JPH05112845A - High strength cold rolled steel for deep drawing good in face shapeability after forming and having excellent dent resistance - Google Patents

Abstract

PURPOSE:To obtain a high strength cold rolled steel sheet for deep drawing excellent in strength, workability or the like, good in the face shapeability after forming and excellent in dent resistance by adding Mn, Cr, Ti, Nb or the like to a dead soft steel base to regulate its compsn. into a specified one. CONSTITUTION:This cover is a steel contg., by weight, 0.0005 to 0.01% C and <0.09% P, contg. one or two kinds of 0.5 to 3.0% Mn and 0.2 to 3.0% Cr and furthermore contg. one or two kinds of 0.005 to 0.10% Ti and 0.005 to 0.10% Nb. The steel has 15 to 25kgf/mm<2> yield strength, <=0.55 yield ratio, <=0.2%, yield elongation at the time of press forming, >=4kgf/mm<2> quantity of work hardening WH and 2 to 5kgf/mm<2> quantity of baking hardening BH at the time of coating hardening as well as excellent secondary workability of >=6kgf/ mm<2> WH+BH, >=1.5 average (r) value, >=35% T.El and -50 deg.C ductile-brittle transition temp. The above steel is subjected to cold rolling and annealing for one time, by which the excellent high strength cold rolled steel sheet for deep drawing constituted of a ferrite single phase structure can be obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention has a high strength and an excellent moldability, and has a high paint bake hardening ability and a tensile strength of 35 to 5.
It is related to 0kgf / mm ² grade high strength cold rolled steel sheet.

The high-strength cold-rolled steel sheet to which the present invention relates is used by press forming in automobiles, household electric appliances, buildings and the like. And, in a narrow sense, cold-rolled steel sheet that is not surface-treated, and for rust prevention, for example, Zn plating or alloyed Zn
Includes both cold-rolled steel sheets that have undergone surface treatment such as plating.
Since the steel sheet according to the present invention is a high-strength cold-rolled steel sheet for deep drawing that has good surface shape after forming and has excellent dent resistance, it is possible to reduce the sheet thickness in use, that is, to reduce the weight. And can contribute to global environment conservation.

Since the cold-rolled steel sheet according to the present invention is used for automobile panels and the like, first of all, excellent workability is required. There are various workability, but a) the surface shape of the molded product is good, especially the surface is free from surface distortion and the shape is well frozen, b) it can be molded without cracking, and c) the surface. It is essential that the stretcher strain does not appear in the.
For that purpose, YP-El at the time of press molding is preferably 0.2% or less and low YP. For example, from the viewpoint of surface strain, Y
It is necessary that P is 25 kgf / mm ² or less. Now TS
Assuming that the steel sheet is of 35 to 50 kgf / mm ² grade, a low YR material having a yield ratio (YR = YP / TS) of 0.55 or less is preferable. In terms of tensile properties, the propriety of forming is mainly the average plastic strain value (hereinafter referred to as the average r value) and the T.I. El., T. If El is 35% or more, the target component can be molded. On the other hand, from the viewpoint of strength, it is more preferable that the strength after press molding and baking is higher. This is because the product has excellent dent resistance. Here, the dent resistance means resistance to permanent dent deformation that occurs in a steel plate when a completed automobile is hit with stones or the like. For that purpose, first, it is well work-hardened at the time of pressing (high WH), and has a high BH property at the time of baking the paint, and the strength after baking (YP + W
H + BH) needs to be high. Here, in order to make it possible to reduce the plate thickness by using a high-strength steel plate, which is the final purpose, WH is 4 kgf / mm ² or more and BH is 2 kgf / mm ² or more, and therefore WH + BH is 6 kgf / mm ² More is required. However, in general, the amount of BH is 5 kgf / mm
^If it exceeds ^2, stretcher strain will occur, so be careful. Here, the WH amount is an amount obtained by subtracting YP from the deformation stress at the time of 2% tensile deformation, while the BH amount is reheated by heat treatment equivalent to coating baking of 170 ° C × 20 minutes after 2% prestrain. This is the amount of increase in stress when tension is applied.

As is apparent from the above situation, in order to be able to reduce the plate thickness of the panel steel sheet, it is necessary to have excellent strength and workability and satisfy many conditions at the same time.

