JP2013185240A - High-tension cold-rolled steel sheet, high-tension plated steel sheet, and method for producing them - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a low carbon BH steel sheet which has a low yield ratio, does not generate yield elongation, and is suitable for press forming of an automobile outer plate etc.SOLUTION: A high-tension cold-rolled steel sheet has: chemical composition including, in mass%, 0.01-0.04% of C, 0.5% or less of Si, 1.5-2.0% of Mn, 0.05% or less of P, 0.01% or less of S, and 0.10% or less of sol.Al, and 0.005% or less of N, 0.36-0.70% of Cr, and 0.0003-0.0030% of B which satisfy the following formula (1); steel structure comprising ferrite as the main phase and a low-temperature transformation-forming phase including at least 0.5 area% of martensite as a second phase; and mechanical characteristics, such as a yield ratio of 0.65 or less and elongation at yield point of 0%. A high-tension plated steel sheet is obtained by performing electroplating on the cold-rolled steel sheet. (1) sol.Al≥2.04×N-2.63×B+0.024, wherein sol.Al, N, and B are mass% of respective elements.

Description

  The present invention relates to a high-tensile cold-rolled steel sheet, a high-tensile plated steel sheet, and methods for producing them, which are suitable as a material for an automotive outer panel. In particular, the present invention relates to a high-tensile cold-rolled steel sheet and a high-tensile plated steel sheet that have the formability required for an automotive outer panel, and excellent surface properties, and methods for producing them.

  In the present invention, “high tension” means that “tensile strength is 390 MPa or more”, which is a general standard for high-strength steel. The value measured in the rolling direction is adopted as the tensile strength of the steel sheet.

  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 automotive steel sheets, the demand for high-tensile steel sheets has been significantly increased in order to reduce the weight of the vehicle body and improve fuel efficiency in consideration of the global environment.

  For example, automobile outer panel such as a door outer or a fender is required to have dent resistance, that is, 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 increased, and as the plate thickness of the steel plate is increased. For this reason, if a steel plate having a high yield stress after being press-molded and subjected to a paint baking process can be used, it is possible to reduce the thickness. Therefore, a steel plate used as a material for an automobile outer panel is required to have such performance.

  On the other hand, in press molding, it is required to be well-familiar with the press die and to generate less springback when the molded product is removed from the press die, that is, to have good shape freezing property. For this reason, the steel sheet used as a raw material for an automobile outer panel is required to have a low yield stress before press forming.

Therefore, as a mechanical characteristic value, a steel sheet having a low initial yield stress and having a high yield stress after press forming and paint baking is suitable as a steel sheet for automobiles.
Furthermore, the steel sheet used for the outer panel of automobiles is required to have a beautiful appearance after painting. For this reason, the surface property of the steel plate itself used is strictly controlled, and a steel plate without surface defects is required. Insufficient mechanical properties such as press formability can be compensated to some extent by adjusting the manufacturing conditions such as molds. However, if surface defects exist in the steel sheet, this is compensated for. There is no way to do this, and it tends to be directly related to the abandonment of press-formed products, that is, the yield reduction.

  In general, various alloy elements are added to a high-strength steel sheet in order to ensure the intended strength, but the added elements are often easily oxidizable elements and have a higher tendency to generate scale flaws. For this reason, generally, the higher the tension, the higher the possibility of surface defects, and it becomes difficult to stably produce a steel sheet having high-quality surface properties. Such a problem becomes more prominent because the demands on the surface properties of steel sheets for automobile outer panel are very severe.

  With respect to these problems, in the prior art, there is a bake hardenable steel plate (BH steel plate) as a steel plate having dent resistance. The BH steel sheet is a steel sheet using a so-called strain age hardening phenomenon in which solute C and N atoms segregate on dislocations to fix the dislocations and increase yield stress. That is, in the process of press forming and baking coating using BH steel sheet, the dislocations introduced during press forming are fixed and fixed by solute C and N during coating baking. This increases the yield stress after baking.

