JP2005179703A - High strength steel sheet having excellent elongation and stretch-flange formability - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high strength steel sheet which can satisfy the balance of a stretch and stretch flange formability on a level higher than that in the conventional case. <P>SOLUTION: The high strength steel sheet contains retained austenite and/or martensite with a mean grain size of 500 nm or under in the crystal grains as a second phase structure at a space factor of 3 to 20% to the whole structure. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a high-strength steel sheet excellent in elongation and stretch flangeability. Specifically, for example, the elongation (total elongation) in a high-strength region of about 780 MPa class or more (preferably about 980 MPa class or more) is 30% or more, The present invention provides a high-strength steel sheet having a stretch flangeability (λ) satisfying 50% or more.

  Aiming to reduce the weight and improve safety performance of automobiles, etc., it is a high-strength steel plate of about 780 MPa or higher, and further about 980 MPa or higher, with elongation (total elongation) and stretch flangeability [hole expandability (local ductility )]] Is highly desired.

  Conventionally, as a steel sheet that achieves both strength and ductility, a ferrite-martensitic composite structure steel sheet [Dual-Phase (DP Steel plate] is known (for example, refer to Patent Document 1). The 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 The spreadability was inferior to λ).

Recently, attention has been focused on TRIP steel sheet (Transformation Induced Plasticity). A TRIP steel sheet generates retained austenite (γ _R ) in a structure, and this γ _R induces transformation during deformation (strain-induced transformation: TRIP) and exhibits excellent ductility. For example, polygonal 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 are known. However, these have the disadvantage of being inferior in stretch flangeability.

  In view of this, various studies have been made to provide a steel sheet having excellent formability such as stretch flangeability while maintaining an excellent balance between strength and elongation due to retained austenite. The present applicant has already disclosed a TRIP steel sheet having tempered martensite, tempered bainite, etc. as a parent phase structure and residual austenite as a second phase structure as a high-strength steel sheet having such required characteristics. (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 generate residual austenite by cooling in a specific pattern from the ferrite-austenite two-phase temperature. It is manufactured by letting

  On the other hand, a technique for improving mechanical characteristics by forming a fine (nano-level) second phase structure has been proposed.

  For example, Non-Patent Document 1 reports the carbide behavior during slab heating in a high-strength hot-rolled steel sheet precipitation strengthened with carbide (cementite) having a nano-level (size that does not become a starting point of fracture) size. According to this method, since heat treatment is performed in which all carbides dissolve during slab heating, a hot-rolled steel sheet in which nano-level carbides are finely dispersed in crystal grains can be obtained. Is also expected to improve. However, the above document only discloses a technique for finely dispersing nano-level carbides in crystal grains, and the desired ductility cannot be obtained because the carbides are hard.

Also, FIG. 10 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 process in which cementite is spheroidized in the grains. However, the retained austenite is shown in FIG. As shown in the schematic diagram of FIG. 9, since it is not dispersed in the matrix and hard carbide is surrounded around the retained austenite, the elongation is low even though the strength is as low as about 590 MPa. It can be seen that the excellent ductility effect due to retained austenite is not obtained (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 Steel Research, Klaus Eberle, Pierre Cantineauz and Philippe Harlet, “New to produce high-strength, low-alloy multiphase steels with transformation-induced plasticity (TRIP) New thermomechanical strategies for the production of high strength low alloyed multiphase steel showing a transformation induced plasticity (TRIP) effect ”, 1999, 70, 6, p. 233-238 pages

  The present invention has been made paying attention to the above circumstances, and its object is to provide a high-strength TRIP steel sheet that can satisfy the balance between elongation and stretch flangeability at a higher level than before. is there.

  The high-strength steel sheet excellent in elongation and stretch flangeability according to the present invention that has solved the above problems is a retained austenite and / or martensite having an average particle size of 500 nm or less as a second phase structure in the crystal grains. Is contained in an area ratio of 3 to 20% with respect to the entire tissue.

  The second phase structure contains an austenite stabilizing element (for example, at least one selected from the group consisting of Co, N1, Cu, Ag, Au, Pt, and Pd) and occupies the second phase structure The content rate of an austenite stabilization element is 20 mass% or more higher than the content rate of the austenite stabilization element which occupies for the whole steel plate.

