JP5503346B2 - Ultra-high strength thin steel sheet with excellent hydrogen embrittlement resistance - Google Patents

Description

  The present invention relates to an ultra-high-strength steel sheet excellent in hydrogen embrittlement resistance, and more particularly to an ultra-high-strength thin steel sheet in which fracture caused by hydrogen, which is a problem of placement cracking and delayed fracture, which is a problem in ultra-high-strength steel sheets having a tensile strength of 1180 MPa or more is suppressed Is. The steel plate of the present invention is suitably used as a material for, for example, automobiles and transportation equipment.

  In particular, TRIP (Transformation Induced Plasticity) steel sheets have attracted attention as ultra high strength steel sheets of 1180 MPa or more. The TRIP steel sheet has an austenite structure remaining, and when deformed at a temperature higher than the martensite transformation start temperature (Ms point), the retained austenite (residual γ) is transformed into martensite by stress, resulting in a large elongation. It is a steel plate. TRIP steel sheets are classified according to the type of parent phase. For example, TPF steel with polygonal ferrite as the parent phase, TAM steel with tempered martensite as the parent phase, and TBF steel with bainitic ferrite as the parent phase are known. It has been. Among these, TBF steel has been known for a long time, and it is easy to obtain high strength by hard bainitic ferrite, and it has excellent elongation due to fine retained austenite formed at the boundary of lath-like bainitic ferrite. Is obtained. Furthermore, TBF steel also has a manufacturing merit that it can be easily manufactured by a single heat treatment (continuous annealing process or plating process).

  On the other hand, in the ultra-high strength region of 1180 MPa class or higher, it is known that the TRIP steel plate as well as other high-strength steel materials has a new problem of delayed fracture due to hydrogen embrittlement. 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. It is a phenomenon, and as a result, it causes adverse effects such as a decrease in the ductility and toughness of the metal material. As the strength of the steel sheet increases, the hydrogen embrittlement resistance tends to decrease.

  In view of such circumstances, the applicant of the present application is a TRIP steel sheet containing residual austenite with bainitic ferrite and martensite as a parent phase, and by appropriately controlling the area ratio and dispersion form of residual austenite, TRIP steel sheets with improved hydrogen embrittlement resistance are disclosed (for example, Patent Document 1 and Patent Document 2). Specifically, we focused on bainitic ferrite and retained austenite, which have a very high hydrogen trapping capacity and hydrogen storage capacity. It is in the shape.

  Although the hydrogen embrittlement resistance is remarkably improved by the steel sheet, further improvement is required.

JP 2007-197819 A JP 2006-207018 A

  The present invention has been made in view of the above circumstances, and its purpose is to significantly increase hydrogen embrittlement resistance in an ultrahigh strength region where the tensile strength is 1180 MPa or more without impairing the excellent ductility characteristic of the TRIP steel sheet. It is to provide an ultra-high strength thin steel sheet.

  The ultra-high-strength steel sheet of the present invention that has solved the above problems is C: 0.10 to 0.25% (meaning mass%, hereinafter the same), Si: 1.0 to 3.0%, Mn: 1.0 to 3.5%, P: 0.010% or less (excluding 0%), S: 0.002% or less (not including 0%), or 0.004% or more and 0.01% or less Al: 1.5% or less (excluding 0%), Cr: 0.003 to 2.0%, balance: iron and inevitable impurities, Mn amount is [Mn], S amount is [S] When [Mn] × 1000 [S] satisfies 2.2 or less, or 12.5 or more and 25 or less, and contains 1% or more of retained austenite as an area ratio with respect to the entire structure, the retained austenite crystal grains Has an average axial ratio (major axis / minor axis) of 5 or more, an average minor axis length of 1 μm or less, and a maximum distance between the retained austenite grains. It has a gist where the adjacent distance satisfies 1 μm or less and the tensile strength is 1180 MPa or more.

  In a preferred embodiment of the present invention, the steel sheet has an area ratio with respect to the entire structure, the total of bainitic ferrite and martensite is 80% or more, and the total of ferrite and pearlite is 9% or less (including 0%). Is satisfied.

  In preferable embodiment of this invention, the said steel plate is further Cu: 0.003-0.5%, Ni: 0.003-1.0%, Mo: 1.0% or less, Nb: 0.1% Hereinafter, from the group consisting of Ti: 0.003-1.0%, V: 0.003-1.0%, Zr: 0.003-1.0%, and W: 0.003-1.0% Including at least one selected.

  In a preferred embodiment of the present invention, the steel sheet further comprises Ca: 0.0005 to 0.005%, Mg: 0.0005 to 0.01%, and REM: 0.0005 to 0.01%. Including at least one selected from.

  In a preferred embodiment of the present invention, the steel sheet further includes B: 0.0002 to 0.01%.

  According to the present invention, P and S in steel are remarkably reduced, and the amount of Mn and S are appropriately controlled, so that the ultrahigh strength thin film having a tensile strength of 1180 MPa or more with significantly improved hydrogen embrittlement resistance. A steel plate could be provided.

FIG. 1 is a schematic diagram for explaining the short axis length and long axis length of retained austenite and the nearest neighbor distance between retained austenite crystal grains. FIG. 2 is a graph showing the relationship between [Mn] × 1000 [S] and hydrogen embrittlement resistance in Examples.

