Dual phase hot rolled steel sheet having excellent formability and stretch flangeability

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to hot-rolled dual phase-structure (ferrite/martensite) steel sheet products a method for their production. In particular, the hot rolled steel sheet has excellent formability and stretch flangeability, as well as improved surface quality, weldability and fatigue property.

[0003] 2. Brief Description of the Prior Art

[0004] Currently, applications of high strength steel sheets to automotive parts are increasing in order to reduce vehicle weight and thus improve the fuel efficiency. Among these high strength steels, dual phase steel, which possesses a microstructure consisting of martensite islands embedded in a ferrite matrix, is attracting more and more attention due to its superior combination of high strength, excellent ductility, continuous yielding and low yield ratio. In particular, dual phase steels help to improve vehicle crashworthiness and durability and therefore improve passenger safety and vehicle life. Although the existing dual phase steel products exhibit good formability, their stretch flangeability is relatively inferior to bainitic type steels. Therefore, there exists a limitation of applying dual phase steel sheets to the manufacture of the parts which must undergo both forming (or otherwise press forming) and stretch flanging process. For this reason, dual phase steel sheets having both excellent formability and stretch flangeability are desired.

[0005] Several method for producing hot-rolled dual phase steel sheets are known. U.S. Pat. No. 4,790,889 discloses a method of producing hot-rolled steel sheet having a dual-phase structure from a slab previously produced by ingot or continuous casting. The slab is heated up to the rolling temperature, hot rolled at a temperature above Ar3, rapidly cooled immediately after finish-rolling from the final rolling temperature down to the coiling temperature at a mean rate in the range from 30° to 70° C./sec and without interruption, and then coiled at a temperature in the range from 350° to 190° C.

[0006] U.S. Pat. No. 4,561,910 discloses a method of producing dual phase hot rolled steel sheet having a composition consisting of 0.03-0.15% by weight of C, 0.6-1.8% by weight of Mn, 0.04-0.2% by weight of P, not more than 0.10% by weight of Al, not more than 0.008% by weight of S, and the remainder being substantially Fe. The heating temperature is kept to 1,1000°-1,250° C., the finishing hot rolling temperature is kept to 800°-900° C., the coiling temperature is kept not higher than 450° C., preferably 400°-100° C., and the cooling rate from beginning of cooling following to hot rolling to coiling is kept to 10° to 200° C./sec, according to an ordinary cooling pattern by air-cooling or water-cooling.

[0007] U.S. Pat. No. 4,421,573 discloses a dual phase high-tensile steel sheet having a martensite and ferrite composite structure and a tensile strength of the order of 50-80 kg/mm2 in an as-hot-rolled state. The steel sheet is produced by a method which comprises preparing as a starting material a slab comprising 0.03-0.15% C, 0.5-1.0% Mn, 0.8-2.0% Cr, 0.01-0.1% Al, the balance being essentially Fe and accompanying impurities, heating said slab at a temperature of 1,050°-1.220° C., hot rolling the heated slab, completing the hot rolling at a temperature of 800°-900° C., thereafter cooling the hot rolled sheet to a temperature of 350°-500° C., and winding the sheet into a coil at the latter temperature.

[0008] U.S. Pat. No. 4,502,897 discloses a method for producing hot rolled steel sheets having a low yield ratio and a high tensile strength due to dual phase structure by finishing the final rolling at a temperature of 780° C., rapidly cooling the steel sheet at a cooling rate of more than 40° C./sec to the temperature range wherein the transformation of γ to α is efficiently caused, holding the steel sheet at this temperature range for more than 5 seconds and rapidly cooling the steel sheet at a cooling rate of more than 50° C./sec from the held temperature to a coiling temperature of 550-200° C. Although this patent allows a slight increase of coiling temperature to as high as 550° C., a step cooling pattern is involved, the latter of which could results in a lower productivity and also is practically difficult to be conducted during hot rolling where the steel sheet continuously travels at a relatively high speed. With respect to mill facility, this method entails expensive cooling sections.

[0009] U.S. Pat. No. 4,407,680 discloses a method for producing a dual phase steel sheet in which the steel sheet is hot rolled and cooled to exhibit a substantially bainite structure throughout its cross-section, and in which the steel sheet is subsequently continuously annealed in the two phase ferrite/austenite field and cooled to transform the austenite to martensite. This method entails an extra continuous annealing process, with a corresponding production cost increase.

