JP2000080440A - High strength cold rolled steel sheet and its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength cold rolled steel sheet having >=780 MPa tensile strength and >=70 MPa amount of baking hardening and combining excellent stretch-flange formability, spot weldability, delayed fracture resistance, and impact resistance. SOLUTION: A steel stock, having a composition in which 1.5-3.5%, by weight, Mn and 0. 005-0.10% Nb are contained and further the amounts of C, Si, P, S, Al, and N are regulated to proper values, respectively, is heated to a temperature at which the amount of Nb unentered into solid solution becomes >=0.003%, finish rolled at 950 to 800 deg.C finish rolling delivery-side temperature, coiled at 700 to 400 deg.C coiling temperature, and cold rolled. The resultant steel sheet is annealed at >=800 deg.C annealing temperature, rapidly cooled continuously down to <=350 deg.C at (15 to 150) deg.C/s cooling rate, cooled slowly down to <=200 deg.C at >=15 deg.C/min cooling rate, and then cooled rapidly down to room temperature. By this procedure, a structure composed essentially of fine bainitic structure of <=2.5 μm average grain size is provided.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an automobile part, such as an impact beam or a bumper, which is manufactured through light drawing, bending, roll forming, etc., and which effectively absorbs impact at the time of collision. The present invention relates to a high-strength cold-rolled steel sheet having a tensile strength of 780 MPa or more and a method for producing the same. The steel sheet referred to in the present invention includes a steel strip, and further includes a steel sheet and a steel strip obtained by subjecting these steel sheets to electroplating and hot-dip plating.

[0002]

2. Description of the Related Art In recent years, there has been a strong demand for the use of high-strength steel sheets for automobile parts in order to improve the safety of automobiles and reduce the weight of vehicle bodies. In particular, steel sheets used for bumper parts and impact beams used to suppress deformation of the cabin during a collision have been required to have a high strength exceeding 590MPa. In order to achieve the requirements, it is further required that the tensile strength has 780 MPa or more. Further, a steel sheet used for a bumper part, an impact beam or the like is required to have not only high strength but also excellent local ductility, that is, excellent properties corresponding to bending formability or stretch flangeability.

Further, automobile parts are often assembled by spot welding, and these steel sheets are required to have excellent spot weldability such as high joint strength at spot welds. Depending on the part, not only spot welding but also TIG welding, MAG welding, laser welding, etc. may be applied. In these weldings, it is also desirable to have excellent welding workability and mechanical properties of the joint part. ing.

In recent years, the structure of an automobile body has been changed with an emphasis on improving the safety of passengers, and a high energy absorbing ability at the time of collision, that is, an impact resistance is an extremely important characteristic. This impact resistance is an important characteristic not only in the base material but also in the welded part,
It is necessary for the welded portion to have high impact energy absorption characteristics as in the base metal portion.

[0005] As described above, steel sheets used for bumper parts, impact beams, and the like are intended to have high strength and also to have high local ductility, weldability and impact resistance. In order to obtain, for example, JP-A-59-143027 and JP-A-60-100630
Japanese Patent Application Laid-Open No. 7-188767 discloses a method for producing a high-strength steel sheet having good ductility and workability, and Japanese Patent Application Laid-Open No. 7-188767 discloses a method for producing a high-strength steel sheet having excellent stretch flangeability. The method is described in JP 59-143027 discloses, after the steel sheet, the heating rate up to 600 ° C. to Ac ₃ transformation point and 5 ° C. / s or higher, and heated to Ac ₃ transformation point or above the soaking temperature Bainite containing one part of ferrite and martensite is maintained by holding at that temperature for 10 sec to 10 min, and then cooling at an average cooling rate of 600 to 300 ° C. at a critical cooling rate or more specified by the alloy element content. As a mixed organization with
A steel sheet having high strength of 60 kgf / mm ² or more and excellent workability.

In the method described in Japanese Patent Application Laid-Open No. 60-100630, a steel sheet is heated to a soaking temperature of not less than the Ac ₃ transformation point (Ac ₃ transformation point + 50 ° C.) and held at that temperature for 3 to 60 seconds. After that, it is cooled to the temperature range between the Ar ₃ transformation point and the Ar ₁ transformation point, and is kept at that temperature range for at least 20 seconds, and the average cooling rate between 600 and 300 ° C. is determined by the critical cooling rate specified by the alloy element content. The steel is annealed by cooling as described above, and has a mixed structure of martensite containing one part of ferrite and bainite.
A thin steel sheet having high strength of f / mm ² or more and excellent workability.

Further, the method described in Japanese Patent Application Laid-Open No. 7-188767 discloses that a steel sheet is heated at a temperature of not less than the Ar ₃ transformation point and not more than 900 ° C.
After annealing for sec to 5 min, the temperature is raised to a
It is rapidly cooled to 200-300 ° C at a cooling rate of 500 ° C / s, and then is kept or reheated at a temperature of 200-400 ° C for 1-30 minutes.
A high-strength cold-rolled steel sheet excellent in stretch flangeability of 780 N / mm ² or more.

However, Japanese Patent Application Laid-Open No. Sho 59-143027 discloses
In steel sheets manufactured by the techniques described in JP-A-60-100630 and JP-A-7-188767, required for steel sheets used for bumper parts and impact beams,
It has been difficult to satisfy all four characteristics of high strength, local ductility, weldability and impact resistance. For example, the methods described in JP-A-59-143027 and JP-A-60-100630 disclose a ferrite phase and a low-temperature transformation structure (
(Martensite, bainite, etc.) to form a mixed structure to obtain high strength with the addition of a relatively small amount of alloying elements and to achieve both workability and high strength. The formation of a mixed structure of different phases itself caused a reduction in local ductility.

