JP2009030147A - Cold rolled steel sheet, plated steel sheet, and method for producing the steel sheet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a cold rolled steel sheet and a plated steel sheet with a composite structure having satisfactory surface properties, free from the generation of pinholes in a product after coating, also having excellent baking hardenability and cold aging resistance, and having a tensile strength of ≥340 MPa, and to provide methods for producing these steel sheets. <P>SOLUTION: The steel sheet has a structure where the main phase is composed of a ferritic phase and the second phase is composed of a low-temperature transformation-generation phase, wherein the size of surface defects including oxide is <25 μm, and preferably has a chemical composition containing, by mass, 0.0025 to <0.04% C, ≤0.5% Si, 0.5 to 3.0% Mn, ≤0.05% P, ≤0.01% S, ≤0.15% sol.Al, <0.008% N, 0.02 to 2.0% Cr, and the balance Fe with impurities. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a cold-rolled steel sheet and a plated steel sheet that are formed into various shapes by, for example, press molding, and are used after being painted. Specifically, the present invention relates to a cold-rolled steel sheet and a plated steel sheet that can all have good bake hardenability, room temperature aging resistance, and surface properties.

  Today, the technical fields of industry are highly divided, and materials used in each technical field require special and high performance. For example, in many cases, a cold-rolled steel sheet used by being formed into various shapes by press working or the like is required to have high strength, specifically, tensile strength of 340 MPa or more. For this reason, use of a high-tensile cold-rolled steel sheet has been studied. In particular, in automobiles, it is an important issue to improve fuel efficiency by reducing the weight of the vehicle body in order to protect the global environment. For this reason, the demand for high-tensile cold-rolled steel sheets capable of reducing the thickness of automobile steel sheets is increasing. Examples of the application destination of such a high-tensile cold-rolled steel sheet include an automobile outer panel and an automobile parts panel.

  Among these applications, steel plates used for automotive outer panels such as door outers and fenders are required to have dent resistance, that is, a property that does not cause permanent deformation even when pressed with a finger or hit with a stone. . This dent resistance is improved as the yield stress after coating baking after press molding is higher and as the plate thickness is thicker. For this reason, if a steel plate having a high yield stress is used as the automobile outer panel, the required dent resistance can be ensured even if the thickness is reduced.

  On the other hand, steel plates used for automotive outer panel are well suited to press molds in press working and have less springback when the molded product is removed from the press mold, that is, good shape freezing property. It is also required. For this reason, the steel plate used for an automobile outer panel is also required to have a low yield stress before press working.

  As described above, a steel plate for an automobile outer panel is required to have a low yield stress before press working and a high yield stress after press working and paint baking.

  As a steel plate having such characteristics, a bake hardenable steel plate (BH steel plate) is known. A BH steel sheet is a steel sheet that utilizes a so-called strain age hardening phenomenon in which the yield stress increases when solid solution C and N atoms segregate on the dislocations and fix the dislocations. When a BH steel plate is used as a steel plate for an automobile outer panel, the dislocation introduced at the time of press forming is fixed by solute C and N during coating baking, so that the yield stress after baking is increased. In addition, improving the bake hardenability of the high-tensile steel plate also leads to improving the dent resistance and the shape freezing property.

  Many proposals have been made so far regarding BH steel sheets. For example, in Patent Documents 1 and 2, a method for producing a BH steel sheet excellent in deep drawability, in which Ti and Nb are added to ultra-low carbon steel and further tensile strength is increased by adding Si, Mn, and P. Is disclosed. However, this method has the following problems.

  (A) Since solid solution strengthening elements such as Si, Mn, and P are added to increase the tensile strength, not only the tensile strength but also the yield stress increases. As a result, shape freezing property is deteriorated and surface distortion is likely to occur.

  (B) It is difficult to achieve both bake hardenability and room temperature aging resistance, and the bake hardening amount obtained is limited in order to ensure room temperature aging resistance.

  On the other hand, Patent Documents 3 to 5 disclose a method for producing a low carbon Al killed steel sheet (hereinafter referred to as “composite structure steel sheet”) having a composite structure in which martensite is dispersed in ferrite. This composite steel sheet has a high tensile strength, a low yield stress, can secure non-aging at room temperature even when the bake hardening amount is large, and is excellent in ductility. For this reason, the problem mentioned above is improved by using this composite structure steel plate.

  In addition to the above-mentioned mechanical property requirements, there are also surface property requirements. Steel panels for automotive exterior panels are usually press-molded and then subjected to three layers of baking coating: electrodeposition coating, intermediate coating, and top coating. However, when a coating defect called pinhole occurs in the top coating film There is. When this defect occurs in an automobile outer panel such as a roof, a hood, an outer panel of a door, etc., which requires a beautiful exterior quality, it becomes a serious coating defect that necessitates repainting. For this reason, it is strongly desired to prevent this type of surface defect.

