JP3419000B2 - High-strength cold-rolled steel sheet excellent in deep drawability and secondary work brittle resistance, and method for producing the same - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength steel sheet and a method for producing the same, which are suitable for applications such as automobiles where relatively severe processing is performed. Such a high-strength steel sheet is, for example, as a steel sheet for automobiles, in order to reduce the thickness of the vehicle body while ensuring the required strength and reduce the weight of the vehicle body and the fuel consumption thereof, and further improve the strength of each member. This is useful for improving reliability and safety. Thus, in recent years, the momentum for the preservation of the global environment and the improvement of passive safety has been increasing, and it has received a great deal of attention.

[0002]

2. Description of the Related Art Conventionally, as a cold-rolled steel sheet having excellent formability, as described in, for example, JP-A-56-139654, an ultra-low carbon steel is used as a base to improve workability and aging. In order to achieve this, many steel sheets have been proposed in which a carbonitride forming component such as Ti or Nb is contained, and a reinforcing component such as P is contained within a range that does not impair the workability to enhance the strength. However, the tensile strength (TS) of these steel sheets was at most 40 kgf / mm ² , and there was a limit to the increase in strength. Therefore, in order to further strengthen the steel plate,
For example, Japanese Patent Application Laid-Open No. 59-193221 proposes a cold-rolled steel sheet further containing Si.
It is difficult to avoid another problem resulting from the large content of (i.e., mainly surface quality problems (for example, deterioration of chemical conversion processability and deterioration of plating property), and it cannot be used at all as a target automobile steel sheet. Further, with respect to a steel sheet of a component system containing a large amount of P as a reinforcing component, there is a problem that the secondary work brittleness resistance is deteriorated.

Regarding cold-rolled steel sheets for use in automobile panels and the like, the ferrite single-phase structure steel as described above has both high r-value, high ductility, normal temperature non-aging property and high BH property. Cold-rolled steel sheet with different composite structure is specially 3-222
No. 4 is disclosed. However, there is a problem in that it is difficult to achieve stable manufacturing because the annealing temperature dependency after cold rolling is large.

[0004]

According to the present invention, as a high strength steel sheet used for automobiles or the like, particularly, the tensile strength is 40 kg.
The main development purpose is to exceed f / mm ² . In such a high-strength steel sheet, it is, of course, necessary to satisfy not only the workability (mechanical properties) but also all the properties required for its use, such as platability and secondary work brittleness resistance. It is a requirement. Some of these characteristics are in conflict with each other, but the other characteristics are required to be even better without being inferior to the conventional steel sheet.
Among them, the secondary work brittleness resistance is apt to deteriorate when the workability is improved, so that it is required to satisfy extremely severe conditions compared with other properties.

In order to satisfy the above conditions, the present invention uses ultra-low carbon steel as a basic component and contains Si, P, and Mn, which are solid solution strengthening components, in appropriate amounts to achieve the target strength. Solid solution strengthening and controlling the contents of Ti, Nb, and B to secure a stable material, and controlling the Ni content to improve the material quality of the composite structure. It is an object of the present invention to propose a high-strength cold-rolled steel sheet having excellent secondary working brittleness and further having bake hardenability, and a method for producing the same.

[0006]

The skeleton of the present invention has a C content of 0.0005 to 0.0050 wt% in order to improve workability.
% Of ultra-low carbon steel as a basic component, the strength is increased by the addition of P and Si, more preferably Mo, and a relatively large amount of Mn is added to stabilize the material In order to obtain a good composite structure steel and to secure the secondary work brittleness resistance, a relatively large amount of Ni is used.
By containing B and B and controlling the Ti and Al contents to be smaller than in the conventional case, the workability is further improved by preventing the impurity phase from intervening in the steel more than necessary.

In the manufacturing process, hot rolling, cold rolling, and annealing are appropriately performed on the component steel having the above composition, including heating and cooling conditions during each process.

