KR102132205B1 - Grater - Google Patents

Abstract

The steel sheet has a predetermined chemical composition, has a steel structure containing ferrite and bainite in an area fraction of 2% or more, and the average dislocation density in ferrite and the average dislocation density in bainite are all 3×10 ¹² m/m 3 to 1 X 10 ¹⁴ m/m 3, and the average particle diameter of ferrite and bainite is 5 μm or less.

Description

Grater

The present invention relates to a steel sheet that is excellent in collision properties suitable for a member of an automobile.

In the case of manufacturing a vehicle body using a steel plate, molding, welding, and painting baking of the steel plate are generally performed. Therefore, it is required that the steel sheet for automobiles has excellent moldability, high strength after coating baking, and excellent collision properties. Conventionally, as a steel sheet used in automobiles, a dual phase (DP) steel sheet having a two-phase structure of ferrite and martensite, and a transformation induced plasticity (TRIP) steel sheet are exemplified.

However, the DP steel sheet and the TRIP steel sheet have a problem that mechanical properties after coating baking may fluctuate in a member. That is, in the forming of the steel sheet, since deformation is added according to the shape of the member to be obtained, the steel sheet after forming includes a part to which deformation is strongly added and a part to which deformation is hardly added. And the larger the added strain, the greater the amount of strain aging hardening by paint baking, which increases the hardness. As a result, there is a case where the difference in yield strength after coating baking is large between a portion to which deformation is added by molding and a portion to which little deformation is added. In this case, a portion where little strain is added is soft, so that bending occurs at this portion, so that sufficient reaction force characteristics and collision characteristics are not obtained.

Japanese Patent Publication No. 2009-185355 Japanese Patent Publication No. 2011-111672 Japanese Patent Publication No. 2012-251239 Japanese Patent Publication No. Hei 11-080878 Japanese Patent Publication No. Hei 11-080879 International Publication No. 2013/047821 Japanese Patent Publication No. 2008-144233 International Publication No. 2012/070271

An object of the present invention is to provide a steel sheet capable of obtaining stable yield strength after coating baking while obtaining good moldability.

The present inventors have made extensive studies to solve the above problems. As a result, it was found that, when the dislocation density in ferrite and the dislocation density in bainite were high, the yield strength was improved by the aging accompanying coating baking even in a portion where little deformation was added during molding. It has also been found that yield strength is further improved by aging when the average particle diameter of ferrite and bainite is small.

Based on such knowledge, the inventors of the present application conducted further studies, and as a result, came to the first aspect of the invention shown below.

(One)

In mass%,

C: 0.05% to 0.40%,

Si: 0.05% to 3.0%,

Mn: 1.5% to 4.0%,

Al: 1.5% or less,

N: 0.02% or less,

P: 0.2% or less,

S: 0.01% or less,

Nb and Ti: 0.005% to 0.2% in total,

V and Ta: 0.0% to 0.3% in total,

Cr, Mo, Ni, Cu and Sn: 0.0% to 1.0% in total,

B: 0.00% to 0.01%,

Ca: 0.000% to 0.005%,

Ce: 0.000% to 0.005%,

La: 0.000% to 0.005%, and

Balance: Fe and impurities

Has a chemical composition represented by,

Has a steel structure containing ferrite and bainite in an area fraction of 2% or more in total,

The average dislocation density in ferrite and the average dislocation density in bainite are both 3×10 ¹² m/m 3 to 1×10 ¹⁴ m/m 3,

A steel sheet characterized in that the average particle diameter of ferrite and bainite is 5 µm or less.

(2)

The steel structure comprises ferrite and bainite in an area fraction: 2% to 60% in total, and martensite: 10% to 90% in total,

The area fraction of retained austenite in the steel structure is 15% or less,

The steel sheet according to (1), wherein the ratio of the area fraction of ferrite to the area fraction of martensite is 0.03 to 1.00.

(3)

In the chemical composition,

V and Ta: 0.01% to 0.3% in total

The steel sheet according to (1) or (2), characterized in that is formed.

(4)

In the chemical composition,

Cr, Mo, Ni, Cu and Sn: 0.1% to 1.0% in total

The steel sheet according to any one of (1) to (3), characterized in that is formed.

(5)

In the chemical composition,

B: 0.0003% to 0.01%

The steel sheet according to any one of (1) to (4), characterized in that is formed.

(6)

In the chemical composition,

Ca: 0.001% to 0.005%,

Ce: 0.001% to 0.005%,

La: 0.001% to 0.005%, or

Alternatively, the steel sheet according to any one of (1) to (5), characterized in that any combination of these is established.

According to the present invention, since the average dislocation density in ferrite and the average dislocation density in bainite are appropriate, stable yield strength can be obtained even after coating baking.

Hereinafter, embodiments of the present invention will be described.

First, the chemical composition of the steel sheet according to the embodiment of the present invention and the steel used for its production will be described. Although the details will be described later, the steel sheet according to the embodiment of the present invention is manufactured through hot rolling, cold rolling, annealing and temper rolling of steel. Therefore, the chemical composition of the steel sheet and steel considers these properties as well as the properties of the steel sheet. In the following description, "%", which is a unit of the content of each element contained in the steel sheet, means "mass%" unless otherwise specified. The steel sheet according to the present embodiment is, in mass%, C: 0.05% to 0.40%, Si: 0.05% to 3.0%, Mn: 1.5% to 4.0%, Al: 1.5% or less, N: 0.02% or less, P: 0.2% or less, S: 0.01% or less, Nb and Ti: 0.005% to 0.2% in total, V and Ta: 0.0% to 0.3% in total, Cr, Mo, Ni, Cu and Sn: 0.0% to 1.0 in total %, B: 0.00% to 0.01%, Ca: 0.000% to 0.005%, Ce: 0.000% to 0.005%, La: 0.000% to 0.005%, and balance: Fe and impurities. As an impurity, what is contained in raw materials, such as ore and scrap, and what is contained in a manufacturing process are illustrated.

