The present invention relates to a high strength cold-rolled steel sheet, favorable for use in a structural member of machine, particularly in a structural member of automobile, which has a tensile strength of 780MPa or more, and a manufacturing method thereof.
From the point of view of achieving weight reduction of automobile for the purpose of reduction in fuel consumption and ensuring safety for occupants of automobile, application of a high strength cold-rolled steel sheet having a tensile strength of 780MPa or more to a structural member of automobile has been studied. However, since such a high strength cold-rolled steel sheet as described above is inferior in ductility and stretch-flangeability to a mild cold-rolled steel sheet, it is difficult to subject the high strength cold-rolled steel sheet to press-forming. The term "stretch-flangeability" as used herein means a property resisting to generation of cracks on a blank end face of steel sheet when it is press-formed and is evaluated, based on a hole-expanding ratio measured by means of hole-expanding test defined by the Japan Iron and Steel Federation Standard: JFST 1001-1996.
To date, various methods for improving stretch-flangeability of a high strength cold-rolled steel sheet have been disclosed as described below.
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Further, prior arts as described below in regard to a high strength cold-rolled steel sheet which, though not referring to stretch-flangeability, aims for enhancement of formability and the like are also disclosed.
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However, such conventional prior arts as described above have problems as described below.
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An object of the present invention is to provide a high strength cold-rolled steel sheet having an elongation of 18% or more, a hole-expanding ratio of 60% or more, and a tensile strength of 780MPa or more and a manufacturing method thereof
This obj ect can be achieved by a high strength cold-rolled steel sheet comprising the features of claim 1.
Further, this high strength cold-rolled steel sheet can be realized by a method comprising the features of claim 2.
In a high strength cold-rolled steel sheet having a tensile strength of 780MPa or more, it is necessary to allow structure to substantially be a dual-phase structure of ferrite phase and martensite phase. For such necessity, as described above, it is necessary to increase an amount of C, thereby deteriorating stretch-flangeability, spot-weldability and, further, phosphatability.
The present inventors have studied on a steel sheet which, even though an amount of C is reduced, has a tensile strength of 780MPa or more and, further, excellent ductility in which an elongation is 18% or more, and excellent stretch-flangeability in which a hole-expanding ratio is 60% or more, and found that the steel sheet can be realized by a steel sheet consisting essentially of, in terms of percentages by mass, 0.04 to 0.10% C, 0.5 to 1.5% Si, 1.8 to 3% Mn, 0.02% or less P, 0.01% or less S, 0.01 to 0.1% Sol. Al, 0.005% or less N, and the balance being iron and inevitable impurities and having a structure substantially comprising ferrite phase and martensite phase. Only components of 0.04 to 0.07 % are within the scope of the appending claims.
Hereinafter, the present invention will be described in detail.
C: C is an important element for giving a great influence on tensile strength, and reinforcing martensite phase which is generated at quenching. When an amount of C is less than 0.04%, a tensile strength of 780MPa or more can not be obtained, while, when it is over 0.10%, stretch-flangeability and spot-weldability are remarkably deteriorated. Accordingly, the amount of C is set to be 0.04 to 0.10%. Further, in order to obtain a tensile strength of from 780MPa to less than 980MPa without deteriorating stretch-flangeability or spot-weldability, it is preferable to set the amount of C to be 0.04% to less than 0.070%.
Si: Si is effective in enhancing ductility of dual phase type steel sheet comprising ferrite phase and martensite phase. When an amount of Si is less than 0.5%, effectiveness thereof becomes insufficient, while, when it is over 1.5%, a large amount of Si oxide is formed on a surface of steel sheet in a hot rolling step, thereby generating surface defects. Accordingly, the amount of Si is set to be 0.5 to 1. 5%. Further, from the point of view of phosphatability, the amount of Si is desirably set to be 1.0% or less.
Mn: Mn is an important element for suppressing generation of ferrite phase in a cooling step of continuous annealing. When an amount of Mn is less than 1.8%, effectiveness thereof becomes insufficient, while, when it is over 3%, a slab crack is generated at the time of continuous casting. Accordingly, the amount of Mn is set to be 1.8 to 3%. Further, in order to consistently produce the steel sheet in a continuous annealing step, the amount of Mn is desirably set to be 2.0 to 2.5%.
P: when an amount of P is over 0.02%, spot-weldability is remarkably deteriorated and, accordingly, the amount of P is set to be 0.02% or less.
S: when an amount of S is over 0.01%, spot-weldability is remarkably deteriorated and, accordingly, the amount of S is set to be 0.01% or less.
Sol. Al: Al is added for performing deoxidization of steel or precipitating N as AlN. When an amount of Sol. Al is less than 0.01%, the deoxidization or the precipitation of AlN is not sufficiently performed, while, when it is over 0.1%, effectiveness thereof is saturated, thereby inviting a cost increase. Accordingly, the amount of Sol. Al is set to be 0.01 to 0.1%.