  The present inventors have further studied after the filing of the patent document, based on the tissue form described in the patent document. As a result, P and S are significantly reduced for the components in steel (specifically, 0.010% or less for P; 0.002% or less, or 0.004% or more and 0.01% or less for S), and , Mn amount and S amount are appropriately controlled (specifically, when Mn amount is [Mn] and S amount is [S], [Mn] × 1000 [S] is 2.2 or less, or 12.5 25 or less), it was found that an ultra-high strength thin steel sheet having a tensile strength of 1180 MPa or more with further improved hydrogen embrittlement resistance as compared with the above-mentioned patent document was obtained, and the present invention was completed.

  First, the components in steel will be described.

  First, P, S, and [Mn] × 1000 [S] that characterize the present invention will be described.

P: 0.010% or less (excluding 0%)
P is an element that not only promotes grain boundary fracture due to grain boundary segregation but also serves as a starting point of corrosion. As a result, hydrogen concentration increases at grain boundaries and the like, and delayed fracture resistance deteriorates. According to the examination results of the present inventors, it has been found that hydrogen embrittlement resistance is remarkably improved when the amount of P is reduced to a lower level (upper limit is 0.010%) than before. If the amount of P exceeds 0.010%, the hydrogen embrittlement resistance cannot be greatly improved even if the amount of Mn and the amount of S are appropriately controlled. The lower the upper limit of the P content, the better, preferably 0.005%, and more preferably 0.003%. From the viewpoint of hydrogen embrittlement resistance, it is better that the amount of P is small. On the other hand, when 0.03% or more is added, corrosion resistance is improved. Therefore, when considering corrosion resistance, 0.03% or more should be added. preferable. Specifically, it is preferable to appropriately control the lower limit of the P amount in consideration of the balance between hydrogen embrittlement resistance and corrosion resistance.

S: 0.002% or less (excluding 0%), or 0.004% or more and 0.01% or less S is an element that exhibits a complicated behavior with respect to delayed fracture resistance, and particularly includes residual austenite. In TRIP steel sheets, the degree of contribution to delayed fracture varies depending on the content, shape, carbon concentration, etc. of retained austenite. Generally, however, it is better to add less S for the following reasons (a) to (c). Is said to improve.
(A) S is a starting point of corrosion in a corrosive environment and is an element that promotes corrosion generation and hydrogen absorption.
(I) S produces inclusions such as MnS, and these inclusions become the starting point of cracking and deteriorate the mechanical strength.
(C) S has the effect of destabilizing retained austenite, and it is better not to add so much in TRIP steel such as TBF steel.

  On the other hand, according to the examination results of the present inventors, the MnS has a strong ability to trap hydrogen at the boundary (interface) between MnS and its surroundings, and when MnS is finely dispersed, It was found that it contributes to the improvement of delayed fracture resistance.

Furthermore, (e) when the steel contains Ni or Cu, sulfides such as Ni ₃ S ₂ and Cu ₂ S formed by reaction with S prevent hydrogen on the steel surface from entering the steel. It has been found that in some cases it is desirable to improve delayed fracture resistance because of its action to prevent.

  In view of the action (advantages and disadvantages) of S on hydrogen embrittlement resistance, in the present invention, the amount of S is (i) S: 0.002% or less, or (ii) S: 0.004% or more and 0. Two aspects of less than 0.01% were identified.

  First, considering the adverse effect (cons) of S on hydrogen embrittlement resistance, (i) S: 0.002% or less, S was reduced to a very small level. Thus, if the amount of S is reduced to the limit, it does not become a starting point of corrosion, so there is no need for a hydrogen trap. In the above (i), the smaller the amount of S, the better, and preferably 0.001% or less.

  On the other hand, considering the usefulness (advantage) of S on hydrogen embrittlement resistance and taking into account the balance with the adverse effect of S, (ii) S: 0.004% or more and 0.01% or less. That is, in order to effectively exhibit the actions (d) and (e), the lower limit of the amount of S is set to 0.004%. However, if the amount of S becomes excessive, the above-mentioned adverse effects (a) to (c) appear, so the upper limit of the amount of S is made 0.01%. That is, S: ≦ 0.002% or S: 0.004% ≦ S ≦ 0.01%. In the above (ii), a preferable amount of S is 0.005% or more and 0.009% or less.

[Mn] × 1000 [S]: 2.2 or less, or 12.5 or more and 25 or less In the present invention, the P amount and the S amount are controlled as described above, and the relationship between the Mn amount and the S amount is also appropriately controlled. doing. This is because MnS that affects hydrogen embrittlement resistance also changes depending on the amount of Mn, and therefore the relationship between the amount of Mn and the amount of S must be appropriately controlled separately from the amount of S. Here, the parameter “[Mn] × 1000 [S]” was determined based on many basic experiments conducted to investigate the relationship between the amount of Mn and the amount of S that affect hydrogen embrittlement resistance. When the coefficient of [S] is set to 1000 with respect to the coefficient of 1], the contribution to the hydrogen embrittlement resistance (defect ratio) is that the S amount is significantly larger than the Mn amount. It is because it became clear as a result of the basic experiment.

  Specifically, first, considering the adverse effects on hydrogen embrittlement resistance caused by MnS, the upper limit of (i) [Mn] × 1000 [S] was set to 2.2. In the above (i), the smaller the value of [Mn] × 1000 [S] is better, preferably 2.0 or less, more preferably 1.8 or less. The lower limit is not particularly limited, and those determined based on [Mn] and [S] can be adopted, but mechanical properties such as strength and clean steel (low S steel) are obtained. Considering the limit, 1.0 or more is preferable.