(C: 0.05% to 0.40%)

C contributes to the improvement of tensile strength. When the C content is less than 0.05%, sufficient tensile strength, for example, 980 MPa or higher tensile strength is not obtained. Therefore, the C content is 0.05% or more. In order to obtain higher tensile strength, the C content is preferably 0.08% or more. On the other hand, when the C content is more than 0.40%, dislocations having a sufficient density in ferrite are not obtained, and it is difficult to obtain a desirable steel structure. Therefore, the C content is 0.40% or less. From the viewpoint of weldability, the C content is preferably 0.35% or less.

(Si: 0.05% to 3.0%)

Si affects the formation of iron carbide and accompanying aging hardening. When the Si content is less than 0.05%, a sufficient amount of solid solution C is not obtained, and the yield strength does not sufficiently increase even by the aging accompanying coating baking. Therefore, the Si content is 0.05% or more. In order to further increase the yield strength, the Si content is preferably 0.10% or more. On the other hand, when the Si content is more than 3.0%, dislocations having a sufficient density in ferrite are not obtained, and it is difficult to obtain a desirable steel structure. Therefore, the Si content is set to 3.0% or less. The Si content is preferably 2.5% or less, more preferably 2.0% or less from the standpoint of suppression of the stand cracking of the slab and suppression of end cracking during hot rolling.

(Mn: 1.5% to 4.0%)

Mn suppresses the transformation from austenite to ferrite and contributes to the improvement of tensile strength. When the Mn content is less than 1.5%, sufficient tensile strength, for example, 980 MPa or higher tensile strength is not obtained. Therefore, the Mn content is 1.5% or more. In order to obtain higher tensile strength, the Mn content is preferably 2.0% or more. On the other hand, if the Mn content is more than 4.0%, sufficient moldability cannot be obtained. Therefore, the Mn content is 4.0% or less. In order to obtain better moldability, the Mn content is preferably 3.5% or less.

(Al: 1.5% or less)

Al is not an essential element, but is used, for example, for deoxidation to reduce inclusions, and may remain in the steel. When the Al content is more than 1.5%, ferrite or bainite having an average dislocation density in the range described below is not obtained. Therefore, the Al content is 1.5% or less. The cost of reducing the Al content is high, and if it is desired to reduce it to less than 0.002%, the cost is significantly increased. For this reason, the Al content may be 0.002% or more. When sufficient deoxidation is performed, 0.01% or more of Al may remain.

(N: 0.02% or less)

N is not an essential element, and is contained as an impurity in steel, for example. When the N content is more than 0.02%, a large amount of nitride precipitates, and sufficient moldability cannot be obtained. Therefore, the N content is 0.02% or less. The cost of reducing the N content is expensive, and the cost is remarkably increased if it is intended to be reduced to less than 0.001%. For this reason, the N content may be 0.001% or more.

(P: 0.2% or less)

P is not an essential element and is contained as an impurity in steel, for example. When the P content is more than 0.2%, a large amount of P compound precipitates, and sufficient moldability is not obtained. Therefore, the P content is 0.2% or less. The P content is preferably 0.07% or less from the viewpoint of weldability. It is expensive to reduce the P content, and the cost is significantly increased if the content is reduced to less than 0.001%. For this reason, the P content may be 0.001% or more.

(S: 0.01% or less)

S is not an essential element and is contained as an impurity in steel, for example. When the S content is more than 0.01%, sulfide precipitates in a large amount and sufficient moldability cannot be obtained. Therefore, the S content is 0.01% or less. The S content is preferably 0.003% or less in order to further suppress the decrease in formability. The cost of reducing the S content is expensive, and the cost is significantly increased if the content is to be reduced to less than 0.0002%. For this reason, the S content may be 0.0002% or more.

(Nb and Ti: 0.005% to 0.2% in total)

Nb and Ti contribute to the refinement and precipitation strengthening of the grains of ferrite or bainite. Since Nb and Ti form (Ti, Nb) carbonitrides, the amount of solid solution C and the amount of solid solution N after annealing change depending on the content of Nb and Ti. When the content of Nb and Ti is less than 0.005% in total, ferrite or bainite having an average particle diameter in the range described below is not obtained, and yield strength does not sufficiently increase even by aging accompanying paint baking. Therefore, the content of Nb and Ti is 0.005% or more in total. In order to sufficiently raise the yield strength by aging, the content of Nb and Ti is preferably 0.010% or more in total. On the other hand, when the content of Nb and Ti is more than 0.2% in total, (Ti, Nb) carbonitride precipitates in a large amount, and sufficient moldability cannot be obtained. Therefore, the content of Nb and Ti is 0.2% or less in total. The content of Nb and Ti is preferably 0.1% or less in total.

V, Ta, Cr, Mo, Ni, Cu, Sn, B, Ca, Ce, and La are not essential elements and are arbitrary elements that may be appropriately contained in a predetermined amount in the steel sheet and steel.

