JP3424619B2 - High tensile cold rolled steel sheet and method for producing the same - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a door impact beer.
Suitable for parts for ensuring collision safety of automobiles such as bumpers and bumper reinforcements.
The present invention relates to a high-tensile cold-rolled steel sheet having a formability of 0.8 or less and a tensile strength of 980 N / mm ² or more and excellent delayed fracture properties, and a method for producing the same.

[0002]

^2. Description of the Related Art In recent years, a high-strength cold-rolled steel sheet having a tensile strength of 980 N / mm ² or more has been strongly desired from the viewpoint of automobile safety measures and weight reduction measures. However, as the strength of the steel sheet increases, deterioration of formability and deterioration of delayed fracture characteristics due to hydrogen embrittlement become a problem.

For example, Japanese Patent Application Laid-Open No. 7-102341 discloses, as a method for realizing an ultrahigh strength cold-rolled steel sheet having a tensile strength of 980 N / mm ² or more with improved hydrogen embrittlement properties, "Ti-added cold-rolled steel sheet On the other hand, continuous annealing in which a martensite transformation is generated by rapidly cooling to a temperature below the Ms point with a slight gradual cooling period from the soaking state of Ac ₃ points or more and then tempering at 150 to 300 ° C for 1 to 15 minutes is performed. The method of realizing a structure centered on tempered martensite in which the amount of ferrite is 20% or less in area ratio and which does not have coarse carbides in the structure is shown by this method. Cold rolled steel sheet has a high yield ratio and low elongation.
It does not fully satisfy the above-mentioned moldability required for automobile parts.

Further, Japanese Unexamined Patent Publication No. 8-134549 discloses a method of producing an ultra-high strength thin steel sheet having improved hydrogen embrittlement characteristics by performing the same treatment on a Ca-added cold-rolled steel sheet. However, it was difficult to obtain a high-strength cold-rolled steel sheet having both satisfactory formability and delayed fracture properties by this method.

Further, Japanese Patent Application Laid-Open No. 9-111396 discloses that "Ti or Nb is added as a ferrite refining element, the hot rolling conditions of steel and the heat treatment conditions after cold rolling are devised, and the structure of the steel sheet is refined. Impact resistance (dynamic yield strength) by forming a two-phase structure of fine ferrite and fine martensite
The invention of "high-strength steel sheet with improved strength" is disclosed. However, even with this technology, it is not possible to combine high-strength steel sheet with a tensile strength of 980 N / mm ² or more, which has a problem of delayed fracture, with sufficient formability. It was difficult.

In view of the above, the object of the present invention is to provide a high tensile strength of 980 N / mm ² or more, a low yield ratio, good elongation and bendability, and excellent delayed fracture. It is to establish a means for realizing a high-strength cold-rolled steel sheet that also has characteristics.

[0007]

[Means for Solving the Problems] Accordingly, the inventors of the present invention have conducted extensive studies to achieve the above-mentioned object, and as a result, "high tensile strength of 980 N / mm ² or more, low yield ratio, good elongation and bending High tensile strength cold rolled steel sheet that combines excellent properties and excellent delayed fracture characteristics, the chemical composition of the steel sheet is adjusted, and the continuous annealing conditions after cold rolling are strictly adjusted in relation to the chemical composition of the steel sheet.
It is possible to realize the structure of the steel sheet by making fine ferrite, fine martensite, and austenite coexist at a predetermined ratio. "

