JP4445095B2 - Composite structure steel plate excellent in burring workability and manufacturing method thereof - Google Patents

Description

【００５８】
【表２】

【００５９】
【発明の効果】
以上詳述したように、本発明は、バーリング加工性に優れた引張強度５４０ＭＰａ以上の複合組織鋼板およびその製造方法を提供するものであり、これらの熱延鋼板を用いることにより、疲労特性を十分に確保しつつバーリング加工性（穴拡げ性）の大幅な改善が期待できるため、工業的価値が高い発明である。
【図面の簡単な説明】
【図１】本発明に至る予備実験の結果を、フェライト平均粒径、第二相の大きさと穴拡げ率の関係で示す図である。
【図２】本発明に至る予備実験の結果を、第二相の炭素濃度と穴拡げ率の関係で示す図である。
【図３】疲労試験片の形状を説明する図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a composite steel sheet having excellent burring workability and a tensile strength of 540 MPa or more, and a method for producing the same, and in particular, it is required to satisfy both hole expansion workability and durability of automobile undercarriage parts and road wheels. The present invention relates to a composite structure steel plate excellent in hole expansibility (burring workability) and a method for producing the same.
[0002]
[Prior art]
In recent years, application of light metals such as Al alloys and high-strength steel sheets to automobile members has been promoted for the purpose of reducing the weight in order to improve the fuel efficiency of automobiles. However, although light metals such as Al alloys have the advantage of high specific strength, they are extremely expensive compared to steel, so their application is limited to special applications. Therefore, in order to promote weight reduction of automobiles in a wider range, application of inexpensive high-strength steel sheets is strongly demanded.
[0003]
In response to such demands for high strength, steel sheets that have both strength and deep drawability in the field of white bodies that account for about 1/4 of the weight of the vehicle body and cold-rolled steel sheets used for panels. Development of steel plates with bake hardenability has been promoted, which has contributed to weight reduction of vehicle bodies. However, at present, the object of weight reduction has shifted to structural members and suspension members that account for about 20% of the weight of the vehicle body, and the development of high-strength hot-rolled steel sheets used for these members has become an urgent task.
[0004]
However, increasing strength generally degrades material properties such as formability (workability), so the key to developing a high-strength steel sheet is how to increase strength without deteriorating material properties. In particular, hole expandability, fatigue durability, corrosion resistance, and the like are important as properties required for structural members and steel plates for suspension members, and how to balance high strength with these properties at a high level is important.
[0005]
For example, hole expansibility and fatigue durability are particularly emphasized as characteristics required for a steel plate for a road wheel disc. This is because the burring process (hole expansion process) in hub hole molding is particularly strict in the road wheel disk molding process, and it is fatigue durability that is managed according to the strictest standards of wheel member characteristics. It is.
[0006]
Currently, as a high-strength hot-rolled steel sheet for road wheel discs, a 590 MPa class ferritic-martensitic composite structure steel sheet (so-called dual phase steel) is used which emphasizes fatigue durability of members and is excellent in fatigue characteristics. However, the strength level required for these steel plates for members is increasing from 590 MPa class to 780 MPa class. On the other hand, not only does the hole expandability tend to decrease with increasing strength, but the composite steel sheet is said to be disadvantageous in terms of hole expandability due to its non-uniform structure. Therefore, there is a possibility that the hole expandability, which was not a problem in the 590 MPa class, becomes a problem in the 780 MPa class.
[0007]
In other words, in the application of high-strength steel plates to undercarriage parts such as road wheels, hole expansibility is an important consideration in addition to fatigue durability. However, with the exception of some exceptions, the invention described for a high-strength steel sheet having a ferrite-martensite composite structure for improving fatigue durability and excellent hole expansibility is hardly seen. Is the current situation.
[0008]
For example, Japanese Patent Laid-Open No. 5-179396 discloses that the microstructure is ferrite and martensite or retained austenite to ensure fatigue durability, and the ferrite is strengthened with precipitates of TiC and NbC, so that the ferrite grains and the martensite phase Has been disclosed in which the difference in the strength is reduced and the concentration of local deformation on the ferrite grains is suppressed to ensure the hole expandability.
