EP1411140A1 - Process for manufacturing of cold-rolled steel strips or sheets having excellent formability - 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

The invention relates to a method for producing a cold rolled steel strip or sheet. For example such tape or sheet for the manufacture of im Vehicle body parts required.

The forming properties of such flat steel products ever increasing demands are made to complex shapes with reduced workload to be able to realize. Good formability is included characterized by a high stretchability characterizing n-value and a likewise high that R-value characterizing deep-drawability.

From the specialist literature ("Production of hot rolled Flat Products ", Association of German Ironworkers, 1972, pages 271-303; "Manufacture of cold rolled Band ", Association of German Ironworkers, 1970, Part 2, Pages 33 - 54; "Materials science of common steels", Association of German Ironworkers, 1977, Part 1, pages 237 - 258) it is known that with conventional Production method in addition to the selection of a suitable one Steel composition the deformation properties crucial by setting the conditions at Pre-rolling of the slabs can be influenced. In the Practice shows, however, that even with steel sheets, the have been generated using these measures increased requirements are not easily met can be.

In addition to the prior art explained above, DE 37 04 828 C2 discloses a process for producing a hot-rolled steel strip with a high r-value, in which a not more than 0.015% by weight C, 1.0-2.0 % By weight Mn, 0.005-0.10% by weight Al, 0.01-0.06% by weight Nb, 0.01-0.1% by weight Ti and the balance iron and inevitable impurities Steel is heated in a post-heating furnace to a temperature of not less than 1150 ° C. The steel which has been heated in this way is then rough-rolled, the roughing being carried out in a temperature range from 980 ° C. to 1100 ° C. with a reduction in thickness of not less than 20% per pass. The pre-rolled steel is then hot-rolled in a temperature range of Ar ₃ to 930 ° C while maintaining an overall reduction of not less than 90% before being coiled at 600 ° C to 800 ° C. The hot strip produced in this way should have a high formability, the essential feature of the method known from DE 37 04 828 C2 being seen in the fact that the IF steel developed has a high Mn content and is free from dissolved C and N and that the steel thus assembled is subjected to a large reduction within a certain temperature range, then cooled and then rewound within a certain temperature range.

DE 101 17 118 C1 discloses a process for the production of fine metal sheet with a final cold strip thickness of at most 0.3 mm. In this method, a steel of 0.0015 to 0.008 mass% C, 0.15 - 0.25 mass% Mn, ≤ 0.02 mass% P, 0.005 - 0.03 mass% S, ≤ 0 , 02 mass% Si, 0.0080 - 0.06 mass% Al, 0.0010 - 0.020 mass% N, ≤ 0.05 mass% Ca, ≤ 0.5 mass% Ni, ≤ 0, 05 mass% Cu, ≤ 0.02 mass% Sn, ≤ 0.01 mass% Mo, ≤ 0.0005 mass% Ti, ≤ 0.0005 mass% Mb, ≤ 0.002 mass% V, ≤ 0.007 mass% B and ≤ 0.05 mass% Co, as well as optionally other elements and the rest iron and unavoidable impurities, cast into slabs or thin slabs. These slabs or thin slabs are then reheated to a temperature in the range from 1080 ° C to 1150 ° C for a maximum holding time of eight hours. The reheated slabs or thin slabs are then hot-rolled several passes in a hot rolling mill at a final rolling temperature above the Ar ₃ temperature to form a hot strip and at temperatures ranging from 590 ° C to 660 ° C. After coiling, the hot strip obtained is cold-rolled into cold strip and subjected to continuous annealing.

The object of the invention was therefore a To create procedures that deal with little Production engineering cold strips or sheets let generate one versus conventional manufactured products further improved formability have.