[0010] U.S. Pat. No. 4,325,751 discloses a steel sheet displaying high strength and formability properties. This product is fabricated by coiling a steel sheet which has been previously processed through a hot strip mill from an initial steel having a very low amount of alloying compounds and having a temperature of between 750° and 900° C., the coiled steel sheet being maintained at a temperature of between 800° and 650° C. for a period of at least one minute, and thereafter cooled to a temperature of below 450° C., the cooling being accomplished at a rate exceeding 10° C./sec. The method includes the additional process of step cooling after coiling, which not only increases the production cost but also requires adding extra facility to most existing hot strip mills.

[0011] The previously known methods or low coiling temperature method could only produce a hot rolled dual phase steel sheet with a limited range of mechanical properties from a single chemistry matrix. In other words, these methods could only provide a single grade of hot rolled dual phase steel sheet based on one chemistry design. A variety of chemistry designs are thus required to produce different grades of hot rolled dual phase steel sheet with different levels of mechanical properties. This would prolong the production time cycle and increase production cost should different grades of hot rolled dual phase steel sheet products be requested for various applications by different customers.

OBJECTS OF THE INVENTION

[0012] Despite the concerted activities in obtaining dual phase hot rolled steel sheet, as evidenced in part by the patents noted above, a need still exists to develop new manufacturing methods to produce this type of steel sheet under conventional hot rolling operation conditions.

[0013] As a principal object, the present invention has thereof been made in order to advantageously avoid the above described problems of the prior methods and has the provision of an alloy design and manufacturing method, which has less demanding or restrictive facility and processing requirements, for producing hot rolled dual phase steel sheet at higher coiling temperatures without any specific annealing and/or cooling processes following the conventional hot rolling and coiling steps.

[0014] Although the hot rolled dual phase steel sheets obtained by the previous patents or methods exhibit low yield ratio and good ducility, no improved stretch flangeability has been demonstrated. In order to meet the current property requirements of the automotive industry, a dual phase steel sheet having the combined properties of excellent formability and stretch flangeability as well as improved surface quality, weldability and fatigue property is desired.

[0015] A further object of the present invention is this to provide a manufacturing method to produce a hot rolled dual phase steel sheet having excellent formability and stretch flangeability as well as improved surface quality, weldabilty, and fatigue properties.

[0016] Another object of the present invention is to provide a practical manufacturing method, including properly adjusting coiling temperature, to produce a family of dual phase hot rolled steel sheets, including hot rolled dual phase steel 550, 600, and 800, using a single chemistry matrix, the details of which will be further demonstrated below by examples.

SUMMARY OF THE INVENTION

[0017] The above and other objects of the present invention are achieved by a method for producing a dual phase hot rolled steel sheet having excellent formability and stretch flangeability as follows:

[0018] (a) providing as a starting material a steel slab or ingot of a composition comprising (in weight percentages) about 0.02-0.15% carbon (C), about 0.3-2.5% manganese (Mn), about 0.1-2.0% silicon (Si), about 0.1-2.0% chromium (Cr), not more than about 0.1% phosphorous (P), not more than about 0.2% titanium (Ti), not more than about 0.2% vanadium (V), not more than about 0.2% niobium (Nb), not more than about 0.5% molybdenum (Mo), not more than about 0.5% copper (Cu), not more than about 0.5% nickel (Ni), and 0.001-0.010 calcium (Ca), the remainder essentially being iron (Fe) and unavoidable impurities;

[0019] (b) reheating the steel slab obtained in the above step to a temperature in the range between 1050° C. (1922° F.) and 1350° C. (2462° F.) and then holding at this temperature for a time period of not less than 10 minutes;

[0020] (c) hot rolling the reheated steel slab into hot rolled steel sheet and completing the hot rolling process at a temperature in the range between 800° C. (1472° F.) and 1000° C. (1832° F.);

[0021] (d) cooling the hot rolled steel sheet, immediately after completing hot rolling, at a mean rate not slower than 10° C./sec (18° F./sec) without requiring specific cooling patterns;

[0022] (e) coiling the cooled hot rolled steel sheet at a temperature not lower than 450° C. (842° F.);

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The foregoing and other features and advantages will be more apparent from the detailed description contained herein below, taken in conjunction with the drawings, in which:

[0024]
FIG. 1 is a graph showing the relationship of tensile strength of hot rolled dual phase steel sheets to the coiling temperature;

[0025]
FIG. 2 is a graph showing the relationship between total elongation and tensile strength obtained on different grades of hot rolled dual phase steel sheets manufactured according to the present invention;

[0026]
FIG. 3 is a graph showing the relationship between hole expansion ratio and tensile strength obtained on hot rolled dual phase steel sheets manufactured according to the present invention as well as the comparison of this relationship with that measured on the conventional hot rolled dual phase steel sheets produced using the prior methods; and

