JP2009001909A - Manufacturing method of high-strength cold-rolled steel sheet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a manufacturing method of a high-strength cold-rolled steel sheet achieving a tensile strength of 980 MPa or higher and superior in strength-ductility balance and workability such as stretch-flange-workability. <P>SOLUTION: The method includes preparing a billet containing 0.05-0.20 mass% of C, 0.2-0.8 mass% of Si, 2.0-4.0 mass% of Mn, 0.02 mass% or less of P, 0.0030 mass% or less of S, 0.05 mass% or less of Al, 0.10-1.2 mass% of Ni, and 0.005-0.030 mass% of Ti, hot-rolling and cold-rolling the billet to a plate, applying a continuous annealing at the temperature range of (Ac<SB>3</SB>-50) to (Ac<SB>3</SB>+50)°C, and then cooling the plate to the temperature range of 350-550°C at a cooling speed of 10°C/s or higher and retaining it for 15 sec or longer in this temperature range. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a method for producing a high-strength cold-rolled steel sheet that has a tensile strength of 980 MPa or more and is excellent in workability such as stretch flangeability, which is suitable for use mainly in automobile parts that are press-formed.

  In recent years, high-strength cold-rolled steel sheets with a tensile strength of 980 MPa or more have been actively used mainly for automobile body reinforcements in order to improve automobile fuel efficiency or passenger safety. Against this background, demands for workability of high-strength cold-rolled steel sheets are becoming increasingly severe. However, although the material strength can be ensured by various strengthening methods, the actual situation is that the workability decreases as the strength increases. In particular, in conventional high-strength steel sheets, there are many places where cracks are generated during processing due to uneven structure or local mixing of hard and soft phases. It is said that this is a cause of deterioration of workability. In addition, such workability generally decreases greatly as the strength of the steel plate increases. For this reason, the conventional steel sheet manufacturing technology increases the strength, ductility, bending characteristics, stretch flangeability, etc. It was difficult to achieve compatibility with workability.

Here, as a technique of a high-strength steel sheet, for example, Patent Document 1 discloses a steel sheet having a tensile strength of 980 MPa or more, but it is difficult to say that it has sufficient workability. Patent Document 2 discloses a hot-rolled steel sheet having excellent stretch flangeability, but the strength-ductility balance is low. Furthermore, Patent Document 3 discloses a steel sheet having excellent stretch flangeability, but has a tensile strength of less than 780 MPa.
JP-A-3-277742 JP 61-19733 A JP-A-4-350

  In general, strength and workability tend to contradict each other, and at present there is a high-strength cold-rolled steel sheet that has good workability and has a tensile strength of 980 MPa or more and a method for producing the same. It is not done.

  Therefore, the present invention proposes a way to solve such problems of the prior art, achieves a tensile strength of 980 MPa or more, and is excellent in workability such as strength-ductility balance and stretch flangeability. It aims at providing about the manufacturing method of a high strength cold-rolled steel plate.

In order to achieve the above-mentioned object, the inventors have conducted various experiments from various aspects such as steel components, production conditions, and metal structures, and repeatedly studied. As a result, it has a predetermined amount of ferrite phase and bainite phase, further has martensite phase and a certain amount of residual γ phase, and also controls the nano hardness of ferrite phase, bainite phase, martensite phase and residual γ phase. For the first time, the difference in local deformability is eliminated by making the steel plate made to be easy to adjust the distribution of strains between the different phases, and the strength is not lowered by uniformly deforming macroscopically. It has been found that excellent stretch flangeability and high strength-ductility balance can be satisfied, and in particular, press formability can be improved.
However, since the martensite phase and the retained austenite phase cannot be distinguished from different structures at the time of hardness measurement, they are defined as the hardness of the MA phase composed of the martensite phase and / or the retained austenite phase.
The stretch flangeability was evaluated by the ultimate deformability. In other words, the ultimate deformability is determined by performing a tensile test in accordance with JIS Z2241 using a tensile test piece in accordance with JIS Z2201, measuring the plate thickness t and the plate width w from the fracture surface after fracture, From the initial plate thickness t ₀ and the initial plate width w _{0 ,}
Ultimate deformability (%) = − {ln (t / t ₀ ) + ln (w / w ₀ )} × 100
Is required.
Here, the evaluation of stretch flangeability is often evaluated simply by the hole expansion ratio, but the same structure is evaluated along with the tensile strength (also referred to as TS or strength) and total elongation (also referred to as El or elongation). Therefore, it was set as the evaluation by the ultimate deformability which can evaluate tensile strength, elongation, and stretch flangeability simultaneously in one tensile test.
Moreover, it discovered that the said steel plate was manufacturable by controlling appropriately a component composition and manufacturing conditions, especially the heating temperature in continuous annealing, the cooling stop temperature, and the holding time there.

