JP2006336074A - High strength and high ductility steel sheet having excellent chemical convertibility - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel sheet having satisfactory ductility and excellent hole expansibility in addition to the characteristics of high strength, and further having excellent chemical convertibility and plating properties. <P>SOLUTION: Regarding the steel sheet having high strength and excellent ductility, and also having excellent chemical convertibility and plating properties, retained austenite with a mean grain size of ≤500 nm is comprised in crystal grains at a space factor of 3 to 20% as a second phase structure, and the concentration of Si is ≤0.8 mass%. The second phase structure comprises an austenite stabilization element(s), and the content of the austenite stabilization element(s) in the second phase structure is preferably higher than 10 mass% the content of the austenite stabilization element(s) in the whole of the steel sheet. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a high-strength and high-ductility steel sheet excellent in chemical conversion processability. Specifically, the steel sheet exhibits high strength of, for example, about 700 MPa class or more, has good ductility, excellent hole expansibility, and chemical conversion processability and plating. The present invention relates to a high-strength steel sheet having excellent properties.

  Aiming at weight reduction and safety improvement of automobiles etc., for example, high strength steel plate of 700MPa class or higher, and 900MPa class or higher, with good ductility, excellent hole expansibility (λ) and chemical conversion processability. A strong steel sheet is desired.

  As a steel sheet that achieves both strength and ductility, a ferrite-martensite composite structure steel sheet [Dual-Phase (DP) steel sheet in which the parent phase has a ferrite structure and coarse island martensite is dispersed at the three major points of the ferrite. ] Is known (for example, Patent Document 1). This DP steel sheet has a low yield ratio (YR), high tensile strength (TS), and excellent elongation (El) characteristics. However, since coarse martensite is the starting point of fracture, stretch flangeability (hole Expandability: Inferior to λ).

Recently, TRIP (Transformation Induced Plasticity) steel sheets have attracted attention. This TRIP steel sheet produces retained austenite (γ _R ) in the structure and this γ _R induces transformation (strain-induced transformation; TRIP) during work deformation and exhibits excellent ductility. For example, polygonal Known are TRIP type composite structure steel (PF steel) composed of ferrite + bainite + residual austenite structure and TRIP type bainite steel (BF steel) composed of bainetic ferrite + residual austenite + martensite. However, these have the disadvantage that ductility, particularly stretch flangeability, is poor.

  Therefore, various studies have been made to provide a steel sheet having excellent formability while maintaining an excellent balance between strength and ductility due to retained austenite. The present applicant also previously proposed a TRIP steel sheet having such required properties as a high-strength and high-ductility steel sheet with tempered martensite and tempered bainite as the parent phase structure and retained austenite as the second phase structure. (Patent Documents 2 to 5). These steel sheets introduce martensite and bainite (and also ferrite) by adjusting the cooling rate after hot rolling, etc., and cool them in a specific pattern from the ferrite-austenite two-phase temperature to generate residual austenite. Made of glue.

  On the other hand, a technique in which mechanical properties are improved by forming a fine second-phase structure at a nano level has been proposed.

  For example, Non-Patent Document 1 reports carbide behavior during slab heating in a high-strength hot-rolled steel sheet precipitation strengthened with nano-level (size that does not become a starting point of fracture) carbide (cementite). In this paper, heat treatment is performed to dissolve all carbides during slab heating, so it is possible to obtain hot-rolled steel sheets in which nano-level carbides are finely dispersed in crystal grains, resulting in improved stretch flangeability. Expected to be. However, this document only discloses a technique for finely dispersing nano-level carbides in crystal grains, and the carbides are hard and thus are not ductile.

Further, “FIG. 10” of Non-Patent Document 2 shows a photograph of a TRIP steel sheet in which the retained austenite of the second phase is finely dispersed by utilizing a so-called spheroidizing treatment in which cementite is spheroidized in the grains. . However, as shown in the schematic diagram of FIG. 9, the retained austenite is not dispersed in the matrix, and since the hard carbide surrounds the retained austenite, the strength is about 590 MPa. Despite the low degree, the elongation is as small as about 25%, and the excellent ductility effect due to retained austenite is not obtained (see “Table 2”).
JP 55-122821 JP 2002-309334 A JP 2002-302734 A JP 2003-73773 A JP 2003-171735 A Materials and Processes, 2003, 16, 1419 Klaus Ebarle, Pierre Cantinese and Philip Harlet (Klaus Eberle, Pierre Canineauz and Philippe Harlet), Steel Research, “New to produce high strength, low alloy multiphase steel with transformation induced plasticity (TRIP) Thermomechanical strategy (New thermomechanics for the production of high strength low allied multiphase page 38, IP23, 19th IP)

  Under the circumstances as described above, the present inventors have advanced research for the purpose of providing a high-strength TRIP steel sheet in which the strength-ductility balance is improved to a level higher than that of the conventional material. The technology described in the issue was developed.

