JP2013139625A - High-strength cold-rolled steel sheet having excellent aging resistance and bake hardenability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-strength cold-rolled steel sheet having both aging resistance and bake hardenability.SOLUTION: The high-strength cold-rolled steel sheet has composition including, by mass%, 0.0010-0.0080% of C, 1.0% or less of Si, 0.1-1.8% of Mn, 0.100% or less of P, 0.01-0.5% of sol. Al, and 0.0050% or less of N, further containing 0.005-0.050% of Nb so as to satisfy C*=C-(12/92.9)Nb>0, and further containing 0.01-0.10% of Cr so as to satisfy Cr≥60C*-0.03 or 0.010-0.050% of Cu so as to satisfy Cu≥42.8C*+0.018. The high-strength cold-rolled steel sheet has: high strength, with tensile strength in a class of 340-440 MPa; excellent bake hardenability, with a bake hardening amount of at least 30 MPa after application of prestrain followed by a paint-bake hardening process; and excellent aging resistance, with yield elongation of 0.8% or less in a tensile test after 60 days of natural aging at 70°C.

Description

The present invention relates to a cold-rolled steel sheet suitable for automotive panel parts, and in particular, has a high strength with a tensile strength TS of 340 to 440 MPa class, and has excellent aging resistance and excellent bake hardenability (hereinafter referred to as “hardening resistance”). , Also referred to as BH property). The “steel plate” here includes a steel plate and a steel strip.
The “cold rolled steel sheet” includes a cold rolled steel sheet and a cold rolled steel sheet obtained by subjecting the cold rolled steel sheet to a surface treatment such as electroplating. The plating process includes a pure zinc plating process, a zinc-based alloy plating process in which an alloy element is added with zinc as a main component, or an Al-based alloy plating process in which an alloy element is added with Al or Al as a main component.

In recent years, from the viewpoint of the preservation of the global environment, there has been a strong demand for improving the fuel efficiency of automobiles in order to reduce carbon dioxide CO ₂ emissions. Furthermore, recently, in order to ensure the safety of passengers in the event of a vehicle collision, there has been a demand for improved safety centered on improving the collision characteristics of an automobile body.
In response to such demands, the weight reduction and strengthening of automobile bodies are being actively promoted. In order to satisfy both the weight reduction and strengthening of automobile bodies at the same time, it is said that it is effective to increase the strength of the materials used and reduce the thickness within a range where rigidity does not become a problem. High-tensile steel plates are actively used for this purpose.

  In response to such demands for reducing the weight of automobile bodies, for example, steel plates having a tensile strength TS of 390 MPa or more tend to be used as panel materials for inner plates and outer plates. On the other hand, for panel parts such as doors and hoods, so-called bake-hardening type steel plates (BH steel plates) whose yield strength increases after paint baking are used because they are required to have excellent dent resistance. It has become.

A BH steel sheet is generally a low-carbon steel sheet containing Nb in an atomic ratio equal to or less than that of carbon, and solid solution strengthened with Mn or P, and a small amount of solid steel after annealing. Due to the presence of molten C, dislocations introduced by processing such as pressing are fixed in the coating baking process, yield strength is increased, and dent resistance is improved.
As such a bake hardening type steel plate (BH steel plate), for example, in Patent Document 1, C: 0.002 to 0.015%, Si: 1.2% or less, Mn: 0.04 to 0.8%, P: 0.03 to 0.10%, Al : 0.02% or more and N% × 4 or more, Nb: C% × 3 to {C% × 8 + 0.020%}, the balance being substantially Fe, high-tensile cold-rolled steel sheet having excellent formability is described Has been. The high-tensile cold-rolled steel sheet described in Patent Document 1 hot-rolls a steel slab having the above composition at a total rolling reduction of 90% or more and a rolling speed of 40 m / min or more, and is wound at a temperature of 600 ° C. or more. Then, cold rolling is performed, and further continuous annealing is performed at 700 to 900 ° C. for 10 seconds to 5 minutes, and cooling to 500 ° C. is performed at a cooling rate of 60 ° C./min or more. This cold-rolled steel sheet is said to be a steel sheet having a tensile strength TS of 340 to 440 MPa, slow aging and excellent formability.