  On the other hand, considering the usefulness to hydrogen embrittlement resistance by MnS, considering the balance with adverse effects, (ii) [Mn] × 1000 [S]: 12.5 to 25. In order to reduce the amount of Mn and the amount of S as much as in (i) above, the manufacturing cost also increases. Therefore, in (ii) above, the desired effect can be exhibited while suppressing the cost. In the above (ii), [Mn] × 1000 [S] is preferably 13 or more and 20 or less, more preferably 15 or more and 18 or less.

  The elements and relational expressions that characterize the present invention have been described above. In the present invention, the other components are as follows.

C: 0.10 to 0.25%
C contributes to improving the strength of the steel sheet, and is an essential element for securing a predetermined retained austenite. In order to secure the strength of 1180 MPa or more and the retained austenite of 1 area% or more, which is the subject of the present invention, the C content is 0.10% or more. A preferable amount of C is 0.12% or more. However, since corrosion resistance and weldability will fall if it adds excessively, from the viewpoint of ensuring these characteristics, C amount shall be 0.25% or less. A preferable amount of C is 0.23% or less.

Si: 1.0-3.0%
Si is an element that effectively suppresses the generation of carbide by decomposition of retained austenite, and is a substitutional solid solution strengthening element that contributes to hardening of the steel sheet. In order to effectively exhibit such an action, the Si amount is set to 1.0% or more. A preferable Si amount is 1.2% or more, and more preferably 1.5% or more. However, if the amount of Si exceeds 3.0%, the scale formation by hot rolling becomes remarkable, and the removal of wrinkles by the scale is costly and economically disadvantageous, so the upper limit of Si amount is 3.0%. The preferable Si amount is 2.5% or less, and more preferably 2.0% or less.

Mn: 1.0 to 3.5%
Mn is an element useful for stabilizing retained austenite and securing a desired retained austenite amount, and therefore the lower limit of the Mn amount is 1.0%. A preferable amount of Mn is 1.2% or more, and more preferably 1.5% or more. However, if excessively added, segregation of Mn becomes prominent, the stability of retained austenite is lowered, and workability may be deteriorated. Therefore, the upper limit of the amount of Mn is set to 3.5%. In consideration of workability, the preferable amount of Mn is 3.0% or less.

Al: 1.5% or less (excluding 0%)
Al is useful as a deoxidizing element, and for that purpose, the Al content is preferably 0.01% or more. Moreover, Al has not only a deoxidizing action but also an anti-corrosion effect and an anti-hydrogen embrittlement effect. The addition of Al improves the corrosion resistance. As a result, the amount of hydrogen generated under atmospheric corrosion is reduced, so that the hydrogen embrittlement resistance is considered to be improved. Furthermore, it is considered that the addition of Al improves the stability of the lath-like retained austenite and contributes to further improvement of hydrogen embrittlement resistance. In order to effectively exhibit such an effect, the Al content is preferably 0.02% or more. The amount of Al is more preferably 0.2% or more, and further preferably 0.5% or more. However, if added excessively, inclusions such as alumina increase and workability deteriorates, so the upper limit of the Al content is 1.5%.

Cr: 0.003-2.0%
Cr is useful as an element that improves strength by improving hardenability and improves hydrogen embrittlement resistance associated with a reduction in the amount of hydrogen under atmospheric corrosion due to the effect of improving corrosion resistance. Further, in the present invention, since the heat treatment conditions described later are adopted, coarse carbides are not precipitated in the steel even when Cr is added, fine carbides are dispersed in the steel, and retained austenite is effectively generated. become. As a result, the hydrogen trapping capability is improved and crack propagation can be prevented. Such an action is preferably exerted more effectively by coexisting with Cu and / or Ni. In order to effectively exhibit the above action by Cr, the lower limit of the Cr amount is set to 0.003%. A preferable Cr amount is 0.1% or more, and more preferably 0.3% or more. However, the above action is not only saturated when added in excess, but also the workability deteriorates, so the upper limit of the Cr content was set to 2.0%. A preferable Cr amount is 1.5% or less, and more preferably 1.0% or less.

  The steel sheet of the present invention contains the above components, and the balance: iron and inevitable impurities. However, the following selective components can be contained as long as the action of the present invention is not impaired.

Cu: 0.003-0.5%, Ni: 0.003-1.0%, Mo: 1.0% or less, Nb: 0.1% or less, Ti: 0.003-1.0%, V : At least one selected from the group consisting of 0.003 to 1.0%, Zr: 0.003 to 1.0%, and W: 0.003 to 1.0%. It is an element that suppresses generation and contributes to improvement of the corrosion resistance of the base material, and the above elements can be used alone or in combination of at least two or more.

  Specifically, Cu and Ni are elements that can improve the corrosion resistance of the steel sheet itself and sufficiently suppress the generation of hydrogen due to the corrosion of the steel sheet. These elements also have the effect of promoting the production of iron oxide (α-FeOOH), which is said to be thermodynamically stable and protective among rust generated in the atmosphere. By promoting the production of hydrogen, it is possible to further suppress the intrusion of the generated hydrogen into the steel sheet and sufficiently suppress the corrosion cracking due to hydrogen in a severe corrosive environment. In order to exert the above effects, it is preferable to set the lower limits of the Cu amount and the Ni amount to 0.003%, respectively. More preferably, any element is 0.05% or more, more preferably 0.1% or more. However, since workability deteriorates when added excessively, it is preferable to set the upper limits of the Cu amount and the Ni amount to 0.5% and 1.0%, respectively. More preferably, the Cu content is 0.3% or less and the Ni content is 0.5% or less.

