JP4412727B2 - Super high strength steel sheet with excellent hydrogen embrittlement resistance and method for producing the same - Google Patents

Description

  The present invention relates to an ultra-high strength steel plate of 1180 MPa class or higher, which is excellent in hydrogen embrittlement resistance, and a method for efficiently producing the ultra-high strength steel plate.

  Steel plates used for press forming in automobiles, industrial machines, etc. are required to have excellent strength and ductility. In recent years, the need for ultra-high strength steel plates of 1180 MPa class or higher is increasing. . As a steel sheet that meets such needs, a TRIP (Transformation Induced Plasticity) steel sheet has attracted attention.

The TRIP steel sheet has an austenite structure remaining, and when deformed at a temperature equal to or higher than the martensite transformation start temperature (Ms point), the retained austenite (γ _R , residual γ) is transformed into martensite by stress and becomes large. Elongated steel sheet, for example, TRIP type composite structure steel (TPF steel) containing polygonal ferrite as a parent phase and containing retained austenite; TRIP type tempered martens containing tempered martensite as a parent phase and containing retained austenite Sight steel (TAM steel); TRIP type bainite steel (TBF steel) containing bainitic ferrite as a parent phase and containing retained austenite is known. Among these, TBF steel has been known for a long time (for example, Nisshin Steel Technical Report No. 43 issued in 1980), and high strength is easily obtained by a hard bainite structure; Since fine retained austenite is easily generated at the boundary of bainitic ferrite, there is a feature that very excellent elongation can be obtained. TBF steel also has a manufacturing advantage that it can be easily manufactured by a single heat treatment (continuous annealing process or plating process).

  However, it is known that when an ultra-high strength region of 1180 MPa class or higher is reached, the TRIP steel sheet has a new problem of delayed fracture (cracking, etc.) due to hydrogen embrittlement, as in the case of a normal high-strength steel sheet. Delayed fracture is a high-strength steel in which hydrogen generated from a corrosive environment or atmosphere diffuses into defects such as dislocations, vacancies, and grain boundaries, embrittles the material, and breaks when stress is applied. This is a phenomenon, and as a result, it has adverse effects such as a decrease in the ductility and toughness of the metal material.

  Therefore, recently, research on hydrogen embrittlement characteristics of TRIP steel has been advanced (Non-patent Documents 1 and 2). According to these reports, excellent anti-hydrogen embrittlement characteristics are observed in any of the above-mentioned TRIP steels. However, when the amount of hydrogen storage in TBF steel is particularly large and the fracture surface of TBF steel is observed, It is shown that cleavage cleavage is suppressed. This significantly suggests the excellent delayed fracture resistance of TBF steel. The reason is that TBF steel is composed of a bainite structure, so the dislocation density of the parent phase is high, and a large amount of hydrogen is trapped on this dislocation, resulting in a large amount of hydrogen being occluded compared to other TRIP steels. It is estimated that.

However, the delayed fracture characteristics of the TBF steel reported in the above-mentioned document are limited to about 1000 seconds at the time of occurrence of cracking by the cathode charge test, and further improvement of the characteristics is demanded. In addition, the heat treatment conditions in the above document have problems such as poor production efficiency of the actual machine because the heating temperature is set high, and the development of new TBF steel with excellent production efficiency is eagerly desired. ing.
Tomohiko Kitajo, 5 others, "Hydrogen embrittlement of ultra-high strength low alloy TRIP steel (1st report: Hydrogen storage properties and ductility)", Proceedings of the 51st Annual Conference of the Society of Materials Science, 2002, Vol. 8, p.17-18 Tomohiko Kitajima, 5 others, “Effect of austempering temperature on hydrogen embrittlement of ultra-high strength low alloy TRIP steel”, CAMP-ISIJ, 2003, Vol. 16, p.568

  The present invention has been made in view of the above circumstances, and an object of the present invention is a TRIP steel sheet having a bainite structure as a parent phase, which has a super high strength with a tensile strength of 1180 MPa or more and is a characteristic of TRIP steel. It is an object of the present invention to provide a novel steel plate having improved hydrogen embrittlement resistance without impairing ductility; and a method capable of efficiently producing the steel plate.

The ultra-high-strength steel sheet of the present invention that can solve the above problems (ultra-high-strength steel sheet having a tensile strength of 1180 MPa or more and excellent in elongation and hydrogen embrittlement resistance) is mass%. so,
C: 0.06 to 0.6%,
Si + Al: 0.5 to 3%
Mn: 0.5-3%,
P: 0.15% or less,
S: 0.02% or less, and
The area ratio for all tissues
3% or more of retained austenite structure,
Containing 30% or more of bainite structure,
Furthermore, the ferrite structure may contain 50% or less,
Among the bainite structures, the bainite ferrite is judged by comparing the same visual field with EBSP (Electron Back Scatter Diffraction Pattern) and SEM. It is.

