JP2620444B2 - High strength hot rolled steel sheet excellent in workability and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention has a low yield ratio (YR),
The present invention relates to a high-strength hot-rolled steel sheet having excellent ductility (El) and excellent workability and a method for producing the same.

[0002]

2. Description of the Related Art Many attempts have been made to increase the strength of a hot-rolled steel sheet. Particularly, the tensile strength is 35 to 5
In the case of a steel sheet having 0 kgf / mm ² , Si, Mn, Cr
Alternatively, P or the like has been added to increase the strength. On the other hand, recent technological advances have made it easier to melt ultra-low carbon steel, and many techniques for adding Ti or Nb based on this to ensure workability have been announced.

For example, Japanese Patent Application Laid-Open No. 60-197847 discloses that a predetermined amount of Nb and B is added to an ultra-low carbon steel in which Mn, Si, and P are specified, and further, the structure is transformed into a ferrite phase and a low-temperature transformation by hot rolling conditions. A non-aging, low-yield-ratio, high-workability hot-rolled steel sheet having a composite structure of phases is disclosed. That is, although Nb and B are added to form a composite structure according to the hot rolling conditions, there is a problem that the material of the composite structure steel tends to vary depending on the manufacturing conditions.

Japanese Patent Application Laid-Open No. Sho 60-149730 discloses that C ≦ 0.08%, Si: 0.1-1.0%, Mn:
A method for producing a hot-rolled steel sheet having a tensile strength of 35 to 50 kgf / mm ² by adding 0.5 to 1.4% and further adding Cr and Ni and having excellent stretch flangeability and secondary workability is disclosed. . However, any of the steels disclosed in this publication is a material having a high YR (yield ratio) as apparent from the description of the examples, and thus has a problem of shape freezing of the processed product. In addition, expensive Ni
There is also the problem of using.

Further, Japanese Patent Application Laid-Open No. 2-1758386 discloses that an extremely low carbon steel has an Mn content of 0.05 to 3.5% and a Ti content of N or S and some or all of C. A hot-rolled steel sheet for drawing has been disclosed which, when added, has high ductility, low YP, excellent secondary workability, and excellent material uniformity. This steel sheet is used for parts that require ultra-deep drawability. As shown in the examples, the TS is 30 kg.
f / mm ² or less, not applicable to high strength steel sheets.

[0006]

Desirable high-strength hot-rolled steel sheets used for parts and containers for automobiles have a low yield strength, a low yield ratio, excellent ductility, and are extremely work-hardened. It is a steel plate that has both paint bake hardening ability. Needless to say, it is necessary that the yield point elongation during processing is small and the secondary workability is excellent. However, none of the hot rolled steel sheets disclosed in the above publications has such various characteristics.

According to the present invention, Mn and Cr, which are rarely used as solid solution strengthening elements, are added to ultra-low carbon steel in a complex manner, and these are effectively used, and furthermore, Ti and Nb are added to provide excellent workability. High strength hot rolled steel sheet, 30-60
While maintaining the tensile strength (TS) of kgf / mm ² , the yield strength (YP) is 13 to 36 kgf / mm ² , and the yield ratio (YR = YP
/ TS) is set to a low YR of 0.60 or less, and the elongation (T.E.
In l) 33% or more, and excellent secondary workability or when there is no cracking due to impact after machining processing, and applying the paint bake hardenability of 2~8kgf / mm ² if necessary WH (2%), which is an index of work hardening ability in a low strain range
An object of the present invention is to provide a hot-rolled steel sheet having a deformation stress-yield strength of 4 kgf / mm ² or more and having extremely good adhesion to plating applied for rust prevention, and a method for producing the same.

[0008]

Means for Solving the Problems In order to achieve the above object, the present invention has the following features. That by weight, C: 0.0003~0.01%, Si: 0.005~1.0%, Mn: 0.2~3.0%, Cr: it contains 0.01 to 3.0% , And a total of 0.5% or more in total of Mn and Cr
Was added, P: 0.001~0.12%, S: 0.001~0.015%, Al: 0.005~0.10%, further, Ti: 0.003-0.10% and Nb : 0.003 to 0.10% or one kind, and Ti ^* = T. When Ti−3.42N, 2 ≦ (Ti ^* + 0.
52Nb) / C ≦ 20 , if necessary
B contained 0.0001 to 0.0030%, and the balance F
e and a high-strength hot-rolled steel sheet excellent in workability characterized by being composed of unavoidable impurities, and a steel slab comprising the above components, after hot rolling at a finishing temperature of Ar ₃ -100 ° C. or higher, This is a method for producing a high-strength hot-rolled steel sheet having excellent workability, characterized by winding at a temperature of 750 ° C. or lower.

