PROCESS FOR PRODUCING A STEEL STRIP COMPRISING A RELATIVELY HIGH STRENGTH DUAL PHASE STEEL

Abstract

A relatively high strength dual phase steel for a cold-rolled or hot-rolled steel strip with excellent forming properties, in particular for lightweight vehicle construction, contains the elements (contents in mass-%): 0.1 to <0.16 of C, 0.02 to <0.05 of Al, 0.40 to <0.60 of Si, 1.5 to <2.0 of Mn, <0.020 of P, <0.003 of S, <0.01 of N, 0.01 of Nb, 0.02 of V, remainder iron including common incidental steel elements with optional addition of Ti. The demanded dual phase microstructure is produced during continuous annealing, wherein the cold-rolled or hot-rolled steel strip is heated in the continuous annealing furnace in a one-step process to a temperature in the range of 820 to 1000° C., preferably 840 to 1000° C., and the annealed steel strip is then cooled down from the annealing temperature with a rate of cooling between 15 and 30° C./s.

Description

The invention relates to a process for producing a cold-rolled or hot-rolled steel strip of a relatively high strength dual phase steel with excellent forming properties, in particular for lightweight vehicle construction according to the preamble of claim 1.

The hotly contested automobile market forces the manufacturer i.a. to look continuously for solutions to lower the fleet consumption while maintaining a highest possible comfort and greatest possible occupant protection. A crucial role plays hereby weight saving of all vehicle components, on the one hand, but also a beneficial behavior of the individual components when exposed to high static and dynamic stress during operation and in the event of a crash, on the other hand. Suppliers attempt to take this requirement into account in such a way that the wall thickness can be reduced through use of high strength and super high strength steels while at the same time improving the component behavior during its manufacture thereof and at operation. Such steels have to meet therefore comparably high standards with respect to strength, stretching capacity, toughness, energy consumption and workability, for example by cold forming, welding and/or surface treatment.

So-called dual phase steels find increasingly application in this area as a result of their excellent formability and high strength values at the same time. Dual phase steels have hereby mainly ferritic-martensitic structure.

Considered in this context are steel strips of dual phase steel which are cold-rolled as well as hot-rolled.

For economic reasons, cold-rolled steel strips are normally subjected to recrystallization annealing by way of a continuous annealing process into a metal sheet that is easy to shape.

The furnace parameters (run-through speed, annealing temperature, rate of cooling) are adjusted in dependence on the alloy composition and strip thickness in accordance with the demanded microstructure and mechanical-technological properties.

The dual phase microstructure is adjusted by heating the cold bath in the continuous annealing furnace to such a temperature that the required ferritic-martensitic microstructure is formed during cooling.

When high corrosion standards demand that the surfaces of hot or cold strips should be galvanized through hot dipping, the annealing treatment is normally carried out in a continuous annealing furnace upstream of the galvanizing bath.

Also in the case of the hot strip, the required dual phase microstructure is occasionally adjusted depending on the alloying concept only during annealing treatment in the continuous furnace in order to be able to realize the demanded mechanical properties on the basis of an austenitic microstructure which is as homogenous as possible.

The alloying concepts for dual phase steels known for example from the documents EP 0 152 665 B1, EP 0691 415 B1, and EP 0510 718 B1, for use in continuous annealing of hot-rolled or cold-rolled steel strips are problematic because of the presence of only a narrow process window for the annealing parameters to ensure-uniform mechanical properties over the length of the strip.

In order for the steels to attain a transformation inertia that is sufficient for realizing the demanded dual phase microstructure, when the cold strip undergoes recrystallizing annealing, the known steels have respective contents, e.g. of Cr, Mo, Nb, or B. In particular the costly elements Cr and Mo have an adverse impact on the manufacturing costs of the dual phase steel.

A narrow process window is to be understood in this context as a need to adjust the run-through speed in dependence on thickness of the strip to be annealed in order to attain a homogenous temperature distribution in the strip and the demanded dual phase microstructure and the mechanical-technological properties during cooling.

When the process windows are wide, the demanded strip properties can be realized even when the strips to be annealed have different thickness while the furnace parameters remain the same.

During manufacture, it is oftentimes required to anneal successive strips of different thickness, e.g. 1.5 and 2.0 mm, depending on specification.

