ES2230500T3 - PROCEDURE FOR THE MANUFACTURE OF STEEL PRODUCTS OF A HIGH RESISTANCE, CONFORMABLE IN COLD OF A HOT ROLLED BAND WITH A GOOD DUCTIBILITY. - Google Patents

Abstract

The invention relates to a method for producing a high-strength cold-formed product from a hot-rolled strip, said product exhibiting good malleability. According to said method, a steel comprising (in wt.%): C: 0.01 0.25 %, Si: 0.01 1.50 %, Mn: 0.50 2.00 %, P: <= 0.08 %, S: <= 0.01 %, Al: 0.001 1.50 %, Cr: <= 0.60 %, Mo: <= 0.60 %, N: < 0.02 %, at least one micro-alloy element from the group Ti: <= 0.20 %, Nb: <= 0.06 %, V: <= 0.15 %, the remainder being iron and the usual impurities is cast to form an input stock such as slabs, thin slabs or a cast strip. Starting from an initial hot-roll temperature, at which the micro-alloy elements remain essentially dissolved, the input stock is hot-rolled to form a hot-rolled strip. Said strip is firstly wound at a winding temperature of less than 600 DEG C and then cold formed into a product at a cold formation degree which is not less than 2 % and not greater than the deformation limit. The cold-formed product is annealed at a temperature and for a period which are less those required for complete recrystallization. The inventive method enables a cold formed product to be produced, said product exhibiting high strength, malleability and plasticity in the cold formed state.

Description

Procedure for manufacturing products of high strength, cold-formed, single-band steel Hot rolled with good ductility.

The invention relates to a method for manufacturing a cold formed steel product of a hot rolling strip, with high strength, with a Good formability

A procedure of DE 197 10 125 A1 is known for the manufacture of a hot rolling strip that It has a high resistance. According to the procedure known melts a complex phase steel containing (in% of weight) from 0.1 - 0.2% C, 0.3 to 0.6% Si, 1.5 up to 2.0% of Mn,? of 0.08% of P, 0.3 to 0.8% of Cr,? 0.4% of Mo,? 0.2 of Ti,? 0.08% of Nb and as iron rest as well as the inevitable impurities, to roughing and then laminated to a lamination band in hot, where the final hot rolling temperature is It is above 800ºC. Then the band of cools hot rolling obtained at a cooling rate of at least 30 ° C / s and then coil with a temperature of winding of at most 600 ° C. With the lamination band in hot obtained in this way, because of the temperature of relatively low winding, the resistance is at least 900 MPa. Apart from this it has the hot rolling strip generated according to the known procedure a good formability cold

This conformability can still be increased, from  in accordance with the procedure known from DE 197 10 125 A1, by the made the hot rolling strip, after winding undergo an annealing in the temperature range of 500 ° C until 850 ° C. By this annealing a situation of raw material is reached, which offers advantages for construction parts, which as a whole they must still have a high resistance, especially the limit of elongation, with good formability. By choosing one higher annealing temperature can be achieved with it particularly high resistances with limit relationships of extremely low elongation or high solidification equivalent with good elongation values.

According to DE 197 10 125 A1, the hot rolling strip treated by annealing in this way is suitable for cold rolling, with which a degree of formability of at least 30% is achieved. Due to the fact that the cold formed strip in this way is continuously annealed at temperatures of 700 ° C to 900 ° C for recrystallization, a cold forming sheet product with high strength can be generated, a ratio of the elongation limit even improved and a good solidifying behavior
tion.

It certainly shows, that the behavior of product elongation generated by cold forming of according to the known procedure is not enough for many employment cases. For example, it is difficult to comply with requirements in relation to resistance on the one hand and the power absorption power otherwise required parts of construction of this type made of such steels and that must be used to protect the occupants of a vehicle In the case of an accident. The construction pieces generated from according to the known procedure they do not even possess the conformability required after cold forming. When they undergo after cold forming an annealing of crystallization, the formability is certainly increased, but then resistance is no longer enough.

It is the task of the invention to procure a procedure for the manufacture of a product formed in cold, which with increased resistance in a situation of cold forming has high ductility and good corresponding conformability.