  Many proposals have been made regarding BH steel sheets. Several proposals have been made on BH steel sheets excellent in deep drawability, in which Ti and Nb are added to ultra-low carbon steel and Si, Mn, P, etc. are further added to improve tensile strength, and their manufacturing methods. However, in this method, when a solid solution strengthening element such as Si, Mn, or P is added to increase the tensile strength, not only the tensile strength but also the yield stress increases. As a result, the shape freezing property deteriorates and surface distortion is likely to occur.

  In Patent Document 1, Mn and Cr, which are quenching elements, are added to a low carbon steel plate so that Mn + Cr ≧ 1.9 (mass%), and the slab heating temperature in the hot rolling line, the slab cooling step after extraction, A cold-rolled steel strip characterized by a descaling process, having a mechanical property with a yield ratio of 0.65 or less, and further having a surface property with a scale flaw generation rate of 0.5% or less; The manufacturing method is disclosed.

JP 2010-215963 A

  Although the cold-rolled steel strip disclosed in Patent Document 1 is a BH steel sheet having a yield ratio as low as 0.65 or less, subsequent investigations have revealed that yield elongation may occur. Therefore, there is a problem that the appearance of the surface may be impaired due to the generation of stretcher strain during press molding.

  In view of the problems of these conventional technologies, the present invention has a low yield stress in the initial state before press forming, and has a high yield stress after press forming and baking. Along with this, there is provided a high-tensile cold-rolled steel sheet and a high-tensile plated steel sheet having a surface property in which yield elongation does not occur, therefore, the generation of stretcher strain during press molding is suppressed and scale wrinkles are suppressed, and methods for producing the same. The purpose is to do.

  The present inventors add B to the cold-rolled steel sheet described in Patent Document 1 to further improve the hardenability, and secure a solid solution B amount (effective B amount) that is effective for improving the hardenability. Therefore, it has been found that the above object can be achieved by defining the amount of Al added and the annealing temperature in the continuous annealing process.

  More specifically, in order to investigate the effect of each element on the hardenability of the Al—N—B based low alloy steel, the present inventors thermodynamically precipitate AlN and BN in austenite before quenching. Under the assumption that the equilibrium has been reached, the amount of solid solution in the austenite before quenching of each element is calculated, the effect of the amount of B on the hardenability and the annealing temperature in continuous annealing after cold rolling. An impact confirmation test was conducted. As a result, when the coiling temperature in the hot rolling process is 550 ° C. or less, the steel sheet after continuous annealing has mechanical properties including a yield ratio of 0.65 or less and a yield point elongation of 0%. It was found that the following conditions (a) and (b) must be satisfied.

(a) The amount of sol.Al satisfies the following formula (1):
sol.Al ≧ 2.04 × N−2.63 × B + 0.024 (1)
In the formula, sol.Al, N, and B represent sol.Al, N, and B contents (% by mass) in the steel, respectively.

(B) An annealing temperature T in continuous annealing is not less than the Ac ₁ transformation point and less than the Ac ₃ transformation point and satisfies the following formula (2): T ≧ 854-1816 × sol.Al + 3706 × N-4785 × B (2)
T in the formula represents the annealing temperature (° C.) in the continuous annealing line, and sol.Al, N, and B represent sol.Al, N, and B contents (mass%) in the steel, respectively.

Here, in the present invention, by mass%, C: 0.01% to 0.04%, Si: 0.5% or less, Mn: 1.5% to 2.0%, P: 0.05 %: S: 0.01% or less, sol. Al: 0.10% or less, N: 0.005% or less, Cr: 0.36% or more and 0.70% or less, B: 0.0003% or more, 0 .0030% or less, comprising the balance Fe and impurities, satisfying the following formula (1), a ferrite as a main phase and a low-temperature transformation generation phase as a second phase, A steel structure containing a martensite having a transformation generation phase of 0.5 area% or more, and a mechanical property having a yield ratio of 0.65 or less and a yield point elongation of 0%. Tensile cold rolled steel sheet:
sol.Al ≧ 2.04 × N−2.63 × B + 0.024 (1)
In the formula, sol.Al, N, and B represent sol.Al, N, and B contents (% by mass) in the steel, respectively.

