JP3846206B2 - High tensile cold-rolled steel sheet with excellent strain age hardening characteristics and method for producing the same - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
  TECHNICAL FIELD The present invention relates to a high workability high-tensile cold-rolled steel sheet that is suitable mainly for automobile bodies, and in particular, a high-tensile cold-rolled steel sheet having a tensile strength (TS) of 440 MPa or more and excellent strain aging hardening characteristics, and a method for producing the same. About. The high-tensile cold-rolled steel sheet of the present invention is used in a wide range of applications, from those used for relatively light processing such as being formed into pipes by mild bending and roll forming to those used for relatively severe drawing. It is suitable for. In addition, the steel plate in this invention shall include a steel plate and a steel strip.
[0002]
  In the present invention, “excellent strain age hardening characteristics” means that after pre-deformation with a tensile strain of 5%, when subjected to aging treatment at a temperature of 170 ° C. for 20 minutes, the deformation stress increases before and after this aging treatment. The amount (denoted as BH amount; BH amount = (yield stress after aging treatment) − (predeformation stress before aging treatment)) is 80 MPa or more and before and after strain aging treatment (predeformation + the aging treatment) It means that the amount of increase in tensile strength (denoted as ΔTS; ΔTS = (tensile strength after aging treatment) − (tensile strength before pre-deformation)) is 40 MPa or more.
[0003]
[Prior art]
  In connection with recent exhaust gas regulations due to global environmental problems, the reduction of vehicle weight in automobiles has become an extremely important issue. In order to reduce the weight of an automobile body, it is effective to increase the strength of a steel plate used in large quantities, that is, to apply a high-tensile steel plate to reduce the thickness of the steel plate to be used.
[0004]
  However, even automobile parts that use thin high-strength steel sheets must exhibit the necessary and sufficient performance according to their roles. Such performance includes, for example, static strength against bending and torsional deformation, fatigue resistance, and impact resistance. Therefore, a high-tensile steel plate applied to automobile parts needs to have excellent properties after forming.
[0005]
  Also, in the process of making automotive parts, press forming is performed on the steel sheet, but if the strength of the steel sheet is too high,
(I)Shape freezeability decreases,
(B)Due to reduced ductility, problems such as cracking and necking occur during molding.
As a result, the application of high-tensile steel sheets to automobile bodies has been hindered.
[0006]
  As a technique for overcoming this, for example, in the case of cold-rolled steel sheets for outer panel panels, steel sheets are known in which ultra-low carbon steel is used as a raw material, and finally the amount of C remaining in a solid solution state is controlled within an appropriate range. Yes. This kind of steel sheet is kept soft during press forming, ensuring shape freezing and ductility, and yield stress using the strain age hardening phenomenon that occurs in the paint baking process of about 170 ℃ × 20 min. It is intended to obtain a rise and secure dent resistance. In this type of steel plate, C is dissolved and soft in the steel at the time of press forming. On the other hand, after press forming, the solid solution C adheres to dislocations introduced at the time of press forming in the paint baking process, yielding. Stress increases.
[0007]
  However, in this type of steel sheet, the yield stress increase due to strain age hardening is kept low from the viewpoint of preventing the occurrence of the strainer strain that becomes a surface defect. For this reason, the place which actually contributes to the weight reduction of components is small.
  That is, in order to reduce the weight of a part, it is necessary not only to increase the yield stress due to strain aging but also to increase the strength characteristics when the deformation progresses further. In other words, an increase in tensile strength after strain aging is desired.
[0008]
  On the other hand, for applications where the appearance is not a problem, the bake hardenability can be further improved by using a solid solution N to further increase the amount of bake-hardening and making the structure a composite structure of ferrite and martensite. A further improved steel sheet has been proposed.
  For example, JP-A-60-52528 discloses a steel containing C: 0.02 to 0.15%, Mn: 0.8 to 3.5%, P: 0.02 to 0.15%, Al: 0.10% or less, and N: 0.005 to 0.025%. A method for producing a high-strength thin steel sheet with good ductility and spot weldability is disclosed, in which hot rolling is performed at a temperature of 550 ° C. or lower and annealing after cold rolling is controlled cooling heat treatment. A steel sheet manufactured by the technique described in JP-A-60-52528 has a mixed structure composed of a low-temperature transformation product phase mainly composed of ferrite and martensite, has excellent ductility, and is actively added. It is intended to obtain high strength by utilizing strain aging during paint baking with N.
[0009]
  However, in the technique described in JP-A-60-52528, the increase in yield stress YS due to strain age hardening is large but the increase in tensile strength TS is small, and the increase in yield stress YS is also large. Due to the large fluctuations in mechanical properties such as variations, it is not possible to expect the steel sheet to be thin enough to contribute to reducing the weight of automobile parts currently required.
[0010]
  Japanese Examined Patent Publication No. 5-24979 discloses a uniform composition containing C: 0.08 to 0.20%, Mn: 1.5 to 3.5% and the balance Fe and unavoidable impurities, and having a uniform structure with a ferrite content of 5% or less. A bake-hardening high-tensile cold-rolled steel sheet made of bainite or bainite partially containing martensite is disclosed. The cold-rolled steel sheet described in Japanese Patent Publication No. 5-24979 is a bainite-based structure in which the temperature range of 400 to 200 ° C. is rapidly cooled in the cooling process after continuous annealing and then gradually cooled. In order to obtain a high bake hardening amount that has not been achieved in the past.
[0011]
  However, with the steel sheet described in Japanese Patent Publication No. 5-24979, the yield strength is increased after baking, and a high bake hardening amount that has not been obtained in the past can be obtained. When applied to a member, improvement in fatigue resistance and impact resistance after molding cannot be expected. For this reason, the problem that it cannot apply to the use for which fatigue resistance, impact resistance, etc. are requested | required strongly remained.
[0012]
  Further, a steel sheet that is subjected to heat treatment after press forming to increase not only the yield stress but also the tensile strength is proposed, although it is a hot-rolled steel sheet.
  For example, Japanese Patent Publication No. 8-23048 discloses a steel containing C: 0.02 to 0.13%, Si: 2.0% or less, Mn: 0.6 to 2.5%, sol.Al: 0.10% or less, and N: 0.0080 to 0.0250%. , Reheat to 1100 ° C or higher, perform hot rolling to finish finish rolling at 850-900 ° C, then cool down to a temperature of less than 150 ° C at a cooling rate of 15 ° C / s or more, and ferrite and martensite There has been proposed a method for producing a hot-rolled steel sheet having a composite structure mainly composed of. However, the steel sheet manufactured by the technique described in Japanese Patent Publication No. 8-23048 has a tensile strength that increases with the yield stress due to strain age hardening, but it is wound at an extremely low winding temperature of less than 150 ° C. There was a problem that the fluctuation of the mechanical characteristics was large. In addition, there is a large variation in the amount of increase in yield stress after press molding-paint baking treatment, and there is also a problem that the hole expansion rate (λ) is low, stretch flangeability is lowered, and press formability is insufficient.
[0013]
  Moreover, as a high strength steel plate having a relatively high yield stress, there is a so-called precipitation strengthened steel in which carbonitride forming elements such as Ti, Nb, and V are added and strengthened by these fine precipitates. Regardless of hot-rolled steel sheets that have undergone a process of sufficiently retaining heat after coiling, cold-rolled steel sheets are difficult to sufficiently precipitate in a short-time continuous annealing process, and have a high yield ratio (tensile strength). It was difficult to produce a steel sheet having a yield stress ratio to YS / TS). In particular, if the C content is reduced in consideration of weldability, there is a problem that it becomes more difficult to obtain a high yield ratio because the amount of precipitates itself decreases in a region where the C content is low.