JP-A-56-139654

However, the technique described in Patent Document 1 has a problem that yield elongation appears when the amount of Nb is reduced or the aging temperature is increased to increase the seizure curability. That is, there has been a problem that the conventional production methods that have been studied cannot have both aging resistance and seizure curability.
In the present invention, a high-strength cold-rolled steel sheet that solves the problems of the prior art, has a tensile strength TS of 340 to 440 MPa, and has both excellent aging resistance and excellent seizure hardenability (BH property). The purpose is to provide.

  Here, “excellent aging resistance” refers to a case where the yield elongation is as low as 0.5% or less after aging promotion treatment at normal temperature (aging treatment at 70 ° C. × 60 days). In addition, “excellent bake hardenability (BH property)” here refers to the pre-strain of the yield stress after applying pre-strain: 2% and applying heat treatment (paint baking process) at 170 ° C for 20 min. The amount of increase with respect to the arrival stress (BH amount) is 30 MPa or more.

  In order to achieve the above-described object, the present inventors have conducted a detailed study on the influence of various factors on BH properties and aging resistance. As a result, in order to combine excellent aging resistance and excellent seizure curability, it has been found that control of the amount of dissolved C is important. And it was conceived that by including two kinds of alloy elements having different affinity with C, it was possible to combine excellent aging resistance and excellent seizure curability by controlling the amount of dissolved C. . As a result of further study, Cr or Cu, which has a weaker affinity for C than Nb, is used in addition to Nb, which has a stronger affinity for C, as two types of alloy elements having a different affinity for C. I came up with it.

And, in order to properly control the solute C, it is important to adjust the content appropriately, so that the yield elongation after normal temperature aging is small and the seizure hardening amount (BH amount) is high. It was found that a high-strength cold-rolled steel sheet having excellent aging resistance and excellent seizure curability can be obtained.
The present invention has been completed based on such findings and further studies. That is, the gist of the present invention is as follows.
(1) By mass%, C: 0.0010 to 0.0080%, Si: 1.0% or less, Mn: 0.1 to 1.8%, P: 0.100% or less, S: 0.03% or less, sol.Al: 0.01 to 0.50%, N: 0.0050% or less, Nb: 0.005 to 0.050%, Cr: 0.010 to 0.10%, C, Nb and Cr are the following formulas (1) and (2): C *> 0 (1)
Cr> 60C * −0.03 (2)
(Where C * = C− (12 / 92.9) Nb, C *: C amount not fixed by Nb (theoretical value) (mass%), Cr, Nb, C: content of each element (mass%))
A high-strength cold-rolled steel sheet excellent in aging resistance and bake hardenability, characterized in that it has a composition comprising the balance Fe and inevitable impurities.
(2) By mass%, C: 0.0010 to 0.0080%, Si: 1.0% or less, Mn: 0.1 to 1.8%, P: 0.100% or less, S: 0.03% or less, sol.Al: 0.01 to 0.50%, N: 0.0050% or less, Nb: 0.005 to 0.050%, Cu: 0.010 to 0.050%, and C, Nb, and Cu are the following formulas (1) and (3): C *> 0 (1)
Cu> 42.8C * + 0.018 (3)
(Where C * = C− (12 / 92.9) Nb, C *: C amount not fixed by Nb (theoretical value) (mass%), Cu, Nb, C: content of each element (mass%))
A high-strength cold-rolled steel sheet excellent in aging resistance and bake hardenability, characterized in that it has a composition comprising the balance Fe and inevitable impurities.
(3) The high-strength cold-rolled steel sheet according to (1) or (2), further containing B: 0.0050% or less by mass% in addition to the above composition.

  According to the present invention, the tensile strength TS suitable for panel parts of an automobile body is 340 MPa or more, generally high strength of 340 to 440 MPa class, low yield ratio, high elongation and excellent moldability. In addition, a high-strength cold-rolled steel sheet having both excellent aging resistance and excellent bake hardenability can be produced easily and inexpensively, and has a remarkable industrial effect. Moreover, the high-strength cold-rolled steel sheet according to the present invention can be applied to the inner and outer plates of an automobile body, and has the effect of sufficiently contributing to the reduction of the weight of the automobile body and the improvement of collision safety. Moreover, the high-strength cold-rolled steel sheet according to the present invention can be applied as a home appliance or a pipe material.