(V and Ta: 0.0% to 0.3% in total)

V and Ta contribute to the improvement of strength by the formation of carbides, nitrides or carbonitrides, and the atomization of ferrite and bainite. Therefore, V or Ta, or both of these may be contained. However, when the content of V and Ta is more than 0.3% in total, a large amount of carbonitride is precipitated, and ductility decreases. Therefore, the content of V and Ta is 0.3% or less in total. The content of V and Ta is preferably 0.1% or less in total from the viewpoint of suppressing the standing crack of the slab and suppressing the end crack during hot rolling. The content of V and Ta is preferably 0.01% or more in total in order to reliably obtain the effect of the above action.

(Cr, Mo, Ni, Cu and Sn: 0.0% to 1.0% in total)

Cr, Mo, Ni, Cu and Sn, like Mn, are used to suppress the transformation of austenite to ferrite. Therefore, Cr, Mo, Ni, Cu or Sn, or any combination thereof may be contained. However, if the content of Cr, Mo, Ni, Cu and Sn is more than 1.0% in total, the workability is significantly deteriorated and the elongation is reduced. Therefore, the content of Cr, Mo, Ni, Cu and Sn is 1.0% or less in total. From the viewpoint of manufacturability, the content of Cr, Mo, Ni, Cu and Sn is preferably 0.5% or less in total. The content of Cr, Mo, Ni, Cu and Sn is preferably 0.1% or more in order to reliably obtain the effect of the above action.

(B: 0.00% to 0.01%)

B improves the stiffness of the steel sheet, suppresses the formation of ferrite, and promotes the formation of martensite. Therefore, B may be contained. However, when the B content is more than 0.01% in total, a large amount of boride precipitates, and sufficient moldability is not obtained. Therefore, the B content is 0.01% or less. In order to further suppress the decrease in ductility, the B content is preferably 0.003% or less in total. The B content is preferably 0.0003% or more in order to reliably obtain the effect by the above action.

(Ca: 0.000% to 0.005%, Ce: 0.000% to 0.005%, La: 0.000% to 0.005%)

Ca, Ce, and La suppress the reduction of workability, particularly elongation, by making oxides and sulfides in the steel sheet fine or by changing the properties of the oxides and sulfides. Therefore, Ca, Ce, or La, or any combination thereof may be contained. However, if any of the Ca content, Ce content, and La content is more than 0.005%, the effect by the above action is saturated, unnecessarily high cost, and moldability decreases. Therefore, Ca content, Ce content, and La content are all 0.005% or less. Ca content, Ce content, and La content are all preferably 0.003% or less in order to further suppress the decrease in formability. The Ca content, Ce content, and La content are all preferably 0.001% or more in order to reliably obtain the effect of the above action. That is, it is preferable that "Ca: 0.001% to 0.005%", "Ce: 0.001% to 0.005%" or "La: 0.001% to 0.005%", or any combination thereof is satisfied.

Next, the steel structure of the steel sheet according to the embodiment of the present invention will be described. In the following description, "%", which is a unit of the proportion of phases or structures constituting a steel structure, means "area %" of an area fraction unless otherwise specified. In the steel structure of the steel sheet according to the embodiment of the present invention, ferrite and bainite are included in an area fraction of 2% or more in total. The average dislocation density in ferrite and the average dislocation density in bainite are both 3×10 ¹² m/m 3 to 1×10 ¹⁴ m/m 3, and the average particle diameter of ferrite and bainite is 5 μm or less.

As described above, by the present inventors, when the dislocation density in ferrite and the dislocation density in bainite are high, the yield strength is improved by the aging accompanying coating baking even in a place where little deformation is added during molding. It turns out to be. If the average dislocation density in ferrite or the average dislocation density in bainite, or both of these are less than 3×10 ¹² m/m 3, the yield strength of the portion where deformation is hardly added during molding is not sufficiently improved by aging, Sufficient collision properties are not obtained. Therefore, both the average dislocation density in ferrite and the average dislocation density in bainite are 3×10 ¹² m/m 3 or more. In order to obtain better collision properties, both the average dislocation density in ferrite and the average dislocation density in bainite are preferably 6×10 ¹² m/m 3 or more. If the average dislocation density in ferrite or the average dislocation density in bainite, or both of these are greater than 1×10 ¹⁴ m/m 3, the moldability deteriorates or the yield strength of the portion where deformation is hardly added during molding is effective for aging Thereby, it is not improved sufficiently, or sufficient collision characteristics are not obtained. Therefore, both the average dislocation density in ferrite and the average dislocation density in bainite are 1×10 ¹⁴ m/m 3 or less. The average dislocation density in ferrite and the average dislocation density in bainite are both preferably 8×10 ¹³ m/m 3 or less in order to obtain better moldability and collision properties.

The average dislocation density in ferrite and the average dislocation density in bainite can be obtained, for example, by using a transmission electron microscopy (TEM) photograph. That is, when a TEM photograph of a thin film sample is prepared and a line is arbitrarily drawn on the TEM photograph to obtain the average dislocation density in ferrite, the point where this line crosses the dislocation line in ferrite is counted. If the length of the line in the ferrite is L, the number of locations where the line and the potential line intersect in the ferrite is N, and the thickness of the sample is t, the dislocation density in the ferrite in the thin film sample is "2N/(Lt)". Is displayed. Using TEM photographs taken at a plurality of locations of the thin film sample, an average value of dislocation density obtained from these multiple TEM photographs is obtained as an average dislocation density in ferrite. An actual value may be used as the thickness t of the sample, or 0.1 µm may be simply used. The average dislocation density in bainite can be obtained in the same manner as the method for obtaining the average dislocation density in ferrite by counting the intersections in bainite and using the length of the line in bainite.