  Among these elements other than the above, regarding Ti, V, Zr, and W, these preferable contents are all 0.003 to 1.0%.

  Among these elements, Ti is an element having an effect of promoting the generation of protective rust, similar to the above-described Cu, Ni, and Cr. This protective rust has a very beneficial effect of suppressing the production of β-FeOOH which is generated particularly in a chloride environment and adversely affects the corrosion resistance (resulting in hydrogen embrittlement resistance). The formation of such protective rust is promoted particularly by the combined addition of Ti and V (further, Zr, W). Further, Ti is a very excellent element for improving corrosion resistance and has an effect of cleaning steel. Further, V is an element effective in increasing the strength and fineness of the steel sheet, in addition to having the effect of improving hydrogen embrittlement resistance by coexisting with Ti as described above. Zr is an element effective for increasing the strength and refining of the steel sheet, and when it coexists with Ti, the effect of improving hydrogen embrittlement resistance is also exhibited. V is an element effective for increasing the strength and refining of the steel sheet, and has an effective action as a hydrogen trap site by controlling the form of carbonitride. Coexistence of Ti and Zr significantly improves hydrogen embrittlement resistance. W is effective for increasing the strength of the steel sheet, and the precipitates are also effective as hydrogen trap sites. Moreover, since the produced | generated rust has the performance which repels a chloride ion, it contributes to the further improvement of corrosion resistance. When W coexists with Ti or Zr, both corrosion resistance and hydrogen embrittlement resistance are improved.

  In order to effectively exhibit the above-described effects of Ti, V, Zr, and W, the total amount is 0.003% or more (more preferably 0.01% or more) when it is contained alone or in combination of two or more. Preferably there is. However, if excessively added, the precipitation of carbonitride increases and the workability is lowered, so when it is included alone or in combination of two or more, the total amount is preferably 1.0% or less, more preferably 0.5% or less.

  In addition, Mo is a useful element for stabilizing the retained austenite and securing a desired retained austenite amount. In addition, in order to enhance the effect of suppressing delayed hydrogen penetration and improving delayed fracture characteristics and the hardenability of the steel sheet. It is also a useful element. Furthermore, it has the effect of strengthening the grain boundaries and suppressing the occurrence of hydrogen embrittlement. In order to effectively exhibit such an action, the Mo amount is preferably 0.005% or more, more preferably 0.01% or more. However, since the said effect | action will be saturated if it adds excessively, it is preferable to make the upper limit of Mo amount into 1.0%. A more preferable amount of Mo is 0.3% or less.

  Nb is a very useful element for increasing the strength and refining of the steel sheet, and the above action is enhanced by the combined effect with Mo. In order to effectively exhibit such an action, the Nb content is preferably 0.005% or more, and more preferably 0.01% or more. However, the above action is saturated when added in excess, so the upper limit of the Nb content is preferably 0.1%. A more preferable Nb amount is 0.3% or less.

At least one of these elements selected from the group consisting of Ca: 0.0005-0.005%, Mg: 0.0005-0.01%, and REM: 0.0005-0.01% is a decrease in pH. Is an element that contributes to improving corrosion resistance, and the above elements can be used alone or in combination of at least two kinds.

  Specifically, Ca, Mg, and REM are effective elements for suppressing the increase in the hydrogen ion concentration in the interface atmosphere accompanying the corrosion of the steel sheet surface and suppressing the decrease in pH, and as a result, the local corrosivity is improved. . Furthermore, these elements control the form of sulfide in steel and contribute to the improvement of workability. In order to effectively exhibit such action, it is preferable that each element is 0.0005% or more. More preferably, any element is 0.001% or more. However, since the workability decreases when added in excess, the Ca content is preferably 0.005% or less, the Mg content is 0.01% or less, and the REM content is preferably 0.01% or less.

  Here, REM (rare earth element) is a lanthanoid element (a total of 15 elements from La with atomic number 57 to Lu with atomic number 71 in the periodic table) added Sc (scandium) and Y (yttrium). Means an element group.

B: 0.0002 to 0.01%
B is an element effective for improving the strength by improving the hardenability in addition to the effect of improving the corrosion resistance. Furthermore, it has the effect of preventing grain boundary cracking by concentrating at the grain boundaries. In order to effectively exhibit such an action, the B content is preferably 0.0002% or more, and more preferably 0.001% or more. However, since hot workability will fall when it adds excessively, it is preferable to make the upper limit of B amount 0.01%. A more preferable amount of B is 0.005% or less.

  In the above, the component in steel of this invention was demonstrated.

  Next, the organization will be described. As described above, the steel sheet of the present invention contains 1% or more of retained austenite in an area ratio relative to the entire structure, and the retained austenite crystal grains have an average axial ratio (major axis / minor axis) of 5 or more and an average minor axis length. Is 1 μm or less, and the closest distance between the retained austenite crystal grains is 1 μm or less.

  The structure is basically the same as the structure described in the above-mentioned patent document previously disclosed by the applicant of the present application, and the amount and form of retained austenite are set so as to obtain a desired high level of hydrogen embrittlement resistance. In addition to the control, the matrix structure was also a two-phase structure of bainitic ferrite and martensite. Hereinafter, each requirement will be described.

1% or more of retained austenite In the present invention, the area ratio of retained austenite with respect to the entire structure is set to 1% or more from the viewpoint of enhancing hydrogen storage capacity and ensuring high elongation. Preferably it is 2% or more, More preferably, it is 3% or more. However, if the area ratio of retained austenite increases too much, it becomes difficult to ensure the desired strength, so the upper limit is made 15%. The preferred area ratio of retained austenite is 14% or less, more preferably 13% or less.

