Patent Application
20140261914
The present invention is related to a process for making hot rolled high strength dual phase steels, and in particular to a process that quenches hot rolled strip to less than 100° C. before coiling.
Methods for producing dual phase steels are known. Such methods typically include heating a slab to a rolling temperature, hot rolling the slab at a temperature above the Ar3 temperature for the steel to produce hot rolled strip, and rapidly cooling the hot rolled strip within specific cooling rates down to coiling temperatures. Prior art such as U.S. Pat. Nos. 4,790,889; 4,561,910; and 4,421,573 teach current practices in which continuous cooling of a hot rolled strip is performed and high temperature coiling is conducted at temperatures between 200-500° C. In addition, U.S. Patent Application Publication No. 2004/0118489 teaches continuous cooling practices with a range of high coiling temperatures (CTs) greater than 450° C. in order to obtain different dual phase steels having tensile strengths between 550-800 megapascals (MPa).
The relatively high CTs are utilized due to limitations of cooling capability, e.g. limited quantities of quenching medium available along a laminar length of a cooling table that is part of a hot rolling facility. In order to compensate for the limitation of cooling capacities, alloying additions of hardening imparting elements such as carbon, manganese, chromium, niobium and molybdenum are added to the steels and thereby also add to the cost of the steel.
Prior art references (e.g., U.S. Pat. Nos. 4,502,897 and 4,325,751) teach a “step cooling” practice where cooling of the hot rolled strip is interrupted after finishing rolling. However, the step cooling is also coupled with relatively high coiling temperatures such as 300-500° C.
Given the above, a process for making a hot rolled high strength dual phase steel without the deficiencies of heretofore known systems and methods would be desirable.
A process for manufacturing hot rolled high strength dual phase steels is provided. The process includes providing a steel slab with a chemical composition in weight percent within a range of 0.06-0.09 carbon (C), 0.8-2.0 manganese (Mn), 0.4 minimum (min) chromium (Cr), 0.08 maximum (max) silicon (Si), 0.05 max phosphorus (P), 0.005-0.010 nitrogen (N), with the balance of iron (Fe) and incidental melting impurities known to those skilled in the art. The steel slab chemical composition also has an aluminum to nitrogen (Al/N) ratio of 10.0 max and a niobium plus titanium plus molybdenum (Nb+Ti+Mo) content of 0.015 max.
The steel slab is soaked, e.g. using a soaking furnace as known to those skilled in the art, within a temperature range of 1150-1300° C. and then hot rolled using a roughing treatment in order to produce a transfer bar. The transfer bar is hot rolled using a finishing treatment in order to produce hot rolled strip. The finishing treatment has an entry temperature between 950-1120° C. and an exit temperature between 800-850° C. The hot rolled strip is cooled from the temperature at which it exits the finishing treatment down close to room temperature, e.g. <100° C., after which it is coiled. Also, the coiled hot rolled steel strip has a microstructure of ferrite plus martensite and is free of pearlite and bainite.
It is appreciated that the cooling and coiling operations are executed as part of a continuous process. Stated differently, the hot rolled strip exits the finishing treatment and travels to a coiling station and is coiled without delay or intermittent stopping of the hot rolled sheet on the cooling table.
In some instances, the hot rolled strip is continuously cooled from the finishing treatment exit temperature to less than 100° C. (hereafter also referred to as “close to room temperature”). In addition, the hot rolled strip can be subjected to a cooling rate between 40-70° C./second (sec). Using such a cooling rate affords for the steel to have a percent volume fraction of martensite between 10-25%. In addition, cooled hot rolled strip has a tensile strength of at least 590 MPa and a percent elongation to failure of at least 17.5%.
The room temperature hot rolled strip can also be subjected to tension leveling, for example within a continuous pickling line, the tension leveling resulting in an increase in yield strength of the material of at least 40 MPa. Also, the material can be subjected to a bake hardening treatment after it has been tension leveled which affords an additional increase in yield strength of at least 30 MPa.
In other instances, the cooling of the hot rolled strip to less than 100° C. includes a “step cooling” treatment. The step cooling treatment can include cooling the hot rolled strip from the finishing treatment exit temperature to a ferrite formation zone for the steel composition being hot rolled. Also, the hot rolled strip is held within the ferrite formation zone for at least 3 seconds before being subjected to additional cooling to less than 100° C. The ferrite formation zone can be within the temperature range of 640-720° C. Also, steel strip subjected to the step cooling treatment has a percent volume fraction of martensite between 10-25% and strength values similar or equivalent to the steel subjected to the continuous cooling process described above.