Therefore, Patent Documents 6 and 7 disclose inventions relating to a galvanized steel sheet for automobile exterior materials that does not generate pinholes after coating and a method for manufacturing the same.
JP 59-31827 A JP 59-38337 A Japanese Patent Laid-Open No. 55-50455 JP-A-56-90926 JP 56-146826 A JP 11-246915 A JP 2000-144431 A

  The invention disclosed by Patent Documents 6 and 7 suppresses surface cracks called flaps that occur during cold rolling by lowering the coiling temperature after hot rolling or reducing intergranular corrosion due to pickling. This prevents pinholes from occurring after painting. However, according to the examination results of the present inventors, the invention disclosed in Patent Documents 6 and 7 can suppress pinholes in a ferrite single-phase steel sheet, but even in a composite structure steel sheet, even if flaps are suppressed. Pinholes may occur due to another cause.

  The present invention has been made in view of such problems of the prior art. For example, the present invention is formed and applied in various shapes by press molding or the like to improve the surface properties of the product after coating. An object of the present invention is to provide a cold-rolled steel sheet and a plated steel sheet that have both excellent bake hardenability and normal temperature aging resistance, and a method for producing these steel sheets.

  Specifically, the present invention has good surface properties, does not cause pinholes in the product after coating, has excellent bake hardenability and normal temperature aging resistance, and has a tensile strength of 340 MPa or more. It aims at providing the manufacturing method of the cold-rolled steel plate and plated steel plate which have a composite structure, and these steel plates.

  The present inventors conducted a detailed preliminary test in order to investigate the effects of additive elements and hot rolling conditions on the surface properties of a composite structure steel plate after coating. In the present specification, “%” relating to the content of steel components means “mass%”.

  The chemical composition of the test steel used in this preliminary test is as follows: C: less than 0.04%, Si: 0.5% or less, Mn: 3.0% or less, P: 0.05% or less, S: 0.00. 01% or less, sol. Al: 0.15% or less, N: less than 0.008%, Cr: 2.0% or less, balance Fe and impurities.

  After heating the steel slab having this chemical composition to 1000 ° C. or higher at various heating rates, it is hot-rolled at a temperature of 800 ° C. or higher and wound up at 400 ° C. or higher. Pickled and cold-rolled to a thickness of 0.75 mm to obtain a cold-rolled steel sheet. The temperature of the above steel slab is measured by measuring the steel slab size, the atmospheric temperature in the heating furnace and the elapsed time, and is the central part in the width direction and the central part in the length direction. The temperature at the 1/4 inner position was obtained by heat transfer calculation.

  This cold-rolled steel sheet was continuously annealed and electrogalvanized to obtain a plated steel sheet. In the base metal structure of this plated steel sheet, ferrite is the main phase (meaning the phase with the largest volume fraction; the same shall apply hereinafter) and the second phase (meaning a phase other than the main phase, hereinafter the same shall apply). It was a low temperature transformation generation phase containing martensite or martensite and bainite. Moreover, the difference in the composition of each steel slab and plated steel sheet was practically not recognized.

Next, a test piece having a width of 100 mm and a length of 300 mm was sampled from the plated steel sheet, subjected to chemical conversion treatment, and subjected to electrodeposition coating and intermediate coating baking.
The surface of the test piece thus obtained after coating was observed visually and with an optical microscope to observe the presence or absence of pinholes. In addition, the state of the surface of the base steel sheet immediately below the pinhole and below the normal part was observed. In this observation, after marking the pinhole part, the coating film and the plating film are removed, and the base material steel plate is subjected to a field emission scanning electron microscope (FE-SEM) equipped with an optical microscope and an X-ray detector (EDS). Used.

  The results of the following (A) to (E) were obtained by these preliminary tests, and further studies were made to complete the present invention.

  (A) The base material steel plate has a surface defect (flap) having a size of several μm, which is considered to be caused by intergranular corrosion during pickling, and a size of several tens to several hundreds μm in which the shape of the flap is different. The occurrence of pinholes has a correlation with the size of the base material surface defects.

  FIG. 1 is a graph showing the relationship between the presence or absence of pinholes and the size of surface defects observed on the base steel sheet. In the figure, the x mark indicates the size of the surface defect at the lower part of the pinhole, and the Δ mark indicates the size of the surface defect at the lower part of the normal part without the pinhole. From FIG. 1, it can be seen that the pinhole is generated above the surface defect of the base material having a size of 25 μm or more. FIG. 2 shows an example in which the base material defect below the pinhole is observed from the surface and the size of the surface defect is measured.