Average axis ratio (major axis / minor axis) of residual austenite crystal grains is 5 or more, average minor axis length is 1 μm or less, and nearest neighbor distance between residual austenite crystal grains is 1 μm or less. It is important to appropriately control the short axis length and the nearest neighbor distance between residual austenite grains to form a fine lath structure on the order of submicrons. FIG. 1 is a schematic diagram for explaining the short axis length and long axis length of retained austenite and the nearest neighbor distance between retained austenite crystal grains.

  First, the average axial ratio (major axis / minor axis) of retained austenite is 5 or more. From the viewpoint of improving hydrogen embrittlement resistance, the average axial ratio is preferably as large as possible, and is preferably 10 or more. On the other hand, since the thickness of retained austenite is required to some extent in order to effectively exert the TRIP effect, the upper limit of the average axial ratio of retained austenite is preferably 30 from the above viewpoint. More preferably, it is 20 or less.

  Furthermore, the average minor axis length of retained austenite is 1 μm or less. This is because the surface area of the retained austenite is increased and the hydrogen trapping ability is increased when a large number of fine retained austenite crystal grains are dispersed with a short average minor axis length. The preferred average minor axis length of the retained austenite is 0.5 μm or less, more preferably 0.25 μm or less. The lower limit is not particularly limited from the viewpoint of hydrogen embrittlement resistance.

  Furthermore, the nearest neighbor distance between the residual austenite crystal grains is preferably 1 μm or less. This is presumably because the propagation of fracture is suppressed by the fine dispersion of fine lath-like retained austenite crystal grains. The nearest neighbor distance is preferably 0.8 μm or less, more preferably 0.5 μm or less.

  The hydrogen trapping ability of the fine lath-like austenite specified in the present invention is much larger than the ability of so-called carbides, and the hydrogen entering by so-called atmospheric corrosion can be made substantially harmless.

  For the structure other than the retained austenite, the total area ratio of bainitic ferrite and martensite is preferably 80% or more, and more preferably 85% or more, based on the area ratio relative to the entire structure. The upper limit of the matrix structure is determined by the balance with other structures. For example, the amount of ferrite and pearlite should be as small as possible, and is preferably 9% or less in total. When no ferrite structure or the like is contained, the upper limit of the matrix structure is 99% because it is composed only of the matrix structure and retained austenite.

  In this way, delayed fracture characteristics due to hydrogen can be remarkably improved by using bainitic ferrite and martensite as a parent phase and forming a composite structure containing the above-mentioned fine lath-like retained austenite.

  Here, the method for discriminating the tissue is as follows.

  Bainitic ferrite is a plate-like ferrite and means a substructure with a high dislocation density. Polygonal ferrite with no or a very small substructure (in the present invention, this polygonal ferrite is called ferrite). ) Is clearly distinguished by SEM observation. Bainitic ferrite is dark gray in SEM photographs (bainitic ferrite and residual austenite and martensite may not be clearly separated and distinguished), but polygonal ferrite is black in SEM photographs, and many It has a square shape and does not contain retained austenite or martensite.

  Residual austenite means a region observed as FCC (face-centered cubic lattice) by “FESP with Electron Back Scatter Diffraction Pattern (EBSP) detector”. In EBSP, an electron beam is incident on the sample surface and the crystal orientation at the electron beam incident position is determined by analyzing the Kikuchi pattern obtained from the reflected electrons generated at this time. The orientation distribution on the sample surface can be measured by scanning in a dimension and measuring the crystal orientation for each predetermined pitch. An example of measurement is given. An arbitrary measurement area (about 50 μm × 50 μm, measurement interval is 0.1 μm) in a plane parallel to the rolling surface at the position of the plate thickness ¼ is a measurement object. In addition, when polishing up to the measurement surface, electrolytic polishing is performed in order to prevent transformation of retained austenite. Next, using the “FE-SEM equipped with an EBSP detector”, an EBSP image is taken with a high-sensitivity camera and captured as an image on a computer. Image analysis is performed, and the FCC phase determined by comparison with a pattern by simulation using a known crystal system (in the case of retained austenite, FCC (face-centered cubic lattice)) is color-mapped. In this way, the area ratio of the mapped region is obtained, and this is used as the area ratio of the retained austenite structure.

  Next, a method for manufacturing the steel sheet will be described. As described above, the steel sheet of the present invention has the greatest feature particularly when the P amount, S amount, and the relationship between [Mn] and [S] are appropriately determined. The described method can be employed.

  Specifically, in order to form the above-described structure exhibiting ultrahigh strength and excellent hydrogen embrittlement resistance using a steel sheet satisfying the above component composition, the finishing temperature in hot rolling is set to supercooled austenite that does not generate ferrite. The temperature is as low as possible. This is because the austenite of the hot-rolled steel sheet can be refined by performing finish rolling at the temperature, and as a result, the structure of the final product is refined.

Further, after hot rolling or after cold rolling performed thereafter, heat treatment is performed in the following manner. That is, a steel satisfying the above-described composition is heated and held for 10 to 1800 seconds (tl) at a heating and holding temperature (Tl) of Ac ₃ point (ferrite-austenite transformation completion temperature) to (Ac ₃ + 50 ° C.). It is cooled to a heating and holding temperature (T2) from [Ms point (martensitic transformation start temperature) −200 ° C.] to Bs point (bainite transformation start temperature) at an average cooling rate of at least ° C./s. Hold heated for seconds (t2).