That is, the present invention is C: 0.0005 to 0.0050
wt% (hereinafter simply referred to as%), Si: 0.10 to 1.50%, Mn: 1.
00 to 3.50%, Ni: 0.020 to 1.000%, Ti: 0.015 to 0.10
0%, Nb: 0.003 to 0.040%, B: 0.0005 to 0.0050%,
Al: 0.030% or less, P: 0.040 to 0.150%, S: 0.010
% Or less and N: 0.0050% or less, the above Si, Mn, Ni and P
The respective contents of% Si,% Mn,% Ni and% P are

[Formula 5] A = 100 ×% P + 80 ×% Si−60 × {% Mn +% Ni} The content of A satisfies the condition of -50 ≦ A <0, and the balance is Fe and unavoidable. The composition of the impurities and the volume ratio of polygonal ferrite 3
% High-strength cold-rolled steel sheet (first invention) having excellent deep drawability and secondary work brittleness resistance.

The present invention also provides C: 0.0005 to 0.0050%,
Si: 0.10 to 1.50%, Mn: 1.00 to 3.50%, Ni: 0.020 to 1.
000%, Ti: 0.015 to 0.100%, Nb: 0.003 to 0.040
%, B: 0.0005 to 0.0050%, Al: 0.030% or less, P: 0.
040 to 0.150%, S: 0.010% or less, and N: 0.0050% or less with the respective contents of Si, Mn, Ni and P% Si,% Mn,%.
Ni and% P are the following formulas

[Equation 6] A = 100 x% P + 80 x% Si -60 x {% Mn +% Ni} A value calculated to satisfy -50 ≤ A <0, with the balance being Fe and unavoidable. After continuous casting, the steel slab having the composition of static impurities is heated to 1150 to 1300 ° C without being cooled to below 300 ° C, hot-rolled, and finish rolling is performed in the temperature range of 700 to 1000 ° C. Water cooling is started within 3 seconds after finishing rolling, followed by cooling at a cooling rate of 30 ° C / s or more and winding at 500 to 680 ° C, followed by pickling, followed by cold rolling with a rolling reduction of 70% or more. Rolling, further soaking at 900 ℃ or more for 10 to 300 seconds to make the volume fraction of austenite phase at soaking 10 to 70%, and then continuously cooling at a cooling rate of 20 ℃ / s or more and 400 ℃ or less. A method for producing a high-strength cold-rolled steel sheet (second invention) excellent in deep drawing property and secondary work brittleness resistance, which comprises quenching to a temperature range. It

Further, the present invention contains Mo: 0.0150 to 0.5000% in addition to the respective component compositions of the first and second inventions (third and fourth inventions).

[0011]

The function of the present invention will be described. The inventors have set the tensile strength of the steel plate to 40 to 60 kgf / mm ² ,
Various studies have been carried out based on ultra-low carbon steel, and as a result, characteristics such as secondary work brittleness resistance can be improved by containing Ni and B after leaving an appropriate amount of solute C. Also, by containing a relatively large amount of P, the workability represented by the r value can be improved and strengthened, and further Si, M
It was found that by optimizing the combination of n, Ni, and P component contents, the deterioration of various surface properties due to Si content can be suppressed to a minimum. It was also found that the structure of the steel sheet having such good characteristics is composed of polygonal ferrite and a low temperature transformation phase in a predetermined ratio. Further, they have found that a composite microstructure steel having such a good material can be stably obtained by controlling the annealing temperature within an appropriate range and controlling the cooling conditions after annealing, and arrived at the present invention.

The reasons for limiting the component composition range and the manufacturing process in the present invention will be described below. C: 0.0005 to 0.0050% It is desirable to reduce the C content from the viewpoint of improving the elongation and the r value, but if it is less than 0.0005%, the resistance to 2
Subsequent processing is not preferable because it deteriorates brittleness and reduces the strength of the welded portion (welding heat affected zone). Also, industrially, it is not worth the cost to reduce it to less than 0.0005%. On the other hand, when the C content exceeds 0.0050%, even if the equivalent amounts of Ti and Nb are contained, a remarkable effect of improving the material (ductility and r value) cannot be obtained, and there is a possibility that inconvenience may occur in hot rolling and other manufacturing processes. It is not preferable because it is remarkable Therefore, the C content is 0.
It was limited to the range of 0005 to 0.0050%.

Si: 0.10 to 1.50% As the Si content, first, 0.10% was made the lower limit as a limit at which a sufficient strengthening effect can be obtained. The Si content may be basically adjusted according to the target level of tensile strength,
When the content of Al exceeds 1.50%, the hot rolled base plate is significantly hardened, so that the cold ductility is deteriorated and the chemical conversion processability is also significantly deteriorated. Further, various internal defects tend to increase, which is not preferable. Therefore, the upper limit of the Si content is set to 1.50%.

