Patent Application
20060162824
The present invention relates to a method for producing high strength, high ductility steel strip, and particularly to a method for producing steel strip containing 0.05/0.20% carbon, 3 to 8% manganese, less than 0.5% silicon, less than 0.1 aluminum, balance iron and incidental impurities by hot rolling, cooling to form a microstructure of which a major portion consists of martensite, and box annealing in the intercritical temperature range to form ferrite and significant amounts of austenite that is retained upon cooling to room temperature.
Customers for automotive steel sheet and strip are interested in the development of steels that possess higher levels of strength than that of currently available steels, to enable the reduction of steel thickness for automotive applications. Higher strength steels enable the automotive designer to reduce vehicle weight while maintaining structural integrity. However, conventional high strength steels lack sufficient ductility to produce complex parts.
This has led to the development of new types of high strength steel that have greater ductility than conventional high strength steels. For example, dual phase steels have been developed that have a microstructure consisting of ferrite and about 10 to 30 percent martensite. The dual phase steels typically have ultimate tensile strength levels ranging from 600 MPa to 1000 MPa with total elongation ranging from 25 to 30% at the 600 MPa strength level and about 15% at the 1000 MPa strength level. Another type of high strength steel that has been developed is called transformation induced plasticity, or TRIP steel, that has a ferrite and austenite microstructure. The austenite in these steels transforms to martensite during deformation or forming by the customer, thus leading to the name TRIP steel. Currently available TRIP steels have ultimate tensile strength levels ranging from 600 to 800 MPa with total elongation of about 30% at the 600 MPa strength level and about 28% at the 800 MPa strength level. Of particular interest to automotive customers are high strength steels having ultimate tensile strengths in the range of 800 to 1000 MPa with total elongations comparable or exceeding the 28 to 30% of currently available TRIP steels.
U.S. Pat. No. 4,437,902 to Pickens et at., discloses a dual phase steel containing less than 0.2% carbon, less than 2% manganese and at least 0.4% copper and 0.6% nickel that is batch annealed in the intercritical temperature range after either hot or cold rolling and slow cooled to produce a steel having an ultimate tensile strength of at least 80 ksi (551 MPa) and at least 18% total elongation. The disclosed steels are primarily used as skelp for the production of steel tubing, however, Alloy A of the disclosure is cited as suitable for automotive applications. The disclosed steels have a dual phase microstructure consisting of martensite dispersed in a ferrite matrix. U.S. Pat. No. 4,047,979 to Grange et al., assigned to the assignee of the present invention, discloses a steel plate product containing about 2 to 6 percent manganese that is produced by hot rolling and annealing in the intercritical temperature range to provide high strength and enhanced toughness at low temperatures as measured by CVN impact energy. Japanese Patent Application Hei 5-311323 discloses steel strip having high strength and elongation having a composition in weight percent of C 0.10-0.20, Si 0.80-1.60, Mn 3.0-6.0, Al 0.5 or less and the rest iron and unavoidable impurities. The steel is hot rolled and coiled at 400-600 C and then box annealed in the intercritical phase region of Ac1 to Ac1+50 C for 1 to 20 hours and furnace cooled. The steel has a microstructure composed of 10% by volume of residual austenite, 40 to 75% ferrite, and 5 to 40% bainite. The steel has a tensile strength of 780 N/mm2 or higher and 25 to 33% elongation. One disadvantage of this reference steel is that the relatively high Si content causes unfavorable oxidation of the strip surface during hot rolling and coiling. Removal of the oxidized surface during hot rolling and in a subsequent pickling operation is difficult and requires extra processing time. Failure to remove the oxidized strip surface results in rejection of the strip for automotive applications due to undesirable surface appearance or unremoved scale.
Under the current state of the art as described above, it is desirable to provide a steel strip suitable for automotive applications that has an ultimate tensile strength level of at least 800 MPa, with total elongations exceeding 25 percent, and that is not subject to excessive oxidation of the strip surface during hot rolling.
According to the present invention a method for the manufacture of steel strip having high strength and high ductility includes hot rolling steel consisting essentially of in weight percent C 0.05/0.20, Mn 3.0/8.0, Si less than 0.5, Al less than 0.1, the balance Fe and inevitable impurities at a minimum finishing temperature of Ar1+50 C, cooling the hot rolled steel strip at a rate sufficient to ensure that the microstructure of the strip consists of greater than 50 percent by volume of martensite, and then annealing the steel strip at a temperature within the range of Ac1 to Ac1+50° C. for a minimum time at temperature of one hour. The steel has a microstructure after annealing that consists essentially of ferrite and retained austenite.
In an alternate embodiment the range of Mn is 2.0/8.0% and the Al is within the range of 0.1/2.0% max.
In a preferred form of the invention, the hot rolled steel strip is cooled at a rate sufficient to ensure that the microstructure of the strip consists of greater than 90 percent by volume of martensite. In another preferred form of the invention, the annealing cycle time is controlled so that the maximum difference in temperature between an outer lap of the coil and an inner lap of the coil is less than 28° C. (50° F.) during the anneal cycle.