  (B) FIG. 3 and FIG. 4 show examples in which a base material surface defect below the pinhole is observed from a cross section. It can be seen that the surface defect of the base material is an uneven defect, and the plating film is missing or raised at the defect portion.

  (C) Iron oxide is present inside the base material defect below the pinhole, and when steel contains Cr, an oxide containing Cr is mixed.

  (D) Many of the surface defects present in the normal part are flaps and are suppressed by lowering the winding temperature, but the base material surface defects below the pinhole are not suppressed even if the winding temperature is reduced. .

  (E) FIG. 5 shows the relationship between the average heating rate and Cr content of steel slabs up to 300 to 1000 ° C. and the presence or absence of pinholes. The x mark indicates that a pinhole has occurred, and the Δ mark indicates that no pinhole has occurred. It can be seen that the generation of pinholes has a correlation with the average heating rate and the Cr content, and the higher the average heating rate and the greater the Cr content, the easier it is to generate.

  Although these causes are not necessarily clear, they are presumed as follows.

(A) The higher the heating rate of the steel slab, and the higher the Cr content, the easier it is to form an oxide layer enriched with FeCr ₂ O _{4 on} the top of the steel during heating.

(B) FeCr ₂ O ₄ has a high degree of adhesion to the steel, resulting in poor descaling. The oxide is pushed into the steel during hot rolling, and is not removed by pickling. Furthermore, it is pushed in or peeled off with the surrounding steel, resulting in a base material surface defect.

  (C) When the surface defect of the base material is large, a normal plating film is not formed, and a defective plating portion tends to be formed.

  (D) Hydrogen gas generated during electrodeposition coating is easily captured in such base material defects and defective plating portions, and pinholes are likely to occur.

  From these results (A) to (E), in the composite structure steel plate, the surface of the base metal steel plate is likely to have minute surface defects due to the remaining oxide generated when the steel slab is heated. Although pinholes are likely to occur, it can be seen that the generation of pinholes can be prevented by reducing the heating rate of the steel slab according to the Cr content and suppressing the surface defects of the base material.

  In addition, since the cause of the pinhole generation is derived from a defect on the surface of the base material in this way, if the base material is controlled to suppress this defect, a chemical conversion treatment is performed without forming a plating layer on the surface of the base material. Even when electrodeposition coating is performed, the formation of pinned layers by electroplating or hot dipping is performed, and even when electrodeposition coating is performed by chemical conversion treatment, the occurrence of pinholes in the coating film is suppressed. The

  The present invention is a cold-rolled steel sheet having a structure in which a main phase is a ferrite phase and a second phase is a low-temperature transformation generation phase including a martensite phase, and the size of surface defects having an oxide is less than 25 μm. .

  The cold-rolled steel sheet according to the present invention has C: 0.0025% or more and less than 0.04%, Si: 0.5% or less, Mn: 0.5% or more and 3.0% or less, P: 0.05% Hereinafter, S: 0.01% or less, sol. Al: 0.15% or less, N: less than 0.008%, Cr: 0.02% or more and 2.0% or less, preferably having a chemical composition comprising the balance Fe and impurities. Instead of a part, one or more selected from the group consisting of B: 0.003% or less, Mo: 1.0% or less, and W: 1.0% or less, and / or Ti: 0 .1% or less and Nb: 0.1% or less may be contained.

  From another viewpoint, the present invention is a plated steel sheet characterized in that the cold-rolled steel sheet described above is used as a base material and a plating layer is provided on the surface.

From another viewpoint, the present invention heats a steel slab having the above-described chemical composition to 1000 ° C. or higher at a heating rate satisfying the following formula (1), hot-rolls, cold-rolls, and anneals. This is a method for producing a cold-rolled steel sheet having a structure in which a main phase is a ferrite phase and a second phase is a low-temperature transformation generation phase including a martensite phase.
HR (° C./min)≦20.0-17.5×Cr (mass%) (1)
Here, HR in the formula represents the average heating rate of the steel slab from 300 to 1000 ° C., and Cr represents the Cr content in the steel in mass%.

  From another point of view, the present invention is characterized in that the cold-rolled steel sheet manufactured by the above-described method is subjected to a plating treatment, wherein the main phase is a ferrite phase and the second phase includes a martensite phase. It is a manufacturing method of the plated steel plate provided with the structure | tissue which is a production | generation phase.

  According to the present invention, for example, it has sufficient formability that can be applied to processing such as press molding, excellent bake hardenability and normal temperature aging resistance, and is excellent in surface properties, even if it is subjected to a coating treatment, a coating defect Provided is a steel sheet that is less prone to generate.