[0027]
FIG. 4 is a micrograph obtained using a LaPera etching technique, which illustrates the typical dual phase structure (fine martensite islands uniformly distributed in the fine-grained ferrite matrix) presented in the hot rolled steel sheet of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028] The present invention is directed to dual phase hot rolled steel sheet and methods of making such a steel sheet. In a preferred embodiment, the dual phase hot rolled steel sheet manufactured according to this invention possesses a microstructure consisting about 3-30% (in volume percentages) martensite islands as a hard second phase embedded in a fine-grained ferrite matrix, and exhibits excellent formability, stretch flangeability, surface quality, weldability and fatigue property. With respect to the preferred applications, the steel sheet can be used after being formed and/or stretch flanged in an “as-hot-rolled” state or optionally painted state for products such as automobile, electrical appliances and building components.

[0029] As will be shown in more detail below, the ranges of the chemical elements desirably contained in the dual phase hot rolled steel sheet produced according to the present invention can be readily obtained in the conventional manufactured process. The preferred limitations on the composition and the reasons for these desired limitations will now be discussed in more detail below.

[0030] Carbon is an important element affecting the hardenability and strength of the steel sheet. It is necessary in an amount of at least 0.02% in order to provide necessary strength for the steel sheet. Thus, the lower limit of carbon content is 0.02% by weight in the preferred embodiment of the present invention. In order to secure the formation of martensite even at high coiling temperatures, however, a more preferable lower limit of carbon is given as 0.03% by weight in the present invention. Since carbon present in the steel sheet in amounts above about 0.15% could deteriorate the formability and weldability of the steel sheet, the maximum carbon content is limited to 0.15%.

[0031] In general, manganese acts as a basic alloying element enhancing the strength and hardenability of steel sheets and is relatively inexpensive. An amount of at least 0.3% by weight of manganese is necessary in order to ensure the strength and hardenability of the steel sheet. The lower limit of manganese content is thus 0.3% by weight in the preferred embodiment of the present invention. Furthermore, in order to enhance the stability of austenite and to form at least 3% by volume of martensite in the final steel sheet, the amount of manganese needs to be more than 0.5% by weight. Therefore, it is a more preferable to contain at least 0.5% by weight of manganese in the present invention. However, when the amount exceeds 2.5% by weight, the weldability is adversely affected. It is thus of importance to limit the amount of manganese to no more than 2.5% by weight.

[0032] Silicon is an element useful for increasing the strength but not significantly impairing the ductility or formability of the steel sheet. An amount of at least 0.1% by weight of silicon is required in order to improve the balance between the strength and formability of the steel sheet. The lower limit of silicon content is thus 0.1% by weight in the preferred embodiment of the present invention. Moreover, silicon promotes the ferrite transformation and needs at least 0.3% by weight to form at least 70% of ferrite in the final steel sheet. In view of this point, a more preferable lower limit of silicon is 0.3% by weight in the present invention. When the content of silicon exceeds 2.0%, its beneficial effect is saturated and the economical disadvantage is then brought out. Accordingly, the upper limit of silicon content is preferably defined to be 2.0% be weight.

[0033] Chromium is an element for improving the hardenability and strength. It is also useful for stabilizing the remaining austenite and promoting the formation of martensite while having no adverse effects on austenite to ferrite transformation. In order to assure these effects, the lower limit of chromium content is 0.1% by weight in the preferred embodiment of the present invention. It is of note that this element is particularly important in the present invention for modifying the microstructure of the hot rolled dual phase steel sheet to achieve excellent combinations of formability, stretch flangeability, surface quality and weldability. In view of the above benefits, a more preferable lower limit of chromium is determined as 0.3% by weight in the present invention. The upper limit of this element is preferably defined to be 2.0% by weight in this invention for maintaining a reasonable manufacturing cost.

[0034] In principle, phosphorus exerts a similar effect to manganese and silicon in view of solid solution hardening. When large amount of phosphorus is added to the steel, however, the rollability of the steel sheet is deteriorated. Besides, the segregation of phosphorus at grain boundaries at high coiling temperatures results in brittleness of the steel sheet, which in turn impairs its formability, stretch flangeability, and weldability. For these reasons, the preferred upper limit of phosphorus content is defined to be 0.1% by weight.

[0035] Sulfur is not normally added to the steel because lower sulfur content is preferable. However, it is present as a residua element, the amount of which depends on the employed steelmaking techniques. Since the present steel contains manganese, sulfur is precipitated in the form of manganese sulfides. A large amount of manganese sulfide precipitates greatly deteriorates the formability, stretch flangeability and fatigue property of the steel sheet. The preferred upper limit of sulfur content is accordingly defined to be 0.03% by weight.

