JP5194930B2 - High yield ratio high strength cold-rolled steel sheet - Google Patents

Description

  The present invention is a high yield ratio high-strength cold-rolled steel sheet excellent in workability useful for automobile body parts and the like, in particular, the tensile strength TS is 440 MPa or more, and the yield ratio YR which is the ratio of the yield strength YS and TS is 0.8 or more, The present invention relates to a high yield ratio high-strength cold-rolled steel sheet in which TS × El, which is the product of elongation El and TS, is 12000 MPa ·% or more.

In recent years, from the viewpoint of global environmental conservation, the weight reduction of automobile bodies to improve fuel efficiency to reduce CO ₂ emissions and the strengthening of automobile bodies to ensure the safety of passengers in the event of a collision have been actively promoted. Yes. In order to satisfy such weight reduction and strengthening of the car body at the same time, it is effective to increase the strength of the steel plate, which is the material of the car body parts, so that the TS becomes 440 MPa or more and reduce the thickness of the steel plate so long as it does not cause a problem of rigidity. Recently, high-strength steel sheets with thin thickness have been actively used for body parts. In addition, steel sheets for car body parts require TS × El of 12000 MPa ·% or more from the viewpoint of workability, and from the viewpoint of safety at the time of collision, there is little deformation at the time of collision, that is, high-speed deformation amount. Since it is also necessary to be small, there is an increasing demand for high yield ratio high strength steel sheets having excellent workability and having a YR of 0.8 or more.

As a high-yield ratio high-strength steel sheet, for example, in Patent Document 1, in mass%, C: 0.07 to 0.25%, Si: 0.1% or less, Mn: 1.5 to 2.5%, P: 0.05% or less, S: 0.01% Below, Al: 0.010 to 0.100%, N: 0.01% or less, Nb: 0.10% or less, Ti: 0.3% or less, Cr: 0.1% or less, the balance is molten on a steel plate made of Fe and inevitable impurities A high strength, high yield ratio hot dip galvanized steel sheet having a TS of 45 kg / mm ² or more and a YR of 0.8 or more formed by forming a galvanized film or an alloyed hot dip galvanized film is disclosed. Further, in Patent Document 2, in mass%, C: more than 0.028% and less than 0.044%, Si: less than 0.8%, Mn: 1.9 to 2.3%, P: 0.001 to 0.035%, S: 0.0001 to 0.013%, Al: 0.1% or less, N: 0.0001 to 0.008%, Ti: 0.012 to 0.029%, Nb: 0.029 to 0.042%, Mo: 0.05 to 0.25%, B: 0.0008 to 0.0038%, the balance from Fe and inevitable impurities A high yield ratio high strength cold rolled steel sheet is disclosed. Furthermore, Patent Document 3 includes mass%, C: 0.02% or less, Si: 0.3% or less, Mn: 0.5 to 2.0%, P: 0.06% or less, S: 0.005% or less, Al: 0.06% or less, N : 0.006% or less, Ti: 0.015 to 0.40%, the remainder of the steel slab consisting of Fe and inevitable impurities is heated to 1150 ° C or higher, and then hot rolled at a finish rolling exit temperature of 880 ° C or higher to form a hot-rolled sheet The steel sheet is wound at a coiling temperature of 400 to 700 ° C, pickled, cold-rolled, and then annealed at 600 to 720 ° C in a carburizing atmosphere. A method for producing a rolled steel sheet is disclosed.
Japanese Patent Laid-Open No. 10-273754 JP 2006-274378 A Japanese Unexamined Patent Publication No. 2007-9253

  However, the high-strength, high-yield ratio hot-dip galvanized steel sheet described in Patent Document 1 has high strength, YR of 0.8 or higher, TS × El of 12000 MPa ·% or higher, but from the ferrite phase and the pearlite phase. Therefore, it is necessary to strictly control the production method in order to suppress the formation of martensite phase and bainite phase, and it is difficult to stably obtain YR of 0.8 or more. The high yield ratio high strength cold-rolled steel sheet described in Patent Document 2 has high strength but does not have a YR of 0.8 or more. In the manufacturing method of the high yield ratio high tensile cold-rolled steel sheet described in Patent Document 3, a high strength, YR of 0.8 or more, TS × El high yield ratio high-tensile cold-rolled steel sheet of 12000 MPa ·% or more can be obtained. The manufacture requires annealing in a carburizing atmosphere, and it is difficult to stably obtain such characteristics.

  The present invention was made to solve such problems, and can be stably manufactured. TS is 440 MPa or higher, YR is 0.8 or higher, and TS × El is 12000 MPa ·% or higher. It aims at providing a cold-rolled steel plate.