  In the steel sheet, as a chemical component in the steel, further, by mass, Mo: 1% or less (not including 0%), Ni: 0.5% or less (not including 0%), Cu: 0.5% Or less (not including 0%), Cr: not more than 1% (not including 0%); Ti: not more than 0.1% (not including 0%), Nb: 0.1% Or less (not including 0%), V: containing at least one of 0.1% or less (not including 0%); Ca: 0.003% or less (not including 0%), and / or REM : Anything containing 0.003% or less (excluding 0%) is a preferred embodiment of the present invention.

Further, the production method of the present invention that can solve the above-mentioned problems is a method for producing the above-described ultra-high-strength steel sheet by performing a continuous annealing process or a plating process, and a steel that satisfies the above-mentioned components in the steel. After heating and holding at a temperature of A ₃ point to (A ₃ point + 20 ° C.) for 10 to 600 seconds, it is cooled to a temperature of Ms point or more and Bs point or less at an average cooling rate of 3 ° C./s or more. It has a gist where it is heated and held for ˜1800 seconds.

  According to the present invention, an ultra-high strength steel plate having a tensile strength of 1180 MPa or more and having improved hydrogen embrittlement resistance can be manufactured with high productivity.

  In order to further improve the hydrogen embrittlement resistance in an ultrahigh strength region having a tensile strength of 1180 MPa or more, the present inventors have made extensive studies focusing on TBF steel having a bainite structure as a parent phase among TRIP steel sheets. In particular, in the method of Non-Patent Document 2 described above, in view of the fact that the heating temperature is high and the production efficiency of the actual machine is inferior, the furnace is easily damaged, and decarburization is likely to occur. As a result of repeated research, it is possible to prevent austenite grain growth by controlling the heating temperature lower than that of conventional TBF steels; as a result, fine bainite blocks that could not be obtained with conventional TBF steels are formed. As a result, it was found that the toughness of the steel material was improved and the hydrogen embrittlement resistance was improved, and the present invention was completed.

Hereinafter, the present invention will be described in detail.
[Organization]
First, the organization that most characterizes the present invention will be described.

  The ultra-high-strength steel sheet according to the present invention contains a retained austenite structure of 3% or more and a bainite structure of 30% or more in terms of the area ratio relative to the entire structure (thus, it may be composed only of retained austenite and bainite structure. Further, the ferrite structure may further contain 50% or less (including 0%). Among the bainite structure, the average grain size of the bainite block in the bainitic ferrite determined by comparison observation with EBSP and SEM. It is satisfied that the diameter is less than 20 μm.

Bainitic structure As described above, the bainite structure is hard and high strength is easily obtained. In addition, since the dislocation density of the parent phase is high, a large amount of hydrogen is trapped on this dislocation, resulting in an advantage that a larger amount of hydrogen is occluded than other TRIP steels. Further, in the bainite structure, fine retained austenite is easily generated at the boundary of the lath-like bainitic ferrite, so that there is also an advantage that very excellent elongation can be obtained. In order to effectively exhibit such an action, the bainite structure is 30% or more, preferably 40% or more, more preferably 50% or more in terms of the area ratio relative to the entire structure. The upper limit can be determined by balance with other structures, and it is difficult to uniformly determine, but when the ferrite structure is not included, the upper limit is approximately 95% or less, more preferably 93%. It is recommended to control the following. When the ferrite structure is contained, it is recommended that the upper limit be controlled to approximately 92% or less, more preferably 90% or less.

  The area ratio of the bainite structure is that the steel sheet is corroded with nital, and an arbitrary measurement area (about 50 × 50 μm) is observed with a scanning electron microscope (SEM) in a plane parallel to the rolling surface at a thickness of 1/4. (Magnification: 1500 times).

  Here, the SEM used in the present invention is “a high-resolution FE-SEM equipped with an EBSP detector (manufactured by Philips, XL30S-FEG)”, and an EBSP detector simultaneously observes the region observed by the SEM on the spot. There is an advantage that it can be analyzed by. Moreover, the bainite block in bainitic ferrite can be identified by utilizing the FE-SEM (this point will be described later).

Average particle size of bainite block <20 μm
Further, the steel sheet of the present invention satisfies the average grain size of the bainite block of less than 20 μm in the bainitic ferrite identified by the method described later in the bainite structure. As described above, the present invention has the greatest feature in that the hydrogen embrittlement resistance of the TBF steel is improved by refining the bainite block, particularly in the bainite structure. When the average particle size of the bainite block exceeds 20 μm, desired characteristics cannot be obtained. The average particle size of the bainite block is preferably as small as possible, preferably 18 μm or less, more preferably 16 μm or less.

  Here, bainitic ferrite is a plate-like ferrite, but means a substructure with a high dislocation density (a lath structure may or may not have a lath structure); no dislocation density Alternatively, polygonal ferrite having an extremely small substructure (in the present invention, this polygonal ferrite is referred to as “ferrite”) is clearly distinguished by SEM observation as follows.

  Polygonal ferrite: Black in SEM photograph, polygonal shape, and does not contain retained austenite or martensite inside.

  Bainitic ferrite: SEM photographs show a dark gray color, and in many cases, bainitic ferrite cannot be separated from retained austenite or martensite.