The present invention has been completed based on the above findings and the like, and provides the following high-strength cold-rolled steel sheet and its manufacturing method. (1) C: 0.10 to 0.20% by weight, Si: 0.8% or less, Mn: 1.6 to 2.7%, P: 0.03% or less, S:
0.010% or less, Al: 0.005 to 0.10%, N: 0.0020 to 0.0
080%, Ti: made _{^{{48/14 N (%)}} +0.005} balance substantially Fe and inevitable impurities with containing 0.12%,
And the volume ratio of ferrite having a particle size of 5 μm or less is 30 to 7
2% martensite with 0% and particle size of 5 μm or less in volume ratio
0-60%, and austenite 3-20% by volume
980N when the yield ratio is 0.8 or less.
High-strength cold-rolled steel sheet with a tensile strength of / mm ² or more and excellent in formability and delayed fracture properties. (2) Weight ratio C: 0.10 to 0.20%, Si: 0.8% or less, Mn: 1.6 to 2.7%, P: 0.03% or less, S:
0.010% or less, Al: 0.005 to 0.10%, N: 0.0020 to 0.0
^{080%, Ti: {48/} 14 N (%) +0.005} comprise 0.12%, further Cr: 0.01~ 1.0%, Mo: 0.01~0.50%,
Nb: 0.003 to 0.08%, V: 0.003 to 0.08%, B: 0.
0001 to 0.0050%, Ca: 0.0001 to 0.01%, Cu: 0.01 to 0.
50%, Ni: 0.01 to 0.50% of 1 type or more, the balance substantially consisting of Fe and unavoidable impurities, and a ferrite having a particle size of 5 μm or less in a volume ratio of 30 to 70% and a particle size of 5 μm or less Volume ratio of martensite of 20-60
%, And austenite in a volume ratio of 3 to 20%, a high-strength cold-rolled steel sheet having a yield ratio of 0.8 or less and a tensile strength of 980 N / mm ² or more and excellent in delayed fracture property. . (3) C: 0.10 to 0.20% by weight, Si: 0.8% or less, Mn: 1.6 to 2.7%, P: 0.03% or less, S:
0.010% or less, Al: 0.005 to 0.10%, N: 0.0020 to 0.0
^{080%, Ti: {48/} 14 N (%) +0.005} a billet balance being substantially Fe and inevitable impurities with containing 0.12%, after cold rolling and hot rolling, the following It is heated for 5 seconds to 5 minutes at the temperature T (° C) shown by the equation (a), and in the cooling process,
From 600 to 750 ° C, it is cooled to 200 to 420 ° C at a cooling rate of 10 to 200 ° C / s, and then 200 to 420 ° C.
A method for producing a high-strength cold-rolled steel sheet excellent in formability and delayed fracture property as described in the above item (1), which is characterized by holding at 80 ° C. for 80 seconds to 5 minutes and then cooling to room temperature. T ₁ +60 ≧ T ≧ T ₁ (a) where 900 ≧ T and T ₁ = 600 + 70 × [Mn
(%) + 11 × {Ti (%) - 48/14 N (%)} ] (4) at weight ratio C: 0.10~0.20%, Si: 0.8 % or less, Mn: 1.6~ 2.7%, P : 0.03 % Or less, S:
0.010% or less, Al: 0.005 to 0.10%, N: 0.0020 to 0.0
^{080%, Ti: {48/} 14 N (%) +0.005} comprise 0.12%, further Cr: 0.01~ 1.0%, Mo: 0.01~0.50%,
Nb: 0.003 to 0.08%, V: 0.003 to 0.08%, B: 0.
0001 to 0.0050%, Ca: 0.0001 to 0.01%, Cu: 0.01 to 0.
50%, Ni: 0.01 to 0.50% of one or more, and the balance consisting essentially of Fe and unavoidable impurities, hot-rolled and cold-rolled, and then the temperature shown by the following formula (b) T
(° C) for 5 seconds to 5 minutes, during the cooling process, 600
From 750 ℃ to 20 ℃ at a cooling rate of 10-200 ℃ / s
Formability and delayed fracture property according to the above item (2), characterized in that it is cooled to 0 to 420 ° C., then held in a temperature range of 200 to 420 ° C. for 80 seconds to 5 minutes and then cooled to room temperature. A method for producing a high-strength cold-rolled steel sheet excellent in heat resistance. T ₁ +60 ≧ T ≧ T ₁ (b) where 900 ≧ T and T ₁ = 600 + 70 × [Mn
(%) + Cr (%) + 2Mo (%) + 11 × {Ti (%) - 48/14 N (%)
^{} + Nb (%) + 1} /2 V (%) + 80B (%) ]

[0009]

The reason why the chemical composition and structure of the steel sheet and the manufacturing conditions are limited as described above in the present invention will be explained together with the operation. [A] Chemical composition a) C: C is an important element for imparting high tensile strength to a steel sheet, and in order to secure the desired tensile strength, the C content is 0.
It is necessary to set it to 10% or more (hereinafter,% representing the component amount is weight%). However, if C is contained in excess of 0.20%, the toughness and weldability deteriorate. Therefore, the C content is set to 0.10 to 0.20%, preferably 0.12 to 0.18%.