  The present inventors have conducted intensive studies on a high yield ratio high strength cold-rolled steel sheet having TS of 440 MPa or more, YR of 0.8 or more, and TS × El of 12000 MPa ·% or more, and found the following.

(i) (Ti / 48) / (C / 12) = 0.1 to 0.9 and a carbide containing Ti having a particle size of 20 nm or less (hereinafter referred to as Ti-based carbide) is 1.0 × 10 ⁴ pieces / mm in ferrite. ^{When 2} or more are deposited, TS of 440 MPa or more is obtained, and YR of 0.8 or more is stably achieved.

  (ii) When the area ratio of ferrite grains having an aspect ratio of less than 3 in the entire microstructure is 80% or more, TS × El of 12000 MPa ·% or more can be obtained.

The present invention was made based on such findings, and in mass%, C: 0.02 to 0.10%, Si: 1.5% or less, Mn: 1.0 to 2.0%, P: 0.005 to 0.1%, S: 0.01% Hereinafter, Al: 0.005 to 0.1%, N: 0.01% or less, Ti: 0.05 to 0.40%, satisfying the following formula (1), the remainder has a component composition consisting of Fe and inevitable impurities, and The microstructure has ferrite, and the area ratio of crystal grains having an aspect ratio of less than 3 in the ferrite is 80% or more, and the ferrite contains 1.0 × 10 5 Ti-based carbide having a grain size of 20 nm or less. A high yield ratio high strength cold-rolled steel sheet characterized by depositing ⁴ pieces / mm ² or more is provided.
(Ti / 48) / (C / 12) = 0.1-0.9 ... (1)
However, Ti and C in the formula (1) represent the content (% by mass) of each element.

  According to the present invention, a high yield ratio high strength cold-rolled steel sheet having TS of 440 MPa or more, YR of 0.8 or more, and TS × El of 12000 MPa ·% or more can be stably produced. If the high yield ratio high-strength cold-rolled steel sheet of the present invention is applied to automobile body parts, not only the weight of the automobile can be reduced, but also safety at the time of collision can be dramatically improved.

  Details of the present invention will be described below. Note that% representing the amount of a component means mass% unless otherwise specified.

1) Component composition
C: 0.02-0.10%
C is an important element of the present invention together with Ti described later, and is an essential component for securing the strength. In order to precipitate the amount of TiC necessary to obtain a TS of 440 MPa or more, the C amount needs to be 0.02% or more. However, if the amount of C exceeds 0.10%, coarse precipitates are generated, the target TS cannot be obtained, and ductility is reduced. Therefore, the C content is 0.02 to 0.10%.

Si: 1.5% or less
If the Si content exceeds 1.5%, surface defects called red scales will occur during hot rolling, and when wet galvanizing (including alloying) is applied, the wettability of the plating will be deteriorated and uneven plating will occur. It causes generation and deteriorates the surface appearance. Therefore, the Si content is 1.5% or less. In the present invention, the Si amount may be 0%, but since Si is an element effective for solid solution strengthening, the amount is preferably 0.01% or more.

Mn: 1.0-2.0%
The Mn content is 1.0% or more from the viewpoint of solid solution strengthening. However, when the amount exceeds 2.0%, segregation occurs and workability is deteriorated. Therefore, the Mn content is 1.0 to 2.0%.

P: 0.005-0.1%
P is an element having a solid solution strengthening effect. However, if the amount of P is less than 0.005%, the effect cannot be obtained. On the other hand, if the amount of P exceeds 0.1%, P segregates at the grain boundaries and deteriorates the secondary work brittleness resistance and weldability. In addition, when using a hot dip galvanized steel sheet, it is necessary to increase the alloying temperature in order to suppress the diffusion of Fe from the steel sheet to the plating layer during the alloying process after hot dip galvanizing. Rings and chipping are likely to occur. Therefore, the P content is 0.005 to 0.1%, preferably 0.01 to 0.1%.

S: 0.01% or less
In addition to causing hot cracking, S is present as an inclusion in the steel and deteriorates ductility. Therefore, the S content is 0.01% or less, but it is more preferable that S is smaller.

Al: 0.005-0.1%
Al has the effect of improving the normal temperature aging resistance by fixing solid solution strengthening or solid solution N. Al is a ferrite-forming element and is also an effective element for adjusting the temperature in the two-phase region of ferrite and austenite. In order to exert such an effect, the Al content needs to be 0.005% or more. On the other hand, if the Al content exceeds 0.1%, the alloy costs are increased and surface defects are induced. Therefore, the Al content is 0.005 to 0.1%.

