JP2009019231A - High-strength cold-rolled steel sheet and manufacturing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-strength cold-rolled steel sheet which has low yield stress, superior resistance to natural aging and superior bake hardenability. <P>SOLUTION: This high-strength cold-rolled steel sheet has a component composition including, 0.01% or more but less than 0.08% of C, 0.2% or less of Si, 0.8% to 1.7% of Mn, 0.10% or less of P, 0.03% or less of S, 0.1% or less of Al, 0.008% or less of N, 0.8% or more of Cr, while satisfying 1.9≤Mn(mass%)+1.3Cr(mass%)≤3.0, and the balance iron with unavoidable impurities. The high-strength cold-rolled steel sheet has a structure including a ferrite phase and a martensitic phase which occupies 2 to 15% by an area rate. The method for manufacturing the cold-rolled steel sheet includes conducting the steps of hot rolling, cold rolling, annealing, primary cooling, secondary cooling and third cooling, while controlling the respective conditions. For instance, in the secondary cooling step, the steel sheet is cooled in a temperature range between 650°C and a temperatures close to the Ms point which is given by Tc(°C)(=410-40×Mn(mass%)-30×Cr(mass%)) and at an average cooling rate of 10°C/s or higher. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a high-strength cold-rolled steel sheet suitable for the fields of automobiles, home appliances, etc., having a low yield stress and excellent in normal temperature aging resistance and bake hardenability, and a method for producing the same.

2. Description of the Related Art In recent years, steel sheets for automobiles have been made thinner and thinner for the purpose of improving fuel efficiency by reducing the weight of the vehicle body and increasing the strength for improving safety. However, increasing the strength of a steel sheet generally causes a deterioration in press formability, and there is a problem that a distortion of about several tens of μm called a surface distortion occurs, resulting in a deterioration in design.
On the other hand, steel plates (BH steel plates) have been developed that are soft and easy to form during press forming and exhibit high bake hardenability in the paint baking process after press forming. This steel plate is based on ultra-low carbon steel, with Ti and Nb added to control the amount of solute C. Since the strength is 340MPa, the yield stress (hereinafter sometimes referred to as YP) is as low as 240MPa. The surface distortion resistance is good, and the yield stress (YP ') after press molding and paint baking is increased to about 300 MPa to ensure dent resistance.
However, from the viewpoint of weight reduction, a steel sheet that is thinner than the current 340BH steel sheet with a thickness of 0.65 to 0.80 mm is desired.For example, in order to reduce the thickness to 0.05 mm, the yield stress after press molding and paint baking ( YP ') is required to be about 350 MPa or more. In order to obtain a high YP 'while ensuring a low YP, a steel plate having high paint bake hardenability (hereinafter also referred to as BH) and work hardenability (hereinafter also referred to as WH) is required. It becomes.
From such a background, for example, in Patent Document 1, annealing and cooling conditions of steel containing C: 0.005 to 0.0070%, Mn: 0.01 to 4.0%, Cr: 0.01 to 3.0% are appropriately controlled, and annealing is performed. It describes a method for obtaining a steel sheet having a low yield stress, high WH and BH properties, and having excellent room temperature aging resistance by making the subsequent structure a low-temperature transformation product unit structure.
In Patent Document 2, the annealing and cooling conditions of steel containing C: 0.04% or less, Mn: 0.5-3.0%, Mo: 0.01-1.0% are appropriately controlled, and the structure after annealing is expressed in volume ratio. Room temperature resistance with a yield stress of 300MPa or less and high WH and BH properties by including a composite structure containing 0.5% or more and less than 10% retained austenite and the balance being a hard phase of ferrite and bainite and / or martensite A method for obtaining a steel sheet excellent in aging, shape freezing property, and dent resistance is described.
Patent Document 3 states that C: 0.01% or more and less than 0.040%, Mn: 0.3 to 1.6%, Cr: 0.5% or less, Mo: 0.5% or less, and Mn + 1.29Cr + 3.29Mo is 1.3% or more and 2.1% or less. Highly strong and high bake hardenability (BH) by properly controlling the cooling conditions after annealing of the contained steel, and making the structure after annealing 70% ferrite or more and 1-15% martensite in volume fraction ) Is described.
Japanese Unexamined Patent Publication No. 6-12940 JP 2005-281867 JP JP 2006-233294 A