As described above, it has been found by the present inventors that the yield strength is further improved by aging when the particle diameter of ferrite and bainite is small. When the average particle diameter of ferrite and bainite is more than 5 µm, the yield strength of the portion where deformation is hardly added at the time of molding is not sufficiently improved by aging, and sufficient collision properties are not obtained. Therefore, the average particle diameter of ferrite and bainite is 5 µm or more. The average particle diameter of ferrite and bainite is preferably 3 µm or less in order to obtain better collision properties.

The average dislocation density in ferrite and the average dislocation density in bainite are both 3×10 ¹² m/m 3 to 1×10 ¹⁴ m/m 3, and the area of ferrite and bainite even if the average particle diameter of ferrite and bainite is 5 μm or less. If the fraction is less than 2% in total, sufficient moldability is not obtained or sufficient collision performance is not obtained. Therefore, the area fraction of ferrite and bainite is 2% or more in total. The area fraction of ferrite and bainite is preferably 5% or more in total in order to obtain better moldability and collision performance.

In the present application, ferrite includes polygonal ferrite (αp), pseudo polygonal ferrite (αq) and granular bainitic ferrite (αB), and bainite includes lower bainite, upper bainite and bainitic ferrite (α°B). ) Is included. Granular bainitic ferrite has a lased, recovered dislocation substructure, and bainitic ferrite is a structure in which laths are bundles without precipitation of carbides, and the old γ grain boundaries remain as it is (Reference: ``steel bay Night Photo Collection-1」 Japan Steel Association (1992) p.4). This reference includes "Granular bainitic ferrite structure; Dislocated substructure but fairly recovered like lath-less, and "sheaf-like with laths but no carbide;" conserving the prior austenite grain boundary.

Ferrite and bainite also contribute to improving the formability of the steel sheet. However, if the area fraction of ferrite and bainite is more than 60% in total, sufficient collision characteristics may not be obtained. Therefore, the area fraction of ferrite and bainite is preferably 60% or less in total. In order to obtain better collision properties, the area fraction of ferrite and bainite is more preferably 40% or less in total.

Martensite contributes to securing tensile strength. When the area fraction of martensite is less than 10%, sufficient tensile strength, for example, tensile strength of 980 MPa or more, may not be obtained, or the average dislocation density in ferrite may be less than 3×10 ¹² m/m 3. Therefore, the area fraction of martensite is preferably 10% or more. The area fraction of martensite is more preferably 15% or more in order to obtain better tensile strength and collision properties. On the other hand, if the area fraction of martensite is more than 90%, the average dislocation density in ferrite or the average dislocation density in bainite, or both of them may exceed 1×10 ¹⁴ m/m 3, or sufficient ductility may not be obtained. have. Therefore, the area fraction of martensite is preferably 90% or less. The area fraction of martensite is more preferably 85% or less in order to obtain better collision performance and ductility. The martensite includes as ??cheat martensite and tempered martensite, and it is preferable that at least 80% by area of the entire martensite is tempered martensite.

If the ratio (f _F /f _M ) of the area fraction f _F of the ferrite to the area fraction f _M of martensite is less than 0.03, the average dislocation density in the ferrite exceeds 1×10 ¹⁴ m/m 3 or sufficient ductility is obtained. It may or may not. Therefore, the ratio (f _F /f _M ) is preferably 0.03 or more. In order to obtain better collision performance and ductility, the ratio (f _F /f _M ) is more preferably 0.05 or more. On the other hand, if the ratio (f _F /f _M ) is more than 1.00, the average dislocation density in ferrite may be less than 3×10 ¹² m/m 3. Therefore, the ratio (f _F /f _M ) is preferably 1.00 or less. In order to obtain better collision performance, the ratio (f _F /f _M ) is more preferably 0.80 or less.

The retained austenite is effective for improving moldability and improving impact energy absorption properties. The retained austenite also contributes to an improvement in the amount of strain aging during coating baking. However, if the area fraction of retained austenite is more than 15%, the average dislocation density in ferrite may exceed 1×10 ¹⁴ m/m 3 or the steel sheet may embrittle after molding. Therefore, the area fraction of retained austenite is preferably 15% or less. The area fraction of retained austenite is more preferably 12% or less in order to obtain better collision properties and toughness. When the area fraction of the retained austenite is 2% or more, the effect of improving the amount of strain aging hardening can be expected.

Perlite, bainite, martensite, and residual austenite are examples of those included in the steel structure, and pearlite. The area fraction of pearlite is preferably 2% or less.

The area ratios of ferrite, bainite, martensite, and pearlite are, for example, by point counting or image analysis using photographs of steel tissues taken by an optical microscope or scanning electron microscopy (SEM). Can be measured. The discrimination between granular bainitic ferrite (αB) and bainitic ferrite (α°B) can be carried out on the basis of the description of the reference and tissue observation by SEM and transmission electron microscopy (TEM).

The area fraction of retained austenite can be measured, for example, by electron backscatter diffraction (EBSD) or X-ray diffraction. When measured by X-ray diffraction, the diffraction intensity of the (111) plane of ferrite {α(111)} and the diffraction intensity of the (200) plane of residual austenite {γ(200) using Mo-Kα rays }, the diffraction intensity of the (211) plane of the ferrite {α(211)} and the diffraction intensity of the (311) plane of the retained austenite {γ(311)} are measured, and the area fraction of the retained austenite is obtained from the following equation. (f _A ) can be calculated.

f _A =(2/3){100/(0.7×α(111)/γ(200)+1)}+(1/3){100/(0.78×α(211)/γ(311)+1 )}

Next, the mechanical properties of the steel sheet according to the embodiment of the present invention will be described.

