KR100886052B1 - Ultrahigh-strength thin steel sheet superior in hydrogen embrittlement resistance and workability - Google Patents

Abstract

The present invention provides an ultra high strength steel sheet excellent in hydrogen embrittlement resistance and workability.

The ultra-high strength steel sheet of the present invention is a mass%, C 0.25% or more 0.60% or less, Si 1.0 to 3.0%, Mn 1.0 to 3.5%, P 0.15% or less, S 0.02% or less and Al 1.5% or less (0% Not included), and the balance consists of iron and unavoidable impurities,

Metal structure after tensile processing of 3% of processing rates,

(i) residual austenite in an area ratio over the entire tissue: 1% or more,

Bainitic Ferrites and Martensite: 80% or more in total,

Ferrites and perlites: 9% or less (including 0%) in total, and

The average axial ratio (long axis / short axis) of the retained austenite grains: 5 or more, or

(ii) retained austenite: at least 1% by area ratio of the entire tissue,

Average axial ratio (long axis / short axis) of the retained austenite grains: 5 or more,

Average short axis length of the grains: 1 µm or less, and

The closest distance between the remaining austenite grains: satisfies 1 µm or less,

Ultra high strength steel sheet having excellent hydrogen embrittlement characteristics, characterized in that the tensile strength is 1180MPa or more.

Description

ULTRAHIGH-STRENGTH THIN STEEL SHEET SUPERIOR IN HYDROGEN EMBRITTLEMENT RESISTANCE AND WORKABILITY}

BRIEF DESCRIPTION OF THE DRAWINGS It is an overview perspective view of the component used for the breakdown pressure test in Example 1. FIG.

It is a side view which shows typically the aspect of the withstand pressure breakdown test in Example 1. FIG.

3 is an overview perspective view of a part used for the impact resistance test in Example 1. FIG.

4 is a cross-sectional view taken along the line A-A in FIG. 3.

5 is a side view schematically showing an aspect of the impact resistance test in Example 1. FIG.

6 shows No. 1 of Example 1; It is an example of TEM observation photograph (magnification 15,000 times) in 101 (example of this invention).

7 shows No. 1 of Example 1; It is a TEM observation photograph example (magnification 15,000 times) at 120 (comparative example).

8 is No. of Example 2; It is a TEM observation photograph example (magnification 15,000 times) in 201 (example of this invention).

9 is No. of Example 2; TEM observation photograph example (magnification 15,000 times) at 220 (comparative example).

10 is a graph showing the relationship between the average axial ratio of residual austenite grains and the hydrogen embrittlement risk index.

It is a figure which shows typically the closest distance between residual austenite grains.

12 is No. of Example 3; It is a TEM observation photograph example (magnification 15,000 times) in 301 (example of this invention).

13 is a view of No. 3 in Example 3; It is a TEM observation photograph example (magnification 60,000 times) in 301 (example of this invention).

14 is No. of Example 3; TEM observation photograph example (magnification 15,000 times) in 313 (comparative example).

Explanation of symbols for the main parts of the drawings

1: Components for test for breakdown pressure test (test body) 2, 5: Spot welding position

3: mold 4: component for impact resistance test (test)

6: Fall 7: the foundation for testing impact properties

The present invention is an ultra high strength foil having excellent hydrogen embrittlement resistance characteristics (performance of suppressing the weakening of a metal material by absorbing hydrogen), in particular, hydrogen embrittlement resistance characteristics after molding processing, and workability. Steel plate, especially high strength with excellent workability while suppressing fracture due to hydrogen embrittlement (degradation of metal material by absorbing hydrogen), which is a problem in steel plate with tensile strength of 1180 MPa or more. It is about a steel sheet.

In obtaining a high-strength component constituting an automobile, an industrial machine, or the like by press forming or bending, it is required that the steel sheet provided for the above-mentioned process have excellent strength and ductility. In recent years, the demand for ultra-high strength steel sheets of 1180 MPa or more has increased due to further weight reduction of automobiles, and in particular, TRIP (Transformation Induced Plasticity) steel sheets have attracted attention as steel sheets meeting these requirements.

The TRIP steel sheet is a steel sheet in which austenite structure remains, and when it is processed and deformed at a temperature higher than the martensite transformation start temperature (Ms point), residual austenite (residual gamma) is organically transformed to martensite due to stress, thereby obtaining a large elongation. Some examples thereof include, for example, TRIP-type composite tissue steels (TPF steel) containing polygonal ferrite as a matrix and containing residual austenite; TRIP type tempered martensitic steels (TAM steel) which are based on tempered martensite and contain residual austenite; TRIP type bainite steel (TBF steel) etc. which are based on bainitic ferrite and contain residual austenite are known. Among them, the TBF steel has been known for a long time (for example, NISSHIN STEEL TECHNICAL REPORT, No. 43, Dec. 1980, p. 1-10), and high strength is obtained by hard bainitic ferrite. It is easy to be fragile, and it is easy to produce | generate fine residual austenite in the boundary of needle-like bainitic ferrite, and such a tissue form has the characteristic which leads to the very excellent elongation rate. In addition, TBF steel also has a manufacturing advantage that it can be easily manufactured by one heat treatment (continuous annealing process or plating process).

By the way, in the ultra-high strength region of 1180 MPa or more, it is known that a new problem such as delayed fracture due to hydrogen embrittlement occurs in the TRIP steel sheet like other high strength steels. Delayed fracture refers to a phenomenon in which hydrogen generated in a corrosive environment or atmosphere diffuses into defects such as dislocations, voids, and grain boundaries in high-strength steel, embrittles the material, and causes fracture in a stressed state. It causes the badness such as deterioration of ductility and toughness.

In high-strength steels that are conventionally used for bolts, PC steel wires, line pipes, etc., when the tensile strength is 980 MPa or more, hydrogen embrittlement (pickle brittleness, plating brittleness, delayed breakdown, etc.) occurs due to the penetration of hydrogen into the steel. It is widely known. Therefore, most of the techniques for improving the hydrogen embrittlement resistance target steel materials such as bolts. For example, "New Development of Delayed Fracture Elucidation" (Japan Iron and Steel Association, published in January 1997) p.111 to 120 has a temper softening resistance of Cr, Mo, V, etc., with a metal structure as a tempered martensite agent. It is reported that addition of the element to which it represents is effective for the improvement of delayed fracture resistance. This is a technique of depositing alloy carbide and utilizing it as a trap site for hydrogen, thereby shifting the delayed fracture form from grain boundary to intragranular fracture and suppressing fracture.

However, in the case of thin steel sheets, hydrogen embrittlement has not been used in view of workability and weldability, and since the steel sheet is thin in thickness and released in a short time even if hydrogen invades, hydrogen embrittlement is rarely a problem. No active countermeasures have been taken. In recent years, however, as described above, in order to reduce the weight of automobiles and to improve the safety of collisions, there has been a demand for additional high strength of reinforcements such as bumpers and impact beams, seat rails and the like. Furthermore, high strength is also required for parts, such as a filler which performed press molding, bending, etc. Therefore, in order to obtain these components, the demand for ultra high strength steel sheets of 980 MPa or more is increasing, and therefore, there is an urgent need to reliably increase the hydrogen embrittlement resistance characteristics of the ultra high strength steel sheets.

In order to improve the hydrogen embrittlement characteristics of the ultra-high strength steel sheet, it is also conceivable to divert the technology related to the bolt steel and the like, for example, the above-mentioned "New Development of Delayed Fracture Elucidation" (Japan Steel Association, January 1997). Issuance) p.111 ~ 120], since the amount of C is 0.4% or more and also contains a large amount of alloying elements, it is impossible to secure the workability required for the thin steel sheet when the technique of the above document is applied to the steel sheet. do. In addition, there is a problem in the manufacturability because the precipitation heat treatment for several hours or more is required for the precipitation of alloy carbides. Therefore, in order to improve the hydrogen embrittlement property of the steel sheet, it is necessary to establish a unique technology.

In general, in the case of quenched (tempered) martensitic steels conventionally employed as high strength steels, high strength can be achieved relatively easily, but in order to increase the workability without variation, it is essential to provide a tempering process. Temperature and time should be adjusted closely. In addition, there is a high risk of generating tempering brittleness, it is difficult to increase the workability certainly. Examples of the increased ductility include a composite steel of martensite and ferrite, but since the hard and soft phases are mixed, it is difficult to sufficiently increase the hydrogen embrittlement resistance due to the strong notch sensitivity.

In the case of steels containing these martensite, the delayed fracture due to hydrogen is thought to occur due to the accumulation of hydrogen in the former austenite grain boundary and the like, and the formation of voids, starting from this portion, and the susceptibility of delayed fracture. In order to reduce, it is adopted as a general solution to lower the diffusible hydrogen concentration by uniformly and finely dispersing carbides and the like as a hydrogen collection site. However, even if a large number of carbides and the like are dispersed as hydrogen trapping sites in this manner, the trapping capacity is limited, and thus, delayed destruction due to hydrogen cannot be sufficiently suppressed.

As a technique for improving the hydrogen embrittlement resistance of steel sheet, it has been proposed in Japanese Patent Laid-Open No. 1999-293383 that hydrogen defects can be suppressed by the presence of an oxide mainly composed of Ti and Mg. However, the above technique is intended for thick steel sheets, but is considered for delayed fracture after high heat input welding, but it does not sufficiently consider the use environment (for example, corrosive environment, etc.) in automobile parts manufactured using thin steel sheets. . In addition, workability is not considered sufficiently.

Japanese Patent Application Laid-Open No. 2003-166035 discloses the correlation between the dispersion form (standard deviation or average particle diameter from average particle diameter or average particle diameter from average particle diameter) of Mg oxides, sulfides, complex precipitates or composite precipitates, the volume fraction of retained austenite, and the steel sheet strength. When controlled, it is disclosed that ductility and delayed fracture resistance after molding can be improved at the same time. However, it is difficult to improve the hydrogen embrittlement resistance characteristic under the environment where hydrogen is generated by corrosion of the steel sheet only by the collection effect by controlling the shape of the precipitate.

By the way, residual austenite had a tendency to be reduced by adversely affecting hydrogen embrittlement resistance in the past, but recently, attention has been paid to TRIP steel having residual austenite because residual austenite contributes to improvement of hydrogen embrittlement resistance. There is.

For example, Tomohiko Hojo and five others, "Hydrogen embrittlement of ultra-high-strength low alloy TRIP steel (1st hydrogen absorption characteristics and ductility)", Japan Society of Materials Research, 51st Academic Lecture, 2002, 8, p.17-18] and literature [Hojo Tomohiko et al., 5, "Influence of Osstem Treatment Temperature on Hydrogen Brittlement of Ultra High Strength Low Alloy TRIP Steels," CAMP-ISIJ, 2003, Vol. 16, p. [568] discusses the hydrogen embrittlement resistance characteristics of TRIP steels, and particularly, the amount of hydrogen occlusion of TBF steel is high, and when the wavefront of TBF steel is observed, the breakage of pseudo cleavage due to hydrogen occlusion is suppressed. However, it is difficult to say that the delayed fracture characteristics of the TBF steel reported in the above document sufficiently take into consideration the harsh use environment for a long time, such as an automotive part, at about 1000 seconds as the time to crack generation by the negative electrode charge test. In addition, since the heat treatment conditions in the document set the heating temperature high, there is also a problem such as poor production efficiency of the machine, and the development of new TBF steel excellent in production efficiency is urgently desired. In addition, there is a problem that the hydrogen embrittlement resistance is lowered by performing press molding or the like.