Mn: 1.00 to 3.50% Mn, when contained alone, deteriorates the mechanical properties after cold rolling annealing, especially the r value, but it is used in combination with other components to
When it is contained in the range of 00 to 3.50% and satisfying the correlation equation described later, the strength can be increased without causing remarkable deterioration of the material. Mn content here is 1.00%
If it does not meet the requirement, it cannot be strengthened sufficiently, while 3.50
%, The steel sheet is significantly hardened, resulting in great difficulty in the cold rolling process. Therefore, the Mn content is 1.00 to 3.50.
It was limited to the range of%.

Ni: 0.020 to 1.000% Since Ni is an expensive component, it has not been contained in the prior art, but it has an important meaning in the present invention.
In other words, the inclusion of Ni not only improves the secondary work brittleness resistance, but also increases the strength and improves the workability by limiting the content to the prescribed range specified by the relationship with Si, Mn, and P. It is possible to improve the sex. Further, as the most important effect, by containing Ni, a composite structure steel can be stably obtained. These effects become remarkable when Ni is contained in 0.020% or more and tend to be saturated when Ni is contained in excess of 1.000% .Therefore, the Ni content is 0.020 to 1.000% in consideration of a significant increase in cost. And the range.

Ti: 0.015 to 0.100% Ti is an essential component for improving the r value. Ti's
The effect of improving r-value becomes remarkable when the content is 0.015%, but 0.10%
Even if the content exceeds 0%, the effect is saturated, and the deterioration of the surface properties becomes remarkable. Therefore, the lower limit of Ti content is limited to 0.015% and the upper limit is limited to 0.100%. Note that, for the first time in the present invention, it was discovered that the above effect is particularly remarkable in the case of P-containing steel.

Nb: 0.003 to 0.040% Composite addition of Ti and Nb is one of the important conditions in the present invention. By containing 0.003% or more of Nb, a higher r value can be obtained as compared with the case of containing Ti alone. Further, the inclusion of Nb has an effect of suppressing abnormal grain growth during annealing, which is advantageous for obtaining a uniform and fine composite structure steel sheet. However, when Nb is contained in an amount of more than 0.040%, the hot-rolled base plate is significantly hardened, which not only hinders the cold-rolling process, but also tends to deteriorate the mechanical properties after annealing. If the Nb content is less than 0.003%, the effect cannot be obtained. Therefore, the Nb content is 0.003 to 0.040%.
Limited to.

B: 0.0005 to 0.0050% B is one of the important components in the present invention. According to the conventional publicly known document, it has been reported that the content of B exerts a great effect on the secondary work brittleness of steel, but at the same time, deterioration of the material (mainly r value) is inevitable. It was being appreciated. However, although the cause / mechanism of the composite structure steel of the present invention is not clear, the secondary work brittleness resistance is remarkably improved as compared with the case of the ferrite single phase structure. Therefore, the problem of secondary processing brittleness can be solved even if the B content is substantially 0, but in terms of stability during the manufacturing process described later, B is contained in an amount of 0.0005% or more (0.0050% or less). It is more advantageous, and deterioration of the material is small in this range. However, this effect is saturated at 0.0050%, and depending on the annealing conditions, it is rather gratifying that workability deteriorates. Therefore, the B content is limited to 0.0005 to 0.0050%.

Al: 0.030% or less Al is also an important component in the present invention. The detailed mechanism is unknown, but the content is lower than that of the conventional one, 0.03%.
When the content is 0% or less, the composition-based steel of the present invention has an effect of improving workability. The material tends to be improved as the amount of Al is reduced, but if the content falls below 0.001% in general, inclusions increase and the workability eventually decreases accordingly. Therefore, the tentative lower limit is 0.00
1%, but if the inclusions are sufficiently removed,
It is presumed that even Al-free steel does not deteriorate in properties.

P: 0.040 to 0.150% The content of P has a very important meaning in the present invention.
That is, it has been found that by including P, although the detailed mechanism is unknown, the workability (mainly r value) is significantly improved while the strength is increased. This effect is remarkable when the content is 0.040% or more. In addition, P
If the content exceeds 0.150%, the segregation during solidification becomes extremely strong, resulting in saturation of the increase in strength and deterioration of workability. It causes a significant deterioration, and deteriorates to a level that is practically unusable. Therefore, the upper limit was set to 0.150%.