Here, when the heating holding temperature (Tl) exceeds (Ac ₃ point + 50 ° C.) or the heating holding time (tl) exceeds 1800 seconds, austenite grain growth is caused and workability (stretch flangeability) is increased. Since it falls, it is not preferable. On the other hand, when the above (Tl) is lower than the temperature at the Ac ₃ point, a desired bainitic ferrite structure cannot be obtained. Moreover, when (tl) is less than 10 seconds, austenitization is not performed sufficiently, and cementite and other alloy carbides remain, which is not preferable. The above (tl) is preferably 30 seconds or longer and 600 seconds or shorter, more preferably 60 seconds or longer and 400 seconds or shorter.

  Next, the steel sheet is cooled at an average cooling rate of 3 ° C./s or more, in order to avoid the formation of a pearlite structure by avoiding the pearlite transformation region. The higher the average cooling rate, the better, and the effect of reducing the fine lath-like austenite spacing. Preferably it is 5 degrees C / s or more, More preferably, it is 10 degrees C / s or more.

  Next, after rapidly cooling to the heating and holding temperature (T2) at the above cooling rate, a predetermined structure can be introduced by isothermal transformation. When the heating holding temperature (T2) here exceeds the Bs point, a large amount of pearlite which is not preferable in the present invention is generated, and a bainitic ferrite structure cannot be sufficiently secured. On the other hand, when the above (T2) is lower than (Ms point−200 ° C.), retained austenite is decreased, which is not preferable.

  On the other hand, when the heating and holding time (t2) exceeds 1800 seconds, the dislocation density of bainitic ferrite decreases and the amount of trapped hydrogen decreases, and a predetermined retained austenite cannot be obtained. On the other hand, even if the heating and holding time (t2) is less than 60 seconds, a predetermined bainitic ferrite structure cannot be obtained. A preferable heating and holding time (t2) is 90 seconds or longer and 1200 seconds or shorter, and more preferably 120 seconds or longer and 600 seconds or shorter.

  In addition, the cooling method after heat holding is not specifically limited, Air cooling, rapid cooling, air-water cooling, etc. can be performed. In addition, the form of residual austenite in the steel sheet can be controlled by the cooling rate at the time of production, the heating and holding temperature (T2), the heating and holding time (t2), and the like. For example, the retained austenite having a small average axial ratio can be formed by setting the heating and holding temperature (T2) to the low temperature side.

  In consideration of actual operation, it is easy to perform the heat treatment (annealing treatment) using a continuous annealing facility or a batch annealing facility. Moreover, when plating a cold-rolled sheet to form 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.

In addition, the hot rolling step (if necessary, the cold rolling step) prior to the continuous annealing treatment is not particularly limited except for the hot rolling finishing temperature, and usually the conditions to be carried out can be appropriately selected and employed. . Specifically, as the hot rolling step, for example, after hot rolling at Ar ₃ point (austenite-ferrite transformation start temperature) or higher, cooling is performed at an average cooling rate of about 30 ° C./s, and a temperature of about 500 to 600 ° C. The conditions such as winding can be adopted. Moreover, when the shape after hot rolling is bad, cold rolling may be performed for the purpose of shape correction. Here, the cold rolling rate is recommended to be 1 to 70%. Cold rolling exceeding a cold rolling rate of 70% increases the rolling load and makes rolling difficult.

  The present invention also includes steel sheets that have been subjected to plating such as chemical conversion treatment, hot dipping, electroplating, and vapor deposition. The type of plating is not particularly limited, and examples thereof include normal zinc plating and aluminum plating. The plating method is not particularly limited, and either hot dipping or electroplating may be used. Furthermore, an alloying heat treatment may be performed after plating, or a plurality of plating processes may be performed.

  The present invention also includes steel sheets that have been subjected to various types of coating, coating ground treatment, organic coating treatment, and the like. For example, a film lamination process may be performed on a steel plate that is not plated or on a plated steel plate. In the case of painting, chemical conversion treatment such as phosphate treatment or electrodeposition coating may be applied according to various applications. A known resin can be used as the paint, and an epoxy resin, a fluororesin, a silicon 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, from the viewpoint of corrosion resistance, it is recommended to use epoxy, fluorine, or silicon acrylic resin. In addition, known additives added to the paint, such as coloring pigments, coupling agents, leveling agents, sensitizers, antioxidants, UV stabilizers, flame retardants, and the like may be added. Also, the form of the paint is not particularly limited, and can be appropriately selected according to the use such as solvent-based paint, powder paint, water-based paint, water-dispersed paint, and electrodeposition paint. Examples of a method for forming the coating layer using the coating material include known methods such as a dipping method, a roll coater method, a spray method, and a curtain flow coater method. What is necessary is just to control the thickness of a coating layer to an appropriate range according to a use etc.

  The steel sheet of the present invention has very excellent hydrogen embrittlement resistance despite its high strength of 1180 MPa or higher. In addition, the strength parts for automobiles (for example, reinforcing members such as bumpers and door impact beams) formed and processed the steel sheet of the present invention also have sufficient material properties (strength, rigidity, etc.), shock absorption and delay resistance. It was confirmed that the destructibility was also good.

  Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited by the following examples, and can be implemented with modifications within a range that can meet the purpose described above and below. They are all included in the technical scope of the present invention.