N: 0.01% or less
When N is present as solute N, it degrades the room temperature aging resistance. Therefore, if the amount exceeds 0.01%, a large amount of Al, Ti, or the like must be added to fix the solid solution N, resulting in high alloy costs. Therefore, the N content needs to be 0.01% or less, but the smaller the amount, the better.

Ti: 0.05-0.40%
Ti is an important element in the present invention, and combines with C to form TiC, which effectively works to strengthen steel. In order to obtain TS of 440 MPa or more, the Ti amount needs to be 0.05% or more. On the other hand, if the Ti content exceeds 0.40%, high strength and high YR cannot be obtained. Therefore, the Ti content is 0.05 to 0.40%.

  The balance is Fe and inevitable impurities.

2) (Ti / 48) / (C / 12): 0.1-0.9 and the number of Ti carbides with a particle size of 20 nm or less: 1.0 × 10 ⁴ / mm ² or more
Steel X and C with C: 0.04%, Si: tr., Mn: 1.1%, P: 0.01%, S: 0.001%, Al: 0.04%, N: 0.0020%, Ti amount changed to 0.02-0.6% : Steel Y with 0.08%, Si: tr., Mn: 1.3%, P: 0.01%, S: 0.001%, Al: 0.04%, N: 0.0020% and Ti amount changed to 0.05-0.8%, After hot rolling, cold rolling, and heat cycle simulating continuous annealing, hold at 880 ° C for 60s, air cool to 620 ° C, hold at 620 ° C for 240s and air cool to make a steel sheet sample JIS No. 5 tensile test specimens were collected from the sample of 90 ° in the direction of 90 ° with respect to the rolling direction, and in accordance with the provisions of JIS Z 2241, a tensile test was performed at a crosshead speed of 10 mm / min, and TS and YR were measured. . As a result, as shown in FIG. 1, TS and YR vary greatly when (Ti / 48) / (C / 12) is around 0.9, and to make YR 0.8 or more, (Ti / 48) / ( It can be seen that C / 12) needs to be 0.9 or less. In this case, TS ≧ 440 MPa was obtained. When the amount of Ti was small (Steel X: Ti = 0.02%), YR was less than 0.8 and TS was also less than 440 MPa.

As described above, the present inventors considered that the reason why YR decreases with TS when (Ti / 48) / (C / 12) exceeds 0.9 is that Ti-based carbides become coarse. Therefore, in order to improve the strength, it is considered that the fine precipitates among the precipitates, specifically, the fine precipitates having a particle size of 20 nm or less are considered to contribute greatly, and the number of Ti-based precipitates having a particle size of 20 nm or less. And the relationship with YR was investigated. The result is shown in figure 2. As shown in FIG. 2, it can be seen that when the number of Ti carbides having a particle size of 20 nm or less is 1.0 × 10 ⁴ pieces / mm ² or more, a YR of 0.8 or more can be obtained. From the above, to obtain TS of 440 MPa or more and YR of 0.8 or more, 1 × 10 ⁴ Ti-based carbides with (Ti / 48) / (C / 12) of 0.9 or less and a particle size of 20 nm or less / mm ² or more must be deposited. If (Ti / 48) / (C / 12) exceeds 0.9, the reason why TS of 440 MPa or more and YR of 0.8 or more cannot be obtained is considered to be because the Ti-based carbides become coarse as described above. In addition, when (Ti / 48) / (C / 12) is less than 0.1, a sufficient amount of Ti-based carbide is not secured, and TS of 440 MPa or more and YR of 0.8 or more cannot be obtained, so (Ti / 48) / (C / 12) needs to be 0.1 or more.

  In the steel sheet of the present invention, a YR of 0.8 or more is achieved by the fine Ti-based carbide having a particle diameter of 20 nm or less as described above, so that a high YR can be stably obtained even when martensite or bainite is mixed.

  Here, the particle size and the number of Ti-based carbides were measured by observing five or more visual fields with a transmission electron microscope at a magnification of 50,000 to 500,000 using a thin film sample. At this time, since the particle size of the observable carbide is about 1 nm, the number of carbides having a particle size of 1 to 20 nm is substantially measured.

  The fine Ti carbide having a particle size of 20 nm or less was precipitated in the ferrite. Here, the particle diameter is the maximum diameter of each precipitate observed by the above observation, and the precipitate is qualitatively analyzed by energy dispersive X-ray spectroscopy (EDX), and fine precipitates having a particle diameter of 20 nm or less are obtained. Was confirmed to be Ti-based carbide.