It is preferable that the steel sheet according to the present embodiment has a tensile strength of 980 MPa or more. If the tensile strength is less than 980 MPa, it is because it is difficult to obtain the advantage of weight reduction by increasing the strength of the member.

The collision characteristics after forming and coating baking of the steel sheet can be evaluated by parameter P ₁ represented by (Equation 1). "YS _BH5 " is the yield strength after aging when 5% tensile pre-strain is added, "YS _BH0 " is the yield strength after aging when tensile pre-strain is not added, "TS ”Is the maximum tensile strength (㎫). The aging temperature is 170°C and the time is 2 hours. The parameter P ₁ corresponds to the ratio of the difference between the yield strength YS _BH5 after coating baking of the portion to which the pre-deformation was added and the yield strength YS _BH0 after coating baking of the portion to which the pre-deformation was not added to the maximum tensile strength TS. . The smaller the value of the parameter P _1, the smaller the difference in yield strength in the member obtained through molding and painting baking. When the size of the tensile pre-strain is set to 5%, it is considered that in the manufacture of a member for a skeleton of an automobile, a molding deformation of 5% or more is generally introduced into a bending or drawing part. When the value of the parameter P ₁ is more than 0.27, when the member produced through molding and painting baking undergoes collision deformation, buckling or deformation occurs from a portion having a low local hardness, so that proper reaction force characteristics and energy absorption are not obtained. There are cases. For this reason, the value of the parameter P ₁ is preferably 0.27 or less. In order to obtain better collision performance, the value of the parameter P ₁ is more preferably 0.18 or less.

P ₁ =(YS _BH5 -YS _BH0 )/TS… (Equation 1)

The formability of the steel sheet can be evaluated by parameter P ₂ represented by (Equation 2). "UEl" is the uniform elongation (%) obtained in a tensile test, and correlates with stretch formability, stretch flange formability, and drawing formability. When the value of the parameter P ₂ is less than 7000, cracks are often generated due to molding or collision, and thus it is difficult to contribute to the weight reduction of automobile members. For this reason, the value of the parameter P ₂ is preferably 7000 or more. In order to obtain better formability, the value of the parameter P ₂ is more preferably 8000 or more.

P ₂ =TS×uEl… (Equation 2)

Next, a method of manufacturing the steel sheet according to the embodiment of the present invention will be described. When manufacturing the steel sheet according to the embodiment of the present invention, it is particularly important to control the average particle diameter of ferrite and bainite, the average dislocation density in ferrite and the average dislocation density in bainite. As a result of careful examination of these controls by the present inventors, it is possible to introduce dislocations in ferrite and bainite using the volume expansion accompanying martensite transformation, and the average dislocation density is the temperature at which martensite is formed and martensite It turned out to depend on the amount of. It has also been found that the average dislocation density in bainite also depends on the temperature at which bainite is formed. It has also been found that the average dislocation density in ferrite and the average dislocation density in bainite can be controlled by adjusting the elongation rate of temper rolling and the line load/tension ratio in temper rolling. Therefore, in this manufacturing method, hot rolling, cold rolling, annealing and temper rolling of the steel having the above chemical composition are performed.

First, a slab having the above chemical composition is produced and hot rolled. The slab provided for hot rolling can be produced by, for example, a continuous casting method, a crushing method, or a thin slab caster. A process such as continuous casting-direct rolling, in which hot rolling is performed immediately after casting, may be employed.

When the temperature of the slab heating is less than 1100°C, remelting of the carbonitride precipitated during casting may become insufficient. Therefore, the temperature of the slab heating is 1100°C or higher. After slab heating, rough rolling and finish rolling are performed. The conditions for rough rolling are not particularly limited, and can be performed, for example, by a conventional method. The reduction ratio in the finish rolling, the time between passes, and the rolling temperature are not particularly limited, but the finish rolling temperature is preferably Ar ₃ or more. The conditions for descaling are also not particularly limited, and can be performed, for example, in a conventional method.

After the finish rolling, the steel sheet is cooled and wound up. When the coiling temperature is more than 680°C, the average particle diameter of ferrite and bainite cannot be 5 µm or less, and the yield strength may not sufficiently increase even with the aging accompanying coating baking. Therefore, the coiling temperature is set to 680°C or lower.

After winding up, the steel sheet is cooled and pickled and cold rolled. You may perform annealing between pickling and cold rolling. When the temperature of this annealing is more than 680°C, the average particle diameter of ferrite and bainite cannot be 5 µm or less, and the yield strength may not sufficiently increase even with the aging accompanying paint baking. Therefore, when annealing is performed between pickling and cold rolling, the temperature thereof is set to 680°C or lower. For this annealing, a continuous annealing furnace or a batch annealing furnace can be used, for example.

The number of rolling passes of cold rolling is not particularly limited, and is performed in the same way as in the normal method. When the rolling reduction ratio of cold rolling is less than 30%, the average particle diameter of ferrite and bainite cannot be 5 µm or less, and the yield strength may not sufficiently increase even with the aging accompanying coating baking. Therefore, the rolling reduction of cold rolling is set to 30% or more.