[0009] Further, since the yield strength (YS) of the base material is low, it is disadvantageous for components used for applications in which absorbed energy up to a small strain range is regarded as important. Further, when welding such as spot welding is performed, the heat-affected zone softens, that is, the strength of the heat-affected zone decreases. This is an important problem not only in the case of a static load but also in the case of a shock load. Furthermore, when a hard low-temperature transformation structure is dispersed in a ferrite phase, a so-called band-like structure is often formed, and cracks are easily propagated along the structure. .

Further, as described in Japanese Patent Application Laid-Open No. Hei 7-188767, a method is employed in which a structure mainly composed of bainite is applied after annealing, followed by water quenching or quenching at a cooling rate comparable to that, followed by tempering. In this method, high strength can be obtained with a relatively small amount of alloy element, and a uniform structure can be obtained in principle. However, in practice, it is difficult to keep the quenching rate uniform throughout the hot-rolled steel strip in the manufacturing process (particularly, distribution occurs in the width direction of the strip), and the material varies greatly depending on the location, resulting in poor shape during press forming. There was a drawback that it caused. Furthermore, the tempering process necessary for ensuring workability is also a short-time process in a relatively low temperature range of about 100 to 300 ° C, which further increases the unevenness of materials in actual process production. .

Such non-uniformity of the material naturally leads to variations in various properties such as local ductility, and as a result, the average material is reduced. Also, since the strength is increased by rapid cooling, when welding such as spot welding is performed,
There has been a problem that the heat-affected zone is remarkably softened and the static and dynamic strengths are reduced. Furthermore, since the prior austenite grain boundary is likely to be a starting point of cracks for delayed fracture or a propagation path of the cracks, there is a problem that the delayed fracture resistance is extremely low.

[0012]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned problems and provides a composition having a relatively low alloy addition amount.
It has a tensile strength exceeding 0 MPa and has excellent formability such as stretch flange formability and bending workability, good weldability in spot welding, etc., impact energy that matches strength not only in the base material but also in the welded part An object of the present invention is to propose a high-strength cold-rolled steel sheet having excellent shock resistance and absorption properties.

Further, in addition to the above-mentioned characteristics, the present invention can reduce the load at the time of molding, has high paint bake hardenability in order to obtain sufficient component strength, and often poses a problem in high-strength steel. An object of the present invention is to propose a steel sheet having excellent characteristics with respect to delayed fracture resistance. The main application of the steel sheet of the present invention is a strength member of an automobile used after being subjected to light working such as a bumper part or an impact beam.

[0014]

Means for Solving the Problems In order to achieve the above-mentioned object, the present inventors have conducted intensive studies on the composition of the steel sheet and the manufacturing process. As a result, the composition, the hot rolling conditions and the cold-rolled sheet annealing conditions were changed. As an appropriate range, the structure has an average crystal grain size of 2.5 μ
It has been found that a steel sheet having high strength and having both local ductility, weldability and impact resistance can be manufactured by using a structure mainly composed of uniform and fine bainite of m or less. The present inventors applied a continuous rolling technique of joining and rolling a preceding sheet bar and a succeeding sheet bar to finish rolling of hot rolling, which is effective for improving the shape and dimensional accuracy of a steel sheet. At the same time, using sheet bar edge heaters and sheet bar heaters to make the steel plate temperature uniform in the longitudinal and width directions, and to mask the cooling water on the width direction edges of the steel plate to prevent overcooling of the edges To do
It was also found that it was effective in making the material uniform.

The present invention has been made based on the above findings. That is, in the present invention, C: 0.
05 to 0.30%, Si: 1.0% or less, Mn: 1.5 to 3.5%, P:
0.02% or less, S: 0.005% or less, Al: 0.150% or less,
N: 0.0200% or less, Nb: 0.005 to 0.10%, balance
It has a composition consisting of Fe and unavoidable impurities, and has a structure mainly composed of a fine bainite structure with an average crystal grain size of 2.5 μm or less, a tensile strength of 780 MPa or more, and a paint bake hardening amount (BH amount): 70 MPa. A high-strength cold-rolled thin steel sheet excellent in weldability, stretch flange formability, delayed fracture resistance and impact resistance, characterized by having the above,
In the present invention, in addition to the above composition, Ti:
0.005 to 0.20%, B: 0.0005 to 0.0040%, V: 0.005 to
It preferably contains one or more of 0.05%, and in addition to the above composition, further contains Cu:
0.02% or more, Ni: 0.02% or more, Cr: 0.02% or more, Mo: 0.
One or more selected from 02% or more may contain 1.0% or less in total and / or Ca: 0.0005 to 0.0050%.