The present invention has been completed based on such knowledge, and the gist thereof is as follows.
(1) C: 0.05-0.20 mass%, Si: 0.2-0.8 mass%, Mn: 2.0-4.0 mass%, P: 0.02 mass% or less, S: 0.0030 mass% or less, Al: 0.05 mass% or less, Ni: A steel slab containing 0.1 to 1.2 mass% and Ti: 0.005 to 0.030 mass% is subjected to hot rolling and then cold rolling, and then heated to a temperature range of (Ac ₃ −50) to (Ac ₃ +50) ° C. After the continuous annealing is performed, the steel sheet is cooled to a temperature range of 350 to 550 ° C. at a cooling rate of 10 ° C./s or more, and maintained in this temperature range for 15 seconds or more. .

(2) In the above (1), the steel slab further comprises
Cu: 0.50 mass% or less,
Mo: 0.50mass% or less and
Cr: A method for producing a high-strength cold-rolled steel sheet, comprising one or more selected from 0.50 mass% or less.

(3) In the above (1) or (2), the steel piece is further
Nb: The manufacturing method of the high intensity | strength cold-rolled steel plate characterized by containing 0.010 mass% or less.

(4) In the above (1), (2) or (3), the steel slab further comprises: V: 0.010 mass% or less and
Zr: A method for producing a high-strength cold-rolled steel sheet, comprising at least one selected from 0.010 mass% or less.

(5) In any one of the above (1) to (4), the steel slab further has a composition containing B: 0.0050 mass% or less.

(6) In any one of the above (1) to (5), the steel piece further includes
Ca: 0.0050 mass% or less and
REM: A method for producing a high-strength cold-rolled steel sheet, comprising at least one selected from 0.0050 mass% or less.

  According to the present invention, the tensile strength is 980 MPa or more, the workability, particularly strength-elongation (TS × El) is balanced, and the stretch flangeability is excellent. A steel plate can be provided. Therefore, by applying the cold-rolled steel sheet of the present invention as a material for automobile parts, it becomes possible to reduce the weight and fuel consumption of the automobile, greatly contributing to the improvement of the safety of automobile occupants.

Next, the high-strength cold-rolled steel sheet of the present invention will be described in detail.
First, the reasons for limiting the component composition will be described for each component.
C: 0.05-0.20 mass%
C is indispensable for strengthening steel by using the low temperature transformation generation phase. That is, in order to achieve a tensile strength of 980 MPa or more, it is necessary to contain 0.05 mass% or more. On the other hand, if it exceeds 0.20 mass%, the weldability deteriorates remarkably, so the upper limit is made 0.20 mass%.

Si: 0.2-0.8mass%
Si is an element that contributes to strength improvement, and the effect is not exhibited at less than 0.2 mass%. On the other hand, when the content exceeds 0.8 mass%, the ferrite transformation is promoted, and the strengthening by the low temperature transformation generation phase becomes insufficient. In addition, Si has an action of promoting the concentration of C in austenite, the second phase is hardened, and a large amount of hard retained austenite phase is likely to exist in the steel sheet structure finally obtained. Thus, the stretch flangeability is deteriorated. In the present invention, the Si content is in the range of 0.2 to 0.8 mass% in order to exhibit high workability with a low Si content.

Mn: 2.0-4.0mass%
Mn is an element that lowers the transformation point and improves the hardenability of austenite, and contributes to the refinement of crystal grains through the action of lowering the Ar ₃ transformation point, and has the effect of increasing the strength-ductility balance. Here, in order to stably obtain a low-temperature transformation generation phase in order to ensure a predetermined strength, it is necessary to contain 2.0 mass% or more. On the other hand, if more than 4.0 mass% is contained, Since the development of the band-like structure accompanying segregation becomes remarkable and adversely affects workability, the upper limit was set to 4.0 mass%.