Next, a method for identifying a bainite block from bainitic ferrite will be described with reference to FIGS. These are No. 1 in Example 1 described later. 2 (examples of the present invention), the above-mentioned “FE-SEM equipped with an EBSP detector” was used to show the results when the same region was observed. Of these, FIG. SEM observation photograph measured based on (magnification: 1500 times); FIG. 2 is a cross-sectional photograph in the plate thickness direction (magnification: 1500 times) when the same area as the measurement area observed by SEM was simultaneously subjected to EBSP analysis. The hardware and software related to the detection, measurement, and analysis of the EBSP was an OIM (Orientation Imaging Microscopy ^™ ) system manufactured by TexSEM Laboratories Inc. The measurement interval is 0.1 μm.

  As shown in FIG. 1, according to SEM observation, polygonal ferrite and bainitic ferrite can be distinguished, so the SEM photograph of FIG. 1 and the EBSP photograph of FIG. 2 are compared and mapped by EBSP analysis. In the structure of FIG. 2, a region (bainitic ferrite) excluding polygonal ferrite that can be identified by SEM can be easily determined.

  Of the bainitic ferrite thus determined, a region having an orientation difference of 15 ° or more between adjacent structures (in the present invention, such a region is considered to be a region having the same crystal orientation). The texture of the grain boundary (Grain Boundaries: Min 15 °, Max 180 °) is added to the 001 inverse pole figure. The region thus mapped (region having an azimuth difference of 15 ° or more) is defined as “bainite block” in the present invention. That is, the bainite block in the present invention is a region (with an inclination angle of 15 ° or more) having the same crystal orientation by EBSP analysis among bainitic ferrite identified by SEM when the same visual field is observed by SEM observation and EBSP analysis. Defined as an area having a heading difference).

  The EBSP method will be briefly described here. The EBSP determines the crystal orientation of the electron beam incident position by making an electron beam incident on the sample surface and analyzing the Kikuchi pattern obtained from the reflected electrons generated at this time. If the electron beam is scanned two-dimensionally on the sample surface and the crystal orientation is measured at every predetermined pitch, the orientation distribution on the sample surface can be measured. According to this EBSP observation, there is an advantage that textures in the plate thickness direction, which are determined to be the same in normal microscope observation and have different crystal orientation differences, can be identified by color tone differences. When the bainite block defined based on the crystal orientation is defined as a constituent feature of the invention as in the present invention, the structure observation by the EBSP method is required.

  Next, for the bainite block detected in this way, the diameter (equivalent circle diameter) of a circle having the same area as the bainite block is calculated. In calculating the diameter of the bainite block, an EBSP analysis photograph with a magnification of 5000 times is used. Similarly, the diameters (equivalent circle diameters) of all bainite blocks existing in the measurement target area (about 50 × 50 μm) are obtained, and the average is defined as “the average particle diameter of bainite blocks” in the present invention.

Residual austenite structure (γ _R , residual γ)
Residual austenite is useful for improving the total elongation, and in order to effectively exhibit such an effect, the area ratio to the entire structure is 3% or more (preferably 5% or more, more preferably 7% or more). . However, since a desired ultra-high strength cannot be ensured if present in a large amount, it is recommended that the upper limit is preferably 30% (more preferably 25%).

In addition, it is preferable that the form of the retained austenite in this invention is a lath shape. Here, “the shape is lath” means that the average axial ratio (major axis / minor axis) is 2 or more (preferably 4 or more, more preferably 6 or more). Such lath-like retained austenite Is extremely useful in that it exhibits not only the TRIP effect similar to that of conventional retained austenite, but also a remarkable effect of improving delayed fracture resistance. Although the upper limit of the average axial ratio is not particularly defined, the preferable upper limit is 30 in view of the fact that a certain amount of retained austenite is necessary to effectively exert the effect of TRIP. Preferably it is 20.

  Further, in order to effectively exhibit the effect of the lath-like retained austenite, the larger the space factor of the lath-like retained austenite, the better. Specifically, it is determined by the balance with other structures (bainite, ferrite, etc.), and it is recommended to control appropriately so that the desired characteristics can be exhibited. For example, the space factor of the lath-like retained austenite may be 50% or more, more preferably 60% or more, still more preferably 70% or more, still more preferably 80% or more, and still more preferably 85% or more. Recommended. Although all of the retained austenite may be composed of lath-like retained austenite, it is recommended that the upper limit be about 95% at a practical level in consideration of restrictions on heating facilities and cooling facilities.

Furthermore, it is recommended that the C concentration (Cγ _R ) in the retained austenite is 0.8% or more. This C gamma _R is largely affects the characteristics of the TRIP (strain-induced transformation process), controlling over 0.8%, in particular, is effective in improving the elongation and the like. Preferably it is 1% or more, More preferably, it is 1.2% or more. Although preferred as the content of the C gamma _R is large, the actual operation, adjustable upper limit is believed to roughly 1.6%.