As described above, the TRIP steel sheet containing the retained austenite exhibits excellent workability in forming parts, and in consideration of the harsh use environment for a long time after molding, such as automotive parts, for hydrogen embrittlement after forming processing. Few development cases have been devised.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and its object is to produce an ultra-high strength steel sheet having a tensile strength of 1180 MPa or more, which exhibits excellent hydrogen embrittlement resistance and high machinability under severe use environment for a long time after molding the steel sheet into parts. To provide.

In order to achieve the above object, the present inventors earnestly studied to obtain a steel sheet exhibiting excellent hydrogen embrittlement resistance even after molding processing, and fully exhibiting excellent workability, which is a characteristic of TRIP steel sheet during molding processing. As a result, in order to secure the excellent hydrogen embrittlement resistance property after the molding process, it is very important to control the structure after the molding process. Specifically, the metal structure after the tensile processing is an area ratio with respect to the whole structure.

Residual austenite: 1% or more

Bainitic ferrite and martensite: 80% or more in total,

Ferrite and perlite: 9% or less (including 0%) in total, and

It was found that it is important to satisfy the average axial ratio (long axis / short axis) of the residual austenite grains: 5 or more.

That is, the first ultra-high strength thin steel sheet having excellent hydrogen embrittlement resistance according to the present invention is more than C 0.25% and 0.60% or less (meaning of the mass%, the same as the component composition below), Si 1.0 to 3.0%, Mn 1.0 to 3.5 %, P 0.15% or less, S 0.02% or less and Al 1.5% or less (not including 0%), and the balance consists of iron and inevitable impurities,

Metal structure after tensile processing of 3% of processing rates at area ratio with respect to whole structure,

Residual austenite: 1% or more

Bainitic ferrite and martensite: 80% or more in total,

Ferrite and perlite: 9% or less (including 0%) in total, and

The average axial ratio (long axis / short axis) of the residual austenite grains: 5 or more is satisfied,

It is also characterized by a tensile strength of 1180 MPa or more.

In addition, the present inventors earnestly studied from the above aspect, it is very important to control the structure after the molding process as follows in order to secure excellent hydrogen embrittlement resistance characteristics after the molding process, specifically, the structure after the molding process this,

Residual austenite: 1% or more

Average axial ratio (long axis / short axis) of the residual austenite grains: 5 or more,

Average short axis length of the retained austenite grains: 1 µm or less, and

The closest distance between the remaining austenite grains: 1 µm or less

It was found that it was important to satisfy all of them. By controlling the structure in this way, even if an alloying element is not added excessively, the hydrogen embrittlement resistance property in the ultrahigh strength steel sheet can be sufficiently improved. On the other hand, the "after the molding process" refers to after the tensile processing of 3% of the processing rate, the specific conditions of the tensile processing is to give a 3% strain (engineering) by the uniaxial tension at room temperature (hereinafter, Tensile processing with a 3% machining rate is sometimes referred to simply as "machining").

That is, the second ultra-high strength thin steel sheet having excellent hydrogen embrittlement resistance according to the present invention is more than C 0.25% 0.60% or less, Si 1.0 to 3.0%, Mn 1.0 to 3.5%, P 0.15% or less, S 0.02% or less and Al Satisfying 0.5% or less (not including 0%), and the balance consists of iron and unavoidable impurities,

Metal structure after tensile processing of 3% of processing rates,

Residual austenite: 1% or more by area of the whole tissue,

Average axial ratio (long axis / short axis) of the residual austenite grains: 5 or more,

Satisfies the average short axis length of the residual austenite grains: 1 µm or less, and the closest distance between the residual austenite grains: 1 µm or less,

It is further characterized by the fact that the tensile strength is 1180 MPa or more.

(Embodiment 1)

The first ultra-high strength thin steel sheet according to the present invention is more than C 0.25% 0.60% or less (meaning of the mass%, the same as the component composition below), Si 1.0 to 3.0%, Mn 1.0 to 3.5%, P 0.15% or less, S Satisfies 0.02% or less, Al 1.5% or less (does not contain 0%), Mo 1.0% or less (does not contain 0%), and Nb 0.1% or less (does not contain 0%), the balance is iron and It consists of inevitable impurities,

(i) The metal structure after tensile processing of 3% of the processing rate is the area ratio with respect to the whole structure,

Residual austenite: 1% or more

Bainitic ferrite and martensite: 80% or more in total,

Ferrite and perlite: 9% or less (including 0%) in total, and

The average axial ratio (long axis / short axis) of the residual austenite grains: 5 or more is satisfied,

(ii) The steel contains a prescribed amount of Mo and / or Nb.

Each requirement has the following meaning.

<Metal structure after tensile processing of 3% of processing rate>

As a result of various experiments assuming the machining situation of the actual part, the best correlation between the laboratory test and the crack of the actual part is obtained when tensile machining is performed at the machining rate of 3%. Because I did.

<Residual austenite: 1% or more by area ratio over whole tissue>

In order to exert excellent hydrogen embrittlement resistance even in a severe use environment for a long time after forming parts, it is necessary to first make residual austenite in the metal structure after processing to be 1% or more. The retained austenite is preferably at least 2%, more preferably at least 3%. On the other hand, if the amount of retained austenite after processing is large, desired ultra high strength cannot be ensured, so the upper limit is preferably set to 20% (more preferably 15%).

<Average axial ratio (long axis / short axis) of residual austenite grains: 5 or more>

If the residual austenite after processing is needle-shaped, the hydrogen trapping capacity is overwhelmingly larger than that of carbide, and especially in the case where the shape is 5 or more on average axial ratio (long axis / short axis), hydrogen invasion due to so-called atmospheric corrosion is substantially harmless and hydrogen embrittlement characteristics. It has been found that can be significantly improved. The average axial ratio of the retained austenite is preferably 10 or more, more preferably 15 or more.

The residual austenite means a region observed as FCC phase (face-centered cubic lattice) by the FE-SEM / EBSP method described later. As one specific example of the measurement by EBSP, the measurement target is an arbitrary measurement area (about 50 x 50 µm and a measurement interval of 0.1 µm) on a plane parallel to the rolled surface at a position of sheet thickness 1/4. Can be. On the other hand, when polishing to the measurement surface, electrolytic polishing is performed to prevent transformation of residual austenite (變態: crystal structure changes). Next, the electron beam is irradiated to the sample set in the mirror cylinder of SEM using "FE-SEM with an EBSP detector" (it mentions later for details). The EBSP image projected on the screen is photographed with a high sensitivity camera (VE-1000-SIT manufactured by Dage-MTI Inc.) and inserted into the computer as an image. Image analysis is performed by a computer, and color mapping of the FCC image determined by comparison with the pattern by simulation using a known crystal system (in the case of residual austenite, the FCC image (face-centered cubic lattice)) is performed. In this way, the area ratio of the mapped area | region is calculated | required, and this is set as "the area ratio of residual austenite." On the other hand, TIMSEM Laboratories Inc. OIM (Orientation Imaging Microscopy ^™ ) system was used as the hardware and software according to the above analysis.

In addition, the average axial ratio was measured by a transmission electron microscope (TEM) (magnification of 150,000 times), and the axial ratio was obtained by measuring the long and short axes of the residual austenite grains present at three arbitrarily selected views. It calculated and it was set as the average axial ratio.

<Bainitic Ferrite and Martensite: 80% or more in total>

In order to reduce the starting point of grain boundary fracture in steel and to reliably lower the diffusible hydrogen concentration to the level of detoxification, and to easily ensure ultra high strength, the martensite generally employed in high strength steels is used to form a matrix of metal structure after processing. It is preferable not to use the site single phase structure but to use the "two-phase structure of bainitic ferrite and martensite" mainly composed of bainitic ferrite.

In the case of the martensitic single-phase structure, carbides (eg, film-like cementite, etc.) are precipitated at grain boundaries, and grain boundary breakage is likely to occur, whereas "bainitic ferrite and martensite two-phase structure" mainly composed of bainitic ferrite, Since the bainitic ferrite is hard, the strength of the entire structure can be easily increased as in the case of the martensitic single phase, and since hydrogen is collected in large amounts at this potential, the hydrogen embrittlement resistance can be enhanced. In addition, the presence of the bainitic ferrite and the residual austenite described later makes it possible to prevent the formation of carbides, which are the starting point of grain boundary fracture, and the needle-like austenite tends to be formed at the boundaries of the acicular bainitic ferrite. There is also an advantage.

Accordingly, in the present invention, the two-phase structure of the bainitic ferrite and martensite is ensured to be 80% or more even after the tensile processing with a processing rate of 3%. Preferably it is 85% or more, More preferably, you may be 90% or more. On the other hand, the upper limit can be determined by the balance with other tissues (residual austenite), and the upper limit is controlled to 99% when it contains no tissues (ferrite, etc.) other than the residual austenite.

The bainitic ferrite as used in the present invention is a plate-like ferrite, which means a substructure having a high dislocation density, and is clearly distinguished from polygonal ferrite having no dislocation or having a very low substructure by SEM observation as follows. .

The area ratio of the bainitic ferrite structure is calculated as follows. That is, the steel is corroded with nital and an arbitrary measurement area (about 50 x 50 µm) in the plane parallel to the rolled surface at the position of 1/4 of the sheet thickness is scanned by SEM (Scanning Electron Microscope). ) It is calculated by observation (1500 times magnification).

Although the bainitic ferrite shows a dark gray color in the SEM image (in the case of SEM, the bainitic ferrite may not be distinguished from the retained austenite or martensite), but the polygonal ferrite is black in the SEM image and has a polygonal shape. It does not contain residual austenite or martensite inside.

The SEM used in the present invention is a "High Resolution FE-SEM (Field Emission Type Scanning Electron Microscope, Philips, XL30S-FEG) equipped with an Electron Back Scatter Diffraction Pattern (EBSP) detector". The advantage is that it can be interpreted by the EBSP detector at the same time. Herein, the EBSP method will be described briefly. The EBSP determines the direction of determination of the electron beam incidence position by analyzing the Kikuchi pattern obtained from the reflected electrons generated at this time by injecting an electron beam to the sample surface. The orientation distribution of the sample surface can be measured by scanning in two dimensions at a predetermined pitch, and according to the EBSP observation, the plate orientation direction in which the crystal orientation differences are different as the tissues judged to be the same in the conventional microscope observation. There is an advantage that the tissue of can be identified by the color tone difference.

