ES2275046T3 - PROCEDURE FOR THE MANUFACTURE OF A COLD LAMINATED STEEL FLOOR OR SHEET THAT HAVE EXCELLENT FITNESS FOR CONFORMATION. - Google Patents

Abstract

Production of a steel strip or sheet comprises casting a steel melt into slabs or thin slabs, heating to at least 1000degrees C, hot rolling using a strip end speed of at least 10 m/s into a hot strip, coiling, cold rolling to form a cold strip, and recrystallization annealing. The steel melt contains (in wt.%) less than 0.1, less than 0.5 Mn, less than 0.03 P, less than 0.03 S, less than 0.1 Al, less than 0.01 N, less than 0.1 Ti, less than 0.05 Nb and less than 0.01 B with a balance of Fe and the usual impurities.

Description

Procedure for the manufacture of a strap or cold rolled steel sheet that have excellent fitness For conformation.

The invention relates to a method for the manufacture of a cold rolled steel strip or sheet. TO As an example, such a strap or sheet is used for manufacture of body parts necessary in construction vehicular.

More and more requirements are demanded from forming properties of flat steel products of this type to also be able to make complex designs with reduced work effort In this respect, good formability It is characterized by a high value of n, which characterizes the ability to stretch on a mold, and also a high value of r, that characterizes the ability of deep drawing.

From the technical bibliography ("Herstellung von warmgewalzten Flacherzeugnissen ", Verein deutscher Eisenhüttenleute, 1972, pages 271-303; "Herstellung von Kaltgewalztem Band ", Verein deutscher Eisenhüttenleute, 1970, part 2, pages 33-54; "Werkstoffkunde der gebräuchlichen Stähle ", Verein deutscher Eisenhüttenleute, 1977, part 1, pages 237-258) it is known that, in addition to choosing a suitable steel composition, in manufacturing mode Conventional conformation properties can be influenced decisively by adjusting the conditions in the laminate previous of roughing. However, in practice it is shown that with steel sheets, which were produced with the application of these measures, nor can the high demands be met without further ado.

In addition to the prior art mentioned, a procedure of DE3704828C2 is known for the manufacture of a hot rolled steel strip with a high value of r, in which a steel containing no more than 0.015% by weight of C, 1.0-2.0% by weight of Mn, 0.005-0.10% by weight of Al, 0.01 - 0.06% by weight of Nb, 0.01 - 0.1% by weight of Ti and iron as rest, as well as inevitable impurities, is heated in an oven of reheat to a temperature of not less than 1150 ° C. TO then, the steel so heated is roughed out, the roughing in a temperature range of 980ºC to 1100ºC with a thickness reduction of not less than 20% per pass. TO then the rough steel is finally hot rolled in a temperature range of Ar 3 at 930 ° C while maintaining a total reduction of not less than 90%, before it coils to 600 ° C to 800 ° C. Hot rolled strip produced in this way must have a high conformability, in which the essential feature of the known document procedure DE3704828C2 is that the processed IF steel has a high Mn content and is free of dissolved C and N and in which steel of such a composition is submitted within a range of determined temperature at a large reduction, then it cools and then coils again within a range of set temperature

From DE10117118C1 a process for the manufacture of thin sheet is known which can be well formed with a final thickness of the cold rolled strip of a maximum of 0.3 mm. In this procedure, a steel containing from 0.0015 to 0.008% by mass of C, 0.15-0.25% by mass of Mn,? 0.02% by mass of P, 0.005-0.03% by mass of S,? 0.02% by mass of Si, 0.0080 - 0.06% by mass of Al, 0.0010 - 0.020% by mass of N,? 0.05% by mass of Ca , ≤ 0.5% by mass of Ni, ≤ 0.05% by mass of Cu, ≤ 0.02% by mass of Sn, ≤ 0.01% by mass of Mo, ≤ 0.0005 % by mass of Ti, ≤ 0.0005% by mass of Mb, ≤ 0.002% by mass of V, ≤ 0.007% by mass of B and ≤ 0.05% by mass of Co, as well as optionally others elements and iron as rest and unavoidable impurities, sneaks to give rough or fine slabs. Then, these roughing or fine slabs are heated again, after cooling, to a temperature that is in the range of 1080 ° C to 1150 ° C, during a maintenance time that amounts to a maximum of eight hours. Then, the newly heated fine slabs or slabs are hot rolled several passes in a hot rolling train at a final rolling temperature that is above the Ar 3 temperature to respectively give a hot rolled strip and is coil at temperatures that are in the range of 590 ° C to 660 ° C. After winding, the hot rolled strip obtained is cold rolled to give a cold rolled strip and is subjected to an annealing which takes place in
continuous.