[0014]
  Furthermore, although the above-described conventional steel plate is excellent in strength evaluation after the baking treatment by a simple tensile test, there is a large variation in strength when plastically deformed according to actual press conditions, and reliability is high. It was not always sufficient to apply to the required parts.
[0015]
[Problems to be solved by the invention]
  The present invention breaks the limitations of the prior art described above, has high moldability, or even higher impact resistance and stable quality characteristics, and has sufficient strength as an automobile part after being molded into an automobile part. To provide a high-tensile cold-rolled steel sheet excellent in strain age hardening characteristics that can sufficiently contribute to weight reduction of the obtained automobile body, and a manufacturing method that can manufacture these steel sheets industrially at low cost and without disturbing the shape. Objective. The strain age hardening characteristics in the present invention are targeted at a BH amount of 80 MPa or more and a ΔTS of 40 MPa or more under the aging conditions of holding at a temperature of 170 ° C. for 20 minutes after pre-deformation with a tensile strain of 5%. In addition, since it can be advantageously applied to parts to which relatively small strain is applied, a high yield ratio type high with a yield ratio of 0.7 or higher is aimed at increasing the yield stress in the original sheet and aiming to stabilize the part strength. It is also possible to use a tension cold-rolled steel sheet.TargetTo do.
[0016]
[Means for Solving the Problems]
  In order to achieve the above-mentioned problems, the present inventors manufactured steel sheets with various compositions and manufacturing conditions, and conducted many material evaluation experiments. As a result, in the field where high workability is required, N, which has not been actively used so far, is used as a strengthening element, and the large strain age hardening phenomenon expressed by the action of this strengthening element is advantageously utilized. It has been found that it is possible to easily achieve both improvement in moldability and increase in strength after molding.
[0017]
  Furthermore, in order to make the best use of the strain age hardening phenomenon caused by N, the present inventors advantageously combined the strain age hardening phenomenon caused by N with the paint baking conditions of automobiles or more actively with the heat treatment conditions after molding. For that purpose, it was found that it is effective to control the microscopic structure of steel sheet and the amount of solute N within a certain range by optimizing hot rolling conditions, cold rolling and cold rolling annealing conditions. . In addition, in order to stably develop the strain age hardening phenomenon due to N, it has been found that it is important to control the Al content according to the N content particularly in terms of composition. In addition, the present inventors have made full use of N without the problem of room temperature aging degradation, which has been a problem in the past, by making the microstructure of the steel sheet the main phase of ferrite and the average grain size to be 10 μm or less. I found what I could do.
[0018]
  That is, the present inventors use N as a reinforcing element, control the Al content to an appropriate range according to the N content, optimize the hot rolling conditions, cold rolling and cold rolling annealing conditions, By optimizing the visual structure and solid solution N, the formability that is much superior to conventional solid solution strengthened C-Mn steel plates and precipitation strengthened steel plates, and strain aging not found in the above conventional steel plates Curing characteristicsAndIt has been found that a steel plate having it can be obtained.
[0019]
  In addition to optimizing the above-described microstructure and solid solution N (solid solution N), the present inventors include Nb to optimize the precipitation Nb (precipitation state Nb). Thus, it has been found that a yield ratio reaching a high level of 0.7 or more can be obtained together with excellent formability compared to conventional steel sheets and strain age hardening characteristics not found in conventional steel sheets.
  In addition, the steel sheet of the present invention is higher in strength after paint baking treatment by a simple tensile test than the conventional steel sheet, and further, there is little variation in strength when plastically deformed according to actual press conditions, and stable component strength Characteristics are obtained. For example, a portion where the strain is greatly applied and the plate thickness is reduced has a larger curing allowance than the other portions, and it is in a direction to equalize when evaluated with a load capacity of (plate thickness) x (strength), and the strength as a part is stable To do. On the other hand, the strength increase due to strain aging is small in the portion where the strain is small, but the necessary strength can be secured because the original strength is high, so the strength stability is improved in actual parts formed with various ranges of strain. Contributes favorably.
[0020]
  The present invention has been completed with further studies based on the above findings. That is, the gist of the high-tensile cold-rolled steel sheet according to the first invention is as follows.
(1) By mass%, C: 0.15% or less, Si: 2.0% or less, Mn: 3.0% or less, P: 0.08% or less, S: 0.02% or less, Al: 0.02% or less, N: 0.0050 to 0.0250%, Nb: 0.007 to 0.04%, N / Al is 0.3 or more, solid solution N is contained 0.0010% or more, and precipitated Nb is 0.005% or more, with the balance being Fe and inevitable impurities. Contains 50% or more of the composition and ferrite phase with an average crystal grain size of 10 μm or less, with the remainder beingIn addition, a pearlite single-phase structure or an area ratio of 2% or less of bainite or martensite and the remainder from pearliteA high-tensile cold-rolled steel sheet with a tensile strength of 440 MPa or more and excellent strain age hardening characteristics, characterized by having a structure of
(2) The high-tensile cold-rolled steel sheet according to (1), further including one group or two or more groups of the following e group to h group in mass% in addition to the composition.
[0021]
                                  Record
    e group: One or more of Cu, Ni, Cr and Mo in total 1.0% or less
    f group: 1 or 2 types of Ti and V in total 0.1% or less
    g group: B is 0.0030% or less.
    h group: One or two of Ca and REM in total 0.0010 to 0.010%
(3) The high-tensile cold-rolled steel sheet has a thickness of 3.2 mm or less (1)Or(2)The high-tensile cold-rolled steel sheet described.
(4(1)Or(2)A high-tensile cold-rolled steel sheet obtained by subjecting the high-tensile cold-rolled steel sheet to electroplating or hot-dip plating.
[0022]
  Moreover, the summary of the manufacturing method of the high-tensile cold-rolled steel sheet according to the second invention is as follows.
  That is, the second aspect of the present invention is in mass%., C: 0.15% or less, Si: 2.0% or less, Mn: 3.0% or less, P: 0.08% or less, S: 0.02% or less, Al: 0.02% or less, N: 0.0050-0.0250%, Nb: 0.007-0.04% included N / Al is 0.3 or more, or the following e group to h group
    e group: One or more of Cu, Ni, Cr and Mo in total 1.0% or less
    f group: 1 or 2 types of Ti and V in total 0.1% or less
    g group: B is 0.0030% or less.
    h group: One or two of Ca and REM in total 0.0010 to 0.010%
A steel slab containing one group or two or more groups selected from the above, preferably having the composition of the balance Fe and unavoidable impurities, is heated to a slab heating temperature of 1100 ° C. or higher, and is roughly rolled into a sheet bar. The sheet bar is subjected to finish rolling with a final rolling reduction of 25% or more and a finish rolling outlet temperature of 800 ° C. or more. After finishing rolling, cooling is preferably started within 0.5 seconds, and cooling rate: A hot rolling process in which the steel sheet is rapidly cooled at 40 ° C./s or more and wound at a winding temperature of 750 ° C. or less, preferably at a winding temperature of 650 ° C. or less, and pickling and cold rolling are performed on the hot rolled plate. Cold rolling process to make a cold-rolled sheet, annealing the cold-rolled sheet at a temperature not lower than the recrystallization temperature and not higher than 900 ° C. and holding time: 10 to 90 s, and then cooling to a temperature range of not higher than 600 ° C. : The cold rolled sheet annealing process, which is cooled at 70 ° C / s or less, is performed sequentially, and the yield ratio is 0.7 or more. And cold-rolled steel sheet toTensile strength 440MPa This is a method for producing a high-tensile cold-rolled steel sheet with excellent strain age hardening characteristics.. In the second aspect of the present invention, it is preferable to further perform temper rolling or leveler processing at an elongation of 1.0 to 15%, preferably an elongation of 1.5 to 15%, following the cold-rolled sheet annealing step. In the second aspect of the present invention, it is preferable to join adjacent sheet bars between the rough rolling and the finish rolling. In the second aspect of the present invention, the rough rolling and the finish rolling are performed. It is preferable to use either or both of a sheet bar edge heater for heating the width end portion of the seat bar and a sheet bar heater for heating the length end portion of the seat bar..