  In the present invention, an appropriate amount of an austenite stabilizing element such as Cu or Ni is contained in the steel component, and fine retained austenite is formed as a second phase in the crystal grains, so that it has high strength, good ductility, and formability. In addition, an excellent steel sheet is provided. However, this prior invention does not take into account chemical conversion properties and plating properties.

  Incidentally, in the invention disclosed in the above Japanese Patent Application No. 2003-418354, the retained austenite is finely dispersed in the crystal grains as a means for increasing the strength and ductility. However, since retained austenite is generally low in stability and easily decomposes and transforms into ferrite, a certain amount of C (usually about 0.1% or more) is added to keep the retained austenite stable.

  On the other hand, C is an element that easily forms a carbide called cementite by reacting with Fe, and when C forms cementite, C cannot contribute to the stabilization of austenite. Therefore, in a TRIP steel sheet in which residual austenite is finely dispersed, a relatively large amount of Si is added in order to suppress precipitation of C as cementite. Incidentally, since Si has an action that delays the formation of cementite kinetically, the increase of Si suppresses the formation of cementite due to the reaction between C and Fe and stabilizes retained austenite.

  On the other hand, since Si is an element that is very easily oxidized, it concentrates on the surface layer of the steel sheet to form an oxide containing Si on the surface, which significantly impairs chemical conversion properties and plating properties. For these reasons, the retained austenite-dispersed TRIP steel sheet having excellent strength-ductility generally has poor chemical conversion properties and plating properties, and the TRIP steel sheet could not be applied to applications in which these characteristics are important.

  The present invention has been completed as a result of further research under the circumstances as described above, and its purpose is to effectively utilize the technical idea of the invention of the prior application, in addition to the characteristics of high strength and high ductility. An object of the present invention is to provide a steel sheet that can exhibit excellent properties in chemical conversion treatment and plating properties.

  The high-strength and high-ductility steel sheet excellent in chemical conversion treatment according to the present invention that has solved the above-mentioned problems is a residual austenite having an average grain size of 500 nm or less as a second phase structure in the crystal grains in a space ratio of 3 to 3. It is characterized by being 20% dispersed and having a Si content of 0.8% by mass or less.

  In the high-strength and high-ductility steel sheet according to the present invention, one having a Si content suppressed to 0.2% by mass or less is preferable because it exhibits excellent performance not only in chemical conversion properties but also in plating properties. The two phases contain an austenite stabilizing element, and the content of the austenite stabilizing element in the second phase structure is preferably 10% by mass or more higher than the content of the austenite stabilizing element in the entire steel sheet. Note that Cu, Ni, Ag, Au, Pt, Pd and the like are preferable as the austenite stabilizing element, and these can be added alone or in combination of two or more in any combination as required. it can.

  According to the present invention, the presence of the nano-sized (500 nm or less) second phase structure (residual austenite) in the crystal grains has an excellent strength / elongation balance, and the Si content is 0.8 mass%. The chemical conversion processability is ensured by reducing to a level or lower, and the Si content is suppressed to a level of 0.2% by mass or less. A steel sheet excellent in all of ductility (formability), chemical conversion treatment, and plating properties can be provided.

