JP2005120437A - High-strength steel thin sheet superior in hole-expandability and ductility - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-strength steel sheet superior in hole-expandability and ductility with a tensile strength of 980 N/mm<SP>2</SP>or higher. <P>SOLUTION: The high-strength steel sheet superior in hole-expandability and ductility comprises, by mass%, 0.01-0.20% C, 1.5% or less Si, 1.5% or less Al, 0.5-3.5% Mn, 0.2% or less P, 0.0005-0.009% S, 0.009% or less N, 0.0006-0.01% Mg, 0.005% or less O, one or two of 0.01-0.20% Ti and 0.01-0.10% Nb, and the balance iron with unavoidable impurities, and satisfies all of the following three expressions: [Mg%]≥([O%]/16×0.8)×24 --- (1), [S%]≤([Mg%]/24-[O%]/16×0.8+0.00012)×32 --- (2), and [S%]≤0.0075/[Mn%] --- (3); and has a metallographic structure consisting mainly of a bainite phase. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

The present invention is mainly intended for automotive steel plates to be pressed, has a thickness of about 6.0 mm or less, has a tensile strength of 980 N / mm ² or more, and has high strength hot rolling excellent in hole expansibility and ductility. The present invention relates to a steel plate and a manufacturing method thereof.

  In recent years, there has been an increasing need for weight reduction as a vehicle fuel efficiency improvement measure and cost reduction by integral molding of parts, and development of hot-rolled high-strength steel sheets excellent in press formability has been promoted. Conventionally, as a hot-rolled steel sheet for processing, a dual-phase steel sheet having a ferrite / martensite structure is known. The dual phase steel sheet is composed of a composite structure of soft ferrite phase and hard martensite phase, and voids are generated from the interface of both phases with extremely different hardness, causing cracks and poor hole expandability. In addition, it is unsuitable for applications requiring high hole expansibility such as undercarriage parts. In contrast, JP-A-4-88125 and JP-A-3-180426 propose a method of manufacturing a hot-rolled steel sheet having excellent hole expansibility with a structure mainly composed of bainite. There are restrictions on the applicable parts due to inferior properties.

  Japanese Patent Laid-Open No. 6-293910, Japanese Patent Laid-Open No. 2002-180188, Japanese Patent Laid-Open No. 2002-180189, and Japanese Patent Laid-Open No. 2002-180190 disclose a steel sheet having a mixed structure of ferrite and bainite. However, with the aim of further reducing the weight of automobiles and complicating parts, higher hole expansibility is required, and high workability and high strength that cannot be handled by the above technology are required. .

In addition, in the Japanese Patent Laid-Open Nos. 2001-342543 and 2002-20838, the inventors of the present invention are not concerned with the elongation, and the state of the crack in the punched hole is important as a means for improving the hole expandability. It is possible to improve the hole expandability by relieving the concentration of stress during hole expansion by generating fine uniform voids in the cross-section of the punched hole by refining (Ti, Nb) N. I found. And the utilization of Mg-type oxide was proposed as a means of refinement | miniaturization of this (Ti, Nb) N. However, in the present invention, only the oxide is controlled. However, oxygen control is less flexible, and since the limited free oxygen after deoxidation is used, the total amount is small and it is difficult to obtain a predetermined dispersion state. It was difficult to obtain a sufficient effect.
JP-A-4-88125 Japanese Patent Laid-Open No. 3-180426 JP-A-6-293910 JP 2002-180188 A JP 2002-180189 A JP 2002-180190 A JP 2001-342543 A JP 2002-20838 A

The present invention has been made to solve the above-mentioned conventional problems, and relates to a thin steel sheet of 980 N / mm ² class or higher, and provides a high-strength thin steel sheet having both excellent hole expansibility and ductility. It is something to try.

