JPH0572460B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a high-strength steel plate with excellent stretch flangeability, fatigue properties, and resistance weldability. In recent years, various measures have been considered to reduce the fuel consumption of automobiles, and among them, reducing the weight of the car body is the most effective measure. In parallel with miniaturization, many attempts have been made to change materials such as using high-strength steel plates or Al alloys. Among these, reducing the weight of wheels is an extremely effective means for reducing fuel consumption, and the application of high-strength steel plates to wheel rims and disks is being actively studied. As such a high-strength steel plate, a composite structure steel plate consisting of ferrite and martensite, that is,
Dual Phase steel plates have been proposed. This composite structure steel sheet has a low yield ratio, high elongation compared to strength, and excellent formability and shape fixability, but is inferior in stretch flangeability. Therefore, when applied to wheel discs, cracks may occur from the hole enlarged portion during disc molding, and
Problems such as the occurrence of cracks at the hole enlarged portion during fatigue tests or running tests remain unresolved. Furthermore, when applied to wheel rims, there is a problem in that cracks occur during roll forming performed after flash butt welding. For example, it is said that the incidence of cracking from the heat-affected zone during molding reaches approximately 50%. The present inventors, in order to improve stretch flangeability,
We have conducted detailed research into the relationship between the steel sheet structure and these properties, and have already found that it is desirable to have the steel sheet structure as a three-phase composite structure of polygonal ferrite, bainite, and martensite. Therefore, as a result of further research, the present inventors found that even when the area ratio of each structure was adjusted to an appropriate amount in the above three-phase structure, the size of martensite had a large effect on stretch flangeability. It was found that The present invention was made based on this knowledge, and consists of a three-phase composite structure of polygonal ferrite, bainite, and martensite, and it specifies the area ratio of each structure and also specifies the size of martensite. The object of the present invention is to provide a composite structure high-strength steel plate with excellent stretch flangeability. The high-strength steel sheet with excellent stretch flangeability, fatigue properties, and resistance weldability according to the present invention has (a) C 0.01-0.2%, Mn 0.6-2.5%, Si 0.02-0.
(b) Cr 0.1-1.5%, Cu 0.1-0.6%, and B
Contains at least one element selected from the group consisting of 0.0005 to 0.1%, with the balance consisting of iron and unavoidable impurities, and S.
A steel whose content is regulated to 0.02% or less, whose structure consists of three phases: polygonal ferrite, bainite, and martensite, and whose bainite area ratio is 4 to 45.
%, the martensite area ratio is 1 to 15%, the martensite area ratio is smaller than the bainite area ratio, and the average diameter of martensite is
It is characterized by being 6μ or less. In the present invention, bainite includes not only bainite containing carbide, but also bainitic ferrite and so-called axial ferrite. Furthermore, martensite also includes some retained austenite. In the high-strength steel plate according to the present invention, the bainite area ratio is 4, especially from the viewpoint of stretch flangeability.
It is said to be in the range of ~45%. A three-phase composite steel sheet consisting of polygonal ferrite, bainite, and martensite has a low yield ratio similar to that of a dual phase steel sheet consisting of ferrite and martensite, and the strength-elongation balance is similar to that of a dual phase steel sheet consisting of ferrite and martensite. It has a characteristic of having a good value similar to that of a steel sheet with a composite structure made of bainite, but as a result of investigating its stretch flangeability, it has been found that it has good stretch flangeability, that is, a bainite area ratio of 4, which indicates a hole expansion rate.
Particularly excellent values were found even within the range of ~45%. Therefore, in order to clarify the cause of this, we focused on the difference in the size of martensite for steel plates in the above range and investigated the relationship between the average diameter of martensite and the hole expansion rate. As a result, the average diameter of martensite was It has become clear that when the hole diameter is 6 μm or less, especially 5 μm or less, it has an excellent hole expansion rate, that is, excellent stretch flangeability. Therefore, in the three-phase composite steel sheet according to the present invention, the average diameter of martensite is limited to 6 μm or less, preferably 5 μm or less. Next, the martensite area ratio is in the range of 1 to 15%. When the martensite area ratio exceeds 15%, the yield ratio increases, while when it is less than 1%, the effect of introducing martensite is small.
In particular, the martensite area ratio is preferably in the range of 1 to 10%, and the bainite area ratio is in the range of 11 to 35%.