[0009]
[Problems to be solved by the invention]
However, in some parts steel plates such as road wheel disks, a balance between formability such as burring workability and a high level of fatigue durability is very important, and satisfactory characteristics can be obtained with the above-mentioned conventional technology. Absent. Moreover, even if both characteristics are satisfied, it is important to provide a manufacturing method that can be stably manufactured at low cost, and the above-described conventional technique is insufficient.
[0010]
That is, the above-mentioned Japanese Patent Application Laid-Open No. 5-179396 is not only able to obtain sufficient elongation because precipitation strengthening of ferrite grains, but also high-density movable dislocations introduced around the martensite phase during production are caused by precipitates. Since the movement is hindered, a characteristic unique to a ferrite-martensite composite structure having a low yield ratio cannot be obtained. Further, the addition of Ti and Nb is not preferable because the manufacturing cost increases.
[0011]
Therefore, the present invention can stably solve the above-mentioned problems of the prior art, and can stably produce a hot-rolled steel sheet having excellent fatigue characteristics and burring workability (hole expansibility) with a tensile strength of 540 MPa or more, and the steel sheet at low cost. The object is to provide a manufacturing method.
[0012]
[Means for Solving the Problems]
The present inventors have made both the burring workability and fatigue characteristics of hot-rolled steel sheet compatible with the manufacturing process of hot-rolled steel sheet produced on an industrial scale by the continuous hot rolling equipment that is currently normally employed. We worked hard to achieve it. As a result, the microstructure is a composite structure in which the phase with the largest volume fraction is ferrite and the second phase is mainly martensite, the ferrite average particle diameter is 2 μm or more and 20 μm or less, and the average particle diameter of the second phase is It is very effective for improving the burring workability that the value divided by the average ferrite particle diameter is 0.05 or more and 0.8 or less and the carbon concentration of the second phase is 0.2% or more and 2% or less. Is newly found and the present invention has been made.
[0013]
That is, the gist of the present invention is as follows.
(1) In mass%,
C: 0.01 to 0.2%, Si: 0.01 to 2%,
Mn: 0.05 to 3%, P ≦ 0.1%,
S ≦ 0.01%, Al: 0.005 to 1%,
Cu: 0.2-2%, B: 0.0002-0.002%,
Ni: 0.1 to 1%
Wherein the remainder Ri Do Fe and unavoidable impurities, the steel sheet strength level is 780Mpa class, a steel hole expansion ratio is not less than 62%, the microstructure, the volume fraction up phase and ferrite , A composite structure mainly composed of martensite in the second phase, the ferrite average particle diameter is 2 μm or more and 20 μm or less, and the value obtained by dividing the average particle diameter of the second phase by the ferrite average particle diameter is 0.05 or more and 0.8 A composite structure steel sheet having excellent burring workability, wherein the carbon concentration of the second phase is 0.2% or more and 2% or less.
[0014]
( 2 ) Upon hot rolling of the steel slab having the component described in (1) above, after finishing the hot finish rolling at Ar3 transformation point temperature or higher and Ar3 transformation point temperature + 100 ° C or lower, Ar3 transformation point temperature or higher and Ar3 transformation temperature is reached. It stays in the temperature range below the point temperature for 1 to 20 seconds, then cools at a cooling rate of 20 ° C./s or more and winds up at a winding temperature of 350 ° C. or less, and its microstructure has the largest volume fraction. A composite structure in which the phase is ferrite and the second phase is mainly martensite, the ferrite average particle size is 2 μm or more and 20 μm or less, and the value obtained by dividing the average particle size of the second phase by the ferrite average particle size is 0.05. A method for producing a composite structure steel plate excellent in burring workability, comprising obtaining a steel plate having a carbon concentration in the second phase of 0.2% or more and 2% or less.
( 3 ) In the hot rolling, after finishing the rough rolling, high pressure descaling is performed, and the hot finish rolling is finished at an Ar3 transformation point temperature or higher and an Ar3 transformation point temperature + 100 ° C or lower ( 2 ). The manufacturing method of the composite structure steel plate excellent in the burring workability of description.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
The basic research results that led to the present invention will be described below.
First, the influence of the average ferrite grain size and the size of the second phase on the hole expandability was investigated. The test material for that purpose was prepared as follows. That is, 0.07% C-1.6% Si-2.0% Mn-0.01% P-0.001% S-0.03% Al slab was prepared by adjusting the composition of the slab. After finishing hot finish rolling at any temperature above the point temperature, it stays for 1 to 15 seconds in any temperature range from the Ar1 transformation point temperature to the Ar3 transformation point temperature, and then cooled to 20 ° C / s or more. Cooled at a speed and wound up at room temperature.