Annealing is performed after cold rolling. When the maximum reaching temperature of this annealing is less than (Ac ₃ -60)° C., the solid solutions of C and N are insufficient, and even with the aging accompanying paint baking, the yield strength does not sufficiently rise, and it is difficult to obtain a desirable steel structure. . Therefore, the maximum reaching temperature is (Ac ₃ -60)°C or higher. In order to obtain better collision properties, the maximum reaching temperature is preferably (Ac ₃ -40)°C or higher. On the other hand, when the maximum reaching temperature is more than 900°C, the average particle diameter of ferrite and bainite cannot be 5 µm or less, and the yield strength may not sufficiently increase even with the aging accompanying coating baking. Therefore, the maximum reaching temperature is 900°C or less. In order to obtain better collision properties, the maximum reaching temperature is preferably 870°C or lower. In order to make the average particle diameter of ferrite and bainite 5 µm or less, it is preferable to set the holding time at the highest reaching temperature for 3 seconds to 200 seconds. In particular, the holding time is preferably 10 seconds or longer, and preferably 180 seconds or shorter.

In cooling after annealing after cold rolling, the average cooling rate between 700°C and 550°C is 4°C/s to 50°C/s. When this average cooling rate is less than 4°C/s, the average dislocation density in bainite is less than 3 x 10 ¹² m/m3. On the other hand, if this average cooling rate is more than 50°C/s, the average dislocation density in bainite becomes more than 1x10 ¹⁴ m/m3. Therefore, the average cooling rate is 4°C/s to 50°C/s.

Subsequently, temper rolling of the steel sheet is performed. The temper rolling is performed under the condition that the parameter P ₃ represented by (Equation 3) is 2 or more and the elongation is 0.10% to 0.8%. "A" is a line load (N/m), and "B" is a tension (N/m2) applied to a steel sheet.

P ₃ =B/A… (Equation 3)

The parameter P ₃ affects the uniformity of dislocation density in the steel sheet. When the parameter P ₃ is less than 2, a sufficient dislocation is not introduced into the ferrite at the center of the sheet thickness of the steel sheet, and the yield strength may not sufficiently increase even by the aging accompanying coating baking. Therefore, the parameter P ₃ is set to 2 or more. In order to obtain better collision characteristics, the parameter P ₃ is preferably 10 or more.

When the elongation rate of temper rolling is less than 0.10%, a sufficient dislocation is not introduced into ferrite, and the yield strength may not sufficiently increase even by the aging accompanying coating baking. Therefore, the elongation rate is set to 0.10% or more. The elongation is preferably 0.20% or more in order to obtain better collision properties. On the other hand, when the elongation is more than 0.8%, sufficient moldability may not be obtained. Therefore, the elongation rate is set to 0.8% or less. The elongation is preferably 0.6% or less in order to obtain better moldability.

In this way, the steel sheet according to the embodiment of the present invention can be produced.

The steel plate may be subjected to a plating treatment between annealing after cold rolling and temper rolling. The plating treatment may be performed, for example, in a plating facility provided in a continuous annealing facility, or in a dedicated plating facility different from the continuous annealing facility. The composition of the plating is not particularly limited. As the plating treatment, for example, hot dipping treatment, alloyed hot dipping treatment, or electroplating treatment can be performed.

According to this embodiment, since the average dislocation density in ferrite and the average dislocation density in bainite are appropriate, stable yield strength can be obtained after coating baking.

In addition, all of the above-described embodiments are merely examples of the embodiment in carrying out the present invention, and the technical scope of the present invention should not be limitedly interpreted by them. That is, the present invention can be implemented in various forms without departing from its technical spirit or its main characteristics.

Example

Next, examples of the present invention will be described. The conditions in the examples are one conditional examples employed to confirm the feasibility and effect of the present invention, and the present invention is not limited to these one conditional examples. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

(First test)

In the first test, steel pieces having a chemical composition shown in Table 1 were melted to prepare steel pieces, and the steel pieces were heated to 1200°C to 1250°C to perform hot rolling. In hot rolling, rough rolling and finish rolling were performed. The blanks in Table 1 indicate that the content of the element was below the detection limit, the balance being Fe and impurities. The underline in Table 1 indicates that its numerical value is outside the scope of the present invention.

The hot-rolled steel sheet obtained by hot rolling was cooled and wound up at 550°C to 700°C. Next, pickling of the hot rolled steel sheet was performed to remove the scale. Thereafter, cold rolling was performed at a rolling reduction rate of 25% to 70% to obtain a cold rolled steel sheet having a thickness of 1.2 mm. For some hot rolled steel sheets, annealing at 550°C was performed between pickling and cold rolling.

Annealing was performed after cold rolling. In this annealing, the temperature was set to 780°C to 900°C and the time was 60 seconds, and cooling was performed such that the average cooling rate between 700°C and 550°C was 20°C/s. Subsequently, temper rolling was performed on condition that the elongation was 0.3% and the parameter P ₃ was 80.

For some steel sheets, hot-dip galvanizing or alloying hot-dip galvanizing was performed during or after continuous annealing, and electrogalvanization was applied to some other steel sheets after continuous annealing. Table 2 shows the steel types corresponding to the plating treatment. "GI" in Table 2 represents a hot-dip galvanized steel sheet subjected to hot dip galvanizing treatment, "GA" represents an alloyed hot-dip galvanized steel plate subjected to alloying hot-dip galvanizing treatment, and "EG" represents electric galvanized treatment. A galvanized steel sheet is shown, and "CR" indicates a cold rolled steel sheet without plating treatment.