[0023]
DETAILED DESCRIPTION OF THE INVENTION
  The steel sheet of the present invention is a high-tensile cold-rolled steel sheet having a tensile strength of 440 MPa or more and excellent strain age hardening characteristics. First, the reasons for limiting the composition of the steel sheet of the present invention will be described. Hereinafter, the mass% is simply referred to as%.
  The steel sheet of the present invention has a composition containing Al: 0.02% or less, N: 0.0050 to 0.0250%, N / Al being 0.3 or more, and solid solution N being 0.0010% or more.
[0024]
  Al: 0.02% or less
  Al is an element that acts as a deoxidizer and is effective in improving the cleanliness of the steel, and is also an element that refines the structure of the steel sheet. In the present invention, Al is preferably contained in an amount of 0.001% or more. On the other hand, excessive Al content deteriorates the surface properties of the steel sheet, further reduces the solid solution N, which is an important constituent element of the present invention, and causes a shortage of solid solution N that contributes to the strain age hardening phenomenon. When the conditions vary, the strain age hardening characteristic, which is a feature of the present invention, tends to vary. For this reason, in the present invention, the Al content is limited to as low as 0.02% or less. From the viewpoint of material stability, Al is preferably 0.015% or less.
[0025]
  N: 0.0050-0.0250%
  N is an element that increases the strength of the steel sheet by solid solution strengthening and strain age hardening, and is the most important element in the present invention. N also has a function of lowering the transformation point of steel, and the content of N is also useful for stabilizing the operation in a situation where rolling with a thin material that greatly interrupts the transformation point is avoided. In the present invention, an appropriate amount of N is contained and the production conditions are controlled, thereby securing a necessary and sufficient amount of N in a solid solution state in a cold-rolled product or a plated product, thereby strengthening solid solution and strain aging. Strength of steel (YS, TS) increase effect in hardening, TS440MPa or more, bake hardening amount (BH amount) 80MPa or more, increase in tensile strength before and after strain aging treatment ΔTS40MPa or more It is possible to stably satisfy the physical property requirements.
[0026]
  When N is less than 0.0050%, the above-described strength increasing effect is not likely to appear stably. On the other hand, if N exceeds 0.0250%, the rate of occurrence of internal defects in the steel sheet increases, and slab cracking during continuous casting occurs frequently. For this reason, N was made into the range of 0.0050-0.0250%. Note that N is more preferably in the range of 0.0070 to 0.0170% from the viewpoint of improving the stability and yield of the material considering the entire manufacturing process. If the N amount is within the range of the present invention, there is no adverse effect on weldability such as spot welding and arc welding.
[0027]
  Solid solution N: 0.0010% or more
  An amount (concentration) of N in a solid solution state (also referred to as solid solution N) of 0.0010% or more in steel in order to ensure sufficient strength with cold-rolled products and to fully exhibit strain age hardening by N. Need to exist in.
  Here, the solute N amount is obtained by subtracting the precipitated N amount from the total N amount in the steel. As an analysis method for the amount of precipitated N, it is effective to obtain by an electrolytic extraction analysis method using a constant potential electrolysis method according to the results of a comparative study of various analysis methods by the present inventors. In addition, there are an acid decomposition method, a halogen method, and an electrolysis method as a method for dissolving the base iron used for the extraction analysis. Among these, the electrolytic method can dissolve only the iron core stably without decomposing extremely unstable precipitates such as carbides and nitrides. Electrolysis is performed at a constant potential using an acetylacetone system as the electrolytic solution.
In the present invention, the result of measuring the amount of precipitated N using a constant potential electrolysis method showed the best correspondence with the actual component strength.
[0028]
  For this reason, in the present invention, the residue extracted by the constant potential electrolysis method is chemically analyzed to determine the amount of N in the residue, which is used as the amount of precipitated N.
  In order to obtain a higher BH amount and ΔTS, the solid solution N amount is 0.0020% or more. To obtain a higher value, 0.0030% or more. To obtain a higher value, 0.0050% or more. It is preferable to do this.
[0029]
  N / Al (N content to Al content ratio): 0.3 or more
  In order to make solid solution N stable and remain at 0.0010% or more in the product state, it is necessary to limit the amount of Al which is an element that strongly fixes N. As a result of examining a steel sheet in which the combination of N content and Al content within the composition range of the present invention is changed over a wide range, in order to make the solid solution N in cold-rolled products and plated products 0.0010% or more, the Al content is It was found that N / Al needs to be 0.3 or more when limited to 0.02% or less. That is, the Al content is limited to (N content) /0.3 or less.
[0030]
  Furthermore, the steel sheet of the present invention has a structure containing a ferrite phase having an average crystal grain size of 10 μm or less in an area ratio of 50% or more. The structure of the steel sheet of the present invention will be described.
  Ferrite phase area ratio: 50% or more
  The cold-rolled steel sheet of the present invention is intended for use in automobile steel sheets and the like that require a high degree of workability. In order to ensure ductility, the ferrite phase contains a ferrite phase in an area ratio of 50% or more. And an organization. If the area ratio of the ferrite phase is less than 50%, it becomes difficult to ensure the ductility necessary for a steel sheet for automobiles that requires high workability. In addition, when better ductility is required, the area ratio of the ferrite phase is preferably 75% or more. In addition, the ferrite referred to in the present invention includes not only ordinary meaning ferrite (polygonal ferrite) but also bainitic ferrite and acicular ferrite not containing carbide..
[0031]
  Average grain size of ferrite phase: 10 μm or less
  In the present invention, as a crystal grain size, a value calculated by a quadrature method prescribed in ASTM from a cross-sectional structure photograph and a nominal grain size obtained by a cutting method prescribed in ASTM from a cross-sectional structure photograph (for example, Umemoto et al .: Heat treatment, 24 ( 1984), 334), whichever is greater.
[0032]
  The cold-rolled steel sheet of the present invention secures a predetermined amount of solute N as a product, but according to the results of experiments and examinations by the present inventors, the average of the ferrite phase is maintained even if the amount of solute N is kept constant. It has been found that when the crystal grain size exceeds 10 μm, the strain age hardening characteristics vary greatly. In addition, the deterioration of mechanical properties when stored at room temperature becomes significant. The details of this mechanism are currently unknown, but one of the causes of the variation in strain age hardening characteristics is the crystal grain size. Segregation and precipitation of alloy elements at the grain boundaries, as well as the processing and heat treatments affecting them. It is presumed to be related to the effects of Therefore, in order to stabilize the strain age hardening characteristics, the average crystal grain size of the ferrite phase needs to be 10 μm or less. In order to stably obtain a further increase in the BH amount and ΔTS amount, the average crystal grain size of ferrite is preferably 8 μm or less.
[0033]
  The cold-rolled steel sheet of the present invention having the composition and structure described above is a cold-rolled steel sheet having a tensile strength TS of 440 MPa or more and excellent strain age hardening characteristics.
  A steel sheet having a TS of less than 440 MPa cannot be widely applied to members having structural members. In order to further expand the applicable range, it is desirable that TS is 500 MPa or more.