<Ferrite and pearlite: 9% or less in total (including 0%)>

The metal structure after the processing may be composed of only the above structure (i.e., mixed structure of bainitic ferrite + martensite and residual austenite), but the ferrite (where, The term " ferrite " may mean polygonal ferrite, i.e. a ferrite having no or no dislocation density) or pearlite. These are tissues which may inevitably remain in the manufacturing process of the present invention, but the smaller the number, the more preferable, and the present invention is suppressed to 9% or less. It is preferably less than 5%, more preferably less than 3%.

Thus, in order to ensure excellent hydrogen embrittlement resistance even after the molding process, for example, the presence of a large amount of retained austenite in the steel sheet before the molding process is 5% or more, or the presence of the retained austenite in a large amount and fine dispersion. Can be mentioned. In addition, controlling the conditions at the time of forming processing (for example, processing by bending forming, controlling the molding temperature or deformation rate), etc., so that residual austenite is hardly transformed, etc. may be used. In order to make austenite nearly constant within an appropriate amount range and to improve the workability and hydrogen embrittlement resistance after molding at the same time while securing other characteristics (high strength, etc.), the following particular requirements (A) and (B) are satisfied. Is recommended.

(A) The chemical component is made high C system and C concentration in residual austenite is raised.

The residual austenite is transformed to martensite by deformation (processing) of the steel sheet, but when the amount of C in the retained austenite is high, it is stable and difficult to be transformed more than necessary. As a result, residual austenite can be secured after the molding process, and excellent hydrogen embrittlement resistance can be maintained.

In the present invention, C is contained in an amount greater than 0.25% in order to obtain such an effect. C is also an element necessary for securing high strength of 1180 MPa or more. Preferably it is 0.27% or more, More preferably, it is 0.30% or more. However, in view of securing corrosion resistance, the amount of C is suppressed to 0.60% or less. Preferably it is 0.55% or less, More preferably, it is 0.50% or less.

Thus, it is recommended to raise C content in a steel plate and to make C concentration (CγR) in residual austenite into 0.8% or more. By controlling CγR to 0.8% or more, the elongation rate can be effectively increased. Preferably it is 1.0% or more, More preferably, it is 1.2% or more. On the other hand, although the said C (gamma) R is so preferable that it is high, it is thought that the upper limit which can actually adjust in operation is roughly 1.6%.

(B) It is needle-shaped while refine | miniaturizing the shape of residual austenite.

If the shape of the retained austenite is needle-shaped while miniaturizing the shape, the retained austenite can be secured because it is not transformed more than necessary during processing.

In conventional TRIP steels, although sufficient residual austenite is present, hydrogen embrittlement resistance may be undesirable. For this reason, the residual austenite present in conventional TRIP steels generally has a micron size (agglomerate level). It is easy to transform into martensite at the time of a stress load because it is a block of a true state, and it is easy to become a starting point of mechanical breakdown because of its shape. As a result of the investigation by the present inventors, it was found that when the retained austenite is needle-shaped, it is more difficult to transform into martensite than the conventional bulk retained austenite even when the same amount of deformation is stable. As a mechanism of the above phenomenon, it is inferred to be due to the difference in the manner in which the stress due to the shape action is applied or the space restraint, but it is not completely explained. On the other hand, stabilization of residual austenite at the time of processing does not affect the fall of the organic transformation processability of a TRIP steel plate. In the present invention, when the austenite is made fine in the form of needles as described above, the organic transformation is efficiently performed with little reduction of the retained austenite, thereby exhibiting excellent workability.

Specifically, when the average axle ratio (long axis / short axis) of the retained austenite grains is 5 or more (preferably 10 or more, more preferably 15 or more), needle-shaped retained austenite reduces the amount of retained austenite during processing. In addition, the average axial ratio (long axis / short axis) of residual austenite grains can be easily achieved even after processing, and the hydrogen absorbing ability inherent in the residual austenite is sufficiently exhibited to significantly improve hydrogen embrittlement resistance. Can be. On the other hand, the upper limit of the average axial ratio is not particularly defined in terms of enhancing the hydrogen embrittlement resistance characteristics, but in order to effectively exhibit the TRIP effect during processing, the thickness of the retained austenite is required to some extent. It is preferable to set it as 30, More preferably, it is 20 or less.

In a preferred embodiment of the invention, Mo and Nb are included for residual austenite refinement. Mo has the effect of suppressing hydrogen embrittlement by strengthening the grain boundary in addition to miniaturization of residual austenite. Moreover, it is an element effective also in improving the hardenability of a steel plate. In order to exhibit such an effect effectively, it is recommended to contain Mo 0.005% or more. More preferably, it is 0.1% or more. However, even if Mo amount exceeds 1.0%, the above effect is saturated and economically useless. Preferably it is 0.8% or less, More preferably, you may be 0.5% or less.

In addition, Nb is particularly effective in refining tissues by the complex effect with Mo. Nb also has the effect of increasing the strength of the steel sheet, and in order to exhibit these effects, it is recommended to contain 0.005% or more. More preferably, it is 0.01% or more. However, even when Nb is contained in excess, these effects are saturated and suppressed to 0.1% or less because they are economically useless. Preferably it is 0.08% or less.

On the other hand, in order to easily obtain the structure after the above-described processing, the structure other than the above retained austenite of the steel sheet before processing is 50% or more (preferably 85% or more, more preferably 90) in total in bainitic ferrite and martensite: % Or more), ferrite and pearlite: It is recommended to be 9% or less in total (preferably less than 5%, more preferably less than 3% and including 0%). This is because, of course, it is preferable that the hydrogen embrittlement resistance is excellent even before processing, and the strength to be specified can be easily achieved.

Although the present embodiment is characterized in that the metal structure after processing is controlled as described above, the metal structure can be easily formed to efficiently increase the hydrogen embrittlement resistance and strength, and to secure the ductility required for the thin steel sheet. The other components should be controlled as follows.

<1.0 to 3.0% of Si>

Si is an important element that effectively suppresses decomposition of residual austenite to form carbides. It is also a substitutional solid solution strengthening element effective for sufficiently hardening the material. In order to express such an effect effectively, it is necessary to contain 1.0% or more. Preferably it is 1.2% or more, More preferably, it is 1.5% or more. However, if the amount of Si is excessive, scale formation in hot rolling becomes remarkable, and it is suppressed to 3.0% or less since it is cost-effective and economically unfavorable to remove a flaw. Preferably it is 2.5% or less, More preferably, it is 2.0% or less.

<Mn 1.0 to 3.5%>

Mn is an element necessary for stabilizing austenite to obtain the desired residual austenite. In order to exhibit such an effect effectively, it is necessary to contain 1.0% or more. Preferably it is 1.2% or more, More preferably, it is 1.5% or more. On the other hand, when the amount of Mn becomes excessive, segregation becomes remarkable and workability may deteriorate, so 3.5% is taken as an upper limit. Preferably it is 3.0% or less.

<P 0.15% or less (0% not included)>

Since P is an element that promotes grain boundary fracture due to grain boundary segregation, the smaller one is preferable, and the upper limit thereof is 0.15%. It is preferably at most 0.1%, more preferably at most 0.05%.

<S 0.02% or less (does not include 0%)>

Since S is an element which promotes hydrogen absorption of the steel sheet in a corrosive environment, the smaller one is preferable, and the upper limit thereof is made 0.02%.

<1.5% Al (0% not included)> (for steel sheet 1 of the present invention)

<Al 0.5% or less (0% not included)> (for steel sheet 2 of the present invention)

Al may add 0.01% or more for deoxidation. In addition, Al is an element having not only deoxidation but also corrosion resistance and hydrogen embrittlement resistance.

As a mechanism of the corrosion resistance improvement effect, the effect by the produced | generated rust produced by the corrosion resistance improvement and atmospheric corrosion of the base material itself can be considered specifically, It is estimated that the effect by the latter produced | generated rust is especially large. As a reason, since the produced rust is more dense and superior in protection than conventional iron rust, atmospheric corrosion is suppressed, and as a result, the amount of hydrogen generated by the atmospheric corrosion is reduced, and hydrogen embrittlement, that is, delayed destruction is effectively suppressed. I think.

In addition, although the details of the mechanism of improving the hydrogen embrittlement resistance of Al are not clear, the concentration of Al on the surface of the steel sheet makes it difficult to penetrate hydrogen into the steel, and the diffusion rate of hydrogen in the steel decreases, and the movement of hydrogen is reduced. It is estimated that hydrogen embrittlement is difficult to occur due to difficulty. In addition, the increase in the stability of acicular residual austenite by Al addition is thought to contribute to the improvement of hydrogen embrittlement resistance.

In order to effectively exhibit such corrosion resistance improvement effect and hydrogen embrittlement resistance improvement effect of Al, it is preferable to make Al amount into 0.02% or more, Preferably it is 0.2% or more, More preferably, it is 0.5% or more.

However, in order to secure the workability by suppressing the increase and the enlarging of inclusions such as alumina, to secure the production of fine residual austenite, and to suppress the corrosion based on the Al-containing inclusions and to suppress the increase in manufacturing cost, It is necessary to suppress Al amount to 1.5% or less. It is preferable to adjust so that A3 point may be 1000 degrees C or less from a manufacturing viewpoint.

On the other hand, when the Al content increases as described above, inclusions such as alumina increase and workability deteriorates. Therefore, in order to sufficiently suppress inclusions such as alumina and to obtain a steel sheet having better workability, the amount of Al is suppressed to 0.5% or less. . Preferably it is 0.3% or less, More preferably, it is 0.1% or less.

The containing elements (C, Si, Mn, P, S, Al, Mo, Nb) defined in the present embodiment are as described above, and the balance component is substantially Fe, but the raw materials, materials, manufacturing facilities, etc. It is allowed to include N (nitrogen) of 0.001% or less as an unavoidable impurity included depending on the situation, and actively contains another element as follows in the range that does not adversely affect the operation of the present invention. It is also possible.

<B 0.0002 to 0.01%>

B is an element effective for increasing the strength of the steel sheet, and in order to exhibit the above effects, it is preferable to contain B at least 0.0002% (more preferably at least 0.0005%). On the other hand, when B is contained excessively, since hot workability deteriorates, it is preferable to make it contain in 0.01% or less (more preferably 0.005% or less).

<1 or more types selected from the group consisting of Ca 0.0005 to 0.005%, Mg 0.0005 to 0.01%, and REM 0.0005 to 0.01%>

Ca, Mg, and REM (rare earth elements) are effective elements for suppressing an increase in hydrogen ion concentration in an interfacial atmosphere due to corrosion of the steel plate surface, that is, suppressing a decrease in pH to increase corrosion resistance of the steel sheet. In addition, it is effective for controlling the form of sulfide in steel to improve workability, and in order to fully exhibit the effect, it is preferable to contain 0.0005% or more in any of Ca, Mg and REM. On the other hand, if excessively contained, workability is deteriorated. Therefore, Ca is preferably 0.005% or less, and Mg and REM are preferably 0.01% or less, respectively.