The chemical composition may further contain one or more elements selected from the following in mass% instead of a part of Fe:
(A) Mo: 1.0% or less and W: 1.0% or less selected from 1% or less,
(B) One or two selected from Ti: 0.1% or less and Nb: 0.1% or less, and (C) Bi: 0.05% or less, Sb: 0.05% or less, and Sn: One or more selected from 0.05% or less.

In the present invention, “main phase” means a phase or structure having the largest volume ratio (usually 50% or more), and the second phase means a phase or structure other than the main phase.
“Low-temperature transformation generation phase” refers to a phase and structure produced by low-temperature transformation such as martensite and bainite. In addition to these, bainitic ferrite is also included in the low-temperature transformation generation phase. Bainitic ferrite is distinguished from ferrite (= polygonal ferrite) in that it has a lath or plate-like form and a high dislocation density, and is distinguished from bainite in that there is no iron carbide inside and at the interface. .

The high-tensile cold-rolled steel sheet according to the present invention is a method in which hot rolling, pickling, cold rolling and continuous annealing are performed on a steel ingot or steel slab having the above chemical composition, and the coiling temperature in the hot rolling is It can be manufactured by a method characterized in that it is 550 ° C. or lower, the annealing temperature during the continuous annealing is not less than the Ac ₁ transformation point and less than the Ac ₃ transformation point, and satisfies the following formula (2):
T ≧ 854-1816 × sol.Al + 3706 × N-4785 × B (2)
T in the formula represents the annealing temperature (° C.) in the continuous annealing line, and sol.Al, N, and B represent sol.Al, N, and B contents (mass%) in the steel, respectively.

The high-tensile steel plate according to the present invention can also be manufactured by a method characterized by including the following steps:
(A) A slab heating step in which a slab having the above chemical composition 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 in which the slab is extracted from the heating furnace and subjected to a water cooling process for cooling the surface temperature of the slab by 50 ° C. or more and 200 ° C. or less within 10 seconds after the 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 or higher and 950 ° C. or lower and a winding temperature of 550 ° C. or lower 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) the cold rolled steel sheet has an Ac ₁ point or more and less than Ac ₃ point and the above An annealing step in which recrystallization annealing is performed in a temperature range that satisfies Equation (2).

According to the present invention,
A high-tensile plated steel sheet comprising an electroplating layer on the surface of the high-tensile cold-rolled steel sheet; anda high tension characterized by performing a plating treatment on the high-tensile cold-rolled steel sheet produced by the above method. Manufacturing method of plated steel sheet,
Is also provided.

  According to the present invention, it has a low yield stress in the initial state before press molding, but has bake hardenability that yield stress increases after press molding and paint baking, and the yield point elongation is 0%. A high-tensile cold-rolled steel sheet and a high-tensile plated steel sheet having mechanical properties and surface properties with reduced scale wrinkles are obtained.

  Therefore, these steel sheets are less likely to generate springback during press forming and have excellent shape freezing properties.After baking, the yield stress increases and exhibits good tending resistance. It has the characteristic required for the steel plate of the raw material of the panel for motor vehicle outer panels, such as a door outer and a fender panel, to have the beautiful surface property which was suppressed.

  For this reason, the high-tensile cold-rolled steel sheet and the high-tensile plated steel sheet according to the present invention are suitable as a material for a panel for an automobile outer plate, and can contribute to solving global environmental problems through weight reduction of an automobile body. It greatly contributes to.