3) Area ratio of ferrite grains with an aspect ratio of less than 3: 80% or more After the above steel Y is hot-rolled and cold-rolled and kept at 840-920 ° C for 60 s in a thermal cycle simulating continuous annealing The steel plate was air-cooled to 620 ° C., held at 620 ° C. for 240 s and then air-cooled to produce a steel plate sample, a tensile test was performed in the same manner as described above, and TS × El was measured. In order to ensure good workability, it is necessary to contain ferrite, but as shown in Fig. 3, the area ratio of the crystal grains having an aspect ratio of less than 3 in the ferrite should be 80% or more. For example, TS × El of 12000 MPa ·% or more can be obtained.

  Here, the area ratio and aspect ratio of the ferrite crystal grains were measured by an image analysis method by etching a plate thickness section parallel to the rolling direction with nital and observing three or more fields at 1000 times with a scanning microscope. At this time, the aspect ratio was obtained by (longest particle size in rolling direction) / (longest particle size in plate thickness direction).

  In the present invention, if the above-described microstructure conditions are satisfied, the objective steel sheet characteristics can be secured, and the structure other than ferrite crystal grains having an aspect ratio of less than 3 does not need to be specified. It may be ferrite grains having an aspect ratio of 3 or more, or pearlite.

  As described above, in the high yield ratio high strength cold-rolled steel sheet of the present invention, it is necessary to appropriately control the number of fine Ti-based carbides having a grain size of 20 nm or less and the proportion of ferrite grains having an aspect ratio of less than 3. For that purpose, it is important to control the annealing temperature and the cooling conditions when annealing the steel slab having the composition of the present invention after hot rolling and cold rolling to obtain a cold rolled sheet. . That is, in order to suppress the formation of ferrite grains having an aspect ratio of 3 or more, it is effective to perform high temperature annealing in the two-phase region of ferrite + austenite. In addition, high-temperature annealing in the two-phase region requires re-dissolution of relatively large Ti carbides precipitated after hot rolling, and subsequent cooling requires the number of fine Ti carbides having a particle size of 20 nm or less. It is also extremely effective for precipitation. Ti-type carbides re-dissolved by high-temperature annealing in the two-phase region are fine particles with a particle size of 20 nm or less by maintaining the temperature near the nose of the ferrite transformation of the CCT curve after air cooling, that is, around 600 ° C. in the component system of the present invention. Reprecipitates as Ti-based carbide, contributing to high strength and high YR. Further, since it is held near the nose of the ferrite transformation, a ferrite-based microstructure is formed, and high ductility is achieved.

  Steel Nos. A to R having the compositions shown in Table 1 were melted in a converter and made into slabs by a continuous casting method. These slabs were heated to 1250 ° C., roughly rolled into sheet bars, finished and rolled at a finishing temperature of 940 ° C., and then wound at a winding temperature of 550 ° C. to obtain hot rolled sheets. These hot-rolled sheets were pickled and cold-rolled at a reduction rate of 75% to obtain cold-rolled sheets having a thickness of 1.0 mm. Subsequently, these cold-rolled sheets were annealed at the annealing temperature shown in Table 2 in a continuous annealing line, then cooled from the annealing temperature to 620 ° C at an average cooling rate of 15 ° C / s, and then held at 620 ° C for 240s. Steel plates No. 1 to 23 were produced. And the area ratio of the ferrite grain of the steel plate after annealing, the number of Ti type carbides, and the tensile characteristic value were investigated by the method similar to the above.

  The results are shown in Table 2.

  In the examples of the present invention, TS is 440 MPa or more, YR is 0.8 or more, and TS × El is 12000 MPa ·% or more.

It is a figure which shows the relationship between (Ti / 48) / (C / 12), TS, and YR. It is a figure which shows the relationship between the number of Ti type carbide | carbonized_materials with a particle size of 20 nm or less, and YR. FIG. 5 is a diagram showing the relationship between the area ratio of ferrite grains having an aspect ratio of less than 3 and TS × El.

Claims (1)

In mass%, C: 0.02 to 0.10%, Si: 1.5% or less, Mn: 1.0 to 2.0%, P: 0.005 to 0.1%, S: 0.01% or less, Al: 0.005 to 0.1%, N: 0.01% or less, Ti: 0.05 to 0.40%, satisfying the following formula (1), the balance has a component composition consisting of Fe and inevitable impurities, the microstructure has ferrite, the aspect ratio in the ferrite is 3 The area ratio of the crystal grains less than 80% or more of the entire microstructure is 80% or more, and the ferrite containing Ti with a grain size of 20 nm or less (Ti-based carbide) is precipitated 1.0 × 10 ⁴ pieces / mm ² or more. High yield ratio high strength cold-rolled steel sheet, characterized by:
(Ti / 48) / (C / 12) = 0.1-0.9 ... (1)
However, Ti and C in the formula (1) represent the content (% by mass) of each element.

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