[0005]

2. Description of the Related Art Many efforts have been made in the past for such purposes. The first is a composite structure strengthened cold rolled steel sheet. This steel sheet is a composite structure steel composed of a soft phase and a hard phase, and has a good balance between strength and ductility. further,
Since high-density mobile dislocations are locally present at the interface between the soft phase and the hard phase, it has a characteristic of being a low YR and high strength steel sheet with low YP and high strength. Further, such a movable dislocation suppresses the occurrence of yield point elongation even if strain aging occurs, so that it is non-aging and high B
It has the characteristic of being a steel sheet that also has H characteristics. However, the biggest drawback is that the average r value is low. This is because C and Mn are frequently used to form a hard phase, but on the other hand, these elements significantly reduce the average r value. Further, the presence of the hard phase causes a significant increase in the drawing resistance of the soft phase.

Among the composite structure steel sheets known to date, the ones having a high average r value are as follows. That is, Japanese Examined Patent Publication No. 2-6814 discloses a method of manufacturing a steel sheet having a high average r-value composite structure by batch annealing a cold rolled sheet and then continuously annealing the cold rolled sheet. The best tensile property value among the examples is YP:
^{20kgf / mm 2, TS: 40kgf} / mm 2, YR: 0.
5, average r value: 1.8 and YP: 22 kgf / mm ² , T
S: 44 kgf / mm ² , YR: 0.5, average r value: 1.6
Is. Although it is a steel plate with extremely good characteristic values,
Since it is a composite structure steel sheet, the BH amount is also expected to be 5 kgf / mm ² . On the other hand, in the case of a steel sheet having a ferrite single phase structure, which is the object of the present invention, there is a problem that non-aging does not occur when the BH amount exceeds 5 kgf / mm ² . Furthermore, in order to manufacture such a steel sheet, it is necessary to perform cold rolling and annealing twice, which complicates the process and increases the cost.

As another example of the composite structure steel sheet, Japanese Patent Publication No. 2-
Japanese Patent No. 5812 discloses a method for producing a high average r-value composite structure steel sheet composed of two phases of ferrite and bainite by continuous annealing. Typical tensile properties are YP: 23
^{kgf / mm 2, TS: 39kgf} / mm 2, YR: 0.65,
Average r value: 1.7, BH: 5.5 kgf / mm ² and Y
P: 25 kgf / mm ² , TS: 49 kgf / mm ² , YR:
0.58, average r value: 1.7, BH: 5.6 kgf / mm ²
Is. Although these characteristic values are excellent, YR is high as a steel plate for a panel, and therefore there is a possibility that a problem may occur in the surface shape of the molded product. Also, JP-A-63-23248
The publication discloses a method for producing a high-strength cold-rolled steel sheet composed of a ferrite phase and a martensite phase having a high average r value and BH property by continuous annealing followed by batch annealing. A typical characteristic value is YP: 38 kgf / mm ² ,
TS: 50 kgf / mm ² , YR: 0.76, average r value:
1.6, BH: 4.7 kgf / mm ² . But YP
Therefore, YR is high and there is a problem in moldability. Furthermore,
Japanese Patent Publication No. 3-2224 also discloses a composite microstructure cold-rolled steel sheet having excellent deep drawability, which is based on an ultra-low carbon steel to which Nb and B are added in combination, and a manufacturing method thereof. It is not a cold-rolled steel sheet with a ferrite single-phase structure. From the viewpoint of dent resistance, the WH amount is a very important characteristic, but it is usually not described in the above-mentioned publicly known documents.

On the other hand, it is well known that a high-strength cold-rolled steel sheet which is solution-strengthened with P or the like based on an ultra-low carbon steel to which Ti or Nb is added has both good strength and workability. In this case, the average r value is generally good, but P
Since it is reinforced with Si and Si, not only TS but also YP remarkably rises, which causes a problem in the surface shape property after molding. Furthermore, the average r
Since a small amount of C and N is fixed with Ti or Nb in order to improve the workability such as the value, it is difficult to impart the BH property. For example, Japanese Unexamined Patent Publication No. 56-139654 discloses a high-strength cold-rolled steel sheet excellent in formability by adding P to an Nb-added ultra-low carbon steel and a method for producing the same. Its typical characteristic value is YP: 25kg
f / mm ² , TS: 41 kgf / mm ² , YR: 0.61, average r value: 1.9, AI (aging index): 0 kgf / mm ² , but YP and YR are high and surface strain occurs. There is a fear. Further, it has no bake hardening ability. In addition, JP-A-56
Japanese Patent Publication No. 142852 discloses a high-strength cold-rolled steel sheet for deep drawing having a low yield ratio YR, but it is mainly reinforced with P and simply supplementarily reinforced with Mn, Mo, and Cr. The average r value is 1.6 or more and the YR is 0.6 or less, which shows good moldability, but since Ti is sufficiently added to C and N, there is a problem that BH property is not imparted at all.