In the present invention, C: 0.05 to 0.5% by weight.
30%, Si: 1.0% or less, Mn: 1.5-3.5%, P: 0.02%
Below, S: 0.005% or less, Al: 0.150% or less, N: 0.02
A steel material containing not more than 00% and Nb: 0.005 to 0.10%, and having a balance of Fe and unavoidable impurities, having an undissolved Nb amount of 0.1%.
It is heated to a temperature of 003% or more, preferably 1150 ° C. or less, and subjected to hot rolling including finish rolling in which the finish-rolling exit temperature is in a temperature range of 950 to 800 ° C. to form a hot-rolled sheet, and a winding temperature of 700. After winding at ~ 400 ° C, the hot-rolled sheet is subjected to cold rolling to form a cold-rolled sheet. The cold-rolled sheet is annealed at an annealing temperature of 800 ° C or more, and 15 to 150 ° C / s
Continuously quenching to a temperature of 350 ° C or less and 200 ° C or more at a cooling rate of 350 ° C, and then slowly cooling to a temperature of 200 ° C or less at a cooling rate of 15 ° C / min or more. Tensile strength: 780MPa or more, Paint baking hardening amount (BH amount): More than 70MPa,
This is a method for producing a high-strength cold-rolled thin steel sheet having excellent weldability, stretch flangeability, delayed fracture resistance and impact resistance.
In the present invention, the composition of the steel material is expressed as a percentage by weight.
C: 0.05 to 0.30%, Si: 1.0% or less, Mn: 1.5 to 3.
5%, P: 0.02% or less, S: 0.005% or less, Al: 0.150
%, N: 0.0200% or less, Nb: 0.005 to 0.10%, Ti: 0.005 to 0.20%, B: 0.0005 to 0.0040
%, V: 0.005 to 0.050%, one or more of which may be contained, and the composition may be composed of the balance of Fe and unavoidable impurities.
C: 0.05 to 0.30%, Si: 1.0% or less, Mn: 1.5 to 3.5
%, P: 0.02% or less, S: 0.005% or less, Al: 0.150%
Hereinafter, N: 0.0200% or less, Nb: 0.005 to 0.10%,
Further, one or two or more selected from Cu: 0.02% or more, Ni: 0.02% or more, Cr: 0.02% or more, and Mo: 0.02% or more are contained in a total of 1.0% or less, with the balance being Fe and inevitable. The composition of the steel material may be composed of impurities, and the composition of the steel material is expressed in terms of% by weight: C: 0.05 to 0.30%, Si: 1.0% or less, Mn: 1.5 to 3.5%, P: 0.02% or less, S: 0.005% %
Below, Al: 0.150% or less, N: 0.0200% or less, Nb: 0.00
5 to 0.10%, and may further contain Ca: 0.0005 to 0.0050%, and may have a composition consisting of the balance of Fe and inevitable impurities.
0.30%, Si: 1.0% or less, Mn: 1.5-3.5%, P: 0.
02% or less, S: 0.005% or less, Al: 0.150% or less, N:
0.0200% or less, Nb: 0.005 to 0.10%, Ti:
0.005 to 0.20%, B: 0.0005 to 0.0040%, V: 0.005 to
Contains one or more of 0.050%,
Cu: 0.02% or more, Ni: 0.02% or more, Cr: 0.02% or more, M
o: One or two or more selected from 0.02% or more may be contained in a total of 1.0% or less, and the composition may be composed of the balance of Fe and unavoidable impurities. And C: 0.05 to 0.30%, Si: 1.0% or less, M
n: 1.5 to 3.5%, P: 0.02% or less, S: 0.005% or less, Al: 0.150% or less, N: 0.0200% or less, Nb: 0.005%
0.10%, Ti: 0.005 to 0.20%, B: 0.00
05-0.0040%, V: 0.005 to 0.050%, one or more of the following, Ca: 0.0005 to 0.0050%, the balance may be a composition comprising Fe and unavoidable impurities. The composition of the steel material is expressed by weight%, C: 0.
05 to 0.30%, Si: 1.0% or less, Mn: 1.5 to 3.5%, P:
0.02% or less, S: 0.005% or less, Al: 0.150% or less,
N: 0.0200% or less, Nb: 0.005 to 0.10%,
Cu: 0.02% or more, Ni: 0.02% or more, Cr: 0.02% or more, M
o: One or two or more selected from 0.02% or more are contained in a total of 1.0% or less, Ca: 0.0005 to 0.0050%,
The balance may be composed of Fe and unavoidable impurities, and the composition of the steel material is C: 0.05 to
0.30%, Si: 1.0% or less, Mn: 1.5 to 3.5%, P: 0.02
%, S: 0.005% or less, Al: 0.150% or less, N: 0.
0200% or less, Nb: 0.005 to 0.10%, and Ti: 0.
005 to 0.20%, B: 0.0005 to 0.0040%, V: 0.005 to 0.0
Containing one or more of 50% and further Cu:
0.02% or more, Ni: 0.02% or more, Cr: 0.02% or more, Mo: 0.
One or more selected from 02% or more, 1.0% or less in total, Ca: 0.0005-0.0050%, balance Fe
The composition may be composed of unavoidable impurities.

In the present invention, it is preferable that the finish rolling is continuous rolling in which a preceding sheet bar and a following sheet bar are joined and continuously rolled on the entry side of the finishing rolling machine. In the invention, in the finish rolling,
It is preferable to use one or both of a sheet bar edge heater and a sheet bar heater on the entry side of the finishing rolling machine, and it is preferable that the finishing rolling is lubricating rolling using lubricating oil.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION
It is a cold rolled steel sheet of 2.0 mm or less. If the sheet thickness exceeds 2.0 mm, the cooling rate after annealing decreases, and it is difficult to obtain the target high strength at a cooling rate of about gas jet, and cooling with strong cooling means such as water quenching has already been described. There is such a problem. On the other hand, if the sheet thickness exceeds 2.0 mm, it is possible to manufacture the sheet at a lower cost by hot rolling, and there is little advantage of manufacturing with a cold-rolled steel sheet.

The steel sheet of the present invention is a high-strength cold-rolled steel sheet having a tensile strength of 780 MPa or more, and has a bake hardening amount (BH
Is a steel sheet having an amount of 70 MPa or more. The upper limit of the tensile strength is not particularly defined, but includes up to 1480 MPa, which is the strength at which the steel sheet is used. Next, the reasons for limiting the component composition of the steel sheet of the present invention will be described.

C: 0.05 to 0.30% C is an austenite stabilizing element and effectively acts to strengthen the transformed structure. Transformation structure strengthening is recognized at a content of 0.05% or more. On the other hand, when the content exceeds 0.30%, the weldability is remarkably reduced, for example, the nugget portion of spot welding is significantly hardened. Especially when an impact load is applied,
The risk of fracture of the weld increases with low energy absorption below the expected value. From the viewpoint of impact resistance, the upper limit of C was set to 0.30%. Therefore, C is 0.05 to 0.30.
%. In addition, from the viewpoint of stably obtaining high tensile properties and weldability, the content is preferably 0.20% or less.

Si: 1.0% or less Si is an element for increasing the tempering softening resistance, and is contained in the present invention in an amount of 1.0% or less. When the content exceeds 1.0%, the hot deformation resistance of the steel is significantly increased, and it becomes difficult to hot-roll a thin material. In applications where surface scale defects are a problem, the content of Si is desirably 0.8% or less. Further, from the viewpoint of improving the cleanliness of steel, it is desirable that Si be contained at 0.01% or more.