However, the above prior art has the following problems.
For example, the technique described in Patent Document 1 is evaluated by the recovery amount of yield point elongation after artificial aging treatment of 100 ° C. × 1 hr (hereinafter sometimes referred to as YPEl) as an evaluation of normal temperature aging resistance. 30 using the Hundy equation shown in equation (2) (Source: Hundy, BB “Accelerated Strain Aging of Mild Steel”. J. Iron & Steel Inst., 178, p. 34-38, (1954).) As a result of calculating the equivalent aging time at ℃, it is equivalent to 18 days at 30 ℃, and it cannot be said that the aging resistance at room temperature is necessarily excellent. Further, in order to obtain a low temperature transformation product unit structure, for example, in the examples, annealing is performed in a very high temperature range of 860 to 980 ° C. In this case, there is a concern about troubles such as plate breakage. For these reasons, it is necessary to develop a steel sheet that does not require high-temperature annealing and has excellent room temperature aging resistance.
log ₁₀ (t _r / t) = 4400 (1 / T _r -1 / T) -log ₁₀ (T / T _r ) (2)
T: Accelerated aging temperature (K)
T _r : Evaluation target temperature (K)
t: Aging time (hr) at accelerated aging temperature T
_tr : Equivalent aging time (hr) when converted into evaluation target temperature _Tr (K)
The technique described in Patent Document 2 contains Mo in an amount of 0.01% to 1.0%, preferably 0.1% to 0.6%, and uses retained austenite as a metal structure from the viewpoint of improving work hardening (WH). . However, Mo is a very high element, and adding 0.18 to 0.56% of Mo as in the examples causes a significant increase in cost. On the other hand, in the comparative examples in which the amount of Mo added in the examples is very low, YR is high and WH is extremely low. Therefore, it is necessary to develop a steel sheet having a low YR and a high WH without requiring expensive Mo.
The technology described in Patent Document 3 controls the martensite fraction and the solid solution C in the ferrite, and as a cooling after annealing to obtain high bake hardenability, a temperature of 550 to 750 ° C to 200 ° C or less Cooling at a cooling rate of up to 100 ° C / s. However, in order to satisfy such cooling conditions, a special method such as quenching in jet water as described in Patent Document 3 is necessary, and it is difficult to manufacture the current continuous hot dip galvanizing line. It is.

  The present invention was made in order to solve the above-mentioned problems, without using expensive Mo, without requiring special equipment, with low yield stress, room temperature aging resistance and bake hardenability. An object is to provide an excellent high-strength cold-rolled steel sheet and a method for producing the same.