  Hereinafter, the chemical composition, steel structure, mechanical properties, surface properties, plating layer, and production conditions thereof of the high-tensile cold-rolled steel sheet and the high-tensile plated steel sheet according to the present invention will be described in more detail. In the present specification, as described above, “%” indicating the chemical composition is all “mass%”. Further, “%” indicating the steel structure is all “area%”, and is a ratio of the entire steel structure.

(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 set to 0.010% or more. Preferably it is 0.015% or more. On the other hand, when the C content exceeds 0.040%, the deep drawability is significantly deteriorated. Therefore, the C content is set to 0.040% or less. Preferably it is 0.025% 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 exceeds 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: 1.5% to 2.0%]
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 martensite. If the Mn content is less than 1.5%, it becomes difficult to obtain the intended steel structure. Therefore, the Mn content is 1.5% or more. On the other hand, if the Mn content exceeds 2.0%, the ductility and deep drawability deteriorate significantly. Therefore, the Mn content is 2.0% or less. Preferably it is 1.8% or less.

[P: 0.05% or less]
P is an element that is generally contained as an impurity. However, P has an effect of increasing the strength of the steel sheet at a low cost without deteriorating the deep drawability so much, and therefore may be positively contained. However, if 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: not less than a value satisfying the formula (1) and not more than 0.10%]
Al has the effect | action which deoxidizes molten steel and makes steel healthy, The effect which suppresses precipitation of BN by fixing N as AlN, and increases the amount of solute B effective in improving hardenability. There is. When the sol.Al content does not satisfy the formula (1), the effect of increasing the amount of dissolved B becomes insufficient. Therefore, the sol.Al content is set to a value that satisfies the formula (1).

sol.Al ≧ 2.04 × N−2.63 × B + 0.024 (1)
On the other hand, even if the sol.Al content exceeds 0.10%, the effect of the above action is saturated and uneconomical. Therefore, the sol.Al content is 0.10% or less.

[N: 0.005% 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 0.005% or less. Preferably it is less than 0.003%.

[Cr: 0.36% to 0.70%]
Cr is an element that has the effect of improving the hardenability of steel without significantly reducing the ductility, and consists of a ferrite that is the main phase and a low-temperature transformation generation phase that is the second phase, which is the object of the present invention. 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 0.36%, it is difficult to obtain the target steel structure, so the Cr content is 0.36% or more. On the other hand, when the Cr content exceeds 0.70%, the generation rate of scale flaws increases. Therefore, the Cr content is set to 0.7% or less. Preferably it is 0.6% or less, More preferably, it is 0.5% or less.

[B: 0.0003% or more and 0.0003% or less]
B is an element having an effect of improving the hardenability of steel. To obtain the effect, 0.0003% or more is necessary, so the B content is set to 0.0003% or more. On the other hand, since the effect is saturated when the B content exceeds 0.0003%, the B content is set to 0.0003% or less. Preferably it is 0.0025% or less. In order to more reliably obtain the effect of the above action, the B content is preferably set to 0.0008% or more.
The elements described below are optional elements that may be added as necessary in the steel composition of the steel sheet of the present invention.

[Mo: 1 type or 2 types selected from 1.0% or less and W: 0.1% or less]
Mo and W are optional elements and may be contained because they have the effect of improving the hardenability of the steel. However, even if Mo and W are 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 Mo and W, the content shall be 1.0% or less, respectively. Preferably it is 0.5% or less. In order to more reliably obtain the effect of the above action, the Mo and W contents are preferably set to 0.02% or more, and more preferably set to 0.05% or more.

[One or two selected from Ti: 0.1% or less and Nb: 0.1% or less]
Ti and Nb are optional elements, and by suppressing precipitation in N as TiN and NbN and suppressing strain aging due to N and improving normal temperature aging resistance, Ti and Nb can be contained. Good. However, even if Ti and Nb are contained in an amount exceeding 0.1%, the effect of the above action is saturated and uneconomical. Therefore, when Ti and Nb are contained, the content is set to 0.1% or less. Preferably it is 0.05% or less, More preferably, it is 0.02% or less. In order to more reliably obtain the effect of the above action, the content of Ti and Nb is preferably set to 0.003% or more.