In addition to the prior art mentioned, from document JP07-018381A a known procedure for manufacturing a rolled steel sheet in cold that can be deeply embedded in an excellent way in the that it is a steel that, in addition to iron (in% by weight), has an ultra-low C content of at most 0.015%, a content of Mn of a maximum of 0.40%, a P content of a maximum of 0.020%, an Al content of at most 0.080%, a content of N of at most 0.005%, a Ti content of 0.005 - 0.020% and an S content of at most 0.015%. In this meaning, the contents of N, S, Mn and Ti must comply with it time condition 3.42 N + S / 16 (Mn + 0.10) <Ti <10 N + S / 8 (Mn + 0.10) to make possible the formation of TiN rainfall, which decisively influences capacity Stretched onto treated steel mold known.

According to JP07-018381A, composite steel in this way is continuously cast with a determined extraction speed and then laminated in hot. In this sense, in the penultimate stage of rolling in hot finishing should be achieved a degree of conformation of at minus 40% and in the last stage of hot rolling of finishing a degree of conformation of at least 20%. The temperatures Hot rolled are in the known procedure respectively above the temperature Ar 3 for ensure a dispersion of properties as small as possible in the Straps obtained after hot rolling. To laminate in hot is followed by a specific cooling of the laminated strip in hot in which cooling speeds of at least 30ºC / s.

The steel sheets and strips produced with the known procedure of document JP07-018381A specifically present values of r that are identified especially as suitable for a drawing treatment deep But for this not only a certain mode is necessary and continuous casting, but with the desired result or even a narrowly limited spectrum of steel compositions

Therefore, the objective of the invention was in getting a procedure with which they could occur, with little effort of production engineering, strapping or laminated sheets cold with a widely dispersed composition that they possess, in comparison with conventionally manufactured products, with a conformability further improved.

This objective is achieved according to the invention. by means of a process for manufacturing the strip or the sheet of cold rolled steel comprising the following stages:

- Strain a steel melt containing (in% by weight) ≤ 0.1% of C, ≤ 0.5% of Mn, <0.03% of P, <0.03% of S, ≤ 0.1% of Al, <0.01% of N, <0.1% of Ti, <0.05% of Nb, ≤ 0.01% of B, iron as the remainder and usual impurities to give a semi-finished product, such as roughing or fine roughing,

- Thoroughly heat the semi-finished product until a preheating temperature that amounts to at least 1000 ° C,

- Hot rolled the semi-finished product in a finishing hot rolling step comprising the minus four lamination boxes to give a laminated strip in hot, in which

- The degree of total conformation \ varepsilon_ {h4} reached during the last four boxes of lamination traversed before leaving the lamination step hot amounts to at least 80%,

- The degree of total conformation \ varepsilon_ {h3} reached during the last three boxes of lamination traversed before leaving the lamination step hot amounts to at least 65%,

- The degree of total conformation \ varepsilon_ {h2} reached during the last two boxes of lamination traversed before leaving the lamination step hot amounts to at least 50%,

- The degree of total conformation \ varepsilon_ {h1} reached during the last rolling box crossed before leaving the hot rolling step amounts to at least 25%,

- Hot rolling in the last four rolling boxes crossed before leaving the step of hot rolling takes place at a rolling temperature that amounts to 880ºC at 950ºC,

- Hot rolling is finished at final hot rolling temperature which is ≥ that the Ar3 temperature, and

- The final strapping strap speed hot rolling obtained by leaving the rolling step hot it amounts to at least 10 m / s,

- Wind the hot rolled strip,

- Cold laminate the hot rolled strip to give a cold rolled strip,

- Recrystallize the laminated strip cold

The invention is based on the knowledge that a decisive improvement of the ability to form steel strips and steel plates of the type in question can not only be achieved through a suitable transformation previously connected to the hot rolled semi-finished product finish respectively used, but especially by choosing of the hot rolling conditions according to the invention. So, cold rolled sheets produced according to the invention stand out for outstanding r values and a drawing capacity correspondingly excellent deep.