FIG. 1 shows an arrangement of the average ferrite grain size and the size of the second phase based on the results of the hole expansion test performed on these steel sheets.
[0016]
From this result, there is a strong correlation between the ferrite average particle size and the size of the second phase (the value obtained by dividing the average particle size of the second phase by the ferrite average particle size) and the hole expandability. It has been newly found that the hole expandability is remarkably improved when the size of the two phases is 2 μm or more and 20 μm or less and 0.05 or more and 0.8 or less, respectively.
[0017]
This mechanism is not always clear, but if the second phase is too large, voids are likely to be generated at the interface between the second phase and the parent phase, which becomes the starting point of cracks when expanding the hole, and if it is too small, it correlates with the hole expansion rate. Since the local ductility is lowered, it is estimated that the hole expansion rate is improved at the optimum size and interval. In addition, if the average ferrite grain size is too small, the yield stress increases, which adversely affects the shape freezing property after molding. If it is too large, the microstructure is lost, and the local ductility correlated with the hole expansion rate decreases. It is thought to do.
In addition, the measuring method of a ferrite average particle diameter followed the cutting method as described in the ferrite crystal grain size test method of JIS G 0552 steel. The average particle diameter of the second phase was defined as the average equivalent circle diameter, and a value obtained from an image processing apparatus or the like was adopted.
[0018]
Furthermore, the effect of the carbon concentration of the second phase on the hole expandability was investigated. FIG. 2 shows the steel sheet in which the hole expandability is arranged by the carbon concentration of the second phase. From this result, there is a strong correlation between the carbon concentration of the second phase and the hole expandability, and it is newly found that the hole expandability is significantly improved when the carbon concentration of the second phase is 0.2% or more and 2% or less. did.
[0019]
This mechanism is not always clear, but if the carbon concentration of the second phase is too high, the difference in strength between the second phase and the parent phase becomes large, and voids are likely to form at the interface during punching. The starting point. On the other hand, if the carbon concentration of the second phase is too low, the ductility of the ferrite phase is inevitably lowered, and the local ductility correlated with the hole expansion rate is lowered, so that the hole expansion rate is lowered. Therefore, it is presumed that the hole expansion rate is improved at the optimum carbon concentration of the second phase.
[0020]
However, if the carbon concentration of the second phase is more than 1.2%, the heat-affected zone may be significantly softened during welding such as spot welding, which may become a starting point for fatigue failure. Is preferably in the range of 0.2% to 1.2%.
The hole expandability (burring workability) was evaluated according to the hole expansion test method described in Japan Iron and Steel Federation Standard JFS T 1001-1996.
[0021]
Next, the microstructure of the steel sheet and the carbon concentration of the second phase in the present invention will be described in detail.
The microstructure of the steel sheet was a composite structure in which the phase with the largest volume fraction was ferrite and the second phase was mainly martensite in order to achieve both fatigue characteristics and burring workability (hole expansibility). However, the second phase is allowed to contain unavoidable bainite and retained austenite.
[0022]
In order to secure good fatigue properties, the volume fraction of bainite and / or retained austenite is preferably 5% or less. Here, the volume fraction of the ferrite and the second phase is an area fraction of the microstructure in the microstructure observed at 200 to 500 times with an optical microscope at a quarter thickness of the steel sheet in the rolling direction. Defined.
[0023]
Moreover, the carbon concentration of the second phase is a value obtained by the calibration curve method described in the literature (Electron Beam Microanalysis: written by Keiyoshi Soejima, published by Nikkan Kogyo Shimbun) using EPMA (Electron Probe Micro Analyzer). It is. However, the measured second phase grains were 5 or more, and the carbon concentration was the average value.
[0024]
On the other hand, as a simple measurement method that replaces the above method, the carbon concentration of the second phase may be obtained by the following method. That is, it is a method of calculating the carbon concentration of the second phase from the carbon content (average carbon concentration in the whole steel) of the whole steel (phase with the largest volume fraction and the second phase) and the carbon concentration in the ferrite.