Further, Japanese Patent Publication No. 61-276931 discloses a method for producing a bake hardenable steel sheet for ultra deep drawing using an ultra low carbon steel to which Ti and Nb are added in combination. In the above publication, the strength is increased by adding Si or P, and Ti is used only for fixing N.
Furthermore, since Nb is added to fix a part of C, BH property is imparted by utilizing excess solid solution C which is always present. Its typical characteristic value is YP: 21.5kg.
f / mm ² , TS: 42.5 kgf / mm ² , YR: 0.5
1, average r value: 2.0, BH: 7.1 kgf / mm ² . Stretcher strain and corresponding yield point elongation were evaluated under the aging condition of 38 ° C x 8 days,
It has not occurred under this condition. However, the BH amount of 5 kgf / mm ² or more described in the examples of the above-mentioned publication generally indicates the usual aging condition (promoted artificial aging condition: 1
If it is 00 ° C. × 1 h), stretcher strain may occur during pressing.

Furthermore, in JP-A-63-190141 and JP-A-64-62440, T
Mn of more than 0.9% and 3% or less based on i-added ultra-low carbon steel
And 0.04 to 0.15 of P, and optionally 0.05 to 1.2% of Si, a high-strength cold-rolled steel sheet having good formability and a method for producing the same are disclosed. ing. Typical characteristic values are YP: 27.1 kgf / mm ² , TS:
43.2 kgf / mm ² , YR: 0.63, average r value: 2.
15 and YP: 26.0 kgf / mm ² , TS: 42.1
kgf / mm ² , YR: 0.62, average r value: 2.35. However, since YP and YR are high, it is feared that surface strain will occur, and since Ti is added excessively with respect to C and N, it is expected that the amount of BH is extremely low even if it is present.

Even in the publicly-known documents cited above, it is usual that the WH amount which is important from the viewpoint of dent resistance is not described.

[0012]

From the above, the desirable high-strength steel sheet used for automobile panels and the like is a steel sheet which has not so high yield strength, is significantly work-hardened, and has a paint bake hardenability. .. Of course, when YP-El is 0.2% or less during press molding, stretcher strain does not occur, excellent workability such as average r value (deep drawing property) and elongation (overhang property), and secondary processing It also needs to be excellent.

The object of the present invention is to provide a yield strength (YP) of 15 to 25 kgf / mm ² and a yield ratio (YR = YP).
/ TS, TS: tensile strength) 0.55 or less, elongation at yield (YR-El) 0.2% or less during press molding, work hardening amount (WH) 4 kgf / mm ² or more, baking during coating baking Curing amount (BH) is ² to 5 kgf / mm ² , WH + BH and 6 kgf / mm ² or more, average r value is 1.5 or more, T.I. El
Of 35% or more and a ductility-brittleness transition temperature of -50 ° C or less, excellent secondary workability, good surface shape after forming, and excellent dent resistance. It is to be.

[0014]

[Means for Solving the Problems] In order to achieve the above-mentioned objectives, the inventors of the present invention have conducted earnest research and obtained the following new findings. That is, 35 to 50 kgf / mm ²
Assuming a panel steel sheet having a grade TS, the average r
From the viewpoint of workability such as value and elongation, it was judged that the steel sheet with a composite structure basically has a limit because of the reasons already described. Therefore, an ultra low carbon steel containing Ti and Nb, which have good workability, alone or in combination, was used as a base.