  According to the present invention, since the nano-sized (500 nm or less) second phase structure (residual austenite and / or martensite) is contained in the crystal grains, the balance between elongation and stretch flangeability is extremely excellent. A strength steel plate can be manufactured.

In order to further improve the elongation and stretch flangeability of the high-strength steel sheet, the present inventors stated that “the reason why the conventional DP steel sheet and TRIP steel sheet are inferior in stretch flangeability (λ) although ductility (elongation) is good. This is because the second phase structure (residual austenite and / or martensite) of these steel sheets is coarse and therefore acts as a starting point of fracture ”. As a result, in order to obtain the desired characteristics,
(I) If the size of the second phase structure is refined to the nano level and the stress concentration at the interface between the matrix (matrix phase structure) and the second phase structure is reduced, the second phase structure becomes a starting point of fracture. To stop working;
(Ii) Moreover, based on the knowledge that such a nano-sized second phase structure should be generated not in the fragile matrix interface (grain boundaries) but in the matrix (crystal grains), Further studies have been made on the basis of whether the nano-level second phase structure can be stably dispersed in the crystal grains. As a result, in order to embody the above knowledge, a segregation part (concentration region) of austenite stabilizing element / carbon / martensite stabilizing element is previously formed in a crystal grain with a size of several nm to several hundred nm. It was found that a high-strength steel sheet having excellent elongation and particularly stretch flangeability can be obtained if heat treatment is performed so that the segregation part does not disappear in the subsequent heat treatment step. .

In particular,
(A) By adding an austenite stabilizing element such as Cu or Ni to the steel, the austenite stabilizing element is finely segregated (concentrated) in the crystal grains, and then subjected to a predetermined heat treatment. , A method for producing a TRIP steel sheet containing nanosized retained austenite (further martensite) in crystal grains;
(B) bainite (especially lower bainite); or by utilizing spherical cementite, a carbon-enriched region at a nano level is introduced into the crystal grains, and then subjected to a predetermined heat treatment to obtain nano-sized retained austenite (further A method for producing a TRIP steel sheet containing martensite in crystal grains;
(C) By adding a martensite formation promoting element to the steel, the martensite formation promoting element is finely segregated (concentrated) in the crystal grains, and then subjected to a predetermined heat treatment to obtain a nanosize. Thus, the present inventors have completed the present invention by adopting a method such as a method for producing a DP steel sheet containing martensite in crystal grains.

  Hereinafter, the high strength steel sheet of the present invention will be described in detail.

  As described above, the characteristic part of the present invention is that “residual austenite and / or martensite having an average particle size of 500 nm or less is generated in the crystal grains as the second phase structure”. Thereby, elongation, especially stretch flangeability can be remarkably improved. First, the second phase structure in the present invention will be described.

  Here, “inside crystal grains” means inside crystal grains excluding crystal grain boundaries, and includes, for example, block interfaces in crystal grains, lath interfaces in the blocks, and the like. However, in order to further improve the elongation and stretch flangeability, it is recommended that the region excluding these block interfaces and lath interfaces be used.

Further, the “second phase structure” in the present invention is retained austenite and / or martensite. Specifically, the second phase structure can be determined in relation to the matrix structure, but in the present invention, both the DP steel sheet and the TRIP steel sheet (the steel sheets described in Patent Documents 2 to 5 described above) are targeted. Because
(I) In the case of DP steel sheet (matrix structure is bainite or ferrite), the second phase structure is martensite;
(II) In the case of a TRIP steel sheet (the parent phase structure is a single structure of tempered martensite or bainite; or a mixed structure of tempered martensite and ferrite or bainite and ferrite), the second phase structure is retained austenite, or Residual austenite and martensite. It should be noted that the structure (matrix and second phase) in the present invention is preferably substantially formed of the structure described above, but other structures (pearlite, matrix structure) inevitably remain in the manufacturing process. In the case of tempered martensite, bainite, tempered martensite in the case where the matrix structure is bainite, etc.) and the inclusion of precipitates are not excluded.