S: 0.010% or less S is a component to be reduced as much as possible in the present invention. By reducing the amount of S, it contributes to the reduction of the precipitates in the steel to improve the workability and the effective amount of Ti that fixes C. Such an effect can be obtained by setting the S content to 0.010% or less.

N: 0.0050% or less N is a component to be reduced as much as possible in the present invention. By reducing the N content, the material (especially ductility, r
Value) can be expected to improve. However, if it is reduced to 0.0050% or less, almost satisfactory effect can be obtained.
It was set to 0.0050%.

Mo: 0.0150 to 0.5000% In the third and fourth inventions, Mo is 0.0150 to 0.5000%.
It is contained in the range of. Mo is an effective component for improving the strength, but if its content is less than 0.0150%, it has the disadvantage that the target strength increasing effect cannot be obtained, while if it exceeds 0.5000%, it is As a result of the hardened remarkably hardened base plate, there arises a disadvantage that cold rolling becomes difficult. Therefore, the Mo content is set to the range of 0.0150 to 0.5000%.

For the above Si, Mn, Ni and P, the respective contents% Si,% Mn,% Ni and% P are

## EQU00007 ## It is necessary that the A value calculated by A = 100.times.% P + 80.times.% Si-60.times. {% Mn +% Ni} is a content satisfying -50≤A <0. By including each component in the range satisfying this condition, the detailed mechanism is unknown, but the required high strength is obtained, and the r value is hardly deteriorated, and high strength and high r value are obtained. Steel sheets can be manufactured. FIG. 1 is a graph showing the effect of the A value on the average r value of the steel sheet. This steel plate is
Various continuous cast slabs with different Si, Mn, Ni and P contents were heated to 1180 to 1280 ℃ (maintained at 350 ℃ or higher after continuous casting), and then finish rolling temperature: 880 ℃ Rolled,
Start quenching within 2 seconds after finish rolling, cooling rate approx. 35 ℃ /
After cooling for 5 seconds and winding at 540 ° C, cold rolling with a rolling reduction of 73% to a plate thickness of 0.7 mm, annealing at 900-1000 ° C for 20 seconds, and subsequent cooling rate of 35 ° C / second It is manufactured by cooling to 320 ℃. It is clear from FIG. 1 that by maintaining the A value in the range of −50 ≦ A <0, it is possible to manufacture a steel sheet having a high r value.

The cold-rolled steel sheet according to the present invention having the above composition range has a structure of polygonal ferrite and a volume ratio of 3
It is characterized by having a composite structure with a low temperature transformation phase of not less than%. In this way, by using a composite structure of polygonal ferrite and a low temperature transformation phase with a volume ratio of 3% or more using ultra-low carbon steel as a material, a high r value can be maintained despite high strength being obtained. Of. Here, the low temperature transformation phase is a structure similar to that of martensite bainite that appears in conventional low carbon steel, but it is a structure that appears in the transformation of 700 ° C. or less in the steel of the present invention. Further, if the volume ratio of the low-temperature transformation phase is less than 3%, the desired sufficient strengthening effect cannot be obtained, so the volume ratio of the low-temperature transformation phase is limited to 3% or more. Note that the upper limit of the volume ratio of the low temperature transformation phase is preferably about 70% or less from the viewpoint that if it is excessive, ductility is significantly deteriorated and it is not preferable.

Next, the reasons for limiting each manufacturing condition in the preferred method for manufacturing a cold rolled steel sheet according to the present invention will be described. -Slab heating temperature: 1150 to 1300 ° C If the heating temperature of the continuous casting slab prior to hot rolling is less than 1150 ° C, it is difficult to secure a sufficiently high hot rolling finishing temperature as described below. However, if this hot rolling temperature is secured, it is advantageous from the viewpoint of the material to further lower the slab heating temperature. However, since the load during hot rolling will also increase, the lower limit is set to 1150 ° C assuming the current equipment. On the other hand, if the slab heating temperature exceeds 1300 ° C, the properties of the steel sheet surface will deteriorate significantly in the end. Therefore, the upper limit was set to 1300 ° C. Further, in the steel of the present invention, since the low temperature toughness of the slab is deteriorated, it is necessary to avoid lowering the temperature to 300 ° C. or lower before charging into the slab heating furnace. This also contributes to homogenization of the structure.