[0009]

In order to obtain a hot-rolled steel sheet having a TS of 30 to 60 kgf / mm ² and having excellent workability, the present inventors have developed a single- or multi-added Ti or Nb alloy having good workability. We conducted intensive research on low carbon steel and obtained the following new findings.

First, Si, P, Mn, and Cr, which are typical solid solution strengthening elements, were added to the base steel, and the tensile properties after hot rolling, particularly the yield behavior and the work hardening phenomenon, were investigated in detail. . As a result, Si and P, which are conventionally frequently used as solid solution strengthening elements, are as follows: a) First, the addition of a very small amount significantly increases the yield strength, and b) As a result, the work hardening rate in a low strain region is significantly reduced. There was found.

On the other hand, when Mn and Cr, which are conventionally rarely used as solid solution strengthening elements, are added in combination , a) the yield strength hardly increases, and b) the tensile strength increases. C) As a result, in the low strain region, A very important new finding is that the work hardening rate is rather increased by these additions. As a result of studying these mechanisms, a) yield strength is Fe
Determined by the difference in atomic radius between the element and the added X element, and increases as the difference in the atomic radius increases. B) The work hardening rate is deeply related to the slip behavior of dislocations. The basic principle was constructed that when the temperature decreases, the dislocation cross-slip becomes difficult and the work hardening rate increases. According to this, the atomic radii of Si and P are much smaller than that of Fe, and hence the difference in atomic radii becomes larger, so that the yield strength increases remarkably. The atomic radii of Mn and Cr are very close to those of Fe, so that the yield strength changes almost. It can be understood that it was not made. On the other hand, the effect on the stacking fault energy is not always clear, but from the observation of the dislocation structure after the initial work hardening, Si and P hardly affect the stacking fault energy within the range of the addition amount investigated. Mn, C
It became clear for the first time that r tends to reduce this.

By the above mechanism, it is considered that when Mn and Cr are added in combination, the yield strength hardly changes, the work hardening rate increases, and the tensile strength increases. Such characteristic behavior is an object of the present invention described above, i.e. to achieve the characteristics of low YP, high WH is conventional Si, it is difficult only the addition of P, Mn, complex of Cr It means that addition is essential. Therefore, in the present invention, active utilization of Mn and Cr is a basic solution to the problems of the prior art. However, a desired addition of Mn and Cr alone may not provide the desired strength or increase the production cost. Therefore, the combined use of Si and P is also considered.

Second, the present inventors have also obtained a new finding that the baking hardenability of the coating is improved by the active addition of Mn and Cr. In the present invention, in order to ensure workability, the base is based on ultra-low carbon steel to which Ti or Nb is added alone or in combination. However, in this case, since C and N are generally fixed to Ti and Nb, seizure is performed. No or no curing ability.
This is because the affinity between C and N and Ti and Nb is very strong, so that (Ti + N)
b) When the amount is larger than the (C + N) amount, the solid solution (C
+ N) is unlikely to remain. However, even in such a case, Mn and Cr are positively added in a complex manner to increase the B content.
The new fact that H property was able to be added is that TiC and NbC
This suggests that a change in the solubility product has occurred. That is, since Mn and Cr have an interaction of attractive force with C,
It is considered that the solid solution C in the matrix equilibrated with iC and NbC was further stabilized, and the solubility product of these precipitates was increased. As a result, the amount of the remaining solid solution C was increased and the BH property was improved. Therefore, the composite addition of Mn and Cr can be utilized as a new means for imparting paint bake hardenability.

The solid solution C that contributes to paint baking is B
Since it strengthens the crystal grain boundaries in the same manner as described above, it is also effective as means for preventing secondary working embrittlement, which is known as a disadvantage of ultra-low carbon steel. It is well known that secondary working embrittlement is promoted by the presence of P segregated at the grain boundaries.
Has an interaction between P and attractive force, and reduces the amount of segregated P, so that it was found that it is also effective in preventing embrittlement in secondary working.

