JP5245948B2 - Cold rolled steel strip manufacturing method - Google Patents

Description

The present invention relates to a method for manufacturing a cold-rolled steel strip . In particular, the present invention relates to a method for producing a cold-rolled steel strip suitable for a material for an automotive outer panel.

  Now that the industrial field is highly divided, materials used in each field are required to have special and high performance. High strength is also required for steel plates used by press forming, and application of high-tensile steel plates is being studied. In particular, regarding steel plates for automobiles, there is a high demand for high-strength steel plates in order to reduce the weight of the vehicle body and improve fuel efficiency in consideration of the global environment. Regarding automobile steel plates, various performances are further required depending on the part to be applied.

  For example, automobile outer panel such as a door outer or a fender is desired to have dent resistance, that is, to have a property that does not cause permanent deformation when pressed with a finger. The dent resistance is improved as the yield stress after press molding and paint baking is higher, and as the plate thickness of the steel plate is thicker. Therefore, it is possible to reduce the thickness by increasing the yield stress after press molding and baking, so from the viewpoint of reducing the weight of the vehicle body, steel sheets with high yield stress after press molding and baking are used for automobiles. It is suitable as a material for the outer panel.

  On the other hand, from the viewpoint of press formability, it is well suited to the mold, and the amount of spring back generated when the press-molded product is released from the mold is small, that is, the shape freezing property is good. desired. Therefore, from the viewpoint of press formability, a steel plate having a low yield stress before press forming is suitable as a material for an automotive outer panel. Here, the absolute evaluation of shape freezing property is performed by yield stress, but since yield stress usually increases with increasing tensile strength, the shape freezing property between steel plates of different tensile strength is Evaluation by means of lack of validity. Therefore, a general evaluation of shape freezing should be performed by the yield ratio, which is a relative evaluation.

  From the above, the steel sheet used as the material for the outer panel of an automobile has a low yield ratio in the initial state before press forming, and also has mechanical characteristics having high yield stress after press forming and paint baking. Is preferred.

  The exterior panel for automobiles is also required to have a beautiful appearance. For this reason, it is requested | required that the steel plate used for the raw material of the outer plate | board panel for motor vehicles should not have a surface defect. When mechanical properties such as the press formability mentioned above are insufficient, it may be possible to cope by changing external conditions such as mold adjustment, but when surface defects such as scale wrinkles exist. It is difficult to respond by changing external conditions. Therefore, it is important to prevent the occurrence of surface defects such as scale wrinkles.

  However, high-strength steel plates usually contain a considerable amount of alloy elements in order to ensure high strength, and these alloy elements are often oxidizable elements that promote the generation of scale flaws. For this reason, in general, the higher the strength, the greater the rate of occurrence of surface defects, making it difficult to stably produce high-tensile steel sheets having high-quality surface properties. In particular, such a tendency becomes apparent because the level of surface texture required for the steel plate as the material of the outer panel of an automobile is very high.

  Regarding these problems, there is a bake-hardening steel sheet (hereinafter also referred to as “BH steel sheet”) as a conventional technique related to dent resistance. This is a steel sheet that positively utilizes the so-called strain age hardening phenomenon in which the yield stress rises when the solid solution C or solid solution N atoms segregate on the dislocations and fix the dislocations. A general BH steel sheet promotes segregation of atoms of solute C and solute N during paint baking, and thereby increases the yield stress by fixing the dislocations to dislocations introduced during press forming. . Many proposals have been made regarding high-strength steel sheets of BH steel sheets.

  Excellent drawability by adding solid solution C and solid solution N by adding Ti and Nb to ultra low carbon steel and further adding solid solution strengthening elements such as Si, Mn, P, etc. BH steel sheet is a typical example. However, since such a BH steel sheet is added with a solid solution strengthening element such as Si, Mn, and P in order to increase the tensile strength, not only the tensile strength but also the yield ratio is high. For this reason, the shape freezing property is inferior and surface distortion is likely to occur. In addition, non-plating due to addition of Si, deterioration of alloying processability due to addition of P, and the like are caused, so that it is difficult to apply to hot dip galvanization. Furthermore, it is difficult to achieve both bake hardenability and room temperature aging resistance, and the upper limit of bake hardening must be limited to about 50 MPa in order to ensure room temperature slow aging.

  By the way, Patent Document 1 discloses an invention relating to a low carbon Al killed steel sheet having a composite structure in which martensite is dispersed in ferrite. A steel sheet having such a composite structure has characteristics that it has a high tensile strength, a low yield ratio, and can ensure non-aging at room temperature even when the bake hardening amount is large.

  In order to obtain a composite structure, it is necessary to improve the hardenability of the steel sheet, and particularly in the case of a hot dip plated steel sheet manufactured in a continuous hot dip plating apparatus where rapid cooling is difficult due to equipment restrictions, it is necessary to remarkably improve the hardenability. For this reason, it is difficult to ensure sufficient hardenability with only typical Mn, which is a quenching element, and it may be necessary to add a large amount of easily oxidizable elements such as Cr and Mo. However, as described above, when a considerable amount of such an easily oxidizable element is contained, the occurrence of surface defects such as scale flaws becomes prominent, and the beautiful surface properties required for the material of the outer panel for automobiles are ensured. Is not easy. Although the invention described in Patent Document 1 is a very excellent invention, there is room for further improvement from the viewpoint of ensuring stable surface properties.