Mn: 1.5 to 3.5% Mn is an effective element that binds to S and prevents hot cracking due to S and must be added according to the amount of S contained. Further, Mn has an action of refining crystal grains, and is positively added in the present invention. Further, Mn improves the hardenability of the steel, generates a low-temperature transformation phase mainly composed of bainite, and significantly increases the strength of the steel. In order to produce a high-strength cold-rolled product sheet having a tensile strength of 780 MPa or more, the content of Mn must be 1.5% or more. On the other hand, if the content exceeds 3.5%, the hot deformation resistance increases and the rolling load increases significantly, making hot rolling difficult. In addition, the strength of the hot-rolled sheet increases, and the cold-rolling process is hindered. Further, if the content of Mn exceeds 3.5%, there are problems such as deterioration of weldability and deterioration of formability of a welded portion. For these reasons, Mn is limited to the range of 1.5 to 3.5%. From the viewpoint of corrosion resistance and moldability, Mn is preferably set to 3.2% or less.

P: 0.02% or less P hardens steel and deteriorates stretch flangeability of a steel sheet. Further, P has a strong tendency to segregate in the central portion of the steel sheet in the continuous cast material, and embrittles the welded portion. For this reason, in the present invention, it is desirable to reduce P as much as possible, but up to 0.02% is acceptable. When the stretch flangeability and weld toughness are emphasized, the content is preferably 0.01% or less.

S: 0.005% or less S is present as nonmetallic inclusions in steel, and reduces the ductility of the steel sheet and further deteriorates the corrosion resistance. Further, in a high-strength steel such as the steel sheet of the present invention, the notch sensitivity tends to increase. Therefore, it is necessary to reduce inclusions such as MnS which are sources of stress concentration. For this reason, it is very important to make the S low, but up to 0.005% is acceptable. In addition, from the viewpoint of workability, the content is desirably 0.002% or less.

Al: 0.150% or less Al is a useful element that acts as a deoxidizing agent, improves the cleanliness of steel, and refines the structure. Although it depends on the deoxidation technique, the deoxidation effect becomes remarkable when the Al content is preferably 0.01% or more, and the effect of refining the structure becomes remarkable. On the other hand, if the addition exceeds 0.150%, the surface properties are degraded, leading to a reduction in the strength of the steel sheet.
% As the upper limit. In addition, 0 from the viewpoint of material stability.
Preferably, it is in the range of 010 to 0.080%. In addition, as a deoxidizing agent, Ti, Ca, etc. may be used in combination with Al or in place of Al. In this case, it is preferable that the content of Al is not more than 0.010% because uniform fine dispersion of the oxide can be achieved.

N: 0.0200% or less When N is contained in a large amount, the decrease in hot ductility, the increase in internal defects, and the occurrence of slab cracks during continuous casting become remarkable. did. In addition, material stability,
From the viewpoint of improving the yield, it is preferable that N is set to 0.0150% or less. In addition, N has a function of lowering the transformation point, and in rolling of a thin material in which the rolling temperature is liable to be lowered, in order to avoid hot rolling at a temperature lower than the Ar ₃ transformation point, a range of 0.0200% or less. It is effective to increase the N content within. In this case, N is preferably at least 0.0020% or more.

Nb: 0.005 to 0.10% Nb is an important element in the present invention. By adding Nb after increasing the Mn content, a fine and uniform structure mainly composed of bainite is obtained. Can be. In order to obtain such a structure, the content of Nb must be 0.005% or more. However, if the content exceeds 0.10%, the effect is saturated and the rolling load during hot rolling increases. For this reason,
Nb was limited to the range of 0.005 to 0.10%.

In the present invention, the following elements can be contained, if necessary, in addition to the above composition. Ti: 0.005 to 0.20%, B: 0.0005 to 0.0040%, V: 0.00
One or two or more of 5 to 0.050% Ti, B, and V all contribute to the refinement of the structure and have an effect of suppressing the ferrite transformation of the steel. It is a useful element. The refinement of the structure is observed when the content of Ti is 0.005% or more, the content of B is 0.0005% or more, or the content of V is 0.005% or more. However, the content of Ti exceeds 0.20%, the content of B exceeds 0.0040%, or the content of 0.05% or more.
Even if V is contained in excess of 0%, the effect tends to be saturated, and an effect commensurate with the added amount cannot be expected, which is economically disadvantageous. Therefore, Ti is 0.005 to 0.20% and B is 0.0005 to 0.00%.
40% and V are desirably in the range of 0.005 to 0.050%. The addition of B significantly increases the strength in combination with Nb. Even if Ti, B, and V are added in combination, their effects are not canceled out.

Cu: 0.02% or more, Ni: 0.02% or more, Cr: 0.2%
One or more selected from 02% or more and Mo: 0.02% or more 1.0% or less in total Cu, Ni, Cr and Mo are elements that suppress ferrite transformation and improve hardenability. It is a useful element for enhancing the strength of a product plate by strengthening the transformation structure. Such an increase in strength due to the strengthening of the transformed structure is recognized when the content of each element is 0.02% or more. However, a large amount of each element causes an increase in hot deformation resistance, an increase in the hardenability of the weld, or a deterioration in the surface treatment properties (chemical conversion properties, plating properties, etc.), so that Cu, Ni, Cr, Mo 1.0% in total content of each element
Was set as the upper limit.

Ca: 0.0005 to 0.0050% Ca controls the form of the sulfide inclusions and contributes to the improvement of the ductility of the steel sheet. In particular, in the steel sheet of the present invention having a high Mn composition and a bainite structure, the stretch flangeability is significantly improved by the addition of Ca. Such an effect is more than 0.0005%
Although Ca addition is observed, the addition exceeding 0.0050% saturates the effect and tends to degrade the surface properties, increasing the risk of deteriorating appearance characteristics after surface treatment. From this,
Ca is desirably in the range of 0.0005 to 0.0050%. In addition, the range of 0.0010 to 0.0035% is more preferable.