A homogenous temperature distribution is difficult to realize in particular in the transition zone from one strip to another, when different thicknesses are involved, and lead in the event of alloy compositions with too small process window to a situation in which the advance of the thinner strip through the furnace is too slow, causing a lower productivity, or the advance of the thicker strip through the annealing furnace is too fast, posing the risk of failure to realize a homogenous temperature distribution and thus the demanded mechanical-technological properties. As a result, increasing waste and even customer complaints are encountered.

The problem of an excessively narrow process window is especially egregious during annealing treatment when load-optimized components of hot or cold strip should be produced which have varying sheet thicknesses in length and, optionally, across the width of the strip, i.e. have been rolled flexibly. A process for producing a steel strip of varying thickness over the strip length is described, e.g., in DE 100 37 867 A1.

When applying the known alloying concepts for dual phase steels, the presence of the narrow process window renders the realization of uniform mechanical properties difficult to achieve over the entire strip length of the respective strip when a continuous annealing of strips of varying thicknesses is already involved.

When the process window is too small, the regions of smaller sheet thickness in flexibly rolled hot or cold strips of steel of known compositions have strengths that are too low as a result of the substantial proportion of ferrite in view of the transformation processes during cooling, or the regions of greater sheet thickness reach values that are too high as a result of the substantial proportion of martensite. Homogenous mechanical-technological properties over the strip length or across the strip width are virtually impossible to attain, when using the known alloying concepts during continuous annealing.

The invention is therefore based on the object to provide a different more cost-efficient alloying concept for a relatively high-strength steel with dual phase microstructure that allows a broadening of the process window for continuous annealing of hot or cold strips in such a way that in addition to strips of varying thickness also steel strips of varying thickness over the strip length and, optionally, across the strip width can be produced having mechanical-technological properties which are as homogenous as possible.

According to the teaching of the invention, this object is solved by a steel having the following contents in mass-%:

C 0.1 to ≦0.16

Al 0.02 to ≦0.05

Si 0.40 to ≦0.60

Mn 1.5 to ≦2.0

P≦0.020

S≦0.003

N≦0.01

Nb=0.01

V=0.02

remainder iron including common incidental steel elements with optional addition of Ti, wherein the demanded dual phase microstructure is produced during continuous annealing, and wherein the cold-rolled or hot-rolled steel strip is heated in the continuous annealing furnace in a one-step process to a temperature in the range of 820 to 1000° C., preferably 840 to 1000° C., and the annealed steel strip is then cooled down from the annealing temperature with a rate of cooling between 15 and 30° C./s.

The relatively high strength dual phase steel in accordance with the invention for the lightweight vehicle construction is characterized in that the targeted addition of V and Nb while omitting the cost-intensive alloying elements Mo or CR results in a transformation inertia which is high enough to enable with very high process reliability an adjustment of the demanded dual phase microstructure with homogenous mechanical-technological properties during continuous annealing from a completely austenitic matrix even when strips are involved having a thickness which varies over the strip length or across the strip width.

Comprehensive laboratory experiments have surprisingly found that a targeted addition of V in combination with Nb provides a dual phase steel which allows a significantly broader process window during continuous annealing. Same microstructure formations and mechanical-technological properties of the strips can be realized even when strips of different thickness or strips with varying thickness are annealed at otherwise constant furnace parameters.

The steel according to the invention offers the benefit of a significantly greater process window compared to known steels. As a result, process reliability is enhanced during continuous annealing or hot dip galvanizing of cold and hot strips with dual phase microstructure. Thus, homogenous mechanical-technological properties in the strip can be assured in hot-galvanized as well as continuously annealed hot or cold strips. This applies for continuous annealing of successive strips with different strip thickness and in particular for strips with varying sheet thickness over the strip length and/or strip width.

When in accordance with the invention relatively high-strength hot or cold strips of varying sheet thicknesses are produced by a continuous annealing process, load-optimized components can be advantageously manufactured from this material through shaping.

In accordance with the invention, a dual phase steel is involved having approx. 20% martensite embedded in the form of islands in the strength class of about 800 MPa. in particular for hot dip galvanizing as well as for the application in a continuous annealing facility.