[0034]
  In the present invention, “excellent in strain age hardening characteristics” means that, as described above, after pre-deformation with a tensile strain of 5%, when subjected to aging treatment at a temperature of 170 ° C. for 20 minutes, before and after this aging treatment. Tensile stress before and after strain aging treatment (pre-deformation + aging treatment) when deformation stress increase (BH amount; BH amount = yield stress after aging treatment-pre-deformation stress before aging treatment) is 80 MPa or more It means that the amount of increase in strength (denoted as ΔTS; ΔTS = tensile strength after aging treatment−tensile strength before pre-deformation) is 40 MPa or more.
[0035]
  When the strain age hardening characteristic is specified, the amount of pre-strain (pre-deformation) is an important factor. Assuming the deformation mode applied to the steel sheet for automobiles, the present inventors investigated the influence of the amount of pre-strain on the strain age hardening characteristics, and as a result,(i)Deformation stress in the deformation mode can be roughly organized by a uniaxial equivalent strain (tensile strain), except in the case of extremely deep drawing.(ii)In actual parts, the amount of strain equivalent to one axis exceeds approximately 5%.(iii)It was found that the component strength corresponds well with the strength (YS and TS) obtained after the strain aging treatment with a pre-strain of 5%. Based on this knowledge, in the present invention, the pre-deformation of the strain aging treatment is set to 5% tensile strain.
[0036]
  Conventional coating baking conditions of 170 ° C x 20 min have been adopted as standard. In general, in order to earn a hardened amount, it is advantageous to hold at a higher temperature for a longer time unless softened by excessive aging. When strain of 5% or more is applied, curing can be achieved even by a more gradual (low temperature side) treatment, in other words, a wider range of aging conditions can be taken.
[0037]
  Specifically, in the steel sheet of the present invention, the lower limit of the heating temperature at which hardening becomes significant after pre-deformation is approximately 100 ° C. On the other hand, when the heating temperature exceeds 300 ° C., the curing reaches its peak, and on the contrary, there is a tendency to slightly soften, and the occurrence of thermal distortion and temper color becomes conspicuous. As for the holding time, when the heating temperature is about 200 ° C., if it is about 30 seconds or longer, substantially sufficient curing can be achieved. In order to obtain larger and more stable curing, it is preferable that the holding time is 60 seconds or longer. However, if the holding time exceeds 20 minutes, further curing cannot be expected, and the production efficiency is significantly reduced, which is disadvantageous in practical use.
[0038]
  From the above, in the present invention, it was determined that the aging treatment conditions were evaluated at 170 ° C. which is the heating temperature of the conventional paint baking treatment conditions and the holding time was 20 minutes. Even with the low temperature heating and short-time aging treatment conditions in which sufficient hardening cannot be achieved with conventional paint-baked steel sheets, large hardening is stably achieved with the steel sheets of the present invention. The heating method is not particularly limited, and any of induction heating, heating with a non-oxidizing flame, laser, plasma, etc., for example, can be preferably used in addition to atmospheric heating with a furnace employed for ordinary paint baking.
[0039]
  The strength of parts for automobiles must be able to withstand complex stress loads from the outside. Therefore, in a steel plate, not only strength characteristics in a small strain range but also strength characteristics in a large strain range are important. In view of this point, the present inventors set the BH amount of the steel sheet of the present invention to be a material for automobile parts to 80 MPa or more and the ΔTS amount to 40 MPa or more. More preferably, the BH amount is 100 MPa or more and ΔTS50 MPa or more. In order to increase the BH amount and the ΔTS amount, the heating temperature during the aging treatment may be set to a higher temperature side and / or the holding time may be set to a longer time side.
[0040]
  In addition, the steel sheet of the present invention has a conventional structure in which it is extremely difficult to cause aging deterioration (a phenomenon in which YS increases and El (elongation) decreases) even if it is left for about a year at room temperature when not formed. Has no advantages.
  By the way, the effect of the present invention can be exhibited even when the product plate thickness is relatively thick. However, when the product plate thickness exceeds 3.2 mm, a necessary and sufficient cooling rate can be secured in the cold-rolled plate annealing process. However, strain aging occurs during continuous annealing, making it difficult to obtain the target strain age hardening characteristics as a product. Therefore, the thickness of the steel sheet of the present invention is preferably 3.2 mm or less.
[0041]
  In the present invention, there is no problem even if the surface of the cold-rolled steel sheet of the present invention is electroplated or hot-plated. These plated steel sheets also exhibit the same amount of TS, BH, and ΔTS as before plating. As the type of plating, any of electrogalvanizing, hot dip galvanizing, alloying hot dip galvanizing, electrotin plating, electrochromic plating, electronickel plating, etc. can be preferably applied..
[0042]
  The present inventionSteel plate,in frontIn addition to the composition containing Al: 0.02% or less, N: 0.0050 to 0.0250%, N / Al 0.3 or more, and solid solution N containing 0.0010% or more, C: 0.15% or less, Si: 2.0 % Or less, Mn: 3.0% or less, P: 0.08% or less, S: 0.02% or less, Nb: 0.007 to 0.04%, further containing 0.005% or more of precipitated Nb, or further e group to h group
    e group: One or more of Cu, Ni, Cr and Mo in total 1.0% or less
    f group: 1 or 2 types of Ti and V in total 0.1% or less
    g group: B is 0.0030% or less.
    h group: One or two of Ca and REM in total 0.0010 to 0.010%
In which the balance is composed of Fe and inevitable impurities, and the structure contains a ferrite phase with an average crystal grain size of 10 μm or less in an area ratio of 50% or more, and the balance is mainly pearlite. It is a high-tensile cold-rolled steel sheet characterized by having a strain age-hardening property and having a yield ratio of 0.7 or more.
[0043]
  AlThe reasons for limiting the composition other than N will be described.
  C: 0.15% or less
  C is an element that increases the strength of the steel sheet, and in order to achieve an average grain size of ferrite of 10 μm or less, which is an important constituent of the present invention, from the viewpoint of further securing a desired strength, 0.005% or more It is preferable to do this. In addition, More preferably, it is 0.03% or more. On the other hand, if it exceeds 0.15%, the carbide fraction in the steel sheet becomes excessive, the ductility is remarkably reduced and formability is deteriorated, and further, spot weldability, arc weldability, etc. are remarkably lowered. From the viewpoints of formability and weldability, C is limited to 0.15% or less. In the second preferred embodiment, it is preferably 0.08% or less, and preferably 0.05% or less for applications requiring better ductility.
[0044]
  Si: 2.0% or less
  Si is a useful element that can increase the strength of a steel sheet without significantly reducing the ductility of the steel. In the second preferred embodiment, it is 0.005% or more, and 0.1% or more if higher strength is desired. It is preferable to contain. On the other hand, Si is an element that greatly increases the transformation point during hot rolling and makes it difficult to ensure quality and shape, or adversely affects the beauty of the steel sheet surface such as surface properties and chemical conversion properties. Then, it limited to 2.0% or less. If Si is 2.0% or less, a remarkable increase in transformation point can be suppressed by adjusting the amount of Mn added in combination, and good surface properties can be secured.
[0045]
  Mn: 3.0% or less
  Mn is an effective element for preventing hot cracking due to S, and is preferably added according to the amount of S contained as in the first preferred embodiment, and Mn is an important constituent of the present invention. It has a great effect on the refinement of crystal grains, and it is preferable to add it positively and use it for improving the material. From the viewpoint of stably fixing S, Mn is preferably contained in an amount of 0.2% or more.