In still yet other instances, the hot rolled strip is continuously cooled from the exit temperature of the finishing treatment to less than 100° C. using an increased cooling rate between 70-100° C./sec. Steel strip subjected to the enhanced cooling rate has a percent volume fraction of martensite between 20-35%, a tensile strength of at least 690 MPa, and a percent elongation to failure of at least 17.5%. The cooled hot rolled strip can also be subjected to tension leveling, for example within a continuous pickling line, which increases the yield strength of the material by at least 50 MPa. Also, the material can be bake hardened to further increase its yield strength by at least 30 MPa.
A process for manufacturing hot rolled high strength dual phase steels is provided. As such, the present invention has use for producing hot rolled steel strip.
The process affords for the manufacture of hot bands and/or pickled and oiled products with ferrite-martensite microstructures. The ferrite-martensite microstructures can be obtained via close to room temperature coiling practices in combination with high cooling rates. In some instances, hot rolled strip is continuously cooled to less than 100° C. before coiling or, in the alternative, the hot rolled strip is subjected to a “step cooling” pattern. In a preferred embodiment, the continuous cooling and step cooling of the hot rolled strip is performed using a superior laminar cooling system having reinforced cooling zones at the beginning and end of a relatively long laminar cooling length/table available after the last finishing stand of a hot rolling finishing treatment.
The step cooling pattern interrupts the continuous cooling and holds the material in a ferrite phase formation region for a given steel composition. It is appreciated that a particular ferrite phase formation region for a given steel can be obtained from a continuous cooling transformation (CCT) curve for the particular grade of steel being hot rolled. The step cooling pattern affords for ferrite percent volume fractions greater than 80% and thus provides greater flexibility for a desired and intended strength-ductility balance of the material.
The close to room temperature coiling practice provides material properties analogous to cold rolled and annealed strip that is cooled to room temperature. However, the close to room temperature cooling of the hot rolled strip as disclosed herein avoids additional downstream cold rolling and annealing processes.
The combination of steel alloy compositions and processes disclosed herein provides hot rolled dual phase steels with exceptional mechanical properties not currently available with similar low alloyed steel compositions. The improvement is obtained by the formation of higher volume fractions of martensite while bypassing the formation of other low transformation products such as pearlite and upper and/or lower bainite.
The hot rolled dual phase steels also exhibit high sensitivity to tension elongation/work hardening (e.g., via tension leveling), for example during pickling. As such, the inventive hot rolled dual phase steels can exhibit an additional increase, sometimes referred to as “kick”, in yield strength when compared to as-rolled hot band yield strengths. This kick in the yield strength can be in the range of 50 MPa.
In the alternative, when such an increase in yield strength is not desired, the hot rolled dual phase steels are subjected to less than 0.5% tension elongation, e.g. in the pickling line, and thereby give a low yield ratio pickled product. It should be appreciated that the sensitivity to work hardening by the inventive steels disclosed herein can be utilized to give higher strength after forming operations in a final formed part that is different than the increase in strength of approximately 30 MPa imparted by traditional prestraining and paint baking.
Regarding the chemical composition of steels disclosed herein, no titanium is intentionally added since it is known that titanium results in the formation of titanium carbonitrides which deteriorates fatigue properties. As such, the hot rolled dual phase steels of the present invention can be used for automotive applications requiring high fatigue resistance, e.g. wheels and rims, along with intended usage for other components such as frames and body parts.
The process includes providing a steel slab having a chemical composition in weight percent within a range of 0.06-0.09 C, 0.8-2.0 Mn, 0.4 min Cr, 0.08 max Si, 0.05 max P, 0.005-0.010 N, with the balance of Fe and incidental melting impurities known to those skilled in the art. The steel slab chemical composition also has an Al to N (Al/N) ratio of 10.0 max and a Nb+Ti+Mo content of 0.015 max.
Thereafter, the steel slab is soaked within a temperature range of 1150-1300° C. followed by hot rolling the slab through a roughing mill and then through a finishing mill in order to produce hot rolled strip. The finishing mill may or may not be a seven-stand finishing mill. After exiting the finishing mill, the hot rolled strip is subjected to a continuous cooling treatment or a step cooling treatment until the strip is cooled to less than 100° C. prior to coiling.