• Vergießen einer (in Gew.-%) ≤ 0,1 % C, ≤ 0,5 % Mn, < 0,03 % P, < 0,03 % S, ≤ 0,1 % Al, < 0,01 % N, < 0,1 % Ti, < 0,05 % Nb, ≤ 0,01 % B, Rest Eisen und übliche Verunreinigungen enthaltenden Stahlschmelze zu einem Vormaterial, wie Brammen oder Dünnbrammen,
• Durchwärmen des Vormaterials auf eine mindestens 1000 °C betragende Vorwärmtemperatur,
• Warmwalzen des Vormaterials in einer mindestens vier Walzgerüste umfassenden Fertigwarmwalzstaffel zu einem Warmband, wobei
• der über die letzten vier vor dem Verlassen der Warmwalzstaffel passierten Walzgerüste erzielte Gesamtumformgrad ε_h4 mindestens 80 %,
• der über die letzten drei vor dem Verlassen der Warmwalzstaffel passierten Walzgerüste erzielte Gesamtumformgrad ε_h3 mindestens 65 %,
• der über die letzten beiden vor dem Verlassen der Warmwalzstaffel passierten Walzgerüste erzielte Gesamtumformgrad ε_h2 mindestens 50 %,
• der über das letzte vor dem Verlassen der Warmwalzstaffel passierten Walzgerüste erzielte Gesamtumformgrad ε_h1 mindestens 25 %,
• das Warmwalzen in den letzten vier vor dem Verlassen der Warmwalzstaffel passierten Walzgerüsten bei einer 880 °C bis 950 °C betragenden Walztemperatur erfolgt,
• das Warmwalzen bei Warmwalzendtemperatur beendet wird, die ≥ der Ar₃-Temperatur ist, und
• die Bandendgeschwindigkeit des erhaltenen Warmbands beim Verlassen der Warmwalzstaffel mindestens 10 m/s beträgt,
• Haspeln des Warmbandes,
• Kaltwalzen des Warmbandes zu einem Kaltband,
• Rekristallisationsglühen des Kaltbands.

This object has been achieved according to the invention by a method for producing cold-rolled steel strip or sheet, which comprises the following steps:

• Casting one (in% by weight) ≤ 0.1% C, ≤ 0.5% Mn, <0.03% P, <0.03% S, ≤ 0.1% Al, <0.01% N , <0.1% Ti, <0.05% Nb, ≤ 0.01% B, balance iron and steel impurities containing common impurities to a starting material, such as slabs or thin slabs,
• Warming the primary material to a preheating temperature of at least 1000 ° C,
• Hot rolling the primary material in a finished hot rolling mill comprising at least four roll stands to form a hot strip, wherein
• the total forming _degree ε _h4 achieved over the last four rolling stands passed before leaving the hot rolling mill, at least 80%,
• the total forming _degree ε _h3 achieved over the last three rolling stands passed before leaving the hot rolling mill at least 65%,
• the total forming _degree ε _h2 achieved over the last two roll stands passed before leaving the hot rolling mill at least 50%,
• the total forming _degree ε _h1 achieved over the last rolling stand passed before leaving the hot rolling mill at least 25%,
• hot rolling in the last four rolling stands passed before leaving the hot rolling mill takes place at a rolling temperature of 880 ° C to 950 ° C,
• the hot rolling is stopped at the hot rolling end temperature which is ≥ the Ar ₃ temperature, and
• the final strip speed of the hot strip obtained when leaving the hot rolling mill is at least 10 m / s,
• Coiling the hot strip,
• Cold rolling the hot strip into a cold strip,
• Cold recrystallization annealing.

The invention is based on the knowledge that not only by a suitable, upstream of the finish hot rolling Processing of the pre-product used, but in particular by the choice of the invention Hot rolling conditions a decisive improvement in the Formability of steel strips and steel sheets in question standing type can be achieved. So draw yourself Cold sheets produced according to the invention by excellent r values and a correspondingly excellent one Thermoformability.

The method according to the invention can be based on this on conventionally composed switches Carry out carbon steels or IF steels. Accordingly, an embodiment of the invention provides that a soft steel alloy is processed, which each (in% by weight) 0.01-0.1% C, 0.1-0.5% Mn, up to 0.03% P, up to 0.03% S, 0.01-0.1% Al, less than 0.005% Ti, less than 0.005% Nb and up to 0.01% B and the balance iron and unavoidable impurities may contain. If an IF steel is processed, it can this (in% by weight) typically less than 0.01 each % C, less than 0.2% Mn, less than 0.02% S, less less than 0.02% P, less than 0.05% Al, less than 0.005% N, 0.02-0.1% Ti, up to 0.05% Nb and less than 0.001% B contain. Of course, the individual Alloy components within the through the Invention generally predetermined areas so on each other vote that an optimal work result is achieved. So it can be cheap in individual cases, each time only one of the specified in the dependent claims Modifications of the alloy to take up by the particular element influenced property emphasize or suppress while the others Alloy elements still in the broader Ranges can be varied.