In the above-mentioned prior application, the present inventors, in order to increase the ductility of a high-strength steel sheet, “the conventional DP steel sheet and TRIP steel sheet are inferior in stretch flangeability (λ) although ductility (elongation) is good. The reason is that the second phase structure (residual austenite) of these steel sheets is coarse and therefore acts as a starting point of fracture, ”and has been studied repeatedly. As a result, in order to obtain predetermined characteristics, (i) if the size of the second phase structure is reduced to the nano level and the stress concentration at the interface with the second phase structure is reduced as a matrix (matrix phase structure), The phase 2 structure should no longer act as a starting point for destruction;
(Ii) “Such a nano-sized second phase structure may be generated not in the fragile matrix interface (grain boundary) but in the matrix (crystal grains)”
Based on this finding, further research has been conducted from the viewpoint of “how to stably disperse the nano-level second phase structure in the crystal grains”. As a result, in order to embody the above knowledge, if a segregation part (concentration region) of an austenite stabilizing element is introduced into a crystal grain in a size of several nanometers to several hundred nanometers in advance, It was clarified that a high-strength steel sheet with significantly improved ductility, particularly stretch flangeability (λ), can be obtained by performing heat treatment so that the segregation part does not disappear. In the present invention, these basic ideas are essentially the same as those of the prior invention.

  Specifically, if an austenite stabilizing element such as Cu, Ni or the like is added to steel and the austenite stabilizing element is finely segregated (concentrated) in the crystal grains and then subjected to a predetermined heat treatment, the crystal grains A high-strength and highly ductile TRIP steel sheet containing nanosized retained austenite therein is obtained.

  That is, the high-strength and high-ductility steel sheet obtained by the above-mentioned method is the one in which retained austenite having an average particle size of 500 nm or less is generated as the second phase in the crystal grains, and thereby, ductility, particularly stretch flangeability is achieved. It was possible to raise significantly. Here, “inside crystal grains” means inside crystal grains excluding crystal grain boundaries, and includes, for example, block interfaces in crystal grains and lath interfaces in the blocks. However, in order to further improve the stretch flangeability, the region excluding these block interfaces and lath interfaces is preferable.

  The “second phase structure” is retained austenite. Although the second phase structure is determined in relation to the matrix structure, the present invention is mainly directed to the TRIP steel sheet (the steel sheet described in Patent Documents 2 to 5), and the TRIP steel sheet (the matrix structure is sintered). In the case of a single structure of tempered martensite or bainite; or a tempered martensite and ferrite, a mixed structure of bainite and ferrite, or a single structure of ferrite), the second phase structure is retained austenite. In addition, although the said structure | tissue (matrix and 2nd phase) in this invention is preferably formed with the structure | tissue substantially mentioned above, the other structure | tissue (pearlite, mother phase structure) inevitably remain | survives in a manufacturing process. Does not exclude the inclusion of bainite when tempered martensite and tempered martensite when the matrix structure is bainite) and precipitates.

  Furthermore, the average particle diameter of the “second phase structure” satisfies 500 nm or less. The average particle size is measured by first corroding the steel sheet with nital and identifying the second phase structure by observation with a transmission electron microscope (TEM; magnification 40,000 times), and then in a field of view of 2.3 μm × 1.9 μm. The average value of the particle diameter (maximum diameter) of the second phase structure present is calculated, and the average particle diameter in a total of five fields is calculated in the same manner. These average values are defined as “average particle diameter of second phase structure”.

  When the average particle diameter of the second phase structure measured in this way exceeds 500 nm, the second phase structure becomes a starting point of fracture, and satisfactory ductility cannot be obtained. Therefore, the structure can be identified by the above-described second-phase structure observation method (40,000 times TEM observation) and the average particle diameter can be calculated is almost the lower limit.

  Furthermore, the space factor of the “second phase structure” in the entire structure is set in the range of 3 to 20%. When the space factor of the second phase structure is less than 3%, the improvement effect such as ductility due to the formation of the second phase structure is not effectively exhibited, while when the space factor of the second phase structure exceeds 20%, This is because the two-phase particles are close to each other or coalesced to form a cluster and easily become a starting point of destruction. A more preferable space factor of the second phase structure is 5% or more and 15% or less.

  Next, the matrix structure will be described. The matrix structure in the TRIP steel sheet includes a total of five types of tempered martensite, bainite or ferrite single structure; or tempered martensite and ferrite or bainite and ferrite mixed structure. Can be mentioned.

  The tempered martensite which is one of the matrix structures in the TRIP steel sheet is as described in Patent Documents 2 to 5, and the tempered martensite has the characteristics (strength and ductility) intended in the present invention. It is extremely useful to secure. That is, tempered martensite is characterized in that it has a lath-like crystal grain and high hardness, but has a lower dislocation density and is softer than ordinary martensite. In the present invention, “tempered martensite” and normal “martensite” can be distinguished by, for example, observation with a transmission electron microscope (TEM).