(Embodiment 2)

The second ultra-high strength thin steel sheet according to the present invention has a C 0.25% or more and 0.60% or less (meaning of the mass%, the same as the component composition below), Si 1.0 to 3.0%, Mn 1.0 to 3.5%, P 0.15% or less, S Satisfies 0.02% or less and Al 1.5% or less (not including 0%), and the balance is made of iron and inevitable impurities,

(i) the structure after the molding process,

Residual austenite: 1% or more

Average axial ratio (long axis / short axis) of the residual austenite grains: 5 or more,

Bainitic ferrite and martensite: 80% or more in total,

Ferrites and Perlites: 9% or less (including 0%) in total

Satisfied, and also

(ii) The steel contains a prescribed amount of Cu and / or Ni.

The reason for defining the above (i) is as described above.

Next, the reason which prescribes said (ii) is explained in full detail.

As described above, while maintaining the retained austenite after processing, the shape is controlled to improve the hydrogen trapping ability,

(a) fully suppressing the generation of hydrogen from the steel in a corrosive environment;

(b) inhibit the intrusion of generated hydrogen into steel

In order to surely reduce the diffusible hydrogen concentration in the steel sheet to the level of detoxification, specific means were examined.

As a result, in order to achieve the above (a) and (b), it is very effective to contain 0.003 to 0.5% of Cu and / or 0.003 to 1.0% of Ni. Hydrogen embrittlement under control of structure by making the element an essential component It was found that the improvement of the characteristics can be further enhanced.

In detail, since corrosion resistance of steel material itself improves by presence of Cu and Ni, hydrogen generation by corrosion of a steel plate can fully be suppressed. In addition, these elements also have the effect of promoting the production of iron oxide α-FeOOH, which is thermodynamically stable and protective among the rust generated in the atmosphere, and suppresses the intrusion of hydrogen generated into the steel sheet by promoting the formation of the rust. It is possible to sufficiently improve the hydrogen embrittlement resistance in harsh corrosive environments. The said effect is especially easy to express by coexisting Cu and Ni.

In order to exhibit the said effect, when it contains Cu, it is necessary to set it as 0.003% or more. Preferably it is 0.05% or more, More preferably, it is 0.1% or more. In addition, when it contains Ni, it is necessary to set it as 0.003% or more. Preferably it is 0.05% or more, More preferably, it is 0.1% or more.

On the other hand, if any element is excessively contained, the workability is lowered, so it is suppressed to 0.5% or less in the case of Cu and 1.0% or less in the case of Ni.

As described in the above (i), in order to achieve excellent hydrogen embrittlement resistance by securing a predetermined residual austenite after the molding process, for example, a large amount of residual austenite occupied in the steel sheet before the molding process is present in 5% or more. The presence of austenite in a large amount and fine dispersion is mentioned. In addition, it is possible to control the conditions during molding so that residual austenite is hardly transformed (for example, to be processed by bending molding, or to control the molding temperature and the deformation rate). Particularly satisfying the above-mentioned (A) and (B) as a specific means in order to make residual austenite nearly constant within an appropriate range and to improve processability and hydrogen embrittlement resistance after molding simultaneously while securing other characteristics (high strength, etc.). It is recommended.

Although the present embodiment is characterized in that the metal structure after processing is controlled as described above and contains a prescribed amount of Cu and / or Ni, the metal structure can be easily formed to effectively improve hydrogen embrittlement resistance and strength. In order to increase and further secure the ductility required for the steel sheet, other components must be controlled as described above.

Containing elements (C, Si, Mn, P, S, Al and Cu and / or Ni) defined in the present embodiment are as described above, and the balance component is substantially Fe, but the raw material, material, and manufacture in steel It is allowed to include N (nitrogen) of 0.001% or less as an unavoidable impurity included in accordance with the situation of the facility, and of course, other elements may be further added as follows in the range that does not adversely affect the operation of the present invention. It is also possible to contain actively.

<Ti and / or V: 0.003 to 1.0% in total>

Ti has the effect of promoting the formation of protective rust similarly to Cu and Ni. The protective rust has a particularly beneficial effect of inhibiting the production of β-FeOOH, which is produced under a chloride environment, which adversely affects corrosion resistance (resulting in hydrogen embrittlement resistance). The formation of such protective rust is particularly promoted by the complex addition of Ti and V (or Zr). Ti is also an element which gives very excellent corrosion resistance and also has the advantage of cleaning steel.

In addition, as mentioned above, V is an element that is effective in increasing strength and refining of the steel sheet in addition to coexisting with Ti to improve hydrogen embrittlement resistance.

In order to fully exhibit the effects of Ti and / or V, the total content is preferably 0.003% or more (more preferably 0.01% or more). Particularly, in view of improving hydrogen embrittlement resistance, it is preferable to add Ti to more than 0.03%, and more preferably, 0.05% or more of Ti. On the other hand, even if Ti is added excessively, the effect is saturated, so it is not economically desirable. Moreover, when V is added excessively, precipitation of carbonitride increases, resulting in deterioration of workability and hydrogen embrittlement resistance. Therefore, it is preferable to add Ti and / or V within the range of 1.0% or less in total. More preferably, it is 0.5% or less.

<Zr 0.003 to 1.0%>

Zr is an effective element for increasing the strength and refining of the steel sheet, and coexists with Ti to improve hydrogen embrittlement resistance. In order to exhibit such an effect effectively, it is preferable to contain Zr 0.003% or more. On the other hand, when Zr is contained excessively, it is preferable to add it within 1.0% or less because precipitation of carbonitride increases and workability and hydrogen embrittlement resistance fall.

<Mo 1.0% or less (does not include 0%)>

Mo is effective in stabilizing austenite to secure residual austenite and inhibiting hydrogen intrusion to improve hydrogen embrittlement resistance. It is also an effective element for increasing the hardenability of steel sheets. In addition, it is effective in suppressing hydrogen embrittlement by strengthening grain boundaries. In order to exhibit such an effect effectively, it is recommended to contain Mo 0.005% or more. More preferably, it is 0.1% or more. However, even if Mo amount exceeds 1.0%, the above effect is saturated and economically useless. Preferably it is 0.8% or less, More preferably, you may be 0.5% or less.

<Nb 0.1% or less (does not include 0%)>

Nb is a very effective element for increasing the strength of steel sheet and refining the structure. Particularly, the effect is sufficiently exhibited by the complex addition with Mo. In order to exhibit such an effect, it is recommended to contain 0.005% or more. More preferably, it is 0.01% or more. However, even when Nb is contained in excess, these effects are saturated and suppressed to 0.1% or less because they are economically useless. Preferably it is 0.08% or less.

<B 0.0002 to 0.01%>

B is an element effective for increasing the strength of the steel sheet, and in order to exhibit the above effects, it is preferable to contain B at least 0.0002% (more preferably at least 0.0005%). On the other hand, when B is contained excessively, since hot workability deteriorates, it is preferable to make it contain in 0.01% or less (more preferably 0.005% or less).

<1 or more types selected from the group consisting of Ca 0.0005 to 0.005%, Mg 0.0005 to 0.01%, and REM 0.0005 to 0.01%>

Ca, Mg, and REM (rare earth elements) are effective elements for suppressing an increase in hydrogen ion concentration in an interfacial atmosphere due to corrosion of the steel plate surface, that is, suppressing a decrease in pH to increase corrosion resistance of the steel sheet. In addition, it is effective for controlling the form of sulfide in steel to improve workability, and in order to fully exhibit the effect, it is preferable to contain 0.0005% or more in any of Ca, Mg and REM. On the other hand, if excessively contained, workability is deteriorated. Therefore, Ca is preferably 0.005% or less, and Mg and REM are preferably 0.01% or less, respectively.

(Embodiment 3)

The third ultra-high strength thin steel sheet according to the present invention is C 0.25% or more and 0.60% or less (meaning of the mass%, the same as the component composition below), Si 1.0 to 3.0%, Mn 1.0 to 3.5%, P 0.15% or less, S Satisfies 0.02% or less and Al 1.5% or less (not including 0%), and the balance is made of iron and inevitable impurities,

(iii) the structure after the molding process,

Residual austenite: 1% or more

Average axial ratio (long axis / short axis) of the residual austenite grains: 5 or more,

Average short axis length of the retained austenite grains: 1 µm or less, and

Nearest distance between the retained austenite grains: 1 µm or less

Satisfies all. By controlling the structure in this way, even if an alloying element is not added excessively, the hydrogen embrittlement resistance property in the ultrahigh strength steel sheet can be sufficiently improved.

On the other hand, the "after the molding process" refers to after the tensile processing of 3% of the processing rate as described above, the specific conditions of the tensile processing is to give a 3% strain (engineering) by uniaxial tension at room temperature (hereinafter, Tensile processing with a 3% machining rate is sometimes referred to simply as "machining").

Hereinafter, the said definition of the retained austenite in this invention is explained in full detail.

<Residual austenite: 1% or more>

<Average axial ratio (long axis / short axis) of residual austenite grains: 5 or more>

In order to exert excellent hydrogen embrittlement resistance even in a severe use environment for a long time after forming parts, it is necessary to first make residual austenite in the metal structure after processing to be 1% or more. Residual austenite not only contributes greatly to the improvement of the hydrogen embrittlement resistance as described above, but is also useful for improving the overall elongation as is generally known, preferably 2% or more, more preferably 3% or more. desirable. On the other hand, if a large amount of retained austenite is present, desired ultra high strength cannot be secured, so the upper limit is preferably set to 15% (more preferably 10%).

And if the retained austenite after needle processing is needle-shaped, hydrogen collection capability will be overwhelmingly larger than carbide. 1 is an average axial ratio of residual austenite grains measured by the method described below, and the hydrogen embrittlement risk index, which is an indicator of hydrogen embrittlement resistance characteristics (measured by the method shown in Examples described later. The graph shows the relationship between the two layers, which indicates that the hydrogen embrittlement risk index decreases sharply when the average axial ratio (long axis / short axis) of the retained austenite grains is 5 or more. This is because the average axial ratio of the retained austenite grains is increased to 5 or more, thereby exhibiting sufficient hydrogen absorbing ability of the retained austenite, and the hydrogen trapping ability is overwhelmingly larger than that of carbides. It is considered that the conversion characteristics have a significant improvement effect. The average axial ratio of the retained austenite is preferably 10 or more, more preferably 15 or more.

<Average short axis length of residual austenite grains: 1 µm or less>

In addition, in the present invention, fine dispersion of the needle-like retained austenite is effective for the improvement of hydrogen embrittlement resistance, and specifically, the needle-like retained austenite grain is 1 µm or less (submicron size). It was found that by dispersing, the hydrogen embrittlement resistance can be surely improved. This is considered to be because the surface area (interface) of the retained austenite crystal grains is increased in the case where a large number of fine retained austenite crystal grains having a short average shortening length are dispersed, thereby increasing the hydrogen trapping ability. The average short axis length of the retained austenite grains is preferably 0.5 µm or less, more preferably 0.25 µm or less.