Starting from the position of the technique previously clarified this task is solved by a procedure for the manufacture of a cold forming product of a Highly resistant hot rolling strip with a good ductility, in which a steel is melted with (in% weight):

0.01-0.25% C,

0.01 - 1.50% of Si,

0.50 - 2.00% of Mn,

≤ 0.08% of P,

≤ 0.01% of S,

0.001 - 1.5% of Al,

le 0.60% Cr,

le 0.60% Mo,

<0.02% Mo, as well as at least one microalloy element of the group of Ti, Nb, V where the content  of Ti ≤ 0.20%, the content of Nb ≤ 0.06%, the V ≤ 0.15% content,

the rest is iron and the usual impurities, at a previous material, such as roughing, thin roughing or a band cast, in which the previous material, starting from a hot rolling start temperature, at which essentially the dissolved microalloy elements remain, hot rolled to form a lamination band in hot, where the hot rolling strip is wound to a winding temperature of less than 600 ° C, where the band of hot rolling then cold conforms to a product with a degree of cold forming corresponding to the minus 2% and at most the modification of the limit of deformation, and where an annealing of the product is carried out generated by cold forming at temperatures and during times that are below temperatures and of the necessary times for a complete recrystallization.

The invention is based on the verification surprising that the increase properties of the resistance of the microalloy elements contained in the steels according to the invention with a generation according with the invention of the hot rolling strip by a Annealing of the product obtained from the lamination band in hot after winding and cold forming and with this can be made cold forming products with particularly high resistance and good power of conformation, also in cold forming situation, without that an improvement by a procedure was necessary for this expensive technician

For the heat treatment carried out later  of the cold forming, according to the invention, is widely compensates for the loss of conformability that originates on a regular basis with cold forming, without loss of resistance. This applies for example also for hardening in the area of sharp edges with cutting or die cutting as well as Widely increased sensitivity to cracking. Instead it is shown that according to heat treatment carried out after cold forming originates from a increased resistance, so that with a product cold formed generated according to the invention can be combine in a directed way a high resistance with a high formability power. This combination of properties advantageous, for example, is possible by cold reshaping cold formed and heat treated product Then, to take it to its final form. Is also available in all its volume the good conformation power by example in construction parts relevant to shocks, such as windshield, side impact brackets, reinforcement elements, to, in the case of a blow with other objects, be able to transform kinetic energy in a high volume at an energy of conformation / transformation.

In the same way, the procedure of according to the invention particularly for the manufacture of construction pieces, whose maintenance of the measures and the form also set high requirements as well as Your job resistance. A typical example for such employment is they represent the racks for screens, which have to keep the image tubes of televisions or computer monitors the shadow mask due to thermal load during its composition and its use under a voltage equalized in a defined fixed plane. The high strength required is obtained by the construction piece cold formed, according to the heat treatment led to according to the invention after forming in cold, so that in the result an element of high strength construction, accurately formed, with a high dilation limit An additional improved, which can lead to deformation of the construction piece and to a surface influence, it is no longer necessary then.

It is a precondition for the result to obtain by the invention, that during hot processing of the steel used in accordance with the invention, the operating parameters, including winding temperature, of such that the increase in possible hardness by itself by the presence of a sufficient amount of microalloy elements it is not reached in the course of winding, but the elements of microalloys are present after winding in situation dissolved, not precipitated. For this reason, the winding temperature according to the invention, in such a way which is with a clear distance below the temperature in the which would reach the maximum precipitate of the elements of the micro alloy.

The steel alloy used in accordance with the invention encompasses a steel composition, from which, depending of the composition of the case and the conditions of production, they can manufacture bainitic steels, double phase steels or steels complex phase What is particularly intended with it are basically steels with a multi-phase structure of a fine granulation in which bainite, ferrite, are contained martensite and remaining austenite, in a volume dependent on the type Steel case.

After a previous material has been generated of a steel with a composition used in accordance with the invention, this material is hot rolled prior to a web Hot rolling. The starting temperature of the hot rolling is chosen with it in such a way that the microalloy elements remain as widely as possible dissolved within the steel, so that they are available for effects produced in the last step of the procedure, according with the invention

With slabs melted in a conventional manner, it should be the start temperature of the lamination at least 1050 ° C for this purpose. They certainly fit particularly for the manufacture of the previous material, procedures of thin roughing or thin-band casting procedures, with which, because of the circumstance, there is no longer the need for a new heating of the previous material respectively of a dissolution of the microalloy elements and on the other hand the speeds of rope cooling, it is particularly possible, keep The microalloy elements are dissolved widely until Start of the hot rolling process.

The end temperature of the lamination in hot preferably should be in the area of austenitization, therefore above the Arar temperature for get the multi-phase structure of fine granulation intended according to the invention.