  The present inventors have refined (Ti, Nb) N in order to improve the hole expandability by reducing the concentration of stress during hole expansion by generating fine uniform voids in the cross-section of the punched hole. As a result of various experiments and examinations on the above method, it is conventionally said that sulfides cause deterioration of hole expansibility, but Mg-based sulfides precipitate at high temperatures (Ti, Nb) N precipitation. Those that act as product nuclei and precipitate at low temperatures have the effect of suppressing the growth of (Ti, Nb) N due to competitive precipitation with (Ti, Nb) N, and improve the hole expansibility by refinement of TiN. I found that it contributed. And in order to avoid the precipitation of conventional Mn-based sulfides and obtain the above-mentioned action with Mg-based sulfides, it is necessary to put the addition balance of O, Mg, Mn and S under certain conditions. Thus, the present invention has been made by finding that a uniform finer (Ti, Nb) N can be easily achieved as compared with the use of Mg-based oxide alone.

(1) By mass% C: 0.01% or more, 0.20% or less,
Si: 1.5% or less,
Al: 1.5% or less,
Mn: 0.5% or more, 3.5% or less,
P: 0.2% or less,
S: 0.0005% or more, 0.009% or less,
N: 0.009% or less,
Mg: 0.0006% or more, 0.01% or less,
O: 0.005% or less,
And Ti: 0.01% or more and 0.20% or less,
Nb: 0.01% or more, 0.10% or less,
The steel structure characterized by satisfying all of the following three formulas having a strength mainly composed of bainite phase is over 980 N / mm ² . High-strength thin steel sheet with excellent hole expandability and ductility.
[Mg%] ≧ ([O%] / 16 × 0.8) × 24 (1)
[S%] ≦ ([Mg%] / 24− [O%] / 16 × 0.8 +0.00012) × 32 (2) [S%] ≦ 0.0075 / [Mn%] ( 3)

(2) In the steel of (1), among the composite precipitates of MgO, MgS, and (Nb, Ti) N, precipitates having a size of 0.05 μm or more and 3.0 μm or less are 5. A high-strength thin steel sheet excellent in hole expansibility and ductility in which the steel structure mainly contains a bainite phase and has a strength exceeding 980 N / mm ² , including 0 × 10 ² or more and 1.0 × 10 ⁷ or less.

(3) Further, the expansibility of the steel structure described in (1) or (2) in which the relationship between Al and Si satisfies the formula (4) in mass% and the strength mainly including the bainite phase exceeds 980 N / mm ² . High strength steel sheet with excellent ductility.
[Si%] + 2.2 × [Al%] ≧ 0.35 (4)

(4) In the steel of (1) or (2) or (3), Ti, C, Mn, and Nb satisfy the formulas (5), (6), and (7), respectively.
0.9 ≦ 48/12 × C / Ti <1.7 (5)
50227 × C-4479 × Mn> −9860 (6)
811 × C + 135 × Mn + 602 × Ti + 794 × Nb> 465 (7)
A high-strength thin steel sheet with excellent hole expansibility and ductility with a steel structure mainly composed of a bainite phase and a strength exceeding 980 N / mm ² .

(5) The steel structure according to (1) to (4) further containing 0.0005% or more and 0.01% or less of one or more of Ca, Zr, and REM in mass%. A high-strength thin steel sheet with excellent hole expansibility and ductility with a main strength exceeding 980 N / mm ² .

(6) Further in mass%,
Cu: 0.04% or more, 0.4% or less,
Ni: 0.02% or more, 0.3% or less,
Mo: 0.02% or more, 0.5% or less,
V: 0.02% or more, 0.1% or less,
Cr: 0.02% or more, 1.0% or less,
B: 0.0003% or more, 0.0010% or less,
A high-strength thin steel sheet excellent in hole expansibility and ductility in which the steel structure described in any one of (1) to (5) mainly contains a bainite phase and has a strength exceeding 980 N / mm ² .

According to the present invention, a hot rolled high-strength steel sheet having a strength level of 980 N / mm ² class or higher and having an unprecedented elongation-ductility balance can be supplied, which is extremely useful industrially.

The present invention focuses on the end face property of the punched hole for improving the hole expandability, and adjusts the addition balance of O, Mg, Mn, and S to uniformly and finely precipitate Mg-based oxides and sulfides. It suppresses the generation of coarse cracks at the time of punching, and improves the hole expandability by making the end face properties uniform. The individual constituent requirements of the present invention will be described in detail below.
First, the reasons for limiting the components of the present invention will be described.