  The matrix structure in the TRIP steel sheet includes a mixed structure containing ferrite in addition to these tempered martensite and bainite. This ferrite means polygonal ferrite, that is, ferrite having a low dislocation density, and the ductility is further improved by the formation of ferrite.

  The space factor of the matrix structure in the steel sheet of the present invention is preferably controlled by the balance with the above-described second phase structure and appropriately adjusted so that predetermined characteristics are obtained.

  Next, components in the steel sheet of the present invention will be described. Hereinafter, all the units of chemical components are mass%.

C: 0.12% or less C is useful for securing the strength, and particularly in the case of a TRIP steel sheet, it is an important element for securing a predetermined retained austenite. However, as far as chemical properties and plating properties intended as other important characteristics are added in addition to strength and ductility in the present invention, C clearly shows a negative effect. Therefore, in the present invention in which it is important to ensure chemical conversion properties and plating properties, the C content is preferably minimized, and is at most 0.12%. More preferably, it is less than 0.10%, More preferably, it is 0.08% or less.

  As described above, C in the steel material plays an important role as a strength improving element and further as a retained austenite stabilizing element. Therefore, in the present invention, the lack of action due to the reduction of the C content can be compensated in some way. I need it. However, in the present invention, an austenite stabilizing element (one or more of Cu, Ni, Ag, Au, Pt, and Pd) that is added to ensure retained austenite is precipitated in the matrix in a finely dispersed state. Complementing the reduction of the amount, it exhibits a strong reinforcing effect and can secure a high level of strength. Therefore, in the present invention, it is specified that the C content is at most 0.12% or less, preferably less than 0.10%, and the lower limit is not particularly specified. It is desirable to contain about 001%.

Si: 0.8% or less Si is a useful element as a solid solution strengthening element while reducing the amount of solid solution C in the ferrite phase and improving ductility. Not only has a significant adverse effect on plating properties. Therefore, in order to ensure a practical level of chemical conversion treatment as a TRIP steel sheet, the Si content should be suppressed to 0.8% or less, more preferably 0.6% or less. Further, Si has a further remarkable influence on the plating property, and in order to ensure satisfactory plating property, it should be suppressed to 0.2% or less, more preferably 0.05% or less.

Mn: 0.7-4%
Mn is also useful as a solid solution strengthening element like Si, and is an indispensable element for stabilizing the austenite phase by suppressing the structural transformation that occurs during the cooling process. Moreover, in the case of a TRIP steel plate, it contributes to the generation of retained austenite similarly to Si. In order to exhibit these effects effectively, the content must be 0.7% or more. Preferably it is 1.0% or more, More preferably, it is 1.5% or more. However, their effects saturate at about 4%, so excess addition beyond that is economically wasteful. Preferably it is 3.0% or less, More preferably, it is 2.0% or less.

One or more of austenite stabilizing elements (Cu, Ni, Ag, Au, Pt, Pd):
These elements are effective elements for achieving high strength while maintaining a high strength-ductility balance, and are particularly useful as austenite stabilizing elements in TRIP steel sheets. These elements may be added singly or in combination of two or more in any combination. In the case of a TRIP steel sheet, it is particularly preferable to add Cu and Ni individually or in combination, and their preferred addition amounts are each 0.1% or more. On the other hand, if these elements are excessive, productivity is impaired such as cracking during hot rolling, so the total is preferably 10% or less (preferably 2% or less for both Ni and Cu).

  The present invention basically contains the above components, with the balance being substantially Fe. “Substantially” means that impurities that are inevitably brought in from steel raw materials, materials, manufacturing equipment, etc. are allowed. For example, P is about 0.02% or less, and S is about 0.01%. Hereinafter, N may be included in an amount of about 0.008% or less. In addition, for example, the following elements may be positively contained within a range that does not adversely affect the operation of the present invention described above.

Cr: 1.0% or less Cr is an element that contributes to strength improvement, and 0.1% or more (more preferably 0.2% or more) can be added in order to effectively exhibit its action. However, the effect is not only saturated at about 1.0%, but if it is too much, the ductility is deteriorated, and in the case of TRIP steel sheet, excessive amount of Cr generates carbides and inhibits the formation of residual austenite (γ _R ). To do. Therefore, Cr should be suppressed to 1.0% or less. More preferably, it is 0.8% or less.