[0025]
The carbon content of the whole steel (phase with the largest volume fraction and the second phase) is the carbon content of the steel component, and the carbon concentration in ferrite can be estimated from the bake hardening index (hereinafter referred to as BH). However, the amount of BH (MPa) is a value obtained by using a JIS No. 5 tensile test piece, giving a pre-strain of 2.0%, performing a heat treatment at 170 ° C. for 20 minutes, and performing a tensile test again. It is the difference between the previous 2.0% flow stress and the yield point after heat treatment.
The amount of BH in the composite structure steel is considered to be correlated with the amount of solid carbon in the ferrite because it is considered that the hard second phase does not cause plastic deformation at a pre-strain of about 2.0%.
[0026]
Literature Foemable HSLA and Dual-Phase Steels (1977), ATDAVENPORT, page 131, FIG. 4 shows the relationship between the solute carbon content and the BH content of the composite structure steel. From this relationship, the relationship between the amount of BH and the amount of solute carbon in the composite structure steel is Cs (solute carbon amount) = 1.5 × 10 ⁻⁴ exp (0.033 × BH)
And can be approximated. Therefore, the carbon concentration of the second phase is Cm = [C (carbon content in steel) −Cs] / fM (second phase volume fraction).
Can be estimated. Moreover, the carbon concentration of the second phase estimated from the above formula and the carbon concentration measured by EPMA show a very good correlation.
[0027]
Then, the reason for limitation of the chemical component of this invention is demonstrated. The component content is% by mass.
C is an element necessary for obtaining a desired microstructure. However, if the content exceeds 0.2%, workability and weldability deteriorate, so the content is made 0.2% or less. Moreover, since intensity | strength will fall that it is less than 0.01%, it is made 0.01% or more.
[0028]
Si is effective for increasing the strength as a solid solution strengthening element as well as being necessary for obtaining a desired microstructure. In order to obtain a desired strength, it is necessary to contain 0.01% or more. However, if it exceeds 2%, workability deteriorates. Therefore, the Si content is set to 0.01% or more and 2% or less.
[0029]
Mn is effective for increasing the strength as a solid solution strengthening element. In order to obtain a desired strength, 0.05% or more is necessary. Further, if over 3% is added, slab cracking occurs, so the content is made 3% or less.
[0030]
P is preferably an impurity and is preferably as low as possible. If contained over 0.1%, the workability and weldability are adversely affected and the fatigue characteristics are also reduced.
[0031]
S is preferably an impurity and is preferably as low as possible. If it is too large, A-based inclusions that deteriorate the hole expansibility are generated. Therefore, S should be reduced as much as possible, but 0.01% or less is acceptable.
[0032]
Al needs to be added in an amount of 0.005% or more for deoxidation of molten steel, but the cost is increased, so the upper limit is made 1.0%. Moreover, when adding too much, a nonmetallic inclusion will be increased and elongation will be degraded, Therefore Preferably it is 0.5% or less.
[0033]
Cu is added in there is an effect of improving the fatigue characteristics in the solid solution state. However, if the content is less than 0.2%, the effect is small, and even if the content exceeds 2%, the effect is saturated. Therefore, the Cu content is in the range of 0.2 to 2%.
[0034]
B is added in the an effect of increasing the fatigue limit by combined addition with Cu. However, if it is less than 0.0002%, it is insufficient for obtaining the effect, and if added over 0.002%, slab cracking occurs. Therefore, the addition of B is set to 0.0002% or more and 0.002% or less.
[0035]
Ni is added to prevent hot brittleness due to Cu inclusion. However, if the content is less than 0.1%, the effect is small, and even if added over 1%, the effect is saturated.
[0039]
Next, the reasons for limiting the production method of the present invention will be described in detail below.
In the present invention, the slab obtained by casting the molten steel whose components are adjusted so as to have the desired component content may be sent directly to a hot rolling mill with a high-temperature slab, or after cooling to room temperature, heating You may hot-roll after reheating in a furnace. The reheating temperature is not particularly limited, but if it is 1400 ° C. or higher, the scale-off amount becomes large and the yield decreases, so the reheating temperature is preferably less than 1400 ° C. In addition, since heating below 1000 ° C. significantly impairs the operation efficiency on the schedule, the reheating temperature is desirably 1000 ° C. or higher.