B) Si: Si is also an element effective in increasing the strength of the steel sheet, but if the Si content exceeds 0.8%, the deterioration of the chemical conversion treatment becomes noticeable. Therefore, the Si content is set to 0.8% or less, preferably 0.5% or less, more preferably
0.3% or less is good.

B) Mn: Mn has the function of facilitating the formation of martensite by stabilizing austenite and ensuring high tensile strength of the steel sheet, but the Mn content is 1.6%.
If it is less than the above range, martensite is not sufficiently formed and a desired tensile strength cannot be secured. However, the Mn content is 2.7%
If it exceeds, the band structure develops, and the formability, especially the bendability decreases. Therefore, although the Mn content is set to 1.6 to 2.7%, it is preferably adjusted to 1.9 to 2.4%.

C) P: P is an undesired impurity element that segregates at the grain boundaries and deteriorates the delayed fracture characteristics of the steel sheet, and is also an element that forms a band structure and adversely affects bendability. Therefore, the P content is set in the range of 0.03% or less at which these adverse effects are not remarkable, but is preferably 0.0
It is preferably 20% or less, more preferably 0.015% or less. d) S: S is also an undesirable impurity element that forms MnS and deteriorates the bendability of the steel sheet. Furthermore, since hydrogen that collects in MnS is the starting point, delayed fracture tends to progress,
S is an element also involved in the deterioration of delayed fracture characteristics. Therefore, the S content is set to 0.010% or less at which these adverse effects do not become remarkable, but is preferably 0.0040% or less, and more preferably 0.0015% or less.

E) Al: Al is an element added as a deoxidizing agent for steel, but if its content is less than 0.005%, the deoxidizing effect is insufficient, while if it exceeds 0.10%, it is contained. Since the effect is saturated and it is economically disadvantageous, the Al content is
It was set at 0.005 to 0.10%.

F) Ti: Ti is one of the important elements in the present invention because it brings about the effect of improving delayed fracture characteristics and bendability of the steel sheet through the grain refining action. That is, Ti
Combines with N in the steel and precipitates as TiN, which suppresses coarsening of crystal grains. Ti also bonds with C in steel.
Since TiC is finely precipitated and delays recrystallization, it contributes to the refinement of the crystal grain also from this point, and the precipitated carbide acts to suppress the grain growth and prevents the coarsening of the crystal grain. When the grain boundary area increases due to the refinement of the crystal grains, the concentration of the impurity element that causes delayed fracture at the grain boundary and the precipitation density of carbide at the grain boundary decrease, which results in good delayed fracture resistance. In addition, in a structure in which fine crystals are mixed, stress concentration in a specific phase during bending is relieved, which improves the bendability. However, in order to exert these effects, the Ti content must be strictly {
It is necessary to secure _{^{48/14 N (%) +}} 0.005}% or more. However, if Ti is contained in excess of 0.12%, weldability problems occur. Therefore, Ti content is _{^{"{48/14 N (%)}} +0.00
5} to 0.12% ”.

G) N: N combines with Ti as described above to give T
Since it becomes iN and has the effect of suppressing the coarsening of crystal grains, a content of 0.0020% or more is secured, but if it exceeds 0.0080%, it causes cracking of the slab. Therefore, the N content is set to 0.0020 to 0.0080%.

H) Cr, Mo: Cr and Mo, like Mn, have the effect of stabilizing martensite by stabilizing austenite and contributing to higher tensile strength of the steel sheet. It is an element added as necessary. However, the effect is not sufficient if the Cr and Mo contents are less than 0.01%, and the Cr content is 1.0% and the Mo content is 0.50%.
If the content of Cr is 0.01 to 0.01%, the effect is saturated and the cost becomes disadvantageous even if the content exceeds 0.01%.
In the case of adding Mo to 1.0%, its content is 0.01 to 0.50%
Specified in each.