In this regard, the procedure according to the invention can be realized based on mild carbon steels of conventional composition or IF steels. Correspondingly it is foreseen a configuration of the invention that treats a steel alloy soft containing respectively (in% by weight) 0.01 - 0.1% of C, 0.1 - 0.5% of Mn, up to 0.03% of P, up to 0.03% of S, 0.01 - 0.1% of Al, less than 0.005% of Ti, less than 0.005% of Nb and up to 0.01% of B, as well as iron as rest and inevitable impurities. If an IF steel is treated, then it may normally contain (in% by weight) respectively less than 0.01% C, less than 0.2% of Mn, less than 0.02% of S, less than 0.02% of P, less than 0.05% of Al, less than 0.005% of N, 0.02-0.1% of Ti, up to 0.05% of Nb and less than 0.001% of B. Obviously, the components of Alloy separately can be combined in this within the intervals previously provided in general by the invention of such that an optimal work result is achieved. So in isolated cases it may be favorable to retake respectively only one of the alloy modifications indicated in the dependent claims to especially highlight and / or delete the property influenced by the respective element, while the other alloy elements vary just like before at the other intervals collected.

The molten steel alloy so suitable sneaks to give a semi-finished product that can hot rolled, such as roughing or fine roughing. These will heat up to a preheat temperature that is preferably in the range of 1000 ° C to 1300 ° C. Then, the semi-finished product is led to a rolling step in hot in which it is laminated to give a hot rolled strip. This hot rolling comprises a finishing laminate that is performs in several stages, which if necessary can be connected previously a roughing.

Regardless of possible alloys which are used in the predetermined frames according to the invention, the improvement intended by the invention is achieved respectively because in the last four stages of hot rolling of finished the degree of total remaining conformation to be overcome During these four stages it is possible to stop. Low "degree of total conformation \ varepsilon_ {h4} "is generally understood in this context the relationship of the difference between the thickness of the d_ {e4} hot rolled strip when entering the last fourth box and thickness of hot rolled strip d_ {e7} al leave the last box with respect to the thickness of the laminated strip in hot d_ {e4} when entering the fourth last box (\ varepsilon_ {h4} = (d_ {e4} -d_ {e7}) / d_ {e4}).

The invention prescribes that with the four Latest hot rolling step boxes the degree of Total conformation should amount to at least 80%. Therefore, according to the invention, in a classic lamination step comprising seven rolling boxes during boxes 4, 5, 6 and 7 must adjust a degree of total conformation of at least 80%, preferably 85%, to achieve the desired increase in the value of r.

However, the distribution of the decline of thicknesses separately in the lamination boxes crossed separately until the end by hot rolled strip is not random, but follows an exactly staggered scheme. So, the invention indicates, in addition to the degree of total conformation that should reached during the last four boxes, that the degree of total conformation that must expire in total during the three Last rolling boxes should still amount to at least 65%, preferably at 75%, the degree of total conformation during last two boxes of the rolling step at least 50%, preferably at 60%, and the degree of total conformation achieved during the last step box at least 25%, preferably at least 30% or even at least 35%. Surprisingly it shows that the impact of the way to proceed according to the invention is all the more favorable the higher the respectively degree of total conformation remaining for the last four boxes crossed.

In addition to the high degree of total conformation that must be exceeded respectively during rolling boxes in heat separately, the temperature at which it is perform hot rolling in the last four boxes crossed by the hot rolling step. This must chosen according to the invention in such a way that the affected passes Hot rolled are made in deep austenite. By therefore, the invention provides for rolling the last four passes to temperatures that are set in the range of 950 ° C at 880 ° C in a close vicinity to the temperature of Ar_ {3} transformation. In this sense, the established rolling temperatures especially close to the temperature Ar 3, which are sufficient from 930 ° C to below the temperature Ar 3. In this case, rolling temperatures respective hot are in a range in which the steel respectively treated is still surely austenitic but which at the same time is at a level that guarantees that work is done on Deep austenite favorable for the purposes of the invention.