[Bi: 0.05% or less, Sb: 0.05% or less, and Sn: 0.05% or less selected from the group consisting of 0.05% or less]
Bi, Sb, and Sn are concentrated at the solidification interface in the casting process, narrowing the dendrite interval and reducing solidification segregation, and thus have the effect of improving the formability of the steel sheet. In addition, when hot dip galvanizing is performed on the steel sheet surface, the melting point of the above elements is lower than that of zinc, so that the hot dip galvanizing process tends to dissolve in the hot dip galvanizing bath in the hot dip galvanizing process. Has the effect of improving wettability. Therefore, you may contain 1 type, or 2 or more types of these elements. However, if the content exceeds the above range, Bi, Sb, or Sn existing in the crystal grain boundary may melt and grain boundary embrittlement may become significant. Therefore, the contents of Bi, Sb, and Sn are each 0.05% or less. Preferably it is 0.01% or less, More preferably, it is 0.0050% or less. In addition, in order to acquire the effect by the said action | operation more reliably, it is preferable that content of either Bi, Sb, and Sn shall be 0.0003% or more.

The rest of the chemical composition is Fe and impurities.
(2) Steel structure The high-tensile cold-rolled steel sheet and the high-tensile plated steel sheet according to the present invention 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 is 0.5%. It has a steel structure containing the above martensite. 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 and obtaining good shape freezing property and surface distortion resistance. it can.

  If the ratio of the low-temperature transformation generation phase in the steel structure is too small, it is difficult to obtain this effect. Therefore, the area ratio of 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%. Therefore, the ratio of the ferrite phase of the main phase is preferably 88% or more, more preferably 90% or more.

  Since martensite is a very important phase in reducing the yield stress and yield ratio of a steel sheet, 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 0.66 or less, it is necessary to contain at least 0.5% or more martensite. The area ratio of martensite is preferably 1% or more, more preferably 2% or more.

  The low temperature transformation product phase is preferably composed of martensite and bainite. Therefore, a preferred metal structure is a structure composed of three phases of main phase ferrite, low-temperature transformation-forming martensite, and bainite.

(3) Mechanical properties The high-tensile cold-rolled steel plate and the high-tensile plated steel plate according to the present invention have mechanical properties such that the yield ratio is 0.65 or less and the yield point elongation is 0%.

  By setting the yield ratio of the steel sheet to 0.65 or less, good shape freezing property and surface distortion resistance can be ensured. Further, by setting the yield point elongation to 0%, the generation of stretcher strain during press molding can be suppressed.

  Relative evaluation of the shape freezing property and the surface strain resistance can be performed based on the yield ratio. Therefore, in the present invention, the yield stress and the tensile strength are not particularly specified. 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. A preferable lower limit of the yield stress is 230 MPa, and a preferable lower limit of the tensile strength is 390 MPa.

(4) Plating layer The plating layer of the high tension plated steel sheet according to the present invention is an electroplating layer. Examples of the electroplating layer include electrogalvanizing and electro-Zn—Ni alloy plating. The amount of plating adhesion is not particularly limited and may be the same as the conventional one, but is usually preferably in the range of 30 to 90 g / m ² per side.

(5) Manufacturing conditions The manufacturing method of the high-tensile cold-rolled steel sheet and the high-tensile plated steel sheet according to the present invention preferably includes the following steps (A) to (G).