On the other hand, from the viewpoint of surface properties, Patent Document 2 discloses that in the finish rolling step after descaling in hot rolling, the surface temperature of the rough bar is not reheated between 920 ° C. and 970 ° C. for 5 seconds or more. _An invention relating to a method for suppressing scale wrinkles by ending rolling at _three or more points is disclosed. According to this method, generation of a new scale in the finish rolling process and generation of CO gas at the scale / base metal interface are suppressed, and generation of blisters is suppressed.

JP 2001-303184 A JP 11-290905 A

  However, in the example of Patent Document 2, as is understood from the fact that the measurement of the surface temperature of the rough bar is performed at one place in the middle of each stand, the invention disclosed in Patent Document 2 It is assumed that the surface temperature of the bar is maintained at the temperature measured at this one point and is maintained only for the time of passing between the stands. However, since the surface temperature of the rough bar fluctuates naturally between the stands, the condition that “the surface temperature of the rough bar is not reheated between 920 ° C. and 970 ° C. for 5 seconds or more” is actually satisfied. It is not clear whether it can be done, and it is also unclear whether this numerical limitation is critical. For this reason, the invention disclosed in Patent Document 2 is difficult to put into practical use.

The present invention has been made in view of the problems of these conventional techniques, and has a low yield stress in the initial state before press molding, and a high yield stress after press molding and paint baking. It is an object of the present invention to provide a method for producing a high-tensile cold-rolled steel strip having surface properties that have mechanical properties and scale wrinkles are suppressed.

The present inventor conducted the following preliminary test to solve the above-mentioned problems, and the influence of (a) alloy elements and (b) production conditions, particularly hot rolling conditions, on the mechanical properties and surface properties of the composite steel sheet. Detailed investigations were made using actual production equipment.
(A) Influence of alloying element The present inventor C: 0.01% or more and 0.04% or less (Unless otherwise specified in this specification, “%” relating to composition means “% by mass”) ), Si: 0.5% or less, Mn: 2.5% or less, P: 0.05% or less, S: 0.01% or less, Al: 0.15% or less, N: 0.008% or less, A 260 mm-thick slab having a chemical composition of Cr: 1.0% or less, the balance Fe and impurities is heated to a surface temperature of 1150 ° C. or higher, and then a finishing temperature: 800 ° C. or higher and 900 ° C. or lower, and winding Taking temperature: Hot rolled under conditions of 400 ° C. or higher and 700 ° C. or lower to obtain a hot rolled steel plate having a thickness of 3.4 mm, and the obtained hot rolled steel plate is pickled by a conventional method to obtain a 0.7 mm plate. Cold-rolled to a thickness, and the resulting cold-rolled steel sheet is ferrite and austenite in a continuous annealing facility A cold rolled steel sheet was obtained by annealing at a temperature of 800 ° C., which is a two-phase region. And the tensile test of the obtained cold-rolled steel plate was done, and the mechanical characteristic was investigated.

FIG. 1 is a graph showing the relationship between the amount of (Mn + Cr), which is the total content of Mn and Cr, and the yield ratio. As shown in the graph in FIG. 1, by setting the chemical composition so that the amount of (Mn + Cr) is 1.9% or more, the cold-rolled steel sheet is composed of ferrite as a main phase and a low-temperature transformation generation phase as a second phase. At the same time, it was found that this low-temperature transformation product phase becomes a steel structure containing martensite, yields mechanical properties with a yield ratio of 0.65 or less, and a good shape freezing property is obtained before press forming. It has also been found that a BH amount of 40 MPa or more is secured, and that good dent resistance can be obtained after press molding and paint baking.
(B) Influence of manufacturing conditions The present inventor examined a method for suppressing scale wrinkles from the viewpoint of surface properties.

  It is known that scale wrinkles are generally caused by scale biting during rolling, and scale biting occurs when descaling is poor in the hot rolling process. Therefore, the optimal descaling position and conditions were examined.

  This inventor investigated in detail the surface condition of the slab in the heating process at the time of subjecting to hot rolling, and the surface condition of the slab after extracting from a heating furnace and performing descaling. As a result, a slab having a chemical composition containing a large amount of Mn and Cr has a sufficient thickness scale generated on the surface of the slab in the heating furnace by simply performing descaling to the extent that it is usually applied before hot rolling. It could not be removed, and it has been found that there are cases where the process proceeds to the hot rolling process with the scale remaining. And it turned out that the iron oxide containing especially Cr exists in the inside of the scale soot generated in the final product.

This steel sheet was equivalent amount is contained Cr in order to obtain a composite structure steel sheet, easy oxide layer on top of the base steel FeCr ₂ O ₄ was concentrated is formed, the FeCr ₂ O ₄ is the base steel Therefore, the descaling failure occurs only by applying the descaling to the extent that it is usually applied. Estimated to trigger.

  Therefore, as a result of examining in detail the descaling conditions immediately after extraction from the heating furnace, the slab is subjected to a water cooling treatment for cooling the surface temperature to 50 ° C. or more and 200 ° C. or less within 10 seconds after extracting the slab from the heating furnace, If the slab is descaled within 0.1 to 10 seconds after the water-cooling treatment, even if it is a steel plate containing a large amount of Cr, the scale peelability of the slab before being subjected to the hot rolling process is remarkably increased. I found out that it can be improved.

  That is, the slab in a high temperature state immediately after being extracted from the heating furnace is subjected to water cooling treatment for cooling to 50 ° C. or more and 200 ° C. or less to embrittle the scale, and after the embrittled scale is grown to an appropriate thickness, the debris is grown. By performing the scaling process, the scale peelability is remarkably improved.