The appropriately alloyed steel melt becomes a hot-rolled raw material, such as slabs or Thin slabs, shed. These are on a Preheating temperature warmed, which is preferably in the range of 1000 ° C to 1300 ° C. Then that will Raw material passed into a hot rolling mill, in which it is too Hot strip is rolled. This hot rolling includes an in finish rolling carried out in several stages, the if necessary, a preliminary rolling can be carried out.

Regardless of which of the alloys possible within the scope of the present invention are used, the improvement sought by the invention is achieved in each case by the fact that in the last four stages of finish hot rolling the remaining degree of deformation to be managed over these four stages is as high as possible. In this context, the "overall degree of _forming ε _h4 " is generally understood to mean the ratio of the difference between the hot strip thickness d _e4 when entering the fourth last stand and the hot strip thickness d _e7 when leaving the last stand to the hot strip thickness d _e4 when entering the fourth last stand (ε _h4 = (d _e4 -d _e7 ) / d _e4 ).

The invention dictates that over the past four Stands of the hot rolling mill the total degree of forming at least Should be 80%. With a classic, seven Rolling stand comprising roll stands is according to the invention therefore a total degree of deformation over the stands 4, 5, 6 and 7 of at least 80%, preferably 85%, to adjust the to achieve the desired increase in the r value.

The distribution of the individual reductions in thickness individual mill stands last run through by the hot strip is not arbitrary, but follows you exactly tiered plan. So the invention overwrites the the last four frames to be formed before that over the last three mill stands in total The total degree of deformation to be managed is still at least 65%, preferably 75%, of the last two scaffolds the rolling season at least 50%, preferably 60% and the Degree of deformation reached over the last frame of the season at least 25%, preferably at least 30% or even must be at least 35%. Has been surprising shown that the impact of the invention Procedure the cheaper the higher the for the last four scaffolds passed through each remaining Total forming is.

In addition to the high overall degree of forming that can be managed via the individual hot rolling stands, the temperature at which the hot rolling is carried out in the last four passes of the hot rolling mill is important. According to the invention, this should be chosen so that the relevant passes of hot rolling are carried out in deep austenite. The invention therefore provides for the last four passes to be rolled at temperatures which are in the range from 950 ° C. to 880 ° C. in close proximity to the transition temperature Ar ₃ . Rolling temperatures that are particularly closely related to the Ar ₃ temperature and range from 930 ° C. down to the Ar ₃ temperature are advantageous. In this case, the respective hot rolling temperatures are in a range in which the steel processed is still certainly austenitic, but at the same time is at a level which ensures that the deep austenite favorable for the effects of the invention is used.

One that is also to be observed according to the invention Operating parameters of importance for the properties of the product obtained is the Bandendgeschwindigkeit. This is at least 10 m / s, for unalloyed very soft steel, however preferred at least 15 m / s to ensure recrystallization in the Avoid finishing series between the roll passes. The high rolling or final rolling speed is required so that it is particularly suitable for those who like to recrystallize Steel after the roll stitches between each No recrystallization in the whale Finishing season is coming. This way, a high one cumulative degree of hot deformation reached, which the Texture basis for the very high r values on hot strip produced according to the invention is set.

Should the cold strip be rolled in one after cold rolling Continuous furnace to be recrystallized annealed, so it did with regard to the high r-values aimed for Steel sheets or strips produced according to the invention as proven advantageous if the reel temperature at least Is 700 ° C. On the other hand, should one for certain Deformation tasks favorable "pan cake structure" in the cold rolled sheet by a in the waistband under the hood recrystallizing annealing taking place, so it is favorable to have the lowest possible temperature, maximum 550 ° C to select the reel temperature. "Pan-Cake-fabric" is characterized by an elongated, relatively coarse Grain out. The structure and the so created associated texture is particularly beneficial when at the cold deformation of the cold sheet into a high component Forming degrees must be achieved. A high Coiling temperature causes rough, low Coil temperature, however, fine carbides and nitrides. For the Cold strip recrystallization is basically a few rough ones Excretions are cheaper than many fine ones because they The recrystallization process is less disruptive. to Creation of a pan cake structure on cold strip over the Hood annealing, however, requires that the Nitride deposits (AlN) on the hot strip through a deep Reel temperature can be suppressed because of the AlN excretion in cold strip recrystallization annealing during the slow heating-up phase as a malfunction Grain orientation is needed and this way set even higher r values. The annealing of the steel strip in a continuous anneal then proves to be special economical if such a continuous annealing with a Hot-dip coating plant, especially one Strip hot-dip galvanizing plant, is coupled.