Al: 2.0% or less Al is an element that contributes to deoxidation. However, if it exceeds 2.0%, cracking is likely to occur during continuous casting, so it should be suppressed to less than that, and more preferably 1.0%. % Or less.

At least one selected from the group consisting of Ti, Nb, and V: 0.1% or less in total. These elements all act as precipitation strengthening elements. At least one (one or two or more) may be added in a total of 0.01% or more (more preferably 0.05% or more). However, when the total amount exceeds 0.1%, the carbide is generated can not be secured a suitable gamma _R content, and more preferably suppressed to 0.08% or less in total.

  Next, the manufacturing method of this invention steel plate is demonstrated.

  In the present invention, a method of adding an austenite stabilizing element is employed as a method for stably generating the second phase structure in the crystal grains in a finely dispersed state. That is, a matrix in which an austenite stabilizing element (specifically, at least one selected from the group consisting of Cu, Ni, Ag, Au, Pt, and Pd) is added to steel and these elements are dissolved in a supersaturated state. Then, the austenite stabilizing element is precipitated as a metal phase or a carbide phase by a predetermined aging treatment, and a nano-size (several nm to several hundred nm) segregation part (concentration region) is introduced. Finally, a TRIP steel sheet containing residual austenite as the second phase structure is produced by applying a predetermined heat treatment (the method described in Patent Documents 2 to 5 described above) while paying attention so that the segregated portion does not disappear. Is the method.

  This method is greatly different from the methods described in Patent Documents 2 to 5 described above. In the above method, before hot rolling, a matrix in which an austenite stabilizing element is dissolved in supersaturation is generated in advance, and the austenite stabilization is performed. This is a point in which a step of introducing a segregation part (concentration region) in which elements are deposited in nanosize [specifically, solution treatment, quenching treatment, and aging treatment as described later] is added.

  This process utilizes the property that the diffusion of the austenite stabilizing element is slower than that of carbon. That is, the austenite stabilizing element is a substitutional element and has a diffusion coefficient slower than that of an interstitial element such as carbon and is difficult to diffuse compared to carbon or the like even during heat treatment. Can be easily generated.

  Hereinafter, it demonstrates along each process.

(1) Step of introducing a segregation part (concentrated region) of nano-sized austenite stabilizing element into a matrix First, a steel material containing the above chemical components (however, an austenite stabilizing element is an essential component) is solutionized. Process. This solution treatment (soaking) is very useful as a means of preventing central segregation of Mn and the like, and dissolving all components in the steel uniformly, and is also important because it ultimately contributes to segregation of austenite stabilizing elements. It is a process.

  In order to exert such an action effectively, it is important to appropriately control the temperature and time of the solution treatment. In the present invention, the solution treatment is performed at about 1100 ° C. or more for 5 hours or more. If the temperature is too low, a sufficient effect cannot be obtained, and if the time is too short, the diffusion time until the solute elements are uniformly distributed is insufficient, so that a satisfactory effect cannot be obtained. These temperatures and times are such that a predetermined effect is exhibited only when both are appropriately controlled, and are preferably set at 1150 ° C. or higher for 10 hours or longer, more preferably 1200 ° C. or higher for 15 hours or longer. The upper limit is not particularly limited from the viewpoint of preventing segregation of Mn. The higher the processing temperature and the longer the processing time, the better. However, in consideration of productivity, cost, etc., the upper limit is 1400 ° C. or lower and 20 hours or shorter. Good.

  Next, after forming into a thin steel plate by hot rolling and cold rolling as necessary, quenching treatment is performed as necessary, and aging treatment is further performed.

  Of these, the quenching process is performed to obtain a matrix in which the austenite stabilizing element is supersaturated, and when the matrix has already been obtained by the solution treatment, the quenching process is omitted. Also good. The quenching treatment conditions are not particularly limited, and the conditions that are usually performed (heating to the austenitizing temperature and then rapidly cooling) can be employed.

  Next, when an aging treatment is performed, the austenite stabilizing element dissolved in supersaturation in the matrix is precipitated in the matrix as a metal or carbide of several to several hundred nm, and the segregation part (concentration region) of the austenite stabilizing element Is formed. This aging treatment is an important step for finally stably dispersing fine austenite of 500 nm or less in the crystal grains. If the aging treatment is omitted, not the crystal grains but the crystal grain boundaries and the lath interface. It has been confirmed that coarse retained austenite exceeding 500 nm is formed, and the intended strength and ductility cannot be obtained in the present invention.