  For this reason, an automobile outer panel using such a steel plate has dent resistance while having shape freezing properties during press forming, and has excellent appearance quality and corrosion resistance. Therefore, the automobile using the steel plate according to the present invention can contribute to the solution of the global environmental problem through weight reduction of the vehicle body.

Hereinafter, the best mode for carrying out the cold-rolled steel sheet and the plated steel sheet according to the present invention will be described in detail.
The reasons for limiting the (a) metal structure, (b) base material surface state, (c) chemical composition, and (d) production conditions of the cold-rolled steel sheet and plated steel sheet of the present embodiment will be sequentially described.

(A) Metal structure The cold-rolled steel sheet and the plated steel sheet of the present embodiment have a composite structure in which a low-temperature transformation generation phase containing a martensite phase is dispersed in a ferrite phase. By having this composite structure, the yield stress of the steel sheet can be reduced, good press formability and surface distortion resistance can be obtained, and high bake hardenability can be obtained without impairing normal temperature aging resistance. .

Here, the “low temperature transformation generation phase” refers to a structure produced by low temperature transformation such as martensite phase or bainite phase. In addition to these, an acicular ferrite phase is exemplified.
This low temperature transformation product phase may contain two or more phases, for example, a martensite phase and a bainite phase. Moreover, it is preferable that the volume ratio of the whole low temperature transformation production | generation phase is more than 1%. When it is 1% or less, the yield stress of the steel sheet is not sufficiently lowered, and it becomes difficult to obtain good press formability and surface distortion resistance. More preferably, the volume fraction of the low temperature transformation product phase is more than 3%. On the other hand, if the volume fraction of the low temperature transformation product phase is excessively increased, the tensile strength is excessively increased, and ductility and deep drawability are deteriorated. For this reason, the volume ratio of the low temperature transformation product phase is preferably less than 15%, more preferably less than 12%.

  Moreover, when the volume ratio of a martensite phase increases too much, a yield stress will rise and it may show the tendency for shape freezing property and surface distortion resistance to deteriorate. For this reason, the volume ratio of the martensite phase is preferably less than 10%, more preferably less than 5%.

  From the viewpoint of improving the balance between press formability and surface strain resistance and tensile strength, it is particularly preferable that both the martensite phase and the bainite phase are included, and the volume ratio of the martensite phase is less than 3%.

  As another feature of the metal structure, in addition to the ferrite phase and the low temperature transformation generation phase, a residual austenite phase may be included. In this case, in order to maintain good room temperature aging resistance, the volume fraction of the retained austenite phase is preferably smaller than the entire volume fraction of the low temperature transformation product phase and less than 3%. More preferably, it is less than%.

  By having the above characteristics on the metal structure, it is possible to obtain a steel sheet having a yield stress of 300 MPa or less. In this case, excellent surface strain resistance is realized, which is preferable. From this viewpoint, it is more preferable to have a yield stress of 270 MPa or less. From the viewpoint of press formability, the steel sheet preferably has a tensile strength of less than 590 MPa.

(B) Base material surface state In the cold-rolled steel sheet of the present embodiment, the size of the surface defect having an oxide on the surface of the base steel sheet is less than 25 μm. Thereby, generation | occurrence | production of the pinhole at the time of painting is prevented. A preferred size is less than 20 μm.

Here, the “oxide” is an element formed in steel, which is generated by oxidation in a hot rolling process including heating of a steel slab, and the types thereof are Fe ₂ O ₃ and Fe ₃ O. ₄ , FeO, FeCr ₂ O ₄ , Fe ₂ SiO ₄ , FeAl ₂ O _{4 and the} like, and mixtures thereof.

  The “surface defect size” is the maximum when the surface of the steel sheet is observed with an optical microscope or a scanning electron microscope (SEM) and the defect is sandwiched between two parallel straight lines as shown in FIG. Is obtained by measuring the distance between the straight lines. In the case of a plated steel plate, the plating film may be dissolved with hydrochloric acid or the like to expose the base steel plate, and the surface of the base steel plate may be similarly measured.

  The presence or absence of oxide can be determined using an SEM equipped with an X-ray detector such as EDS, but the oxide exists from the inside to the end of the surface defect of the base material and is exposed to the surface. In some cases, surface defects should be observed from the cross section. At this time, it is preferable to use a focused ion beam processing apparatus (FIB) so that the oxide does not fall off for the preparation of the cross-sectional observation sample. For cross-sectional observation / analysis, an electric field suitable for elemental analysis of a micro region is used. It is preferable to use a radiation scanning electron microscope (FE-SEM).

  The surface state other than the size of the surface defect caused by the oxide is not particularly defined. Since the flap deteriorates the chemical conversion property and the plating property, the smaller the size, the better. However, the flap having a size of about 10 μm or less, which is inevitably generated by pickling, may be scattered.