Other than the above components, the balance is Fe and inevitable impurities. O: 0.00 as inevitable impurities
70% or less, Sn: 0.01% or less, Zn: 0.01% or less are acceptable. The structure of the steel sheet of the present invention having the above composition is a structure mainly composed of a fine bainite structure having an average crystal grain size of 2.5 μm or less.

The structure of the steel sheet of the present invention is a structure having fine bainite as a main phase, and ferrite, martensite and pearlite other than bainite as sub-phases in a total area ratio.
It may contain up to 20%, preferably up to 10%. By adopting a structure mainly composed of bainite, high strength can be ensured and compared to a tempered martensite structure of the same strength,
Stretch flange workability is significantly improved. In the present invention, bainite refers to a structure in which carbides are mainly precipitated at lath boundaries or laths, and tempered martensite refers to a structure in which carbides are frequently precipitated at former austenite grain boundaries.

Although bainite and tempered martensite have the same strength, according to the investigations of the present invention, the structure mainly composed of bainite is superior in stretch flange properties. This is considered to be due to the fact that carbides precipitated at the former austenite grain boundaries in the tempered martensite structure and coarsely precipitated adversely affect stretch flangeability. Further, by adopting a structure mainly composed of bainite for the same reason, delayed fracture resistance and impact resistance are remarkably improved. In addition, it is preferable that bainite has a so-called lower bainite structure with few harmful coarse carbides.

The structure of the steel sheet of the present invention is a fine structure having an average crystal grain size of 2.5 μm or less. In the present invention, the measurement of the crystal grain size is measured for the entire thickness of the sheet. Further, the crystal grain size is measured on a cross section in the rolling direction and a cross section in a direction perpendicular to the rolling direction in accordance with the provisions of JIS G 0552, and the average value thereof is displayed. By forming a microstructure having an average crystal grain size of 2.5 μm or less, stretch flangeability, delayed fracture resistance, and impact energy absorption characteristics during high-speed molding are remarkably improved. In addition, even when subjected to a welding heat cycle, a uniform and fine particle size distribution is substantially maintained, so that these characteristics, particularly the impact energy absorption characteristics at the time of high-speed molding, are remarkably improved even in a welded portion.
When higher stretch flangeability is required, the average crystal grain size is desirably 2.0 μm or less. The average crystal grain size is 2.5 μm or less, preferably 2.0 μm or less.
In order to obtain a structure with fine bainite of less than μm as the main phase, an appropriate amount of elements such as Mn and Nb is added as described above, and the annealing conditions under hot rolling and the cooling conditions after annealing are appropriately controlled as described later. It is necessary to. Tensile strength: 780MPa or more To satisfy the demand for thinning due to high strength, the tensile strength is made 780MPa or more. By making the structure mainly bainite, a tensile strength of 780 MPa or more is inevitably obtained.

Baking hardening amount (BH amount): 70 MPa or more The BH amount in the present invention is obtained by adding 2% tensile strain,
It is defined as the increase in yield stress after aging at ℃ 20 min. When the BH amount is 70 MPa or more, when used as a part, it appears as a difference in absorbed energy particularly at the time of large deformation. That is, even if the same amount is deformed by impact, more energy can be absorbed and more excellent properties can be exhibited as the impact energy absorbing member. Thereby, if the absorption energy is the same, the thickness of the steel sheet can be reduced. BMP amount is 70MPa
If it is less than 30, such an effect is not sufficiently exerted. In order to secure a BH amount of 70 MPa or more, it is preferable to control the particle size more finely. In addition, the amount of BH increases due to the presence of a small amount of a martensite phase or the like. However, since the BH amount tends to decrease as the yield strength (YS) increases, it is desirable to control the BH amount in consideration of these factors.

Next, the manufacturing conditions of the steel sheet of the present invention will be described. It is desirable to smelt the molten steel having the above-described composition by a commonly known smelting method such as a converter or an electric furnace, and to form a steel material such as slab by a continuous casting method. If solidified by continuous casting,
Macro segregation of components can be prevented. In addition, it goes without saying that an ingot making method, a thin slab casting method, or the like may be used instead of the continuous casting method.

The produced steel material is once cooled to room temperature and then reheated and rolled, or is directly cooled without being cooled down to room temperature and charged into a heating furnace while heating and then rolled. Rolling or direct rolling in which a slight heat retention is performed and then rolling is performed immediately. When direct rolling is performed, from the viewpoint of microstructure uniformity and miniaturization, it is necessary to temporarily stop the austenite (γ) → ferrite (α) transformation and then heat again to the γ region. desirable.

Heating temperature of hot rolling: the amount of undissolved Nb is 0.003
% The steel material is heated to a temperature at which the amount of undissolved Nb becomes 0.003% or more. Preferably it is 1150 ° C or lower. By performing rough rolling at a relatively low temperature with some undissolved Nb remaining, dynamic recrystallization occurs during rough rolling, and as a result, a remarkably fine and uniform hot-rolled structure can be obtained. .