  The aging treatment conditions (temperature and time) vary depending on the type and content of the added austenite stabilizing element, and the size of the obtained austenite varies depending on the aging treatment conditions. In general, a range of from 400 to 750 ° C. for 20 to 720 minutes is preferable.

  After the above treatment is performed, austenite stabilizing elements are segregated (concentrated) in nano size by a predetermined hot rolling treatment, and cold rolling treatment is performed as necessary. A TRIP steel sheet having a desired second phase structure is obtained. In any case, the segregation part (concentrated region) of the austenite stabilizing element formed in the matrix by the above method is heat-treated particularly by controlling the hot rolling temperature so that it does not diffuse / disappear in the subsequent heat treatment. Thus, the TRIP steel plate intended in the present invention can be obtained.

  When the matrix structure is tempered martensite (which may contain ferrite), martensite (martensite that has not been tempered) is obtained by hot rolling, and the segregated portion of the austenite stabilizing element. What is necessary is just to implement the method employ | adopted when obtaining the austenite in which (concentration part) was introduce | transduced.

  First, the steel sheet is heated in the austenite region, and it is preferable that the heating temperature and time are approximately 800 to 1000 ° C. and the heating time is 1 to 20 minutes. This is because, when heated at a high temperature for a long time, the segregated portion of the austenite stabilizing element generated in the above process diffuses and disappears.

  Next, martensite is generated by rapidly cooling the steel sheet to a temperature below the Ms point. When it is desired to generate ferrite in addition to martensite, the cooling rate may be controlled so as to pass through the ferrite transformation region in the continuous cooling transformation curve (CCT curve). However, since the pearlite structure is undesirable for the present invention, it is preferable to appropriately control the cooling rate so as to avoid the pearlite transformation region.

  As for the cooling rate, when it is desired to generate only martensite (no generation of ferrite), a method of rapidly cooling to a predetermined temperature (one-stage cooling method) is simple. However, when it is desired to further generate ferrite, it is difficult to stably generate ferrite by one-step cooling, so it is preferable to employ a multi-stage cooling method in which the cooling rate is set in a plurality of times, particularly austenite-ferrite 2 A method of cooling again after holding at the phase temperature is recommended. In either case of adopting the above-described cooling pattern of the single-stage cooling method or the multi-stage cooling method, the cooling rate is preferably 10 ° C./second or more (preferably 20 ° C./second or more).

  By such a hot rolling treatment, a steel sheet into which martensite (and also ferrite) has been introduced and a segregation part (concentration part) of the austenite stabilizing element in the matrix is obtained.

Next, this steel sheet is heated at a temperature of A ₁ point or more. Thereby, only the segregation part of the austenite stabilizing element is austenitized by reverse transformation, and the part excluding the segregation part is maintained as martensite (tempered martensite).

  The heating temperature at this time is preferably 1000 ° C. or less. This is because when the heating temperature is high, the segregated portion (concentrated region) of the austenite stabilizing element formed in the matrix by the above method diffuses and disappears.

  In addition, the heating time is appropriately selected according to the set amount of the target second phase structure (residual austenite), and varies depending on the heating temperature and the subsequent cooling rate. However, it is usually selected from the range of 10 seconds or more (preferably 20 seconds or more, more preferably 30 seconds or more) and 600 seconds or less (preferably 500 seconds or less, more preferably 400 seconds or less). If the heating time is short, the residual austenite is insufficient, and if it is too long, the tempered martensite is insufficient, or the lath-like structure characteristic of the tempered martensite is damaged, and the residual austenite is coarsened or remains. Carbide is easily generated by the decomposition of austenite.

  The steel sheet is then air cooled to room temperature while avoiding ferrite and pearlite transformations. Thereby, the austenite part produced | generated by the heating mentioned above will be maintained and austenite (residual austenite), and it will be cooled, and finally, tempered martensite is made into a parent phase, and 2nd phase structure (inside crystal grain) In addition, a TRIP steel sheet having a residual austenite of 500 nm or less is obtained.

  In the second phase structure obtained as described above, the austenite stabilizing element is segregated (concentrated), and the content of the austenite stabilizing element in the second phase structure is the austenite stabilizing element in the entire steel sheet. Compared with the content of the chemical element, the content is 10% by mass or more.