Thus, a sample of a steel sheet was produced. Then, the steel structure of the sample was observed to measure the average dislocation density in ferrite and the average dislocation density in bainite.

In the observation of the steel structure, the area fractions of ferrite, bainite, martensite and residual austenite, and the average particle diameter of ferrite and bainite were measured. In this observation, an analysis by point counting method or image analysis using a photograph of a tissue photographed by SEM or TEM, or analysis by X-ray diffraction was performed on a portion of the thickness of the steel sheet. At this time, for ferrite and bainite, an area surrounded by grain boundaries with a hard angle of 15° or more was used as one crystal grain, and the average nominal particle diameter of each of 50 or more crystal grains was defined as an average particle diameter d. Total area fraction of ferrite and bainite f _F+B , area fraction of ferrite f _F , area fraction of martensite f _M , area fraction of retained austenite f _A , area fraction ratio (f _F /f _M ) It is shown in 2. The underline in Table 2 indicates that its numerical value is outside the scope of the present invention.

The average dislocation density was obtained from (Equation 4) using a TEM photograph. The thin film sample for TEM observation was taken from a part of 1/4 thickness from the surface of the steel sheet. As the thickness t of the thin film sample, 0.1 µm was simply used. For each of ferrite and bainite, TEM photographs were taken at five or more locations for each thin film sample, and the average value of dislocation density obtained from these TEM photographs was taken as the average dislocation density in the thin film sample. Table 2 also shows the average dislocation density ρ _{F in} ferrite and the average dislocation density ρ _{B in} bainite. The underline in Table 2 indicates that its numerical value is outside the scope of the present invention.

ρ=2N/(Lt)… (Equation 4)

Thereafter, a tensile test according to JIS Z 2241 was performed for each sample. In this tensile test, a tensile test piece conforming to JIS Z 2201 was used, with the plate width direction (direction perpendicular to the rolling direction) as the longitudinal direction. In this case, for each sample the maximum tensile strength TS, yield strength YS, uniform elongation uEl, 5% yield strength after aging in the case of a tensile pre-strain a portion YS _BH5, and tensile yield strength after aging in the case pre-deformation is not added YS _BH0 was measured. And the parameter P ₂ of moldability represented by the parameters P _1, and (formula 2) on the yield strength is represented by (formula 1) was calculated. Table 3 shows the results. The underline in Table 3 indicates that its numerical value is outside the target range.

As shown in Table 3, Sample No. which is an invention example. 1, No. 2, No. 10 to No. 13, No. 20 to No. 23, No. 25 to No. 27 exhibited excellent collision properties and formability because the requirements of the present invention were provided. Sample No. in which the ratio of the total area fraction of ferrite and bainite, the area fraction of martensite, the area fraction of retained austenite, and the area fraction of ferrite to the area fraction of martensite are within a preferred range. 1, No. 2, No. 12, No. 13, No. 21 to No. 23, No. 26, No. At 27, the parameter P ₂ was 8000 or more, and the moldability was particularly excellent.

Sample No. 3, No. In 14, sufficient formability was not obtained because the average dislocation density ρ _B was excessive. Sample No. 4, No. 5, No. 7, No. 16, No. In 17, sufficient collision characteristics were not obtained because the average dislocation density ρ _F was too small. Sample No. In 6, sufficient collision characteristics were not obtained because the average dislocation density ρ _F was excessive. Sample No. 8, No. In 18, sufficient moldability was not obtained because the average particle diameter d was excessive. Sample No. 9, No. In 19, sufficient moldability was not obtained because the total area fraction f _F+B of ferrite and bainite was too small. Sample No. At 15, sufficient collision characteristics were not obtained because the average dislocation density ρ _F and the average dislocation density ρ _B were too small. Sample No. In 24, sufficient collision characteristics were not obtained because the average dislocation density ρ _F and the average dislocation density ρ _B were excessive.

Sample No. At 28, sufficient tensile strength was not obtained because the C content was too small. Sample No. At 29, since the C content was excessive, the average dislocation density ρ _F was excessive and sufficient collision characteristics were not obtained. Sample No. At 30, sufficient collision properties were not obtained because the Si content was too small. Sample No. At 31, since the Si content was excessive, the average dislocation density ρ _F was too small to obtain sufficient collision characteristics. Sample No. In 32, sufficient tensile strength was not obtained because the Mn content was too small. Sample No. In 33, since the Mn content was excessive, the average dislocation density ρ _F and the average dislocation density ρ _B were excessive, so that sufficient moldability was not obtained. Sample No. At 34, because the Al content was excessive, the average dislocation density ρ _F and the average dislocation density ρ _B were too small to obtain sufficient collision characteristics. Sample No. At 35, sufficient moldability was not obtained because the N content was excessive. Sample No. At 36, sufficient moldability was not obtained because the P content was excessive. Sample No. At 37, sufficient moldability was not obtained because the S content was excessive. Sample No. 38 and No. In 39, sufficient moldability was not obtained because the total contents of Ti and Nb were excessive. Sample No. At 40, since the total contents of Ti and Nb were too small, the average dislocation density ρ _F was too small, and sufficient collision characteristics were not obtained.

(Second test)

In the second test, the steel of the symbol A was used, and the conditions of the treatment other than temper rolling were sample No. The sample was prepared in the same manner as in 1 and the elongation rate and parameter P ₃ of temper rolling were changed. And various measurements similar to 1st test were performed. Table 4 shows the results. The underline in Table 4 indicates that the numerical value thereof is outside the predetermined range of temper rolling, the scope of the present invention, or the target range.