[0040]
In the hot rolling process, after the rough rolling is finished, finish rolling is performed, but the final pass temperature (FT) needs to be finished in a temperature range of Ar3 transformation point temperature to Ar3 transformation point temperature + 100 ° C. or less. This is because when the rolling temperature falls below the Ar3 transformation point temperature during hot rolling, strain remains and the ductility is lowered and the workability deteriorates. When the finishing temperature exceeds the Ar3 transformation point temperature + 100 ° C, the finish rolling is performed. Since the subsequent austenite grain size is increased, the ferrite transformation is not sufficiently promoted in the two-phase region performed in the subsequent cooling step, and the desired microstructure cannot be obtained. Accordingly, the finishing temperature is set to Ar3 transformation point temperature or higher and Ar3 transformation point temperature + 100 ° C or lower.
[0041]
Here, when high-pressure descaling is performed after the end of rough rolling, it is preferable that the condition of high-pressure water collision pressure P (MPa) × flow rate L (liter / cm ² ) ≧ 0.0025 on the steel plate surface is satisfied.
The collision pressure P of high-pressure water on the steel sheet surface is described as follows (see “Iron and Steel”, 1991, vol. 77, No. 9, P1450).
P (MPa) = 5.64 × P ₀ × V / H ²
However,
P ₀ (MPa): Fluid pressure V (L / min): Nozzle flow rate H (cm): Distance between steel plate surface and nozzle
The flow rate L is described as follows.
L (liter / cm ² ) = V / (W × v)
However,
V (liter / min): Nozzle flow rate W (cm): Width of spray liquid per nozzle hitting the steel plate surface v (cm / min): Upper limit of plate speed collision pressure P × flow rate L is the effect of the present invention Although there is no particular need to determine the value in order to obtain the above, it is preferable that the amount be 0.02 or less because inconveniences such as increased wear of the nozzles occur when the nozzle flow rate is increased.
[0043]
Furthermore, the maximum height Ry of the steel sheet after finish rolling is preferably 15 μm (15 μm Ry, l2.5 mm, ln12.5 mm) or less. For example, as described in “Handbook of Fatigue Design for Metallic Materials”, edited by the Japan Society of Materials Science, page 84, the fatigue strength of a hot-rolled or pickled steel sheet correlates with the maximum height Ry of the steel sheet surface. It is clear from this. Further, the subsequent finish rolling is desirably performed within 5 seconds in order to prevent the scale from being generated again after descaling.
[0044]
In the process after finishing rolling, first, the process stays for 1 to 20 seconds in the temperature range (two-phase region of ferrite and austenite) from the Ar3 transformation point to the Ar1 transformation point. The retention here is performed in order to promote the ferrite transformation in the two-phase region, but if it is less than 1 second, the ferrite transformation in the two-phase region is insufficient, so that sufficient ductility cannot be obtained. On the other hand, if it exceeds 20 seconds, pearlite is generated, and a composite structure in which the target phase with the maximum volume fraction is ferrite and the second phase is mainly martensite cannot be obtained.
[0045]
Further, the temperature range for retaining for 1 to 20 seconds is preferably Ar1 transformation point or more and 800 ° C or less in order to facilitate the ferrite transformation, and for that purpose, at the cooling rate of 20 ° C / s or more after finish rolling is finished. It is preferable to quickly reach the temperature range. Further, the residence time of 1 to 20 seconds is preferably 1 to 10 seconds so as not to extremely reduce productivity.
[0046]
Next, from the temperature range to the coiling temperature (CT), cooling is performed at a cooling rate of 20 ° C./s or more, but at a cooling rate of less than 20 ° C./s, pearlite or bainite is generated and sufficient martensite. Thus, a microstructure having the target ferrite as the phase with the maximum volume fraction and martensite as the second phase cannot be obtained.
The upper limit of the cooling rate up to the coiling temperature is not particularly defined, but the effects of the present invention can be obtained.
[0047]
When the coiling temperature exceeds 350 ° C., bainite is generated and sufficient martensite cannot be obtained, and a microstructure with the target ferrite as the phase with the largest volume fraction and martensite as the second phase cannot be obtained. Therefore, the coiling temperature is limited to 350 ° C. or less.
Further, the lower limit value of the coiling temperature is not particularly limited. However, if the coil is wet for a long time, there is a concern about poor appearance due to rust.