  Furthermore, the average particle diameter of the “second phase structure” satisfies 500 nm or less. The average particle size is measured as follows. First, after corroding a steel plate with nital and identifying the second phase structure by observation with a transmission electron microscope (TEM; magnification 40,000 times), the second phase present in a visual field of 2.3 μm × 1.9 μm. The average value of the particle size (maximum diameter) of the tissue is calculated. Similarly, the average particle diameter in a total of 5 fields of view was calculated, and these average values were defined as “average particle diameter of the second phase structure” in the present invention. 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 desired elongation and stretch flangeability cannot be obtained. The smaller the average particle size of the second phase structure, the better, and no lower limit is specified. Accordingly, the lower limit can be determined by the above-described method for observing the second phase structure (40,000-fold TEM observation) where the structure can be identified and the average particle diameter can be calculated.

  Further, the space factor of the “second phase structure” with respect to the whole structure is 3 to 20%. When the space factor of the second phase structure is less than 3%, the effect of improving ductility and the like due to the formation of the second phase structure cannot be effectively exhibited. On the other hand, when the space factor of the second phase structure exceeds 20%, the second phase particles come close to each other or coalesce to form a cluster, which becomes a starting point of destruction.

  Next, the matrix structure in the present invention will be described.

  As described above, the present invention is intended for both DP steel plates and TRIP steel plates, and the matrix structure in DP steel plates is bainite or ferrite, while the matrix structure in TRIP steel plates is tempered martensite or bainite alone. There are four types of structures; or a mixed structure of tempered martensite and ferrite or bainite and ferrite.

  Among these, 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 desired characteristics (elongation and stretch flangeability). It is extremely useful to secure The tempered martensite in the present invention is characterized in that the crystal grains are in a lath shape and the hardness is high, but the dislocation density is small and soft compared to normal martensite. “Tempered martensite” and normal “martensite” in the present invention can be distinguished by, for example, observation with a transmission electron microscope (TEM).

  Further, the matrix structure in the TRIP steel sheet includes a mixed structure containing ferrite in addition to these tempered martensite and bainite. The above ferrite means polygonal ferrite, that is, ferrite having a low dislocation density, and stretch flangeability can be further enhanced by the formation of ferrite.

  The space factor of the parent phase structure in the DP steel plate and TRIP steel plate of the present invention is controlled by the balance with the second phase structure described above, and it is recommended to adjust appropriately so as to obtain desired characteristics.

  In the foregoing, the second phase structure and the matrix structure that characterize the present invention have been described.

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

C: 0.05-0.6%
C is useful for securing the strength, and in particular in the case of a TRIP steel sheet, it is an essential element for securing a predetermined retained austenite. Preferably, the C content is 0.08% or more, more preferably 0.10% or more. On the other hand, when C is excessive, not only the effect is saturated, but also defects due to center segregation easily occur at the casting stage. Accordingly, the C content is 0.6% or less, preferably 0.5% or less, and more preferably 0.4% or less. In addition, when the amount of C exceeds 0.3%, the weldability decreases. In consideration of weldability, it is recommended that the C content be 0.3% or less, preferably 0.28% or less, and more preferably 0.25% or less.

Si: 0.5 to 3.5%
Si is a useful element as a solid solution strengthening element while reducing the amount of solid solution C in the ferrite phase and contributing to improvement in ductility such as elongation. On the other hand, in the TRIP steel sheet, Si is an element that contributes to the formation of retained austenite. Preferably it is 1.0% or more, More preferably, it is 1.2% or more. However, if added over 3.5%, there is a risk of cracking, and the workability also deteriorates. Preferably it is 3% or less, More preferably, it is 2.5% or less, More preferably, it is 2.0% or less.

Mn: 0.7-4%
Mn is also useful as a solid solution strengthening element like Si, and is an element necessary for stabilizing the austenite phase by suppressing transformation in the cooling process. Moreover, in the case of a TRIP steel plate, it is an element which contributes to the production | generation of a retained austenite like Si. In order to exhibit such an action effectively, it is necessary to add 0.7% or more. Preferably it is 1.0% or more, More preferably, it is 1.5% or more. However, if the addition exceeds 4%, the above effect is saturated, which is economically wasteful. Preferably it is 3.0% or less, More preferably, it is 2.0% or less.