P: 0.02 mass% or less P is preferably reduced because it causes internal cracks and deterioration of the workability of the steel sheet, but 0.02 mass% is acceptable because it is acceptable up to 0.02 mass%.

S: 0.0030 mass% or less S is present as non-metallic inclusions (MnS) in steel and serves as a stress concentration source during stretch flange molding, so its content is preferably low. However, in the range where the amount of S is 0.030 mass% or less, even if the strength is high, the stretch flange characteristics are not adversely affected, so 0.0030 mass% was made the upper limit.

Al: 0.05 mass% or less
Al is used for deoxidation of steel, but if it is less than about 0.01 mass%, inclusions may remain and workability may deteriorate, so it is preferable to set it to 0.01 mass% or more. However, if it exceeds 0.05 mass%, not only the effect is saturated but also the surface shape is deteriorated, so the upper limit was set to 0.05 mass%.

Ni: 0.10 to 1.2 mass%
Ni is an element effective for improving the strength without reducing the elongation, and exhibits its effect at 0.10 mass% or more. However, even if the content exceeds 1.2 mass%, the effect is saturated and an effect commensurate with the content cannot be expected, so the upper limit was set to 1.2 mass%.

Ti: 0.005-0.030 mass%
Ti is an effective element for achieving finer and uniform texture and improving stretch and stretch flangeability. These effects are seen with addition of 0.005 mass% or more. On the other hand, when Ti exceeding 0.030 mass% is contained, a hard carbide is formed and stretch flangeability is lowered, so the upper limit is made 0.030 mass%.

The above are the essential components, but the following components can be selectively added as necessary.
Cu: 0.50 mass% or less
Mo: 0.50mass% or less
Cr: One or more selected from 0.50 mass% or less
Cu, Mo and Cr are effective elements for improving the strength without reducing the elongation. In order to acquire the effect in any case, it is preferable to contain 0.01 mass% or more. On the other hand, even if contained in a large amount exceeding 0.50 mass%, there is no further effect and it is economically disadvantageous, so the contents of Cu, Mo, and Cr are preferably 0.50 mass% or less, more preferably 0.01%, respectively. -0.50 mass%.

Nb: 0.010 mass% or less
Nb is an element that affects the precipitation form and recrystallization temperature, such as NbC. In particular, in the present invention, Nb effectively acts on finer and uniform tissue. Therefore, it has the effect of providing high elongation and stretch flangeability despite high strength. In order to acquire such an effect, it is preferable to contain at 0.005 mass% or more. On the other hand, if it exceeds 0.010 mass%, a large amount of hard precipitates are formed in the steel, and the stretch flangeability is deteriorated. More preferably, it is 0.005-0.010 mass%.

V: 0.010 mass% or less
Zr: At least one selected from 0.010 mass% or less These elements are effective elements for increasing the strength of the steel sheet by suppressing the coarsening of the crystal grain size of the carbide. In order to acquire such an effect, all contain 0.005 mass% or more. On the other hand, if it exceeds 0.010 mass%, a large amount of hard precipitates are formed in the steel and stretch flangeability is deteriorated. Therefore, V and Zr are preferably contained as 0.010 mass% or less. More preferably, it is set as the range of 0.005 mass%-0.010 mass%, respectively. These elements exhibit the same behavior when used alone or in combination.

B: 0.0050 mass% or less B is also an element effective for increasing the strength, and contributes to improvement of hardenability. Moreover, by adding B, it becomes easy to form a low-temperature transformation generation phase. In order to acquire such an effect, 0.0003 mass% or more is preferable. On the other hand, since even if it contains exceeding 0.0050 mass%, a further effect is not acquired and it becomes economically disadvantageous, it is good to contain 0.0050 mass% as an upper limit.

Ca: 0.0050 mass% or less
REM: At least one selected from 0.0050 mass% or less
Ca and REM have an effect of reducing the stress concentration and suppressing the deterioration of stretch flangeability by spheroidizing precipitates such as sulfides, for example, MnS and reducing sharp precipitates. In order to acquire the said effect, it is preferable to contain 0.0003 mass% or more, respectively. On the other hand, when the content of these elements exceeds 0.0050 mass%, the effect is saturated and it is economically disadvantageous. Therefore, it is preferable that both Ca and REM are 0.0050 mass% or less, More preferably, it is set as the range of 0.0003-0.0050 mass%, respectively.
The balance other than the above-described components is substantially the composition of Fe, that is, the composition of Fe and inevitable impurities.