  Steel of the components shown in Table 1 (remainder: iron and inevitable impurities) is vacuum-melted to form a slab, and then a hot-rolled steel sheet having a thickness of 3.2 mm is obtained according to the following process (hot rolling → cold rolling → continuous annealing). After that, the surface scale was removed by pickling and cold rolled to 1.2 mm.

<Hot rolling process>
Start temperature: 1150-1250 ° C. hold for 30 minutes Finishing temperature: 860 ° C.
Cooling rate: 40 ° C / s
Winding temperature: 550 ° C
<Cold rolling process>
Cold rolling rate: 62.5%
<Continuous annealing process>
For each steel, after holding for 120 seconds at a temperature of Ac ₃ point to Ac ₃ point + 30 ° C., and cooled at an average cooling rate of 20 ° C. / s up to T2 (° C.) shown in Table 2, 240 at T2 (° C.) After holding for 2 seconds, it was cooled with air to room temperature. Here, T2 is a temperature corresponding to the heating and holding temperature, and was controlled in the range of (Ms−200 ° C.) to Bs point as described above.

In addition, in order to investigate the influence of the manufacturing conditions on the steel sheet structure, no. 28, no. After the cold-rolled steel sheet was held for 120 seconds at a temperature of Ac ₃ point-50 ° C. (temperature corresponding to the heating holding temperature T1), the average cooling rate was 2 ° C./s. The product was cooled to T2 (° C.) described in 1. and maintained at T2 (° C.) for 240 seconds, and then air-water cooled to room temperature. That is, no. No. 28 is an example in which T1 is set to a temperature lower than the preferred range, and the average cooling rate after being held at T1 is made slower than the preferred range.

  The metal structure, tensile strength (TS), elongation [total elongation (EL)], hydrogen embrittlement resistance (hydrogen embrittlement risk evaluation index), and weldability of each steel sheet thus obtained were as follows. Each was examined and evaluated.

(Metal structure)
FE-SEM (manufactured by Philips, XL30S-FEG) for an arbitrary measurement region (about 50 μm × 50 μm, measurement interval is 0.1 μm) in a plane parallel to the rolling surface at a thickness of 1/4 of each steel plate The area ratio of bainitic ferrite (BF) and martensite (M) and the area ratio of residual austenite (residual γ) were measured according to the above method. And it measured similarly in the 2 visual fields selected arbitrarily, and calculated | required the average value. Further, other structures (ferrite, pearlite, etc.) were obtained by subtracting the area ratio occupied by the structures (BF, M, residual γ) from the total structure (100%).

  Furthermore, the average axis ratio, average minor axis length, and nearest neighbor distance between crystal grains of the residual γ crystal grains were measured according to the above method, the average axis ratio was 5 or more, and the average minor axis length was 1 μm (1000 nm ) Hereinafter, those having the nearest neighbor distance of 1 μm (1000 nm) or less satisfy the requirements of the present invention (◯), the average axial ratio is less than 5, the average minor axis length is more than 1 μm (1000 nm), and the nearest neighbor distance is 1 μm. Those exceeding (1000 nm) were evaluated as (x) not satisfying the requirements of the present invention.

(Tensile strength and elongation)
The tensile test was performed using a JIS No. 5 test piece, and the tensile strength (TS) and elongation (EL) were measured. The strain rate in the tensile test was 1 mm / sec. In the present invention, a steel sheet having a tensile strength measured by the above-described method of 1180 MPa or more was evaluated as having an elongation of 10% or more as “excellent in elongation”.

(Hydrogen embrittlement resistance)
Using a flat plate test piece with a thickness of 1.2 mm, a low strain rate tensile test method (SSRT) with a crosshead speed of 2 μm / min is performed, and a hydrogen embrittlement risk index (%) defined by the following formula is obtained. The hydrogen embrittlement resistance was evaluated.
Hydrogen embrittlement risk index (%) = 100 × (1−E1 / E0)
Where
E0 indicates the elongation at break of the test piece in a state of substantially not containing hydrogen in the steel,
E1 indicates the elongation at break of a steel material (test piece) that was corroded by 7 cycles of a combined cycle test (5% NaCl spray->dry->wet; 8 hours 1 cycle).

  If the hydrogen embrittlement risk index exceeds 70%, there is a risk of causing hydrogen embrittlement during use. Therefore, in the present invention, 70% or less was evaluated as being excellent in hydrogen embrittlement resistance.

These results are summarized in Table 2. No. in Table 2 Is the steel type No. described in Table 1. For example, No. 2 in Table 2. 1 is No. 1 in Table 1. 1 is an example. For reference, each No. is shown in the right column of Table 2 to be described later. Ac ₃ points, Bs points, and Ms points are also shown. The calculation method of the Ac ₃ point, the Bs point, and the Ms point is as follows. These are formulas quoted from Lesley Steel Material Science (published by Maruzen Co., Ltd., William C. Leslie), where [C], [Ni], etc. are the contents of elements such as C, Ni (mass) %).
Ac ₃ points = 910−203 × √ [C] −15.2 × [Ni] + 44.7 × [Si] + 104 × [V] + 31.5 × [Mo] + 13.1 × [W] −30 × [ Mn] -11 × [Cr] −20 × [Cu] + 700 × [P] + 400 × [Al] + 120 × [As] + 400 × [Ti]
Bs point = 830-270 × [C] −90 × [Mn] −37 × [Ni] −70 × [Cr] −83 × [Mo]
Ms point = 561-474 × [C] −33 × [Mn] −17 × [Ni] −17 × [Cr] −21 × [Mo]

  From the results in Table 2, it can be considered as follows.