Here, the retained austenite in the present invention means a region observed as an FCC phase (face-centered cubic lattice) by the FE-SEM / EBSP method described above. Specifically, as in the bainite structure, an arbitrary measurement area (about 50 × 50 μm, measurement interval is 0.1 μm) on a plane parallel to the rolling surface at the position of the plate thickness ¼ is set as a measurement target. Incidentally, when polishing up to the measurement surface, electrolytic polishing is performed to prevent transformation of retained austenite. Next, using the FE-SEM, the sample set in the SEM column is irradiated with an electron beam. EBSP projected on the screen is shot with a high-sensitivity camera (VE-1000-SIT made by Dage-MTI Inc.) and captured as an image on a computer. Image analysis is performed with a computer, and the FCC phase determined by comparison with a simulation pattern using a known crystal system [in the case of retained austenite, FCC phase (face-centered cubic lattice)] is color-mapped. The area ratio of the region mapped in this manner is obtained, and this is defined as “area ratio of residual austenite”. The hardware and software for the above analysis used an OIM (Orientation Imaging Microscopy ^™ ) system manufactured by TexSEM Laboratories Inc.

For reference, FIG. 3 shows an EBSP analysis photograph (magnification: 1500 times) obtained by mapping the EBSP analysis photograph of FIG. 2 described above as an FCC phase. In FIG. 3, the region indicated by the arrow (←) is retained austenite (γ _R ).

Ferrite as "ferrite" in the present invention, polygonal ferrite, that is, whether there dislocation density or mean very little ferrite.

  In the present invention, ferrite is an arbitrary structure and may contain 0%. In order to effectively exhibit the effect of improving the elongation characteristics due to ferrite, the area ratio of ferrite is preferably 5% or more. More preferably, it is 10% or more. In particular, from the viewpoint of improving the elongation characteristics, it is better to have more ferrite. However, if it exceeds 50%, it becomes difficult to ensure the required ultra-high strength, so the upper limit is made 50%. Preferably it is 40% or less, More preferably, it is 30% or less.

The area ratio of the ferrite is calculated by the following equation.
Ferrite area ratio (%) = 100− [bainite area ratio (%)] − [residual austenite
Area ratio (%)]
Note: The bainite area ratio and residual austenite area ratio are determined by the methods described above.
It is to be measured.

In addition, the steel sheet of the present invention may be composed of only the above structure (that is, a mixed structure of bainite and retained austenite, or a mixed structure of bainite, ferrite, and retained austenite), but does not impair the function of the present invention. Therefore, it may have another organization (for example, martensite). These are structures that can inevitably remain in the production process of the present invention, but are preferably as small as possible (for example, the upper limit of the total area ratio is at most 10%).

  Next, basic components constituting the steel plate of the present invention will be described. Hereinafter, all the units of chemical components are mass%.

C: 0.06 to 0.6%
C is an essential element for securing high strength and securing retained austenite. More specifically, it is an important element for containing a sufficient amount of C in the austenite phase and allowing the desired austenite phase to remain even at room temperature, and is useful for increasing the strength-elongation balance. In particular, when the amount of C is added by 0.25% or more, the amount of retained austenite is increased and the concentration of C in the retained austenite is increased, so that extremely high strength-elongation can be obtained.

  However, if added over 0.6%, not only the effect is saturated, but also defects due to center segregation during casting and the like are observed. Moreover, when it adds 0.25% or more, weldability will deteriorate.

  Therefore, considering mainly weldability, it is preferable to control C: 0.06 to 0.25% (more preferably 0.2% or less, still more preferably 0.15% or less), When high elongation or the like is required without requiring spot welding, it is recommended to control C: 0.25 to 0.6% (more preferably 0.3% or more).

Si + Al: 0.5 to 3%
Si and Al are elements that effectively suppress the decomposition of residual austenite and the formation of carbides. In particular, Si is useful as a solid solution strengthening element. In order to effectively exhibit such an effect, it is necessary to add 0.5% or more in total of Si and Al. Preferably it is 0.7% or more, More preferably, it is 1% or more. However, even if the above elements are added in total exceeding 3%, the above effects are saturated, and it is economically wasteful. If added in a large amount, hot brittleness is caused, so the upper limit is 3%. And Preferably it is 2.5% or less, More preferably, it is 2% or less.

Mn: 0.5 to 3%
Mn is an element necessary for stabilizing austenite and obtaining desired retained austenite. In order to exhibit such an action effectively, it is necessary to add 0.5% or more. Preferably it is 0.7% or more, More preferably, it is 1% or more. However, if it is added in excess of 3%, adverse effects such as slab cracking are observed. Preferably it is 2.5% or less, More preferably, it is 2% or less.

P: 0.15% or less (excluding 0%)
P is an element effective for securing desired retained austenite. In order to effectively exhibit such an action, it is recommended to add 0.03% or more (more preferably 0.05% or more). However, if it exceeds 0.15%, the secondary workability deteriorates. More preferably, it is 0.1% or less.

S: 0.02% or less (excluding 0%)
S is an element that forms sulfide inclusions such as MnS and degrades workability as a starting point of cracking. Preferably it is 0.02% or less, More preferably, it is 0.015% or less. Note that the workability deterioration suppression effect due to the reduction of S is saturated when S is reduced to 0.003% or less, and conversely, considering that the cost for reducing S is high, the lower limit is 0. It is recommended to exceed 0.003%, more preferably 0.005% or more.