[0048]
【Example】
The following examples further illustrate the present invention.
The steels D to J having chemical components shown in Table 1 are melted in a converter and continuously cast, then reheated at the heating temperature (SRT) shown in Table 2, and after rough rolling, the finish rolling shown in Table 2 is also performed. After rolling at a temperature (FT) to a plate thickness of 1.2 to 5.4 mm, each was wound at a winding temperature (CT) shown in Table 2. In some cases, high pressure descaling was performed under conditions of a collision pressure of 2.7 MPa and a flow rate of 0.001 liter / cm ² after rough rolling. However, the display about the chemical composition in a table | surface is the mass%.
[0049]
The tensile test of the hot-rolled sheet thus obtained was performed by first processing the specimen into a No. 5 test piece described in JIS Z 2201, and following the test method described in JIS Z 2241. Table 2 shows the test results. Here, the volume fraction of the ferrite and the second phase is an area fraction of the microstructure in the microstructure observed at 200 to 500 times with an optical microscope at a quarter thickness of the steel sheet in the rolling direction. Defined. In addition, the measurement method of the ferrite average particle diameter is based on the cutting method described in the ferrite crystal grain size test method of JIS G 0552 steel, the average particle diameter of the second phase is defined as the average equivalent circle diameter, and the image processing apparatus, etc. The value obtained was adopted.
[0050]
The carbon concentration of the second phase is obtained by using a calibration curve method described in the literature ("Electron Beam Microanalysis", published by Nikkan Kogyo Shimbun Co., Ltd.) using EPMA (Electron Probe Micro Analyzer). Value. However, the measured second phase grains were 5 or more, and the carbon concentration was the average value.
On the other hand, for some samples, the carbon concentration of the second phase is measured by the above-described simple measurement method.
[0051]
Further, a complete double-bending plane bending fatigue test was performed on a plane bending fatigue test piece having a length of 98 mm, a width of 38 mm, a minimum cross-sectional width of 20 mm, and a notch curvature radius of 30 mm as shown in FIG. It was. The fatigue properties of the steel sheet were evaluated by a value obtained by dividing the fatigue limit σW at 10 × 10 ⁷ times by the tensile strength σB of the steel sheet (fatigue limit ratio σW / σB).
However, the surface of the fatigue test piece was left as pickled without any grinding.
On the other hand, burring workability (hole expandability) was evaluated according to the hole expansion test method described in Japan Iron and Steel Federation Standard JFS T 1001-1996.
[0052]
Consistent with the present invention is steel G, which contains a predetermined amount of steel components, and its microstructure is a composite structure in which the phase with the largest volume fraction is ferrite and the second phase is mainly martensite. Yes, the average ferrite particle size is 2 μm or more and 20 μm or less, the average particle size of the second phase divided by the average ferrite particle size is 0.05 or more and 0.8 or less, and the carbon concentration of the second phase is 0.2%. A composite structure steel sheet having excellent burring workability, characterized by being 2% or less, is obtained.
[0053]
Steels other than the above are outside the scope of the present invention for the following reasons.
That is, steel C-1 has a finish rolling finish temperature (FT) higher than the range of the present invention, and the ferrite grain size (Df), the size of the second phase (dm / Df), and the second phase carbon concentration (Cm). Is outside the scope of the present invention, a sufficient hole expansion ratio (λ) and fatigue limit ratio (σW / σB) are not obtained.
Steel C-2 has a finish rolling end temperature (FT) lower than the range of the present invention and the size of the second phase (dm / Df) is outside the range of the present invention. Further, the fatigue limit ratio (σW / σB) is not obtained. Furthermore, strain remains and ductility (El) also decreases.
[0054]
Steel C-3 has a cooling rate (CR) after residence lower than the range of the present invention, and the coiling temperature (CT) is also higher than the range of the present invention. Therefore, since the ferrite grain size (Df), the size of the second phase (dm / Df) and the second phase carbon concentration (Cm) are outside the scope of the present invention, a sufficient hole expansion ratio (λ) and fatigue limit ratio (ΣW / σB) is not obtained.
Steel C-4 has a residence temperature (MT) lower than the range of the present invention, and the second phase size (dm / Df) and the second phase carbon concentration (Cm) are outside the range of the present invention. The hole expansion rate (λ) and the fatigue limit ratio (σW / σB) are not obtained.