  FIG. 2 shows the slab with a surface temperature of 1280 ° C. charged in the heating furnace, extracted from the heating furnace, subjected to water cooling treatment, and after descaling treatment is performed 2 seconds after the water cooling treatment. It is a graph which shows the relationship between the time until a water cooling process start, and the scale residual rate in the slab surface after a descaling process. The scale remaining rate was evaluated by image determination after imaging the slab surface before entering the hot rolling process with a video camera.

  As is apparent from the graph of FIG. 2, the time from the extraction in the heating furnace to the start of the water cooling treatment is within 10 seconds, and the slab surface temperature is cooled to 50 ° C. or more and 200 ° C. or less, so that the scale is brittle due to rapid cooling from a high temperature state. And the scale peelability in the subsequent descaling process is improved. On the other hand, when the time from the heating furnace extraction to the start of the water cooling process exceeds 10 seconds or the cooling of the water cooling process is less than 50 ° C., the scale peelability in the descaling process is poor and the scale remaining rate is large.

  FIG. 3 shows a case where a slab having a surface temperature of 1280 ° C. is extracted from the heating furnace, subjected to water cooling treatment after 4 seconds, and descaling treatment is performed after water cooling treatment. It is a graph which shows the relationship between the time until a scaling process start, and the scale residual rate in the slab surface after a descaling process.

  As is clear from the graph of FIG. 3, a water cooling process is performed to cool the slab surface temperature to 50 ° C. or more and 200 ° C. or less, and the time from the water cooling process to the start of the descaling process is set to 0.1 seconds or more and 10 seconds or less. As a result, the scale embrittled by the water-cooling process grows moderately and is efficiently removed by the descaling process, improving the scale peelability in the descaling process and reducing the rate of scale flaws in the final product. It can be suppressed to 0.5% or less.

  On the other hand, when the time from the water cooling treatment to the start of the descaling treatment exceeds 10 seconds or the cooling of the water cooling treatment is less than 50 ° C., the scale peelability in the descaling treatment is poor, and the scale in the final product The incidence of soot increased to 1% or more.

  As described above, within 10 seconds after extracting the slab from the heating furnace, a water cooling treatment for cooling the surface temperature to 50 ° C. or more and 200 ° C. or less is performed on the slab, and after the water cooling treatment within 0.1 seconds or more and 10 seconds, By applying a descaling process to the slab, even if it is a steel plate containing a large amount of Cr, the scale peelability of the slab before being subjected to the hot rolling process can be remarkably improved.

The present invention has been made on the basis of these new findings obtained by the preliminary test.
This invention is a manufacturing method of the cold-rolled steel strip characterized by having the following process (A)-(G).
(A) C: 0.010% to 0.040%, Si: 0.5% or less, Mn: 1.0% to 2.5%, P: 0.05% or less, S: 0.01 % Or less, sol. Al: 0.001% or more and 0.15% or less, N: 0.008% or less and Cr: 0.25% or more and 1.0% or less, with the balance being Fe and impurities, and formula (1) : A slab heating step in which a slab having a chemical composition satisfying Mn + Cr ≧ 1.9 is charged into a heating furnace so that the surface temperature is 1150 ° C. or higher and 1350 ° C. or lower;
(B) A slab cooling step of extracting a slab from a heating furnace and performing a water cooling treatment for cooling the surface temperature of the slab within a range of 50 ° C. to 200 ° C. within 10 seconds after the extraction;
(C) A descaling step in which the slab is descaled within 0.1 seconds to 10 seconds after the water cooling treatment;
(D) A hot rolling step in which hot rolling of a finishing temperature: Ar ₃ points or more and 950 ° C. or less and a winding temperature: 400 ° C. or more and 700 ° C. or less is performed on the slab to form a hot rolled steel sheet;
(E) A step of pickling the hot-rolled steel sheet to obtain a pickled steel sheet;
(F) a cold rolling step in which the pickled steel sheet is cold rolled at a reduction rate of 50% or more to obtain a cold rolled steel sheet; and
(G) An annealing process for subjecting a cold-rolled steel sheet to recrystallization annealing in a temperature range of Ac ₁ point or more and less than Ac ₃ point.

In the method for producing a cold-rolled steel strip according to the present invention, the chemical composition is changed from Mo: 1.0% or less, B: 0.0020% or less, and W: 1.0% or less, instead of part of Fe. It is preferable to contain 1 type (s) or 2 or more types selected from the group which consists of.

In these methods for producing a cold-rolled steel strip according to the present invention, the chemical composition is selected from the group consisting of Ti: 0.1% or less and Nb: 0.1% or less, instead of a part of Fe. It is preferable to contain seeds or two kinds.

  In the method for manufacturing a cold-rolled steel strip according to the present invention, it is preferable to perform plating on the surface of the cold-rolled steel strip.

  According to the present invention, it has a low yield stress in the initial state before press forming, a high yield stress after press forming and paint baking, and a surface property with reduced scale wrinkles. A high-tensile cold-rolled steel strip can be obtained. Therefore, according to the present invention, for example, required for a steel plate of an automotive outer panel such as a door outer or a fender panel, before the press molding, the yield stress is low and the shape freezing property is excellent. After coating and baking, it is possible to obtain a high-tensile cold-rolled steel strip having high yield stress, excellent dent resistance, beautiful surface properties with suppressed scale wrinkles, and excellent room temperature aging resistance.