(C) Composition The cold-rolled steel sheet of the present embodiment preferably has the following composition in order to further improve ductility, room temperature aging resistance, and the like.

C: 0.0025% or more and less than 0.04% When the C content is less than 0.0025%, it is difficult to obtain the above composite structure. On the other hand, when the C content is 0.04% or more, the ductility and deep drawability of the steel sheet tend to be impaired. Therefore, the C content is preferably 0.0025% or more and less than 0.04%. A more desirable range is 0.011% or more and 0.029% or less, and a particularly desirable range is 0.016% or more and 0.029% or less.

Si: 0.5% or less Si is an element inevitably contained in steel, and deteriorates ductility and significantly deteriorates the chemical conversion property of cold-rolled steel sheets and the plateability of plated steel sheets. Therefore, the smaller the Si content, the better. However, since Si has the effect | action which strengthens a steel plate, in order to strengthen steel, you may contain 0.5% as an upper limit. More preferably, it is 0.1% or less, Most preferably, it is 0.02% or less.

Mn: 0.5% or more and 3.0% or less Mn has an effect of improving the hardenability of steel, and is preferably contained in an amount of 0.5% or more in order to disperse the low-temperature transformation generation phase in the ferrite phase. . On the other hand, if it is excessively contained, ductility and deep drawability deteriorate. Therefore, in this embodiment, it is preferable that the upper limit of the Mn content is 3.0%. A more preferable range is 1.0% or more and less than 2.0%, and a particularly preferable range is 1.0% or more and less than 1.5%.

P: 0.05% or less P is an element inevitably contained in steel, and segregates at the grain boundaries to deteriorate secondary work brittleness and weldability. Therefore, the smaller the P content, the better. However, P can be strengthened at a low price and without deeply degrading the deep drawability. Therefore, P may be contained in a range of 0.05% or less in order to obtain a desired strength. A more preferred range is 0.005% or more and less than 0.035%, and a particularly preferred range is 0.010% or more and less than 0.020%.

S: 0.01% or less S is an impurity inevitably contained in the steel, and segregates at the grain boundaries and embrittles the steel. Therefore, the smaller the S content, the better. Therefore, the S content is preferably 0.01% or less.

sol. Al: 0.15% or less Al is used for deoxidizing molten steel. However, if it exceeds 0.15%, the effect is saturated and uneconomical. For this reason, sol. The Al content is preferably 0.15% or less. In addition, in order that Al may combine with N to form AlN and prevent aging degradation due to N, it is desirable to contain 10 times or more the N content.

N: Less than 0.008% N is an element inevitably contained in steel, and an increase in N content deteriorates ductility, deep drawability, and normal temperature aging resistance. Therefore, the N content is preferably less than 0.008%. A more preferred range is less than 0.005%, and a particularly preferred range is less than 0.004%.

Cr: 0.02% or more and 2.0% or less Cr has the effect of improving the hardenability of the steel without impairing the ductility, and 0.02% or more in order to disperse the low temperature transformation generation phase in the ferrite phase. It is preferable to contain. On the other hand, Cr deteriorates chemical conversion property in cold-rolled steel sheets, and deteriorates plating properties in plated steel sheets. Therefore, it is preferable that the upper limit of the Cr content is 2.0%. A more preferable range is 0.05% or more and less than 1.15%. A particularly preferable range is 0.15% or more and 0.90% or less. In order to further improve the ductility, it is preferable to contain 1/10 or more of the Mn content.

  The steel plate according to the present embodiment may contain the elements listed below as optional additional elements.

One or more selected from the group consisting of B: 0.003% or less, Mo: 1.0% or less, and W: 1.0% or less B, Mo, and W improve the hardenability of the steel. In order to further improve, you may contain 1 type, or 2 or more types of these. However, since B deteriorates the deep drawability, the upper limit is made 0.003%. A desirable range is 0.0002% or more and less than 0.002%. Further, if Mo and W are contained in excess of 1.0%, the effect is saturated and uneconomical, so 1.0% or less. A desirable range is 0.02% or more and less than 0.5%.

One or two of Ti: 0.1% or less and Nb: 0.1% or less Ti and Nb are bonded to N to form TiN or NbN, thereby preventing aging deterioration due to N. Both may be included. However, if the content exceeds 0.1%, the effect is saturated and uneconomical. For this reason, each content shall be 0.1% or less. There is no specific lower limit. A preferable range is 0.003% or more and 0.025% or less.

  Other than the elements described above, Fe and impurities.