A range of % is preferred. Furthermore, it is desirable that the martensite area ratio is smaller than the bainite area ratio. The above structure can be adjusted by adjusting the hot rolling and subsequent cooling conditions in consideration of the chemical components described below, or by adjusting the subsequent annealing conditions (continuous annealing, continuous annealing, Batch annealing). That is, according to the conventional hot rolling method for manufacturing high-strength steel sheets, the finishing temperature is set at ₃ points or higher, the cooling rate is set at 40 to 70°C/second, and the coiling temperature is set at 550 to 650°C.
It is said that However, when the coiling temperature is 550° C. or higher, the structure other than ferrite becomes one or both of bainite and pearlite, making it impossible to obtain the desired three-phase ferrite-bainite-martensite structure. In contrast to such conventional methods, the steel according to the invention can be obtained, for example, by the following method. First, the first method is to heat the slab in the usual way, and then heat it to a finishing temperature of ₃ points or more, usually at
Examples include a method of hot rolling at 830 to 860°C, cooling at an average cooling rate of 5°C/second or more until winding, and winding at a temperature of 500°C or lower. In this method, the finishing temperature is preferably as low as ₃ Ar points or higher in order to reduce the austenite grain size after hot rolling as much as possible.In this way, by reducing the austenite grain size, transformation The grain size of martensite can also be reduced. During cooling, the austenite after rolling is first
It transforms into ferrite, and part of the untransformed austenite transforms into bainite. Bainite also contains fine austenite with concentrated carbon. By winding this structure at 500° C. or lower, preferably 300 to 450° C., the remaining austenite transforms into martensite and finally forms a ferrite-bainite-martensite three-phase structure.
Martensite is generated at ferrite grain boundaries and in bainite (grain boundaries and inside grains), and in particular, martensite generated in bainite is fine, so its average diameter is 6 μm or less. When the cooling rate is too slow, the amount of bainite and martensite increases because ferrite formation is controlled, and as a result, the average diameter of martensite becomes 6 μm or more. Cooling rate is 10~
A range of 50°C/sec is preferred. The above cooling rate is 5
℃/second or less and when the winding temperature is 500℃ or more,
Untransformed austenite forms either pearlite or bainite, or both, making it impossible to obtain the desired structure. As described above, the amount of bainite and martensite and the average diameter of martensite are regulated by the average cooling rate, and the production of martensite is regulated by the coiling temperature. Next, as a second method, after finishing with Ar ₃ points or more, cooling Ar ₁ to _{Ar 3} at 10℃/second or more, and cooling that temperature range for 2 to 20 seconds without water injection. , cooling at 10℃/second or more until winding, 500℃
A method of winding at a temperature of .degree. C. or lower can be mentioned. In this method, austenite produced after finish rolling becomes ferrite and untransformed austenite during cooling in Ar ₁ to Ar ₃ without water injection for 2 to 20 seconds. In this case, the austenite grain size is a mixture of large and small ones, and relatively large austenite grains have a lower concentration of carbon in austenite than fine grains, so during subsequent cooling or winding. transforms into Bainite. On the other hand, fine-grained untransformed austenite does not transform until it is rolled up due to its high carbon concentration, and becomes martensite during winding, and thus finally becomes ferrite.
A bainite-martensite three-phase structure is formed. Moreover, since fine-grained untransformed austenite becomes martensite, most of it has an average diameter
It becomes fine particles of 6 μm or less. Cooling from finish rolling to Ar ₁ to Ar ₃ is 10℃/
Rapid cooling of seconds or more is preferable, and slow cooling of 10° C./second or less causes the second phase to become a pearlite structure. The temperature of non-injected water is Ar ₁ ~ where ferrite transformation is likely to occur.
Ar ₃ is preferably set at 700 to 800°C; outside this range, bainite and martensite increase more than specified. When the time without water injection is 2 seconds or less, the amount of bainite and martensite increases, and the average diameter of martensite also increases to 6 μm or more. On the other hand, when no water is poured for more than 20 seconds, the amount of bainite and martensite is small, or the structure becomes pearlite, making it impossible to obtain the desired three-phase structure. Unable to obtain martensite average particle size. Further, the cooling before winding is preferably performed at a rate of 10°C/sec or more, and the winding temperature is 500°C or less, preferably 300 to 450°C. When the cooling rate and coiling temperature are other than these, the untransformed austenite becomes a composite structure of either bainite or pearlite, or both. Furthermore, a third method includes annealing a hot-rolled steel plate and a cold-rolled steel plate. According to this method, for example, hot-rolled steel sheets or
Cold-rolled steel sheets that have been cold-rolled by 40% or more are usually
After holding the annealing temperature at 800-950℃ for more than 1 minute,