In the present invention, as described above, by controlling the average axial length of the retained austenite grains together with the average axial ratio, the hydrogen trapping ability of the fine acicular austenite of the present invention is carbide even when residual austenite of the same volume ratio is present. Can be made overwhelmingly larger than in the case of dispersing, so that hydrogen invading by atmospheric corrosion can be substantially harmless.

<Closest distance between residual austenite grains: 1 µm or less>

In the present invention, it has also been found that by controlling the closest distance between the retained austenite grains, the hydrogen embrittlement resistance can be further improved. Specifically, it was found that the hydrogen embrittlement resistance can be surely improved when the closest distance between the residual austenite grains is 1 µm or less. This is considered to be because the propagation of fracture (crack) is suppressed by forming a state in which the fine acicular needle-like austenite is dispersed in large numbers, and a structure having high resistance to fracture is obtained. The closest distance between the retained austenite grains is preferably 0.8 µm or less, more preferably 0.5 µm or less.

The residual austenite means a region observed as FCC phase (face-centered cubic lattice) by the FE-SEM / EBSP method described later. As one specific example of the measurement by EBSP, the measurement target is an arbitrary measurement area (about 50 x 50 µm and a measurement interval of 0.1 µm) on a plane parallel to the rolled surface at a position of sheet thickness 1/4. Can be. On the other hand, when polishing to the measurement surface, electrolytic polishing is performed to prevent transformation of residual austenite. Next, the electron beam is irradiated to the sample set in the mirror cylinder of SEM using "FE-SEM with an EBSP detector" (it mentions later for details). The EBSP image projected on the screen is photographed with a high sensitivity camera (VE-1000-SIT manufactured by Dage-MTI Inc.) and inserted into the computer as an image. Image analysis is performed by a computer, and color mapping of the FCC image determined by comparison with the pattern by simulation using a known crystal system (in the case of residual austenite, the FCC image (face-centered cubic lattice)) is performed. In this way, the area ratio of the mapped area | region is calculated | required, and this is set as "the area ratio of residual austenite." On the other hand, TIMSEM Laboratories Inc. OIM (Orientation Imaging Microscopy ^™ ) system was used as the hardware and software according to the above analysis.

In addition, the average axial ratio, average short axis length, and the closest distance between the retained austenite grains of the retained austenite grains were obtained as follows. First, the average axial ratio of residual austenite grains was observed with a transmission electron microscope (TEM) (magnification of 150,000 times), and the axial ratio was determined by measuring the long and short axes of the residual austenite grains present at three arbitrarily selected views. Was computed and it was set as the average axial ratio. The average short axis length of the retained austenite grains was calculated by calculating the average value of the short axes measured as described above. In addition, the closest distance between the retained austenite grains was observed by TEM (10,000 times magnification), and in three fields selected arbitrarily, as shown by (a) in FIG. 11, parallel to the major axis direction (that is, (b in FIG. 11). The distance such as) is not the closest distance.] The closest distance of the retained austenite grains was measured and averaged by the closest distances of the three visual fields.

In order to reduce the starting point of grain boundary fracture in steel and to ensure that the diffusible hydrogen concentration is lowered to the detoxification level and to ensure ultra-high strength easily, the martensite generally employed in high-strength steels is used. It is preferable not to use the site single phase structure but to use the "two-phase structure of bainitic ferrite and martensite" mainly composed of bainitic ferrite.

In the case of the martensitic single-phase structure, carbides (eg, cementite, etc. on the film) tend to precipitate at grain boundaries, and grain boundary breakage is likely to occur, whereas bainitic ferrite is mainly composed of "two-phase structure of bainitic ferrite and martensite". When the bainitic ferrite is hard, the strength of the entire structure can be easily increased in the same way as in the case of martensite single phase, and since hydrogen is collected at this potential, hydrogen embrittlement resistance can be enhanced. . In addition, the presence of the bainitic ferrite and the residual austenite described later makes it possible to prevent the formation of carbides, which are the starting point of grain boundary fracture, and the needle-like austenite tends to be formed at the boundary of the acicular bainitic ferrite. There is also an advantage.

Accordingly, in the present invention, the two-phase structure of the bainitic ferrite and martensite is ensured to be 80% or more even after the tensile processing with a processing rate of 3%. Preferably it is 85% or more, More preferably, you may be 90% or more. On the other hand, the upper limit can be determined by the balance with other tissues (residual austenite), and the upper limit is controlled to 99% when it contains no tissues (ferrite, etc.) other than the residual austenite.

The bainitic ferrite as used in the present invention is a plate-like ferrite, which means a substructure having a high dislocation density, and is clearly distinguished from polygonal ferrite having no dislocation or having a very low substructure by SEM observation as follows. .

The area ratio of the bainitic ferrite structure is calculated as follows. That is, the steel is corroded with nital and an arbitrary measurement area (about 50 x 50 µm) in the plane parallel to the rolled surface at the position of 1/4 of the sheet thickness is scanned by SEM (Scanning Electron Microscope). ) It is calculated by observation (1500 times magnification).

Although the bainitic ferrite shows a dark gray color in the SEM image (in the case of SEM, the bainitic ferrite may not be distinguished from the retained austenite or martensite), but the polygonal ferrite is black in the SEM image and has a polygonal shape. It does not contain residual austenite or martensite inside.

The SEM used in the present invention is a "High Resolution FE-SEM (Field Emission Type Scanning Electron Microscope, Philips, XL30S-FEG) equipped with an Electron Back Scatter Diffraction Pattern (EBSP) detector". The advantage is that it can be interpreted by the EBSP detector at the same time. Here, the EBSP method will be described briefly. The EBSP determines the direction of determination of the electron beam incident position by injecting an electron beam onto the sample surface and analyzing the Kikuchi pattern obtained from the reflected electrons generated at this time. When scanning is carried out and the crystal direction is measured for every predetermined pitch, the azimuth distribution of the sample surface can be measured. According to this EBSP observation, there is an advantage that the tissue in the plate thickness direction having different crystal orientation differences as the tissues judged to be the same in the ordinary microscope observation can be identified by the color tone difference.

The metal structure after the processing may be composed of only the above structure (i.e., mixed structure of bainitic ferrite + martensite and residual austenite), but the ferrite (where, The term " ferrite " may mean polygonal ferrite, i.e. a ferrite having no or no dislocation density) or pearlite. These are tissues which may inevitably remain in the manufacturing process of the present invention, but the smaller the number, the more preferable, and the present invention is suppressed to 9% or less. It is preferably less than 5%, more preferably less than 3%.

Thus, in order to ensure excellent hydrogen embrittlement resistance even after the molding process, for example, the presence of a large amount of retained austenite in the steel sheet before the molding process is 5% or more, or the presence of the retained austenite in a large amount and fine dispersion. Can be mentioned. In addition, controlling the conditions at the time of forming processing (for example, processing by bending forming, controlling the molding temperature or deformation rate), etc., so that residual austenite is hardly transformed, etc. may be used. In order to make austenite nearly constant within an appropriate amount range and to improve the workability and hydrogen embrittlement resistance after molding at the same time while securing other characteristics (high strength, etc.), the following particular requirements (A) and (B) are satisfied. It is likewise recommended.

Although the present embodiment is characterized in that the metal structure after processing is controlled as described above, the metal structure can be easily formed to efficiently increase the hydrogen embrittlement resistance and strength, and to secure the ductility required for the thin steel sheet. It goes without saying that the other components must be controlled as described above.

Although the present invention does not prescribe the manufacturing conditions, it can be easily processed using a steel material that satisfies the above composition, and at the same time after the hot rolling in order to form the structure having ultra high strength and excellent hydrogen embrittlement resistance after processing. Or after the cold rolling carried out afterwards, it is recommended to perform heat treatment in the following manner. That is, Ms point-Bs point at the average cooling rate of 3 degree-C / s or more after holding and maintaining the steel which satisfy | fills the above-mentioned component composition for 10 to 1800 second (t1) at temperature T1 of A3 point-(A3 point +50 degreeC). It is recommended to cool it to the temperature T2 of and to keep heating (t2) for 60 to 1800 seconds in the said temperature range.

When T1 exceeds (A3 point + 50 ° C) or t1 exceeds 1800 seconds, it is not preferable because it causes grain growth of austenite and deteriorates workability (elongation flange property). On the other hand, if the T1 is lower than the temperature at the A3 point, no predetermined bainitic ferrite structure is obtained. In addition, when t1 is less than 10 seconds, it is not preferable because austenitization is not sufficiently performed and cementite or other alloy carbides remain. Preferably, t1 is 30 seconds or more and 600 seconds or less, and more preferably 60 seconds or more and 400 seconds or less.

Subsequently, the steel sheet is cooled, and cooling at an average cooling rate of 3 ° C./s or more is for avoiding the pearlite transformation region and preventing the formation of pearlite structure. This average cooling rate is so good that it is large, Preferably it is 5 degrees C / s or more, More preferably, it is recommended to set it as 10 degrees C / s or more.

Next, after cooling at the said speed | rate to Ms point-Bs point, a constant temperature transformation can make a mother phase into the biphasic structure of bainitic ferrite + martensite. When the heating holding temperature T2 here exceeds the Bs point, a large amount of undesirable pearlite is produced in the present invention, and the bainitic ferrite structure cannot be sufficiently secured. On the other hand, if the T2 is less than the Ms point, the residual austenite decreases, which is not preferable.

In addition, when the heating holding time t2 exceeds 1800 seconds, the dislocation density of the bainitic ferrite becomes small, the amount of hydrogen trapped decreases, and a predetermined residual austenite is not obtained. Further, even if t2 is less than 60 seconds, a predetermined bainitic ferrite structure is not obtained. Preferably, said t2 is made into 90 second or more and 1200 second or less, More preferably, it is 120 second or more and 600 second or less. It does not specifically limit about the cooling method after heat holding, Air cooling, quenching, water cooling, etc. can be performed.

In consideration of practical operation, the annealing treatment can be easily carried out using a continuous annealing facility or a batch annealing facility. In addition, in the case of performing hot dip galvanizing by plating the cold rolled plate, the plating condition may be set to satisfy the heat treatment condition, and the heat treatment may be performed in the plating process.

In addition, the above-mentioned hot rolling process (cold rolling process as needed) before continuous annealing treatment is not specifically limited, Usually, the conditions implemented are selected suitably and can be employ | adopted. Specifically, as the hot rolling step, for example, after completion of hot rolling at an Ar 3 point or more, cooling at an average cooling rate of about 30 ° C./s and winding at a temperature of about 500 to 600 ° C. may be adopted. In addition, when the shape after hot rolling is bad, cold rolling may be performed for the purpose of shape correction. Here, it is recommended that the cold rolling rate be performed at 1 to 70%. This is because cold rolling with a cold rolling ratio of more than 70% increases the rolling load and makes rolling difficult.