  The steel of the present invention basically contains the above components, and the balance: substantially iron and impurities, but the following allowable components can be added as long as the effects of the present invention are not impaired.

Mo: 1% or less (not including 0%), Ni: 0.5% or less (not including 0%), Cu: 0.5% or less (not including 0%), Cr: 1% or less (0 These elements are not only useful as steel strengthening elements, but also effective in stabilizing retained austenite and securing a predetermined amount. In order to effectively exhibit such an action, Mo: 0.05% or more (more preferably 0.1% or more), Ni: 0.05% or more (more preferably 0.1% or more), Cu : 0.05% or more (more preferably 0.1% or more) and Cr: 0.05% or more (more preferably 0.1% or more) are recommended to be added. However, even if Mo and Cr are added in excess of 1% and Ni and Cu are added in excess of 0.5%, the above effect is saturated, which is economically wasteful. More preferably, Mo is 0.8% or less, Ni is 0.4% or less, Cu is 0.4% or less, and Cr is 0.8% or less.

At least one of these elements : Ti: 0.1% or less (not including 0%), Nb: 0.1% or less (not including 0%), V: 0.1% or less (not including 0%) Has an effect of precipitation strengthening and refinement of structure, and is an element useful for increasing the strength. In order to effectively exhibit such an action, Ti: 0.01% or more (more preferably 0.02% or more), Nb: 0.01% or more (more preferably 0.02% or more), V : 0.01% or more (more preferably 0.02% or more) is recommended to be added respectively. However, if any element is added in excess of 0.1%, the above effect is saturated, which is economically useless. More preferably, Ti is 0.08% or less, Nb is 0.08% or less, and V is 0.08% or less.

Ca: 0.003% or less (excluding 0%), and / or
REM: 0.003% or less (excluding 0%)
Ca and REM (rare earth elements) are elements that control the form of sulfide in steel and are effective in improving workability. Here, examples of rare earth elements used in the present invention include Sc, Y, and lanthanoids. In order to effectively exhibit the above action, it is recommended to add 0.0003% or more (more preferably 0.0005% or more). However, even if added over 0.003%, the above effect is saturated, which is economically useless. More preferably, it is 0.0025% or less.

  Next, a method for producing the steel sheet of the present invention will be described.

In the production method of the present invention, steel that satisfies the above-described component composition is heated and held at a temperature of A ₃ point to (A ₃ point + 20 ° C.) for 10 to 600 seconds, and then Ms at an average cooling rate of 3 ° C./s or more. It is characterized in that it is cooled to a temperature not lower than the point and not higher than the Bs point, and is heated and held in this temperature range for 1 to 1800 seconds. Hereinafter, each process will be described in detail with reference to a schematic diagram (FIG. 4) of the method of the present invention.

First, steel satisfying the above-described component composition is heated and held at a temperature of A ₃ point to (A ₃ point + 20 ° C.) (T 1 in FIG. 4) for 10 to 600 seconds (t 1 in FIG. 4). Here T1 (soaking temperature) and t1 (soaking time) is extremely important in order to obtain the desired bainite block, or T1 exceeds the temperature of (A ₃ point + 20 ° C.), t1 is 600 seconds If it exceeds, austenite grain growth will be caused and coarse bainite blocks will be formed.

On the other hand, T1 becomes lower than the temperature of the _three points A, not predetermined bainitic structure is obtained. Moreover, if t1 is less than 10 seconds, austenitization is not sufficiently performed and cementite and other alloy carbides remain.

  Considering such points, T1 (soaking temperature) is preferably 650 ° C. or more and 900 ° C. or less; t1 (soaking time) is preferably 30 seconds or more and 300 seconds or less, more preferably 60 seconds. This is 240 seconds or less.

  Next, the steel sheet is cooled. In the present invention, the steel sheet is cooled to a temperature not lower than the Ms point and not higher than the Bs point (T2 in FIG. 1) at an average cooling rate of 3 ° C./s or higher (CR1 in FIG. 4). Heat and hold in the temperature range for 1 to 1800 seconds (t2 in FIG. 4).

  This step ensures a desired bainite structure (which may also include a ferrite structure) and avoids the formation of a pearlite structure that is not preferable for the present invention (in the present invention, the area ratio of the pearlite structure is 10% or less at the maximum). This is set especially for the purpose of suppression.

  First, the steel plate heated as described above is cooled at an average cooling rate (CR1) of 3 ° C./s or more, in order to avoid the formation of a pearlite structure by avoiding a pearlite transformation region. It is recommended that the average cooling rate be as large as possible, preferably 10 ° C./second or more (more preferably 20 ° C./second or more). In cooling, as shown in FIG. 1, it may be rapidly cooled monotonously to a predetermined temperature (T2) (one-stage cooling). However, when it is desired to generate a ferrite structure, the ferrite structure is stabilized by one-stage cooling. Since it is difficult to introduce, it is recommended to adopt a multi-stage cooling method in which the cooling rate is set in a plurality of times. The average cooling rate of each step in this case is also recommended to be 3 ° C./s or higher (preferably 10 ° C./s or higher, more preferably 20 ° C./s or higher), as in the case of single-stage cooling. .