The exact composition of the structure of the Hot rolling strip obtained is determined by the chemical properties, heating conditions, of lamination and cooling as well as the temperatures of winding It has been shown that the action sought in accordance with the invention then fits particularly safely the increased strength and conformability with steels double phase, when the winding temperature is at most 300 ° C With steels with mainly a bainitic structure, safely reach this action with winding temperatures that they are from 350ºC to 450ºC. Preferred complex phase steels They should be wound with temperatures from 450ºC to 550ºC.

Already in a thermally shaped situation they have steels of the type employed in accordance with the invention a high resistance of at least 550 N / mm2. In the course of the subsequent cold forming solidify the steels very strongly. At the same time the dilation is greatly reduced, so that a highly resistant product is obtained, without However, only difficult to deformable. Building parts Cold rolling formed in this way possess, due to its reduced conformability only a reduced conformation reserve and correspondingly a reduced energy collection power. As such, they can only be used in a reduced way, for example for  manufacture of building elements, to be used as bumpers in areas susceptible to collisions of a car.

Due to the fact that according to the invention, after cold forming an annealing of the product obtained, at temperatures and for some time, with which safely prevents complete recrystallization, by one part is further increased the elongation limit of steel employee. On the other hand by annealing, their ductility values at approximately the level at which I found the ductility of the hot rolling strip before cold forming. In this way they can be converted products, originally hardly conformable immediately after cold forming, to construction pieces, which not only do they have high resistance, but because of their good ductility properties also have high power of conformation. They are suitable as such for those applications, for which before the annealing treatment were not appropriate.

Typical annealing temperatures during annealing subsequent to cold forming are found in the 450ºC to 700ºC zone, preferably in the 550ºC zone up to 650 ° C. Annealing time and annealing temperature are interchangeable with each other in a limited range, that is, with high temperatures only maintenance times are required reduced and the opposite. Correspondingly the heat treatment as a treatment step carried out by separated into a dome or continuous annealing device. A annealing treatment carried out in a conventional manner, with conditions that would lead to complete recrystallization, would void the action achieved by the invention.

It is particularly advantageous to incorporate the step of annealing in a heat treatment, to which the cold forming product from the band hot rolling in the course of subsequent processing without more. In this step you can for example try a zinc plating or another procedure for coating or passivating the cold formed product surface, which is carried out in the zone of the expected temperatures for the treatment of annealing.

The degrees of conformation intended in the cold forming course are at least 2% until its boundary deformation, where the effects achieved by the mode of proceed according to the invention then have particularly its impact, when the degree of conformation in cold is 5% up to 20%, in particular 5% up to 10%. Cold forming can be carried out for example as a roll banding, stretching, pressing or forming by high internal pressure. A cold rolled with comparatively A low degree of conformation is also suitable for this.

As a microalloy element it is used in the steel according to the invention at least one of the elements Ti, Nb and / or V. It is clear that these elements can be added, so combined, in quantities that are sufficient for laying in operation of the favored precipitation process at the end of the process according to the invention. Is certainly preferred the use of Ti, which is characterized by a behavior of precipitation particularly advantageous. If Ti is used according to a preferred form of the invention as unique microalloy element, then its content should be at minus 3.4 times the content of N, to ensure a potential for sufficient dissolution of the microalloy element. Preferably The Ti content of the steel for this purpose is found in the 0.07 zone up to 0.15% weight, while the content of  Nitrogen is preferably less than 0.007% by weight. With Ti content in this magnitude optimally supports the formation of finer rainfall, with an action hardener

Depending on the type of steel produced in the case Al content is preferably in the area of 0.015 to 0.08% by weight, preferably from 0.5 to 1.5% of weight.

Steel sheets generated in accordance with the invention are preferably suitable for cold forming of construction parts, of which the same is required time a good ductility behavior and correspondingly a good deformation behavior as well as high requirements for maintenance of measures geometric In addition the construction elements are characterized made of the plates generated in accordance with the invention by reduced weight and high energy absorption power.

An example for an advantageous use of sheet metal Steel generated according to the invention is the manufacture of Racks for flat screens. They can also be manufactured from steel sheets manufactured according to the invention elements of construction for motor vehicle bodies or parts of  assembly, which on the one hand must be particularly solid and on the other hand, for example in the case of a collision with another vehicle, transform the kinetic energies that are released with the blow in conformation energy. These are among others the longitudinal supports, the so-called "crash boxes", the chassis parts, hollow structure parts and parts of reinforcement, such as columns A, B or C of the bodywork of cars.

The hot rolling strip generated from according to the invention it is basically adapted for zinc plating. Without However, it is also necessary to provide a galvanized fire that takes place relatively low temperatures, which takes place in particular per piece, galvanized whose temperatures are presented by above 750ºC, they are not very suitable for this, since with this they can exceed the critical temperatures of the achieved effect of according to the invention in the elaborated part of the case.