First, Si, P, Mn, and Cr, which are typical solid solution strengthening elements, are added to the above base steel, and tensile properties, particularly yield behavior after cold rolling, annealing, and temper rolling are added. The work hardening phenomenon was investigated in detail. As a result, Si and P, which have been frequently used as solid solution strengthening elements from the past, a) first increase the yield strength remarkably by adding a small amount, b)
As a result, it was found that the work hardening rate in the low strain region was significantly reduced.

On the other hand, when Mn and Cr, which have not been used so far as solid solution strengthening elements, are added, a) yield strength is hardly increased, b) tensile strength is increased, and c) as a result,
We obtained a very important new finding that the work hardening rate in the low strain region is increased by these additions. As a result of investigating these mechanisms as well, a) the yield strength is determined by the difference in atomic radius between the Fe element and the added X element, and increases as the difference in atomic radius increases. B) Work hardening rate is dislocation It was deeply related to the slip behavior of the above, and the basic principle was established that, when the stacking fault energy decreases due to the addition of the X element, dislocation cross slip becomes difficult and the work hardening rate increases.
According to this, Si and P have a remarkably smaller atomic radius than Fe, and therefore the difference in atomic radius becomes large, so that the yield strength increases remarkably, and Mn and Cr have an atomic radius very close to that of Fe, so that the yield strength changes almost. You can understand that you did not let me. On the other hand, although the influence on the stacking fault energy is not always clear, from the observation result of the dislocation structure after the initial work hardening, Si and P have almost no influence on the stacking fault energy within the investigated addition amount range, but Mn , Cr
For the first time, it became clear that there was a tendency to reduce this.

Due to the above mechanism, it is considered that when Mn and Cr are added, the yield strength hardly changes, the work hardening rate increases, and the tensile strength increases. Such characteristic behavior is due to the above-mentioned object of the present invention, that is, low YP and high W.
In order to achieve the characteristics of H, it means that it is difficult to add Si and P in the related art, and the addition of Mn and Cr is essential. Therefore, in the present invention, the positive use of Mn and Cr is the basic solution to the problems of the prior art. However, if only Mn and Cr are added, the desired strength may not be obtained, or the manufacturing cost may increase, so consideration should be given to the combined use with Si and P.

Secondly, the present inventors have obtained a new finding that coating bake hardenability is also improved by positively adding Mn and Cr. In the present invention, Ti and Nb are used to ensure workability.
Is used as a base, but in this case, since C and N are generally fixed to Ti and Nb, there is no bake hardening ability, or there is little.
This is because the affinity between C, N and Ti, Nb is very strong, so that (Ti + N
b) When the amount is larger than the amount of (C + N), the solid solution (C
This is because + N) hardly remains. However, even in such a case, the new fact that the BH property can be imparted by positively adding Mn and Cr suggests that the solubility products of TiC and NbC have changed. That is,
Since Mn and Cr have an interaction of attractive force with C, TiC
It is considered that the solid solution C in the matrix equilibrated with NbC and NbC is further stabilized and the solubility product of these precipitates is increased, and as a result, the amount of remaining solid solution C is increased and the BH property is improved. Therefore, addition of Mn and Cr can be utilized as a new means for imparting paint bake hardenability.

The solid solution C that contributes to the baking property of the coating is B
Since the grain boundaries are strengthened similarly to the above, it is also effective as a means for preventing secondary work embrittlement, which is known as a drawback of ultra-low carbon steel. It is well known that secondary work embrittlement is promoted by the presence of P segregated at the grain boundaries, but Mn, Cr
It has been also found that since P has an interaction with P and reduces the amount of P segregated, it is also effective in preventing secondary work embrittlement.

Nowadays, the trend toward the use of surface-treated steel sheets is remarkable, as the anticorrosion of automobile bodies is progressing. Surface treatment methods for steel sheets are roughly classified into electroplating and hot dipping. The former generally has good plating adhesion regardless of the type of steel sheet, but has a problem that it is difficult to make a thick basis weight and the cost becomes high. On the other hand, the latter does not have the drawback of the former, but depending on the type of steel sheet, the plating adhesion may be deteriorated or the Fe—Zn alloying reaction may be significantly affected. The present inventors have found that the steel of the present invention which suppresses the addition amounts of Si and P, which are often used as strengthening elements in conventional steel, and utilizes Mn and Cr, is alloyed and melted by a continuous hot dip galvanizing process of a Zenzimer system. In the manufacture of galvanized steel sheets, we also gained new knowledge with the following advantages. That is, since Si and P suppress the alloying reaction between Zn and Fe, when manufacturing a steel sheet containing a large amount of these elements, the line speed must be reduced and productivity must be reduced. Further, the addition of Si deteriorates the adhesion of plating and causes various problems during press molding. On the other hand, it was found that the addition of Mn and Cr does not have such an adverse effect. Although the mechanism is not always clear, a) Mn segregated on the surface changes the type and morphology of the oxide remaining after reduction to improve plating adhesion, and b) Mn.
It is speculated that Cr and Cr play a role of reducing the amount of P segregated at the grain boundaries through the interaction with P and normalizing the alloying reaction. This point was also utilized as a solution to the problems of the conventional method.