  Incidentally, in the above-mentioned patent documents 2 to 5, in the annealing process, so-called austempering treatment is performed for the purpose of “stabilizing residual austenite by generating a C-enriched region”, but in the above method employed in the present invention. The austempering process is not always essential. In the present invention, since the austenite concentration region is formed in advance by the addition of the austenite stabilizing element, it remains without performing the austempering treatment that positively imparts the C concentration region as in the non-patent document. This is because austenite is stabilized. Of course, the austempering treatment may be performed for the purpose of further stabilizing the retained austenite.

  Considering the actual operation, it is easy to perform the annealing treatment after the cold rolling using a continuous annealing facility or a batch annealing facility. When plating a cold-rolled sheet, 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.

  When the parent phase structure is bainite or ferrite, the steel sheet is heated in the same manner as described above, and then rapidly cooled to a temperature not lower than the Ms point and not higher than the Bs point while avoiding the ferrite transformation and the pearlite transformation. Treatment (bainite transformation) is performed. By this treatment, bainite is generated, and the segregation part of the austenite stabilizing element is maintained as austenite.

  If it is desired to generate ferrite in addition to bainite, the cooling rate may be controlled so as to pass through the ferrite transformation region in the continuous cooling transformation curve (CCT curve). However, since the pearlite structure is not desirable for the present invention, the cooling rate is appropriately controlled so as to avoid the pearlite transformation region. The cooling rate at this time may be controlled in the same manner as described above.

  Here, after cooling to a temperature not lower than the Ms point and not higher than the Bs point [for example, a temperature not lower than 300 ° C. (preferably not lower than 350 ° C.) and not higher than 480 ° C. (preferably not higher than 450 ° C.), the temperature is maintained for a predetermined time (austempering). The reason for this is to secure a predetermined amount of retained austenite while transforming the parent phase to bainite. The holding time in the temperature range can be appropriately set according to the amount of retained austenite in the target TRIP steel sheet, and cannot be uniformly defined, but is, for example, 10 seconds or longer (preferably 50 seconds or longer). If the holding time is too long, the bainite transformation proceeds and the amount of retained austenite decreases. Therefore, the holding time is 1200 seconds or less, preferably 600 seconds or less.

  Further, when air-cooled to room temperature after the austempering treatment, the segregated portion of the austenite stabilizing element is maintained as it is, and a second phase structure containing the desired retained austenite is obtained. The second phase structure obtained in this way contains a predetermined amount of residual austenite of 500 nm or less in the crystal grains.

  In addition, it is easy to perform the series of heat treatments described above using a continuous annealing facility or a batch annealing facility. When plating a cold rolled plate, 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.

  Hereinafter, the configuration and operational effects of the present invention will be described in more detail with reference to experimental examples.However, the present invention is not limited by the following experimental examples, and is appropriate within a range that can be adapted to the purpose described above and below. It is also possible to carry out the invention with modifications, and these are all included in the technical scope of the present invention.

  The physical property test methods employed in the following experimental examples are as follows.

[Tensile strength (TS) and total elongation (El)]
It measured using the JIS5 test piece.

[Hole expansion rate (λ)]
A 70 mm long x 70 mm wide x 2.0 mm thick test piece was punched out and a hole with a diameter of 10 mm was punched in the center. The hole expansion ratio (λ) was measured (Iron and Steel Federation Standard JFST 1001).

[Tissue observation]
The area ratio of the matrix structure (tempered martensite and bainite) in each steel material was measured after corroding the steel material with nital and identifying the structure by observation with a scanning electron microscope (SEM: magnification 1000 times or 2000 times). . Residual austenite was measured for volume fraction (%) by a saturation magnetization measurement method [Japanese Patent Laid-Open No. 2003-90825, R & D Kobe Steel Technical Report / Vol. 52, no. 3 (Dec. 2002)].

[Chemical conversion processability]
After the chemical conversion treatment of the surface of each test steel plate under the following conditions, the steel plate surface was observed with a SEM at 1000 times, and the adhesion status of zinc phosphate crystals was examined for 10 arbitrarily selected visual fields. evaluated.
Chemical treatment liquid: using a treatment liquid “Palbond L3020” manufactured by Nihon Parkerizing Co., Ltd.
Chemical conversion treatment process: degreasing → water washing → surface adjustment → chemical conversion treatment,
Evaluation criteria:
○: Zinc phosphate crystals adhere to all 10 fields of view with an area of 95% or more.
×: 5% of non-adhered portion of zinc phosphate crystal (referred to as skein) even in one of ten views
There are more.