The continuous cooling treatment employs average cooling rates in the range of 40-70° C./sec after the finishing treatment. In some instances, the continuous cooling treatment is executed within a laminar cooling section of a hot strip mill cooling table. In the alternative, the step cooling treatment employs a first phase of fast or rapid cooling after the final stage of finishing rolling followed by a hold of the hot rolled strip within a ferrite formation zone for the particular steel chemistry being processed. The hot rolled strip is held within the ferrite formation zone for at least 3 seconds which aids in efficient austenite to ferrite transformation.
The hot rolled strip, whether continuously cooled or specifically when step cooled, is subjected to a final water quench and cooling to a target coiling temperature. The final or third stage is a fast rapid quenching step that utilizes high cooling rates that ensure pearlite and upper and/or lower bainite are avoided in the final room temperature microstructure. It is appreciated that such lower temperature products hamper the strength and ductility of the material and thus their absence is an improvement provided by the inventive process disclosed herein that is not present in the prior art.
The microstructure of the hot rolled dual phase steels processed according to embodiments of the present invention has a volume fraction percentage of ferrite greater than 80%, which in turn provides for an ideal strength-ductility balance for hot rolled dual phase steels with tensile strengths greater than 590 MPa. Again, the microstructures so obtained for the hot rolled dual phase steels are completely ferrite plus martensite with no pearlite, bainite or any other lower transformation products present.
In a preferred embodiment, cooling of the hot rolled dual phase steels is achieved by utilizing six reinforced cooling zones during a first step or first phase of cooling, followed by eight micro cooling zones, which is then further followed by another six reinforced cooling zones towards the end of a cooling table laminar length. In addition, an available water capacity of greater than 15,000 cm3/hr is used to achieve desired high cooling rates.
The term “reinforced cooling zones” refers to cooling zones having two additional valves at both top and bottom locations. The two additional valves provide a total of six top and six bottom water cooling valves at each of the six cooling zones at the start and towards the end of the laminar length of the cooling table. Also, the term “micro cooling zones” refers to traditional laminar cooling arrangements with four headers per cooling zone location.
A preferred embodiment also employs a long laminar cooling section with a length greater than 150 meters (m) with approximately 130 m of water cooling length available when using a step of “air cooling” during a step cooling practice.
It is appreciated that the long laminar cooling section with reinforced cooling zones affords for desired temperatures and cooling rates for continuous cooling and/or step cooling to obtain desired ferrite-martensite dual phase structure. For example, austenite to ferrite transformation aided during a hold zone of cooling in air, in combination with reinforced cooling zones during the third or final phase of cooling, allows for the avoidance of pearlite and other undesired transformation products after the hold zone. In addition, cooling practices during the first and third phases of the step cooling aid in obtaining hard martensite islands in a ferritic matrix, which gives an ideal dual phase strength-ductility balance of properties.
The inventive process affords for hot rolled strip to be cooled to temperatures of less than 70° C., which is analogous to a cold rolled and annealed product cooled to room temperature. However, it is appreciated that the inventive process disclosed herein allows for the additional production steps of cold rolling and intercritical annealing to be eliminated. As such, the novel hot rolling practice provides a product that is immediately available to be pickled and/or shipped as hot band. Stated differently, the inventive process avoids the traditional cooling period of 48-72 hours for hot rolled coiled steel strip and thus reduces inventory, logistic complications, and the like.
In another embodiment of the present invention, steels falling within the chemistry range described above and having tensile strengths greater than 690 MPa are provided. Such steels are subjected to close to room temperature cooling; however, enhanced cooling rates in the range of 70-100° C./sec are used. The higher cooling rates afford for higher martensite volume fractions to be present in the room temperature microstructure which affords for the increase in tensile strength. Therefore, heretofore hot rolled dual phase steels that use higher alloying contents to achieve such tensile strengths are greatly improved upon.
Regarding 590 MPa grades of hot rolled steel sheet, a slab of steel having a chemical composition within the range disclosed above can be soaked at elevated temperatures within 1150-1300° C. in order to ensure that most if not all alloying elements are in solid solution. The slab is then subjected to a roughing treatment and/or a finishing treatment to produce hot rolled strip having a thickness between 2.0-5.5 millimeters (mm). The finishing treatment can have an entry temperature between 950-1120° C. and an exit temperature between 800-850° C. In some instances, the lower range of temperatures for the finishing treatment marks the temperature at which possible two-phase rolling can take place.