The present invention was constructed on the basis of such an idea and new knowledge, and the gist thereof is as follows.
% By weight, C: 0.0005 to 0.01%, P: 0.0
Contains less than 9%, Mn: 0.5-3.0%, Cr:
One or two of 0.2 to 3.0%, and T
i: 0.005-0.10% and Nb: 0.005-
A steel having a structure containing one or two of 0.10% and having a yield strength (YP) of 15 to 25 kgf / m.
m ² , the yield ratio (YR = YP / TS) is 0.55 or less, and the yield point (YP-El) during press molding is 0.2% or less,
In work hardening amount (WH) is 4 kgf / mm ² or more, bake hardening amount (BH) is in 2~5kgf / mm ^2, and WH + BH is 6kg
f / mm ² , average r value of 1.5 or more, T.F. El is 35% or more, and the ductility-brittleness transition temperature is -50 ° C or less and excellent secondary workability. Good surface shape after molding and excellent dent resistance. It is a strength cold rolled steel sheet. Further, this steel sheet has a ferrite single-phase structure and is obtained from one cold rolling / annealing.

[0022]

The reason why the components and the characteristic values are limited as described above in the present invention will be further described. C content: C is an extremely important element that determines the material properties of the product. The present invention is premised on a vacuum degassed ultra low carbon steel, but when C is less than 0.0005%, the grain boundary strength decreases, secondary work brittleness occurs, and the manufacturing cost significantly increases. Therefore, the lower limit is made 0.0005%.
On the other hand, when the C content exceeds 0.01%, the strength increases,
Since the moldability remarkably decreases, the upper limit is made 0.01%.

Mn content: Mn is an effective solid solution strengthening element that does not significantly increase the yield strength and increases the strength, and also imparts bake hardenability, prevents secondary work brittleness, chemical conversion treatability and Since it also has the effect of improving the hot-dip galvanizing property, it is positively added in the present invention. If the addition amount is less than 0.5%, the above-mentioned effects do not remarkably appear, so the lower limit is made 0.5%. On the other hand, if it exceeds 3.0%, the low temperature transformation product after annealing increases, the yield strength remarkably increases, and the ductility decreases. Further, the average r value also decreases, so the upper limit is made 3.0%.

Cr content: Cr and Mn are effective elements that increase the yield strength with almost no increase in yield strength.
In addition, since it imparts bake hardenability and prevents secondary work brittleness, it is positively used in the present invention. However, if the addition amount is less than 0.2%, the effect is not exhibited, so the lower limit value is made 0.2%. On the other hand, if it exceeds 3%, the pickling property of the hot-rolled sheet deteriorates or the chemical conversion treatment property of the product sheet deteriorates, so the upper limit is made 3%.

P content: P is known as an element that increases strength at low cost, and is often used in conventional solid solution strengthened high strength steel sheets. However, if the addition amount exceeds 0.90%, even if Mn and Cr are positively added as in the present invention, a) the yield strength is excessively increased and the surface shape defect is caused during pressing for panel applications. Or b) secondary processing embrittlement occurs. In addition, c) the alloying reaction becomes extremely slow during continuous hot-dip galvanizing, resulting in reduced productivity.
Therefore, the upper limit value is set to 0.90%. Ti, N
b amount: Ti, Nb is T or part or all of C, N
By fixing as i, C, TiN or Nb (CN), it has a role of ensuring the workability and non-aging property of the ultra low carbon steel sheet. If the amount of Ti or Nb is less than 0.005%, the effect of addition does not appear, so this is made the lower limit. On the other hand, if the amount of Ti or Nb exceeds 0.10%, the alloy cost is significantly increased, the recrystallization temperature is increased, and the workability is deteriorated. Therefore, the upper limit is set to 0.10%.