Furthermore, the structure of the steel sheet will be described.
The steel sheet of the present invention has a ferrite phase volume fraction of 30-60%, a bainite phase volume fraction of 20-40%, a total volume fraction of martensite phase and residual austenite phase of 10-20%, and The volume fraction with respect to the entire structure of the residual austenite phase is 2.0 to 8.0%, and the ratio of the nanohardness H _nB of the bainite phase to the nanohardness H _nF of the ferrite phase: H _nB / H _nF is 1.5 or less The ratio of the nano-hardness H _nM of the M-A phase composed of the ferrite phase and / or the retained austenite phase to the nano-hardness H _nF of the ferrite phase: it is important to have a structure in which H _nB / H _nF is 3.0 or less It is.

Ferrite phase volume fraction: 30-60%
When the fraction of the soft ferrite phase is less than 30%, the elongation decreases. On the other hand, when it exceeds 60%, the soft ferrite phase increases and the tensile strength decreases. Therefore, in order to optimize the strength-elongation balance, the ferrite volume fraction is set in the range of 30 to 60%.

Bainite phase volume fraction: 20-40%
Due to the presence of the bainite phase, it is possible to suppress the growth and progress of cracks due to voids generated during forming (voids generated in the steel when the amount of deformation increases). If the volume fraction of this bainite phase is less than 20%, the effect of suppressing the growth and propagation of cracks is poor, while if it exceeds 40%, the elongation decreases, so the bainite volume fraction is in the range of 20-40%. did.

Total volume fraction of martensite phase and retained austenite phase: 10-20%
The martensite phase, which is a hard phase, and the residual austenite phase that transforms into martensite during processing and becomes hard are cracking sources during molding, so the total volume fraction of the martensite phase and the residual austenite phase is 20 % Or less. On the other hand, if the total volume fraction is less than 10%, the tensile strength decreases, so 10 to 20% was made the optimum range.

Volume fraction of retained austenite phase: 2.0-8.0%
By including the residual austenite phase in the range of 2.0 to 8.0% in terms of volume fraction relative to the entire structure, transformation-induced plasticity occurs and elongation is improved. That is, if the volume fraction is less than 2.0%, there is no effect of transformation-induced plasticity, whereas if it exceeds 8.0%, the stretch flangeability is lowered, so 2.0 to 8.0%.

H _nB / H _nF ≦ 1.5
H _nMB / H _nF ≦ 3.0
Ratio of nano hardness H _nB of bainite phase to nano hardness H _nF of ferrite phase: H _nB / H _nF exceeds 1.5, or nano hardness H _nM of MA phase and nano hardness of the ferrite phase When the ratio of H _nF : H _nM / H _nF exceeds 3.0, the extreme deformation decreases. The reason for this is that if the hardness difference between the soft ferrite phase and the hard bainite phase or MA phase is large, the difference in deformability between the soft phase and the hard phase is large. Since the progress and propagation of cracks occur easily, the stretch flangeability deteriorates. In the present invention having a plurality of hard phases, the ratio of the nano hardness H _nB of the bainite phase to the nano hardness H _nF of the ferrite phase: H _nB / H _nF ≦ 1.5, the nano hardness H _{nM of the MA} phase The ratio of the nano hardness H _nF of the ferrite phase: H _nM / H _nF ≦ 3.0 showed good strength-elongation balance and stretch flangeability.
Since the bainite phase and the MA phase are harder than the ferrite phase, H _nB / H _nF > 1 and H _nM / H _nF > 1 are necessarily _satisfied .

  Here, nano hardness is the hardness measured about the micro area | region whose diameter is 1 micrometer or less. By adopting this nano-hardness, it is possible to evaluate the hardness of the structure more accurately than the Vickers hardness as measured in the past in the composite structure steel having a fine second phase. It is possible to obtain a steel sheet that achieves high structure and workability by achieving no microstructure control.