As a consequence of the optional addition of Ti in contents of ≦0.01%, the fine-grained configuration of the microstructure and the mechanical-technological properties can be adjusted in accordance with the invention via the formation of nitrides or carbonitrides in dependence on the N-content of the steel.

The field of application of the steel for rolling with flexible strip thicknesses in longitude and transverse directions with respect to the rolling direction is opened up as a result of its insensitivity against process fluctuations during heat treatment.

This insensitivity is effectuated by the use of Nb and in particular V which cause a transformation-inert or transformation-free zone during cooling.

In order to attain a respective effect, the steel has in accordance with the invention a V content of at least 0.02% and a Nb content of at least 0.01%. Nb acts hereby as grain refining element, with the extent of the Nb addition being suited to the actual C and N contents of the steel.

The addition of V is also adjusted in accordance with the invention to the contents of C and N, with the extent of the addition being suited however in such a way that enough V is kept in solution in order to realize a sufficient transformation inertia. When desiring a behavior that is as transformation-inert as possible and thus to realize a broadest possible process window during continuous annealing, the V content amounts to at least 0.06 to 0.10% and the Nb content to more than 0.02 to 0.05%. Further increase of the contents of V and Nb does not provide any further benefits as far as a further retarded transformation of the steel is concerned and thus for the broadness of the process window during continuous annealing.

In order to attain a substantially homogenous starting microstructure for adjustment of the dual phase microstructure, the annealed strip is first heated to a temperature that causes a completely austenitic microstructure. The annealing temperatures range hereby for the steel according to the invention between approx. 820 and approx. 1000° C., depending on the concrete alloy composition.

Performed tests have shown that this steel has a zone which does not undergo a reverse transformation of austenite into ferrite, bainite, or martensite despite temperatures of less than 800° C. Important is hereby in particular the temperature range of about 450° C. because the galvanizing bath temperature is hereby at a level for hot dip galvanizing.

The adjusted content of ferrite and (residue) austenite during cooling is maintained until after the process step “galvanizing”. The still present proportion of austenite is then fully transformed into martensite during continued cooling. The galvanizing parameters may vary over a wide range. The galvanizing speeds range between 60 and 120 m/min depending on the sheet thickness. The rate of cooling before and after the galvanizing bath ranges at fairly low 10 to 30° C./sec.

The produced material may be processed as cold bath as well as also hot bath, in dressed and undressed but also heat-treated state (intermediate annealing) via a hot dip galvanizing line or a pure continuous annealing facility.

At the same time, there is the possibility to vary the cooling conditions in a targeted manner before the galvanizing bath in order to increase or decrease the proportion of ferrite. As a result, it is possible to, e.g., produce partly martensitic steels (PM).

Claims

1.-8. (canceled)

9. A process for producing a cold-rolled or hot-rolled steel strip of a high strength dual phase steel having excellent forming property, comprising the steps of: 0.1 to ≦0.16 of C, 0.02 to ≦0.05 of Al 0.40 to ≦0.60 of Si 1.5 to ≦2.0 of Mn ≦0.020 of P ≦0.003 of S ≦0.01 of N 0.01 of Nb 0.02 of V, remainder iron including common incidental steel elements;

selecting a cold-rolled or hot-rolled steel strip with a composition comprising, in mass-%,

heating the steel strip in a continuous annealing furnace in a one-step process to an annealing temperature in the range of 820 to 1000° C.; and

cooling the annealed steel strip from the annealing temperature with a rate of cooling between 15 and 30° C./s.

10. The process of claim 9, further comprising the step of adding Ti.

11. The process of claim 9, wherein the steel strip is heated to a temperature of 840 to 1000° C.

12. The process of claim 9, wherein the content of V is 0.06%.

13. The process of claim 9, wherein the content of V is 0.08%.

14. The process of claim 9, wherein the content of Nb is 0.02%.

15. The process of claim 9, wherein the content of Nb is 0.04%.

16. The process of claim 10, wherein the content of Ti is ≦0.01%.

17. The process of claim 9, further comprising the step of refining the steel strip by hot dipping.

18. The process of claim 17, further comprising the step of dressing the steel strip.

19. The process of claim 9, wherein the cold-rolled or hot-rolled steel strip of dual phase steel is used for lightweight vehicle construction.