The reason for limiting the composition of the cold-rolled steel sheet of the present invention will be described first. Hereinafter, unless otherwise specified, mass% is simply expressed as%.
C: 0.0010 to 0.0080%
C is an important element in the present invention in order to combine excellent aging resistance and excellent BH properties. In order to ensure the desired excellent BH property, the content of 0.0010% or more is required. On the other hand, if the content exceeds 0.0080%, it is necessary to keep the Nb content high from the viewpoint of securing the desired excellent aging resistance, which leads to an increase in material cost. In addition, when the Nb content is low, the large content of C as described above may cause aging at room temperature, which reduces aging resistance. For this reason, C was limited to the range of 0.0010 to 0.0080%. In addition, Preferably it is less than 0.0060%, More preferably, it is 0.0040% or less.

Si: 1.0% or less
Si is an element that has the effect of increasing the strength of the steel sheet by solid solution strengthening and improving the work hardening ability. In order to acquire such an effect, it is desirable to contain 0.1% or more. On the other hand, a large content exceeding 1.0% tends to generate a red scale during hot rolling to deteriorate the surface appearance of the steel sheet, and further promotes non-plating during galvanization. Moreover, the above-described large amount of Si also reduces chemical conversion properties. For this reason, Si was limited to 1.0% or less. In addition, Preferably it is 0.5% or less.

Mn: 0.1-1.8%
Mn has a function of increasing the strength of the steel sheet by solid solution and fixing S as MnS to prevent hot cracking due to S. In order to obtain such an effect, a content of 0.1% or more is required. On the other hand, an excessive content exceeding 1.8% lowers the ductility and the r value. For this reason, Mn was limited to the range of 0.1 to 1.8%. In addition, Preferably it is 1.2% or less.

P: 0.100% or less P has the effect of solid-solution and strengthening the steel sheet, but segregates at the grain boundaries and reduces secondary work embrittlement resistance and weldability. Such an adverse effect of P becomes prominent with an excessive content exceeding 0.100%. For this reason, P was limited to 0.100% or less. In addition, Preferably it is 0.080% or less.
S: 0.03% or less S causes hot cracking, and also exists as sulfide inclusions in steel, thereby reducing the ductility and the like of the steel sheet. Therefore, in the present invention, it is desirable to reduce as much as possible, but 0.03% is acceptable. For these reasons, S is limited to 0.03% or less. In addition, Preferably it is 0.01% or less.

sol.Al: 0.01-0.50%
Al acts as a deoxidizer and forms nitrides to fix solute N and improve aging resistance. In order to acquire such an effect, 0.01% or more of content is required. On the other hand, a large content exceeding 0.50% raises the material cost (alloy cost) and causes frequent surface defects. For this reason, sol.Al was limited to the range of 0.01 to 0.50%. In addition, Preferably it is 0.30% or less.

N: 0.0050% or less N is an element that increases the strength of steel by solid solution, but inclusion exceeding 0.0050% lowers aging resistance. Therefore, in the present invention, N is limited to 0.0050% or less.
Nb: 0.005 to 0.050%
Nb has a high ability to form carbides, combines with C to form carbides and fixes C, and has the effect of refining the hot-rolled structure and increasing the r value, thereby contributing to improved formability. In order to acquire such an effect, 0.005% or more of content is required. On the other hand, an excessive content exceeding 0.050% increases the hot deformation resistance and increases the rolling load during hot rolling. For this reason, Nb was limited to 0.005 to 0.050% of range. In addition, Preferably it is 0.006% or more.
In addition, Nb is contained in the above-mentioned range and adjusted to satisfy the following formula (1) from the meaning of fixing C.

C *> 0 (1)
(Where C * = C− (12 / 92.9) Nb, C *: maximum value of C amount not fixed by Nb (mass%), Nb, C: content of each element (mass%))
C * is the amount of C that is not fixed by Nb (theoretical value) (mass%), and means the maximum value of the solid solution C amount when all Nb is precipitated as NbC. When C * does not satisfy the formula (1), the desired BH property cannot be secured if C * is 0 or less. For this reason, the Nb content is adjusted in relation to the C content so that C * exceeds 0.

Cr: 0.010-0.10%
Cr is not as strong as Nb, but is an element that interacts with C. Cr traps (captures) C at room temperature and delays its diffusion. On the other hand, Cr separates from C and contributes to the improvement of BH properties during the coating baking process. In order to acquire such an effect, Cr needs to contain 0.010% or more. On the other hand, if the content exceeds 0.10%, C is excessively fixed and the BH property is lowered. For this reason, Cr was limited to the range of 0.010 to 0.10%.