Tests were conducted in the laboratory to determine whether a high strength, high ductility steel could be produced utilizing conventional hot rolling practices and box annealing techniques. Table 1 below presents the composition of the steel materials studied in these laboratory tests.
Laboratory ingots of the above compositions were reheated to 1260° C. (2300° F.) and rolled from 178 mm (7) inch thickness to an intermediate slab thickness of 51 mm (2 inches). The slabs were reheated again to the same temperature and hot rolled in seven passes to 4 mm (0.16 inch) thickness to simulate a conventional hot rolling process. The hot rolled strips were then placed in a programmable furnace which was set to cool at 28° C./hour (50° F./hour) from the prescribed “coiling temperature” to simulate coiling on a commercial hot strip mill. Three different furnace temperatures were used so as to simulate three different coiling temperatures, namely, 538° C. (1000° F.), 593° C. (110° F.), and 649° C. (1200° F.). After cooling to room temperature, some of the strips were examined to determine the microstructure in the “as hot rolled condition”. This examination revealed that strips had a fully martensitic microstructure after hot rolling and cooling as just described. The strips were then annealed in a furnace simulating commercial box anneal thermal cycles at one of the following aim peak temperatures 510° C. (950° F.), 538° C. (1000° F.), 566° C. (1050° F.), 593°C. (1100° F.), 621° C. (1150° F.), and 649° C. (1200° F.). The annealed strips were tested to determine mechanical properties and the amount of retained austenite was determined in the microstructure. The results for each steel composition are provided in the tables below.
Table 2 below presents the results for Steel A at the three simulated coiling temperatures and six peak annealing temperatures along with an as hot rolled sample that was not annealed after hot rolling.
Table 3 below presents the results for Steel B at the three simulated coiling temperatures and six peak annealing temperatures along with an as hot rolled sample that was not annealed after hot rolling.
Table 4 below presents the results for Steel C at the three simulated coiling temperatures and six peak annealing temperatures along with an as hot rolled sample that was not annealed after hot rolling.
The results of the laboratory tests show that it is possible through the selection of hot roll finish and coiling temperature, and box anneal temperature to obtain exceptionally high ultimate tensile strength and total elongation in a plain low carbon steel composition containing manganese and having low levels of silicon. An important feature is that the rate of cooling after finish hot rolling and the coiling temperature are sufficient to obtain a microstructure after hot rolling that consists of at least 50% martensite by volume. Preferably, the microstructure after finish hot rolling and coiling consists of at least 90% martensite and most preferably 100% martensite by volume. For the composition of the steels shown in Table 1 the coiling temperature may range from 538 to 649° C. The results are more sensitive to box annealing temperature, with excellent tensile strength and total elongation being obtained at box annealing temperature within the range of 593 to 654° C. The total elongation seems to drop to lower values at box anneal temperatures of 677° C.
Based on the results of these tests, a steel strip product consisting essentially in weight percent of C 0.05/0.20, Mn 3.0/8.0, Si less than 0.5, Al less than 0.1, the balance Fe and incidental impurities, can be produced through the selection of hot roll finish and coiling temperature and box anneal temperature so as to have tensile strength greater than 800 MPa and total elongation greater than 25%. The steel can achieve a combination of ultimate tensile strength in MPa multiplied by total elongation percent of 25,000 or more. These represent exceptional properties for a plain carbon steel that is suitable for automotive and other critical applications where exceptionally clean surface is required. To achieve various strength levels the combination of carbon and manganese content may be selected based on desired ultimate tensile strength. For a given carbon level increasing the manganese content results in increased ultimate tensile strength and causes only a slight decrease in total elongation. In one preferred embodiment, a steel strip product having an ultimate tensile strength of greater than 800 MPa and total elongation of at least 35% can be produced from strip consisting essentially in weight percent of C 0.09/ 0.11, Mn 5.7/6.0, Si less than 0.5, Al less than 0.1, balance Fe and incidental impurities. To achieve these properties the hot roll finishing temperature should be within the range of 871° C./982° C. (1600° F./1800° F.), with a coiling temperature within the range of 566° C./621° C. (1050° F./1150° F.), and a box annealing temperature within the range of 635° C./663° C. (1175° F./1225° F.) for at least one hour at temperature. In a second preferred embodiment, a steel strip product having an ultimate tensile strength of greater than 900 MPa and total elongation of at least 30% can be produced from strip consisting essentially in weight percent of C 0.09/0.11, Mn 7.0/7.2, Si less than 0.5, Al less than 0.1, balance Fe and incidental impurities. To achieve these properties the hot roll finishing temperature should be within the range of 871° C./982° C. (1600° F./1800° F.), with a coiling temperature within the range of 566° C./621° C. (1050° F./1150° F.), and a box annealing temperature within the range of 635° C./663° C. (1175° F./1225° F.) for at least one hour at temperature.
While one or more preferred embodiments of the invention have been identified, other configurations and modifications can be provided within the scope of the present invention.