Cool to 500℃ or less at an average cooling rate of 50℃/second or less. The heating temperature is in the range of 800 to 950°C in order to make most of the structure during heating to be austenite.
When the heating temperature is 800℃ or less, ferrite occupies the majority, the amount of austenite is small, and the particles become fine. Therefore, the austenite after cooling tends to change to martensite, and eventually becomes a ferrite-martensite structure. Become. On the other hand, the heating temperature is 950
When the temperature is above .degree. C., the desired three-phase structure can be obtained, but the austenite grains become coarse and the average martensite diameter increases to 6 .mu.m or more.
The heating time may be 1 minute or more, since it is sufficient to form the material into austenite. When the cooling rate is faster than 50℃/sec, ferrite transformation is suppressed, and the amount of bainite and martensite increases more than specified.
Furthermore, the average diameter of martensite is 6 μm or more. On the other hand, by setting the temperature to 50℃/second or less,
The ferrite and bainite transformation progresses, and the austenite that remains at the end is small in amount and is fine grained, so it becomes fine grained martensite with a three-phase structure and a size of 6 μm or less. The cooling stop temperature is 500° C. or less; if it exceeds this temperature, the untransformed austenite becomes a composite structure of one or both of bainite and pearlite, making it impossible to obtain the desired structure. still,
The annealing method may be continuous annealing or batch annealing. Next, the chemical components of the high-strength steel plate according to the present invention will be explained. C is an essential element for maintaining the necessary strength and forming a low-temperature transformation product consisting of bainite and martensite, but when it exceeds 0.2%, the ductility deteriorates significantly and weldability is impaired.
In addition, if moldability is particularly required, the amount of C is
It is desirable to keep it at 0.09% or less. The lower limit amount of C is set to 0.01% in order to effectively exhibit the effects of strengthening and improving hardenability. Mn is an essential element to increase hardenability and obtain the desired structure. In order to effectively obtain these effects, 0.6% or more is required, however,
When it exceeds 2.5%, it causes difficulty in welding, impairs ductility and weldability, and further increases the price of the steel plate. Si is an element necessary for deoxidizing molten steel, and
It is the most effective element as a substitutional solid solution element, and is an essential element in order to obtain a steel plate with high strength and high ductility. Furthermore, Si also has the effect of favoring the formation of clean polygonal ferrite. In order to effectively obtain such an effect, 0.02
% to prevent embrittlement of the weld (increase in transition temperature), and on the other hand, the upper limit to 1.5% to prevent deterioration of the surface scale condition. S is set to 0.020% or less in order to improve formability, especially stretch flangeability, and preferably 0.009% or less when strict stretch flangeability is required. Cr, Cu, and B are all elements that contribute to strengthening steel, and especially improve hardenability.
It is an element useful in obtaining a desired structure. The lower limit of the amount of these elements to be added is determined based on the amount that can effectively exhibit their effects, and the upper limit is determined from an economic standpoint at which the effects are saturated. Therefore,
Addition amount is Cr0.1~1.5%, Cu0.1~0.6%, B0.0005
~0.1% range. In the present invention, in addition to the above elements, the steel plate may contain the following elements. Rare earth elements (REM) and Ca have the effect of improving ductility, particularly stretch flangeability, through sulfide morphology control. The lower limit of the amount of these elements to be added is the minimum necessary amount to effectively exhibit their effects, and the upper limit is from an economic standpoint that saturates the effect, and on the other hand, from an amount that worsens cleanliness. It is determined. Therefore, the amount added is REM0.005~0.2
%, Ca is preferably in the range of 0.005 to 0.1%. Al may be contained as a deoxidizing agent in a range of 0.005 to 0.06%. Further, P can be added within a range that does not cause grain boundary embrittlement, that is, within a range of 0.1% or less. P has a strong reinforcing effect and, like Si, has an effect of purifying ferrite and is useful for improving elongation and the like. The present invention will be explained below with reference to Examples, but the present invention is not limited to these Examples in any way. Example A sample material having the chemical composition shown in Table 1 was melted in a vacuum melting furnace, and roughly rolled into a 30 mm thick slab.
A hot-rolled plate with a thickness of 4 mm was obtained by passing. In addition, some hot-rolled sheets were cold-rolled (75%). For these hot-rolled steel sheets and cold-rolled steel sheets, 780~
Heat at each temperature of 950℃ for 5 to 10 minutes, then
Test materials with different structures were prepared by cooling at various cooling rates. Heat treatment conditions for these test materials