(D) Manufacturing conditions The steel plate which concerns on this embodiment will not be specifically limited to a manufacturing method, if it has the characteristics in the above metal structures and the characteristics in the surface state of a base material. However, if the following manufacturing method is employ | adopted about the steel plate which has said chemical composition, obtaining the steel plate which concerns on this embodiment efficiently and stably will be implement | achieved.

  The steel having the above-described composition is made into a steel ingot by a continuous casting method after being melted by a known means, or a steel slab by a method in which the steel ingot is made into a steel ingot by any casting method and then rolled into pieces. It is said. The steel ingot or steel slab is re-heated or hot-rolled by performing auxiliary heating on the high-temperature steel ingot after continuous casting or the high-temperature steel slab after rolling. In the present specification, such steel ingots and steel slabs are collectively referred to as “steel slabs” as materials for hot rolling. In order to keep the surface properties good, it is preferable to clean the surface of the slab in the cold or warm state before heating.

The steel slab is preferably heated to 1000 ° C. or higher at a heating rate that satisfies the following formula (1).
HR (° C./min)≦20.0-17.5×Cr (mass%) (1)
Here, HR in the formula represents the average heating rate of the steel slab from 300 to 1000 ° C., and Cr represents the Cr content in the steel in mass%. When heated so as to satisfy this condition, surface defects of the cold-rolled steel sheet or the plated steel base material are suppressed, and it is possible to stably achieve a steel sheet having no pinholes after coating.

  The heating condition of 1000 ° C. or higher is not particularly specified, but if the heating temperature is low, the steel plate temperature during rolling decreases and the rolling load increases, making it difficult to control the shape of the steel plate. It is preferable. In addition, if the heating temperature is too high, the productivity is reduced due to scale loss, so the heating temperature is preferably less than 1300 ° C. More preferably, the heating temperature is less than 1200 ° C.

  In measuring the heating rate of the steel slab, the steel slab thickness is 1/4 or more inside of the steel slab thickness in the thickness direction from the steel slab surface, and the steel slab width is 1/4 or more inside in the width direction. Install a thermocouple in the range that is more than 1/4 of the length of the billet in the direction, and measure the temperature of the billet directly, or measure the ambient temperature and elapsed time in the heating furnace and are in the same range What is necessary is just to obtain | require an average heating rate from the time required to heat-up the temperature of a steel piece, and to heat up from 300 degreeC to 1000 degreeC.

The conditions for hot rolling are not particularly specified. However, it is preferable to perform finish rolling in a low temperature range of austenite to refine the crystal grains of the hot-rolled steel sheet, thereby developing a recrystallized texture preferable for deep drawability during annealing. Therefore, it is desirable to perform the final reduction in a temperature range not lower than the Ar ₃ transformation point and not higher than (Ar ₃ transformation point + 100 ° C.). Further, in order to suppress surface defects of the cold-rolled steel sheet or the plated steel base material, the difference between the finish rolling start temperature and the finish rolling end temperature is preferably 100 ° C. or more, and the finish rolling end temperature is preferably less than 850 ° C. .

  In addition, in order to perform finish rolling in these temperature ranges, you may heat a rough rolling material between rough rolling and finish rolling. At this time, it is desirable to heat the rear end of the rough rolled material at a higher temperature than the front end, and to suppress the temperature variation over the entire length of the rough rolled material at the start of finish rolling to 140 ° C. or less. Thereby, the uniformity of the product characteristic in a coil improves.

  The heating method of the rough rolled material may be performed using a known means. For example, a solenoid induction heating device may be installed between the rough rolling mill and the finish rolling mill, and the heating temperature increase amount may be controlled based on the temperature distribution in the longitudinal direction on the upstream side of the induction heating device.

  In this way, when the steel sheet is cooled and wound into a coil after finishing the hot rolling, it is desirable to wind it at less than 650 ° C. in order to reduce the yield due to the generation of scale. Moreover, in order to suppress a flap, it is more preferable to wind up at less than 550 ° C. On the other hand, in order to sufficiently precipitate AlN and suppress aging deterioration due to N, it is preferable to set the lower limit of the coiling temperature to 450 ° C.

  The steel sheet hot-rolled as described above is descaled by pickling or the like, and then cold-rolled according to a conventional method. Cold rolling is preferably rolled to a sheet thickness of less than 1.0 mm at a reduction ratio of 70% or more in order to develop a recrystallized texture preferable for deep drawability by recrystallization annealing performed after cold rolling.

  The steel sheet after the cold rolling is subjected to treatment such as degreasing according to a known method, if necessary, and then recrystallization annealing. The temperature control in this recrystallization annealing is important for obtaining a steel sheet having a suitable metal structure.