  First, no. Nos. 1 to 17 are examples that satisfy the requirements of the present invention, and show extremely excellent hydrogen embrittlement resistance even with an ultrahigh strength steel plate of 1180 MPa or more, and the elongation that is a characteristic of TRIP steel plate is also good. . Therefore, it turns out that this invention steel plate is very useful as a reinforcement component etc. of the motor vehicle used by an atmospheric corrosive atmosphere.

  In contrast, no. 18 to 28 are examples in which the value of [Mn] × 1000 [S] defined in the present invention (hereinafter sometimes referred to as Z value) exceeds the upper limit, and hydrogen embrittlement resistance was lowered.

  Specifically, no. 18 to 21 are examples simulating Patent Document 1 and Patent Document 2 described above, and the amount and form of retained austenite satisfy the requirements of the present invention. Furthermore, since the S amount (Nos. 18 and 19), the P amount and the S amount (Nos. 20 and 21) do not satisfy the scope of the present invention, the desired hydrogen embrittlement resistance cannot be secured. That is, in order to realize the high hydrogen embrittlement resistance specified in the present invention, it is not sufficient to control the amount and form of retained austenite, and it is extremely important to appropriately control the components in the steel. I understand that.

  No. 22 to 25, 27, and 28, the amount of S satisfies the scope of the present invention, but the Z value is deviated, and the contents of C, Si, Mn, P, and Cr in the steel are also deviated and remain. The amount and form of austenite are also examples outside the scope of the present invention, and hydrogen embrittlement resistance is reduced, and TS and El are also reduced. For example, it was considered that the corrosion resistance deteriorated because No22 had an excessive amount of C, and the hardenability deteriorated because No23 had an insufficient amount of Mn, and sufficient strength could not be secured. Since No25 had insufficient C amount, sufficient intensity | strength was not obtained. In No. 27, the Cr amount is excessive, and it is assumed that coarse carbides are precipitated and the workability becomes difficult.

  No. No. 26 is an example in which only the Z value is deviated for the components in the steel, and the form of retained austenite deviates from the scope of the present invention. The hydrogen embrittlement resistance is lowered and TS is also lowered.

  In addition, No24 is an example using martensitic steel with a small amount of Si. However, residual austenite is hardly obtained, and the Z value is also outside the scope of the present invention, so that it is inferior in hydrogen embrittlement resistance.

  In No. 28, the component in steel is No. Although it was the same example as 1 but was not manufactured under the recommended conditions of the present invention (out of the heating and holding temperature T1 and out of the average cooling rate after the heating and holding), the desired matrix structure and retained austenite morphology were obtained. The hydrogen embrittlement resistance decreased. That is, no. Residual austenite No. 28 did not satisfy the average axial ratio defined in the present invention and was agglomerated, and the matrix did not have a two-phase structure of bainitic ferrite and martensite, and the strength decreased.

  For reference, FIG. 2 shows a graph showing the relationship between [Mn] × 1000 [S] and hydrogen embrittlement resistance. This figure is a plot of the results of Table 2, and when [Mn] × 1000 [S] satisfies 2.2 or less, or 12.5 or more and 25 or less, hydrogen is an index of hydrogen embrittlement resistance. It can be seen that the embrittlement risk index is 70% or less and the hydrogen embrittlement is excellent. In this test, it is presumed that the adverse effect on the corrosion resistance was larger than that of the MnS hydrogen trap, and as a result, the delayed fracture resistance also deteriorated.

Claims (5)

C: 0.10 to 0.25% (meaning mass%, hereinafter the same),
Si: 1.0-3.0%,
Mn: 1.0 to 3.5%
P: 0.010% or less (excluding 0%),
S: 0.002% or less (excluding 0%), or 0.004% or more and 0.01% or less,
Al: 1.5% or less (excluding 0%),
Cr: 0.003 to 2.0%,
The rest: iron and inevitable impurities
When the amount of Mn is [Mn] and the amount of S is [S],
When S: 0.002% or less (excluding 0%), [Mn] × 1000 [S] is 2.2 or less ,
S: When 0.004% or more and 0.01% or less, [Mn] × 1000 [S] satisfies 12.5 or more and 25 or less,
It contains 1% or more of retained austenite in the area ratio to the whole structure
Said residual austenite grains have an average axial ratio (long axis / short axis) of 5 or more, the average minor axis length is 1μm or less, the nearest neighbor distance between the residual austenite crystal grains satisfies the 1μm or less, and,
An ultra-high-strength thin steel sheet excellent in hydrogen embrittlement resistance, characterized by having a tensile strength of 1180 MPa or more.   The ultra-high-strength thin film according to claim 1, wherein the total area ratio of bainitic ferrite and martensite is 80% or more, and ferrite and pearlite are 9% or less (including 0%) in total. steel sheet.   Furthermore, Cu: 0.003-0.5%, Ni: 0.003-1.0%, Mo: 1.0% or less, Nb: 0.1% or less, Ti: 0.003-1.0% Or at least one selected from the group consisting of V: 0.003-1.0%, Zr: 0.003-1.0%, and W: 0.003-1.0%. The ultra-high strength thin steel sheet described in 1.   Furthermore, it contains at least one selected from the group consisting of Ca: 0.0005-0.005%, Mg: 0.0005-0.01%, and REM: 0.0005-0.01%. 4. The ultra high strength thin steel sheet according to any one of 3 above.   Furthermore, B: The ultra-high-strength thin steel plate in any one of Claims 1-4 containing 0.0002-0.01%.

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