N: since N deteriorates formability of steel sheet, an amount of N is desirably as low as possible. However, when the amount thereof is reduced more than necessary, a refining cost is increased. Accordingly, the amount of N is set to be 0.005% or less such that it does not substantially deteriorate the formability.
Besides the aforementioned elements, when at least one element selected from 0.01 to 1.0% Cr, 0.01 to 0.5% Mo, 0.0001% to 0.0020% B, 0.001 to 0.05% Ti, 0.001 to 0.05% Nb, 0.001% to 0.05% V, and 0.001 to 0.05% Zr is allowed to be contained, there is an advantage in that structure adjustment at the time of continuous annealing is facilitated, or stretch-flangeability is enhanced by suppressing an occurrence in which a carbide or a nitride is formed in the steel sheet during casting or in a hot rolling step and, then, crystal grains come to be coarse. When a content of each element is less than the lower limit, the aforementioned effects are not sufficiently performed, while it is over the upper limit, the ductility is liable to be deteriorated.
Structure of steel sheet substantially comprises two phases of: ferrite phase and martensite phase. Besides these two phases, bainite phase in which iron is a main constitutional element or austenite phase may not deteriorate effectiveness of the present invention, so long as it is contained in an amount of less than 2% in terms of volume fraction. Further, compounds containing iron such as cementite may be contained in the ferrite phase, the martensite phase or an interface between ferrite and martensite phases. Still further, compounds such as AlN and MnS may not impair the effectiveness of the present invention, so long as each of the composition elements or impurity elements is within the scope of the invention.
When a volume fraction of martensite phase is 30 to 45%, in the range of from 780MPa to less than 980MPa of tensile strength, or when it is 45 to 60%, in the range of from 980MPa to 1180MPa of tensile strength, more excellent stretchflangeability can be obtained. Only a volume fraction of martensite phase from 30 to 45 % is within the scope of the appending claims
Further, in the range in which desired strength can be achieved, a tempering treatment can appropriately be performed on the martensite phase.
Firstly, a slab having the aforementioned compositions is produced by continuous casting method or ingot making plus blooming method and, then, either after reheating or directly, the resultant slab is hot-rolled. A final rolling temperature (finishing temperature) at hot rolling is desirably from Ar3 transformation temperature to 870 DEG C, in order to allow structure to be finer to thereby enhance ductility or stretch-flangeability. The hot-rolled steel sheet is cooled and, then, coiled. A coiling temperature is desirably 620 DEG C or less for the purpose of enhancing ductility or stretch-flangeability.
Next, the resultant steel sheet is cold-rolled to be in a desired thickness. At this time, a cold-rolling reduction rate is desirably 55% or more for the purpose of enhancing ductility or stretch-flangeability by allowing structure to be finer.
Finally, the cold-rolled steel sheet is annealed under conditions as described below in a continuous annealing furnace.
When a heating temperature is less than 750°C, a sufficient amount of austenite phase is not generated and, accordingly, high strength can not be aimed for, while, when it is over 870°C, transformation into austenite single phase occurs allowing structure to be coarse, thereby deteriorating ductility or stretch-flangeability. Further, when a heating time is less than 10 seconds, austenite phase is not sufficiently generated and, accordingly, high strength can not be aimed for.
When a cooling terminal temperature is less than 550°C, a volume fraction of ferrite phase becomes unduly high, strength becomes insufficient, while, when it is over 750°C, not only ductility is deteriorated by subsequent rapid cooling, but also flatness of steel sheet is deteriorated. A cooling rate at this time is desirably set to be 20°C/sec in the range of from 550°C to 750°C depending on compositions such that a volume fraction of austenite phase can be adjusted to be from 30% to 45% or from 45% to 60%, namely, a volume fraction of martensite phase can ultimately be adjusted to be from 30% to 45% or from 45% to 60%.
When a cooling rate is 100°C/sec or less, quenching becomes insufficient and, accordingly, high strength can not be aimed for. In order to consistently aim for high strength, rapid cooling is desirably performed at a cooling rate of 500°C/sec or more. Further, when a cooling terminal temperature is over 300°C, either bainite phase is generated, or austenite phase remains, thereby deteriorating stretch-flangeability. In order to obtain consistent excellent stretch-flangeability, the cooling terminal temperature is preferably set to be 100°C or less.
After the rapid cooling, the resultant steel sheet may be held at the cooling terminal temperature for from 5 minutes to 20 minutes or subjected to tempering treatment at from 150°C to 390°C for from 5 minutes to 20 minutes. By performing the tempering treatment, the martensite phase which has been generated at the rapid cooling is tempered, thereby enhancing ductility and stretch-flangeability. Further, when a tempering temperature is less than 150°C, or a tempering time is less than 5 minutes, such effect as described above can not sufficiently be obtained. On the other hand, when the tempering temperature is over 390°C, or the tempering time is over 20 minutes, strength is remarkably decreased, thereby being sometimes incapable of obtaining a tensile strength of 780MPa or more.
Still further, it is preferable that the obtained steel sheet is subjected to temper rolling at a reduction rate of from 0.1% to 0.7% to thereby eliminate yield elongation completely.