First, it is preferable that the soaking temperature in the recrystallization annealing is equal to or higher than the Ac ₁ transformation point. By performing soaking in this temperature range, it is possible to efficiently and stably obtain a composite structure in which the main phase is a ferrite phase and the second phase is a low-temperature transformation generation phase containing martensite. . Here, the Ac ₁ transformation point means the start temperature of the ferrite → austenite transformation during heating.

However, if the soaking temperature becomes too high, the ferrite becomes excessively coarse and rough skin occurs during press molding. Therefore, the upper limit of the soaking temperature is preferably less than (Ac ₃ transformation point + 100 ° C.), and Ac ₃ More preferably, it is less than the transformation point. Here, the Ac ₃ transformation point means the completion temperature of the ferrite → austenite transformation during heating.

  Moreover, it is preferable that the heating rate to the soaking temperature in recrystallization annealing shall be less than 60 degreeC / s. If the heating rate until the soaking temperature is reached is too high, the ferrite may become finer and the ductility may be deteriorated.

  Further, in the cooling process after soaking in recrystallization annealing, a temperature range of 650 ° C. or lower and 450 ° C. or higher is set to 15 ° C./s or higher and 200 ° C./s in order to suppress generation of ferrite and improve normal temperature aging resistance. It is preferable to cool at the following cooling rate. A more preferable cooling rate is more than 60 ° C./s and less than 130 ° C./s.

  The cooling method from the soaking temperature to 650 ° C. is not particularly limited. However, it is desirable to cool at a cooling rate of less than 10 ° C./s in order to increase the stability of austenite and easily obtain a low temperature transformation product phase.

  The cold-rolled steel sheet thus obtained may be subjected to temper rolling according to a conventional method. However, when the elongation rate of temper rolling is high, ductility is deteriorated. Therefore, the elongation of temper rolling is preferably 1.0% or less. A more preferable elongation is 0.5% or less.

  The cold-rolled steel sheet manufactured by the above-described method may be electroplated according to a conventional method to manufacture an electroplated steel sheet. In the case of producing a hot-dip galvanized steel sheet, the cold-rolled steel sheet produced by the above-described method may be subjected to hot-dip plating according to a conventional method, or after performing cold rolling by the above-described method, continuous melting is performed. Recrystallization annealing may be performed in a plating facility to perform plating. When manufacturing a hot-dip galvanized steel sheet in a continuous hot-dip plating facility, in the cooling process after soaking in recrystallization annealing, it is cooled at a cooling rate of 4 ° C./s or higher to a predetermined temperature in the range of 460 to 600 ° C. It is preferable to perform hot dipping after holding at the predetermined temperature for 10 seconds or more. By doing so, C concentration in austenite is promoted, and a low-temperature transformation generation phase containing martensite is easily obtained.

  Although the kind of plating is not specifically limited, it is preferable to set it as the zinc-type plating which is excellent in the corrosion resistance after coating. When Cr is contained, electroplating is preferable from the viewpoint of plating properties. Moreover, you may perform temper rolling after plating. Also in this case, the elongation of temper rolling is preferably 1.0% or less, and more preferably 0.5% or less.

  The structure of the steel sheet manufactured in this way has a main phase of a ferrite phase and a low-temperature transformation generation phase containing a martensite phase as a second phase.

The present invention will be described more specifically with reference to examples.
Steel slabs adjusted to the chemical composition shown in Table 1 were produced by continuous casting. These steel slabs were heated under the conditions shown in Table 2, then hot-rolled and wound into a coil to obtain a hot-rolled sheet having a thickness of 3.0 mm. The obtained hot-rolled sheet was pickled and then cold-rolled to a sheet thickness of 0.75 mm (rolling rate 75%). Subsequently, the cold-rolled plate was annealed at 750 to 800 ° C. for about 30 seconds with continuous annealing equipment. The cooling conditions after soaking were 5 ° C / s for the average cooling rate from the soaking temperature to 650 ° C, and 80 ° C / s for the average cooling rate from 650 ° C to 450 ° C. The obtained annealed sheet was temper-rolled at an elongation of 0.5% to obtain a cold-rolled steel sheet. Furthermore, some of the cold-rolled steel plates were subjected to electrogalvanizing to obtain a galvanized steel plate having a plating adhesion amount per side of 70 g / m ² .

  The surface of the obtained cold-rolled steel sheet was observed using an FE-SEM equipped with EDS, and the size of surface defects and the presence or absence of oxides in the defects were investigated. Moreover, after dissolving the plating film of a plated steel plate using hydrochloric acid, the surface defect which exists in the base material surface was investigated similarly to the case of a cold-rolled steel plate.