As a result of earnest research to solve the above problems, the following findings were obtained.
Even if a small amount of pearlite or bainite is generated during annealing and cooling, the yield stress increases. For this reason, by controlling Mn and Cr having high hardenability within a specific range, generation of pearlite and bainite is suppressed, and low work yield and high work hardenability are obtained.
At the same Mn equivalent, the smaller the amount of Mn added, the more the A1 and A3 lines in the Fe-C phase diagram shift to a higher temperature and higher C side, so the solid solution C in the ferrite increases. Therefore, by reducing the amount of Mn added, the bake hardening characteristic, which is a strain aging phenomenon of solute C, is improved. On the other hand, if the amount of Mn added is excessively reduced, the aging property deteriorates. Therefore, it is important to control Mn within an appropriate range in order to achieve both room temperature aging resistance and bake hardenability.
In other words, by properly controlling the Mn content, the room temperature aging resistance and bake hardenability are balanced to a high level, and the Mn equivalent (= Mn + 1.3Cr) by adjusting the Cr content is controlled to an appropriate range. The inventors have found that a high-strength cold-rolled steel sheet having a yield stress and high work hardenability can be produced, and the present invention has been completed.
The present invention has been made based on the above findings, and the gist thereof is as follows.
[1] Component composition is C: 0.01% or more and less than 0.08%, Si: 0.2% or less, Mn: 0.8% or more, 1.7% or less, P: 0.10% or less, S: 0.03% or less, Al: 0.1% by mass% In the following, N: 0.008% or less, Cr: 0.8% or more and 1.9 ≦ Mn (mass%) + 1.3Cr (mass%) ≦ 3.0, with the balance being iron and inevitable impurities, A high-strength cold-rolled steel sheet having a ferrite phase and a martensite phase with an area ratio of 2 to 15%.
[2] In the above [1], one or more of Mo: 0.15% or less, V: 0.5% or less, B: 0.01% or less, Ti: 0.1% or less and Nb: 0.1% or less A high-strength cold-rolled steel sheet comprising:
[3] A steel slab having the composition described in [1] or [2] above is hot-rolled and cold-rolled, then annealed at an annealing temperature of more than 750 ° C. and less than 820 ° C., and then from the annealing temperature to 650 The temperature range up to ℃ is primary cooled at an average cooling rate of 2 ℃ / s to 20 ℃ / s, and then the temperature range from 650 ℃ to the temperature Tc given by the following formula (1) is 10 ℃ / s Secondary cooling is performed at the above average cooling rate, and third cooling is performed at an average cooling rate of 0.2 ° C / s to 10 ° C / s in the temperature range from temperature Tc to 200 ° C. A method for producing rolled steel sheets.
Tc (℃) = 410-40 × Mn-30 × Cr (1)
Here, the element symbol in the formula represents the element content in steel in mass%.
In the present specification, “%” indicating the component of steel is “% by mass”. The high strength cold rolled steel sheet is a cold rolled steel sheet having a tensile strength of 340 MPa or more.

According to the present invention, a high-strength cold-rolled steel sheet having a low yield stress and excellent in room temperature aging resistance and bake hardenability can be obtained without using Mo and without requiring special equipment. As a result, when used for automobile inner and outer plate applications, it is possible to reduce the weight by reducing the thickness.
And since the high intensity | strength cold-rolled steel plate of this invention has the above outstanding characteristics, it can utilize widely for household appliances etc. including the steel plate for motor vehicles, and is industrially useful.

In the present invention, the component composition is defined as Mn: 0.8% to 1.7%, Cr: 0.8% or more, and the Mn equivalent is controlled within an appropriate range of 1.9 ≦ Mn (mass%) + 1.3Cr (mass%) ≦ 3.0 To do. And a structure | tissue is made into a martensitic phase of 2-15% by a ferrite phase and an area ratio. This is a feature of the present invention and is the most important requirement. By setting it as such a component composition and structure, a high-strength cold-rolled steel sheet having low yield stress and excellent room temperature aging resistance and bake hardenability can be obtained.
Also, in order to produce such a high-strength cold-rolled steel sheet with low yield stress and excellent room temperature aging resistance and bake hardenability, it is essential to control the annealing and cooling conditions. Annealing at an annealing temperature of less than ℃, primary cooling (cooling from annealing temperature to 650 ℃ with an average cooling rate of 2 ℃ / s to 20 ℃ / s), secondary cooling (from 650 ℃ to Tc (℃) ( = 410-40 × Mn (mass%)-30 × Cr (mass%)), the temperature range up to the temperature near the Ms point is cooled at an average cooling rate of 10 ° C / s or more), 3rd cooling (above The temperature is cooled from Tc to 200 ° C. at an average cooling rate of 0.2 ° C./s to 10 ° C./s).