(A) A slab heating step in which a slab having the above-described chemical composition 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 in which the slab is extracted from the heating furnace and subjected to a water cooling process for cooling the surface temperature of the slab by 50 ° C. or more and 200 ° C. or less within 10 seconds after the 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 or higher and 950 ° C. or lower and a winding temperature of 550 ° C. or lower 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) the cold rolled steel sheet has an Ac ₁ point or more and less than Ac ₃ point and a formula An annealing step in which recrystallization annealing is performed in a temperature range that satisfies (2).

T ≧ 854-1816 × sol.Al + 3706 × N-4785 × B (2)
Among the above steps, particularly important conditions for obtaining a steel sheet having a metal structure and mechanical properties according to the present invention are the coiling temperature in the hot rolling step (D) and the temperature condition in the annealing step (G). . Therefore, more generally, the method for producing a high-strength steel sheet according to the present invention is a method of subjecting a steel ingot or steel piece having the above chemical composition to hot rolling, pickling, cold rolling and continuous annealing. The method wherein the coiling temperature in the hot rolling is 550 ° C. or lower, the annealing temperature during the continuous annealing is not less than the Ac ₁ transformation point and less than the Ac ₃ transformation point, and satisfies the above formula (2). It can be specified that

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

  It is possible to embrittle the scale by subjecting the slab extracted from the heating furnace to a high temperature state by subjecting it to a water cooling treatment. If the surface temperature of the slab is less than 1150 ° C., even if the water cooling treatment is performed in the subsequent slab cooling step. It may be difficult to embrittle the scale. Therefore, the slab surface temperature is preferably 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. Therefore, the slab surface temperature is preferably 1350 ° C. or lower, more preferably 1250 ° C. or lower.

  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, it is preferable to subject the slab extracted from the heating furnace to a water cooling treatment for cooling the surface temperature by 50 ° C. or more and 200 ° C. or less within 10 seconds after the extraction. The water-cooled slab extracted from the heating furnace is embrittled by water-cooling, and the embrittled scale is grown to an appropriate thickness to improve the scale removal performance. By performing the removal, the scale can be efficiently removed.

  If the time from extraction in the heating furnace to the start of water cooling exceeds 10 seconds, the decrease in the surface temperature of the slab during that time becomes large, and it may be difficult to sufficiently embrittle the scale even if water cooling treatment is performed. Therefore, it is preferable that the time from the extraction in the heating furnace to the start of water cooling is within 10 seconds.

  Further, if the cooling temperature width 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 temperature width in the water cooling treatment is preferably 50 ° C. or more.

  When the cooling temperature width in the water cooling process exceeds 200 ° C., the scale does not grow sufficiently, and the scale may not be sufficiently removed in the subsequent descaling process. Therefore, the cooling temperature width in the water cooling treatment is preferably set to 200 ° C. or less.

(C) Descaling step In the descaling step, it is preferable that the slab subjected to the water cooling treatment is subjected to a descaling treatment within 0.1 seconds to 10 seconds after the water cooling treatment. The descaling process can be performed by jetting high-pressure water according to a conventional method.

  The scale that has been embrittled by water-cooling treatment is grown to an appropriate thickness to enhance the descaling property, and then the descaling process is performed to remove the scale. If it is less than 1 second, the scale does not grow sufficiently, and even if the descaling process is performed, the scale may not be sufficiently removed. Therefore, it is preferable that the time from the water cooling process to the start of the descaling process is 0.1 second or longer.

  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, it is preferable that the time from the water cooling process to the start of the descaling process is within 10 seconds.

(D) Hot rolling process In the hot rolling process, the slab subjected to the descaling process is preferably hot-rolled at a finishing temperature: Ar ₃ point or more and 950 ° C or less, and the coiling temperature is 550 ° C or less. As 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 point, the hot-rolled steel sheet becomes coarse. Accordingly, the finishing temperature is preferably Ar ₃ point or higher and 950 ° C. or lower.

  If the coiling temperature exceeds 550 ° C., intergranular corrosion occurs in the pickling process after hot rolling, and surface defects (flaps) due to intergranular corrosion tend to occur after cold rolling. Hydrogen defects generated during electrodeposition coating are easily trapped by surface defects, causing defects in the coating surface called pinholes. Therefore, the coiling temperature is set to 550 ° C. or lower.