The following clarifies in greater detail the invention with the help of embodiments.

A composite steel according to table 1 is has melted in a continuous casting facility to roughing previous, which then as previous material has been laminated in heat at a temperature of 1125 ° C to a lamination band Hot in a hot rolling train. Final temperature of the hot rolling was 925 ° C. The temperature of winding was around 475 ° C. After winding it has pickling the hot rolling strip and pickling situation It has been supplied to a cold rolling mill.

TABLE 1

one

There have been cold rolled several test pieces of Hot rolling band band with grades of 5% cold rolling, 10%, 20%, 30%, 40%. Of the cold rolling bands obtained in this way have been composed below groups G1 through G5, where each group G1 to G5 of the case has been awarded a rolling band in  cold, one without cold deformation (degree of cold deformation = 0%), one with 5%, one with 10%, one with 20%, one with a 30% and one with a 40% degree of cold deformation. For reasons For comparison, the G1 group plates have not been subjected to none of the normalization anneals that follow the cold forming while they have been annealed in an oven dome plates of group G2 at 500 ° C, plates of group G3 at 550 ° C, the plates of the G4 group at 600 ° C and the plates of the G4 group at 650 ° C, for normalization for one hour and then They have allowed the air to cool.

In table 2 they have been incorporated for each of the sheets of groups G1 - G5 the properties of matter premium found in the tensile test.

TABLE 2

3

It is shown that resistance values are adjusted clearly improved with the plates made in the manner indicated in the invention G1.1 - G1.5 as a consequence of heat treatment carried out after cold forming, in comparison with sheets only cold formed and not heat treated. At the same time it is achieved by the heat treatment led to out after cold forming a homogeneous dilation A_ {gl}, which corresponds roughly to homogeneous dilation A_ {gl} of the sheet without cold deformation and without treatment thermal G11.0. Correspondingly they present the steels generated according to the invention, cold formed and with a subsequent heat treatment, conformability particularly good and an energy collection power equal to good

A second example has been indicated in table 3 of a steel composition used in accordance with the invention.

TABLE 3

4

A corresponding alloy steel melted and It was cast in roughing. The following have overheated roughing at a temperature of more than 1150 ° C and has been laminated in hot at a hot rolling temperature of at least 850 ° C. After winding at a temperature of less than 600 ° C, they found out, for a first T2.0 test strip of untreated band, for a T2.1 band test tube, annealed for 30 minutes at 580 ° C and then air cooled, for a T2.2 band specimen, cold rolled only, with a cold forming degree of a 10% and for a T2.3 band specimen in a manner according to the invention with a degree of cold forming of 10% of cold rolling, then annealed for 30 minutes at 580 ° C and then air cooled, the mechanical properties R_ {p0,2}, R_ {m}, R_ {e} / R_ {m} A5, A_ {50}, respectively A_ {80}. The determined values of the Mechanical properties are indicated in Table 4.

TABLE 4

5

These examples also justify, that by combination according to the invention of cold forming and subsequent heat generates highly resistant steels, that because of their good ductility they have a power of particularly high conformation. In this way it exceeds the resistance of the T2.3 band specimen generated in accordance with the invention the resistance of the untreated band T2.0 specimen, while its expansion values correspond to those of the T2.0 band test tube not formed. In the same way it presents that the T2.2 cold-deformed band specimen alone compared to the T2.3 band test tube generated in accordance with the invention does not only shows a ductility widely below, but also a lower resistance. With the T2.1 band specimen with only a heat treatment is the ductility certainly at a level comparable with the T2.3 band specimen generated in accordance with the invention At the same time, however, there is resistance of the T2.1 band specimen well below the resistance of the band test generated in accordance with the invention.

The composite steel with the alloy indicated in the  Table 5 has also been melted and roughed out.

TABLE 5

6

These roughings have been reheated  as in the example indicated in context of tables 3/4 to a temperature of more than 1150ºC, with a rolling temperature in hot at least 850ºC and a winding at less than 600ºC. TO then six T3.0 band specimens have been taken up to T3.5.

The subsequent elaboration steps taken to out with the band specimens in question from T3.0 to T3.2 they have also been indicated in table 6 as well as the values corresponding mechanical properties for a test tube Taken in the center of the band.

TABLE 6

7

Also in this case it documents the test tube of band T3.2 clearly, that with the procedure according to the invention you can obtain sheet metal products highly resistant cold, which, as a comparison of Ductility values, with respect to their conformability, are superior to those of the test tube band T3.0 not formed. I also know  achieve, according to the invention, improvements in the properties when the cold rolled strip is annealed for a few times much longer with comparatively lower temperatures, for example 450 ° C to 550 ° C, in a dome.