I) Nb, V, B: Nb, V and B all have an action of delaying recrystallization and refining crystal grains, and are elements added as necessary. However, the effect is that the Nb content is less than 0.003% and the V content is
If the B content is less than 0.003% and the B content is less than 0.0001%, the Nb content is 0.08% and the V content is 0.08%.
However, even if the B content exceeds 0.0050%, the above effect is saturated and the cost becomes disadvantageous. Therefore, when Nb is added, the content is set to 0.003 to 0.08%, when V is added, the content is set to 0.003 to 0.08, and when B is added, the content is set to 0.0001 to 0.01%. It was

J) Ca: Ca is an element added as necessary because it has a function of combining with S in steel to make sulfides spherical and improve bendability and delayed fracture resistance. If the content is less than 0.0001%, the effect due to the above action is insufficient, and even if the content exceeds 0.01%, the effect is saturated and it is disadvantageous in terms of cost, so the Ca content is 0.0001 to
It was set at 0.01%.

K) Cu, Ni: Cu and Ni each have a corrosion inhibiting effect and have an action of suppressing the invasion of hydrogen which is concentrated on the surface and causes delayed fracture. It is an added element. However, the effect is that Cu and Ni
Also, if the content is less than 0.01%, it is not sufficient, and in any case, if the content exceeds 0.50%, the effect is saturated and the cost becomes disadvantageous. Therefore, when Cu is added, the content is 0.01 to 0.50%, and when Ni is added, the content is 0.
It was set to 01 to 0.50 respectively.

[B] Structure The cold-rolled steel sheet according to the present invention has a volume fraction of ferrite having a grain size of 5 μm or less of 30 to 70% and a volume of martensite having a grain size of 5 μm or less of 20 to 60%. It is important that the structure has austenite in a volume ratio of 3 to 20%, whereby high tensile strength of 980 N / mm ² or more, and
It has a yield ratio of 0.8 or less leading to excellent formability, good elongation, good bendability, and excellent delayed fracture resistance.

That is, the structure of the cold-rolled steel sheet is "30 μm each of fine ferrite having a grain size of 5 μm or less and martensite.
To 70% by volume and 20 to 60% by volume, the "concentration at the grain boundary of the impurity element such as P that causes delayed fracture" and "precipitation of carbide at the grain boundary" can be obtained. "Density" decreases, and this leads to ensuring excellent delayed fracture resistance. In order to sufficiently suppress delayed fracture, it is necessary to suppress the precipitation of carbides, and for that purpose, the main structure of cold-rolled steel sheet is required. It is important to use ferrite and martensite instead of bainite or pearlite.

Further, 30 to 70% by volume of ferrite is necessary to obtain a "yield ratio of 0.8% or less" which is advantageous for formability of steel sheet, and 20 to 60% by volume of martensite is 980 N / mm. Necessary to obtain a tensile strength of ² or more. By the way, refining the grain size of ferrite and martensite to 5 μm or less leads to the effect of alleviating the stress concentration on the ferrite during bending, and is important for ensuring good bendability.

For the above reasons, in the cold-rolled steel sheet according to the present invention, ferrite having a grain size of 5 μm or less is used in a volume ratio of 30 to 7 μm.
2% martensite with 0% and particle size of 5 μm or less in volume ratio
It has been determined that the content is 0 to 60%, but the particle size is preferably 5 μ
The volume ratio of ferrite having a particle size of m or less is preferably 40 to 60%, and that of martensite having a particle size of 5 μm or less is preferably 30 to 60%, more preferably 35 to 50%.
It is good to