  This nano hardness can be measured using TRIBOSCOPE of Hysitron. In general, when the indentation size is extremely small, hardness may depend on the indentation size. To avoid this, the measurement can be performed with the indentation size substantially the same. Specifically, the load is adjusted so that the depth of the indentation (= Contact Depth) proportional to the size of the indentation falls within a predetermined range, and the hardness Hn may be determined based on the load at this time.

Next, a method for manufacturing the steel sheet will be described.
The production method of the present invention includes the above-described essential components or components that can be selectively added, and heats the steel slab immediately after casting or once after cooling, and then hot rolling and then cold rolling. After annealing, perform continuous annealing.
At this time, it is important to regulate the conditions of continuous annealing and subsequent heat treatment as described later. The conditions of the remaining steps are not particularly limited, but the finish rolling finishing temperature and the coiling temperature at the time of heating the steel slab and hot rolling are preferably the following conditions.

Steel bill heating temperature (SRT): 1100-1250 ° C
In order to refine the initial austenite grains, the heating temperature of the steel slab before hot rolling is preferably 1250 ° C. or less, more preferably 1200 ° C. or less. In order to secure the finish rolling temperature, the heating temperature is preferably 1100 ° C. or higher.

Finishing rolling finish temperature (FDT): 850-950 ° C
When hot-rolling the steel slab, if the finish rolling finish temperature in hot rolling is less than 850 ° C, the deformation resistance during rolling is large, and the structure becomes non-uniform, resulting in a layered structure, even after cold rolling annealing. It tends to be a non-uniform structure, and the workability tends to decrease. On the other hand, at a high temperature exceeding 950 ° C., the austenite grains become coarse, a uniform and fine structure cannot be obtained, and the workability tends to decrease. Therefore, the finish rolling finish temperature is preferably in the range of 850 to 950 ° C.

Winding temperature (CT): 450-650 ° C
When the coiling temperature is lower than 450 ° C., a large amount of a hard martensite phase is generated, the rolling load during cold rolling is increased, and the cold rolling property after hot rolling is lowered, so that productivity is lowered. On the other hand, when it exceeds 650 ° C., the carbide becomes coarse. For this reason, it is preferable that winding-up temperature shall be the range of 450-650 degreeC.

  In the present invention, cold rolling is performed after hot rolling to obtain a cold rolled sheet. The cold rolling conditions need not be particularly limited, and may be according to ordinary methods. The cold rolling reduction ratio is preferably about 30% to 60% from the viewpoint of ease of manufacturing.

Next, the essential conditions in manufacturing will be described.
Continuous annealing temperature: (Ac ₃ −50) to (Ac ₃ +50) ° C.
The cold-rolled sheet is annealed continuously and needs to be annealed by heating to an annealing temperature in a temperature range of (Ac ₃ −50) to (Ac ₃ +50) ° C. That is, by highly controlling the annealing temperature in such a narrow range, coarsening of crystal grains can be prevented, and a cold-rolled annealing plate having a desired structure can be obtained. When the annealing temperature is less than (Ac ₃ −50) ° C., a sufficient bainite phase cannot be obtained. In addition, since the influence of the cold-rolled structure remains and becomes a band-shaped structure, the target structure cannot be obtained and the stretch flangeability is remarkably lowered. On the other hand, when the temperature exceeds (Ac ₃ +50) ° C., the crystal grains are abruptly coarsened and a uniform fine structure cannot be obtained, and desired structure characteristics, that is, high strength-elongation balance and stretch flangeability can be obtained. It becomes difficult.
The Ac ₃ transformation point can be obtained by a differential thermal dilatometer or the like.

Cooling rate from 350 ° C to 550 ° C: 10 ° C / s or more If the cooling rate from the annealing temperature to 350 ° C to 550 ° C is too slow, crystal grains are likely to be coarsened. Since pearlite is contained, it becomes difficult to ensure the strength-ductility balance. Accordingly, the cooling rate here is set to 10 ° C./s or more.

Holding temperature: 350 ° C to 550 ° C
When the holding temperature after cooling is stopped is higher than 550 ° C, a soft phase such as pearlite is formed, while at a temperature lower than 350 ° C, a martensite phase is excessively formed and the workability is lowered, so the holding temperature is reduced. Was 350-550 ° C.