Finishing rolling temperature: 700 to 1000 ° C. The finishing rolling temperature is required to be at least 700 ° C. in order to improve workability represented by r value after cold rolling and annealing.
When hot-rolled at a temperature of less than 700 ° C., the rolling structure in the hot-rolled sheet remarkably remains, and finally a texture undesired for workability is formed, which is not preferable. On the other hand, if the finish rolling temperature exceeds 1000 ° C., the roll damage of the rolling mill becomes large, which is a serious obstacle to actual production. It is also disadvantageous in terms of collective organization. Therefore, the finish rolling temperature of the hot rolling was set in the range of 700 to 1000 ° C.

Cooling condition after hot rolling For cooling after hot rolling, water cooling is started within 3 seconds after finishing rolling, and subsequently at a cooling rate of 30 ° C / s or more, at 500 to 680 ° C described below. It is necessary to cool to the coiling temperature. If air cooling is performed for more than 3 seconds after finishing rolling, Ti
The precipitation of phosphides of the steel markedly progresses, and not only the strength of the steel sheet but also the workability such as r value is deteriorated. Further, unless the rapid cooling is continued at a cooling rate of 30 ° C./s or more, precipitation of Ti phosphide will remarkably progress and the material will deteriorate. In addition, although the detailed mechanism is unknown, the final material, especially the r value, deteriorates probably because the transformation mechanism during cooling after hot rolling changes.

Winding temperature: 500 to 680 ° C. If the winding temperature is less than 500 ° C., the unevenness of cooling causes disorder of the plate shape, which interferes with pickling and cold rolling in the next step. From the viewpoint of the material, it is considered that the precipitation of TiC is excessively suppressed, but the material is deteriorated. On the other hand, when the winding temperature exceeds 680 ° C, Ti phosphide is formed and not only the deterioration of the material but also the deterioration of the pickling property with the increase of the scale thickness becomes remarkable. Furthermore, various problems due to the surface concentration of Si etc. will become apparent. Therefore, the winding temperature was set in the range of 500 to 680 ° C.

Cold rolling reduction rate: 70% or more The cold rolling reduction rate after pickling is limited to 70% or more. If it is less than 70%, the r value is not improved and sufficient deep drawability cannot be obtained.
It is preferably 80% or more.

Annealing conditions Annealing conditions are defined as conditions under which a composite structure showing a stable and good material can be obtained. It is necessary to make the volume fraction of the austenite phase during soaking at 10 to 70% by soaking at 900 ° C or higher for 10 to 300 seconds. By setting the volume fraction of the austenite phase at the time of soaking to 10 to 70%, the final strength and r value can be kept within a suitable range. Although the detailed mechanism is unknown, it is considered that the transformation texture was optimized under the conditions of the present invention steel and the present invention, and a texture desirable for deep drawability was obtained. On the other hand, when the soaking temperature is lower than 900 ° C, the normal polygonal ferrite structure is formed, and the polygonal ferrite targeted in the present invention is obtained.
A composite structure of ferrite and low temperature transformation phase cannot be obtained. In addition, the bake hardenability is low. Similarly, if the soaking time is less than 10 seconds, there is a disadvantage that the structure during soaking becomes unstable and the material changes easily.On the other hand, if it exceeds 300 seconds, surface thickening and surface treatment deterioration due to high temperature annealing deteriorate. It causes the inconvenience. In any case, although the detailed mechanism is unknown, from the relationship between the development of the texture accompanying the transformation during annealing and soaking, and the stability of austenite during cooling,
It is estimated that the optimum range exists for the annealing conditions. In addition, when high temperature annealing is performed, in addition to the remarkable surface concentration of additive components (for example, Si, Al, etc.),
Since it becomes difficult to maintain stable annealing,
It is estimated that the limit will be 00 ° C.

Cooling conditions: The cooling rate from the temperature in the above-mentioned annealing to 400 ° C.
20 ℃ / sec or more. . If the cooling rate is less than 20 ° C / sec, a stable composite structure cannot be obtained. Further, unless such cooling is rapidly cooled to 400 ° C. or less, the change in r value is small, but the desired high strength cannot be obtained and sufficient bake hardenability cannot be obtained, which is not preferable. By rapidly cooling to a sufficiently low temperature range at a sufficient cooling rate, a high-strength composite structure steel sheet having a good material can be manufactured.