[0036] Aluminum is employed for deoxidation of the steel and fixing nitrogen to form aluminum nitrides. Theoretically, the acid-soluble amount of (27/14)N, i.e., 1.9 times the amount of nitrogen, is required to fix all nitrogen as aluminum nitrides. Practically, however, the use of at least 0.01% of aluminum by weight is effective as a deoxidation element. Therefore, the lower limit of aluminum content is preferably defined to be 0.01% by weight. When the content of aluminum exceeds 0.2%, on the other hand, the formability of the steel sheet is significantly decreased. The preferred amount of aluminum is thus at most about 0.2% by weight.

[0037] The alloying elements, titanium, vanadium, and niobium, have strong effect for retarding austenite recrystallization and refining grains. When a moderate amount of these elements is added, the strength of the final steel sheet is properly increased. These elements are also useful to accelerate the transformation of austenite to ferrite after the final hot rolling. However, when the content of each of these element exceeds 0.2% by weight, large amounts of the respective precipitates are formed during hot rolling, cooling and/or coiling the steel sheet. The corresponding precipitation hardening becomes very high and the formability of the steel sheet is markedly deteriorated. It is therefore preferable to contain each of these elements not more than 0.2% by weight.

[0038] The alloying elements, molybdenum, copper, and nickel, are useful for improving hardenability and strength of the steel sheet. However, all of these elements are expensive and thus the preferred upper limit for each of these elements is defined to be 0.5% by weight for economic reasons.

[0039] Calcium is another important element in this invention because it helps to modify the shape of sulfides. Thus, it reduces the harmful effect of sulfur and eventually improves the stretch flangeability and fatigue property. Since an amount of at least 0.001% by weight is needed to secure this beneficial effect, the lower limit of calcium content is established at 0.001% by weight in the preferred embodiment of the present invention. It is also of note that this beneficial effect is saturated when the amount of calcium exceeds 0.01% by weight, so that the preferred upper limit of this element is defined as 0.01% by weight.

[0040] Other impurities should be kept to as small a concentration as is practicable.

[0041] By employing a steel falling within the above compositional or chemistry constraints, the process will have less demanding or restrictive facility and processing requirements. In fact, the process can be carried out at most existing hot strip mills without requiring any additional equipment or capital cost. A more specific recitation of a preferred process includes the following steps:

[0042] (1) Prepare a steel melt having a composition falling within the ranges discussed above.

[0043] (2) Use a conventional continuous slab caster or a conventional ingot caster to produce a slab (or ingot) having a thickness suitable for hot rolling into a hot rolled band, alternatively referred to as a hot rolled steel sheet.

[0044] (3) Heat the steel slab obtained in th above step to a temperature in the range between the 1050° C. (1922° F.) and 1350° C. (2462° F.).

[0045] (4) Hold the steel slab at the above temperature for a time period of not less than 10 minutes, and preferably not less than 30 minutes to assure the uniformity of the initial microstructure of the slab before hot rolling.

[0046] (5) Hot roll the reheated steel slab into a steel sheet and complete the hot rolling process at a temperature in the range between 800° C. (1472° F.) and 1000° C. (1832° F.), and preferably in the range between 800° C. (1472° F.) and 950° C. (1742° F.) in order to obtain a fine-grained ferrite matrix and avoid the formation of pearlite phase.

[0047] (6) Cool the hot rolled steel sheet, immediately after completing hot rolling, at a mean rate not slower than 10° C./sec (18° F./sec). Since the steel composition and the final product properties in accordance with the present invention are not dependent on control of specific cooling patterns, and the conventional runout table cooling conditions at most existing hot strip mill are suitable for the process.

[0048] (7) Coil the cooled hot rolled steel sheet at a temperature not lower than 450° C. (842° F.). The employed coiling temperature determines the mechanical properties (yield strength, tensile strength, and total elongation) of the final steel sheet. Through properly adjusting the coiling temperature within the range of 460° C. to 650° C., a family of dual phase hot rolled steel sheets, say hot rolled dual phase steels having yield strengths of 550, 600, and 800 MPa, can be produced using a single chemistry design, the details of which will be further demonstrated below by example.

[0049] Hot rolled steels produced by the above process can be formed and/or stretch flanged into a desired shape for a final application. If desired, the final component can be painted.