Holding time: 15 seconds or more Holding in the above temperature range is important for sufficient transformation from austenite to bainite phase. If this holding time is less than 15 seconds, a large amount of hard martensite phase will be produced. It takes 15 seconds or more to form and reduce stretch flangeability. In addition, even if it is held for more than 10 minutes, the effect is saturated and the production cost only rises.

Using steel slabs (steel slabs) having the composition shown in Table 1, cold-rolled steel sheets (thickness 1.2 mm) were manufactured under the conditions shown in Table 2 at a cold-rolling reduction rate of 50%.
The Ac ₃ transformation point (° C.) was measured by collecting a test piece from a steel slab having the composition shown in Table 1. That is, the test piece was heated from room temperature to around 1200 ° C. at a heating rate of 1 ° C./s, and the Ac ₃ transformation point was measured with a differential thermal dilatometer. The Ac ₃ transformation points obtained are also shown in Table 1.
The cold rolled steel sheet thus obtained was evaluated for material characteristics and the like by the following method. The ultimate deformability was as described above, and was determined using a test piece subjected to investigation of mechanical properties. The evaluation results are also shown in Table 2.

[Mechanical properties]
A tensile test based on JIS Z2241 was performed using a JIS No. 5 test piece with the perpendicular direction of rolling as the longitudinal direction.
[Volume fraction of ferrite phase, bainite phase and martensite phase]
Ten SEM images with a thickness of 5,000 times from the surface of the steel sheet (section parallel to the rolling direction) are taken from the surface of the steel sheet. The fraction was determined. In addition, the average of the volume fraction calculated | required in each of 10 visual fields was made into the volume fraction of each phase.
[Volume fraction of retained austenite phase]
The amount of retained austenite was measured by grinding from the surface of the steel plate to a depth of 1/4 of the plate thickness, and then etching the ground surface, followed by X-ray analysis.
[Nano hardness]
The measurement position is a depth part of the plate thickness 1/4 from the surface of the steel plate, and the hardness of each phase was measured using TRIBOSCOPE of Hysitron. Here, when the indentation size is very small, the indentation size dependency of hardness may occur. Therefore, in order to avoid this, the measurement was performed with the indentation size being almost the same. Specifically, the load was adjusted so that the depth of the indentation (= Contact Depth) proportional to the size of the indentation was 50 nm ± 10 nm, and the hardness Hn was measured based on the load. At this time, a section of the indentation is about 350 nm. The nano-hardness ratio was calculated between the other ratios thus obtained.

Claims (6)

C: 0.05-0.20 mass%,
Si: 0.2-0.8mass%
Mn: 2.0-4.0mass%,
P: 0.02 mass% or less,
S: 0.0030 mass% or less,
Al: 0.05 mass% or less,
Ni: 0.1-1.2mass% and
Ti: 0.005-0.030 mass%
Is subjected to hot rolling, then cold rolling, and then subjected to continuous annealing to be heated to a temperature range of (Ac ₃ −50) to (Ac ₃ +50) ° C., and then 10 ° C./s or more. A method for producing a high-strength cold-rolled steel sheet, wherein the steel sheet is cooled to a temperature range of 350 to 550 ° C. at a cooling rate of and maintained in this temperature range for 15 seconds or longer. The billet according to claim 1, further comprising:
Cu: 0.50 mass% or less,
Mo: 0.50mass% or less and
Cr: A method for producing a high-strength cold-rolled steel sheet, comprising one or more selected from 0.50 mass% or less. The billet according to claim 1 or 2, further comprising:
Nb: The manufacturing method of the high intensity | strength cold-rolled steel plate characterized by containing 0.010 mass% or less. In Claim 1, 2 or 3, the steel slab further comprises: V: 0.010 mass% or less and
Zr: A method for producing a high-strength cold-rolled steel sheet, comprising at least one selected from 0.010 mass% or less. The method for producing a high-strength cold-rolled steel sheet according to any one of claims 1 to 4, wherein the steel slab further has a composition containing B: 0.0050 mass% or less. The steel slab according to any one of claims 1 to 5,
Ca: 0.0050 mass% or less and
REM: A method for producing a high-strength cold-rolled steel sheet, comprising at least one selected from 0.0050 mass% or less.