【table】

[Table] is shown in Table 2. Furthermore, Table 3 shows the mechanical properties and microscopic structure observation results of the obtained steel plate. As is clear from Table 3, all of the test materials according to the present invention have an appropriate balance of polygonal ferrite, bainite, and martensite, and the average diameter of martensite is 6 μm or less. High strength and low yield ratio.
Excellent elongation balance and stretch flangeability. Table 3 also shows the results of flash butt welding, fatigue properties, and wheel disc forming test (n=25) for some hot rolled steel sheets. The conditions for flash butt welding are as follows. Flashing allowance: 3mm Flashing time: 3 seconds Setup allowance: 3mm Setup time: 2/60 seconds Upset speed: 150mm/second Blank shape: 3.2mm thickness x 30mm width x 75mm length

【table】

[Table] Next, a hot-rolled steel sheet was produced using a test material in which 0.02% of Ce was added to the corresponding components of test material 1 under the hot rolling, cooling and winding conditions shown in Table 4. Table 5 shows the mechanical properties and microscopic structure observation results of this hot rolled steel sheet. Furthermore, Table 5 shows the results of flash butt welding, fatigue properties, and wheel disc forming test (n=25) of this hot rolled steel plate.

[Table] Shows elites.

【table】

【table】

【table】

【table】

【table】

【represent. The conditions for flash butt welding are the same as described above. As is clear from Table 5, in addition to the above-mentioned properties, test materials 2-2 to 2-4 according to the present invention have excellent flat butt weldability and fatigue strength, and when formed into a wheel disc. It also has the characteristic of having an extremely low defective rate.

Claims (1)

[Claims] 1% by weight (a) C 0.01~0.2%, Mn 0.6~2.5%, Si 0.02~
(b) Cr 0.1-1.5%, Cu 0.1-0.6%, and B
Contains at least one element selected from the group consisting of 0.0005 to 0.1%, with the balance consisting of iron and inevitable impurities,
A steel whose S content is regulated to 0.02% or less, whose structure consists of three phases of polygonal ferrite, bainite and martensite, with a bainite area ratio of 4 to 45% and a martensite area ratio of 1 to 15%, A high-strength steel sheet with excellent stretch flangeability, fatigue properties, and resistance weldability, characterized in that the martensite area ratio is smaller than the bainite area ratio, and the average diameter of martensite is 6μ or less. 2 In weight% (a) C 0.01~0.2%, Mn 0.6~2.5%, Si 0.02~
(b) Cr 0.1-1.5%, Cu 0.1-0.6%, and B
0.0005~0.1% at least one element selected from the group consisting of (c) rare earth elements 0.005~0.2%, and Ca 0.005~
A steel containing at least one element selected from the group consisting of 0.01%, the balance consisting of iron and unavoidable impurities, and with S controlled to 0.02% or less, and whose structure is polygonal ferrite, bainite, and martensite. It consists of three phases: bainite area ratio 4-45%, martensite area ratio 1-15%.
%, the area ratio of martensite is smaller than the area ratio of bainite, and the average diameter of martensite is 6 μ or less. High-strength steel plate with excellent stretch flangeability, fatigue properties, and resistance weldability. . 3 In weight% (a) C 0.01~0.2%, Mn 0.6~2.5%, Si 0.02~
(b) Cr 0.1-1.5%, Cu 0.1-0.6%, and B
Contains at least one element selected from the group consisting of 0.0005 to 0.1%, with the balance consisting of iron and inevitable impurities,
A steel whose S content is regulated to 0.02% or less, whose structure consists of three phases of polygonal ferrite, bainite and martensite, with a bainite area ratio of 4 to 45% and a martensite area ratio of 1 to 15%, A high-strength steel sheet with excellent stretch flangeability, fatigue properties, and resistance weldability, characterized in that the martensite area ratio is smaller than the bainite area ratio, and the average diameter of martensite is 6μ or less. 4 In weight% (a) C 0.01~0.2%, Mn 0.6~2.5%, Si 0.02~
(b) Cr 0.1-1.5%, Cu 0.1-0.6%, and B
0.0005~0.1% at least one element selected from the group consisting of (c) rare earth elements 0.005~0.2%, and Ca 0.005~
A steel containing at least one element selected from the group consisting of 0.01%, the balance consisting of iron and unavoidable impurities, and with S controlled to 0.02% or less, and whose structure is polygonal ferrite, bainite, and martensite. It consists of three phases: bainite area ratio 4-45%, martensite area ratio 1-15%.
%, the area ratio of martensite is smaller than the area ratio of bainite, and the average diameter of martensite is 6 μ or less. High-strength steel plate with excellent stretch flangeability, fatigue properties, and resistance weldability. .