[0046]
  Mn is an element that increases the strength of the steel sheet, and is preferably contained in an amount of 1.2% or more for the strength requirement exceeding TS500 MPa. From the viewpoint of stably securing the strength, it is more preferably 1.5% or more. When the Mn content is increased to this level, variations in the mechanical properties and strain age hardening characteristics of the steel sheet with respect to fluctuations in production conditions including hot rolling conditions are reduced, which is effective in stabilizing the quality.
[0047]
  On the other hand, if Mn is contained in a large amount exceeding 3.0%, the hot deformation resistance of the steel sheet tends to increase, the spot weldability and the formability of the welded portion tend to deteriorate, and further, the formation of ferrite occurs. Since it is suppressed, the ductility tends to decrease remarkably. For this reason, Mn was limited to 3.0% or less. In particular, in applications where better corrosion resistance and formability are required, Mn is desirably 1.5% or less.
[0048]
  Further, Mn has a function of lowering the transformation point during hot rolling, and inclusion with Si can offset an increase in transformation point due to the inclusion of Si. Especially for products with thin plate thickness, quality and shape change sensitively due to changes in transformation point, so it is important to balance Mn and Si contents precisely. For these reasons, it is more preferable that Mn / Si is 3.0 or more.
[0049]
  P: 0.08% or less
  P is an element useful as a solid solution strengthening element of steel. From this viewpoint, P is preferably contained in an amount of 0.001% or more. However, if contained in excess, it causes the steel to become brittle and further reduces the stretch flangeability of the steel sheet. Moreover, since P has a strong tendency to segregate in steel, it causes embrittlement of the weld due to it. For this reason, P was limited to 0.08% or less. In addition, when the stretch flange workability and weld toughness are particularly important, 0.04% or less is preferable. Further, it is more preferably 0.02% or less from the viewpoint of weld zone toughness.
[0050]
  S: 0.02% or less
  S is an element that exists as an inclusion in the steel sheet and causes deterioration of the ductility and corrosion resistance of the steel sheet. In the present invention, S is limited to 0.02% or less. In applications where good workability (especially stretch flangeability, hole expandability, etc.) is required, the content is preferably 0.015% or less. Further, when the required level of stretch flangeability is high, S is preferably 0.008% or less. Further, in order to stably maintain the strain age hardening characteristics at a high level, the detailed mechanism is unknown, but it is preferable to reduce S to 0.008% or less.
[0051]
  Nb: 0.007 to 0.04%
  Nb is one of the important elements that remarkably refine crystal grains to increase YS, improve the yield ratio (YR = YS / TS) to 0.7 or more, and develop large strain age hardening due to N. In order to acquire an effect, it is preferable to contain 0.007% or more. On the other hand, in the present invention, it is preferable to limit the Nb amount to 0.04% or less when combined with other nitride forming elements in order to secure the necessary amount of solute N.
[0052]
  Precipitation Nb: 0.005% or more
  The present inventionThe Nb content in the steel sheet is also important for the presence of Nb in the steel. That is, the presence of a certain amount of Nb present in the precipitated state (also referred to as precipitated Nb) is necessary to obtain stable strain age hardening characteristics and to obtain a high yield ratio. With the Nb content of the second preferred embodiment of the present invention, it is desirable that at least 0.005% or more of precipitated Nb is present. Nb is determined by dissolving and extracting by electrolytic extraction using an acetylacetone solvent. There are various melting methods, but the reason is that the value obtained by this method showed the best correlation with the strain age hardening characteristics of steel. Main departureClearThe box estimates that Nb is more tied to C than N, but the details are unknown.
[0053]
  Main departureMeikoIn the plate, in addition to the above chemical components, the following e group to h group
    e group: One or more of Cu, Ni, Cr and Mo in total 1.0% or less
    f group: 1 or 2 types of Ti and V in total 0.1% or less
    g group: B is 0.0030% or less.
    h group: One or two of Ca and REM in total 0.0010 to 0.010%
Including one or more groupsMube able to.
[0054]
  Group e elements: Cu, Ni, Cr, and Mo are all elements that contribute to strength increase without significant reduction in ductility of the steel sheet.ThisThese effects are recognized when Cu: 0.01% or more, Ni: 0.01% or more, Cr: 0.01% or more, Mo: 0.01% or more, and can be selected alone or in combination as required. However, when the content is too large, the hot deformation resistance increases, or the chemical conversion property and the surface treatment characteristics in a broad sense are deteriorated, and the welded portion is hardened to deteriorate the weldability. For this reason, it is preferable that the elements of the e group be 1.0% or less in total.
[0055]
  Group f elements: Ti and V are elements that contribute to the refinement and uniformity of crystal grains, and this effect is recognized at each of Ti: 0.002% or more and V: 0.002% or more, and is selected as necessary. And can be contained alone or in combination. However, when there is too much content, hot deformation resistance will increase and chemical conversion property and the surface treatment characteristic in a broad sense will deteriorate. For this reason, it is preferable that the total amount of elements in the f group is 0.1% or less.
[0056]
  Group g element: B is an element having the effect of improving the hardenability of the steel, and is contained as needed for the purpose of increasing the strength of the steel by increasing the fraction of the low-temperature transformation phase other than the ferrite phase. be able to. This effect is observed when B: 0.0002% or more is added. However, when the amount is too large, the hot deformability is lowered, and the solute N is reduced by generating BN. For this reason, B is preferably 0.0030% or less.
[0057]
  The elements of group h: Ca and REM are all elements that are useful for controlling the shape of inclusions, and are particularly preferably contained alone or in combination when there is a demand for stretch flange formability. In that case, if the total of the elements in the h group is less than 0.0010%, the effect of controlling the shape of the inclusion is insufficient. On the other hand, if it exceeds 0.010%, the occurrence of surface defects becomes conspicuous. For this reason, it is preferable that the elements of the h group be in the range of 0.0010 to 0.010% in total.
[0058]
  The balance other than the chemical components described above is Fe and inevitable impurities. As an inevitable impurity, O: 0.0050% or less is acceptable.
  In addition to the above composition, the steel sheet of the present invention has a ferrite phase with an average crystal grain size of 10 μm or less in an area ratio of 50% or more, and the balance is pearlite.A single-phase structure, or bainite or martensite with an area ratio of 2% or less, and the balance is pearlite.Have an organization. In addition to having an average crystal grain size of 10 μm or less and a ferrite phase of 50% or more in area ratio, as a phase other than the ferrite phase (second phase), Pa-A structure composed of a single phase of lite, or a bainite or martensite with an area ratio of 2% or less, and the remainder composed of pearlite.RukoAs a result, the strength and the yield ratio are improved.
[0059]
  The present invention having the above composition and structureMeikoThe plate has a tensile strength TS of 440 MPa or more and a yield ratio of 0.7 or more, and is a high yield ratio type high-tensile cold-rolled steel plate with excellent strain age hardening characteristics..
  Next, the manufacturing method of this invention steel plate is demonstrated.