For this reason, the cold-rolled steel strip manufactured according to the present invention is suitable as a material for a panel for an automobile outer plate, and contributes greatly to the development of industries, such as being able to contribute to solving global environmental problems through weight reduction of automobile bodies. Is.

FIG. 1 is a graph showing the relationship between the amount of (Mn + Cr) and the yield ratio. FIG. 2 is a graph showing the relationship between the time until the water cooling treatment start time after extraction from the heating furnace and the scale remaining rate on the slab surface. FIG. 3 is a graph showing the relationship between the time from the start of the water cooling process to the start of the descaling process and the rate of occurrence of scale defects in the final product.

Hereinafter, the chemical composition of the cold-rolled steel strip to be more prepared in the present invention, the steel structure, mechanical properties, surface properties, in detail the reasons for limitation of the plating layer and the production conditions. In the present specification, as described above, “%” indicating the chemical composition is all “mass%”, and “%” indicating the steel structure is all “area%” and occupies the entire steel structure. It is a ratio.

(1) Chemical composition [C: 0.010% or more and 0.040% or less]
C is an important element that determines the steel structure, and is composed of ferrite as a main phase and a low-temperature transformation generation phase as a second phase, which is an object of the present invention, and has a low-temperature transformation generation phase of 0.5% or more. It is an important element for obtaining a steel structure containing martensite. If the C content is less than 0.010%, it is difficult to obtain a target steel structure. Therefore, the C content is 0.010% or more. On the other hand, when the C content exceeds 0.040%, the deep drawability deteriorates significantly. Therefore, the C content is 0.040% or less.

[Si: 0.5% or less]
Si is an element that is generally contained as an impurity, but since it has an effect of increasing the strength of the steel sheet by solid solution strengthening, it may be positively contained. However, when the Si content is more than 0.5%, the chemical conversion property of the steel sheet is significantly deteriorated. Moreover, when performing a plating process, the fall of plating adhesiveness becomes remarkable. Therefore, the Si content is 0.5% or less. Preferably it is 0.1% or less, More preferably, it is 0.02% or less.

[Mn: 1.0% to 2.5%]
Mn is an element having an effect of improving the hardenability of steel, and is composed of a ferrite as a main phase and a low-temperature transformation generation phase as a second phase, which is an object of the present invention. It is an important element for obtaining a steel structure containing 0.5% or more of martensite. If the Mn content is less than 1.0%, it is difficult to obtain a target steel structure. Therefore, the Mn content is 1.0% or more. On the other hand, if the Mn content exceeds 2.5%, the ductility and deep drawability deteriorate significantly. Therefore, the Mn content is 2.5% or less. Preferably it is 2.0% or less, More preferably, it is 1.8% or less.

[P: 0.05% or less]
P is an element generally contained as an impurity, but it does not deteriorate the deep drawability so much and has an effect of increasing the strength of the steel sheet at a low cost. However, when the P content exceeds 0.05%, the secondary work brittleness resistance and the weldability deteriorate significantly. Therefore, the P content is 0.05% or less. Preferably it is 0.025% or less.

[S: 0.01% or less]
S is an element contained as an impurity and has an action of segregating at the grain boundaries and embrittlement of the steel. Therefore, the S content is 0.01% or less.

[Sol. Al: 0.001% or more and 0.15% or less]
Al has the effect | action which deoxidizes molten steel and makes steel healthy. sol. When the Al content is less than 0.001%, deoxidation is insufficient. Therefore, sol. The Al content is 0.001% or more. On the other hand, sol. Even if the Al content exceeds 0.15%, the effect of the above action is saturated and uneconomical. Therefore, sol. The Al content is 0.15% or less.

[N: 0.008% or less]
N is an element contained as an impurity and has an effect of deteriorating ductility, deep drawability, and normal temperature aging resistance. Therefore, the N content is 0.008% or less. Preferably it is less than 0.005%, More preferably, it is less than 0.003%.

[Cr: 0.25% to 1.0%]
Cr is an element that has the effect of improving the hardenability of steel without significantly reducing the ductility. From the purpose of the present invention, ferrite as a main phase and a low-temperature transformation generation phase as a second phase. At the same time, it is an important element for obtaining a steel structure containing martensite having a low temperature transformation generation phase of 0.5% or more. If the Cr content is less than 0.25%, it is difficult to obtain a target steel structure. Therefore, the Cr content is 0.25% or more. Preferably it is 0.30% or more, more preferably 0.35% or more. On the other hand, if the Cr content is more than 1.0%, the chemical conversion property of the steel sheet is significantly deteriorated. Moreover, when performing a plating process, the fall of plating adhesiveness becomes remarkable. Therefore, the Cr content is 1.0% or less. Preferably it is 0.80% or less.

[Mn + Cr ≧ 1.9]
As described above, Mn and Cr have an effect of improving the hardenability of steel, and are important elements for realizing the steel structure intended by the present invention. Therefore, the contents of Mn and Cr need to be taken into account in consideration of the effects of both. Therefore, in order to obtain the target steel structure of the present invention, the total content of Mn and Cr is set to 1.9% or more. The upper limit of the total content of Mn and Cr does not need to be specified, but is preferably 2.5% or less from the viewpoint of chemical conversion properties and plating adhesion.

[Mo: 1.0% or less]
Mo is an optional element and may be contained because it has the effect of improving the hardenability of the steel. However, even if Mo is contained in an amount exceeding 1.0%, the effect by the above action is saturated and uneconomical, and the chemical conversion property is deteriorated. Therefore, when it contains Mo, the content shall be 1.0% or less. Preferably it is 0.5% or less. In order to more reliably obtain the effect of the above action, the Mo content is preferably 0.02% or more, and more preferably 0.05% or more.