  The present invention basically contains the above components, and the balance is substantially iron, but P is 0.02% or less as an inevitable impurity brought into the steel depending on the situation of raw materials, materials, manufacturing equipment, etc. S may be included in a range of 0.01% or less, N in a range of 0.008% or less, and the like. In addition, the following elements can also be positively contained within a range that does not adversely affect the operation of the present invention.

At least one element selected from the group consisting of Co, Ni, Cu, Ag, Au, Pt, and Pd is an element effective for achieving high strength while maintaining a high strength-ductility balance. Especially in TRIP steel sheet, it is useful as an austenite stabilizing element. The above elements may be added alone or in combination of two or more. In particular, in the case of a TRIP steel plate, the use of Ni and / or Cu is recommended, and it is preferable to add Ni: 0.1% or more and / or Cu: 0.1% or more. On the other hand, if these elements are added excessively, productivity deteriorates such as cracking during hot rolling, so the total of these elements is 10% or less (preferably Ni: 2% or less and / or Cu: 2). % Or less).

Cr: 1.0% or less (excluding 0%)
Cr is an element that contributes to strength improvement. In order to effectively exhibit such action, Cr is preferably added in an amount of 0.1% or more (more preferably 0.2% or more). However, even if Cr is added excessively, the effect is saturated and ductility deteriorates. Moreover, in the case of a TRIP steel plate, if Cr is added excessively, carbide is generated and the generation of retained austenite is reduced. From this point of view, it is preferable to suppress Cr to 1% or less. More preferably, it is 0.8% or less.

Al: 2.0% or less Al is an element contributing to deoxidation, but if it exceeds 2.0%, cracking due to continuous casting occurs. More preferably, it is 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 have a precipitation strengthening action. In order to effectively exhibit such an action, at least one of the above elements (one may be used, or two or more may be used in combination) is 0.01% or more in total (more preferably 0.05). % Or more) is recommended. However, the total amount of the element exceeds 0.1%, the carbide is formed and the desired gamma _R content can not be obtained. More preferably, it is 0.08% or less in total.

  The chemical components in the present invention have been described above.

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

  In the present invention, the methods (a) to (c) described above can be used as a method for stably finely dispersing the desired second phase structure in the crystal grains. Hereinafter, description will be made sequentially.

I: Method for generating retained austenite (further martensite) in the grains]
(A) Utilization of austenite stabilizing element In this method, first, an austenite stabilizing element in steel (specifically, at least one selected from the group consisting of Co, Ni, Cu, Ag, Au, Pt, and Pd). To form a matrix in which these elements are dissolved in supersaturation, and then, by a predetermined aging treatment, the austenite stabilizing element is precipitated as a metal phase or a carbide phase. After introducing a segregation part (concentrated region) of several hundred nm (a-1), finally, a predetermined heat treatment (described in Patent Documents 2 to 5 described above) is performed while taking care that the segregation part does not disappear. (A-2) is performed to produce a TRIP steel sheet containing a desired second phase structure. In the method of the present invention, the point greatly different from the methods described in Patent Documents 2 to 5 described above is that, in the present invention, before hot rolling, a matrix in which the austenite stabilizing element is dissolved in supersaturation in advance is generated, and the austenite stabilization This is the point of adding a process [specifically, solution treatment, quenching treatment, and aging treatment as necessary] to introduce a segregation part (concentration zone) in which the crystallization element is deposited in nano size. .

  The step (a) utilizes the characteristic that the austenite stabilizing element has a slower diffusion than carbon. That is, the austenite stabilizing element is a substitutional element and has a slower diffusion coefficient than interstitial elements such as carbon, and is difficult to diffuse compared to carbon or the like even during heat treatment. Is easy to generate.

  Hereinafter, it demonstrates according to each process.

(A-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-described chemical components (however, an austenite stabilizing element is an essential component) Solution treatment. This solution treatment (soaking) prevents center segregation due to Mn and the like, and is extremely useful as a means for uniformly dissolving all components in the steel, and can ultimately contribute to segregation of austenite stabilizing elements, It is an important process.