  In addition, specimens with a width of 100 mm and a length of 300 mm were collected from cold-rolled steel sheets and plated steel sheets, subjected to chemical conversion treatment, applied with electrodeposition coating and intermediate top-coating, and then the surface of the coating film was observed visually and using an optical microscope. The existence of pinholes was investigated.

  Furthermore, a tensile test was performed using a JIS No. 5 tensile specimen taken from the sheet width direction of the cold rolled steel sheet and the plated steel sheet, yield stress (YS), tensile strength (TS), yield point elongation (YPE), and total elongation ( El) was determined.

  Bake hardenability is obtained by taking a JIS No. 5 tensile test piece from the width direction of the annealed plate, applying a 2% tensile pre-strain, and subjecting it to a heat treatment at 170 ° C. for 20 minutes, followed by a tensile test. The difference between the obtained yield stress and 2% deformation stress was defined as the amount of BH, which was used as an index for bake hardenability.

  For aging resistance at normal temperature, a JIS No. 5 tensile test piece taken from the width direction of the annealed plate was taken, held in an electric furnace set at 40 ° C. for 3 months, and then subjected to a tensile test, yield point elongation (YPE) It was evaluated by measuring.

  Table 3 shows the performance evaluation results. The size of the surface defects is determined by measuring the size of the defects using an SEM with a magnification of 500 times at any 10 locations on the surface of the steel plate in the case of a cold-rolled steel plate and on the surface of the base material in the case of a plated steel plate. The maximum value of.

  The metal structure was composed of a low-temperature transformation generation phase in which the main phase was a ferrite phase and the second phase contained a martensite phase, and the size of surface defects was less than 25 μm. No. 10 showed no pinholes after coating, and showed a high BH amount of 46 MPa or more, while the YPE after aging was 0.1% or less, indicating good room temperature aging resistance. . Furthermore, YS was 270 MPa or less, and YPE was 0%, indicating good press formability.

On the other hand, in Test Nos. 2 and 6, since the heating rate of the steel slab was high and the equation (1) was not satisfied, the size of the surface defect having oxide was 25 μm or more, and a pinhole was generated after coating.
Moreover, since the metal structures of the test numbers 3 and 9 were a ferrite single phase, YS and YPE were high and press formability was unsatisfactory, YPE after aging was large, and aging resistance was unsatisfactory.

It is a graph which shows the relationship between the magnitude | size of a base material surface defect, and the presence or absence of pinhole generation | occurrence | production. It is the example which observed the base material surface defect from the surface. This is an example in which a base material surface defect (concave shape) is observed from a cross section. This is an example in which a base material surface defect (convex shape) is observed from a cross section. It is a graph which shows the relationship between the average heating rate of a steel piece, Cr content, and the presence or absence of pinhole generation | occurrence | production.

Claims (7)

  A cold-rolled steel sheet comprising a structure in which a main phase is a ferrite phase and a second phase is a low-temperature transformation generation phase including a martensite phase, and the size of a surface defect having an oxide is less than 25 μm.   C: 0.0025% or more and less than 0.04%, Si: 0.5% or less, Mn: 0.5% or more, 3.0% or less, P: 0.05% or less, S: 0.005% by mass. 01% or less, sol. The cold-rolled steel sheet according to claim 1, having a chemical composition comprising Al: 0.15% or less, N: less than 0.008%, Cr: 0.02% or more and 2.0% or less, the remaining Fe and impurities.   The chemical composition may be one selected from the group consisting of B: 0.003% or less, Mo: 1.0% or less, and W: 1.0% or less in mass%, instead of a part of Fe. The cold-rolled steel sheet according to claim 2 containing two or more kinds.   4. The chemical composition according to claim 2, wherein the chemical composition contains one or two of Ti: 0.1% or less and Nb: 0.1% or less in mass% instead of part of Fe. Cold rolled steel sheet.   A plated steel sheet comprising the cold-rolled steel sheet according to any one of claims 1 to 4 as a base material and a plating layer on a surface thereof. A steel slab having the chemical composition according to any one of claims 2 to 4 is heated to 1000 ° C or higher at a heating rate satisfying the following formula (1), hot-rolled, cold-rolled, and annealed. A method for producing a cold-rolled steel sheet comprising a structure in which a main phase is a ferrite phase and a second phase is a low-temperature transformation generation phase including a martensite phase.
HR (° C./min)≦20.0-17.5×Cr (mass%) (1)
Here, HR in the formula is the average heating rate of the steel slab from 300 to 1000 ° C., and Cr is the Cr content (mass%) in the steel.   A cold-rolled steel sheet manufactured by the manufacturing method according to claim 6 is plated, wherein the main phase is a ferrite phase and the second phase is a low-temperature transformation generation phase including a martensite phase. A method for producing a plated steel sheet having a structure.

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