Yield strength (YP): A steel sheet having a high YP is desirable from the viewpoint of dent resistance. However, surface shape problems such as surface distortion and shape freezing occur. Therefore, from this viewpoint, the upper limit of YP of the steel sheet is set. Generally TS
YP also rises with the rise of, but YP is 25kgf / mm
If it is ² or less, good surface formability can be secured, so Y
The upper limit of P is 25 kgf / mm ² .

Yield ratio (YR = YP / TS) and work hardening amount (WH): When YR exceeds 0.55, the surface shape property deteriorates and the WH amount decreases, so the upper limit is made 0.55. ..
Further, such a low YR steel plate generally has a large WH, but if it is not 4 kgf / mm ² or more, the YP is low, so that a problem of dent resistance occurs. Therefore, the lower limit of WH is 4 kgf
/ Mm ²

Bake hardening amount (BH) and (WH + BH) amount:
From the viewpoint of dent resistance, it is essential that the BH amount is 2 kgf / mm ² or more, and the higher the better. However, the upper limit and 5 kgf / mm ² because there is a possibility that the stretcher strain occurs during press becomes to 5 kgf / mm ² greater. Also, low Y
From the viewpoint of ensuring the dent resistance of the R steel plate, the (WH + BH) amount is set to 6 kgf / mm ² or more.

Average r value and T.S. El: From the viewpoint of workability
The higher these are, the more desirable. The lower limits are 1.
5 and 35%.

Secondary workability: In general, extremely low carbon steel containing Ti or Nb is inferior in secondary workability. Furthermore, if P is added as a strengthening element, it will be further deteriorated. As described above, there are various evaluation methods for secondary work embrittlement, which is an essential problem of ultra-low carbon high-strength cold-rolled steel sheets, but the following method is used in the present invention. That is, the secondary processing brittleness transition temperature is a ductility in the case where a blank having a diameter of 50 mm is punched out from a temper-rolled steel plate, then cup-formed with a punch having a diameter of 33 mm, and subjected to a drop weight test at various temperatures. Brittle transition temperature. It is empirically known that the problem of secondary work embrittlement does not occur if the ductility-brittleness transition temperature determined by this method is -50 ° C or less, and thus this value is set as the lower limit value.

The high-strength cold-rolled steel sheet of the present invention is cold-rolled once.
It is manufactured in the annealing step, and repeated cold rolling and annealing twice or more in order to give an average r value to the composite structure steel sheet is not used because it increases the cost and complicates the manufacturing process.

Thus, according to the present invention, the tensile strength is 3
5 ~ 50kgf / mm ² grade, yield strength 15 ~ 25kgf /
mm ² , yield ratio (YR = YP / TS) is 0.55 or less, YP-El during press molding is 0.2% or less, work hardening amount (WH) is 4 kgf / mm ² or more, and when coating baking Bake hardening amount (BH) is 2-5kgf / mm ² , WH + BH is 6kgf
/ Mm ² , average r value of 1.5 or more T.I. It is possible to provide a high-strength cold-rolled steel sheet having a ferrite single-phase structure having an excellent dual workability with an El of 35% or more and a ductility-brittleness transition temperature of -50 ° C or less.

[0033]

【Example】

(Example 1) A steel having the composition shown in Table 1 was melted and hot-rolled at a slab heating temperature of 1150 ° C, a finishing temperature of 910 ° C and a winding temperature of 650 ° C to obtain a steel plate having a thickness of 4.0 mm. After pickling, cold rolling at 80% rolling reduction to 0.8 mm
Cold rolled sheet, then heating rate 15 ° C / s, soaking 840
Continuous annealing was performed at a cooling rate of 20 ° C / s and a cooling rate of 50 ° C for 50 seconds. Further, temper rolling was carried out at a rolling reduction of 0.5%, and JIS No. 5 tensile test pieces were sampled and subjected to a tensile test. The results of the tensile test are also summarized in Table 2.