  In order to effectively exhibit such an action, it is important to appropriately control the temperature and time of the solution treatment. In the present invention, the solution treatment is performed at 1270 ° C. or more for 5 hours or more. Below 1270 ° C., the solubility curve cannot be reached and the desired effect cannot be obtained. Also, if it is less than 5 hours, the diffusion time until the solute atoms are uniformly distributed is insufficient, so that the desired effect cannot be obtained. These temperatures and times are intended to exhibit the desired effect only when both are appropriately controlled, but preferably solution treatment is performed at 1300 ° C. or higher for 10 hours or longer, more preferably at 1350 ° C. or higher for 15 hours or longer. It is recommended to process. The upper limit is not particularly limited from the standpoint of preventing segregation of Mn and the like. The larger the treatment temperature and the treatment time, the better. However, in consideration of productivity, cost, etc., at 1430 ° C. or less for 25 hours or less ( More preferably, it is recommended to control at 1400 ° C. or lower for 20 hours or shorter.

  Next, after hot rolling and, if necessary, cold rolling to produce a thin steel sheet, a quenching process and an aging process are performed as necessary.

  Of these, the quenching treatment is performed for the purpose of obtaining a matrix in which the austenite stabilizing element is supersaturated, and when the matrix has already been obtained by the solution treatment described above, this quenching treatment is performed. Can be omitted. The quenching process is not particularly limited, and a condition that is usually performed (heating to the austenitizing temperature and then rapidly cooling) can be employed.

  Next, by performing an aging treatment, the austenite stabilizing element dissolved in supersaturation in the matrix is precipitated in the matrix as a metal layer or a carbide layer of several to several hundred nm, and the segregation part of the austenite stabilizing element ( (Enriched area) is generated. The above aging treatment is extremely important for finally stably dispersing fine austenite and the like of 500 nm or less in the crystal grains. If the aging treatment is omitted, the grain boundaries and the lath interface are not in the crystal grains. Furthermore, it has been confirmed by experiments that coarse residual austenite exceeding 500 nm is generated and desired elongation and stretch flangeability cannot be obtained (see No. 7 in Table 1 described later).

  The conditions (temperature and time) of the aging treatment can be changed depending on the type and content of the added austenite stabilizing element, and the size of the obtained austenite can be changed depending on the conditions of the aging treatment. Therefore, although it cannot be determined uniquely, it is generally recommended to perform an aging treatment at 400 to 750 ° C. for 20 to 720 minutes.

(A-2) The nano-sized retained austenite and the like are generated in the matrix, and after performing the process more than the step of obtaining a desired matrix structure , the austenite stabilizing element is nano- sized by a predetermined hot rolling process. By generating austenite segregated (concentrated) by size and performing a cold rolling treatment as necessary, a TRIP steel sheet having bainite or ferrite as a parent phase structure and also having a target second phase structure is obtained. Furthermore, by performing predetermined annealing after the above step, a TRIP steel sheet having a tempered martensite (which may contain ferrite) as a parent phase structure and also having a target second phase structure is obtained. In any case, the segregation part (concentrated region) of the austenite stabilizing element formed in the matrix by the method (a) is particularly hot rolled so that it does not diffuse and disappear by the subsequent heat treatment. Therefore, the desired TRIP steel sheet can be obtained.

  Hereinafter, the production method will be described separately for each aspect of the matrix structure.

(A-2-1) When the matrix structure is tempered martensite (may contain ferrite)
(I) Hot-rolling treatment This hot-rolling treatment aims to obtain martensite (martensite that has not been tempered) and austenite in which a segregation part (concentration part) of an austenite stabilizing element is introduced. To be implemented.

  First, the steel sheet is heated to the austenite region. The upper limit of the heating temperature and time is 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 by the step (a) diffuses and disappears.

  Next, the steel sheet is rapidly cooled to a temperature below the Ms point to generate martensite. In addition, in addition to martensite, when it is desired to further generate ferrite, 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.