  Next, a predetermined bainite structure can be introduced by performing a constant temperature transformation after quenching to a temperature (T2) not lower than the Ms point and not higher than the Bs point. When the heating temperature T2 exceeds the Bs point, a large amount of pearlite which is not preferable for the present invention is generated, and a predetermined bainite structure cannot be secured. On the other hand, when T2 falls below the Ms point, the area ratio of retained austenite decreases.

  On the other hand, when the heating and holding time t2 exceeds 1800 seconds, the dislocation density of bainite becomes small and a predetermined retained austenite cannot be obtained. On the other hand, if t2 is less than 1 second, a predetermined bainite structure cannot be obtained. Preferred t2 is 30 seconds or more and 1200 seconds or less; more preferably 60 seconds or more and 600 seconds or less.

  In consideration of actual operation, it is easy to perform the annealing treatment using a continuous annealing facility or a batch annealing facility. Moreover, when plating a cold-rolled sheet to make hot dip galvanizing, the plating conditions may be set so as to satisfy the heat treatment conditions, and the heat treatment may be performed in the plating step. Further, the plating may be alloyed.

Further, the hot rolling process and the cold rolling process before the above-described continuous annealing treatment are not particularly limited, and usually, the conditions to be performed can be appropriately selected and employed. Specifically, as the hot rolling step, conditions such as cooling at an average cooling rate of about 30 ° C./s and winding at a temperature of about 500 to 600 ° C. are adopted after the hot rolling is completed at _{Ar 3} or higher. be able to.

  Since this steel sheet is excellent in elongation and hydrogen embrittlement resistance and also in collision resistance safety, its application includes structural parts such as automobiles and industrial machines. Front and rear side members, crash parts such as crash boxes, pillars such as center pillar RF (reinforce), roof rail RF (reinforce), side sills, floor members, body parts such as kick parts, bumpers Suitable for manufacturing shock-absorbing parts such as RF (reinforce) and door impact beams.

  Hereinafter, the present invention will be described in detail based on examples. However, the following examples are not intended to limit the present invention, and all modifications made without departing from the spirit of the preceding and following descriptions are included in the technical scope of the present invention.

Example 1: Examination of component composition In this example, test steel No. 1 composed of the component compositions shown in Table 1 was used. A to T (the balance is iron and impurities, and the unit in the table is mass%) is vacuum-melted and used as a slab for experiment, and then according to the following steps (hot rolling → cold rolling → continuous annealing) After obtaining a hot-rolled steel sheet having a thickness of 2 mm (2.5 mm for the test steel Nos. Q to T), the surface scale was removed by pickling and cold rolling was performed until the thickness became 1.2 mm.

Hot rolling process : Start temperature (SRT) 1150 ℃, Finishing temperature (FDT) 850 ℃, Cooling rate 40 ℃ / s, Winding temperature 550 ℃
Cold rolling process : 50% cold rolling rate
Continuous annealing: For each sample steels 120 seconds in Table 2 T1 (A ₃ point + about 15 ° C.) (In Table 2, t1) and held, Table 2 in CR1 in Table 2 (average cooling rate) After cooling to T2 (water cooling), the T2 was maintained for 120 seconds (t2 in Table 2).

  Each steel plate thus obtained was measured for tensile strength (TS), elongation [total elongation (El)], and hydrogen embrittlement resistance (cathode CH life) in the following manner. In addition, the area ratio of the structure | tissue in each steel plate and the average particle diameter of a bainite block were measured in accordance with the method mentioned above.

[Measurement of tensile strength (TS) and elongation]
The tensile test used a JIS No. 5 test piece and measured tensile strength (TS) and elongation (El). The strain rate in the tensile test was 1 mm / sec. In the present invention, those having a tensile strength measured by the above method of 1180 MPa or more are targeted, and those having an elongation of 13% or more are evaluated as “excellent in elongation”.

[Measurement of hydrogen embrittlement resistance]
In measuring the hydrogen embrittlement resistance, a strip test piece in which each steel plate was adjusted to a size of 15 mm × 65 mm was used. A stress of 980 MPa was applied to the strip test piece by four-point bending, and in a mixed solution of (0.5 mol sulfuric acid + 0.01 mol KSCN), a potential of −80 mV, which was lower than natural potential using a potentiostat. The hydrogen embrittlement resistance (cathode CH life) was evaluated by measuring the time at which cracking occurred when an electric potential was applied. In addition, in this invention, the thing whose measurement time by the said method is 1000 second or more is evaluated as "it is excellent in a hydrogen embrittlement resistance".

  These results are shown in Table 2.

  From these results, it can be considered as follows (the following Nos. All mean Nos. In Table 2).

  First, Nos. 2 to 4 and 8 to 19 are all manufactured according to the method defined in the present invention using steel types (No. B to D and H to S in Table 1) that satisfy the scope of the present invention. Although it is an example of an invention, it is excellent in both elongation and hydrogen embrittlement resistance in an ultrahigh strength region of 1180 MPa or more.

  On the other hand, the following examples using steel types (Nos. A and E to G in Table 1) that do not satisfy the scope of the present invention have the following problems.