Here, the WH amount which is important in the present invention is the work hardening amount when a tensile strain of 2% is applied in the rolling direction, and is the amount obtained by subtracting the yield stress (YP) from the 2% deformation stress. is there. The BH content is 2% prestrained material at 170 ° C x 2
It is the amount of increase in stress (value obtained by subtracting 2% deformation stress from the falling yield stress at the time of re-pulling test) when the tensile test is carried out again after the heat treatment equivalent to coating baking for 0 minutes. Also, 2
The next processing brittleness transition temperature is 5 mm from the temper-rolled steel plate.
It is a ductile-brittle transition temperature when a 0 mm blank is punched and then cup-formed with a punch having a diameter of 33 mm and subjected to a drop weight test at various temperatures.

[0035]

[Table 1]

[0036]

[Table 2]

As is clear from Table 2, the steel of the present invention has a low yield strength and a good surface formability as compared with the high-strength steel sheet having the same level of tensile strength as the conventional steel, and the WH and B
Since the amount of H is high, it is an index of dent resistance σ _d = YP + W
H + BH is also a good steel plate which is higher than conventional materials, and is therefore a suitable material for outer / inner panel of automobiles.

That is, as shown in FIG. 1, it can be expected that the steel of the present invention has a lower yield strength and a better surface shape after pressing than the conventional steel, even if the strength is the same. On the other hand, as shown in Fig. 2, even if the yield strength is the same as that of the conventional steel (WH
+ BH) amount is high, so dent resistance characteristics (σ _d = YP + WH
+ BH) is also improved at the same time. Further, as shown in Table 1, the steels of the present invention contain less P and Si than the conventional steels, and
Since a large amount of n or Cr is added, the ductility-brittleness transition temperature is as low as -50 ° C or lower and the secondary workability is excellent.

The invention steel sheet described in this embodiment is
Composite structure steel sheet described in [Prior Art], Ti
It can be seen that the ultra low carbon steel sheet to which Nb or Nb is added has excellent properties as a whole in the intended use of the present invention.

(Example 2) Steel No. 1 in Table 1 1-2,1-
Steel having the composition shown in 3,2-1,2-3,2-5 is melted, and the slab heating temperature is 1150 ° C. and the finishing temperature is 900.
Hot rolling under conditions of ℃ and winding temperature 700 ℃, 4.0
The steel plate has a thickness of mm. After pickling, cold rolling with a reduction rate of 80% is applied to make a cold-rolled sheet of 0.8 mm, and then a heating rate of 15 ° C /
After heating up to a maximum heating temperature of 830 ℃ with
s, followed by conventional hot dip galvanizing at 460 ° C. (Al concentration in the bath is 0.11%) and further heating to 520
After alloying at 20 ° C. for 20 seconds, it was cooled to room temperature at about 10 ° C./s. The obtained galvannealed steel sheet was measured for mechanical properties, plating adhesion, and Fe concentration in the plating film.

Table 3 also shows these results. Here, the plating adhesion was determined by determining the peeling condition of the zinc coating after the cellophane tape was bonded to the bent portion after peeling the 180 ° contact bending, and then peeling and peeling the tape. The evaluation was made into the following 5 grades. 1 ... Large peeling, 2 ... During peeling, 3 ... Small peeling, 4 ... Small peeling,
5 ... No peeling at all The Fe concentration in the plating layer was determined by X-ray diffraction.

[0042]

[Table 3]

As is clear from Table 3, the steel of the present invention has a lower YP, higher WH and BH contents than conventional steels, and has improved dent resistance and corresponding σ _d . These are examples 1
But this is a confirmed point. Further, the steel of the present invention has better plating adhesion than the conventional steel, and the Fe concentration in the alloy layer is in an amount corresponding to that of the δ ₁ phase considered to be a desirable phase. In the present invention, this is Si that deteriorates the plating adhesion or P or Si that suppresses the alloying reaction.
It is considered that the strength is increased by reducing Mn and Cr and adding Mn and Cr.

The invention steel sheet described in this example is
Composite structure steel sheet described in [Prior Art], Ti
It can be seen that the ultra low carbon steel sheet to which Nb or Nb is added has excellent properties as a whole in the intended use of the present invention. Further, in a steel sheet having a composite structure, generally, a large amount of Si is added to achieve a multi-phase structure, but in that case, deterioration of plating characteristics due to Si becomes a problem. The steel sheet of the present invention is
Such a problem does not occur.