  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) In the annealing step annealing process is subjected to recrystallization annealing in the temperature range satisfying Ac less than ₁ point or more Ac ₃ point to the cold-rolled steel sheet obtained by the cold rolling step and the following formula (2).

T ≧ 854-1816 × sol.Al + 3706 × N-4785 × B (2)
If the annealing temperature is less than the Ac ₁ point, a low temperature transformation phase cannot be obtained. On the other hand, if the annealing temperature is not less than the Ac ₃ point, 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. Further, as described above, when the expression (2) is not satisfied, the yield point elongation is not 0%, and the generation of stretcher strain cannot be suppressed during press molding, so the annealing temperature in the annealing process is higher than the Ac ₁ transformation point. The range is less than the Ac ₃ transformation point and satisfies the formula (2).

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 annealing temperature to 650 ° C. is not particularly limited, but 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.

In the case of producing a plated steel sheet, the cold-rolled steel sheet produced by the above-described method may be electroplated according to a conventional method, and the plating method and the chemical composition of the plated film are not limited.
The high-tensile cold-rolled steel plate and high-tensile plated steel plate 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. Furthermore, it is preferable that the elongation of temper rolling is 1.0% or less. More preferably, it is 0.7% or less.

  The steel sheet produced in this manner 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 contains martensite at 0.5 area% or more. With a yield ratio of 0.65 or less and a yield point elongation of 0%.

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 a cold state, the surface temperature was set to 1280 ° C. and extracted from the heating furnace, and after about 5 seconds, water cooling treatment was performed to cool to 60 ° C. Then, after 5 seconds, descaling treatment by high-pressure water injection was performed. Next, the slab was hot-rolled under a condition where the finishing temperature was 820 ° C., and wound into a coil at a coiling temperature of 450 ° C. to obtain a hot-rolled steel plate having a thickness of 3.0 mm.

The hot-rolled steel sheet was pickled, cold-rolled to a thickness of 0.7 mm, and wound to obtain a cold-rolled steel sheet.
The obtained cold-rolled steel sheet was annealed at a annealing temperature shown in Table 2 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 0.7% to obtain a cold-rolled annealed steel sheet.

About some obtained cold-rolled annealing steel plates, the electrogalvanization process was performed in the electroplating installation. The plating adhesion amount per side was 70 g / m ² .
From the cold-rolled steel sheet and electroplated steel sheet thus obtained, a tensile test was conducted using a JIS No. 5 tensile specimen taken from the 0 ° direction (rolling direction) with respect to the rolling direction, and the yield stress (YS). The tensile strength (TS) and the yield point elongation (YPE) were determined, and the yield ratio (YR), which is the ratio of YS / TS, was calculated.

In addition, the steel structure of the plate thickness section was micro-observed by a conventional method to determine the area ratio of each phase and structure.
The bake hardenability is determined by applying a 2% tensile pre-strain to a JIS No. 5 tensile test specimen taken from the rolling direction and subjecting it to heat treatment at 170 ° C., which is a standard baking condition after coating, for 20 minutes. For the test, the difference between the obtained YS and 2% deformation stress was defined as the amount of BH and used as an index for bake hardenability.

  Table 2 also shows the results of steel structure and mechanical properties.

  Sample Nos. 1, 3, 4, 6, 9, 10, and 11 in Table 2 are examples of the invention that satisfy the conditions of the present invention, and Samples No. 2, 5, 7, and 8 satisfy the conditions of the present invention. This is an unsatisfactory comparative example.