  C is an element that affects the workability of steel, and the workability deteriorates as the content increases. In particular, if it exceeds 0.20%, carbides (pearlite, cementite) harmful to the hole expandability are generated, so the content is made 0.20% or less. However, when a particularly high hole expansibility is required, it is desirable to make it 0.1% or less. Moreover, 0.01% or more is necessary in terms of securing strength.

  Si is an effective element for suppressing the formation of harmful carbides and increasing the ferrite fraction and improving the elongation, and it is desirable to add it because it is an effective element for securing the material strength by solid solution strengthening. However, when the addition amount is increased, the chemical conversion treatment property is lowered and the spot weldability is also deteriorated, so 1.5% is made the upper limit.

  Al, like Si, is an effective element for suppressing the formation of harmful carbides and increasing the ferrite fraction and improving the elongation. In particular, it is an element necessary for achieving both ductility and chemical conversion treatment. Al is an element necessary for deoxidation from the past, and is usually added in an amount of about 0.01 to 0.07%. As a result of intensive studies, the present inventors have found that chemical conversion can be improved without deteriorating ductility by adding a large amount of Al even in a low Si system. However, as the amount added increases, not only the effect of improving ductility is saturated, but also the chemical conversion treatment performance decreases, and spot weldability also deteriorates, so the upper limit is 1.5%. It is desirable that the upper limit is 1.0%.

  Mn is an element necessary for ensuring the strength, and at least 0.50% of addition is necessary. In order to secure hardenability and obtain a stable strength, addition of over 2.0% is desirable. However, if added in a large amount, microsegregation and macrosegregation are likely to occur, and these deteriorate the hole expandability. Accordingly, the upper limit is set to 3.50%.

  P is an element that increases the strength of the steel sheet, and is an element that improves corrosion resistance by simultaneous addition with Cu. However, if the addition amount is high, it is an element that causes deterioration of weldability, workability, and toughness. Accordingly, the content is set to 0.2% or less. In particular, when corrosion resistance is not a problem, 0.03% or less is desirable with emphasis on workability.

  S is one of the most important additive elements in the present invention. S combines with Mg to form a sulfide, which becomes a nucleus of (Ti, Nb) N, and by suppressing the growth of (Ti, Nb) N, it contributes to these miniaturization and has a hole expanding property. It is thought to bring about a dramatic improvement. In order to obtain this effect, 0.0005% or more must be added, and 0.001% or more is desirable. However, excessive addition forms a Mn-based sulfide and conversely degrades the hole expandability, so 0.009% or less is desirable.

  Since N contributes to the generation of (Ti, Nb) N, it is preferable that N is small in order to ensure workability. If it exceeds 0.009%, coarse TiN is generated and the workability deteriorates, so the content is made 0.009% or less.

  Mg is one of the most important additive elements in the present invention. With this addition, Mg combines with oxygen to form an oxide and S to form a sulfide. The Mg-based oxides and Mg-based sulfides generated at this time are in a distributed state in which the size of the individual precipitates is small and uniformly dispersed compared to conventional steel to which no Mg is added. These precipitates finely dispersed in the steel contribute to the fine dispersion of (Ti, Nb) N and are considered to be effective in improving the hole expansibility. However, if it is less than 0.0006%, the effect is insufficient, and addition of 0.0006 or more is necessary. In order to sufficiently obtain the effect, 0.0015% or more is desirable. On the other hand, addition over 0.01% not only saturates the improvement for the amount added, but conversely degrades the cleanliness of the steel and degrades the hole expandability and ductility, so the upper limit is made 0.01%.

  O is one of the most important additive elements in the present invention. Bonds with Mg to form an oxide and contributes to improvement of hole expansibility. However, excessive addition degrades the cleanliness of the steel and causes elongation degradation, so it is desirable to make the upper limit 0.005%.

