Patent Application
20090065103
1. Technical Field
This invention relates to the production of galvanized hot dip dual-phase steels that require high tensile strength while retaining good formability and spot welding requirement properties.
2. Description of Prior Art
In the prior art, dual-phase steels having tensile strength of about 650 MPa and below typically have good manufacturing characteristics including formability and spot welding. As such, their shear factors do not limit design attributes. However, dual-phase steels having tensile strengths greater than about 700 MPa, such 800 MPa referred to as DP800 steel, may not have good manufacturing characteristics.
Industries are therefore interested in so-called TRIP-grade steels having high tensile strength with good formability which typically need a higher carbon content, such as greater than about 0.15%, in order to produce sufficient amounts of retained austenite. The carbon content of retained austenite is typically greater than about 1.2% to be stable at room temperature. This high carbon content of steel alloy, however, made it impossible to achieve proper spot welding requirements to meet industry standards. The goal of producing a dual-phase steel having a tensile strength of 800 MPa with formability and weldability characteristics close to a DP600 rated steel has been heretofore illusive.
Additional prior art limiting factors of a dual-phase steel of 800 MPa include reduced bending properties requiring higher bending radius, and a hole expanding ratio that is reduced due to the local hard Martensite Islands formation.
There are design limitations due to the shear factor of the steel sheet. It has been noted that the critical R/T for DP800=8 and for DP600=4 in order for some designs to be formed successfully. Therefore smaller R/T values of DP800 will benefit the manufacturing design capabilities and reduce material gauging with weight saving opportunities and associated cost reduction. The production of the lean-alloyed (spot weldable), formable DP800 grade steels using different alloyed steel chemistries in a hot dip galvanized line with direct air cooling after zinc bath temperature to ambient room temperature will be impossible with the forming requirements due to the hard Martensite phase formed during such traditional cooling configuration.
Prior art literature has established that a softer Martensite produced during aging/tempering at approximately 300° C. will improve bending and hole expanding properties in a dual-phase steel. Tempering, therefore, of material of Martensite established in continuous annealing lines has good bending properties even at 85% of soft martensite in a DP980 steel as opposed to a DP980 having hard Martensite of approximately 70%.
Literature also notes (K. R. Kinsman et al.) presented in 1967 that thermal stabilization of austenite via dislocation pinning of austenite would produce more retained austenite. In the presence of freshly formed Martensite, the chemical potential of carbon is different in Martensite, austenite and in the interface boundary by virtue of the unrelaxed strain associated with the interface dislocations. During aging/reheating the carbon activity and carbon concentration adjusts towards equilibrium.
Accordingly, when steel strip is rapidly cooled from 300° C. to approximately 250° C. which will be close to M50 temperature (approximately 50% austenite will be formed to Martensite). Thereafter, the steel is reheated to the aging temperature of greater than 350° C. During aging, carbon is assumed to segregate to the dislocation interface surrounding the Martensite nucleus sufficient to pin austenite.
B. Cooman and J. Speer have published as earlier as 2006 that quench and partitioning will produce the increased ductility due to the beneficial effect of the interlath austenite, which is ductile and tough constituent. Typically, the steel grades having retained austenite more than 8% are called TRIP steels. The carbon equivalent may be defined by the following equation:
C.E. (carbon equivalent)=C-%+Si-%/30+Mn-%/20+2P-%+4S-%
Due to spot welding requirements C.E. <0.25% the carbon content of steel substrate for DP800 should be low; max. 0.10%. This will mean that Mn-eq values are very high >2.7% for conventional thermal cycle shown in
Mn-eq=Mn-%+Cr-%+2.6Mo-%
Using the new galvanizing technologies the coatability problems of conventional technology are significantly eliminated and quenching will additionally eliminate the need of 0.2% Mo alloying shown in
The present invention is directed to a method and apparatus of producing a dual-phase galvanized steel strip with improved formability while maintaining a high tensile strength. The improved properties are achieved by utilizing increased holding temperatures and times between multiple reheating stations in an improved cooling tower configuration and method after the zinc pot of the hot dip galvanizing strip line. The present invention comprises an additional step of cooling and an additional step of reheating, as compared to the prior art. In the cooling step, the galvanized steel strip has a temperature reduction of from about 300° C. to about 150° C.-250° C. This step of cooling should cool to a maximum extent of about 150° C. different between the initial temperature and the final temperature. This cooling may be accomplished by a water quench, or the use of a cooling tower, or other means. The step of reheating should follow the step of cooling. The step of reheating should heat the galvanized steel strip to a temperature of about 340°-390° C. This reheating causes the martensite in the galvanized steel strip to be tempered at a relatively low temperature, which reduces the Fe—Zn phase formation in the GI-coating.
Referring now to
The invention also provides for an optional cooling station 25 which can be used in which an air/water mist is employed in certain applications cooling the galvanized steel strip 11 from less than 320° C. to less than 200° C. just before a set of return rollers 26 at the bottom of the treatment loop 13. A second induction heating station 27 is provided at the beginning of the second pass return heating the galvanized strip 11 to 340° C.-390° C. as needed. The return second pass of the treatment loop 13 provides additional time for air cooling the galvanized steel strip 11 before the third set of return rollers 28 at the top of the cooling tower loops and directs the steel strip 11 through a set of conventional gasjet coolers 29 for further temperature reduction and finalized coiling at 30.
Referring now to
It will thus be seen that under the improvement of the invention that the addition of a transfer treatment loop 13 (different from a conventional cooling tower 14 configuration) allows for additional multiple treatment of the hot-dip galvanized steel strip 11 versus the traditional paths of the single loop currently available in prior art. It will be seen that the addition of the hot water quench at 22 the beginning of the first pass of the treatment loop 13 and subsequent first reheating by the induction reheaters 24 and time to optional air/water mist cooling at 25 in the first pass and second reheating by induction heating station 27 on the second pass will induce the desired formability attributes of a DP600 grade steel to a desirable high tensile strength grade DP800 grade steel aiding the formability and welding requirements desired in industry.
It will thus be seen that a new and novel cooling tower and method of treatment of dual-phase steel has been illustrated and described and it will be apparent to those skilled in the art that various changes and modification may be made therein without departing from the spirit of the invention.
| Number | Date | Country | |
|---|---|---|---|
| 60967984 | Sep 2007 | US |