It is also possible to use the recrystallization annealed strip an electrolytic surface refinement undergo.

The invention is described below with reference to Embodiments explained in more detail.

In the attached figure there are schematically a seven Rolling stands 1, 2, 3, 4, 5, 6, 7 comprising a hot rolling mill 8 in the conveying direction F behind the hot rolling season 8 arranged roller table 9 and one at the end of the roller table 9 arranged reel device 10 shown.

In the hot rolling season there are slabs for hot strip W hot-rolled, which consists of a 0.025 wt .-% C, 0.15% by weight of Mn, in each case less than 0.01% by weight of P and S, 0.04% by weight Al, 0.003% by weight N, balance iron and mild steel containing inevitable impurities have been poured off.

In a first attempt, the slabs are on one Preheating temperature of 1250 ° C has been warmed through before them in the hot rolling season 8 to hot strip W at one Hot rolling end temperature of 900 ° C has been hot rolled are. The hot strip obtained is then one Reel temperature of 520 ° C coiled and at one Cold rolling degree of 75% has been rolled into cold strip. After this Cold rolling is the cold strip in the coil in the bell annealer at 700 ° C was recrystallized annealed.

In a second attempt, the slabs are on one Preheating temperature of 1100 ° C warmed in the Hot rolling season 8 at a hot rolling end temperature of 900 ° C to hot strip W hot rolled. The hot strip W is coiled at a temperature of 720 ° C, with a cold rolling degree of 75% and cold rolled at 800 ° C in a continuous furnace been recrystallized annealed.

In the course of hot rolling in the hot rolling mill 8, the degrees of deformation ε _h1 to ε _h7 to be achieved via the individual remaining roll stands 1, 2, 3, 4, 5, 6, 7 have been set such that those for the last four roll stands 4 , 5,6,7 of the remaining rolling formations were significantly higher than in conventional production. In order to demonstrate the effects that occur with the inventive setting of the highest possible residual forming for the last four roll stands 4, 5, 6, 7, three hot rolling variants Ea, Eb and Ec according to the invention were compared with a conventionally carried out hot rolling test K. The table below shows the respective degrees of deformation ε _h4 , ε _h5 , ε _h6 , ε _h7 for the forming over the last four roll stands 4, 5, 6, 7 for the tests Ea, Eb, Ec and the conventional ones performed experiment K entered. total degree Remaining mill stands to be completed Try ea Try Eb Trial Ec Attempt K ε _h4 4,5,6,7 approx. 80% approx. 85% > 85% approx. 75% ε _h5 5,6,7 approx. 65% approx. 75% > 75% approx. 55% ε _h6 6.7 approx. 50 % approx. 60% > 60% approx. 35% ε _h7 7 approx. 25% approx. 30% > 35% approx. 15%

In Diag. 1 are the r values r _{0 °} (r value in the rolling direction) determined for the conventionally produced hot strips, r _{45 °} (r value diagonal to the rolling direction), r _{90 °} (r value transverse to the rolling direction), r _m ( r _m = (r _{0 °} + 2r ₄₅ + r _{90 °} ) / 4) compared with the r _{0 °} , r _{45 °} , r _{90 °} , r _{m °} values that could be found in the cold plates produced according to the invention. There is a clear superiority of the sheets produced according to the invention.

Another evidence of the desired with regard to good cold formability, especially deep drawability, superior properties generated according to the invention Diag supplies steel sheets and strips. 2, in which the Result of the texture analysis of those generated according to the invention Sheets of the texture analysis of the conventionally produced Sheets is juxtaposed. It is applied Occupancy density along certain "fibers" on which the most important grain orientations. Overall are strong deposits on the γ fiber in the orientations <111> || Sheet standards, especially for (111) [110] detect. These texture components are for high r values he wishes. Accordingly, the higher r values of the sheets produced according to the invention compared to conventionally produced sheets on a stronger allocation traced. The texture results thus explain the achieved r values. It was also shown that for an annealed aluminum-calmed deep-drawn steel very high r-values of approx. 2.0 am generated according to the invention Cold strip with tightening of the <111> glow structure in the Related. Here too, according to the invention manufactured sheets on significantly better properties than the conventionally manufactured sheets.