  Ti and Nb are one of the most important additive elements in the present invention. Ti and Nb form carbides and are effective in increasing the strength, contributing to uniform hardness and improving the hole expandability. In addition, a nitride is formed finely and uniformly with Mg-based oxides and sulfides as nuclei, and this has the effect of suppressing the occurrence of coarse cracks by forming fine voids at the time of punching and suppressing stress concentration. This is thought to bring about a dramatic improvement in hole expansibility. In order to exhibit these results effectively, it is necessary to add at least 0.01% of both Nb and Ti. However, if these additions become excessive, the ductility deteriorates due to precipitation strengthening. Therefore, the upper limit is set to 0.20% or less for Ti and 0.10% or less for Nb. These elements are effective even when added alone, and are effective even when added in combination.

  Ca, Zr, and REM control the shape of sulfide inclusions and are effective in improving hole expansibility. In order to exhibit this effectively, it is necessary to add at least one or two or more of 0.0005%. On the other hand, addition of a large amount deteriorates the cleanliness of the steel, so that the hole expandability and ductility are impaired. Accordingly, the upper limit is made 0.01%.

  Cu is an element that improves the corrosion resistance when combined with P. In order to obtain this effect, it is desirable to add 0.04% or more. However, the addition of a large amount increases the hardenability and lowers the ductility, so the upper limit is made 0.4%.

  Ni is an essential element for suppressing hot cracking when Cu is added. In order to obtain this effect, it is desirable to add 0.02% or more. However, the addition of a large amount increases the hardenability and lowers the ductility like Cu, so the upper limit is made 0.3%.

  Mo is an element effective for suppressing the formation of cementite and improving the hole expansibility. To obtain this effect, it is necessary to add 0.02% or more. However, since Mo is also an element that enhances hardenability, if added excessively, ductility decreases, so the upper limit is made 0.5%.

  V forms carbides and contributes to securing the strength. In order to obtain this effect, addition of 0.02% or more is necessary. However, since a large amount of addition reduces elongation and costs are high, the upper limit is made 0.1%.

  Cr, like V, forms carbides and contributes to securing strength. In order to obtain this effect, addition of 0.02% or more is necessary. However, since Cr is an element that enhances hardenability, elongation is reduced by adding a large amount. Therefore, the upper limit is made 1.0%.

  B is an element that strengthens the grain boundary and is effective in improving secondary work cracking, which is a problem with ultra high tensile strength. In order to obtain this effect, addition of 0.0003% or more is necessary. However, since B is also an element that enhances hardenability, the ductility is lowered by the addition of a large amount, so the upper limit is made 0.001%.

  As a result of diligent research to solve the above problems, the present inventors have finely dispersed TiN using Mg-based oxides and sulfides by putting the addition balance of O, Mg, Mn, and S under certain conditions. I found out that it is possible. That is, to sufficiently precipitate Mg oxide, to control the precipitation temperature of Mg-based sulfides while suppressing the precipitation of Mn-based sulfides, and to use the above-described action as a nucleus and growth suppressing action. Is possible. For this purpose, the following three relational expressions were derived. This will be described below.

In the present invention, since Mg-based sulfides are used in addition to Mg-based oxides, Mg needs to be added in an amount equal to or more than O. However, since O forms an oxide with other elements such as Al, the inventors have intensively studied. As a result, the effective O combined with Mg is 80% of the amount of analysis, and Mg addition beyond this is a hole. Necessary for forming sufficient sulfides to improve the spreadability, and the amount of Mg added needs to satisfy the formula (1). On the other hand, in the formation of Mg-based sulfides, S is an essential element. However, when the amount of S added is high, S becomes a Mn-based sulfide. It does not affect the deterioration of hole expansibility, but under conditions where a large amount of precipitation occurs, details are not clear, but it affects the characteristics of single precipitation or Mg-based sulfide precipitates and deteriorates the hole expandability. For this reason, S addition amount needs to satisfy | fill Formula (2) with respect to Mg and effective O amount. Furthermore, when Mn and S are both high, Mn-based sulfides precipitate at high temperatures, so that the formation of Mg-based sulfides can be suppressed and sufficient hole expansibility cannot be improved. It is necessary to satisfy equation (3).
[Mg%] ≧ ([O%] / 16 × 0.8) × 24 (1)
[S%] ≦ ([Mg%] / 24− [O%] / 16 × 0.8 +0.00012) × 32 (2) [S%] ≦ 0.0075 / [Mn%] ( 3)