[Plating properties]
Hot dip galvanization was performed by immersing each test steel sheet in a hot dip zinc bath containing 0.13% Al and having a temperature of about 450 ° C., and the surface of each of the obtained plated steel sheets was 10 mm in an area of 500 mm × 500 mm. The visual field was visually observed and evaluated according to the following criteria based on the presence or absence of a region where plating was not attached (referred to as non-plating).
○: No plating is seen in all 10 fields of view,
X: Non-plating exists even in one of ten visual fields.

Experimental Examples 1-7
A steel material having the composition shown in Table 1 was melted and cast, and then roughly rolled at a heating temperature of 1100 ° C. to obtain a slab having a thickness of 30 mm, soaked at 1200 ° C. for 24 hours, and then cooled in a furnace. Next, after the surface oxide scale was ground and removed, hot rolling was performed at a finishing temperature of 900 ° C. to obtain a hot-rolled steel sheet having a thickness of 3.2 mm. After pickling, it was cold-rolled to obtain a thin steel plate having a thickness of 1.2 mm, which was heated at 950 ° C. for 5 minutes and then water-quenched, followed by aging treatment at 500 ° C. for 10 hours. The sample was heated again at 920 ° C. for 5 minutes, rapidly cooled to 400 ° C., held at the same temperature for 4 minutes, and then air-cooled.

  The properties of the obtained steel sheet are as shown in Table 1, and have a structure in which about 20 nm of retained austenite is dispersed in the crystal grains, and the retained austenite fraction is as shown in Table 1, respectively. As is apparent from Table 1, reference numerals 1 to 5 are examples that satisfy all the requirements of the present invention, and have excellent chemical conversion properties. On the other hand, reference numerals 6 and 7 are comparative examples in which the Si content exceeds the prescribed requirement, and the chemical conversion property is poor.

Experimental Examples 8-13
Steel materials having the composition shown in Table 2 were melted and cast, then roughly rolled at a heating temperature of 1100 ° C. to obtain a slab having a thickness of 30 mm, soaked at 1200 ° C. for 24 hours, and then cooled in a furnace. Next, after grinding and removing the oxide scale on the surface, hot rolling was performed at a finishing temperature of 900 ° C. to obtain a hot-rolled steel sheet having a thickness of 3.2 mm. After pickling, it was cold-rolled to obtain a thin steel plate having a thickness of 1.2 mm, which was heated at 950 ° C. for 5 minutes and then water-quenched, followed by aging treatment at 500 ° C. for 10 hours. This sample was again heated at 920 ° C. for 5 minutes, rapidly cooled to 400 ° C. and then allowed to enter the plating bath, then removed from the plating solution, held at 400 ° C. for 4 minutes, and then air-cooled.

  The properties of the obtained steel sheet were as shown in Table 2, and about 20 nm of retained austenite was dispersed in the crystal grains, and the retained austenite fraction was as shown in the same table. As is apparent from Table 2, reference numerals 8 to 11 are examples that satisfy the requirements of the present invention, and have excellent plating properties. On the other hand, the codes | symbols 12 and 13 have a Si content exceeding 0.2 mass%, and have poor plating properties.

Claims (4)

  In the crystal grains, residual austenite having an average grain size of 500 nm or less as a second phase structure is dispersed in a space ratio of 3 to 20%, and the Si content is 0.8% by mass or less. High-strength, high-ductility steel plate with excellent processability.   Plating characterized in that residual austenite having an average grain size of 500 nm or less as a second phase structure is dispersed in the crystal grains in a space factor of 3 to 20%, and the Si content is 0.2% by mass or less. High strength and high ductility steel sheet with excellent properties.   The second phase contains an austenite stabilizing element, and the content of the austenite stabilizing element in the second phase structure is 10% by mass or more higher than the content of the austenite stabilizing element in the entire steel sheet. The high strength and high ductility steel sheet according to claim 1 or 2.   The high-strength and high-ductility steel sheet according to claim 3, wherein the austenite stabilizing element is at least one selected from the group consisting of Cu, Ni, Ag, Au, Pt, and Pd.