  Sample Nos. 1, 3, 4, 6, 9, 10 and 11 whose chemical components and annealing temperatures satisfy the present invention are composed of ferrite as a main phase and a low-temperature transformation generation phase as a second phase. At the same time, the low-temperature transformation generation phase had a steel structure containing martensite of 0.5 area% or more, had a yield characteristic of 0.65 or less, and a yield point elongation of 0%. Furthermore, the BH amount by baking was 50 MPa or more.

  On the other hand, in Sample Nos. 2 and 5, the chemical components satisfy the conditions of the present invention, but the annealing temperature does not satisfy the conditions of the present invention. The yield ratio was high because the components did not satisfy the conditions of the present invention. Therefore, the shape freezing property is inferior and yield elongation occurs, so that stretcher strain may occur during press molding.

Claims (8)

In mass%, C: 0.01% or more and 0.04% or less, Si: 0.5% or less, Mn: 1.5% or more and 2.0% or less, P: 0.05% or less, S: 0.0%. 01% or less, sol.Al: 0.10% or less, N: 0.005% or less, Cr: 0.36% or more and 0.70% or less, B: 0.0003% or more and 0.0003% or less A chemical composition comprising the balance Fe and impurities and satisfying the following formula (1):
A steel structure comprising a ferrite as a main phase and a low-temperature transformation generation phase as a second phase, wherein the low-temperature transformation generation phase contains 0.5% by area or more of martensite;
Mechanical properties with a yield ratio of 0.65 or less and a yield point elongation of 0%;
A high-tensile cold-rolled steel sheet characterized by comprising:
sol.Al ≧ 2.04 × N−2.63 × B + 0.024 (1)
In the formula, sol.Al, N, and B represent sol.Al, N, and B contents (% by mass) in the steel, respectively.   The chemical composition contains one or two selected from the group consisting of Mo: 1.0% or less and W: 1.0% or less in mass%, instead of a part of Fe. The high-tensile cold-rolled steel sheet described in 1.   The chemical composition contains 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 Fe. Or the high-tensile cold-rolled steel sheet according to 2.   The chemical composition is one type selected from the group consisting of Bi: 0.05% or less, Sb: 0.05% or less, and Sn: 0.05% or less in mass%, instead of a part of the Fe. Or the high-tensile cold-rolled steel sheet according to any one of claims 1 to 3, comprising two or more kinds.   An electroplating layer is provided on the surface of the high-tensile cold-rolled steel sheet according to claim 1. In the method for producing a high-tensile cold-rolled steel sheet according to any one of claims 1 to 4, comprising hot rolling, pickling, cold rolling, and continuous annealing on a steel ingot or steel slab,
The steel ingot or steel slab has the chemical composition according to any one of claims 1 to 4,
The coiling temperature in the hot rolling is 550 ° C. or less,
The annealing temperature during the continuous annealing is not less than the Ac ₁ transformation point and less than the Ac ₃ transformation point, and satisfies the following formula (2):
A method for producing a high-tensile cold-rolled steel sheet.
T ≧ 854-1816 × sol.Al + 3706 × N-4785 × B (2)
T in the formula represents the annealing temperature (° C.) in the continuous annealing line, and sol.Al, N, and B represent sol.Al, N, and B contents (mass%) in the steel, respectively. The method for producing a high-tensile cold-rolled steel sheet according to any one of claims 1 to 4, comprising the following steps:
(A) A slab heating step in which the slab having the chemical composition according to any one of claims 1 to 4 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 in which the slab is extracted from the heating furnace and subjected to a water cooling process for cooling the surface temperature of the slab by 50 ° C. or more and 200 ° C. or less within 10 seconds after the 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 or higher and 950 ° C. or lower and a winding temperature of 550 ° C. or lower 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) the cold rolled steel sheet has an Ac ₁ point or more and less than Ac ₃ point and a formula An annealing step in which recrystallization annealing is performed in a temperature range that satisfies (2).   The method for producing a high-tensile plated steel sheet according to claim 5, wherein the cold-rolled steel sheet produced by the production method according to claim 6 or 7 is plated.