  As for the cooling rate, a method of rapidly cooling to a predetermined temperature (one-stage cooling) is convenient when only martensite is to be generated (no ferrite is generated). Since it is difficult to stably produce ferrite, it is recommended to adopt a multi-stage cooling method in which the cooling rate is set in multiple times, especially after holding at the austenite-ferrite two-phase temperature, cooling again The starting method is recommended. Regardless of the cooling pattern of the above-described single-stage cooling or multi-stage cooling method, it is recommended that the cooling rate be, for example, 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) is introduced, and a steel sheet having a segregation part (concentration part) of an austenite stabilizing element in the matrix is obtained.

(Ii) Annealing Next, the steel sheet is heated at a temperature of A ₁ point or higher. 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).

  Here, the upper limit of the heating temperature is preferably set to 1000 ° C. This is because when the heating temperature is high, the segregated portion (concentrated region) of the austenite stabilizing element formed in the matrix diffuses and disappears by the method (a) described above.

  Further, the heating time can be appropriately selected according to the set amount of the desired second phase structure (residual austenite, etc.), and also varies depending on the heating temperature, the subsequent cooling rate, etc., so it is uniformly specified. However, it can be 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 will be insufficient. On the other hand, if the heating time is long, the tempered martensite is insufficient, or the lath-like structure that is characteristic of the tempered martensite is impaired, and the coarsening of the retained austenite, Generation is likely to occur.

  Next, the steel sheet is air-cooled to room temperature while avoiding ferrite transformation and pearlite transformation. Thereby, the austenite part produced | generated by the above-mentioned heating will be maintained and austenite (residual austenite), and it will be cooled. A TRIP steel sheet containing the retained austenite of 500 nm or less in the grains is obtained.

  In the second phase structure obtained as described above, the austenite stabilizing element is segregated (concentrated). Specifically, the content of the austenite stabilizing element in the second phase structure is as high as 20% by mass or more compared to the content of the austenite stabilizing element in the entire steel sheet.

  Incidentally, in the methods of Non-Patent Documents 2 to 5 described above, a so-called austempering treatment is performed in the annealing process for the purpose of “stabilizing retained austenite by generating a C-enriched region”. In the case of an embodiment utilizing an austenite stabilizing element), the austempering process is not necessarily an essential process. In the present invention, by adding the austenite stabilizing element, since the austenite concentration region is generated in advance, even without performing the austempering treatment that positively imparts the C concentration region as in the non-patent document, This is because the retained austenite is stabilized. Of course, the austempering treatment may be performed in the present invention for the purpose of further stabilizing the retained austenite.

  In consideration of 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 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.

(A-2-2) When the parent phase structure is bainite or ferrite In this case, the steel sheet is first heated in the same manner as in the above (i), and then to a temperature not lower than the Ms point and not higher than the Bs point. By rapidly cooling while avoiding ferrite transformation and pearlite transformation and performing a predetermined austempering treatment (bainite transformation) at the temperature, bainite is generated and the segregation part of the austenite stabilizing element is maintained as austenite.

  In addition to the bainite, if it is desired to further generate ferrite, 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 may be controlled in the same manner as (i) 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 reason for the treatment is to secure a desired 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 set amount of retained austenite in the target TRIP steel sheet, and it is difficult to uniformly define, for example, 10 seconds or more (preferably 50 seconds or more) To do. If the holding time is too long, the bainite transformation proceeds and the amount of retained austenite decreases. Therefore, it is recommended that the holding time be controlled to 1200 seconds or less, preferably 600 seconds or less.

  Further, after air-cooling 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 desired residual austenite is obtained. On the other hand, if the austenite treatment is rapidly cooled to room temperature, the segregated portion of the austenite stabilizing element becomes martensite and becomes martensite.

  The second phase structure obtained as described above 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.

(B) Utilization of bainite structure This method is different from the “method of utilizing an austenite stabilizing element” in the above (a), instead of adding an austenite stabilizing element to steel, As previously introduced bainite containing fine carbide in the interior, by utilizing these structures, by introducing a carbon-enriched region in the crystal grains, and then applying a predetermined heat treatment, A TRIP steel sheet containing a second phase structure such as retained austenite is produced. This method utilizes the property that austenite is stabilized by the formation of a segregation part (concentration region) of carbon, and a carbon concentration region can be introduced in advance before the hot rolling treatment. It ’s fine.