After exiting the last finishing stand of the finishing treatment, the strip can be continuously cooled to a temperature less than 100° C. before coiling. The strip is cooled using an average cooling rate of between 40-70° C./sec with the lower cooling rate range marking the limit to prevent pearlite formation, while the higher cooling rate range marking the limit to prevent a higher than desired volume fraction of martensite in the steel microstructure.
In the alternative, hot rolled strip exiting the finishing treatment can be cooled using a step cooling practice that holds the hot strip in a ferrite formation zone between 640-720° C. for a minimum of 3 seconds. The hold step is followed by a fast or rapid water quench. Sufficiently high cooling rates are obtained or employed in the first cooling phase in order for effective targeted cooling of the material into the ferrite formation zone, and in the third cooling phase for preventing the transformation of low transformation products, apart from martensite.
Hot rolled strip, whether cooled via continuous cooling or step cooling, is coiled at temperatures less than 70° C. As such, coils manufactured according to an inventive process disclosed herein are immediately available for shipping as hot band and/or proceeding downstream for pickling. In addition, and as stated above, the microstructure of such steel is dual phase with martensitic islands in a ferritic matrix.
The process and chemical compositions disclosed herein provide hot rolled dual phase steel that is sensitive to tension leveling/work hardening based on the continuous yielding phenomena for steels. As such, hot band or pickled product provided to customers can receive additional work hardening to provide an increase in yield strength of around 50 MPa in a final formed/stamped part compared to heretofore known and similar steel grades. The formed/stamped high strength product can subsequently be paint baked to get an additional bake hardening increase in strength of at least 30 MPa, i.e. BH2≧30 MPa. As such, additional high strength can be obtained or incorporated in as-formed product with the aid of work hardening and bake hardening.
Turning now to
The hot rolling finishing treatment has an entry temperature between 950-1120° C. and an exit temperature between 800-850° C. Also, the hot rolled strip is subjected to a continuous cooling treatment with a cooling rate between 40-70° C./sec until reaching close to room temperature, and then coiled.
With reference to
In order to provide a specific teaching of the invention and yet not limit the scope thereof in any way, examples of the process according to one or more embodiments of the invention are provided below.
Steel slabs from heat chemistries labeled 1, 2, 3, and 4 as defined in Table 1 below and having chemical compositions (wt %) within the range disclosed above and a thickness of approximately 255 mm were soaked at approximately 1240-1280° C. and then subjected to a roughing treatment to produce a transfer bar. The transfer bar was subjected to a finishing treatment with an entry temperature of 1050° C. and an exit temperature of 820° C. in order to produce hot strip with a thickness between 2-5.3 mm.
The strip was then continuously cooled at an average cooling rate of between 40-70° C./sec as illustratively shown in
Steel slabs from heat chemistries identified as 5 and 6 in Table 1 above and having a thickness of approximately 255 mm were soaked at approximately 1240-1280° C. The soaked steel slabs were subjected to roughing and finishing treatments as described above, the finishing treatments having an exit temperature of 820° C. After exiting the finish treatment, the hot rolled strip was subjected to enhanced cooling rates between 70-100° C./sec and continuously cooled to less than 100° C. before coiling.
The higher cooling rates coupled with the close to room temperature quenching practices resulted in the hot rolled strip having a minimum tensile strength of 690 MPa and a yield ratio in the range of 0.55-0.7. It is appreciated that the higher volume fraction of martensite as compared to the volume fraction shown in
In addition to the above,
The effect of tension leveling the hot rolled strip after being subjected to the close to room temperature coiling practice disclosed herein is shown in
Such an increase in yield strength is schematically shown on a plot of yield strength versus percent elongation in
It should be appreciated that the data disclosed herein is unknown in the prior art and thus demonstrates unexpected results. In addition, the above examples and embodiments are not meant to limit the scope of the invention but are simply examples that provide teaching to one skilled in the art. As such, changes, alterations, modifications, and the like will be apparent to one skilled in the art and yet fall within the scope of the present invention. Therefore, it is the claims and all equivalents thereof that define the scope of the invention.
This application claims priority of U.S. Provisional Application Ser. No. 61/791,608 filed on Mar. 15, 2013, which is incorporated in its entirety herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61791608 | Mar 2013 | US |