Hereinafter, the present invention will be described in detail.
First, the reasons for limiting the chemical components of steel in the present invention will be described.
C: 0.01% or more and less than 0.08%
C is effective for increasing the strength and is one of the important elements in the present invention. If the amount of C is less than 0.01%, a desired martensite phase cannot be obtained, and it becomes difficult to apply it to automotive outer panel applications due to the occurrence of yield point elongation and increase in YP. In order to secure a desired martensite phase, the C content is 0.01% or more. On the other hand, if the C content is 0.08% or more, the martensite phase becomes too much, resulting in an increase in YP, a decrease in BH, and a deterioration in weldability. Therefore, the C content is 0.01% or more and less than 0.08%. From the viewpoint of low YP and high BH, it is preferably 0.01% or more and less than 0.06%, and more preferably 0.01% or more and less than 0.05%.
Si: 0.2% or less
Si has a large solid solution strengthening ability, and it is better to be less from the viewpoint of low YP. However, since the Si content is allowed up to 0.2%, the Si content is 0.2% or less.
P: 0.10% or less
P is an element effective for increasing the strength. However, if the P content exceeds 0.10%, YP increases and the surface distortion resistance deteriorates. Moreover, it segregates at the grain boundary of the steel sheet and degrades the secondary work brittleness resistance. Therefore, the P content is 0.10% or less.
S: 0.03% or less
S is better because it lowers the hot workability and increases the hot cracking susceptibility of the slab. In particular, when the amount of S exceeds 0.03%, the ductility deteriorates due to the precipitation of fine MnS, and the press formability deteriorates. Therefore, the S amount is 0.03% or less. From the viewpoint of press formability, the S content is preferably 0.015% or less.
Al: 0.1% or less
Al serves as a deoxidizing element to reduce inclusions in the steel and to fix unnecessary solid solution N in the steel as nitrides. However, when the Al content exceeds 0.1%, cluster-like alumina inclusions increase, ductility deteriorates, and press formability deteriorates. Therefore, the Al content is 0.1% or less.
N: 0.008% or less
Since it is not preferable that N is left in a solid solution state from the viewpoint of room temperature aging resistance, it is preferable that N be less. In particular, if the amount of N exceeds 0.008%, the amount of nitride-forming elements added to fix N increases, leading to an increase in manufacturing cost. Moreover, ductility and toughness deteriorate due to the formation of excess nitride. Therefore, the N content is 0.008% or less.
Cr: 0.8% or more
Cr is an element that improves hardenability and is an extremely important element for the formation of a martensite phase. Furthermore, Cr has a higher effect of improving hardenability than Mn and has a small solid solution strengthening ability, so it is effective for lowering YP, and is actively added in the present invention. If the Cr content is less than 0.8%, the above hardenability improving effect and low YP effect may not be obtained, so the Cr content is 0.8% or more. In addition, as described above, Mn has a limited amount of addition from the viewpoint of achieving both BH characteristics and room temperature aging resistance, and thus suppresses the formation of pearlite and bainite in the cooling process after annealing, thereby reducing YP. Therefore, it is necessary to control to a predetermined Mn equivalent by adjusting the Cr amount.
Mn + 1.3Cr: 1.9% to 3.0%
The value of Mn + 1.3Cr is one of the indexes representing hardenability, and it is important to control within an appropriate range in order to generate a martensite phase. If the value of Mn + 1.3Cr is less than 1.9%, the hardenability becomes insufficient, pearlite and bainite are likely to occur during cooling after annealing, and YP increases. On the other hand, if the value of Mn + 1.3Cr exceeds 3.0, the martensite phase increases and YP increases, so there is a concern about deterioration of surface strain resistance. Further, the addition of an excessive alloy element causes an increase in manufacturing cost. Therefore, the value of Mn + 1.3Cr is 1.9% to 3.0%, and preferably from more than 2.2% to 3.0% from the viewpoint of low YP.
With the above essential additive elements, the steel of the present invention can achieve the desired characteristics, but in addition to the above essential additive elements, the following elements can be added as necessary.