[B: 0.0020% or less]
B is an optional element and may be contained because it has the effect of improving the hardenability of the steel. B has an effect of further improving the bake hardenability. However, if the B content exceeds 0.0020%, the moldability is significantly deteriorated. Therefore, when it contains B, the content shall be 0.0020% or less. Preferably it is 0.0015% or less. In order to more reliably obtain the effect of the above action, the B content is preferably set to 0.0002% or more.

[W: 1.0% or less]
W is an optional element and may be contained because it has the effect of improving the hardenability of the steel. However, even if W is contained in excess of 1.0%, the effect of the above action is saturated and uneconomical, and the chemical conversion property is deteriorated. Therefore, when it contains W, the content shall be 1.0% or less. In order to more reliably obtain the effect of the above action, the W content is preferably set to 0.02% or more.

[Ti: 0.1% or less]
Ti is an optional element, and has the effect of suppressing the strain aging by N and improving the normal temperature aging resistance by precipitating and fixing N in the steel as TiN, so it may be contained. However, even if Ti is contained exceeding 0.1%, the effect by the above action is saturated and uneconomical. Therefore, when Ti is contained, its content is set to 0.1% or less. In order to more reliably obtain the effect of the above action, the Ti content is preferably set to 0.003% or more.

[Nb: 0.1% or less]
Nb is an optional element, and has the effect of suppressing the strain aging by N and improving the normal temperature aging resistance by precipitation fixing N as NbN in the steel, so it may be contained. However, even if it contains Nb exceeding 0.1%, the effect by the said action will be saturated and it will become uneconomical. Therefore, when Nb is contained, the content is 0.1% or less. In order to more reliably obtain the effect of the above action, the Nb content is preferably set to 0.003% or more.
Other chemical compositions are Fe and impurities.

(2) cold-rolled steel strip is more produced steel structure present invention, the main phase ferrite and together comprising a low temperature transformation product phase is the second phase, martensite phase formed by low-temperature transformation is not less than 0.5% It has a steel structure containing sites. By providing such a composite structure, it is possible to obtain a good bake hardenability without impairing the normal temperature aging resistance while reducing yield stress to obtain good shape freezing property and surface distortion resistance. it can.

  Here, the “main phase” is a phase or structure having the largest proportion in the steel structure, that is, a phase or structure exceeding 50%. The “low-temperature transformation generation phase” is a phase and structure produced by low-temperature transformation such as martensite and bainite, and also includes acicular ferrite.

  If the ratio of the low-temperature transformation generation phase in the steel structure is too small, this effect cannot be obtained. Therefore, the low temperature transformation product phase is preferably more than 3%. On the other hand, if the ratio of the low-temperature transformation generation phase in the steel structure is excessive, the yield stress increases on the contrary, and the shape freezing property and the surface strain resistance deteriorate. Furthermore, the tensile strength is excessively increased, and the ductility and deep drawability are significantly deteriorated. Therefore, the low temperature transformation product phase is preferably less than 12%. More preferably, it is less than 10%.

  Since martensite is a very important phase in reducing the yield stress and yield ratio of the steel strip, the low temperature transformation phase contains martensite. All of the low temperature transformation generation phases may be martensite. In order to make the yield ratio 0.65 or less, it is necessary to contain at least 0.5% or more martensite.

Incidentally, cold-rolled steel strip to be more prepared to the invention is made of a ferrite and low temperature transformation product phase is the second phase is the main phase, may unavoidably retained austenite or cementite is mixed is there. Residual austenite deteriorates room temperature aging resistance, so that the retained austenite is less than the low-temperature transformation generation phase and is preferably less than 3%.

(3) cold-rolled steel strip to be more prepared to mechanical properties the invention have mechanical properties yield ratio is 0.65 or less.
By setting the yield ratio of the steel strip to 0.65 or less, good shape freezing property and surface distortion resistance can be ensured. Relative evaluation of the shape freezing property and the surface strain resistance can be performed based on the yield ratio. Therefore, there are no special restrictions on yield stress and tensile strength. However, as an absolute evaluation, the yield stress is preferably 300 MPa or less, and more preferably 280 MPa or less. The tensile strength is preferably less than 590 MPa.

(4) cold-rolled steel strip is more produced surface texture present invention has a surface texture scale defects incidence is 0.5% or less.
If the scale wrinkle generation rate exceeds 0.5%, not only the productivity decreases due to the low yield, but also the productivity increases due to the significant increase in the number of steps required to remove the scale wrinkles. . Therefore, the scale wrinkle generation rate is 0.5% or less.

  Here, the “scale flaw occurrence rate” is the ratio of the area that must be removed due to the presence of scale flaws to the entire steel strip. In order to remove the area where the scale ridge exists, not only the scale ridge but also its peripheral part must be removed. Specifically, the entire width of a predetermined length where the scale ridge exists is rounded down. There is a need. In the “scale wrinkle area ratio” in which the area ratio of only the scale wrinkles is evaluated, the actual yield cannot be accurately evaluated. Therefore, the present invention uses the scale wrinkle occurrence rate.