Furthermore, the steel sheet according to the present invention can be electroplated, hot-dip galvanized or applied with solid lubricant.
Steel Nos. 1 to 10 having respective compositions as shown in Table 1 were each cast into slab. The cast slab was reheated at 1250°C, hot-rolled at a finishing temperature of about 860°C, and slowly cooled at about 20°C/sec to produce a hot-rolled steel sheet having a thickness of 2.8mm by simulating coiling at 600°C for one hour. Next, the hot-rolled steel sheet was cold-rolled to produce a cold-rolled steel sheet having a thickness of 1.2 mm and, then, the cold-rolled steel sheet was subj ected to heating treatment which simulated continuous annealing. The continuous annealing was performed under conditions that a temperature of the steel sheet was elevated at a heating rate of about 20°C/sec and, then, the steel sheet was soaked at 830°C for 300 seconds, slowly cooled down to 700°C at about 10°C/sec and, thereafter, rapidly cooled in jet-flowing water having a temperature of 20°C. A cooling rate of such rapid cooling was about 2000°C/sec. Finally, the steel sheet was subjected to tempering treatment at 300°C for 15 minutes, cooled and, then, subjected to temper rolling of 0.3% to produce steel sheet Nos. 1 to 10. Thereafter, in regard to the steel sheet Nos. 1 to 10, respective tensile characteristics and hole-expanding ratios (λ) were measured.
In regard to the tensile characteristics, a JIS No. 5 test piece (JIS Z 2201) was obtained along each of a rolling direction and a direction at a right angle thereto and subjected to a test in accordance with JIS Z 2241 to determine yield strength (YP), tensile strength (TS), and elongation (El).
In regard to the hole-expanding ratio, a test was conducted in accordance with the evaluation method of stretch-flangeability defined by the Japan Iron and Steel Federation Standard (JFST 1001-1996), to determine the value thereof.
Values to be targeted according to the present invention are as follows:
The results are shown in Table 2.
It is found that each of steel sheet Nos. 2, 3, 9, and 10 which are examples according to the present invention satisfies the relations: TS≥780 MPa; El≥8%; and λ≥60%, and thus has high strength, and is excellent in ductility and stretch-flangeability.
On the other hand, as comparative examples, steel sheet No. 1 is low in TS, due to small amount of C; steel sheet No. 5 is remarkably low in λ, due to large amount of C and small amount of Mn; steel sheet No. 6 is low in λ, due to small amount of Si; steel sheet No. 7 is low in TS and λ, due to small amount of Mn; and steel sheet No. 8 is low in El, due to large amount of Mn.
By using the slab of steel No. 2 as shown in Table 1, the steps up to cold rolling were performed under same conditions as in Example 1 and, then, continuous annealing and tempering treatment were performed under conditions as shown in Table 3. Finally, temper rolling of 0.3% was performed in the same manner as in Example 1 to produce steel sheet Nos. A to H. Thereafter, in regard to steel sheet Nos. A to H, the same tests as in Example 1 were conducted.
The results are shown in Table 4.
It is found that each of steel sheet Nos. A, E, G, and H which are examples according to the present invention satisfies the relations: TS≥780 MPa; El≥18%; and λ≥60%, and thus has high strength, and is excellent in ductility and stretch-flangeability.
On the other hand, as comparative examples, steel sheet No. B is low in TS and λ, due to high heating temperature: this is considered to be caused by that structure having martensite phase as a main component has become coarse; steel sheet No. C is low in TS and λ, due to short heating time: this is considered to be caused by that a sufficient amount of austenite phase was not generated during heating and, after rapid cooling, a sufficient volume fraction of martensite phase was not obtained; steel sheet No. D is low in TS and λ, due to low slow cooling terminal temperature: this is considered to be caused by that ferrite phase was generated during the slow cooling and, after rapid cooling, a volume fraction of martensite phase was reduced; and steel sheet No. F is low in TS and λ, due to low rapid cooling speed and high rapid cooling terminal temperature.
Steel Nos. 1 to 9 having respective compositions as shown in Table 5 were each cast into slab. The cast slab was subjected, under the same conditions as in Example 1, to hot rolling, cold rolling, continuous annealing, and temper rolling to produce steel sheet Nos. 1 to 9. Thereafter, yield strength (YP), tensile strength (TS), elongation (El), and hole-expanding ratio (λ) were measured in the same manner as in Example 1.
The results are shown in Table 6.
It is found that each of steel sheet Nos. 1, 2, 3, 8, and 9 which are examples according to the present invention satisfies the relations: TS≥780 MPa; El≥18%; and λ≥60%, and thus has high strength, and is excellent in ductility and stretch-flangeability.
On the other hand, as comparative examples, steel sheet No. 4 is low in El and λ, due to large amount of C; steel sheet No. 5 is remarkably low in λ, due to large amount of C and small amount of Mn; steel sheet No. 6 is low in λ, due to small amount of Si; and steel sheet No. 7 is low in El and λ, due to large amount of Mn.
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