One or more of Mo: 0.15% or less, V: 0.5% or less, B: 0.01% or less, Ti: 0.1% or less, Nb: 0.1% or less
Mo: 0.15% or less
Mo is an element effective in improving hardenability and stably obtaining a martensite phase, and can be added in an amount of 0.01% or more. However, if the amount of Mo exceeds 0.15%, the effect is saturated and the cost is significantly increased. Therefore, when adding Mo, it is 0.15% or less.
V: 0.5% or less, B: 0.01% or less
V and B are hardenability improving elements, and can be added in an amount of 0.01% or more and 0.0005% or more, respectively, in order to stably produce a martensite phase. However, even if these elements are added excessively, an effect commensurate with the cost cannot be obtained. Therefore, when V and B are added, the content is made 0.5% or less and 0.01% or less, respectively.
Ti: 0.1% or less, Nb: 0.1% or less
Ti and Nb can be added in an amount of 0.005% or more in order to form carbonitrides to reduce the amount of dissolved C and N and prevent deterioration of aging. However, even if they are added excessively in excess of 0.1%, the effect is saturated and an effect commensurate with the cost cannot be obtained. Therefore, when adding Ti and Nb, respectively, it is made 0.1% or less.
The remainder other than the above consists of Fe and inevitable impurities. As an unavoidable impurity, for example, O forms non-metallic inclusions and adversely affects quality, so it is desirable to reduce O to 0.003% or less.

Next, the structure of the high-strength cold-rolled steel sheet having low yield stress and excellent room temperature aging resistance and bake hardenability according to the present invention will be described.
Ferrite phase and martensite phase with area ratio of 2% to 15% The high-strength cold-rolled steel sheet of the present invention has a two-phase structure of ferrite phase and martensite phase with area ratio of 2% to 15%. By controlling the martensite phase within this range, the surface distortion resistance and work hardenability are enhanced, and a steel plate that can be used for automotive outer panel applications is obtained. When the area ratio of the martensite phase exceeds 15%, the strength is remarkably increased, and, for example, it does not have sufficient surface distortion resistance and press formability as a steel sheet for automobile inner and outer panels targeted by the present invention. Therefore, the area ratio of the martensite phase is 15% or less. On the other hand, when the area ratio of the martensite phase is less than 2%, YPEl tends to remain, and the yield stress (YP) increases, resulting in deterioration of surface strain resistance. Accordingly, the area ratio of the martensite phase is 2% to 15%, preferably 2% to 10%.
In the steel sheet of the present invention, a pearlite phase, a bainite phase, a residual γ phase, and an inevitable carbide other than the two phases of the ferrite phase and the martensite phase may be included if the total area ratio is about 2%. .
In addition, the above area ratio is obtained by polishing the L cross section of the steel sheet (vertical cross section parallel to the rolling direction), corroding with nital, observing 12 fields of view at a magnification of 4000 times with SEM, and analyzing the photographed structure photograph. Can be sought. In the structure photograph, ferrite is a region with a slightly black contrast, the region where carbides are generated in a lamellar or dot array is pearlite and bainite, and particles with white contrast are martensite.
Further, for example, by appropriately controlling the Mn equivalent and the cooling condition after annealing, the structure can be controlled within the above-described area ratio range.