In conventional microstructure steels or precipitation-strengthened steels to which Nb or the like is added, it is common to use a heating temperature of a high temperature to utilize the effect of suppressing the grain growth by solid-solution Nb (for example, Japanese Patent Application Laid-Open No. H6-145891). In this case, it is inevitable that the tissue becomes uneven. On the other hand, the present invention is particularly characterized in that it is heated at a low temperature in order to utilize the precipitated Nb. Incidentally, in the conventional high-tensile steel, to reduce the rolling load, to reduce the deformation resistance by heating to a high temperature of 1250 ℃ or more, in the present invention, the progress of dynamic recrystallization suppresses the increase in deformation resistance So
Even at low temperature heating, the increase in rolling load is relatively small. As the undissolved Nb in the present invention, a value quantified by electrolytic extraction analysis on a sample that has been water-quenched from the corresponding heating temperature is used.
The electrolytic extraction analysis method is a method in which an analysis sample is subjected to constant potential electrolytic separation, filtered, and then quantified by an absorption spectrophotometry. (According to the method recommended by the Iron and Steel Institute Joint Research Group Steel Analysis Subcommittee.
For example, Kiichi Narita: Iron and Steel, 66 (1980), P211) At high temperatures where the amount of undissolved Nb is less than 0.003%, or at temperatures exceeding 1150 ° C, the initial γ grains rapidly increase. It becomes coarse and cannot achieve uniform and fine hot rolled structure, resulting in mixed grains. From the viewpoint of optimizing the structure, the heating temperature of the steel material is preferably lower than 1100 ° C, more preferably 1050 ° C or lower. In order to reduce the rolling load, it is preferable to heat to 950 ° C. or higher.

Finishing roll delivery side temperature: 950 to 800 ° C. The heated steel material is then subjected to rough rolling and finish rolling to form a hot rolled sheet. In order to make the structure of the hot-rolled sheet uniform and fine, the exit temperature of the finish rolling is preferably in the temperature range of 950 to 800 ° C. If the temperature on the side of the finish rolling is less than 800 ° C., the structure of the hot-rolled sheet expands and becomes non-uniform, and furthermore, the processed structure remains at the steel sheet edge and the like, and the formability deteriorates. On the other hand, when the finish-rolling exit side temperature exceeds 950 ° C., the structure of the hot-rolled sheet becomes coarse, so that the structure of the cold-rolled annealed sheet (product sheet) becomes coarse and tends to be mixed. For this reason, the finish-rolling exit temperature is limited to the range of 950 to 800 ° C. The temperature is preferably in the range of 900 to 840 ° C. from the viewpoint of improving the mechanical properties of the steel sheet, especially the ductility and the delayed fracture resistance.

Winding temperature: 700 to 400 ° C. The hot-rolled sheet after finishing rolling is wound into a coil. The winding temperature of the hot rolled sheet is limited to the range of 700 to 400 ° C. If the winding temperature exceeds 700 ° C, it becomes difficult to ensure the uniformity of the material, while if it is lower than 400 ° C, the shape of the hot-rolled sheet is disturbed, and the picking and cold rolling in the subsequent process may reduce the line passability. Problems arise. For this reason, the winding temperature of the hot rolled sheet is
The range was 700 to 400 ° C. The temperature is preferably 650 to 450 ° C. from the viewpoint of uniformity of the material.

The hot rolled sheet is then subjected to cold rolling to be a cold rolled sheet. The hot-rolled sheet is cold-rolled after being pickled by a usual method. From the viewpoint of refining the structure by cold rolling recrystallization, the rolling reduction of the cold rolling is preferably set to 40% or more. When the hot-rolled sheet is hard, hot-rolled sheet annealing can be performed. Annealing temperature of cold rolled sheet: 800 ° C or higher The cold rolled sheet is then annealed. Annealing is performed at a temperature not lower than the recrystallization end temperature and not higher than 1000 ° C., but annealing is performed at 800 ° C. or higher to obtain a pre-structure for obtaining a uniform and fine structure. If the annealing temperature is lower than 800 ° C., there is a problem that an unrecrystallized structure is partially mixed and a structure having an average crystal grain size of 2.5 μm or less cannot be obtained on a product plate. On the other hand, when the temperature exceeds 1000 ° C., the structure becomes coarse.

Rapid cooling rate from the annealing temperature to 350 ° C. or lower but higher than 200 ° C .: 15 to 150 ° C./s The cold rolled sheet heated to the annealing temperature is subjected to a rapid cooling treatment. If the cooling rate in the quenching treatment is less than 15 ° C./s, it is difficult to make the structure mainly bainite, and a tensile strength of 780 MPa or more cannot be obtained. On the other hand, the cooling rate from the annealing temperature is 15
If the temperature exceeds 0 ° C./s, the variation in strength increases. The cooling rate is preferably less than 100 ° C./s. In the present invention, the rapid cooling at a cooling rate of 15 to 150 ° C / s is performed at 350 ° C or less.
Conduct continuously up to a temperature range of over 200 ° C. If the quenching stop temperature is higher than 350 ° C., bainite transformation is not completed, and bainite is tempered, carbides are coarsely precipitated and a desired structure cannot be obtained, so that the strength decreases. If the quenching stop temperature is 200 ° C. or less, the structure becomes martensitic, which is not preferable. Here, "continuously" means that the cooling step is continued without interruption within the above-mentioned quenching temperature range.

Slow cooling rate after quenching: 15 ° C./min or more Following the rapid cooling treatment, 15 ° C./min or more, preferably 300 ° C./min
Apply a slow cooling process of min or less. The slow cooling treatment is performed up to a temperature of 200 ° C. or lower, preferably 100 ° C. or higher. By performing the slow cooling process, the tension can be stabilized and uniformized. If the cooling rate is less than 15 ° C./min, the rapidly cooled structure is tempered, and the stretch flangeability is reduced. When the cooling rate exceeds 300 ° C./min, the delayed fracture resistance tends to decrease. Therefore, the cooling rate of the slow cooling process after the rapid cooling is limited to 15 ° C./min or more, and preferably 300 ° C./min or less. Further, when the slow cooling treatment is completed at a temperature higher than 200 ° C., there is a problem that the delayed fracture resistance deteriorates.