The inventors of the present invention have studied the structure suitable for ensuring good elongation and excellent delayed fracture property in the cold-rolled steel sheet, and as a result, the tensile strength of the present invention of 980 N / mm ² or more was obtained. It has been found that the presence of retained austenite in a volume ratio of 3 to 20% is indispensable in order to secure the formability desired in the steel sheet for automobile door impact beams, bumper reinforcements and the like. It was This is because a proper amount of austenite improves elongation due to transformation-induced plasticity (TRIP), which makes it possible to improve the elongation by the above-mentioned JP-A-7-102341 and JP-A-9-111396.
It is possible to obtain a high-tensile steel plate having performance superior to that of the cold-rolled steel plate disclosed in the publication. However, if the amount of austenite is less than 3% by volume, the effect of improving elongation due to transformation-induced plasticity (TRIP) is not sufficient. On the other hand, austenite is a phase that has a high hydrogen absorption capacity and deteriorates the delayed fracture characteristics, and if the content thereof exceeds 20% by volume, the delayed fracture characteristics deteriorate significantly. Therefore, in the present invention, the content ratio of austenite is defined as 3 to 20% by volume, but preferably 5%.
It is good to adjust to ~ 15%.

The remaining phase of the cold-rolled steel sheet according to the present invention may be any of ferrite having a grain size of more than 5 μm, martensite having a grain size of more than 5 μm, bainite, or a mixed phase thereof. There is no problem.

[C] Steel Plate Manufacturing Conditions In the present invention, when manufacturing a cold-rolled steel sheet, first, a steel containing each of the above components and the balance being Fe and inevitable impurities is used in a converter, an electric furnace or Melt in an open hearth furnace. Further, the tailoring of the steel may be any of rimmed steel, capped steel, semi-killed steel or killed steel, and for the production of the billet, either by "ingot-segmentation rolling" or "continuous casting" means. There is no problem.

According to the method of the present invention, the steel slab is hot-rolled in the usual manner, further pickled, cold-rolled, and then heat-treated (continuous annealing). Tension cold rolled steel sheet is manufactured. That is, in the hot rolling, usually, a steel slab heated and held at 1100 to 1280 ° C. is continuously hot rolled, and finish rolling is finished at 750 to 950 ° C., and then a condition of winding in a temperature range of 500 to 700 ° C. Adopted. Incidentally, the steel billet charged into the heating furnace during hot rolling may be a slab that remains at a high temperature after casting,
A slab left at room temperature is also acceptable.

In the method of the present invention, the condition of heat treatment (continuous annealing) after cold rolling is extremely important. That is, in the method of the present invention, the steel sheet after cold rolling is T ₁ +60 ≧ T ≧ T ₁ where 900 ≧ T and T ₁ = 600 + 70 × {Mn +
Cr + 2Mo + 11 heated _{^{× (Ti- 48/14 N)}} + Nb + 1/2 V + 80B} becomes a temperature T represented by the formula (℃) in 5 seconds to 5 minutes,
10 to 200 from 600 to 750 ° C during the cooling process
The heat treatment is performed under the condition that the temperature is cooled to 200 to 420 ° C. at a cooling rate of ° C./s, then held in the temperature range of 200 to 420 ° C. for 80 seconds to 5 minutes, and then cooled to room temperature.

Here, the above formula is a formula obtained by an experiment by the inventors of the present invention, and "T ₁ + 60-
The temperature range of “T ₁ ” is a temperature range immediately above recrystallization, and is a temperature at which the recrystallized ferrite and the austenite produced become the finest. Therefore, by strictly setting the heating temperature during the heat treatment within this range, it becomes possible to finally achieve the desired fine ferrite and martensite accurately. However, when the heating temperature exceeds the "temperature at which the austenite transformation is completed", the above-mentioned structure refining effect becomes small, so the upper limit of the heating temperature is set to 90.
Limited to 0 ° C.

If the heating temperature during the heat treatment falls below the lower limit of the above formula, unrecrystallized ferrite remains and the formability deteriorates, while if the heating temperature exceeds the upper limit of the above formula, the grain growth suppressing effect becomes small. As a result, coarse martensite and coarse bainite are eventually formed, and the yield ratio increases, delayed fracture resistance deteriorates, and bendability decreases. Further, even if the particle size of the obtained ferrite and martensite satisfy the specified conditions of the present invention, if the heating temperature is above the upper limit of the above formula, the amount of austenite transformation during annealing becomes large. Since the C enrichment is insufficient and unstable austenite is formed, the final residual amount of austenite is reduced, and it is not possible to secure desired elongation characteristics for the steel sheet.