[0055]
Steel C-5 has no residence time (Time), and the size of the second phase (dm / Df) and the second phase carbon concentration (Cm) are outside the scope of the present invention. λ) and fatigue limit ratio (σW / σB) are not obtained.
Steel D has a C content outside the range of the present invention, so that the target microstructure cannot be obtained, and sufficient strength (TS) and fatigue limit ratio (σW / σB) cannot be obtained.
Steel E does not have a sufficient strength (TS) and fatigue limit ratio (σW / σB) because the Si content is outside the scope of the present invention.
[0056]
Steel F has a sufficient strength (TS) because the Mn content is outside the scope of the present invention, and the ferrite grain size (Df) and the size of the second phase (dm / Df) are outside the scope of the present invention. ), Hole expansion rate (λ) and fatigue limit ratio (σW / σB) are not obtained.
Steel H does not have a sufficient hole expansion ratio (λ) and fatigue limit ratio (σW / σB) because the S content is outside the range of the present invention.
In Steel I, since the P content is outside the range of the present invention, a sufficient fatigue limit ratio (σW / σB) is not obtained.
In Steel J, the C content is outside the range of the present invention, so that sufficient elongation (El), hole expansion rate (λ) and fatigue limit ratio (σW / σB) are not obtained.
[0057]
[Table 1]

[0058]
[Table 2]

[0059]
【The invention's effect】
As described above in detail, the present invention provides a composite structure steel plate having a tensile strength of 540 MPa or more excellent in burring workability and a method for producing the same. By using these hot-rolled steel plates, sufficient fatigue characteristics are provided. Therefore, a significant improvement in burring workability (hole expansibility) can be expected while ensuring a high industrial value.
[Brief description of the drawings]
FIG. 1 is a diagram showing the results of a preliminary experiment leading to the present invention in relation to the average ferrite grain size, the size of the second phase, and the hole expansion rate.
FIG. 2 is a diagram showing the results of a preliminary experiment leading to the present invention in relation to the carbon concentration of the second phase and the hole expansion rate.
FIG. 3 is a diagram illustrating the shape of a fatigue test piece.

Claims (3)

% By mass
C: 0.01-0.2%
Si: 0.01-2%
Mn: 0.05-3%,
P ≦ 0.1%,
S ≦ 0.01%,
Al: 0.005 to 1%
Cu: 0.2-2%,
B: 0.0002 to 0.002%,
Ni: 0.1 to 1%
Wherein the remainder Ri Do Fe and unavoidable impurities, the steel sheet strength level is 780Mpa class, a steel hole expansion ratio is not less than 62%, the microstructure, the volume fraction up phase and ferrite , A composite structure mainly composed of martensite in the second phase, the ferrite average particle diameter is 2 μm or more and 20 μm or less, and the value obtained by dividing the average particle diameter of the second phase by the ferrite average particle diameter is 0.05 or more and 0.8 A composite structure steel sheet having excellent burring workability, wherein the carbon concentration of the second phase is 0.2% or more and 2% or less.   In the hot rolling of the steel slab having the component according to claim 1, after finishing the hot finish rolling at Ar3 transformation point temperature or higher and Ar3 transformation point temperature + 100 ° C or lower, Ar1 transformation point temperature or higher and Ar3 transformation point temperature or lower. It stays in the temperature range for 1 to 20 seconds, then cools at a cooling rate of 20 ° C./s or more, and winds at a coiling temperature of 350 ° C. or less. The microstructure has the maximum volume fraction as ferrite. , A composite structure mainly composed of martensite in the second phase, the ferrite average particle diameter is 2 μm or more and 20 μm or less, and the value obtained by dividing the average particle diameter of the second phase by the ferrite average particle diameter is 0.05 or more and 0.8 A method for producing a composite structure steel sheet having excellent burring workability, wherein a steel sheet having a second phase carbon concentration of 0.2% or more and 2% or less is obtained.   3. The burring process according to claim 2, wherein in the hot rolling, after the rough rolling is finished, high-pressure descaling is performed, and the hot finish rolling is finished at an Ar 3 transformation point temperature or higher and an Ar 3 transformation point temperature + 100 ° C. or lower. A method for producing a composite steel sheet having excellent properties.

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