Following slow cooling to a temperature of 200 ° C. or lower, preferably 100 ° C. or higher, rapid cooling (referred to as secondary rapid cooling) is preferably performed again to room temperature to facilitate handling.
The conditions for the secondary quenching are not particularly limited, but the starting temperature of the secondary quenching is preferably 200 to 100 ° C. In the present invention, the hot finish rolling is desirably a continuous rolling in which a preceding sheet bar and a succeeding sheet bar are joined at the entry side of the finishing mill and continuously rolled. By performing continuous rolling, unsteady portions at the front and rear ends of the steel sheet (steel strip) are eliminated, and stable hot rolling can be performed over the entire length and the entire width of the steel sheet coil. Thereby, the cross-sectional shape and the dimensional accuracy of the coil of the steel plate are improved, and as a result, the uniformity of the quality after annealing is also improved. The joining of the preceding sheet bar and the succeeding sheet bar is preferably performed by pressure welding, laser welding, electron beam welding, or the like, but is not limited to these methods. Furthermore, by performing continuous rolling, tension can always be applied to the steel sheet during cooling after rolling, so that a good steel sheet shape can be maintained.
Further, since the leading end portion of the steel sheet can be stably passed, lubricating rolling can be applied, and the roll life can be extended by reducing the rolling load and the roll surface pressure.

In order to ensure uniformity of the quality after annealing, it is desirable to use one or both of a sheet bar edge heater and a sheet bar heater on the entry side of the finishing mill in hot finish rolling. Finally, the sheet bar edge heater is desirably heated by the sheet bar edge heater so that the temperature difference in the steel sheet width direction in the finish rolling is desirably 20 ° C. or less. Further, it is desirable to heat the sheet bar with a sheet bar heater so as to compensate for the temperature drop at the front and rear ends of the steel sheet and to make the temperature difference from the central part preferably 20 ° C. or less. Note that there is no problem if the sheet bar edge heater and the sheet bar heater are used at the same time.

It is desirable that the hot finish rolling be lubricated rolling using lubricating oil. By lubricating rolling, in addition to reducing the rolling load, it is possible to improve the shape and dimensional accuracy of the steel sheet and to make the structure uniform in the thickness direction of the steel sheet.

[0048]

EXAMPLES (Example 1) Molten steel having the components shown in Table 1 was smelted in a converter and made into a slab having a thickness of 220 mm by a continuous casting method. The slab was formed into a hot rolled sheet under the hot rolling conditions shown in Table 2. In the hot finish rolling, the preceding sheet bar and the succeeding sheet bar were melt-pressed and joined on the entry side of the finishing mill to form continuous rolling. Further, a sheet bar heater and a sheet bar edge heater were used on the entrance side of the finishing mill to make the sheet bar temperature uniform. After the completion of hot rolling, the steel sheet was water-cooled with a hot run table until the steel sheet was wound up, and the winding temperature was adjusted. In addition, lubrication rolling was performed after the finish rolling. Next, the hot-rolled sheet was pickled and cold-rolled at a cold reduction shown in Table 2 to obtain a cold-rolled sheet. Then, these cold rolled sheets were added to Table 2
Annealed at the temperature shown in the following, after annealing, quenched under the conditions of Table 2,
Slow cooling and secondary quenching were carried out and cooled to room temperature.

With respect to the obtained cold-rolled annealed sheet,
Tensile test, bake hardenability test, stretch flangeability test, delayed fracture resistance test, spot weldability test and impact resistance test were investigated. The amount of undissolved Nb is a value obtained by heating a test piece cut out of a slab at the same temperature as the slab heating temperature and quenching it with water and quantifying it by electrolytic extraction analysis.

The test method is shown below. (1) Microstructure test A test piece was sampled from the rolling direction and the direction perpendicular to the rolling direction of each cold-rolled annealed sheet, and the cross-section in the rolling direction, the structure in the direction perpendicular to the rolling direction, and the crystal grain size were measured. The crystal grain size was measured in accordance with the provisions of JIS G 0552 for the entire thickness of the cross section in the rolling direction and the cross section perpendicular to the rolling direction, and expressed as an average value thereof. (2) Tensile test A JIS No. 5 tensile test piece was collected from each cold-rolled annealed plate, and its tensile properties (yield stress YS, tensile strength TS, elongation El) were examined. (3) Bake hardenability test A JIS No. 5 tensile test specimen was collected from each cold-rolled annealed sheet, subjected to a 2% tensile prestrain, and then subjected to a heat treatment at 170 ° C. for 20 minutes.
Next, a tensile test was performed, and the amount of increase in yield stress before and after the heat treatment was determined to be the BH amount. (4) Stretch flangeability test A test piece (sheet thickness x 100 x 100 m) taken from each cold-rolled annealed sheet
m), a hole with a diameter of 10 mm is punched out, and then a conical punch with a vertex angle of 60 ° is inserted from the opposite side of the burrs (the side with the “burr” on the shear surface) to form a hole to expand the hole. The hole diameter D (mm) when penetrating through is determined, and the critical hole expansion rate λ
Was calculated. Note that the critical hole spread rate λ is λ (%) =
{(D-10) / 10)} × 100. (5) Delayed fracture resistance test A 59 mmφ disk sampled by punching from each cold-rolled annealed plate was subjected to deep drawing using a 33 mmφ punch and a drawing die with a shoulder radius of 4 mm (client: plate thickness + 10%). Then, it was immersed in pure water for one week and observed for cracks. × when cracks were found, ○ when cracks were not found
And (6) Spot weldability test Each cold-rolled annealed plate is overlapped and spot-welded under the following conditions to create a shear tensile test having a width of 25 mm in accordance with JIS Z 3140. It was measured. The case where the shear tensile strength of the welded portion increased in proportion to the tensile strength of the base material and the fracture did not reach inside the nugget was evaluated as ○. The case where there was a decrease in strength, or the case where the inside of the nugget was broken was evaluated as x.

Spot welding conditions: (a) Electrode: 6 mmφCF (b) Energizing time: 8 cycles (c) Initial pressing force: 250 kgf (d) Holding time: 15 cycles (e) Welding current: current immediately below dust generation (7) Impact test (2) for each cold rolled annealed plate and spot weld
Using the same tensile test piece as in the test of (6), high-speed tension was applied at a tensile speed of 7 m / s by a hydraulic servo type high-speed tensile tester, and the breaking strength and the breaking mode were investigated.