Although the present invention is directed to a thin steel sheet, the form of the product is not particularly limited, and in addition to a steel sheet obtained by hot rolling or a steel sheet obtained by further cold rolling, annealing is performed after hot rolling or cold rolling. A chemical conversion treatment may be performed, plating by hot dip plating, electroplating, vapor deposition, or the like, various coatings, unpainting treatments, organic coating treatments, or the like may be performed.

As a kind of said plating, any of general zinc plating, aluminum plating, etc. is preferable. Moreover, as for the plating method, any of hot dip plating and electroplating is preferable, and also alloying heat treatment may be performed after plating, and multilayer plating may be performed. Moreover, a film lamination process can also be performed on an unplated steel plate or a plated steel plate.

When the coating is performed, chemical conversion treatment such as phosphate treatment may be performed or electrodeposition coating may be performed according to various uses. As the paint, a known resin can be used, and an epoxy resin, a fluorine-containing resin, a silicone acrylic resin, a polyurethane resin, an acrylic resin, a polyester resin, a phenol resin, an alkyd resin, a melamine resin, and the like can be used together with a known curing agent. . In particular, in view of corrosion resistance, use of an epoxy resin, a fluorine-containing resin, and a silicone acrylic resin is recommended. In addition, well-known additives added to the paint, such as coloring pigments, coupling agents, leveling agents, sensitizers, antioxidants, ultraviolet stabilizers, flame retardants, and the like, may be added.

Moreover, the form of coating is not specifically limited, either, A solvent type coating material, powder coating material, water based coating material, water dispersion type coating material, electrodeposition coating material, etc. can be selected suitably. In order to form a desired coating layer in steel materials using the said coating material, it is preferable to use well-known methods, such as the immersion method, the roll coater method, the spraying method, and the curtain flow coater method. It is preferable that the thickness of the coating layer employ | adopt an appropriate value well-known according to a use.

The ultra-high strength steel sheet of the present invention can be applied to interior parts such as seat rails as well as automobile strength parts such as reinforcement members of automobiles such as bumpers, door impact beams, fillers, rain forces, and members. Also in the component obtained by shaping | molding process, while exhibiting sufficient material characteristic (strength), it exhibits the outstanding hydrogen embrittlement characteristic.

Hereinafter, although an Example is given and this invention is demonstrated further more concretely, this invention is not limited to the following example, of course, It can also be changed suitably and implemented in the range which may be suitable for the meaning of the previous and the latter, and these are all It is included in the technical scope of the present invention.

Example 1

Test steel No. which consists of the component composition of Table 1 After vacuum-solving A-1 to Y-1 to an experimental slab, a hot rolled steel sheet having a plate thickness of 3.2 mm was obtained according to the following process (hot rolling → cold rolling → continuous annealing), and then the surface scale was removed by pickling. After the cold rolled until the thickness was 1.2mm.

Hot Rolling Process

Start temperature (SRT): hold for 30 minutes at 1150 to 1250 ° C

Finishing Temperature (FDT): 850 ℃

Cooling rate: 40 ℃ / s

Coiling temperature: 550 ℃

Cold rolling process

Cold Rolling Rate: 50%

<Continuous Annealing Process>

After holding for 120 seconds at A3 point + 30 degreeC with respect to each test steel, it air-cooled to To degree of Table 2 at the average cooling rate of 20 degree-C / s, and hold | maintained at the said temperature to 240 degreeC. Thereafter, the water was cooled to room temperature.

On the other hand, No. In 116, as a comparative example, in order to produce martensitic steel, which is a conventional high strength steel, the cold rolled steel sheet was heated to 830 ° C, held for 5 minutes, and then quenched with water, and tempered at 300 ° C for 10 minutes. Also no. In 120, the steel plate after cold rolling was heated to 800 degreeC, hold | maintained for 120 second, and then cooled to 350 degreeC by the average cooling rate of 20 degreeC / s, and hold | maintained for 240 second at the said temperature.

The JIS No. 5 test piece was sampled from the steel sheet thus obtained, and the actual machining was performed to perform tensile processing at a processing rate of 3%, and the metal structure of each sample before and after processing, tensile strength (TS) and elongation rate before processing [ Total elongation (El)] and hydrogen embrittlement resistance after processing were measured by the following method.

[Observation of metal structure]

The metal structure was observed as follows using the test piece before and behind the said process. That is, an arbitrary measurement area (about 50 μm × 50 μm, measurement interval of 0.1 μm) on a plane parallel to the rolled surface was observed and photographed at a position of the product plate thickness 1/4, and the bainitic ferrite (BF ) And the area ratio of martensite (M) and the area ratio of residual austenite (residual γ) were measured in accordance with the above-described method. And the average value was calculated | required similarly in 2 visual fields chosen arbitrarily. In addition, other structures (ferrite, pearlite, etc.) were obtained by subtracting the area ratio occupied by the structure from the total structure (100%).

In addition, the average axle ratio of the retained austenite grains in the steel sheet before and after processing was measured according to the above-described method, satisfying the requirements of the present invention that the average axle ratio was 5 or more, and that the average axle ratio was less than 5. The requirements of the present invention were evaluated as not satisfying (×).

[Measurement of tensile strength (TS) and elongation (El)]

The tensile test was performed using the JIS No. 5 test piece before processing, and the tensile strength (TS) and elongation (E1) were measured. In addition, the strain rate of the tensile test was 1 mm / sec. In the present invention, an elongation rate of 10% or more was evaluated as "excellent elongation rate" for steel sheets having a tensile strength of 1180 MPa or more measured by the above method.

[Evaluation of Hydrogen Embrittlement Characteristics]

The hydrogen embrittlement resistance characteristics were obtained by stretching the JIS No. 5 test piece, applying a deformation of 3% of the machining rate, performing a bending process in which the bend R became 15 mm, loading a stress of 1000 MPa, and immersing in a 5% hydrochloric acid aqueous solution. The time to crack initiation was measured.

In addition, a hydrogen filled four-point bending test was also performed for some steel grades. Specifically, cathodic hydrogen filling was performed by immersing a 65 mm × 10 mm single piece test piece cut out from the steel sheets and giving a deformation rate of 3% in a solution of (0.5 mol / H ₂ SO ₄ +0.01 mol / KSCN). Then, the maximum stress which does not break for 3 hours was measured as limit breaking stress (DFL).

These results are written together in Table 2.

The following can be considered from Tables 1 and 2 (the following No. shows the experiment No. in Table 2).

No. satisfying the requirements specified in the present invention. 101 to 113 (the steel sheet 2 of the present invention) and 121 to 125 (the steel sheet 1 of the present invention) exhibit ultra high strength of 1180 MPa or more, and are also excellent in hydrogen embrittlement resistance characteristics under the harsh environment after processing. Moreover, the elongation rate which should be provided as a TRIP steel plate is also favorable, and the steel plate optimal as a reinforcement part of an automobile exposed to atmospheric corrosion atmosphere, etc. was obtained. In particular, No. 121 to 125 show better hydrogen embrittlement resistance.

On the other hand, No. which does not satisfy the provisions of the present invention. 114-120 and 126 have the following problems, respectively.

That is, No. 114 used steel grade N-1 which lacked C amount, and was not able to ensure excellent workability.

No. Since 115 uses the steel grade O-1 which lacks Mn amount, sufficient retained austenite could not be secured and the hydrogen embrittlement resistance after processing was inferior.

No. 116 is an example in which martensite steel, which is a conventional high-strength steel, was obtained using steel type P-1 having a low Si content, and its hydrogen embrittlement resistance was deteriorated because little residual austenite was present. Moreover, the elongation rate required for the steel sheet was not secured.

No. 117 is an example using the steel grade Q-1 which has an excessive amount of C. Since carbide precipitated, both the moldability and the hydrogen embrittlement resistance after processing fell.

No. 118 uses the steel grade R-1 in which the amount of Mo is excessively contained, and No. 118 used. Since 119 uses the steel grade S-1 in which the Nb amount was excessively contained, all the moldability was remarkably inferior. On the other hand, No. In 118 and 119, the above machining could not be performed and the characteristics after the machining could not be investigated.

No. Although 120 uses the steel material which satisfy | fills the component composition prescribed | regulated by this invention, since it was not manufactured on the recommended condition, the obtained steel plate became a conventional TRIP steel plate. As a result, the residual austenite does not satisfy the average axial ratio specified in the present invention, and since the parental phase does not become a two-phase structure of bainitic ferrite and martensite, it is not a strength level requiring excellent hydrogen embrittlement resistance.

No. Since 126 exceeds the amount of Al specified as the steel sheet 1 of the present invention, a predetermined amount of retained austenite can be secured, but the retained austenite does not satisfy the average axial ratio specified by the present invention and the desired appearance is achieved. In addition, since the inclusions such as AlN were also produced, the hydrogen embrittlement resistance was poor.

Next, the parts are molded using the steel sheets of the steel symbol A-1 and J-1 of Table 1 and the comparative steel sheet (the conventional high-strength steel sheet of 590 MPa class), and the breakdown pressure test and the impact resistance test as described below. Was carried out to investigate the performance (crush resistance and impact resistance characteristics) as a molded article.

[Withstand pressure breakdown test]

First, the parts (test body, hat channel part) 1 as shown in FIG. 1 were created using the steel plate of the steel grade symbols A-1 and J-1 of Table 1, and a comparative steel plate, respectively, and the crushability is as follows. The test was performed. That is, a current of 0.5 kA lower than the dust generation current flowed from the electrode having a tip diameter of 6 mm to the spot welding position 2 of the component shown in FIG. 1, and spot welding was performed at a 35 mm pitch as shown in FIG. And as shown in FIG. 2, the metal mold | die 3 was pressed from the upper part of the longitudinal center part of the component 1, and the maximum load was calculated | required. In addition, the absorbed energy was determined from the area of the load-displacement diagram. The results are shown in Table 3.

From Table 3, it can be seen that the parts (test bodies) prepared using the steel sheet of the present invention exhibited higher loads and higher absorption energy than those of conventional steel sheets having low strength, and thus had excellent collapse resistance. have.

[Impact resistance test]

Using the steel sheets of the steel symbol A-1 and J-1 of Table 1 and the comparative steel plate, the components (test body, hat channel component) 4 shown in FIG. 3 were produced, respectively, and the impact resistance test was done as follows. . 4 shows A-A sectional view of the component 4 in the said FIG. In the impact resistance test, the spot welding is performed at the spot welding position 5 of the component 4 as in the case of the breakdown pressure test, and then the component 4 is placed on the base 7 as schematically shown in FIG. The fall weight (mass 110 kg) 6 was dropped from the position of 11 m in height from the upper part of the said component 4, and the absorption energy until 40 mm deformation | transformation (height direction shrink | contraction) was calculated | required. . The results are shown in Table 4.

