the upper and lower rolls 1a and 1b. The pre-form rolling is performed from one longitudinal end of the clad steel 4, for a length L₀ that is in the range of 20% to 80% of the overall length L of the clad steel 4. After the pre-form rolling process set forth above, reduction rolling is performed from the the opposite longitudinal end of the clad steel 4 over the portion not rolled in the pre-form rolling process. The draft in the reduction rolling is greater than or equal to the draft in the pre-form rolling. In the preferred rolling process, the pre-form rolling and reduction rolling processes are performed in one rolling pass cycle. The rolling process is performed at least for two cycles. In order to compare the performance of the preferred rolling process according to the invention to those of the prior art, comparative rolling processes are performed through the processes shown in Figs. 2(A), 2(B), 2(C), 2(D) and 2(E) and Figs. 3(A), 3(B), 3(C) and 3(D) , in which Figs. 2(A), 2(B), 2(C), 2(D) and 2(E) illustrate the process disclosed in the aforementioned Tokkai Showa 61-232003 and Figs. 3(A), 3(B), 3(C) and 3(D) illustrate the conventional reverse rolling process. In the process of Figs. 2(A), 2(B), 2(C), 2(D) and 2(E), pre-form rolling is performed on both longitudinal ends of the clad steel 4. According to the disclosure of Tokkai Showa 61-232003, pre-form rolling was performed over a distance corresponding to the thickness of the clad steel. On the other hand, in the process of Figs. 3(A), 3(B), 3(C) and 3(D), the clad steel 4 passes, at first, from one end thereof and then from the other end. As observed from Fig. 1(E), the clad steel 4 rolled by the preferred process has a substantially even clad ratio over the entire length. On the other hand, as seen from Fig. 2(E), the clad steel rolled by the process shown in Tokkai Showa 61-232003 had portions at both ends where no layer of the metal 2 is formed. The single layer portions extend for lengths of ℓ₁ and ℓ₂, as seen from Fig. 4. Furthermore, as shown in more detail in Fig. 4, at the regions C₁ and C₂ adjacent both end portion, the clad ratio becomes uneven. The region where single layer is formed, and uneven clad ratio region are hereafter referred to as "inferior quality region". On the other hand, the clad steel 4 processed by the process of reverse rolling of Figs. 3(A), 3(B), 3(C) and 3(D) had metal layers 2 and 3 where difference in length corresponded to the difference between the expansion ratios of the respective layers. Figs. 5 to 7 show results of experimentally performed rolling utilising the preferred processes. In the experiments, clad steel slab composed of a layer of stainless steel and a layer of soft steel was used. The slab was 200 mm thick and 5 m length. The clad slab was a total draft of 50% in total including the reduction in the pre-form rolling and reduction rolling stages. In rolling according to the preferred process, rolling was performed in two rolling pass cycles. In the first experiments, rolling temperature was fixed at 1000 °C. In the sample slabs wherein the layer thickness were 10%, 20%, 30% and 40%, the ratio of the length of single layer regions (ℓ₁ + ℓ₂) to the length L₀ of the portion of clad slab, over which pre-form rolling was performed, was checked. The result is shown in Fig. 5. As will be seen from Fig. 5, the length of the single layer region could be maintained at a minimum value when pre-form rolling was performed over of 20% to 80% of the overall length of the slab. In the secondary experiments, clad ratio was fixed at 30%. Experimental rollings were performed respectively at 900 °C, 1000 °C and 1200 °C. In the second set of experiments, the relationship between the length of the single layer regions and the rolling temperature was checked. The result of the experiments is shown in Fig. 6. In the third experiments, experimental rollings were performed of 5%, 10% and 20% drafts. The relationship between the length of the single layer regions and the draft was checked. The results of the experiments are shown in Fig. 7. The second and third sets of experiments confirm that the pre-form rolling length is preferred in a range of 20% to 80%. The relationship between number of pass cycles and the length of the single layer regions was observed. The result is shown in Fig. 8 . As seen from Fig. 8, by performing of rolling two pass cycles or more, the length of the single layer regions was significantly reduced. Further experimentation was performed to determine the relationship between the layer thickness ratio and length (C₁ + C₂) of the uneven clad ratio region. Clad ratios were respectively 30% and 40%. The results of these tests are shown in Fig. 9. From the results of experiments shown in Fig. 9, it can be confirmed that the preferred range of length of portion of the clad slab, for which the pre-form rolling is to be performed is 20% to 80% of the overall length. Furthermore, similarly to the length of the single layer region, the length of the uneven clad ratio region can be significantly reduced by performing rolling for two pass cycles of rolling or more. In another experiment, rotation speeds of the upper and lower rolls 1a and 1b were differentiated relative to each other. In the experiments performed, upper roll 1a was rotated at higher speed than