In the present invention, Cr is contained within the above-described content range and adjusted according to the amount of C not fixed with Nb so as to satisfy the following formula (2), that is, according to C *. To do.
Cr> 60C * −0.03 (2)
(Where C * is the amount of C that is not fixed by Nb (theoretical value), and the maximum value (mass%) of the solid solution C amount when all Nb is precipitated as NbC, Cr, C: content of each element (mass%))
When the Cr content does not satisfy the formula (2), that is, when Cr is less than (60C * -0.03), C (solid solution C) cannot be trapped sufficiently, and stretcher strain is generated during molding of parts. And the surface quality of the parts is likely to deteriorate. For this reason, Cr is adjusted and contained so as to exceed (60C * −0.03). Note that equation (2) is an experimental equation found by the present inventors.

The above components are basic components, but in the present invention, Cu may be contained instead of Cr.
Cu: 0.010 to 0.050%
Cu, like Cr, is not as strong as Nb, but is an element that interacts with C. Like Cr, Cu traps (captures) C at room temperature and delays its diffusion. On the other hand, Cu dissociates from C and contributes to the improvement of BH properties during paint baking. In order to acquire such an effect, Cu needs to contain 0.010% or more. On the other hand, if the content exceeds 0.050%, C is excessively fixed and the BH property is lowered. For this reason, Cu was limited to the range of 0.010 to 0.050%.

In the present invention, Cu is contained within the above-described range and adjusted according to C * so as to satisfy the following formula (3).
Cu> 42.8C * + 0.018 (3)
(Where C * = C− (12 / 92.9) Nb, C *: C amount not fixed by Nb (theoretical value) (mass%), Cu, Nb, C: Content of each element (mass%)
If the Cu content does not satisfy the formula (3), that is, if Cu is less than (42.8C * + 0.018), C (solid solution C) cannot be trapped sufficiently, and the stretcher strain is formed when the part is molded. Is likely to occur, and the surface quality of the component is liable to deteriorate. For this reason, Cu is contained by adjusting so as to exceed (42.8C * + 0.018). Note that equation (3) is an experimental equation found by the present inventors.

In addition to the above components, B: 0.0050% or less may be further contained.
B: 0.0050% or less B is an element that segregates at the grain boundary and improves the secondary work embrittlement resistance. In order to ensure such an effect, it is desirable to contain 0.0003% or more, but even if it exceeds 0.0050%, the effect is saturated and an effect commensurate with the content cannot be expected, which is economically disadvantageous. Become. For this reason, when it contains, it is preferable to limit B to 0.0050% or less. More preferably, it is 0.0030% or less.

The balance other than the components described above consists of Fe and inevitable impurities. As unavoidable impurities, Ca: 0.01% or less, REM: 0.01% or less, Sb: 0.01% or less, Sn: 0.1% or less, Zn: 0.01% or less are acceptable.
The cold-rolled steel sheet of the present invention has a ferrite single phase structure. The term “ferrite single phase structure” as used herein refers to a case where the ferrite phase contains 95% or more by volume. In addition, Preferably it is 98% or more. Examples of the second phase other than the ferrite phase include martensite, bainite, and pearlite.

Next, a preferred method for producing the cold-rolled steel sheet of the present invention will be described.
In the present invention, a steel material having the above-described composition is used as a starting material, the steel material is heated and hot-rolled to form a hot-rolled sheet, and the hot-rolled sheet is cold-rolled and cold-rolled sheet A cold rolling process, an annealing process for subjecting the cold-rolled sheet to a cold-rolled annealed sheet, and a pressure-adjusting process for subjecting the cold-rolled annealed sheet to temper rolling, in order. It is a rolled steel sheet.

  In addition, the manufacturing method of the steel material is not particularly limited, but the molten steel having the above composition is melted by a conventional melting method such as a converter method or an electric furnace method, and a conventional method such as a continuous casting method is used. It is preferable to use a steel material such as a slab by the casting method. The casting method of the steel material (slab) is preferably a continuous casting method in order to prevent macro segregation of components, but there is no problem even with the ingot casting method or the thin slab casting method.