From Table 4, it can be seen that the component (test body) prepared using the steel sheet of the present invention exhibits higher absorption energy than that of the conventional steel sheet with low strength, and has excellent impact resistance characteristics.

For reference, the TEM observation photograph of the test piece obtained in the present example is shown. 6 is No. which is an example of this invention. It is an example of the TEM observation photograph of 101. From this FIG. 6, the metal structure of the ultra-high-strength steel plate of this invention is a state in which the needle-shaped residual austenite (the black part of a bar-shaped line in FIG. 6) disperse | distributed in this invention was dispersed. Able to know. 7 is No. which is a comparative example. Although it is an example of the TEM observation photograph of 120, From this FIG. Although the retained austenite (slightly round black part in FIG. 7) exists in the ultra-high strength steel plate of 120, it turns out that it is a massive retained austenite which does not satisfy | regulate the specification of this invention.

Example 2

Test steel No. which consists of the component composition of Table 5. After vacuum-solving A-2 to Y-2 to produce an experimental slab, a hot rolled steel sheet having a plate thickness of 3.2 mm was obtained according to the following process (hot rolling → cold rolling → continuous annealing), and then the surface scale was removed by pickling. After the cold rolled until the thickness was 1.2mm.

Hot Rolling Process

Start temperature (SRT): hold for 30 minutes at 1150 to 1250 ° C

Finishing Temperature (FDT): 850 ℃

Cooling rate: 40 ℃ / s

Coiling temperature: 550 ℃

Cold rolling process

Cold Rolling Rate: 50%

<Continuous Annealing Process>

Each test steel was held at A3 point + 30 ° C for 120 seconds, then rapidly cooled (air cooled) to To ° C in Table 6 at an average cooling rate of 20 ° C / s, and held at the To ° C for 240 seconds. Thereafter, the water was cooled to room temperature.

On the other hand, No. In 217, as a comparative example, in order to produce martensitic steel, which is a conventional high strength steel, the steel sheet after cold rolling was heated to 830 ° C, held for 5 minutes, and then quenched with water and tempered at 300 ° C for 10 minutes. Also no. In 220, the cold rolled steel sheet was heated to 800 ° C and held for 120 seconds, then cooled to 350 ° C at an average cooling rate of 20 ° C / s, and held at 240 ° C for the above temperature.

The JIS No. 5 test piece was sampled from the steel sheet thus obtained, and the actual machining was performed to perform tensile processing at a processing rate of 3%, and the metal structure of each sample before and after processing, tensile strength (TS) and elongation rate before processing [ Total elongation (El)] and hydrogen embrittlement resistance after processing were measured by the following method.

[Observation of metal structure]

The metal structure was observed as follows using the test piece before and behind the said process. That is, an arbitrary measurement area (about 50 μm × 50 μm, measurement interval of 0.1 μm) on a plane parallel to the rolled surface was observed and photographed at a position of the product plate thickness 1/4, and the bainitic ferrite (BF ) And the area ratio of martensite (M) and the area ratio of residual austenite (residual γ) were measured in accordance with the above-described method. And the average value was calculated | required similarly in 2 visual fields chosen arbitrarily. In addition, other structures (ferrite, pearlite, etc.) were obtained by subtracting the area ratio occupied by the structure from the total structure (100%).

In addition, the average axle ratio of the retained austenite grains in the steel sheet before and after processing was measured according to the above-described method, satisfying the requirements of the present invention that the average axle ratio was 5 or more, and that the average axle ratio was less than 5. The requirements of the present invention were evaluated as not satisfying (×).

[Measurement of tensile strength (TS) and elongation (El)]

The tensile test was performed using the JIS No. 5 test piece before processing, and the tensile strength (TS) and elongation (E1) were measured. In addition, the strain rate of the tensile test was 1 mm / sec. In the present invention, an elongation rate of 10% or more was evaluated as "excellent elongation rate" for steel sheets having a tensile strength of 1180 MPa or more measured by the above method.

[Evaluation of Hydrogen Embrittlement Characteristics]

The hydrogen embrittlement resistance characteristics were obtained by stretching the JIS No. 5 test piece, applying a deformation of 3% of the machining rate, performing a bending process in which the bend R became 15 mm, loading a stress of 1000 MPa, and immersing in a 5% hydrochloric acid aqueous solution. The time to crack initiation was measured.

In addition, the bending test piece produced as described above in consideration of the actual use environment was subjected to an accelerated exposure test in which a spray spray of a 3% NaCl solution was continuously performed once a day for 30 days to determine the number of days until crack formation. Measured.

In addition, a hydrogen filled four-point bending test was also performed for some steel grades. Specifically, 65 mm x 10 mm single-piece test pieces cut out from the steel sheets and given a deformation rate of 3% were immersed in a (0.5 mol / H ₂ SO ₄ +0.01 mol / KSCN) solution to carry out negative hydrogen filling, The maximum stress that did not break in time was measured as the limit fracture stress (DFL). And Experiment No. The ratio (DFL ratio) with respect to DFL of 203 (steel type symbol C-2) was calculated | required.

These results are written together in Table 6.

The following can be considered from Tables 5 and 6 (the following No. shows the experiment No. in Table 6).

No. satisfying the requirements specified in the present invention. 201 to 214 (the steel sheet 2 of the present invention) and 221 to 225 (the steel sheet 1 of the present invention) exhibit ultra high strength of 1180 MPa or more, and also have excellent hydrogen embrittlement resistance under harsh environment after processing. Moreover, the elongation rate which should be provided as a TRIP steel plate is also favorable, and the steel plate optimal as a reinforcement part of an automobile exposed to atmospheric corrosion atmosphere, etc. was obtained. In particular, No. 221 to 225 show more excellent hydrogen embrittlement resistance.

On the other hand, No. which does not satisfy the provisions of the present invention. 215 to 220 and 226 have the following problems, respectively.

That is, No. Since 215 uses the steel grade O-2 which lacks the amount of C, the residual austenite after processing was remarkably reduced, and the hydrogen embrittlement resistance of the present invention level was not secured.

No. Since 216 uses steel grade P-2 which lacks Mn amount, sufficient residual austenite could not be secured and the hydrogen embrittlement resistance after processing was inferior.

No. 217 is an example in which martensite steel, which is a conventional high-strength steel, was obtained using steel grade Q-2 having a low Si content, and its hydrogen embrittlement resistance was deteriorated because little residual austenite was present. Moreover, the elongation rate required for the steel sheet was not secured.

No. 218 is an example using the steel grade R-2 which has an excessive amount of C. Since carbide precipitated, both the moldability and the hydrogen embrittlement resistance after processing fell.

No. Since 219 uses steel grade S-2 which does not contain Cu and / or Ni, sufficient corrosion resistance cannot be secured and the hydrogen embrittlement resistance of the present invention level cannot be secured.

No. Although 220 uses the steel material which satisfy | fills the component composition prescribed | regulated by this invention, since it was not manufactured on the recommended condition, the obtained steel plate became a conventional TRIP steel plate. As a result, the retained austenite does not satisfy the average axial ratio specified in the present invention and does not become a biphasic structure of bainitic ferrite and martensite, and thus is not a strength level requiring excellent hydrogen embrittlement resistance.

No. Since 226 exceeds the amount of Al specified as the steel sheet 1 of the present invention, a predetermined amount of retained austenite can be secured, but the retained austenite does not satisfy the average axial ratio specified by the present invention, and the desired mother phase In addition, since the inclusions such as AlN were also produced, the hydrogen embrittlement resistance was poor.

Next, the parts are molded using steel sheets of the steel grade symbols A-2 and K-2 of Table 5 and a comparative steel sheet (a conventional high-strength steel sheet of 590 MPa class), and the breakdown pressure test and the impact resistance test as described below. Was carried out to investigate the performance (crush resistance and impact resistance characteristics) as a molded article.

[Withstand pressure breakdown test]

First, the maximum load was calculated | required similarly to Example 1 using the steel plate of the steel grade symbols A-2 and K-2 of Table 5, and a comparative steel plate, respectively. In addition, the absorbed energy was determined from the area of the load-displacement diagram. The results are shown in Table 7.

From Table 7, it can be seen that the parts (test specimens) produced using the steel sheet of the present invention exhibited higher loads than those of conventional steel sheets having low strength and also had high absorption energy, and thus had excellent collapse resistance. have.

[Impact resistance test]

The impact resistance test was carried out similarly to Example 1 using the steel plates of Table 5 A-2 and K-2 and the comparative steel plate of Table 5, respectively. The results are shown in Table 8.

From Table 8, it can be seen that the component (test body) prepared using the steel sheet of the present invention exhibits higher absorption energy than that of a conventional steel sheet with low strength, and has excellent impact resistance characteristics.

For reference, the TEM observation photograph of the test piece obtained in the present example is shown. 8 is No. which is an example of this invention. It is an example of the TEM observation photograph of 201. From this FIG. 8, the metal structure of the ultra-high strength steel plate of this invention is a state in which the needle-shaped residual austenite (the black part of bar shape in FIG. 8) disperse | distributed in this invention was disperse | distributed. Able to know. 9 is No. which is a comparative example. It is an example of the TEM observation photograph of 220. From this FIG. Although the residual austenite (slightly round black part in FIG. 9) exists in the ultra-high strength steel plate of 220, it turns out that it is the bulk residual austenite which does not satisfy | regulate the specification of this invention.

Example 3

Test steel No. which consists of the component composition of Table 9. After vacuum-solving A-3 to Q-3 to form an experimental slab, a hot rolled steel sheet having a plate thickness of 3.2 mm was obtained according to the following process (hot rolling → cold rolling → continuous annealing), and then the surface scale was removed by pickling. After the cold rolled until the width was 1.2mm.

Hot Rolling Process

Start temperature (SRT): hold for 30 minutes at 1150 to 1250 ° C

Finishing Temperature (FDT): 850 ℃

Cooling rate: 40 ℃ / s

Coiling temperature: 550 ℃

Cold rolling process

Cold Rolling Rate: 50%

<Continuous Annealing Process>

About each test steel, it hold | maintained for 120 second at A3 point +30 degreeC, and then it rapidly cooled to To degree of Table 10 (air cooling) at the average cooling rate of 20 degreeC / s, and hold | maintained for 240 seconds at said To degreeC. Thereafter, the water was cooled to room temperature.

On the other hand, No. In 311, the steel sheet after cold rolling was heated to 830 degreeC, hold | maintained for 5 minutes, and tempered at 300 degreeC for 10 minutes in order to produce the martensitic steel which is a conventional high strength steel as a comparative example. Also no. In 312, the cold rolled steel sheet was heated to 800 ° C. and held for 120 seconds, then cooled to 350 ° C. at an average cooling rate of 20 ° C./s, and held at the temperature for 240 seconds.