Next, a method for producing a high-strength cold-rolled steel sheet having low yield stress, excellent room temperature aging resistance and bake hardenability according to the present invention will be described.
The high-strength cold-rolled steel sheet of the present invention is made by melting steel adjusted to the above-described chemical composition range into a slab, then after hot rolling (pickling), cold rolling, then annealing and cooling Can be obtained. Here, annealing is performed at an annealing temperature of more than 750 ° C. and less than 820 ° C., and cooling is primarily cooled at an average cooling rate of 2 ° C./s to 20 ° C./s in the temperature range from the annealing temperature to 650 ° C., The temperature range from 650 ° C to the temperature near the Ms point given by Tc (° C) (= 410-40 × Mn (mass%)-30 × Cr (mass%)) with an average cooling rate of 10 ° C / s or more Secondary cooling is performed, and further, the temperature range from the temperature Tc to 200 ° C. is subjected to tertiary cooling at an average cooling rate of 0.2 ° C./s to 10 ° C./s.
Here, the method for melting steel is not particularly limited, and an electric furnace or a converter may be used. Moreover, the casting method of the steel after melting may be a slab by a continuous casting method, or may be a steel ingot by an ingot forming method.
When the slab is hot-rolled after continuous casting, the slab may be reheated in a heating furnace and then rolled, or may be rolled directly without heating the slab. In addition, the steel ingot may be ingot-rolled and then subjected to hot rolling and then subjected to hot rolling.
Hot rolling can be carried out according to a conventional method, for example, the heating temperature of the slab is 1100 to 1300 ° C, the finishing rolling temperature is Ar3 point or higher, the cooling rate after finishing rolling is 10 to 200 ° C / s, and the winding temperature is It can be set to 400-750 degreeC. About a cold rolling rate, it can carry out by 50 to 85% within a normal operation range.
Hereinafter, details of the annealing and cooling processes important in the present invention will be described.
Annealing temperature: An annealing temperature of more than 750 ° C. and less than 820 ° C. needs to be heated to an appropriate temperature in order to obtain a microstructure of a ferrite phase and a martensite phase. When the annealing temperature is 750 ° C. or lower, a predetermined amount of martensite phase cannot be obtained because the austenite phase is not sufficiently generated. For this reason, the surface strain resistance deteriorates due to YPEl remaining or YP increase. On the other hand, if the annealing temperature is 820 ° C. or higher, the amount of dissolved C in the ferrite may be reduced, and a high BH amount may not be obtained. Therefore, the annealing temperature is more than 750 ° C. and less than 820 ° C.
Primary cooling rate: 2 ° C / s or more and 20 ° C / s or less The primary cooling rate from the annealing temperature to 650 ° C is an average cooling rate of 2 ° C / s or more and 20 ° C / s or less. When the cooling rate is less than 2 ° C./s, the growth of ferrite becomes remarkable, a predetermined amount of martensite phase cannot be obtained, and YP increases due to the remaining YPEl. On the other hand, when the cooling rate exceeds 20 ° C./s, the concentration of C, Mn, and Cr in the austenite becomes insufficient, and the amount of pearlite and bainite increases and YP increases in the subsequent cooling process. Therefore, to reduce the YP, the average cooling rate from the annealing temperature to 650 ° C. should be 2 ° C./s or more and 20 ° C./s or less.
Secondary cooling rate: 10 ℃ / s or more
The secondary cooling rate from 650 ° C. to the temperature near the Ms point given by Tc is 10 ° C./s or more in terms of average cooling rate. When the secondary cooling rate is less than 10 ° C / s, pearlite or bainite is generated around 400 to 500 ° C, and YP increases. On the other hand, the upper limit of the secondary cooling rate is not specified, but if it exceeds 100 ° C / s, the solid solution C in the ferrite remains unreduced and shows a high BH, but the normal temperature aging resistance deteriorates and the YP increases. Invite. Therefore, the secondary cooling rate is preferably 10 ° C./s or more and preferably 100 ° C./s or less. Here, the temperature is given by Tc = 410-40 × Mn-30 × Cr, and the element symbol indicates mass%.
Third cooling rate: 0.2 ℃ / s or more and 10 ℃ / s or less
The temperature range from the temperature in the vicinity of the Ms point given by Tc to 200 ° C should be cooled at an average cooling rate of 0.2 ° C / s or more and 10 ° C / s or less. As a result, it is possible to lower the YP by precipitating excessively dissolved solid solution C in the ferrite. However, if it exceeds 10 ° C / s, solute C in ferrite cannot be reduced and YP may increase. Moreover, while it becomes high BH, deterioration of normal temperature aging resistance is caused. Therefore, the tertiary cooling rate is 0.2 ° C./s or more and 10 ° C./s or less.
Furthermore, in the present invention, the steel sheet of the present invention can be subjected to temper rolling for shape correction after heat treatment. Further, in the present invention, it is assumed that the steel material is manufactured through normal steelmaking, casting, and hot rolling processes. However, for example, thin casting or the like omits part or all of the hot rolling process. You can also