The heating time in this heat treatment (continuous annealing) is
It is necessary to adjust to 5 seconds to 5 minutes. If the heating time is less than 5 seconds, the recrystallization time is insufficient or the austenitization is insufficient, so that the structure defined in the present invention cannot be obtained.
On the other hand, if the heating time exceeds 5 minutes, the above effects are saturated, and there is a problem in equipment such that the heat treatment equipment is unnecessarily lengthened. Further, if the heating is performed for an excessively long time, the austenite is excessively stabilized and the amount of retained austenite exceeds the specified range of the present invention, and the delayed fracture property of the obtained cold rolled steel sheet deteriorates.

According to the method of the present invention, the annealed steel sheet has a temperature range of 600 to 750 ° C. to 200 to 200 ° C. in the cooling process.
It is cooled to a temperature range of 420 ° C at a cooling rate of 10 to 200 ° C / s. If the cooling rate in this region is too slow, a sufficient amount of martensite cannot be obtained and the yield ratio increases, so the formability of the cold rolled steel sheet deteriorates. Even if an amount of martensite within the range specified in the present invention is obtained, if the cooling rate is too slow, excessive concentration of C into austenite occurs and the austenite is stabilized, so that an excessive amount of martensite is obtained. Oh
Stenite remains and causes deterioration of delayed fracture characteristics. The effect of obtaining martensite is saturated even if the cooling rate in the above region exceeds 200 ° C / s. Therefore, after annealing, it is limited to cooling at a cooling rate of 10 to 200 ° C./s from a temperature range of 600 to 750 ° C. to a temperature range of 200 to 420 ° C., but the cooling rate in the above region is preferably 20 to
It is better to adjust the temperature to 120 ° C./s, more preferably 40 to 80 ° C./s. The cooling rate from the annealing temperature to the temperature range of 600 to 750 ° C. is not particularly limited, but usually,
It is gradually cooled at a cooling rate of about 2 to 20 ° C / s.

From the temperature range of 600 to 750 ° C., 10 to 20
Steel plates cooled at a cooling rate of 0 ° C / s once
The temperature is kept at 420 ° C. (preferably 240 to 360 ° C.) for 80 seconds to 5 minutes, and then cooled to room temperature. This short-time holding treatment is carried out in order to stabilize the structure of the obtained steel sheet and suppress the characteristic variation, but if the holding temperature is higher than 420 ° C., bainite and carbides may be produced instead of martensite. The delayed fracture characteristics of are greatly deteriorated. In addition, the holding temperature is 20
If the temperature is lower than 0 ° C, flatness is likely to occur. In addition, the above 2
Hold time for the process of holding at a temperature of 00 to 420 ° C for a short time is 8
If it is less than 0 seconds, it is not possible to sufficiently secure the effect of stabilizing the structure of the obtained steel sheet and suppressing the variation in characteristics. On the other hand, the effect is saturated even if a holding time of more than 5 minutes is secured.

By subjecting the steel of the specific composition specified in the present invention to the above-mentioned treatment, "at a yield ratio of 0.8 or less, 980 N / mm ²
It is possible to stably obtain a “high-strength cold-rolled steel sheet having excellent formability and delayed fracture characteristics exhibiting the above tensile strength”, but a skin pass having an elongation rate of 3% or less is applied to the steel sheet after the heat treatment for straightening. Even if it is applied, there is no problem in terms of characteristics. Of course, even if the surface of the steel sheet after these treatments is subjected to a surface treatment such as galvanization, the characteristics desired by the present invention are not particularly adversely affected.

The present invention will be described below with reference to examples.