EXAMPLE

[0050] In the course of developing the present invention, steel slabs having the following compositions were prepared:

1ComponentWeight %C .061Mn1.330Si1.101Cr0.817P0.015S0.004Al0.045Ti0.012V0.005Mo0.011Cu0.010Ca0.001Febalance

[0051] Each of the steel slabs was reheated to about 1232° C. (2250F) and then held at this temperature for about 2 hours. Subsequently, these slabs were hot rolled using hot rolling termination temperatures (or finishing exiting temperatures) ranged from 850° C. (1562° F.) to 950° C. (1742° F.). Immediately after hot rolling, the hot rolled sheets were water cooled at a runout table using cooling rates ranging from 20° C./sec (36° F./sec) to 70° C./sec (126° F./sec) down to various coiling temperature ranging from 460° C. (860° F.) to 650° C. (1202° F.). The final thickness of the hot rolled steel sheets processed in this way ranged from 2.5 mm to 6.0 mm in order to meet the specific requirements for different applications.

[0052] Test pieces were cut from the resulting hot rolled steel sheets in a direction along the hot rolling direction, and then machined into specimens for standard ASTM tensile testing to measure the corresponding mechanical properties. The obtained testing data have demonstrated that the mechanical properties, especially tensile strength, depend largely on the employed coiling temperature, as shown in FIG. 1. Therefore, a family of dual phase hot rolled steel sheets (hot rolled DP 550, 600, and 800) were produced from these slabs which have the same chemical compositions by properly adjusting coiling temperature. Some typical mechanical property results obtained according to standard ASTM tensile testing on these hot roll steel sheets are presented in TABLE 1 in the first order of product grade and second order of product thickness in view of their final applications. As clearly demonstrated in TABLE 1, several grades of hot rolled dual phase (HR DP) steel sheets with excellent ductility or formability can be readily manufactured using a single chemistry design through properly adjusting the coiling temperature, which is based on the present invention. This is also demonstrated by FIG. 1.

2TABLE 1CoilingCoilingYieldTensileTotalProductThicknessTemperatureTemperatureStrengthStrengthElongationGrade(mm)(° C.)(° F.)(MPa)(MPa)(%)HR DP 5502.5610113039757730.23.7607112541858431.33.9621115038956431.15.5629116539155633.3HR DP 6003.7532 99041862528.95.5502 93539766232.5HR DP 8003.9488 91061483819.6

[0053] The total elongation values obtained during the above standard ASTM tensile testing on the hot rolled dual phase steel sheets with the final thickness of 2.5 mm, 3.7 mm, 3.9 mm, and 5.5 mm are also presented in FIG. 2 as a function of tensile strength. The excellent total elongation-tensile strength relationship is clearly revealed in this figure for different grades of these steel sheet products, which further demonstrates the excellent formability associated with these hot rolled steel sheets.

[0054] In view of application or customer requirements, more test pieces were cut from some of the hot rolled steel sheets in a direction perpendicular to the hot rolling direction, and then machined into specimens for standard JIS No. 5 tensile testing to measure the corresponding mechanical properties. Some typical results obtained during these tests are given in TABLE 2, which demonstrate again the excellent ductility or formability exhibited by the dual phase hot rolled steel sheets produced based on the present invention.

3TABLE 2CoilingCoilingYieldTensileTotalProductThicknessTemperatureTemperatureStrengthStrengthElongationGrade(mm)(° C.)(° F.)(MPa)(MPa)(%)HR DP 5502.5610113041457836.05.5629116534855135.06.0649120035157732.5HR DP 6003.5538100039564029.55.5502 93538666233.8

[0055] Square test specimens of about 80 mm by 80 mm were also cut from some of these hot rolled dual phase steel sheets, and then prepared for standard JSFT hole expansion testing to determine the corresponding stretch flangeability. The obtained results on the hot rolled dual phase steel sheets with the final thickness of 3.9 mm, 3.7 mm and 2.5 mm are given in FIG. 3 as a function of tensile strength. As also compared in this figure, the obtained values of hole expansion ration are much higher than those measured on the conventional hot rolled dual phase steel sheets produced using the prior methods, which demonstrates the excellent stretch flangeability pertinent to the hot rolled dual phase steel manufactured according to the present invention.

[0056] Finally, the microstructure of the presently invented hot rolled steel sheets was examined. One of the typical micrographs obtained using a “LePera” etching technique is given in FIG. 4. As is illustrated by and can be readily observed in this micrograph, fine martensite islands (white) are uniformly distributed in the fine-grained ferrite matrix. It is such a dual phase structure that provides not only the excellent formability but also the excellent stretch flangeability.

Dual phase hot rolled steel sheet having excellent formability and stretch flangeability

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