The JIS No. 5 test piece was sampled from the steel sheet thus obtained, and the actual machining was performed to perform tensile processing at a processing rate of 3%, and the metal structure of each sample before and after processing, tensile strength (TS) and elongation rate before processing [ Total elongation (El)] and the hydrogen embrittlement resistance characteristics (hydrogen embrittlement risk index) after processing were investigated in the following manners, respectively.

[Observation of metal structure]

The metal structure was observed as follows using the test piece before and behind the said process. In other words, at a position of 1/4 of the thickness of the product sheet, an arbitrary measurement area (about 50 μm × 50 μm and a measurement interval of 0.1 μm) on a plane parallel to the rolled surface was observed and photographed to obtain bainitic ferrite (BF). And the area ratio of martensite (M) and the area ratio of residual austenite (residual γ) were measured in accordance with the above-described method. And the average value was calculated | required similarly in 2 visual fields chosen arbitrarily. In addition, other structures (ferrite, pearlite, etc.) were obtained by subtracting the area ratio occupied by the structure from the total structure (100%).

In addition, the average axial ratio of the residual austenite grains, the average short axis length, and the closest distance between the residual austenite grains in the steel sheet before and after processing were measured according to the above-described method. The average axial ratio was 5 or more to satisfy the requirements of the present invention (○), and less than 5 was evaluated as not satisfying the requirements of the present invention (×).

[Measurement of tensile strength (TS) and elongation (El)]

The tensile test was performed using the JIS No. 5 test piece before processing, and the tensile strength (TS) and elongation rate (El) were measured. In addition, the strain rate of the tensile test was 1 mm / sec. In the present invention, an elongation rate of 10% or more was evaluated as "excellent elongation rate" for steel sheets having a tensile strength of 1180 MPa or more measured by the above method.

[Evaluation of Hydrogen Embrittlement Characteristics]

A low strain rate tensile test (SSRT) with a strain rate of 1 × 10 ⁻⁴ / sec was carried out using a plate specimen of 1.2 mm thickness, and hydrogen embrittlement was obtained by obtaining a hydrogen embrittlement risk index (%) defined by the following equation. The properties were evaluated.

Hydrogen Embrittlement Risk Index (%) = 100 × (1-E1 / E0)

Here, E0 shows the elongation at break of the test piece in the state which does not substantially contain hydrogen in steel, and E1 shows the elongation at break of the steel material (test piece) electrochemically filled with hydrogen in sulfuric acid. Meanwhile, the hydrogen filling was performed by immersing the steel (test piece) in a mixed solution of H ₂ SO ₄ (0.5 mol / L) and KSCN (0.01 mol / L), and at room temperature and constant current (100 A / m 2).

Since the hydrogen embrittlement risk index exceeds 50%, there is a risk of causing hydrogen embrittlement during use, and thus, in the present invention, 50% or less is evaluated to have excellent hydrogen embrittlement resistance.

These results are shown in Table 10.

From Tables 9 and 10, it can consider as follows (the following No. shows experiment No. in Table 10).

No. satisfying the requirements specified in the present invention. 301 to 309 (the steel sheet 2 of the present invention) and 313 to 317 (the steel sheet 1 of the present invention) exhibit ultra high strength of 1180 MPa or more, and are also excellent in hydrogen embrittlement resistance under harsh environment after processing. Moreover, the elongation rate which should be provided as a TRIP steel plate is also favorable, and the steel plate optimal as a reinforcement part of an automobile exposed to atmospheric corrosion atmosphere, etc. was obtained.

On the other hand, No. which does not satisfy the provisions of the present invention. 310 to 312 and 318 have the following problems, respectively.

That is, No. 310 is an example using the steel grade J-3 which has an excessive amount of C. Since carbide precipitates and the average short axis length of residual austenite is long, both moldability and hydrogen embrittlement resistance after processing fell.

No. 311 is an example in which martensite steel, which is a conventional high-strength steel, was obtained by using steel grade K-3 having a low Si content, and its hydrogen embrittlement resistance was deteriorated because little residual austenite was present. Moreover, the elongation rate required for the steel sheet could not be secured.

No. Although 312 uses the steel material which satisfy | fills the component composition prescribed | regulated by this invention, since it was not manufactured on the recommended condition, the obtained steel plate became a conventional TRIP steel plate. As a result, the retained austenite is significantly reduced after processing, and also does not satisfy the average axial ratio and average short axis length defined in the present invention, and the strength is low because the phase-like bainitic ferrite and martensite are not two-phase structures. At the same time, the hydrogen embrittlement characteristics were also poor.

No. Since 318 exceeds the amount of Al defined as the steel sheet 1 of the present invention, a predetermined amount of retained austenite can be secured, but the retained austenite does not satisfy the average axial ratio specified in the present invention, and the desired appearance is achieved. In addition, since the inclusions such as AlN were also produced, the hydrogen embrittlement resistance was poor.

Next, the parts are molded by using steel sheets of the steel symbols A-3 and G-3 shown in Table 9 and a comparative steel sheet (a conventional high-strength steel sheet of 590 MPa class), and the breakdown pressure test and the impact resistance test as described below. Was carried out to investigate the performance (crush resistance and impact resistance characteristics) as a molded article.

[Withstand pressure breakdown test]

First, the crushability test was performed similarly to Example 1 using the steel plate of the steel grade symbols A-3 and G-3 of Table 9, and a comparative steel plate, and the maximum load was calculated | required. In addition, the absorbed energy was determined from the area of the load-displacement diagram. The results are shown in Table 11.

From Table 11, it can be seen that the parts (test specimens) produced using the steel sheet of the present invention exhibited higher loads and higher absorption energy than those of conventional steel sheets having low strength, and thus had excellent collapse resistance. have.

[Impact resistance test]

The impact resistance test was carried out in the same manner as in Example 1 using steel sheets of steel sheet symbols A-3 and G-3 of Table 9 and comparative steel sheets, respectively. The results are shown in Table 12.

From Table 12, it can be seen that the component (test body) prepared using the steel sheet of the present invention exhibits higher absorption energy than that of the conventional steel sheet with low strength, and has excellent impact resistance characteristics.

For reference, the TEM observation photograph of the test piece obtained in the present example is shown. 12 is No. which is an example of this invention. 301 is a TEM observation photograph example (15,000 times magnification), and FIG. 13 is an example of a TEM observation photograph (60,000 times magnification) in which a part of the photograph of FIG. 12 is enlarged. From these FIGS. 12 and 13, the ultra-high strength steel sheet of the present invention. It can be seen that the metallic structure of the austenite remains in a finely dispersed state of the austenite (the black portion in the bar shape in FIGS. 12 and 13), and the shape of the retained austenite is a needle that satisfies the requirements defined by the present invention. 14 is No. which is a comparative example. Although it is an example of the TEM observation photograph of 313, From this FIG. Although the retained austenite (slightly round black part in FIG. 14) exists in the ultra-high strength steel plate of 313, it turns out that it is the bulk retained austenite which does not satisfy the prescription | regulation of this invention.

According to the present invention, even after forming a steel sheet into parts, it is possible to produce a super high strength thin steel sheet having a tensile strength of 1180 MPa or more that exhibits excellent workability during molding while maintaining excellent hydrogen embrittlement resistance by dehydrogenating hydrogen invading from the outside. It is possible to provide reinforcing materials such as bumpers and impact beams, and automobile parts such as seat rails, fillers, reinforces and members, for example, as ultra-high strength parts which are extremely hard to cause delayed fracture and the like.

Claims (10)

By mass%, it satisfies C 0.25% or more and 0.60% or less, Si 1.0 to 3.0%, Mn 1.0 to 3.5%, P 0.15% or less, S 0.02% or less and Al 1.5% or less (not including 0%), Group A: at least one selected from 0.003 to 0.5 mass% of Cu and 0.003 to 1.0 mass% of Ni, Group B: 0.003-1.0 mass% in total of at least one selected from Ti and V, Group C: 1.0 mass% or less of Mo (not containing 0 mass%), 0.1 mass% or less of Nb (not containing 0 mass%), D group: B 0.0002 to 0.01 mass%, and Group E: at least one selected from the group consisting of Ca 0.0005 to 0.005 mass%, Mg 0.0005 to 0.01 mass%, and REM 0.0005 to 0.01 mass% At least one group selected from the group consisting of groups A to E, The balance consists of iron and unavoidable impurities, Metal structure after tensile processing of 3% of processing rates at area ratio with respect to whole structure, Residual austenite: at least 1%, and Bainitic ferrite and martensite: meets 80% or more in total, Average axial ratio (long axis / short axis) of the retained austenite grains: 5 or more, Ultra high strength steel sheet excellent in hydrogen embrittlement resistance and workability, characterized in that the tensile strength is 1180MPa or more. The method of claim 1, The ultra-high strength steel sheet further satisfies the average uniaxial length of the residual austenite grains: 1 µm or less, and the closest distance between the residual austenite grains: 1 µm or less of the metal structure after the tensile processing at the processing rate of 3%. . By mass%, it satisfies C 0.25% or more and 0.60% or less, Si 1.0 to 3.0%, Mn 1.0 to 3.5%, P 0.15% or less, S 0.02% or less and Al 1.5% or less (not including 0%), Group A: at least one selected from 0.003 to 0.5 mass% of Cu and 0.003 to 1.0 mass% of Ni, Group B: 0.003-1.0 mass% in total of at least one selected from Ti and V, Group C: 1.0 mass% or less of Mo (not containing 0 mass%), 0.1 mass% or less of Nb (not containing 0 mass%), D group: B 0.0002 to 0.01 mass%, and Group E: at least one selected from the group consisting of Ca 0.0005 to 0.005 mass%, Mg 0.0005 to 0.01 mass%, and REM 0.0005 to 0.01 mass% At least one group selected from the group consisting of groups A to E, The balance consists of iron and unavoidable impurities, Metal structure after tensile processing of 3% of processing rates, Residual austenite: 1% or more by area of the whole tissue, Average axial ratio (long axis / short axis) of the retained austenite grains: 5 or more, Average short axis length of the retained austenite grains: 1 µm or less, and The closest distance between the remaining austenite grains: satisfies 1 µm or less, Ultra high strength steel sheet excellent in hydrogen embrittlement resistance and workability, characterized in that the tensile strength is 1180MPa or more. delete delete delete delete delete delete By mass%, it satisfies C 0.25% or more and 0.60% or less, Si 1.0 to 3.0%, Mn 1.0 to 3.5%, P 0.15% or less, S 0.02% or less and Al 1.5% or less (not including 0%), The balance consists of iron and unavoidable impurities, Metal structure after tensile processing of 3% of processing rates at area ratio with respect to whole structure, Residual austenite: 1% or more, Bainitic ferrites and martensite: at least 80% in total, and Ferrites and Perlites: Meets 9% or less (not including 0%) in total, Average axial ratio (long axis / short axis) of the retained austenite grains: 5 or more, Ultra high strength steel sheet excellent in hydrogen embrittlement resistance and workability, characterized in that the tensile strength is 1180MPa or more.

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