  The steel sheet of the present invention basically has a composition within the above-described range, that is, Al: not more than 0.02%, N: 0.0050 to 0.0250%, and N / Al is not less than 0.3.And C: 0.15 %Less than, Si : 2.0 %Less than, Mn : 3.0 % Or less, P: 0.08 %, S: 0.02 %Less than, Nb : 0.007 ~ 0.04 % Or the following e group to h group
e group: Cu , Ni , Cr , Mo 1 type or 2 types or more in total 1.0 %Less than
f group: Ti , One or two of V in total 0.1 %Less than
g group: B 0.0030 %Less than
h group: Ca , REM 1 type or 2 types in total 0.0010 ~ 0.010 %
1 group or 2 groups or more selected from among, preferably the balance Fe And inevitable impuritiesA steel slab having a composition is heated to a slab heating temperature of 1100 ° C. or higher, roughly rolled into a sheet bar,The rolling reduction in the final pass of finish rolling twenty five %more than,Finishing rolling exit temperature: Finish rolling to 800 ℃ or more, coiling temperature: 750 ℃ or less, hot rolling process to make a hot rolled sheet, and pickling and cold rolling to the hot rolled sheet Cold rolling process to make a cold rolled sheet, and to the cold rolled sheetAbove recrystallization temperature 900 Holding time at temperatures below ℃: Ten ~ 90 sAnnealing, then600 Up to the temperature range below ℃Cooling rate: 70 ℃ / s Cool belowIt manufactures by performing sequentially the cold-rolled sheet annealing process which performs cooling.
[0060]
  The reason for limiting the slab composition used in the production method of the present invention is the same as the reason for limiting the steel sheet of the present invention.
  The slab used in the production method of the present invention is preferably produced by a continuous casting method in order to prevent macro segregation of components, but may be produced by an ingot forming method or a thin slab continuous casting method. In addition to the normal process of once cooling the slab to room temperature and heating it again, it is rolled directly after rolling in a furnace after inserting it into a heating furnace without cooling or rolling immediately after performing a slight heat retention. Energy saving processes such as direct rolling can also be applied without problems. In particular, in order to effectively secure N in a solid solution state, direct feed rolling in which precipitation of N is delayed is one of useful techniques.
[0061]
  First, the reasons for limiting the hot rolling process conditions will be described.
  Slab heating temperature:1100℃ or more
  In order to ensure the necessary and sufficient amount of solute N in the initial state and to meet the target value (0.0010% or more) of the solute slab in the product,1100Over ℃TheFrom the viewpoint of avoiding an increase in loss accompanying an increase in oxidized weight, the slab heating temperature is preferably 1280 ° C. or lower.
[0062]
  The slab heated under the above conditions is made into a sheet bar by rough rolling. The conditions for rough rolling need not be specified, and may be performed according to a conventional method. However, from the viewpoint of securing the solid solution N amount, it is desirable to perform the treatment in as short a time as possible.
  Next, the sheet bar is finish-rolled to obtain a hot-rolled sheet.
[0063]
  In the present invention, it is preferable to join successive sheet bars between rough rolling and finish rolling and continuously finish rolling. As the joining means, it is preferable to use a pressure welding method, a laser welding method, an electron beam welding method or the like.
  As a result, the proportion of unsteady parts (the front end and the rear end of the material to be processed) that are likely to be disturbed in finish rolling and subsequent cooling is reduced, and the stable rolling length (continuous length that can be rolled under the same conditions) is reduced. And the stable cooling length (continuous length that can be cooled with tension applied) are extended, and the shape / dimensional accuracy and yield of the product are improved.
[0064]
  In addition, it has become possible to easily carry out lubrication rolling for thin objects and wide widths, which was difficult to perform due to problems such as sheeting and biting by conventional single rolling for each sheet bar, reducing rolling load and roll surface pressure. This extends the life of the roll.
  In the present invention, either one or both of a sheet bar edge heater that heats the width end portion of the sheet bar and a sheet bar heater that heats the length end portion of the sheet bar is used between rough rolling and finish rolling. Thus, it is preferable to make the temperature distribution in the width direction and the longitudinal direction of the sheet bar uniform. Thereby, the material dispersion | variation in a steel plate can be made still smaller. The sheet bar edge heater and the sheet bar heater are preferably of the induction heating type.
[0065]
  As for the use procedure, it is desirable to first compensate for the temperature difference in the width direction by the sheet bar edge heater. The amount of heating at this time is preferably set so that the temperature distribution range in the width direction on the finish rolling exit side is approximately 20 ° C. or less, although it depends on the steel composition and the like. Next, the temperature difference in the longitudinal direction is compensated by the sheet bar heater. The heating amount at this time is preferably set so that the length end temperature is higher by about 20 to 40 ° C. than the center temperature.
[0066]
  From the viewpoint of reducing the microstructure of the steel sheet and improving workability such as hole expansibility, the rolling reduction of the final finish rolling pass is set to twenty five % Or more. The final pass of finish rolling is one of the important factors governing the microstructure of the steel sheet. Rolling rate in this pass twenty five % Reduction is sufficient From the state of unrecrystallized austenite in which strain is accumulated in the ferrite, ferrite transformation can be performed, thereby achieving a remarkable microstructure refinement of the hot-rolled mother board. By performing cold rolling and annealing using this as a raw material, not only miniaturization of the structure after cold rolling annealing but also homogenization is achieved. That is, the particle size distribution of the ferrite phase is not varied, and the dispersed pearlite phase is also made fine and present uniformly. This has the advantage that the hole expandability is improved.
[0067]
  Finishing rolling delivery temperature: 800 ℃ or more
  The finish rolling outlet temperature FDT is set to 800 ° C. or higher in order to make the structure of the steel plate uniform and fine. When the FDT is below 800 ° C., the structure becomes non-uniform, and the processed structure remains in part. Such remaining of the processed structure can be avoided by increasing the coiling temperature. However, when the coiling temperature is increased, coarse crystal grains are generated and the amount of solute N is greatly reduced, so that it becomes difficult to obtain a target tensile strength of TS440 MPa or more. In order to further improve the mechanical properties, the FDT is desirably 820 ° C. or higher. Further, from the viewpoint of preventing the occurrence of scale flaws and the like due to excessive increase in the rolling temperature, the FDT is preferably set to 1000 ° C. or less. After finish rolling, it is desirable to cool the steel sheet at an early stage in order to refine crystal grains and secure a solid solution N amount.
[0068]
  Cooling after finish rolling: Cooling starts within 0.5 seconds after finishing rolling, rapid cooling at a cooling rate of 40 ° C / s or more
  In the present invention, it is desirable to start cooling immediately after finishing rolling (within 0.5 seconds) and to set the average cooling rate during cooling to 40 ° C./s or more. By satisfying this condition, the high temperature region where AlN precipitates can be rapidly cooled, and N in a solid solution state can be effectively secured. If this cooling start time or cooling rate does not satisfy the above conditions, it is difficult to achieve grain refinement due to excessive grain growth and precipitation of AlN due to strain energy introduced by rolling. This increases the risk that the amount of dissolved N will be deficient. From the viewpoint of ensuring the uniformity of the material and shape, the cooling rate is preferably suppressed to 300 ° C./s or less.
[0069]
  Winding temperature: 750 ℃ or less
  As the coiling temperature CT decreases, the steel sheet strength tends to increase. To ensure the target tensile strength of TS440MPa or higher, CT should be 750 ° C or lower.The From the viewpoint of securing strength 650 It is preferable to set it as below ℃.If the CT is less than 200 ° C., the shape of the steel sheet tends to be disturbed, and there is a high risk of causing problems in actual operation, and the uniformity of the material tends to decrease. For this reason, it is desirable that CT be 200 ° C. or higher. When more uniform material is required, CT is preferably 300 ° C. or higher. More preferably, it is 450 ° C. or higher.
[0070]
  In the present invention, in the finish rolling, lubrication rolling may be performed in order to reduce the hot rolling load. By performing lubrication rolling, there is an effect that the shape and material of the hot-rolled sheet are made more uniform. In addition, it is preferable to make the friction coefficient in the case of lubrication rolling into the range of 0.25-0.10. Moreover, the operation of hot rolling is further stabilized by combining lubrication rolling and continuous rolling.