EXAMPLES First, steel having the chemical composition shown in Table 1 was melted in a converter and then continuously cast into a slab. Then, the slab was heated to 1200 ° C. and then hot-rolled at a finishing temperature of 910 ° C. and a winding temperature of 570 ° C. to produce a hot-rolled steel sheet having a thickness of 2.6 mm. Then, this hot-rolled steel sheet was pickled and cold-rolled to a thickness of 1.2 mm.

[0036]

[Table 1]

Next, the steel sheet after cold rolling was annealed under the conditions shown in Table 2, cooled to 680 ° C. at a cooling rate of 5 ° C./s, and further cooled to the holding temperature at the cooling rate shown in Table 2. Then, a continuous annealing treatment was performed in which the holding temperature was maintained for 180 seconds. In addition, in the test numbers 8 and 15 in which the annealing time was long, the annealing treatment was performed in the laboratory.

[0038]

[Table 2]

Microstructural observation test pieces were taken from the cold-rolled steel sheet manufactured in this manner, and the metallic structure was observed by light source and SEM observation after nital corrosion. The retained austenite was measured by X-ray. In addition, a JIS No. 5 test piece and a bending test piece were sampled from the cold-rolled steel sheet in the direction perpendicular to the rolling direction, and a tensile test and a bending test were performed. Furthermore, after separately squeezing a test piece sampled with a squeezing ratio of 2.0, 0.5
% Sulfuric acid aqueous solution for 24 hours, and the delayed fracture property was evaluated by the number of cracks generated. Table 3 shows the metallographic structure, tensile properties, bending properties (limit bending radius) and delayed fracture properties (number of delayed fracture cracks) of each cold-rolled steel sheet investigated in this manner.

[0040]

[Table 3]

As is clear from the results shown in Table 3, the cold rolled steel sheet according to the present invention has a yield ratio of 0.8 or less and 1
The elongation is 5% or more and the limit bending radius is 2.0t or less, which shows that the formability is excellent. Also, the number of cracks in the delayed fracture test is excellent at 10 or less.

On the other hand, the cold-rolled steel sheets according to Test Nos. 1 and 32 having a low Ti content lacked fine ferrite and fine martensite, and the amount of retained austenite was insufficient. Inferior fracture characteristics.

Further, test numbers 4 and 12 having low annealing temperatures and
The cold-rolled steel sheet according to No. 20 and the cold-rolled steel sheet according to Test No. 6, which has a short annealing time, contained a large amount of coarse ferrite due to insufficient recrystallization during annealing and insufficient austenitization, and especially bendability,
It is inferior in delayed fracture characteristics. On the other hand, the cold-rolled steel sheets according to Test Nos. 5, 13, 21 and 31 having a high annealing temperature have a small amount of austenite and a low elongation.

The cold-rolled steel sheets according to Test Nos. 7, 14 and 22 having a slow cooling rate, and the cold-rolled steel sheets according to Test Nos. 9 and 16 having a high holding temperature have high yield ratio due to the formation of bainite containing carbide. Poor formability and delayed fracture resistance. The cold rolled steel sheet according to Test No. 30 having a low Mn content cannot obtain sufficient amounts of martensite and retained austenite, has a high yield ratio, and is inferior in formability and delayed fracture resistance. Furthermore, Si
The cold-rolled steel sheet according to Test No. 36, which had a high content, had no problems in formability, delayed fracture resistance, etc., but the chemical conversion treatability deteriorated.

[0045]

[Summary of Effects] As described above, according to the present invention, high tensile strength, excellent formability, and excellent delayed fracture characteristics are combined, and it is suitable as a reinforcing part for automobiles such as bumper reinforcements. It is possible to obtain a high-strength cold-rolled steel sheet, which is useful in industry.