  The obtained steel material (slab) is then subjected to a hot rolling process. Heating for hot rolling is preferably a method in which the steel material (slab) is once cooled to room temperature and then reheated, but is not cooled to room temperature, but is charged in a heating furnace as it is. Alternatively, energy-saving processes such as direct feed rolling and direct rolling, in which rolling is performed immediately after performing a slight heat retention, can be applied without any problem.

The hot rolling step is preferably a step of heating the steel material to a predetermined temperature, subjecting the steel material to hot rolling consisting of rough rolling and finish rolling to form a hot rolled plate, and then winding it.
The heating temperature of the steel material is preferably 1000-1300 ° C. When the heating temperature is less than 1000 ° C., the deformation resistance is high and the rolling load increases, and the risk of trouble occurring during hot rolling increases. On the other hand, at a high temperature exceeding 1300 ° C., the scale loss increases as the oxidized weight increases. For this reason, it is preferable to limit the heating temperature of a steel raw material to the range of 1000-1300 degreeC.

The heated steel material is made into a sheet bar by rough rolling. The conditions for rough rolling need not be particularly limited, and can be performed according to a conventional method. From the viewpoint of lowering the steel material heating temperature and preventing problems during hot rolling, it is an effective method to use a so-called sheet bar heater that heats the sheet bar.
Next, the sheet bar is finish-rolled to obtain a hot-rolled sheet. At this time, the finish rolling finish temperature is preferably 860 ° C. or higher. This temperature is the surface temperature of the steel sheet. This is to obtain a fine hot-rolled sheet structure so that excellent deep drawability can be secured after cold rolling and recrystallization annealing. When the finish rolling finish temperature is less than 860 ° C, the surface layer is below the transformation point, coarse grains are formed, the transformation texture from unrecrystallized austenite (γ) develops strongly, and the texture develops after cold rolling annealing. Furthermore, the rolling load at the time of hot rolling becomes high. On the other hand, when the finish rolling finish temperature exceeds 980 ° C. and becomes high, the structure becomes coarse, preventing the formation and development of a recrystallized texture after cold rolling annealing, and a high r value may not be ensured. For this reason, it is preferable that the finish rolling finish temperature is limited to 860 ° C. or higher and 980 ° C. or lower. In addition, More preferably, it is 880-940 degreeC.

  In order to reduce the rolling load during hot rolling, lubrication rolling may be performed between some or all passes of finish rolling. Performing lubrication rolling is also effective from the viewpoint of uniform steel plate shape and uniform material. In addition, it is preferable to make the friction coefficient in the case of lubrication rolling into the range of 0.10-0.25. Furthermore, it is also preferable to use a continuous rolling process in which successive sheet bars are joined and finish-rolled continuously. The continuous rolling process is desirable from the viewpoint of the operational stability of hot rolling.

  After finishing rolling, the hot rolled sheet is wound up. The winding temperature is preferably a temperature in the range of 550 to 720 ° C. This temperature is the surface temperature of the steel sheet. When the coiling temperature is less than 550 ° C., the precipitation of NbC becomes insufficient. On the other hand, at a high temperature exceeding 720 ° C., the crystal grains become coarse and the strength of the steel sheet is reduced. Furthermore, when the coiling temperature is set to a high temperature exceeding 720 ° C., an increase in r value after cold rolling annealing may be hindered. For this reason, the winding temperature is preferably limited to a range of 550 to 720 ° C. The temperature is more preferably 660 ° C. or lower.

The hot-rolled sheet that has finished the hot-rolling process is then subjected to a cold-rolling process.
In addition, it is preferable to perform pickling suitably before a cold rolling process. As the pickling method, any conventional method can be applied.
In the cold rolling process, it is only necessary to obtain a cold rolled sheet having a desired size, and the conditions are not particularly limited. However, the rolling reduction of the cold rolling is preferably 50% or more. In order to obtain a steel sheet having a high r value, it is effective to use a high cold rolling reduction ratio. If the rolling reduction ratio is less than 50%, the {111} recrystallization texture does not develop, and the deep drawability may decrease. is there. On the other hand, as the cold rolling reduction increases, the r value increases. However, when the cold rolling reduction exceeds 85%, the effect is saturated, and the rolling load on the roll increases. For this reason, the cold rolling reduction ratio is preferably 50% or more and 85% or less.