An operating parameter that must also observed according to the invention, which is important for properties of the product obtained, is the final speed of the strap. This amounts to at least 10 m / s in case of very soft steel without alloying, but at least 15 m / s is preferred to avoid a recrystallization in the finishing step between the passes of laminate. The high speed of rolling and / or final rolling is necessary so that, especially in the case of very auspicious steel at recrystallization, recrystallization does not occur in the finishing step after rolling passes between lamination boxes separately. This way you get a high degree of cumulative hot forming by which in the hot rolled strip produced according to the invention, adjust the texture base for very high r values.

If the cold rolled strip should be annealed after cold rolling so that it recrystallizes in an oven of continuous passage, then according to the invention have proved to be advantageous, in relation to the high values of intended r, steel sheets or strips produced when the temperature of winding amounts to at least 700 ° C. If, on the contrary, in the cold rolled sheet is produced by an annealing that will recrystallize that takes place in the coil under the hood a "pancake structure" favorable for certain jobs conformation, then it is favorable to choose a temperature of winding as low as possible that amounts to a maximum of 550ºC. The "pancake structure" stands out for elongated grain, relatively thick. Then, the structure provided in such a way and the texture attached to it are especially favorable when in the cold forming of cold rolled strip to give a piece high levels of conformation must be achieved. A high winding temperature causes carbides and thick nitrides, due to the opposite a low carbide and nitride winding temperature fine. For the recrystallization of cold rolled strip are in principle more favorable rainfall less thick than many fine because in the course of recrystallization they act as less interfering way. However, to produce a structure pancake on a cold rolled strip, during annealing in bell it is necessary that nitride (AlN) precipitation be suppress hot rolled strip by low winding temperature since AlN precipitation is needed in the recrystallization annealing of the cold rolled strip during the slow heating phase as dislocation for orient the grains and in this way values of r are still adjusted Taller. Annealing of the steel strip in continuous annealing proves to be especially profitable when continuous annealing such is coupled with a bath immersion finishing line cast, especially a hot dip galvanizing line of Straps

It is also possible to subject the annealed strap to recrystallization to an electrolytic surface finish.

The following explains in more detail the invention by examples of embodiment.

The attached figure shows schematically a step 8 of hot rolling that It comprises seven boxes 1,2,3,4,5,6,7 lamination, a path 9 of rollers arranged in the direction of transport F behind the hot rolling step 8 and a winding device 10 arranged at the end of the roller path 9.

In the hot rolling stage they have hot rolled roughing to give hot rolled strip C that were molded from a soft steel containing 0.025% by weight of C, 0.15% by weight of Mn, respectively less than 0.01% by weight of P and S, 0.04% by weight of Al, 0.003% by weight of N, iron as rest and inevitable impurities.

In a first experiment, the roughing is heated to a preheat temperature of 1250 ° C before they were hot rolled in lamination step 8 hot to give hot rolled strip C at a temperature 900 ° C hot rolled finish. The strip rolled in hot obtained is then wound at a winding temperature of 520ºC and laminated with a 75% cold rolling grade to Give the cold rolled strip. After cold rolling, the Cold rolled strip is rewound in the coil so that recrystallize in a bell oven to anneal at 700 ° C.

In a second experiment, the roughing is heated to a preheat temperature of 1100 ° C, it hot rolled in step 8 of hot rolling to a final hot rolling temperature of 900ºC to give strapping hot rolled C. The hot rolled strip C is wound to a temperature that rises to 720 ° C, cold rolled with a 75% cold rolling grade and is counted at 800 ° C so recrystallize in a continuous pass through oven continually.