the lower roll 1b. The higher speed upper roll 1a mated the material 2 which has higher deformation resistance. Naturally, the other material 3 having higher malleability mates with the lower roll 1b which rotates at lower speed. The roll speed ratio was varied. Results of the experiments are shown in Fig. 10. As seen from Fig. 10, when the roll speed ratio, i.e. roll speed of the upper roll 1a versus roll speed of the lower roll 1b is greater than or equal to 1.1, the length of inferior quality regions can be substantially reduced in comparison with that obtained from rolling utilizing rolls of equal roll speed. Example In order to further comfirm the improved performance of the preferred process according to the present invention, further experiments were performed for clad steel composed of a layer of stainless steel and a layer of soft steel. The clad steel was in the form of a slab having length of 5m and thickness of 200 mm. Rolling was performed in two pass cycles, each of which pass cycles included pre-form rolling from one longitudinal end of the clad slab and reduction rolling from the other longitudinal end. The draft was 50%. In order to compare this, comparative experiments were performed utilizing the conventional process. In the conventional rolling process, pre-form rolling was performed on one longitudinal end portion of the clad slab. Then, reduction rolling was performed from the other end. After one pass cycle, normal rolling was performed for another pass cycle. Additional experiments were perfromed in which the roll speeds of the upper and lower rolls were different. In the experiments, the ratio of the roll speed of the upper roll versus the roll speed of the lower roll is set at 1.1. Results of the experiments are shown in Figs. 11 and 13. In Figs. 11 and 12, the result of the comparative example is shown by solid line, the result of the preferred process with equal roll speed is shown by the broken line, and the result of the preferred process with different roll speed is shown by the one-dot chain line. As will be seen from Figs. 11 and 12, by setting the pre-form rolling length within the range of 20% to 80% of the overall length of the slab, substantial reduction of the single layer region (ℓ₁ + ℓ₂) and the uneven clad ratio region (C₁ + C₂) can be obtained. Furthermore, the length of single layer region (ℓ₁ + ℓ₂) and the uneven clad ratio region (C₁ + C₂) in the slab rolled by the preferred process of the present invention is much smaller than that of the slab rolled by the conventional process. In addition, the length of the inferior quality region in relation to the number of pass cycles performed for obtaining the desired draft versus the original thickness was observed and the results are shown in Fig. 13. As will be seen from Fig. 13, the rolling processes were performed for obtaining the desired draft, i.e. 50% by two pass cycles or more substantially reduces the length of the inferior quality region. As will be appreciated herefrom, the preferred process for rolling of clad steel provides a substantially increased yield when pre-form rolling is performed from one end on a portion, the length of which is 20% to 80% of the overall length of the clad steel, and the reduction rolling is performed subsequently from the other end with a draft that is greater than or equal to the draft used in the pre-forming. A still greater increase in yield can be obtained by repeating the foregoing rolling process at least two passes. Differentiating of the roll speed to mate the higher speed roll to the layer having lower malleability, further assist for improvement of the yield by reducing length of inferior quality portions. Therefore, the present invention fulfills all of the objects and advantages sought therefor.

Claims: A process for rolling elongate clad steel comprising a first layer of a first material and a second layer of a second material, which second material has a lower malleability than that of the first material, which process comprises the steps of:- (i) pre-form rolling the steel at a first given draft from one end of the clad steel, (ii) reduction rolling the clad steel from the other end of the clad steel, characterised in that the pre-form rolling is effected over a length of from 20% to 80% of the total length of the slab, in that the reduction rolling is effected at a second given draft which is greater than or equal to the first given draft and in that steps (i) and (ii) are repeated as desired. A process as claimed in claim 1, in which the reduction rolling is performed on that portion of the clad steel which is left unrolled in pre-form rolling. A process as claimed in claim 1, in which the reduction rolling is performed over the overall length of the clad steel. A process as claimed in claim 1, 2 or 3 further comprising the steps of :- defining a path between first and second rolls to pass the clad steel therethrough during rolling; driving the second roll which mates with the second layer at a higher speed than that of the first roll which mates with the first layer

Inventors: ABE, HIDEO C/O TECHNICAL RESEARCH DIVISION, HIRUTA, TOSHIKI C/O TECHNICAL RESEARCH DIVISION, YARITA, IKUO C/O TECHNICAL RESEARCH DIVISION
Applicants: KAWASAKI STEEL CORPORATION
Application year: 1988
Main classification: B21B1
IPC codes: B21B1