Uniform refinement of (Nb, Ti) N is important to reduce the concentration of stress during hole expansion by generating fine uniform voids in the cross-section of punched holes and to improve hole expandability. However, when this size is small, it will not be effective since it does not become the starting point of fine voids, and if it is too large, it will become the starting point of coarse cracks.On the other hand, if this precipitate density is small, the number of fine voids generated during punching will be insufficient. It is considered that the effect of suppressing the generation of coarse cracks cannot be obtained. As a result of intensive investigations, the present inventors have found that composite precipitation of MgO and MgS can be used as this method, and the cause is not clear, but in the combined use of sulfides in addition to oxides, the effect is exhibited. The composite precipitate size and precipitate density are MgO, MgS, and (Nb, Ti) N composite precipitates, and a precipitate of 0.05 μm or more and 3.0 μm or less is 5.0 × 10 ² per square mm. It has been found that it is necessary to include at least 1.0 and not more than 1.0 × 10 ⁷ . At this time, even if Al ₂ O ₃ and SiO ₂ are contained in the composite oxide, this effect is not impaired, and even if MnS is contained in a small amount, the effect is not impaired.

Si and Al are very important elements for controlling the structure for ensuring ductility. However, Si may have surface irregularities called Si scales in the hot rolling process, and this may impair the appearance of the product, and the formation of a chemical conversion film is poor in chemical conversion treatment and coating performed after pressing. Cases or poor paint adhesion. For this reason, there are cases where a large amount of Si cannot be added to some applications with severe chemical conversion properties. At this time, Al can be substituted for Si in order to achieve both ductility and chemical conversion, but if both Si and Al are added in large amounts, the ferrite phase fraction increases and the desired strength cannot be obtained. . Therefore, in order to ensure sufficient strength and ensure ductility, the relationship between Si and Al needs to satisfy formula (4). However, it is desirable to set it to 0.9 or more especially when elongation becomes a problem.
[Si%] + 2.2 × [Al%] ≧ 0.35 (4)

  Since the present invention is a technique for improving the cross-sectional properties at the time of punching, the present invention is effective even if the metal structure includes any of a ferrite phase, a bainite phase, and a martensite phase. However, in order to ensure a strength of over 980 MPa, it is necessary to use structural strengthening as the strengthening mechanism, and in order to improve the hole expandability among the workability, it is necessary to mainly use the bainite phase. At this time, if the second phase is a ferrite phase, the ductility is improved. Therefore, it is desirable to include a ferrite phase as the second phase. In the steel of the present invention, even if an austenite phase remains in the structure, the effect of the present invention is not hindered. However, coarse cementite and pearlite phases are not desirable because the effect of improving the end face properties due to Mg-based precipitates is diminished.

Steel with a strength of over 980 N / mm ² shows deterioration in ductility and hole expansibility associated with high tempering. As a result of diligent research to solve the above problems, the present inventors have found that, as a means of ensuring ductility while securing strength and improving the hole expansion property by improving the punched end face properties with Mg-based precipitates, the bainite phase mainly It was found that it is effective to define the ranges of the components of C, Mn, and Ti in the steel structure. That is, the following three relational expressions were derived by clarifying the maximum use of TiC precipitation strengthening and the effect of Mn and C on the structure strengthening material. This will be described below.

  If the amount of addition of C is small compared to Ti, the elongation is deteriorated due to an increase in solid solution Ti, so 0.9 ≦ 48/12 × C / Ti. On the other hand, if C is too high compared to Ti, TiC will precipitate during hot rolling heating and the effect of increasing the strength will not be obtained, and the hole expandability will be degraded due to the increase in the amount of C in the second phase. In addition, this leads to a reduction in the effect of improving the end face properties due to the Mg-based precipitates, so the upper limit is 48/12 × C / Ti <1.7. In particular, when importance is attached to hole expansibility, it is desirable that 1.0 ≦ 48/12 × C / Ti <1.3.