  First, No. 1 is an example using a steel type A with a small amount of C. A predetermined amount of bainite structure (hard) cannot be obtained, the ferrite structure becomes excessive, and the strength is lowered. The hydrogen embrittlement resistance was not measured because the strength was so low that a four-point bending stress could not be applied.

  Moreover, No. 5 is an example using the steel type E with a small total amount of (Si + Al), and the desired retained austenite cannot be obtained and the elongation is lowered.

  No. 6 is an example using the steel type F with a small amount of Mn, and the desired retained austenite cannot be obtained, so the elongation is low.

  No. 7 is an example using steel type G with a large amount of Mn, and because the strength was too high, hot rolling cracks occurred during hot rolling, and subsequent annealing treatment could not be performed.

  No. 20 is a conventional DP (dual phase) steel sheet, which does not have a bainite structure, and therefore has a reduced strength. No. In No. 20, since the predetermined strength was not satisfied, the hydrogen embrittlement resistance was not measured.

  Next, the above No. 17 and No. Parts were formed using 20 steel plates, and as described below, the resistance to fracture test, impact resistance test, and hydrogen embrittlement resistance were evaluated, and the performance as a molded product (breakdown resistance, impact resistance and Hydrogen embrittlement resistance) was investigated.

<Pressure resistance test>
First, no. Parts (test body, hat channel part) 1 as shown in FIG. 5 were prepared using 17 and 19 steel plates, respectively, and the crushability test was performed as follows. That is, spot welding was performed at a pitch of 35 mm as shown in FIG. 5 by applying a current 0.5 kA lower than the dust generation current from an electrode having a tip diameter of 6 mm to the spot welding position 2 of the component shown in FIG. And as shown in FIG. 6, the metal mold | die 3 was pressed from the upper direction center part of the component 1, and the maximum load was calculated | required. Absorbed energy was determined from the area of the load-displacement diagram. The results are shown in Table 3.

  From Table 3, No. The part (test body) created using the steel plate of 17 shows a higher load than the case of using a conventional steel plate with low strength, and also has a high resistance to cracking because the absorbed energy is also high. You can see that

<Impact resistance test>
No. Parts (test body, hat channel part) 4 as shown in FIG. 7 were prepared using 17 and 19 steel plates, respectively, and an impact resistance test was performed as follows. FIG. 8 is a cross-sectional view taken along line AA of the component 4 in FIG. In the impact resistance test, after spot welding is performed at the spot welding position 5 of the part 4 as in the case of the above-mentioned fracture resistance test, the part 4 is set on the base 7 as schematically shown in FIG. The falling energy (mass: 110 kg) 6 was dropped from the position of 11 m in height from above 4, and the absorbed energy until the part 4 was deformed 40 mm (the height direction contracted) was determined. The results are shown in Table 4.

  From Table 4, No. It can be seen that the part (test specimen) prepared using the 17 steel plate shows higher absorbed energy than the case of using the conventional steel plate having low strength and has excellent impact resistance.

<Evaluation of hydrogen embrittlement resistance>
No. 17 and 19 steel plates were formed into actual parts, and the hydrogen embrittlement resistance was evaluated in the state of the molded product. Specifically, no. The center pillar RF, the door impact beam, and the roof rail RF were pressed using 17 and 19 steel plates, immersed in 5% hydrochloric acid, and the presence or absence of cracking up to 24 hours was compared. The results are shown in Table 5.

  From Table 5, no. It can be seen that each of the parts prepared using the 17 steel plates did not generate cracks despite having high strength, and had excellent hydrogen embrittlement resistance.

Example 2: Examination of production conditions In this example, an experimental slab using the steel type B shown in Table 1 (steel satisfying the composition of the present invention) was used, and hot rolling and cold rolling were performed under the same conditions as in Example 1. Then, the cold-rolled steel sheets shown in Nos. 1 to 11 were obtained by performing continuous annealing under various conditions shown in Table 6. The plate thickness is all 1.2 mm.

  Next, the structure and various characteristics of the steel sheet were examined in the same manner as in Example 1. These results are also shown in Table 6.

  First, No. 1 is an example of the present invention produced according to the method defined in the present invention; and No. 11 is further alloyed after the above-described steps (for the purpose of alloying after being immersed in hot dip galvanizing). The examples of the present invention, which were heat-treated at 500 ° C.), each have an ultrahigh strength of 1180 MPa or more, and are excellent in both elongation and hydrogen embrittlement resistance.

  On the other hand, Nos. 2 to 10 which do not satisfy any of the conditions defined in the present invention have the following problems.

Among them, No. 2 has a heating temperature T1 of 930 ° C., which exceeds the upper limit of the present invention (A ₃ point of steel type B is 795 ° C., and thus reaches 815 ° C.), so that a coarse bainite block is formed and The hydrogen embrittlement characteristics decreased.

No. 3 had a heating temperature T1 of 760 ° C., which was lower than the lower limit of the present invention (A ₃ point of steel type B = 795 ° C.), so that a predetermined amount of bainite structure was not obtained and the strength was lowered. The hydrogen embrittlement resistance was not measured because the strength was so low that a four-point bending stress could not be applied.