  For this purpose, for example, a method using a pearlite structure is conceivable. However, in order to obtain the target retained austenite of the second phase in the present invention, it is necessary to generate cementite in the crystal grains. The goal cannot be achieved. Therefore, bainite is utilized in the method (b). Of these, bainite contains fine carbides (cementite) in ferrite crystal grains (in the lath), and in particular, use of lower bainite containing a lot of fine carbides is recommended.

  In addition, the method of (a) mentioned above can be substantially employ | adopted as the method of said (b) except using the steel material which does not add an austenite stabilization element as a steel material. However, since carbon has a very fast diffusion coefficient compared to the austenite stabilizing element, especially the temperature and time of aging treatment, the heating rate during annealing, etc., so that the enriched area of the generated carbon does not diffuse and disappear. It is necessary to manage this more strictly. If the heating time is long, the concentrated carbon has a uniform concentration due to diffusion. On the other hand, if the heating time is short, the carbide remains as it is, and a second phase structure containing the desired retained austenite is obtained. Because there is no.

II: Method for obtaining DP steel sheet containing martensite as the second phase structure
(C) Utilization of martensite formation promoting element In this method, after martensite formation promoting element is added to steel, the martensite formation promoting element is finely segregated (concentrated) in the crystal grains. This is a method for producing a DP steel sheet containing nano-sized martensite in crystal grains by performing a predetermined heat treatment. The method described above is substantially the same as the method (a) described above except that, instead of using a steel material added with an austenite stabilizing element, a steel material added with a martensite formation promoting element is used. The method (a) can be employed. That is, after introducing a segregation part (concentration zone) of the martensite formation promoting element into the matrix by performing a predetermined solution treatment, quenching treatment, and aging treatment as necessary, then usually employed. In manufacturing a DP steel sheet by the above-described method, a desired DP steel sheet may be manufactured while paying particular attention to heating conditions such as a heating rate so that the segregated portion does not diffuse.

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

  In the following examples, TRIP steel sheets having tempered martensite / bainite as a parent phase were produced by adding an austenite stabilizing element, and the austenite stabilizing element, the content of C element, and the aging treatment were performed by a machine. The effects on physical characteristics were investigated.

  Specifically, the steel materials No. 1 to 7 having the composition shown in Table 1 (units in the table are% by mass, the balance: iron and inevitable impurities) are heat-treated under the conditions described later. The structural fraction and mechanical properties of the obtained steel sheet were measured by the following methods.

  First, the tensile strength (TS) and total elongation (El) of a steel plate were measured using a JIS No. 5 test piece.

  For stretch flangeability, a test piece with a length of 70 mm x width 70 mm x thickness 2.0 mm is prepared, a hole with a diameter of 10 mm is punched out in the center, and then the hole is widened on a flash with a 60 ° conical punch. Evaluation was made by measuring the hole expansion rate (λ) at the time of crack penetration (Iron and Steel Federation Standard JFST 1001).

  In addition, the structure was observed as follows.

  The area ratio of the matrix structure (tempered martensite and bainite) in each steel material was measured after the steel material was corroded with nital and the structure was identified by observation with a scanning electron microscope (SEM: magnification 1000 times or 2000 times). did.

  The retained austenite was measured for volume fraction (%) by a saturation magnetization measurement method [see Japanese Patent Application Laid-Open No. 2003-90825, R & D Kobe Steel Engineering Reports / Vol.52, No.3 (Dec. 2002)].

  The results thus obtained are also shown in Table 1. In the table, the area ratio of the matrix structure is omitted, but a value obtained by subtracting the volume ratio of the second phase structure (residual austenite) from 100 is the area ratio of the matrix structure.