(5) cold-rolled steel strip to be more prepared in the plating layer present invention may comprise a plating layer on the surface for the purpose of improvement of corrosion resistance. The plating layer may be an electroplating layer or a hot dipping layer. Examples of the electroplating layer include electrogalvanizing and electro-Zn—Ni alloy plating. Examples of the hot dip plating layer include hot dip galvanizing, alloying hot dip galvanizing, hot dip aluminum plating, hot dip Zn-Al alloy plating, hot dip Zn-Al-Mg alloy plating, hot dip Zn-Al-Mg-Si alloy plating, etc. The

(6) The method for producing a cold-rolled steel strip according to the manufacturing conditions present invention, that having a following step (A) ~ (G).

(A) A slab heating step in which a slab having the chemical composition described in the above item (1) is charged into a heating furnace so that the surface temperature is 1150 ° C. or higher and 1350 ° C. or lower;
(B) The slab cooling process which extracts the said slab from the said heating furnace, and performs the water cooling process which cools the surface temperature of the said slab to 50 to 200 degreeC within 10 second after extraction;
(C) A descaling step in which the slab is descaled within 0.1 seconds to 10 seconds after the water cooling treatment;
(D) A hot rolling step in which the slab is subjected to hot rolling at a finishing temperature of Ar ₃ points or more and 950 ° C. or less and a winding temperature of 400 ° C. or more and 700 ° C. or less to obtain a hot rolled steel sheet;
(E) A step of pickling the hot-rolled steel sheet to obtain a pickled steel sheet;
(F) a cold rolling step in which the pickled steel sheet is cold-rolled at a reduction rate of 50% or more to obtain a cold-rolled steel sheet; and (G) a temperature of Ac ₁ point or more and less than Ac ₃ point on the cold-rolled steel sheet. An annealing process for recrystallization annealing in a range.

Hereinafter, steps (A) to (G) will be sequentially described.
(A) Slab heating step In the slab heating step, it is preferable to charge the slab having the chemical composition described in the above item (1) into a heating furnace so that the surface temperature is 1150 ° C or higher and 1350 ° C or lower.

  In the present invention, the scale is embrittled by subjecting the slab extracted from the heating furnace to a high-temperature state by water-cooling, so even if the slab surface temperature is less than 1150 ° C., even if water-cooling is performed in the subsequent slab cooling step. It may be difficult to embrittle the scale. Therefore, the slab surface temperature is set to 1150 ° C. or higher.

  On the other hand, when the surface temperature of the slab exceeds 1350 ° C., the amount of scale generated in the heating furnace increases remarkably, and it becomes difficult to remove the scale by the descaling process. Accordingly, the slab surface temperature is set to 1350 ° C. or less. Preferably it is 1250 degrees C or less.

  In addition, the said slab can be manufactured by performing the lump rolling after making the molten steel melted by the conventional method by a continuous casting method or making it a steel ingot. The slab charged into the heating furnace may be cooled to room temperature, or may be in a high temperature state after continuous casting or ingot rolling. In order to further improve the surface properties of the final product, it is preferable to cool or warm the slab before charging into the heating furnace.

(B) Slab Cooling Step In the slab cooling step, the slab extracted from the heating furnace is subjected to a water cooling process for cooling the surface temperature to 50 ° C. or more and 200 ° C. or less within 10 seconds after the extraction.

  In the present invention, since the scale is embrittled by subjecting the slab extracted from the heating furnace to a water-cooling treatment, the surface temperature of the slab is lowered if the time from the extraction to the start of water cooling exceeds 10 seconds. In some cases, it becomes difficult to make the scale brittle even if it is subjected to water cooling treatment. Therefore, the time from extraction in the heating furnace to the start of water cooling should be within 10 seconds.

  Further, when the cooling in the water cooling treatment is less than 50 ° C., the thermal stress applied to the scale by the water cooling treatment becomes small, and it may be difficult to make the scale brittle even if the water cooling treatment is performed. Therefore, the cooling in the water cooling treatment is set to 50 ° C. or higher.

  In the present invention, the scale embrittled by the water cooling treatment is grown to an appropriate thickness to enhance the descalability, and the scale removal is performed in the subsequent descaling process. In some cases, the scale does not grow sufficiently, and the scale is not sufficiently removed in the subsequent descaling process. Therefore, the cooling in the water cooling treatment is set to 200 ° C. or less.

(C) Descaling step In the descaling step, the slab subjected to the water cooling process is subjected to a descaling process within 0.1 seconds to 10 seconds after the water cooling process.

  In the present invention, the scale embrittled by the water cooling treatment is grown to an appropriate thickness to improve the descaling property, and then the descaling process is performed to remove the scale. If the time is less than 0.1 seconds, the scale does not grow sufficiently, and even if the descaling process is performed, the scale may not be sufficiently removed. Therefore, the time from the water cooling process to the start of the descaling process is 0.1 seconds or more.

  On the other hand, if the time from the water-cooling process to the start of the descaling process exceeds 10 seconds, the scale growth proceeds excessively, and a thick and strong scale is newly generated below the embrittled scale. In some cases, the scale may not be sufficiently removed. Therefore, the time from the water cooling process to the start of the descaling process is set to be within 10 seconds.

(D) Hot rolling process In the hot rolling process, the slab subjected to the descaling process is subjected to hot rolling at a finishing temperature: Ar ₃ points or more and 950 ° C or less, and a winding temperature: 400 ° C or more and 700 ° C or less. It is preferable to use a hot-rolled steel sheet.

In order to develop a recrystallization texture preferable for deep drawability after cold rolling and recrystallization annealing, it is preferable to make the crystal grain size of the hot-rolled steel sheet fine. Therefore, it is preferable that the finishing temperature in the hot rolling is a low temperature range of the austenite range. If the finishing temperature is higher than 950 ° C. or less than Ar ₃ points, the hot-rolled steel sheet becomes coarse. Therefore, the finishing temperature is preferably Ar ₃ points or more and 950 ° C. or less.