  In No. 4, since the heating time t1 was long, austenite grain growth was induced and a coarse bainite block was generated, and the hydrogen embrittlement resistance was deteriorated.

  In No. 5, since the heating time t1 was short, austenitization did not proceed sufficiently, cementite remained, a predetermined retained austenite was not obtained, elongation was lowered, and hydrogen embrittlement resistance was also lowered.

  In No. 6, since the cooling rate CR1 was small, a large amount of pearlite structure was generated, and a predetermined amount of bainite structure was not obtained, and the desired strength and hydrogen embrittlement resistance could not be ensured. Further, a predetermined amount of retained austenite was not obtained, and the elongation was reduced.

  No. 7 is an example in which the heating temperature T2 after cooling exceeds 600 ° C. and the Ms point, a large amount of pearlite structure is generated, and a predetermined amount of bainite structure cannot be obtained, and a desired strength cannot be ensured. The hydrogen embrittlement resistance also deteriorated. Further, a predetermined amount of retained austenite was not obtained, and the elongation was reduced.

  No. 8 is an example in which the heating temperature T2 after cooling is 200 ° C., which is lower than the Bs point. A predetermined amount of retained austenite is not obtained, elongation is reduced, and hydrogen embrittlement resistance is also reduced.

  No. 9 is an example in which the heating time t2 after cooling is short, a predetermined bainite structure was not obtained, and the hydrogen embrittlement resistance deteriorated. In addition, martensite was formed and the elongation was reduced.

  No. 10 is an example in which the heating time t2 after cooling is long, the decomposition of the retained austenite proceeds, the predetermined amount of retained austenite cannot be obtained, and the elongation becomes low.

  For reference, FIG. 10 shows the results of EBSP photographs (color map: magnification 5000 times) of the cross section in the thickness direction in the present invention example (No. 1) and the comparative example (No. 2). As shown in FIG. No. 1 [FIG. 10 (a)] shows that the desired fine bainite block was produced, whereas the comparative example No. 2 [FIG. 10 (b)], it can be seen that coarse bainite blocks are generated.

No. of Example 1 2 is an SEM photograph (magnification: 1500 times) in 2 (example of the present invention). It is the photograph (magnification: 1500 times) which carried out the EBSP analysis of the same area | region as FIG. 3 is a photograph in which residual austenite γ _R (FCC phase) is mapped in the EBSP analysis photograph of FIG. 2. It is the schematic explaining the typical process of this invention method. FIG. 3 is a schematic perspective view of components used in a pressure-resistant fracture test in Example 1. FIG. 3 is a side view schematically showing a state of a pressure-breaking resistance test in Example 1. FIG. 3 is a schematic perspective view of components used in an impact resistance test in Example 1. It is AA sectional drawing in the said FIG. It is the side view which showed typically the mode of the impact-resistant characteristic test in Example 1. FIG. It is an EBSP photograph (magnification: 5000 times) of an example of the present invention (No. 1) and a comparative example (No. 2) in Example 2.

Explanation of symbols

1 Pressure fracture test parts (test specimen)
2,5 Spot welding position 3 Mold 4 Impact resistance test parts (test body)
6 Falling weight 7 (For impact resistance test)

Claims (5)

% By mass
C: 0.06 to 0.6%,
Si + Al: 0.5 to 3%
Mn: 0.5-3%,
P: 0.15% or less,
S: not more than 0.02%, the balance being iron and impurities, and
The area ratio for all tissues
3% or more of retained austenite structure,
Containing 30% or more of bainite structure,
Furthermore, the ferrite structure may contain 50% or less,
Among the bainite structures, the average grain size of bainite blocks is less than 20 μm in bainitic ferrite determined by comparing the same visual field with EBSP (Electron Back Scatter Diffraction Pattern) and SEM. Super high strength steel plate with excellent hydrogen embrittlement characteristics. Furthermore, in mass%,
Mo: 1% or less (excluding 0%),
Ni: 0.5% or less (excluding 0%),
Cu: 0.5% or less (excluding 0%),
Cr: 1% or less (excluding 0%)
The ultra-high-strength steel sheet according to claim 1, which contains at least one of the following. Furthermore, in mass%,
Ti: 0.1% or less (excluding 0%),
Nb: 0.1% or less (excluding 0%),
V: 0.1% or less (excluding 0%)
The ultra high strength steel sheet according to claim 1 or 2, which contains at least one of the following. Furthermore, in mass%,
Ca: 0.003% or less (not including 0%) and / or REM: 0.003% or less (not including 0%)
The ultra-high strength steel sheet according to any one of claims 1 to 3. A method for producing the ultra high strength steel sheet according to any one of claims 1 to 4,
The steel satisfying the component according to any one of claims 1 to 4 is heated and held at a temperature of A ₃ point to (A ₃ point + 20 ° C) for 10 to 600 seconds, and then at an average cooling rate of 3 ° C / s or more. A method for producing an ultra-high strength steel sheet, wherein the steel sheet is cooled to a temperature not lower than the Ms point and not higher than the Bs point and heated and held in the temperature range for 1 to 1800 seconds.

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