Claims (4)

(57) [Claims] 1. A weight ratio of C: 0.10 to 0.20%, Si:
0.8% or less, Mn: 1.6 to 2.7%, P: 0.03% or less,
S: 0.010% or less, Al: 0.005 to 0.10%, N: 0.00
^{20~0.0080%, Ti: {48/} 14 N (%) +0.005} ~0.12
%, And the balance is essentially Fe and unavoidable impurities, and has a grain size of 5 μm or less in a volume ratio of 3%.
0 to 70%, martensite having a particle size of 5 μm or less is 20 to 60% by volume, and austenite is 3 to 3 by volume.
It has a yield ratio of 0.8 or less and 9
A high-strength cold-rolled steel sheet having a tensile strength of 80 N / mm ² or more and excellent in formability and delayed fracture properties. 2. A weight ratio of C: 0.10 to 0.20%, Si:
0.8% or less, Mn: 1.6 to 2.7%, P: 0.03% or less,
S: 0.010% or less, Al: 0.005 to 0.10%, N: 0.00
^{20~0.0080%, Ti: {48/} 14 N (%) +0.005} ~0.12
%, Cr: 0.01 to 1.0%, Mo: 0.01 to 0.50
%, Nb: 0.003 to 0.08%, V: 0.003 to 0.08%,
B: 0.0001 to 0.0050%, Ca: 0.0001 to 0.01%, Cu: 0.
01-0.50%, Ni: 0.01-0.50%, at least one of which is composed of Fe and unavoidable impurities, and has a grain size of 5 μm or less in a volume ratio of 30-70.
%, Martensite having a particle diameter of 5 μm or less is 20 in volume ratio.
.About.60%, and austenite in a volume ratio of 3 to 20%, and a yield ratio of 0.8 or less and 980 N / m.
A high-strength cold-rolled steel sheet with a tensile strength of m ² or more and excellent formability and delayed fracture properties. 3. A weight ratio of C: 0.10 to 0.20%, Si:
0.8% or less, Mn: 1.6 to 2.7%, P: 0.03% or less,
S: 0.010% or less, Al: 0.005 to 0.10%, N: 0.00
^{20~0.0080%, Ti: {48/} 14 N (%) +0.005} ~0.12
%, And the balance consisting essentially of Fe and unavoidable impurities is hot-rolled and cold-rolled, and then heated to a temperature T (° C.) represented by the following formula (a) for 5 seconds to 5 minutes. In the cooling process, it is cooled from 600 to 750 ° C to 200 to 420 ° C at a cooling rate of 10 to 200 ° C / s, and then 200 to
The method for producing a high-strength cold-rolled steel sheet having excellent formability and delayed fracture properties according to claim 1, which is characterized by holding at a temperature range of 420 ° C for 80 seconds to 5 minutes and then cooling to room temperature. T ₁ +60 ≧ T ≧ T ₁ (a) where 900 ≧ T and T ₁ = 600 + 70 × [Mn
(%) + 11 × {Ti (%) - 48/14 N (%)} ] 4. A weight ratio of C: 0.10 to 0.20%, Si:
0.8% or less, Mn: 1.6 to 2.7%, P: 0.03% or less,
S: 0.010% or less, Al: 0.005 to 0.10%, N: 0.00
^{20~0.0080%, Ti: {48/} 14 N (%) +0.005} ~0.12
%, Cr: 0.01 to 1.0%, Mo: 0.01 to 0.50
%, Nb: 0.003 to 0.08%, V: 0.003 to 0.08%,
B: 0.0001 to 0.0050%, Ca: 0.0001 to 0.01%, Cu: 0.
01 to 0.50%, Ni: 0.01 to 0.50% of one or more, and the balance substantially consisting of Fe and inevitable impurities, hot-rolled and cold-rolled. The sample is heated to the temperature T (° C) shown for 5 seconds to 5 minutes, and in the cooling process, 6
From 00 to 750 ℃, cool to 200 to 420 ℃ at a cooling rate of 10 to 200 ℃ / s, then 200 to 420 ℃
The method for producing a high-strength cold-rolled steel sheet having excellent formability and delayed fracture properties according to claim 2, characterized in that the temperature is maintained for 80 seconds to 5 minutes and then cooled to room temperature. T ₁ +60 ≧ T ≧ T ₁ (b) where 900 ≧ T and T ₁ = 600 + 70 × [Mn
(%) + Cr (%) + 2Mo (%) + 11 × {Ti (%) - 48/14 N (%)
^{} + Nb (%) + 1} /2 V (%) + 80B (%) ]