[0071]
  The hot-rolled sheet that has been subjected to the above-described hot-rolling step is then subjected to pickling and cold-rolling in the cold-rolling step to become a cold-rolled plate.
  The conditions for pickling may be generally known conditions and are not particularly limited. In addition, when the scale of a hot-rolled sheet is extremely thin, cold rolling may be performed immediately without performing pickling.
[0072]
  Further, the cold rolling conditions may be generally known conditions and are not particularly limited. Note that the cold rolling reduction is preferably 40% or more from the viewpoint of ensuring the uniformity of the structure.
  Next, the cold-rolled sheet is subjected to a cold-rolled sheet annealing step by continuous annealing.
  Further, following the cold-rolled sheet annealing step, temper rolling or leveler processing with an elongation of 1.0 to 15% may be performed. By performing temper rolling or leveler processing after the cold-rolled sheet annealing step, strain age hardening characteristics such as BH amount and ΔTS amount can be stably improved. The total elongation in temper rolling or leveler processing is preferably 1.0% or more. If the elongation is less than 1.0%, the strain age hardening property is not improved, while if the elongation exceeds 15%, the ductility of the steel sheet is lowered. Although the processing modes are different between temper rolling and leveler processing, the present inventors have confirmed that there is no significant difference in the effect on the strain age hardening characteristics of the steel sheet..
[0073]
  NextClearlyThe reason for limiting the cold-rolled sheet annealing step will be described.
  Continuous annealing temperature: above recrystallization temperature and below 900 ℃
  The annealing temperature for continuous annealing was set to the recrystallization temperature or higher.
  If the continuous annealing temperature is lower than the recrystallization temperature, the recrystallization is not completed, and the strength satisfies the target, but the ductility is low, so that the formability is lowered and the steel sheet cannot be applied. In order to further improve the formability, the continuous annealing temperature is preferably 700 ° C. or higher. On the other hand, when the continuous annealing temperature exceeds 900 ° C., nitrides such as AlN are precipitated, and the amount of solute N in the steel plate as a product is insufficient. In addition, the crystal grains become coarse and the risk of impairing the surface appearance increases. For this reason, the continuous annealing temperature is preferably not less than the recrystallization temperature and not more than 900 ° C. In particular, when aiming at a high yield ratio, the annealing temperature is preferably set to 850 ° C. or lower from the viewpoint of preventing coarsening of the structure and reducing the solute N loss due to the progress of precipitation.
[0074]
  Holding time at continuous annealing temperature: 10 ~ 90s
  The holding time at the continuous annealing temperature is preferably as short as possible from the viewpoint of refining the structure and securing an amount of solute N that is higher than desired, but is preferably 10 s or longer from the viewpoint of operational stability. . On the other hand, if the holding time exceeds 90 s, it becomes difficult to refine the structure and secure the amount of solute N. For this reason, the holding time at the continuous annealing temperature is preferably in the range of 10 to 90 s.
[0075]
  Cooling after maintaining the annealing temperature (primary cooling): Cooling rate up to a temperature range of 600 ° C or less 70 ° C / s or less
  Cooling after soaking in continuous annealing is important from the viewpoint of refining the structure and securing the amount of dissolved N, and in the present invention, continuous cooling at a cooling rate of 70 ° C / s or less up to a temperature range of 600 ° C or less. Cool down. When the cooling rate exceeds 70 ° C / s, the yield ratio decreases, and further, the uniformity of the material in the width direction of the steel sheet is insufficient. Preferably, the cooling rate is 5 ° C./s or more in order to secure TS and YS. When the cooling stop temperature when cooling at such a cooling rate is higher than 600 ° C., the bake hardenability is lowered, which is not preferable.
[0076]
  After the primary cooling is finished, a so-called overaging treatment in which the temperature is maintained within a predetermined temperature range may be performed or not particularly performed. However, from the viewpoint of further improving the material, particularly ductility, it is desirable to reduce the solid solution C as much as possible to reduce the normal temperature age hardening and to further manifest the influence of the solid solution N on the strain age hardening characteristics. It is preferable to perform an overaging treatment in which the temperature is maintained at 350 to 450 ° C. for 120 s or less.
[0077]
  Furthermore, this departureIn the lightIs,in frontThe elongation is more preferably 1.5% or more.
[0078]
【Example】
Example 1)
  table1Molten steel having the composition shown in FIG. 2 was melted in a converter and made into a slab by a continuous casting method. These slabs are displayed2Heated under the conditions shown in Fig.2The sheet bar has the thickness shown in Fig.2It was set as the hot rolled sheet by the hot rolling process which performs finish rolling of the conditions shown in. In some cases, lubrication rolling was performed by finish rolling.
[0079]
  These hot-rolled sheets can be pickled and2It was set as the cold rolled sheet by the cold rolling process which consists of the cold rolling of the conditions shown in. Next, these cold rolled sheets2The continuous annealing by the continuous annealing furnace was performed on the conditions shown in. In addition, temper rolling was performed following the cold-rolled sheet annealing step. In addition, all the annealing temperatures of continuous annealing were more than the recrystallization temperature.
  About the obtained cold-rolled annealed sheet, SolidThe amount of dissolved N, microstructure, tensile properties, and strain age hardening properties were investigated.
(1) Investigation of solute N content
The amount of solute N was determined by subtracting the amount of precipitated N from the total N amount in steel determined by chemical analysis. The amount of precipitated N was determined by an analysis method using the above-described constant potential electrolysis method.
(2) Microscopic tissue
Test specimens are taken from each cold-rolled annealed plate, and the microstructure (C section) perpendicular to the rolling direction is imaged using an optical microscope or a scanning electron microscope, and the main phase is detected using an image analyzer. The structure fraction and the type of the second phase or further the structure fraction of a certain ferrite were obtained.
[0080]
Further, the crystal grain size of ferrite as the main phase is a value calculated by a quadrature method prescribed in ASTM from a structural photograph of a cross section (C cross section) orthogonal to the rolling direction or a nominal value obtained by a cutting method prescribed in ASTM. The larger of the particle sizes was adopted.
(3) Tensile properties
From each cold-rolled annealed plate JIS Sample No. 5 in the rolling direction, JIS Z 2241 In accordance with the provisions of strain rate: 3 × Ten ^-3 A tensile test was performed at / s, and yield strength YS, tensile strength TS, and elongation El were determined.
(4) Strain age hardening characteristics
From each cold-rolled annealed plate JIS Sample No. 5 was taken in the rolling direction, and 5% tensile pre-strain was applied here as pre-deformation. 170 ℃ × 20min After applying a heat treatment equivalent to the paint baking process, strain rate: 3 × Ten ^-3 / S tensile test, pre-deformation-tensile properties after paint baking (yield stress YS _BH , Tensile strength TS _BH ), BH amount = YS _BH -YS _Five% , ΔTS = TS _BH -TS was calculated. YS _Five% Is the deformation stress when the product plate is predeformed 5%, YS _BH , TS _BH Is the yield stress and tensile strength after the pre-deformation-paint baking process, and TS is the tensile strength of the product plate.
[0081]
  In addition, about No.3-G steel (steel plate No.3-9), the steel plate surface was subjected to hot dip galvanizing to produce a plated steel plate, and various properties were similarly evaluated. The annealing conditions in the continuous plating line were the same as those in the continuous annealing line.
  Table of obtained results3Shown in
[0082]
【table1]
[0083]
【table2]
[0084]
【table3]
[0085]
  In the examples of the present invention, all had excellent ductility, high yield ratio, and excellent strain age hardening characteristics, and exhibited a significantly high BH amount and ΔTS.