No. 1 (Example of the present invention)
After melting No. 1 steel shown in Table 1 and solution treatment at 1350 ° C. for 10 hours, the steel surface was ground and hot rolled (heating temperature 1200 ° C., finishing temperature 850 ° C., coiling temperature about 600 After obtaining a 2 mm thick hot rolled steel sheet by cold rolling, a cold rolled steel sheet having a thickness of 1.5 mm was obtained by cold rolling. Next, this cold-rolled steel sheet was heated at 950 ° C. for 5 minutes, then quenched with water, and then aged at 450 ° C. for 240 minutes. Further, the steel sheet was heated at 950 ° C. for 5 minutes, then quenched in water, rapidly heated to 800 ° C. in an infrared heating furnace, held at that temperature for 10 seconds, and then air-cooled to obtain No. 1 A steel plate was obtained.

  The results are as shown in Table 1. The matrix structure is tempered martensite, and the second phase structure is a TRIP steel sheet in which about 300 nm of retained austenite is dispersed in crystal grains, and the elongation and A high strength steel plate of about 980 MPa class having excellent stretch flangeability was obtained.

No. 2 (Comparative example of Example 1)
No. above. No. 1 except that Cu and Ni as austenite stabilizing elements were not added. No. 1 by the same production method. Two steel plates were obtained.

  The results are as shown in Table 1. Since No. 2 does not contain an austenite stabilizing element, no retained austenite was obtained and elongation was lowered.

No. 3 (Comparative example of No. 1)
No. above. No. 1 except that C in the steel was 0.30%. No. 1 by the same production method. 3 steel plates were obtained.

The results are as shown in Table 1. No. 3 has a large amount of C in the steel, so a lot of retained austenite is produced, and elongation and stretch flangeability deteriorate.
No. 4 (Comparative example of No. 1)
No. above. 1, omitting the aging treatment and the steps after the heating in the infrared heating furnace were: “After rapid heating to 800 ° C. in the infrared heating furnace, hold at that temperature for 5 minutes, and further at 400 ° C. for 3 minutes. Except for “Retained”, no. No. 1 by the same production method. 4 steel plates were obtained.

  The results are as shown in Table 1. Since No. 4 was not subjected to the predetermined aging treatment, 800 nm coarse retained austenite was generated at the lath interface, and elongation and stretch flangeability were deteriorated.

No. 5 (Invention Example 2)
No. above. No. 1 steel material is used. By the method similar to 1, operation to the aging treatment was implemented. Then, after heating the said steel plate for 5 minutes at 950 degreeC, it cooled to 500 degreeC (bainite formation temperature range), and after hold | maintaining for 3 minutes at the said temperature, the steel plate of No. 5 was obtained by air-cooling.

  The results are shown in Table 1. The parent phase structure is bainite, and the second phase structure is a TRIP steel sheet in which about 400 nm of retained austenite is dispersed in crystal grains, and the elongation and stretch flangeability are as follows. A high-strength steel sheet of about 980 MPa class excellent in the above was obtained.

No. 6 (Comparative example of No. 5)
No. above. No. 5 except that Cu and Ni which are austenite stabilizing elements were not added. In the same manufacturing method as in No. 5, no. 6 steel plates were obtained.

  The results are as shown in Table 1. Since 6 does not contain an austenite stabilizing element, no retained austenite was obtained and elongation was lowered.

No. 7 (Invention Example 3)
No. No. 1 except that C in the steel is 0.08%. No. 1 is used as the steel material having the same composition as No. 1. No. 1 by the same production method. Seven steel plates were obtained.

  The results are as shown in Table 1. The matrix structure is tempered martensite, and the second phase structure is a TRIP steel sheet in which about 200 nm of retained austenite is dispersed in crystal grains, the elongation, and A high strength steel sheet of about 780 MPa class having excellent stretch flangeability was obtained.

Claims (3)

  Elongation characterized by containing 3 to 20% of retained austenite and / or martensite having an average particle size of 500 nm or less as a second phase structure in the crystal grains in a space factor with respect to the entire structure, and an elongated flange High strength steel plate with excellent properties.   The second phase structure contains an austenite stabilizing element, and the content of the austenite stabilizing element in the second phase structure is 20% by mass compared to the content of the austenite stabilizing element in the entire steel sheet. The high-strength steel sheet according to claim 1, which is higher than the above.   The high-strength steel sheet according to claim 2, wherein the austenite stabilizing element is at least one selected from the group consisting of Co, Ni, Cu, Ag, Au, Pt, and Pd.