In the course of hot rolling in the 8 step hot rolling, shaping degrees \ varepsilon_ {h1} to \ varepsilon_ {h7} to be reached in the respective boxes 1, 2, 3, 4, 5, 6.7 lamination remaining separately they were adjusted in such a way that the conformations residuals remaining for the last four boxes 4, 5, 6.7 of rolling mill lamination were clearly higher than in the case of conventional manufacturing. In this sense, for determine the effects to be adjusted in the adjustment according to the invention of a residual conformation remaining as high as possible for the last four boxes 4, 5, 6.7 lamination are compared respectively three hot rolled variants Ea, Eb and Ec according to the invention with a hot rolling experiment K performed in a conventional manner. The following table shows respectively the degrees of conformation? h4}, \ varepsilon_ {h5}, \ varepsilon_ {h6}, \ varepsilon_ {h7} remaining that affect the conformation during the last four Lamination boxes 4,5,6,7 for the experiments Ea, Eb, Ec carried out according to the invention and the experiment K performed of conventional way.

Grade of Other boxes Experiment Experiment Experiment Experiment conformation lamination Ea Eb Ec k total that must complete \ varepsilon_ {h4} 4,5,6,7 Approx. 80% Approx. 85% > 85% Approx. 75% \ varepsilon_ {h5} 5,6,7 Approx. 65% Approx. 75% > 75% Approx. 55% \ varepsilon_ {h6} 6.7 Approx. fifty% Approx. 60% > 60% Approx. 35% \ varepsilon_ {h7} 7 Approx. 25% Approx. 30% > 35% Approx. fifteen%

The values of r are compared in diagram 1 r_ {0 ^ {} (value of r in the rolling direction), r_ {45 ^ {}} (value of r diagonal to the rolling direction), r_ {90 ^ {o}} (value of r perpendicular to the rolling direction), r_ {m} (r_ {m} = (r_ {0 ^ {}} + 2r_ {45} + r_ {90 o}} / 4) determined for conventionally produced hot rolled strips with the values r_ {0 ^ {}, r_ {45 ^ {}}, r_ {90 ^ {}, r_ {m o}} that they could be fixed on the cold rolled sheets produced according to the invention. It shows a clear superiority of the plates produced according to the invention.

Diagram 2 presents another proof of the superior properties in relation to the good capacity of desired cold forming, especially the ability to deep drawing of steel sheets and strips produced according to the invention, in which the result of the texture analysis of the plates produced according to the invention is compared with the analysis of texture of conventionally manufactured sheets. This represented the density of occupation along certain "fibers" on which are the grain orientations more important. Together, intense occupations can be recognized on the fiber? in the orientations <111> || normal to the sheet, especially in (111) [110]. These components of texture is desired for high values of r. Correspondingly, the highest r values of the sheets produced according to the invention can be attributed, compared to the plates produced conventionally, to a more intense occupation. The texture results explain with this the values of r reached. In addition, it was also shown that the values of r very high of approximately 2.0 for a deep drawing steel Calmed with Al Calmed in cold rolled strip produced according to the invention are related to an increase in the structure of annealed <111>. The sheets manufactured according to the invention they also have clearly better properties in this regard than conventionally manufactured sheets.

Finally, in diagram 3 it is represented to the sheet produced according to the invention, annealed in an oven of bell for annealing, and annealing to recrystallization in an oven of continuous annealing, the value of r_ {m} with respect to the degree of total conformation achieved in the course of rolling in hot to demonstrate the influence of annealing type of recrystallization and the degree of conformation with respect to the degree of accumulated austenitic conformation of hot rolling in the r_ {m} value of the cold rolled strip obtained. It shows that high degrees of conformation lead to a clear increase in value of r_ {m}. At the same time it is shown that the plates bell annealed have higher r values compared with the plates annealed continuously due to the pancake structure It fits in this type of annealing.

Claims (25)