[0033]

Example 1 Steels having various compositional compositions shown in Table 1 were melted in a converter, and a cold-rolled steel sheet having a thickness of 0.7 mm was produced under the following conditions, and its mechanical properties were evaluated. investigated.

[0034]

[Table 1]

Slab heating temperature: 1200 to 1280 ° C. (maintained at 350 ° C. or higher after continuous casting) Finishing rolling temperature: 750 to 850 ° C. Cooling condition: Start quenching within 2 seconds after finishing rolling, cooling at about 30 ° C./second Winding temperature: 600 ℃ Cold rolling reduction: 76% Annealing condition: 920 ℃, 30 seconds Soaking cooling condition: Cooling from annealing temperature to 280 ℃ at a cooling rate of 30 ℃ / second

The tensile properties of the obtained cold-rolled steel sheet were evaluated by a usual test method using JIS No. 5 tensile test pieces. Also,
Regarding the secondary processing brittleness, after conical cups drawn with a drawing ratio of 2.0 were flange cut, a 5 kg weight was dropped from a height of 80 cm at various temperatures to give an impact load, and The upper limit temperature at which various cracks occur is evaluated. If this temperature is approximately -45 ° C or lower, it can be determined that there is no problem in a normal use environment. BH (bake hardenability) was evaluated by the amount of increase in deformation stress when aged at 170 ° C for 20 minutes after 2% prestrain in a tensile test. AI (aging index) was also evaluated by the amount of increase in deformation stress when aging at 100 ° C x 1 hr after pre-straining 7.5%. The results thus obtained are shown in Table 2.

[0037]

[Table 2]

As is clear from Table 2, the examples according to the present invention have superior El. (Ductility) and high r as compared with the comparative examples.
It can be seen that the steel plate has a good workability and high tensile strength.
Also, regarding the secondary working brittleness resistance, the brittleness temperature is as good as −100 ° C. or less, and it can be said that it has desirable characteristics from the viewpoint of reliability as a strength member. The major difference from the cold-rolled steel sheet annealed in the ferrite region (less than 900 ° C) is that the steel of the present invention has a high BH property, but has a delayed aging property at room temperature. This is clear from the fact that BH is large and AI is small in Table 2. Further, it can be seen that the secondary processing brittleness resistance can be further improved by forming a composite structure.

Example 2 Steels having the components shown in Table 3 were melted in a converter and manufactured with actual equipment under various manufacturing conditions shown in Table 4 to obtain cold-rolled steel plates having a plate thickness of 0.7 mm. Various tests were conducted on these steel sheets to investigate various properties. The results thus obtained are also shown in Table 4.

[0040]

[Table 3]

[0041]

[Table 4]

As is apparent from Table 4, the conforming examples manufactured under the conditions of the present invention have better characteristics than the comparative examples. Further, when the chemical conversion treatment and the plating adhesion of the electroplating were performed on these steel sheets, the levels were almost the same as those of conventional cold-rolled steel sheets for automobiles.

[0043]

EFFECTS OF THE INVENTION The cold-rolled steel sheet according to the present invention has high strength, excellent deep drawability and resistance to secondary work embrittlement, and further has bake hardenability. It is especially useful in applications such as.

[Brief description of drawings]

FIG. 1 is a graph showing the effect of an A value on the average r value of a steel sheet.

─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-6-10094 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C22C 38/00 301 C21D 8/04 C21D 9 / 48 C22C 38/14

Claims (4)