As the amount of Mn added increases, ferrite formation is suppressed, so that the second phase fraction increases and the strength can be easily secured, but the elongation decreases. On the other hand, C hardens the second phase to improve the elongation, although accompanied by deterioration of hole expansibility. Therefore, in order to ensure the elongation required to exceed 980 N / mm ² , it is necessary to satisfy Equation (6).
50227 × C-4479 × Mn> −9860 (6)

In order to ensure workability, it is necessary to satisfy the above two expressions. If the steel plate has a level of 780 N / mm ^2, it is relatively easy to satisfy the above two formulas while ensuring the strength. However, in order to ensure a strength of over 980 N / mm ² , the hole expandability should be improved. Addition of C that deteriorates or Mn that deteriorates elongation is unavoidable. In order to ensure a strength of more than 980 N / mm ^2, it is necessary to adjust the components in a range that satisfies the equation (7) while satisfying the above two equations.
811 × C + 135 × Mn + 602 × Ti + 794 × Nb> 465 (7)

The dispersion state of the composite precipitate defined in the present invention is quantitatively measured by, for example, the following method. An extraction replica sample is prepared from an arbitrary place on the base steel plate, and this is observed using a transmission electron microscope (TEM) at a magnification of 5000 to 20000 times over an area of at least 5000 μm ² . The number is measured and converted to the number per unit area. At this time, the oxide and (Nb, Ti) N are identified by composition analysis by energy dispersive X-ray spectroscopy (EDS) attached to TEM and crystal structure analysis of electron diffraction image by TEM. When it is complicated to perform such identification on all the complex inclusions to be measured, the procedure is simply as follows. First, the number of target sizes is measured according to the above-mentioned procedure for each shape and size. Among these, all of the different shapes and sizes are identified according to the above-mentioned procedure for each of 10 or more, and oxidized. The ratio between the product and (Nb, Ti) N is calculated. Then, this ratio is multiplied by the number of inclusions measured first. When carbides in the steel interfere with the above TEM observation, the carbides can be agglomerated or melted by heat treatment to facilitate observation of complex inclusions.

Next, the general manufacturing method for obtaining this steel plate is demonstrated.
The finish rolling finish temperature needs to be not less than the Ar ₃ transformation point in order to prevent the formation of ferrite and improve the hole expandability. However, if the temperature is too high, the strength is reduced due to the coarsening of the structure and the ductility is lowered. Since the cooling rate suppresses the formation of carbides harmful to the hole expandability, a high hole expansion ratio can be obtained with a cooling rate of 20 ° C./s or more. In addition, the cutting temperature was set to 600 ° C. or lower in order to suppress the formation of pearlite and cementite, which are harmful to hole expansibility.

  Further, by performing air cooling during continuous cooling, it is possible to increase the occupancy ratio of the ferrite phase and improve ductility. However, when pearlite is generated during air cooling, the ductility is reduced, and therefore the air cooling start temperature is set to 650 ° C. or higher and 750 ° C. or lower.

Next, this invention is demonstrated based on an Example.
Steels having the components shown in Table 1 were melted and slabs were obtained by continuous casting according to a conventional method. The steels with the symbols A to Z according to the present invention, the steel with the symbol a, the addition amount of C, the steel with b, the addition amount of Mn, the steel with c, the addition amount of O, the steel with e, the addition amount of S, and the steel with f Is outside the scope of the present invention. The steel of a is the formula (5), the steel of b is the formula (3) and formula (6), the steel of c is the formula (1) and formula (2), the steel of d is the formula (4), For the steel, the formulas (2) and (3), the steel for f is the formula (1), and for the steel g, the formula (7) is out of the scope of the present invention. Further, the number of precipitates in the steel f is outside the scope of the present invention. These steels were heated in a heating furnace at a temperature of 1200 ° C. or higher, and hot rolled steel sheets having a thickness of 2.6 to 3.2 mm were obtained by hot rolling. Table 2 shows the hot rolling conditions.
In Table 2, A4 and J2 are cooling rates, B3 and F3 are air cooling start temperatures, and E3, G3 and Q4 are winding temperatures outside the scope of the present invention.