1. Procedure for manufacturing a strapping or cold rolled steel sheet comprising the following stages: - Strain a steel melt containing (in% by weight) C: \ leq 0.1%, Mn: \ leq 0.5%, P: < 0.03%, S: < 0.03%, To the: \ leq 0.1%, N: < 0.01%, You: < 0.1%, Nb: < 0.05%, B: ≤ 0.01% iron as rest and usual impurities for give a semi-finished product, such as roughing or fine roughing, - Thoroughly heat the semi-finished product until a preheating temperature that amounts to at least 1000 ° C, - Hot rolled the semi-finished product in a finishing hot rolling step comprising the minus four lamination boxes to give a laminated strip in hot, in which - The degree of total conformation \ varepsilon_ {h4} reached during the last four boxes of lamination traversed before leaving the lamination step hot amounts to at least 80%, - The degree of total conformation \ varepsilon_ {h3} reached during the last three boxes of lamination traversed before leaving the lamination step hot amounts to at least 65%, - The degree of total conformation \ varepsilon_ {h2} reached during the last two boxes of lamination traversed before leaving the lamination step hot amounts to at least 50%, - The degree of total conformation \ varepsilon_ {h1} reached during the last rolling box crossed before leaving the hot rolling step amounts to at least 25%, - Hot rolling in the last four rolling boxes crossed before leaving the step of hot rolling takes place at a rolling temperature that amounts to 880ºC at 950ºC, - Hot rolling is finished at final hot rolling temperature which is ≥ that the Ar3 temperature, and - The final strapping strap speed hot rolling obtained by leaving the rolling step hot it amounts to at least 10 m / s, - Wind the hot rolled strip, - Cold laminate the hot rolled strip to give a cold rolled strip, - Recrystallize the laminated strip cold 2. Method according to claim 1, characterized in that the steel melt contains at least 0.01% by weight of C. 3. Method according to claim 1, characterized in that the steel melt contains less than 0.01% by weight of C. Method according to one of the preceding claims, characterized in that the steel melt contains at least 0.1% by weight of Mn. 5. Method according to one of the preceding claims, characterized in that the steel melt contains less than 0.2% by weight of Mn. Method according to one of the preceding claims, characterized in that the steel melt contains less than 0.02% by weight of P and / or less than 0.02% by weight of S. Method according to one of the preceding claims, characterized in that the steel melt contains less than 0.05% by weight of Al. Method according to one of the preceding claims, characterized in that the steel melt contains less than 0.005% by weight of N. Method according to one of the preceding claims, characterized in that the steel melt contains less than 0.005% by weight of Ti. Method according to one of claims 1 to 8, characterized in that the steel melt contains at least 0.02% by weight of Ti. Method according to one of the preceding claims, characterized in that the steel melt contains less than 0.005% by weight of Nb. Method according to one of the preceding claims, characterized in that the steel melt contains less than 0.001% by weight of B. Method according to one of the preceding claims, characterized in that the hot rolling in the last four rolling boxes crossed before leaving the hot rolling step takes place at a rolling temperature that is at least equal to the Ar_ { 3} and at most equal to 930 ° C. 14. Method according to one of the preceding claims, characterized in that the total degree of conformation [epsilon] {h4} reached in the last four rolling boxes crossed before leaving the hot rolling step amounts to at least 85%. 15. Method according to one of the preceding claims, characterized in that the total degree of conformation [epsilon] {h3} reached in the last three rolling boxes crossed before leaving the hot rolling step is at least 75%. 16. Method according to one of the preceding claims, characterized in that the total degree of conformation [epsilon] {h2} reached in the last two rolling boxes crossed before leaving the hot rolling step is at least 60%. 17. Method according to one of the preceding claims, characterized in that the total degree of conformation [epsilon] {h1} reached in the last rolling box crossed before leaving the hot rolling step is at least 35%. 18. Method according to one of the preceding claims, characterized in that the final speed of the strip of the hot-rolled strip obtained when leaving the hot-rolling step is at least 15 m / s. 19. Method according to one of the preceding claims, characterized in that the winding temperature is at least 700 ° C. 20. Method according to one of claims 1 to 18, characterized in that the winding temperature is less than 550 ° C. 21. Method according to one of the preceding claims, characterized in that the degree of cold rolling achieved by cold rolling amounts to at least 40%. 22. Method according to one of the preceding claims, characterized in that the recrystallization annealing takes place continuously. 23. Method according to claim 22, characterized in that the annealed strip crystallization is then subjected to a molten bath immersion finish. 24. Method according to one of claims 1 to 21, characterized in that the recrystallization annealing takes place in the coil in a bell oven. 25. Method according to one of claims 1 to 22 or 24, characterized in that an electrolytic surface finish takes place after recrystallization annealing.