Hereinafter, the present invention will be further described by examples.
Steels having chemical components of steels A to R shown in Table 1 were melted by vacuum melting to produce slabs. These slabs were heated at 1200 ° C, then hot rolled at a finishing temperature of 870 ° C, cooled, and then wound at 600 ° C to produce a hot-rolled steel sheet having a thickness of 2.5 mm. The obtained hot-rolled steel sheet was pickled and then cold-rolled at a rolling rate of 70% to obtain a cold-rolled steel sheet having a thickness of 0.75 mm.
Next, after the sample cut out from the cold-rolled steel plate obtained above was annealed under the conditions of 770 ° C. × 90 seconds in an infrared image furnace, the annealing temperature to 650 ° C. was 1 at an average cooling rate of 5 ° C./s. Next cooling, then secondary cooling to the temperature near the Ms point given by Tc at an average cooling rate of 20 ° C / s, and further from the temperature near the Ms point given by Tc to 200 ° C to an average cooling rate of 1 ° C After tertiary cooling at / s, temper rolling with an elongation of 0.5% was performed.
For the cold-rolled steel sheet obtained as described above, a sample was taken, the area ratio of the martensite phase was measured, the tensile properties, work hardening amount (WH), bake hardening amount (BH), and after the accelerated aging test The yield point elongation (YPEl) was measured. The detailed measurement method is shown below.
(1) Area ratio of martensite phase: L-section (vertical section parallel to the rolling direction) of the sample was mechanically polished and corroded with nital, then 12 times at a magnification of 4000 using a scanning electron microscope (SEM) The field of view was observed and quantified using the photographed tissue photograph (SEM photograph). Here, the particles having white contrast were martensite and the remaining particles having black contrast were ferrite, and the ratio of martensite to the whole was determined.
(2) Tensile properties: JIS No. 5 test specimens were taken in the 90 ° direction (C direction) with respect to the rolling direction, and subjected to a tensile test in accordance with the provisions of JIS Z 2241, yield stress (YP) and tensile strength (TS ) Was measured.
(3) Work hardening (WH): The stress difference between the stress after 2% pre-strain and the yield stress (YP) was measured.
(4) Bake hardening amount (BH): The stress difference between the stress after 2% pre-strain and the yield stress after applying heat treatment equivalent to 170 ° C x 20 minutes baking was measured.
(5) Yield point elongation after accelerated aging test (YPEl): After heat treatment at 100 ° C. for 24 hours, a tensile test (based on JIS Z 2241) was performed to measure YPEl. The accelerated aging conditions are set so that the equivalent aging time calculated by the Hundy equation is 1.2 years at 30 ° C and about 2 months at 50 ° C, assuming that the equator is exceeded when exporting steel sheets, for example. did.
The above measurement results are shown in Table 1 together with the manufacturing conditions.

In Table 1, Nos. 1 to 15 (steel A to O) are examples of the present invention having a structure in which the components and production conditions are within the scope of the present invention, and the area ratio of the martensite phase is 2% to 15%. In the examples of the present invention, low YR, high BH and YPEl after aging are not observed.
On the other hand, Nos. 16 to 18 (steel P to R) whose components depart from the scope of the present invention are inferior in YR, BH, or YPEl after aging.
Specifically, No. 16 (steel P) has a high Mn content and a high amount of solute C, and thus has a high BH, but has a high YPEl after aging. Further, since the value of Mn + 1.3Cr is low, pearlite and bainite are generated during cooling after annealing, and a predetermined amount of martensite cannot be secured, so that YR is high and YPEl after aging is high. Since No. 17 (steel Q) has a large amount of Mn, the amount of solute C in ferrite is small and BH is low. Moreover, since ferrite is solid-solution strengthened, YP is high and surface distortion resistance is inferior.
Since No. 18 (steel R) has a low C content, a predetermined amount of martensite cannot be obtained, YR is high, and YPEl after aging is also high.