As shown in Table 4, the sample No. in which temper rolling was performed in a preferred range. 43 to No. 46, No. At 50, a steel sheet meeting the requirements of the present invention could be produced.

Sample No. 41, No. At 42, the average dislocation density ρ _F and the average dislocation density ρ _B were excessive because the elongation was too small, and sufficient collision characteristics were not obtained. Sample No. In 47, since the elongation was excessive, the average dislocation density ρ _F and the average dislocation density ρ _B were excessive, so that sufficient moldability was not obtained. Sample No. In 48, the average dislocation density ρ _F and the average dislocation density ρ _B were excessive because the elongation was excessive, so that sufficient moldability was not obtained. Sample No. At 49, a sufficient collision characteristic was not obtained because the value of the parameter P ₃ was too small.

The present invention can be used, for example, in industries related to steel sheets suitable for automobile bodies.

Claims (18)

In mass%,
C: 0.05% to 0.40%,
Si: 0.05% to 3.0%,
Mn: 1.5% to 4.0%,
Al: 1.5% or less,
N: 0.02% or less,
P: 0.2% or less,
S: 0.01% or less,
Nb and Ti: 0.005% to 0.2% in total,
V and Ta: 0.0% to 0.3% in total,
Cr, Mo, Ni, Cu and Sn: 0.0% to 1.0% in total,
B: 0.00% to 0.01%,
Ca: 0.000% to 0.005%,
Ce: 0.000% to 0.005%,
La: 0.000% to 0.005%, and
Balance: Fe and impurities
Has a chemical composition represented by,
Has a steel structure containing ferrite and bainite in an area fraction of 2% or more in total,
The average dislocation density in ferrite and the average dislocation density in bainite are both 4×10 ¹² m/m 3 to 1×10 ¹⁴ m/m 3,
The average particle diameter of ferrite and bainite is 5 µm or less,
When the line load is A (N/m) and the tension applied to the steel sheet is B (N/m 2) and B/A is the parameter P, the parameter P is 2 or more and the elongation is 0.10% to 0.8%. In temper rolling,
The yield strength after aging when 5% tensile pre-strain is added is YS _BH5 , and the yield strength after aging when tensile pre-strain is not added ( _㎫ ) is YS _BH0 , and the maximum tensile strength ( ㎫) as TS,
(YS _BH5 -YS _BH0 )/TS≤0.27
A steel sheet characterized in that this holds. The method according to claim 1, wherein the steel structure comprises ferrite and bainite in an area fraction: 2% to 60% in total, and martensite: 10% to 90%,
The area fraction of retained austenite in the steel structure is 15% or less,
A steel sheet characterized in that the ratio of the area fraction of ferrite to the area fraction of martensite is 0.03 to 1.00. According to claim 1 or claim 2, In the chemical composition,
V and Ta: 0.01% to 0.3% in total
Steel sheet characterized in that is established. According to claim 1 or claim 2, In the chemical composition,
Cr, Mo, Ni, Cu and Sn: 0.1% to 1.0% in total
Steel sheet characterized in that is established. According to claim 3, In the chemical composition,
Cr, Mo, Ni, Cu and Sn: 0.1% to 1.0% in total
Steel sheet characterized in that is established. According to claim 1 or claim 2, In the chemical composition,
B: 0.0003% to 0.01%
Steel sheet characterized in that is established. According to claim 3, In the chemical composition,
B: 0.0003% to 0.01%
Steel sheet characterized in that is established. According to claim 4, In the chemical composition,
B: 0.0003% to 0.01%
Steel sheet characterized in that is established. According to claim 5, In the chemical composition,
B: 0.0003% to 0.01%
Steel sheet characterized in that is established. According to claim 1 or claim 2, In the chemical composition,
Ca: 0.001% to 0.005%,
Ce: 0.001% to 0.005%,
La: 0.001% to 0.005%, or
Or any combination of these, characterized in that the steel sheet. According to claim 3, In the chemical composition,
Ca: 0.001% to 0.005%,
Ce: 0.001% to 0.005%,
La: 0.001% to 0.005%, or
Or any combination of these, characterized in that the steel sheet. According to claim 4, In the chemical composition,
Ca: 0.001% to 0.005%,
Ce: 0.001% to 0.005%,
La: 0.001% to 0.005%, or
Or any combination of these, characterized in that the steel sheet. According to claim 5, In the chemical composition,
Ca: 0.001% to 0.005%,
Ce: 0.001% to 0.005%,
La: 0.001% to 0.005%, or
Or any combination of these, characterized in that the steel sheet. According to claim 6, In the chemical composition,
Ca: 0.001% to 0.005%,
Ce: 0.001% to 0.005%,
La: 0.001% to 0.005%, or
Or any combination of these, characterized in that the steel sheet. According to claim 7, In the chemical composition,
Ca: 0.001% to 0.005%,
Ce: 0.001% to 0.005%,
La: 0.001% to 0.005%, or
Or any combination of these, characterized in that the steel sheet. The method of claim 8, wherein in the chemical composition,
Ca: 0.001% to 0.005%,
Ce: 0.001% to 0.005%,
La: 0.001% to 0.005%, or
Or any combination of these, characterized in that the steel sheet. The method of claim 9, wherein in the chemical composition,
Ca: 0.001% to 0.005%,
Ce: 0.001% to 0.005%,
La: 0.001% to 0.005%, or
Or any combination of these, characterized in that the steel sheet. delete