  The hot rolled steel sheet thus obtained was subjected to a tensile test and a hole expansion test using JIS No. 5 pieces. The hole expansibility (λ) is obtained by expanding a punched hole having a diameter of 10 mm with a 60 ° conical punch, and λ = (from the hole diameter (d) and the initial hole diameter (d0: 10 mm) when the crack penetrates the plate thickness. d−d0) / d0 × 100.

Table 2 shows TS, El, and λ of each test piece. FIG. 1 shows the relationship between strength and elongation, and FIG. 2 shows the relationship between strength and hole expansion rate. It can be seen that the steel of the present invention has an elongation compared to the comparative steel 1 and a hole expansion rate higher than that of the comparative steel 2, and is superior in all properties compared to the comparative steel 3. On the other hand, the steel of g1 could not obtain the target strength.
Thus, according to the present invention, a high-strength hot-rolled steel sheet excellent in both hole expansion rate and ductility can be obtained while ensuring a predetermined strength of 980 N / mm ² .

It is a graph which shows the effect of this invention steel on the elongation with respect to tensile strength. It is a graph which shows the effect of this invention steel on the hole expansion ratio with respect to tensile strength.

Claims (6)

In mass% C: 0.01% or more, 0.20% or less,
Si: 1.5% or less,
Al: 1.5% or less,
Mn: 0.5% or more, 3.5% or less,
P: 0.2% or less,
S: 0.0005% or more, 0.009% or less,
N: 0.009% or less,
Mg: 0.0006% or more, 0.01% or less,
O: 0.005% or less,
And Ti: 0.01% or more and 0.20% or less,
Nb: 0.01% or more, 0.10% or less,
The steel structure characterized by satisfying all of the following three formulas having a strength mainly composed of bainite phase is over 980 N / mm ² . High-strength thin steel sheet with excellent hole expandability and ductility.
[Mg%] ≧ ([O%] / 16 × 0.8) × 24 (1)
[S%] ≦ ([Mg%] / 24− [O%] / 16 × 0.8 +0.00012) × 32 (2) [S%] ≦ 0.0075 / [Mn%] ( 3) Further, in the steel according to claim 1, among the composite precipitates of MgO, MgS, and (Nb, Ti) N, precipitates having a size of 0.05 μm or more and 3.0 μm or less are 5.0 × 10 per square mm. A high-strength thin steel sheet excellent in hole expansibility and ductility with a steel structure mainly including a bainite phase and having a strength exceeding 980 N / mm ² , including ² or more and 1.0 × 10 ⁷ or less. Further, the steel structure according to claim 1 or 2, wherein the relationship between Al and Si satisfies the formula (4) at a mass%, and the strength mainly including a bainite phase has a hole expandability and ductility exceeding 980 N / mm ^2. Excellent high-strength thin steel sheet.
[Si%] + 2.2 × [Al%] ≧ 0.35 (4) In the steel of claim 1 or claim 2 or claim 3, further, Ti, C, Mn, and Nb satisfy formulas (5), (6), and (7), respectively.
0.9 ≦ 48/12 × C / Ti <1.7 (5)
50227 × C-4479 × Mn> −9860 (6)
811 × C + 135 × Mn + 602 × Ti + 794 × Nb> 465 (7)
A high-strength thin steel sheet with excellent hole expansibility and ductility with a steel structure mainly composed of a bainite phase and a strength exceeding 980 N / mm ² . The steel structure according to any one of claims 1 to 4, wherein the steel structure according to claim 1 further contains 0.0005% or more and 0.01% or less of one or more of Ca, Zr, and REM in mass%. high strength thin steel sheet strength with excellent 980 N / mm ² greater than the hole expandability and ductility. In mass%,
Cu: 0.04% or more, 0.4% or less,
Ni: 0.02% or more, 0.3% or less,
Mo: 0.02% or more, 0.5% or less,
V: 0.02% or more, 0.1% or less,
Cr: 0.02% or more, 1.0% or less,
B: 0.0003% or more, 0.0010% or less,
A high-strength thin steel sheet excellent in hole expansibility and ductility in which the steel structure according to any one of claims 1 to 5 containing one or more of the above has a strength of mainly bainite phase exceeding 980 N / mm ² .

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