Steels having chemical components of steels B, G, and J shown in Table 1 were melted by vacuum melting, and subjected to hot rolling, pickling, and cold rolling under the same conditions as in Example 1. After maintaining at the annealing temperature shown for 90 seconds, primary cooling, secondary cooling, tertiary cooling and temper rolling were performed under the conditions shown in Table 2.
For the cold-rolled steel sheet obtained from the above, a sample was taken, and in the same manner as in Example 1, the area ratio of the martensite phase, further tensile properties, work hardening (WH), bake hardening ( BH) and YPEl after accelerated aging test.
The obtained results are shown in Table 2 together with the production conditions.

In Table 2, Nos. 19, 20, 23, 24, and 27 are examples of the present invention having a structure in which the components and production conditions are within the scope of the present invention, and the area ratio of the martensite phase is 2% to 15%. In the examples of the present invention, low YR, high BH and YPEl after aging are not observed.
On the other hand, since No. 21 has a low annealing temperature, a predetermined amount of martensite phase cannot be obtained, YR is high, and YPEl after aging is also high.
Since No. 22 has a high annealing temperature, the amount of C dissolved in the ferrite is reduced and BH is low. In No. 25, since the primary cooling rate is slow, the growth of ferrite becomes remarkable, a predetermined amount of martensite phase cannot be obtained, YPEl remains, and YP increases. Moreover, YPEl after aging is also high, and the room temperature aging resistance is inferior.
Since No. 26 has a fast primary cooling rate, element concentration in austenite becomes insufficient, and pearlite and bainite are likely to be formed during subsequent cooling. As a result, the martensite area ratio obtained after cooling decreases, YR increases, and YPEl after aging also increases.
Since No. 28 has a slow secondary cooling rate, austenite decomposes into pearlite and bainite around 400 to 500 ° C., and the amount of these products increases, so the martensite area ratio obtained after cooling decreases. For this reason, YR is high and YPEl after aging is also high.
Since No. 29 has a fast tertiary cooling rate, solid solution C in ferrite cannot be reduced, and YR is higher than that of the invention example of the same strength level. Moreover, since there is much solid solution C, there exists a possibility of causing deterioration of normal temperature aging resistance, while becoming high BH.

  The high-strength cold-rolled steel sheet of the present invention has low yield stress and excellent room temperature aging resistance and bake hardenability. It is preferably used in a required field.

Claims (3)

  Ingredient composition is C: 0.01% or more and less than 0.08%, Si: 0.2% or less, Mn: 0.8% or more, 1.7% or less, P: 0.10% or less, S: 0.03% or less, Al: 0.1% or less, N : Containing 0.008% or less, Cr: 0.8% or more, and satisfying 1.9 ≦ Mn (mass%) + 1.3Cr (mass%) ≦ 3.0, the balance is composed of iron and inevitable impurities, and the structure is the ferrite phase A high-strength cold-rolled steel sheet having a martensite phase with an area ratio of 2 to 15%.   Furthermore, it contains at least one of Mo: 0.15% or less, V: 0.5% or less, B: 0.01% or less, Ti: 0.1% or less, and Nb: 0.1% or less in mass%. Item 2. A high-strength cold-rolled steel sheet according to Item 1. A steel slab having the component composition according to claim 1 or 2 is hot-rolled and cold-rolled, and then annealed at an annealing temperature of more than 750 ° C. and less than 820 ° C., and then the temperature range from the annealing temperature to 650 ° C. Primary cooling is performed at an average cooling rate of 2 ° C / s or more and 20 ° C / s or less, and then the temperature range from 650 ° C to the temperature Tc given by the following equation (1) is set at an average cooling rate of 10 ° C / s or more. A method for producing a high-strength cold-rolled steel sheet, characterized in that the secondary cooling is performed, and then the temperature range from the temperature Tc to 200 ° C is tertiary-cooled at an average cooling rate of 0.2 ° C / s or more and 10 ° C / s or less.
Tc (℃) = 410-40 × Mn-30 × Cr (1)
Here, the element symbol in the formula represents the element content in steel in mass%.