An example for a job particularly advantageous of a steel generated according to the invention exists in the manufacture of racks, in which the shadow masks (lens / lens visors) of tubes TVs For the blackened of a shadow mask held in such a rack exposes the rack for a while sufficient at a temperature, which is typically about 570 ° C. Due to the fact that the frame is generated from a band of hot rolling generated in accordance with the invention, employing the outstanding formability in this situation of the hot rolling strip by a cold forming and then in the course of blackened the shadow mask it holds, it stays for a sufficient time at the temperature necessary for the blackened, frame strength is increased to one level, in which the frame can support safely and with a stability in form the thermal load exerted on it during the manufacture and use of the television tube and you can ensure that the shadow mask is free from deformation.

Other advantageous use of the according to the invention consists in the manufacture of supports of side strokes or other construction elements exposed to blows for bodies of galvanized vehicles. On the other hand it carries out the cold forming of the construction piece of the case immediately from the hot rolling strip generated according to the invention, for example by an operation Deep drawing. The conformation construction piece in cold then undergoes a piece galvanized with temperatures in the area of 470 ° C. In the course of this process of galvanized the construction piece is also maintained at a level of temperature, at which the increase in ductility is adjusted and of the resistance achieved in accordance with the invention. He lateral impact support obtained in this way presented by reason for its good ductility and at the same time high resistance a power collecting power particularly high, whereby in the case of a hit with another vehicle transforms the kinetic energy that is released with it into a great measure in deformation energy and space can be secured necessary for the survival of the occupants.

Claims (17)

1. Procedure for the manufacture of a cold forming product of a laminating belt in hot with high strength, with good deformation power in which it melts - a steel with (in% weight): C: 0.01-0.25%, If: 0.01 - 1.50%, Mn: 0.50 - 2.00%, P: ≤ 0.08%, S: ≤ 0.01%, At: 0.001 - 1.5%, Cr:? 0.60%, Mo:? 0.60%, N: <0.02%, as well as at least one microalloy element of the group Ti:? 0.20%, Nb: ≤ 0.06%, V: ≤ 0.15%, the rest is iron and the usual impurities, at a previous material, such as roughing, thin roughing or a band cast, - in which the previous material, starting from a hot rolling start temperature, at which the microalloying elements remain essentially dissolved, hot rolled to form a lamination band in hot, - in which the hot rolling strip winding at a winding temperature that is less than 600 ° C, - in which the hot rolling strip a then it conforms coldly to a product with a degree of cold forming corresponding to at least 2% and as maximum deformation limit modification, and - in which the product generated by the cold forming is heated at temperatures and for some times that are below temperatures and Times needed for complete recrystallization. 2. Method according to claim 1, characterized in that the steel contains Ti as the sole microalloying element and that the ratio of the Ti content in relation to the N content is at least 3.4. 3. Method according to one of the previous claims, characterized in that the hot rolling temperature is at least equal to the temperature of Ar3. 4. Method according to one of the previous claims, characterized in that the winding temperature is below 300 ° C. 5. Method according to one of claims 1 to 3, characterized in that the winding temperature is 350 ° C - 450 ° C. Method according to one of claims 1 to 3, characterized in that the winding temperature is 450 ° C - 550 ° C. 7. Method according to one of the previous claims, characterized in that the degree of cold forming is from 5% -20%, in particular from 5% to 10%. Method according to one of the previous claims, characterized in that the annealing temperature is 550 ° C - 600 ° C. Method according to one of the previous claims, characterized in that the annealing is carried out continuously. Method according to one of claims 1 to 8, characterized in that the annealing is carried out in a dome oven. 11. Method according to one of the previous claims, characterized in that the previous material is generated as a molten strip in a casting mill. 12. Method according to one of the previous claims, characterized in that the product manufactured by cold forming is a frame for flat screens. 13. Method according to one of claims 1 to 11, characterized in that the product manufactured by cold forming is a construction element for the manufacture of motor vehicle bodies or an assembly part. 14. Method according to one of the previous claims, characterized in that the product is cold generated by a high internal pressure conformation. 15. Method according to one of the previous claims, characterized in that the cold formed product is subjected to zinc plating. 16. The method according to claim 15, characterized in that the galvanizing is carried out as a galvanized fire. 17. Method according to claim 15 or 16, characterized in that the zinc plating is carried out at temperatures of about 460 ° C.