  On the other hand, when the winding temperature is higher than 700 ° C., the yield is significantly reduced due to the scale generation after winding. Therefore, the winding temperature is preferably 700 ° C. or lower. On the other hand, when the element that fixes the solid solution N such as Ti or Nb is not included, it is preferable to fix the solid solution N with Al to ensure normal temperature aging resistance, but when the coiling temperature is less than 400 ° C, N may not be sufficiently fixed by the generation of AlN, and the normal temperature aging resistance may deteriorate. Therefore, the winding temperature is preferably 400 ° C. or higher.

  In addition, when hot rolling consists of rough hot rolling and finishing hot rolling, in order to ensure the said finishing temperature, a rough rolling material is heated between rough hot rolling and finishing hot rolling. May be. For heating the rough rolled material, for example, a solenoid type induction heating device is provided between the rough hot rolling mill and the finishing hot rolling mill, and the longitudinal temperature distribution of the rough hot rolled material before the induction heating device is used. This is possible by controlling the heating temperature rise by the induction heating device.

(E) Pickling process In the pickling process, the hot-rolled steel sheet obtained by the hot rolling process is pickled to obtain a pickled steel sheet. Pickling may be performed by a conventional method. In addition, in order to further reduce scale wrinkles due to Cr oxide, surface grinding may be performed before or after pickling.

(F) Cold rolling step In the cold rolling step, it is preferable that the pickled steel plate obtained by the pickling step is subjected to cold rolling at a reduction rate of 50% or more to obtain a cold rolled steel plate.

  In order to develop a texture preferable for deep drawability by recrystallization annealing in the subsequent annealing step, it is preferable to set the reduction ratio in cold rolling to 50% or more. On the other hand, when the rolling reduction in cold rolling exceeds 85%, the rolling load increases, and the load on the rolling mill may be excessive. Therefore, the rolling reduction in cold rolling is preferably 85% or less. More preferably, it is 80% or less. The cold steel sheet is subjected to a treatment such as degreasing according to a known method as necessary. Moreover, in order to further reduce the scale wrinkles caused by the Cr oxide, surface grinding may be performed after the cold rolling and before the annealing.

(G) Annealing process In an annealing process, recrystallization annealing is performed to the cold-rolled steel plate obtained by the said cold rolling process in the temperature range of Ac ₁ point or more and less than Ac ₃ point.

The annealing temperature of the recrystallization annealing is composed of a ferrite that is a main phase and a low-temperature transformation generation phase that is a second phase, and the low-temperature transformation generation phase has a steel structure containing martensite of 0.5 area% or more. In addition, the temperature range is from ₁ point to less than ₃ points. When the annealing temperature is less than Ac ₁ point, a low temperature transformation phase cannot be obtained. On the other hand, when the annealing temperature is ₃ points or more, a single phase structure consisting only of the low temperature transformation phase tends to be formed. It may deteriorate and deep drawability may be significantly reduced. Therefore, the annealing temperature of recrystallization annealing is set to be ₁ point or more and less than ₃ points of Ac.

What is necessary is just to determine the cooling after recrystallization annealing suitably in order to ensure a low-temperature transformation production | generation phase.
For example, cooling after recrystallization annealing in a continuous annealing facility is performed at a temperature range of 450 ° C. or higher and 650 ° C. or lower in the range of 15 ° C./s to 200 ° C./s in order to suppress the formation of ferrite and secure a low-temperature transformation generation phase. It is preferable to cool at the following average cooling rate. The cooling method from the soaking temperature to 650 ° C. is not particularly limited, but the cooling is performed at an average cooling rate of less than 10 ° C./s in order to increase the stability of austenite and facilitate the securing of the low-temperature transformation generation phase. It is preferable to do.

  In addition, cooling after recrystallization annealing in a continuous hot dip galvanizing facility is performed at a cooling rate of 4 ° C./s or higher to a temperature range of 460 ° C. or higher and 600 ° C. or lower, and held for 10 seconds or longer before applying hot dip galvanizing. It is preferable. By performing such heat treatment, C concentration in austenite is promoted, and a low temperature transformation phase containing martensite is easily obtained. Moreover, in order to improve the corrosion resistance after coating, it is preferable to reheat and perform an alloying treatment after hot dip galvanization.

  The cold-rolled steel strip thus obtained may be subjected to temper rolling according to a conventional method, but in order to suppress an increase in yield ratio and a decrease in elongation due to temper rolling, It is preferable that the elongation is 1.0% or less. More preferably, it is 0.6% or less.

  The cold-rolled steel strip produced according to the method of the present invention may be subjected to electroplating treatment or hot dip plating using this as a base material.

The present invention will be described more specifically with reference to examples.
Slabs having the chemical composition shown in Table 1 were produced by a continuous casting method.

  After surface treatment of these slabs in the cold, after charging into a heating furnace to a surface temperature of 1280 ° C. and extraction from the heating furnace, under the water cooling treatment conditions and descaling treatment conditions shown in Table 2, Except for a part, water cooling treatment and descaling treatment were performed, hot rolled, and wound into a coil shape to obtain a hot rolled steel plate having a thickness of 3.0 mm. Moreover, the slab surface after descaling processing was imaged, and the scale residual ratio of the slab surface was obtained by image processing. The cooling temperature in the water cooling treatment was 50 ° C. or higher and 200 ° C. or lower.