The cold-rolled sheet that has finished the cold-rolling process is subjected to an annealing process that is subjected to an annealing process to form a cold-rolled annealed sheet. In the present invention, the annealing treatment is preferably a continuous annealing line, and after the cold-rolled sheet is soaked at a temperature in the range of annealing temperature: 760 to 900 ° C., the average cooling rate up to 300 ° C. is 10 ° C./s or more. As a cooling process, it is preferable.
When the annealing temperature is less than 760 ° C., an unrecrystallized structure remains and ductility decreases. On the other hand, at a high temperature exceeding 900 ° C., an austenite phase is generated during annealing, and a low-temperature transformation phase after cooling is generated, or the amount of solute C is increased to lower the aging resistance. For this reason, it is preferable to limit the annealing temperature to a temperature in the range of 760 to 900 ° C. The temperature is more preferably 800 ° C. or higher.

On the other hand, if the cooling rate after soaking is an average cooling rate up to 300 ° C. and less than 10 ° C./s, the cooling is too slow, so that solid solution C is reprecipitated and the BH property is lowered. For this reason, it is preferable that the cooling rate after soaking is limited to 10 ° C./s or more at an average cooling rate up to 300 ° C. More preferably, it is 40 ° C./s or less.
Note that the structure of the cold-rolled annealed sheet after the annealing process is a ferrite single-phase structure. When a second phase other than the ferrite phase, such as a martensite phase, is present, the aging resistance changes, and the subsequent production conditions for temper rolling and low-temperature heat treatment deviate from the optimum conditions in the present invention.

The cold-rolled annealed plate after the annealing step may be subjected to a plating process such as electroplating. Examples of the plating treatment include pure zinc plating treatment, zinc-based alloy plating treatment with zinc as the main component and addition of alloy elements, or Al-based alloy plating treatment with addition of alloy elements as the main component of Al or Al.
Next, the cold-rolled annealed plate that has been subjected to the annealing step or the cold-rolled annealed plate (plated plate) that has been subjected to a plating treatment is subjected to a temper rolling step.

  In the temper rolling step, temper rolling is performed on the cold-rolled annealed plate or plated plate for the purpose of shape correction, adjustment of surface roughness, and the like. The elongation ratio in temper rolling is preferably in the range of 0.5 to 2.0%. If the elongation rate is less than 0.5%, the intended purpose of shape correction and surface roughness adjustment cannot be achieved. On the other hand, if the elongation rate exceeds 2.0%, the yield point increases, and surface quality such as surface strain is reduced when the outer panel is formed, for example. For this reason, it is preferable that the elongation rate in temper rolling is 0.5 to 2.0%. In addition, More preferably, it is 1.0% or less.

  Hereinafter, the present invention will be described in detail based on examples.

  Molten steel having the composition shown in Table 1 was melted in a converter, which is a conventional melting furnace, and made into a slab (steel material: thickness 250 mm) by a conventional continuous casting method. These slabs were heated to 1250 ° C. and roughly rolled into sheet bars. Then, these sheet bars were subjected to finish rolling under the conditions shown in Table 2 to form a hot rolled sheet and wound in a coil shape. Next, the hot-rolled sheet wound in a coil shape was subjected to a pickling treatment to remove the surface scale, and then a cold-rolling step of performing cold rolling with a cold reduction ratio of 70% was performed to obtain a cold-rolled sheet.