Finally, in Diag. 3 for the metal sheets produced according to the invention, recrystallized in the bell-type annealing furnace and in the continuous annealing furnace, the r _m value is plotted against the total degree of deformation achieved in the course of hot rolling in order to determine the influence of the type of recrystallization annealing and the amount of deformation via the accumulated austenite deformation degree of the hot rolling r _m value of the cold strip obtained. It can be seen that high degrees of deformation lead to a significant increase in the r _m value. At the same time, it can be seen that the dome-annealed sheets, due to the pan-cake structure that occurs with this type of annealing, have further increased r-values compared to the sheets annealed in a continuous process.

Claims (25)

A method of manufacturing a cold rolled steel strip or sheet comprising the following steps: Shedding one (in% by weight) C ≤ 0.1%, Mn ≤ 0.5%, P <0.03%, S <0.03%, al ≤ 0.1%, N <0.01%, Ti <0.1%, Nb <0.05%, B ≤ 0.01% Remainder iron and conventional impurities containing molten steel to a starting material, such as slabs or thin slabs, Warming the primary material to a preheating temperature of at least 1000 ° C, Hot rolling the primary material in a finished hot rolling mill comprising at least four roll stands to form a hot strip, wherein the total forming _degree ε _h4 achieved over the last four rolling stands passed before leaving the hot rolling mill, at least 80%, the total forming _degree ε _h3 achieved over the last three rolling stands passed before leaving the hot rolling mill at least 65%, the total forming _degree ε _h2 achieved over the last two roll stands passed before leaving the hot rolling mill at least 50%, the total degree of _deformation ε _h1 achieved over the last rolling stand passed before leaving the hot rolling mill is at least 25%, hot rolling in the last four rolling stands passed before leaving the hot rolling mill takes place at a rolling temperature of 880 ° C to 950 ° C, the hot rolling is stopped at the hot rolling end temperature which is ≥ the Ar ₃ temperature, and the final strip speed of the hot strip obtained when leaving the hot rolling mill is at least 10 m / s, Coiling the hot strip, Cold rolling the hot strip into a cold strip, Cold recrystallization annealing. A method according to claim 1, characterized in that the molten steel contains at least 0.01 wt .-% C. A method according to claim 1, characterized in that the steel melt contains less than 0.01 wt .-% C. Method according to one of the preceding claims, characterized in that the steel melt contains at least 0.1 wt .-% Mn. Method according to one of the preceding claims, characterized in that the steel melt contains less than 0.2 wt .-% 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 wt .-% 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 B. Method according to one of the preceding claims, characterized in that the hot rolling in the last four rolling stands passed before leaving the hot rolling season takes place at a rolling temperature which is at least equal to the Ar ₃ temperature and at most equal to 930 ° C. Method according to one of the preceding claims, characterized in that the total degree of _deformation ε _h4 achieved over the last four rolling stands passed before leaving the hot rolling mill is at least 85%. Method according to one of the preceding claims, characterized in that the total degree of _deformation ε _h3 achieved over the last three rolling stands passed before leaving the hot rolling mill is at least 75%. Method according to one of the preceding claims, characterized in that the total degree of _deformation ε _h2 achieved over the last two roll stands passed before leaving the hot rolling mill is at least 60%. Method according to one of the preceding claims, characterized in that the total degree of _deformation ε _h1 achieved over the last rolling stands passed before leaving the hot rolling mill is at least 35%. Method according to one of the preceding claims, characterized in that the final strip speed of the hot strip obtained is at least 15 m / s when leaving the hot rolling season. Method according to one of the preceding claims, characterized in that the reel temperature is at least 700 ° C. Method according to one of claims 1 to 18, characterized in that the reel temperature is less than 550 ° C. Method according to one of the preceding claims, characterized in that the degree of cold rolling achieved by cold rolling is at least 40%. Method according to one of the preceding claims, characterized in that the recrystallization annealing takes place in one pass. A method according to claim 22, characterized in that the recrystallization-annealed strip is then hot-dip coated. Method according to one of claims 1 to 21, characterized in that the recrystallization annealing takes place in the bund in a hood furnace. Method according to one of claims 1 to 22 or 24, characterized in that an electrolytic surface treatment takes place after the recrystallization annealing.

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