The obtained hot-rolled steel sheet was pickled and then cold-rolled to a thickness of 0.7 mm and wound to obtain a cold-rolled steel sheet.
About a part of the obtained cold-rolled steel sheet, annealing was performed soaking at 800 ° C. for 30 seconds in a continuous annealing facility. The cooling conditions after soaking were 5 ° C / s for the average cooling rate from the soaking temperature to 650 ° C, and 60 ° C / s for the average cooling rate from 650 ° C to 450 ° C. After cooling, temper rolling was performed at an elongation rate of 0.6% to obtain a cold-rolled steel strip.

  In addition, the remaining cold-rolled steel sheet was annealed at a temperature of 750 ° C. or higher and 830 ° C. or lower for 50 seconds in a continuous hot dipping facility. Cooling conditions after soaking were such that the average cooling rate from the soaking temperature to 550 ° C. was 7 ° C./s, held at 550 ° C. for 50 seconds, then immersed in a hot dip galvanizing bath to perform hot dipping, and further 520 The alloying treatment was performed by heating to ° C. After the plating treatment, temper rolling was performed at an elongation of 0.8% to obtain a cold-rolled steel strip.

The cold-rolled steel strip thus obtained was dispensed, the surface was observed visually and with an automatic scissor reader, the scale scissors were cut down, and the scale scoring rate was determined.
In addition, a tensile test was conducted using a JIS No. 5 tensile specimen taken from the 90 ° direction with respect to the rolling direction, yield stress (YS), tensile strength (TS), yield point elongation (YPE), and total elongation (El). ) The bake hardenability is obtained by applying a tensile pre-strain of 2% to a JIS No. 5 tensile test piece taken from 90 ° direction with respect to the rolling direction, subjecting it to a heat treatment at 170 ° C. for 20 minutes, and then subjecting it to a tensile test. The difference between the obtained YS and 2% deformation stress was defined as the BH amount and used as an index for bake hardenability.

In addition, the steel structure of the plate thickness section was observed to determine the area ratio of each phase and structure.
Table 2 also shows the results of the steel structure, mechanical properties, and surface properties.
Sample No. in Table 2 1, 4, 5, 7, 8, and 9 are examples of the present invention that satisfy the conditions of the present invention. 2, 3, 6 and 10 are comparative examples which do not satisfy the conditions of the present invention.

  Sample No. 2 has a total content of Mn and Cr of 1.9% or more and water cooling treatment and descaling treatment are suitable conditions. 1, 4, 5, 7, 8, and 9 are composed of ferrite as a main phase and a low-temperature transformation generation phase as a second phase, and the low-temperature transformation generation phase contains martensite of 0.5 area% or more. It has a steel structure, has a mechanical property with a yield ratio of 0.65 or less, has a surface property with a scale flaw generation rate of 0.5% or less, and has a high BH amount of 40 MPa or more. Had.

  In contrast, sample no. In 2, 3 and 6, the mechanical properties were good, but the generation rate of scale wrinkles was high and the surface properties were inferior. Sample No. No. 10 had a low yield ratio and a poor shape freezing property because the total content of Mn and Cr was low.

Claims (4)

A method for producing a cold-rolled steel strip, comprising the following steps (A) to (G):
(A) In mass%, C: 0.010% to 0.040%, Si: 0.5% or less, Mn: 1.0% to 2.5%, P: 0.05% or less, S : 0.01% or less, sol. Al: 0.001% or more and 0.15% or less, N: 0.008% or less and Cr: 0.25% or more and 1.0% or less, with the balance being Fe and impurities, And a slab heating step in which a slab having a chemical composition satisfying) is charged into a heating furnace so that the surface temperature is 1150 ° C. or higher and 1350 ° C. or lower;
(B) The slab cooling process which extracts the said slab from the said heating furnace, and performs the water cooling process which cools the surface temperature of the said slab to 50 to 200 degreeC within 10 second after extraction;
(C) A descaling step of performing a descaling process on the slab within 0.1 seconds to 10 seconds after the water cooling process;
(D) A hot rolling step in which the slab is subjected to hot rolling at a finishing temperature of Ar ₃ points or more and 950 ° C. or less and a winding temperature of 400 ° C. or more and 700 ° C. or less to obtain a hot rolled steel sheet;
(E) A step of pickling the hot-rolled steel sheet to obtain a pickled steel sheet;
(F) a cold rolling step in which the pickled steel sheet is cold-rolled at a reduction rate of 50% or more to obtain a cold-rolled steel sheet; and (G) a temperature at which the cold-rolled steel sheet has an Ac of ₁ point or more and less than Ac ₃ points. An annealing process for recrystallization annealing in a range.
Mn + Cr ≧ 1.9 (1)
Here, Mn and Cr in Formula (1) show content (mass%) of each element. The chemical composition is one selected from the group consisting of Mo: 1.0% or less, B: 0.0020% or less, and W: 1.0% or less in mass% instead of part of the Fe. Or the manufacturing method of the cold-rolled steel strip described in Claim 1 characterized by including 2 or more types. The chemical composition may contain one or two selected from the group consisting of Ti: 0.1% or less and Nb: 0.1% or less in mass%, instead of a part of the Fe. The method for producing a cold-rolled steel strip according to claim 1 or 2, characterized in that it is characterized in that: The method for producing a cold-rolled steel strip according to any one of claims 1 to 3, wherein the surface of the cold-rolled steel strip is plated.

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