These cold-rolled plates were further subjected to an annealing process using a continuous annealing line to obtain cold-rolled annealed plates under the conditions shown in Table 2. Furthermore, these cold-rolled annealed sheets were subjected to a temper rolling step for temper rolling under the conditions shown in Table 2.
Test pieces were collected from the obtained cold-rolled annealed plates and subjected to structure observation, tensile test, bake hardenability test, and aging test, and the microstructure, tensile properties, seizure curability and aging resistance were investigated. The test method was as follows.
(1) Microstructure observation A specimen for microstructural observation is collected from the obtained cold-rolled annealed plate, a cross section (L cross section) parallel to the rolling direction is polished, is subjected to nital corrosion, and is subjected to an optical microscope (magnification: 400 times). The structure was observed, imaged, and the area ratio of the ferrite phase was determined by image analysis, and this was defined as the volume ratio. In the photomicrograph of the optical microscope, the inside of the ferrite phase grains is not corroded and is white. When there was a second phase that corroded in black other than the ferrite phase, the structure was observed with a scanning electron microscope (magnification: 3000 times), imaged, and the volume fraction of the second phase was determined by image analysis. . In the scanning electron micrograph, the ferrite phase exhibits a slightly black contrast, and the martensite phase as the second phase is observed as particles exhibiting a white contrast. Moreover, the area | region where the carbide | carbonized_material produced | generated to the lamellar form or the point sequence form was made into the pearlite and bainite phase. However, a fine second phase having a diameter of 0.2 μm or less was excluded.
(2) Tensile test JIS No. 5 test piece (GL: 50 mm) was sampled from the obtained cold-rolled annealed plate so that the tensile direction of the test piece was 90 ° (C direction) with respect to the rolling direction. Then, in accordance with the provisions of JIS Z 2241, a tensile test was performed at a crosshead speed of 10 mm / min to obtain tensile properties (yield strength YS, tensile strength TS, elongation El).
(3) Bake hardenability test JIS No. 5 test piece (GL: 50 mm) so that the tensile direction of the test piece is 90 ° direction (C direction) with respect to the rolling direction from the obtained cold-rolled annealed plate After applying 2% pre-strain to the test piece, a process corresponding to paint baking at 170 ° C. × 20 min was performed. And it re-tensioned and calculated | required the upper yield point (yield stress) after heat processing. The difference between the obtained upper yield point after the heat treatment and the ultimate stress at the pre-strain was determined and used as the bake hardening amount (BH amount).
(4) Aging test JIS No. 5 test piece (GL: 50mm) was sampled from the obtained cold-rolled annealed plate so that the tensile direction of the test piece was 90 ° (C direction) with respect to the rolling direction. And maintained at an aging temperature (70 ° C.) for 60 days. After the holding, the tensile test was carried out in the same manner as (2) the tensile test to determine the yield elongation. A case where the yield elongation was 0.5% or less was evaluated as “good” as being excellent in aging resistance. Otherwise, it was set as x.

  The obtained results are shown in Table 3.

  Each of the inventive examples has a ferrite single phase structure, a tensile strength TS of 340 to 440 MPa class, an elongation El of 40% or more, excellent workability, and an excellent BH amount of 30 MPa or more. It is a high-strength cold-rolled steel sheet that exhibits hardenability, has a low yield elongation of 0.5% or less after aging treatment, and has excellent aging resistance. On the other hand, in comparative examples that are out of the scope of the present invention, the seizure curability is lowered when the BH amount is less than 30 MPa, or the yield elongation exceeding 0.5% occurs after the aging treatment, and the aging resistance is lowered. Steel plate No. 12 has a high Mn content outside the scope of the present invention, and has a bainite-like structure, a low elongation El, and a low formability.

Claims (3)

% By mass
C: 0.0010 to 0.0080%, Si: 1.0% or less,
Mn: 0.1 to 1.8%, P: 0.100% or less,
S: 0.03% or less, sol.Al: 0.01 to 0.50%,
N: 0.0050% or less, Nb: 0.005 to 0.050%,
Cr: 0.010-0.10%
In which C, Nb, and Cr satisfy the following formula (1) and the following formula (2), and has a composition comprising the balance Fe and inevitable impurities, High strength cold-rolled steel sheet with excellent resistance.
Record
C *> 0 (1)
Cr> 60C * −0.03 (2)
Where C * = C− (12 / 92.9) Nb
C *: Maximum amount of C not fixed by Nb (% by mass)
Cr, Nb, C: Content of each element (mass%) % By mass
C: 0.0010 to 0.0080%, Si: 1.0% or less,
Mn: 0.1 to 1.8%, P: 0.100% or less,
S: 0.03% or less, sol.Al: 0.01 to 0.50%,
N: 0.0050% or less, Nb: 0.005 to 0.050%,
Cu: 0.010 to 0.050%
And C, Nb, and Cu are contained so as to satisfy the following formula (1) and the following formula (3), and have a composition comprising the balance Fe and inevitable impurities, and seizure resistance and seizure High strength cold-rolled steel sheet with excellent curability.
Record
C *> 0 (1)
Cu> 42.8C * + 0.018 (3)
Where C * = C− (12 / 92.9) Nb
C *: Maximum amount of C not fixed by Nb (% by mass)
Cu, Nb, C: Content of each element (% by mass)   The high-strength cold-rolled steel sheet according to claim 1 or 2, further comprising B: 0.0050% or less by mass% in addition to the composition.