  In addition, the tensile properties of the plated steel sheet that has been hot dip galvanized for No.3-G steel (steel sheet No.3-9) show a tendency for TS to be slightly lower than the tensile characteristics without plating. Considering the balance between strength and elongation, almost the same characteristics can be obtained.
[0086]
  In addition, regarding the steel plate No. 3-1 as an example of the present invention and the steel plate No. 3-10 as a comparative example, the aging conditions were changed variously, and the strain age hardening characteristics were investigated. The results are shown in the table4Shown in In addition, the test method changed only the aging temperature and the aging time among the above-mentioned conditions.
[0087]
【table4]
[0088]
  In the example of the present invention (steel plate No. 3-1), the standard aging conditions of 170 ° C. × 20 min aging treatment yielded values of BH amount of 90 MPa and ΔTS 50 MPa, but a wide range of aging treatment conditions as shown in Table 11 However, it was possible to satisfy the BH amount of 80 MPa or more and ΔTS 40 MPa or more. On the other hand, in the comparative example (steel plate No. 3-10), even when the aging temperature was changed in the range of 100 to 300 ° C., the large BH amount and ΔTS as in the present invention example were not shown.
[0089]
  That is, the steel sheet of the present invention can secure a high BH amount and ΔTS even under a wide range of aging conditions.
(Example2)
  table5The steel having the composition shown in FIG.1The slab is formed in the same manner as above, and the slab is displayed.6Heated under the conditions shown in the above, roughly rolled into a 25 mm thick sheet bar,6It was set as the hot rolled sheet by the hot rolling process which performs finish rolling of the conditions shown in. In addition, the sheet | seat bar | burr which flanks on the finishing rolling entrance side after rough rolling was joined by the melt-pressing method, and was continuously rolled. In addition, the temperature of the seat bar was adjusted by using an induction heating type seat bar edge heater and a sheet bar heater at the width end portion and the length direction end portion of the seat bar.
[0090]
  These hot-rolled sheets can be pickled and6A 1.6 mm-thick cold-rolled sheet was obtained by a cold rolling process comprising cold rolling under the conditions shown in FIG. Next, these cold rolled sheets6The continuous annealing by the continuous annealing furnace was performed on the conditions shown in. In addition, all the annealing temperatures of continuous annealing were made into the recrystallization temperature or more.
  About the obtained cold-rolled annealing board, the amount of solid solution N, a micro structure, a tensile characteristic, and a strain age hardening characteristic were investigated by the method similar to Example 1. FIG.
[0091]
  Table the results7Shown in
[0092]
【table5]
[0093]
【table6]
[0094]
【table7]
[0095]
  Each of the inventive examples had excellent ductility, high yield ratio, and excellent strain age hardening characteristics, and exhibited a stable and remarkably high amount of BH and ΔTS, regardless of variations in production conditions. Moreover, in the present invention example, the plate thickness accuracy and shape of the product steel plate were improved by performing continuous rolling and temperature adjustment in the longitudinal direction and width direction of the sheet bar..
[0096]
【The invention's effect】
  According to the present invention, the yield stress is increased to 80 MPa or more and the tensile strength is increased to 40 MPa or more by the pre-deformation-paint baking process.HighHigh yield ratio type high-tensile cold-rolled steel sheet having high strain age hardening characteristics and high formability can be produced at low cost without disturbing the shape, and has a remarkable industrial effect. Furthermore, when the high-tensile cold-rolled steel sheet of the present invention is applied to automobile parts, it is possible to obtain a stable and high part property by increasing the tensile strength as well as the yield stress by paint baking treatment, etc. For example, from 2.0 mm to 1.6 mm thick, it is possible to reduce the grade by 1 grade from the conventional level, and there is also an effect that it can sufficiently contribute to weight reduction of the car body.

Claims (9)

% By mass
C: 0.15% or less, Si: 2.0% or less,
Mn: 3.0% or less, P: 0.08% or less,
S: 0.02% or less, Al: 0.02% or less,
N: 0.0050 to 0.0250%, Nb: 0.007 to 0.04%
N / Al is 0.3 or more, N is in a solid solution state is 0.0010% or more, and Nb in precipitation state is 0.005% or more, with the balance being Fe and inevitable impurities, and the average crystal grain Ferrite phase with a diameter of 10 μm or less contains 50% or more area ratio, the balance is a pearlite single phase structure or the area ratio is 2% or less bainite or martensite and the balance is pearlite structure, yield ratio 0.7 A high-tensile cold-rolled steel sheet with a tensile strength of 440 MPa or more and excellent strain aging hardening characteristics. The high-tensile cold-rolled steel sheet according to claim 1 , further comprising one group or two or more groups of the following e group to h group in mass% in addition to the composition.
Group e: One or more of Cu, Ni, Cr, and Mo total 1.0% or less in total. Group f: One or two of Ti and V total 0.1% or less in total. Group g: B is 0.0030% or less. h group: One or two of Ca and REM in total 0.0010 to 0.010% The high-tensile cold-rolled steel sheet according to claim 1 or 2 , wherein the high-tensile cold-rolled steel sheet has a thickness of 3.2 mm or less. High tensile cold-rolled coated steel sheet comprising applying electroplating or hot dipping in high-strength cold-rolled steel sheet according to claim 1 or 2. In mass%,
C: 0.15% or less, Si: 2.0% or less,
Mn: 3.0% or less, P: 0.08% or less,
S: 0.02% or less, Al: 0.02% or less,
N: 0.0050 to 0.0250%, Nb: 0.007 to 0.04%
Include, and N / Al is not less than 0.3, or even a steel slab having a composition containing more than one group or two groups selected from among the following e group ~h group, slab heating temperature: 1100 ° C. or higher And then rolled into a sheet bar, and the sheet bar is subjected to finish rolling with a final rolling reduction ratio of 25% or more and a finish rolling exit temperature: 800 ° C. or more, and a winding temperature: 750 ° C. A hot rolling step for winding a hot rolled sheet below, a cold rolling step for pickling and cold rolling the hot rolled plate to form a cold rolled plate, and a recrystallization temperature of 900 ° C. or higher on the cold rolled plate A cold-rolled sheet annealing step is performed in which annealing is performed at the following temperature at a holding time of 10 to 90 s, and then cooling is performed at a cooling rate of 70 ° C./s or less to a temperature range of 600 ° C. or less. A method for producing a high-tensile cold-rolled steel sheet with a tensile strength of 440 MPa or more and excellent strain age hardening characteristics .
Group e: One or more of Cu, Ni, Cr, and Mo total 1.0% or less in total. Group f: One or two of Ti and V total 0.1% or less in total. Group g: B is 0.0030% or less. h group: One or two of Ca and REM in total 0.0010 to 0.010% 6. The method for producing a high-tensile cold-rolled steel sheet according to claim 5 , wherein after the finish rolling, cooling is started within 0.5 s, the cooling is performed at a cooling rate of 40 ° C./s or more, and the winding is performed. The method for producing a high-tensile cold-rolled steel sheet according to claim 5 or 6 , further comprising temper rolling or leveler processing with an elongation of 1.0 to 15% following the cold-rolled sheet annealing step. The method for producing a high-tensile cold-rolled steel sheet according to any one of claims 5 to 7 , wherein sheet bars that follow each other are joined between the rough rolling and the finish rolling. Between the rough rolling and the finish rolling, one or both of a sheet bar edge heater for heating the width end portion of the sheet bar and a sheet bar heater for heating the length end portion of the sheet bar are used. A method for producing a high-tensile cold-rolled steel sheet according to any one of claims 5 to 8 .