Table 3 shows the results of each test.

[0053]

[Table 1]

[0054]

[Table 2]

[0055]

[Table 3]

From Table 3, it can be seen that each of the examples of the present invention has a tensile strength.
It has a high strength of 990MPa or more and a large bake hardening amount of 155MPa or more, which indicates that it has excellent stretch flangeability, weldability, delayed fracture resistance and impact resistance. In comparison, comparative examples outside the scope of the present invention have lower strength or bake hardening amount, or stretch flangeability,
Any of the weldability, delayed fracture resistance, and impact resistance was deteriorated, and none of these properties was excellent. (Example 2) A slab having the same composition as steel No. A, Ca and Ca shown in Table 1 (without addition of Ca) was used to obtain a hot-rolled sheet under hot rolling conditions shown in Table 4. In some of the sheets, hot finish rolling was performed by continuous rolling, in which the preceding sheet bar and the succeeding sheet bar were melt-welded and joined at the entry side of the finishing rolling machine and then rolled.
For one part, lubrication rolling was performed after the finish rolling. Next, the hot-rolled sheet was pickled and cold-rolled at a cold rolling reduction shown in Table 4 to obtain a cold-rolled sheet. Thereafter, these cold-rolled sheets were annealed at the temperatures shown in Table 4, and after annealing, quenched, gradually cooled and secondary quenched under the conditions shown in Table 4, and cooled to room temperature.

With respect to the obtained cold-rolled annealed sheet, a structure test, a tensile test, a bake hardening test, a stretch flangeability test, a delayed fracture resistance test, a spot weldability test and an impact resistance test were conducted in the same manner as in Example 1. investigated. Table 5 shows the results.

[0058]

[Table 4]

[0059]

[Table 5]

From Table 5, it can be seen that all of the inventive examples have tensile strengths.
It has a high strength of 985MPa or more, and has a large bake hardening amount of 85MPa or more, indicating that it has excellent stretch flangeability, weldability, delayed fracture resistance and impact resistance. In comparison, comparative examples outside the scope of the present invention have lower strength or bake hardening amount, or stretch flangeability,
Any of the weldability, delayed fracture resistance, and impact resistance was deteriorated, and none of these properties was excellent.

[0061]

According to the present invention, with a relatively small alloy addition amount, it has high tensile strength exceeding 780 MPa, and has excellent formability such as stretch flange formability and bending workability, and spot welding. Excellent weldability, excellent impact resistance with impact energy absorption characteristics commensurate with strength not only in the base metal part but also in the welded part, and also with high paint bake hardenability and excellent delayed fracture resistance A cold-rolled thin steel sheet can be provided. INDUSTRIAL APPLICABILITY The steel sheet of the present invention can be expected to greatly contribute to weight reduction and improvement of safety of automobiles as automobile strength members such as bumper parts and impact beams, and has a remarkable industrial effect. Further, the steel sheet of the present invention can be applied not only to cold-rolled steel sheets but also to various types of plated steel sheets (electroplating, hot-dip coating).

 ──────────────────────────────────────────────────の Continued on the front page (72) Inventor Setsuo Megga 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Prefecture Inside the Chiba Works of Steel Corporation (72) Inventor Takao Uchiyama 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki F term in Chiba Works (reference) 4K037 EA01 EA02 EA05 EA06 EA09 EA11 EA13 EA15 EA16 EA17 EA18 EA19 EA20 EA23 EA25 EA27 EA31 EA32 EB05 EB09 FC03 FC04 FE01 FE02 F03 F07 F05

Claims (4)

[Claims] C. 0.05 to 0.30%, Si: 1.0% or less, Mn: 1.5 to 3.5%, P: 0.02% or less, S: 0.005% or less, Al: 0.150% or less, N: 0.0200 %, Nb: 0.005 to 0.10%, has a composition consisting of the balance of Fe and unavoidable impurities, and has a structure mainly composed of a fine bainite structure having an average crystal grain size of 2.5 μm or less. : High-strength cold-rolled thin steel sheet having excellent weldability, stretch flangeability, delayed fracture resistance and impact resistance, characterized by having a 780 MPa or more and a paint bake hardening amount (BH amount): 70 MPa or more. 2. In addition to the above composition, further in weight%, T
i: 0.005 to 0.20%: B: 0.0005 to 0.0040%, V: 0.005
2. The high-strength cold-rolled thin steel sheet according to claim 1, wherein the steel sheet contains one or more of 0.05% to 0.050%. 3. In addition to the above composition, further in weight percent C
u: 0.02% or more, Ni: 0.02% or more, Cr: 0.02% or more, M
o: One or more selected from 0.02% or more, 1.0% or less in total, and / or Ca: 0.0005 to 0.00
The high-strength cold-rolled thin steel sheet according to claim 1 or 2, which contains 50%. 4. In% by weight, C: 0.05 to 0.30%, Si: 1.0% or less, Mn: 1.5 to 3.5%, P: 0.02% or less, S: 0.005% or less, Al: 0.150% or less, N: 0.0200 %, Nb: 0.005 to 0.10%, steel material is heated to a temperature at which the amount of undissolved Nb becomes 0.003% or more, and the finish rolling exit temperature is in the temperature range of 950 to 800 ° C. And hot-rolled at a temperature of 700 to 400 ° C., and then cold-rolled to form a cold-rolled sheet.
Anneal at an annealing temperature of 800 ° C or more,
Cools continuously at a cooling rate of ~ 150 ° C / s to a temperature of 350 ° C or less and over 200 ° C, and then at a cooling rate of 15 ° C / min or more.
Tensile strength characterized by slow cooling to a temperature of 200 ° C or less: 780MPa or more, paint bake hardening amount (BH amount): 70MPa
A method for producing a high-strength cold-rolled thin steel sheet having the above features and excellent in weldability, stretch flangeability, delayed fracture resistance and impact resistance.