(57) [Claims] 1. C: 0.0005 to 0.0050 wt%, Si: 0.10 to 1.50 wt%, Mn: 1.00 to 3.50 wt%, Ni: 0.020 to 1.000 wt%, Ti: 0.015 to 0.100 wt%, Nb: 0.003 to 0.040. wt%, B: 0.0005 to 0.0050 wt%, Al: 0.030 wt% or less, P: 0.040 to 0.150 wt%, S: 0.010 wt% or less and N: 0.0050 wt% or less of the above Si, Mn, Ni and P. Each content% Si,% Mn,% Ni
And% P are contained under the condition that A = 100 ×% P + 80 ×% Si-60 × {% Mn +% Ni} and the value of -50 ≦ A <0 is satisfied. , The balance is composed of Fe and unavoidable impurities, and the volume ratio is 3 with polygonal ferrite.
% Cold rolled steel sheet with deep drawability and secondary work embrittlement resistance that form a structure with a low temperature transformation phase of not less than%. 2. C: 0.0005 to 0.0050 wt%, Si: 0.10 to 1.50 wt%, Mn: 1.00 to 3.50 wt%, Ni: 0.020 to 1.000 wt%, Ti: 0.015 to 0.100 wt%, Nb: 0.003 to 0.040. wt%, B: 0.0005 to 0.0050 wt%, Al: 0.030 wt% or less, P: 0.040 to 0.150 wt%, S: 0.010 wt% or less and N: 0.0050 wt% or less of the above Si, Mn, Ni and P. Each content% Si,% Mn,% Ni
And% P are contained under the condition that A = 100 ×% P + 80 ×% Si-60 × {% Mn +% Ni} is satisfied and -50 ≦ A <0 is satisfied. , The balance consists of Fe and unavoidable impurities, and the steel slab has a composition of 1150 ~ 13 without being cooled down to below 300 ℃ after continuous casting.
Finish at the temperature range of 700 to 1000 ℃ by heating to 00 ℃ and hot rolling, finish water rolling within 3 seconds after finishing rolling, and continue cooling at a cooling rate of 30 ℃ / s or more. And then pickling, followed by cold rolling with a rolling reduction of 70% or more, and soaking at 900 ° C or more for 10 to 300 seconds to obtain the austenite phase during soaking. Of 10% to 70% in volume, and subsequent rapid cooling to a temperature range of 400 ° C or less at a cooling rate of 20 ° C / sec or higher, high strength with excellent deep drawability and secondary processing brittleness resistance. Manufacturing method of cold rolled steel sheet. 3. C: 0.0005 to 0.0050 wt%, Si: 0.10 to 1.50 wt%, Mn: 1.00 to 3.50 wt%, Ni: 0.020 to 1.000 wt%, Mo: 0.0150 to 0.5000 wt%, Ti: 0.015 to 0.100. wt%, Nb: 0.003 to 0.040 wt%, B: 0.0005 to 0.0050 wt%, Al: 0.030 wt% or less, P: 0.040 to 0.150 wt%, S: 0.010 wt% or less and N: 0.0050 wt% or less Content of Si, Mn, Ni and P% Si,% Mn,% Ni
And% P are contained under the condition that A = 100 ×% P + 80 ×% Si-60 × {% Mn +% Ni} is satisfied and -50 ≦ A <0 is satisfied. , The balance is composed of Fe and unavoidable impurities, and the volume ratio is 3 with polygonal ferrite.
% Cold rolled steel sheet with deep drawability and secondary work embrittlement resistance that form a structure with a low temperature transformation phase of not less than%. 4. C: 0.0005 to 0.0050 wt%, Si: 0.10 to 1.50 wt%, Mn: 1.00 to 3.50 wt%, Ni: 0.020 to 1.000 wt%, Mo: 0.0150 to 0.5000 wt%, Ti: 0.015 to 0.100. wt%, Nb: 0.003 to 0.040 wt%, B: 0.0005 to 0.0050 wt%, Al: 0.030 wt% or less, P: 0.040 to 0.150 wt%, S: 0.010 wt% or less and N: 0.0050 wt% or less Content of Si, Mn, Ni and P% Si,% Mn,% Ni
And% P are contained under the condition that A = 100 ×% P + 80 ×% Si-60 × {% Mn +% Ni} is satisfied and -50 ≦ A <0 is satisfied. , The balance consists of Fe and unavoidable impurities, and the steel slab has a composition of 1150 ~ 13 without being cooled down to below 300 ℃ after continuous casting.
Finish at the temperature range of 700 to 1000 ℃ by heating to 00 ℃ and hot rolling, finish water rolling within 3 seconds after finishing rolling, and continue cooling at a cooling rate of 30 ℃ / s or more. And then pickling, followed by cold rolling with a rolling reduction of 70% or more, and soaking at 900 ° C or more for 10 to 300 seconds to obtain the austenite phase during soaking. Of 10% to 70% in volume, and subsequent rapid cooling to a temperature range of 400 ° C or less at a cooling rate of 20 ° C / sec or higher, high strength with excellent deep drawability and secondary processing brittleness resistance. Manufacturing method of cold rolled steel sheet.