[0045]

As is apparent from the above description, according to the present invention, a high-strength cold-rolled steel sheet excellent in press formability, which has never been obtained, can be obtained by a low-cost manufacturing method. Further, the steel sheet of the present invention is not only suitable as a base plate of an electroplated steel sheet, but also excellent as a hot-dip galvanized steel sheet.
As a result, when the steel sheet of the present invention is used in the body or frame of an automobile, the thickness of the sheet can be reduced, that is, the weight of the vehicle body can be reduced. Therefore, the present invention can greatly contribute to the preservation of the global environment. Thus, the industrial significance of the present invention is extremely great.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between tensile strength and yield strength of a product plate.

FIG. 2 is a diagram showing a relationship between yield strength and σ _d (index of dent characteristic).

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[Procedure amendment]

[Submission date] March 12, 1992

[Procedure Amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0008

[Correction method] Change

[Correction content]

On the other hand, it is well known that a high-strength cold-rolled steel sheet, which is solid solution strengthened with P or the like, based on an ultra-low carbon steel to which Ti or Nb is added, has both good strength and workability. In this case, the r value is generally good, but P and S
Since it will be strengthened by i, not only TS but YP will rise significantly,
There is a problem with the surface shape after molding. Further, in order to improve the workability such as r value, a small amount of C or N is added with Ti or Nb.
Is fixed, it becomes difficult to impart BH property. For example, Japanese Unexamined Patent Publication No. 56-139654 discloses a high-strength cold-rolled steel sheet excellent in formability by adding P to an Nb-added ultra-low carbon steel and a method for producing the same. Its typical characteristic value is YP: 25kgf / m
m ² , TS: 41 kgf / mm ² , YR: 0.61, r:
1.9, AI (aging index): 0 kgf / mm ² , but Y
Therefore, P may have a high YR and surface distortion may occur. Further, it has no bake hardening ability. In addition, JP-A-56-142
Japanese Patent No. 852 discloses a high-strength cold-rolled steel sheet for deep drawing having a low yield ratio YR, but it is mainly reinforced with P and simply supplementarily reinforced with Mn, Mo, and Cr. The r value is 1.
Moldability of 6 or more and YR of 0.6 or less is good, but T
Since i is sufficiently added to C and N, there is a problem that BH property is not imparted at all.

[Procedure Amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0017

[Correction method] Change

[Correction content]

Due to the above mechanism, it is considered that when Mn and Cr are added, the yield strength hardly changes, the work hardening rate increases, and the tensile strength increases. Such characteristic behavior is due to the above-mentioned object of the present invention, that is, low YP and high W.
In order to achieve the characteristics of H, it means that the conventional addition of Si and P is not preferable and the addition of Mn and Cr is essential. Therefore, in the present invention, the positive use of Mn and Cr is the basic solution to the problems of the prior art. However, if only Mn and Cr are added, the desired strength may not be obtained, or the manufacturing cost may increase, so consideration should be given to the combined use with Si and P.

Claims (3)

[Claims] 1. C: 0.0005-0.01 by weight
%, P: less than 0.09%, Mn: 0.5-3.
0%, Cr: 0.2 to 3.0%, one or two
Seed, plus Ti: 0.005-0.10% and N
b: a steel having a composition containing one or two of 0.005 to 0.10% and having a yield strength (YP) of 15 to
25 kgf / mm ² , yield ratio (YR = YP / TS, TS: tensile strength) of 0.55 or less, yield point elongation (YP-El) of 0.2% or less during press molding, and work hardening amount (WH )
Is 4 kgf / mm ² or more, and the bake hardening amount (B
H) is ² to 5 kgf / mm ² , and WH + BH is 6 kgf /
mm ² or more, average r value of 1.5 or more, T.I. El is 35% or more, and the ductility-brittleness transition temperature is -50 ° C or less and excellent secondary workability. Good surface shape after molding and excellent dent resistance. Strength cold rolled steel sheet. 2. The high-strength cold-rolled steel sheet for deep drawing according to claim 1, which has a ferrite single-phase structure. 3. The high-strength cold-rolled steel sheet for deep drawing according to claim 1, which is manufactured by a single cold rolling / annealing method.