Today, rust prevention of automobile bodies and the like is progressing, and the trend of surface-treated steel sheets is remarkable. Surface treatment methods for steel sheets are roughly classified into electroplating and hot-dip plating. The former generally has good plating adhesion irrespective of the type of steel sheet, but has a problem that it is difficult to thicken it and the cost is high. On the other hand, the latter does not have the drawback of the former, but depending on the type of the steel sheet, the plating adhesion deteriorates and the alloying reaction of Fe-Zn is greatly affected. The present inventors have suppressed the addition amounts of Si and P, which are conventionally frequently used as reinforcing elements in steel,
The present invention utilizing Mn and Cr has also obtained new knowledge having the following advantages, particularly in the production of alloyed hot-dip galvanized steel sheet by a continuous hot-dip galvanizing process of the Sendzimer method. 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 to lower the productivity. Further, the addition of Si deteriorates the plating adhesion, and causes various problems during press molding. On the other hand, it has been found that the composite addition of Mn and Cr does not have such an adverse effect. The mechanism is not always clear,
a) Mn segregated on the surface changes the type and form of oxide remaining after reduction to improve plating adhesion, and b) Mn or C
It is presumed that r plays a role in reducing the amount of P segregated at the grain boundary through the interaction with P and making the alloying reaction normal. This was also used as a solution to the problems of the conventional method.

The present invention has been constructed based on such ideas and new findings.

Hereinafter, the reason why the components and characteristic values are limited in the present invention as described above will be further described. C content: C is a very important element that determines the material properties of the product. The present invention presupposes ultra-low carbon steel subjected to vacuum degassing, but when C is less than 0.0003%, the grain boundary strength decreases, secondary working embrittlement occurs, and the production cost increases significantly. . On the other hand, if the C content exceeds 0.01%, the strength is increased, but the moldability is significantly reduced. Therefore, the range is set to 0.0003% to 0.01%.

Si content: When the amount of Si is increased, YP
Rises excessively, and shape freezing occurs during processing. Furthermore, problems such as a decrease in chemical conversion treatment properties, a decrease in adhesion of hot-dip galvanizing, and a decrease in productivity due to a delay in alloy reaction occur. Therefore, the upper limit is set to 1.0%.

Mn content: Mn is an effective solid solution strengthening element that increases the yield strength without significantly increasing the yield strength. It also imparts bake hardening ability, prevents secondary work brittleness, prevents chemical conversion treatment, Since it also has the effect of improving hot-dip galvanizing properties, it is positively added in the present invention. However, if added in an excessively large amount, the yield strength is significantly increased and the ductility is reduced, so the upper limit is made 3.0%. If the amount of Mn is less than 0.2%, the hardening is slight, so the lower limit is made 0.2%.

Cr content: Like Cr and Mn, Cr and Mn are also effective elements that increase the strength without increasing yield strength, and impart bake hardenability and prevent secondary work brittleness. The invention actively uses it. However, when the amount of Cr increases, the pickling property of the hot-rolled sheet decreases, and the chemical conversion property of the product sheet deteriorates. Therefore, the upper limit is set to 3%. The above-mentioned Mn and Cr are added for the same functional purpose. However, in order to obtain appropriate properties by these elements , Mn + Cr is added.
0.5% or more.

P content: P is known as an element that increases strength at low cost, and is often used in conventional high strength steel sheets reinforced with solid solution. However, when the addition amount exceeds 0.12%, even if Mn and Cr are positively added as in the present invention, a) the yield strength is excessively increased, causing poor shape freezing during processing, and b). Secondary working embrittlement may occur. Further, c) the alloying reaction during continuous hot-dip galvanizing becomes extremely slow, and the productivity is reduced. Therefore, the upper limit is set to 0.12%. The lower limit is made 0.001% from the viewpoint of steelmaking technology.

S content: S is preferably low, but 0.00
If it is less than 1%, the production cost increases, so this is set as the lower limit. On the other hand, if it exceeds 0.015%, MnS or T
Since a large number of sulfides such as iS precipitate and workability deteriorates, this is set as the upper limit.

Al content: Al is used for adjusting deoxidation.
If it is less than 0.005%, the yield of Ti addition is reduced. On the other hand, if it exceeds 0.1%, the cost is increased.

Ti and Nb contents: Ti and Nb are obtained by partially or entirely converting C and N into TiC, TiN or Nb (C
By fixing as N), it has a role of ensuring workability and non-aging property of the ultra-low carbon steel. Therefore, Ti ^* =
T. When Ti-3.42N, 2 ≦ (Ti ^* +
0.52Nb) / C ≦ 20 . At this time, the content range of each element is preferably set to 0.003 to 0.10%.
If it is less than 003%, the effect of the addition is not exhibited, while
% Or more causes a significant increase in alloy cost.

Although the amount of N is not particularly defined, it is preferable that the amount is lower. If the amount increases, excessive addition of Ti or Al becomes necessary, or workability deteriorates.
060% or less. In the present invention, B may be added as necessary. In this case, if N is fixed in advance, it segregates at the crystal grain boundaries and is effective in preventing embrittlement in secondary processing. However, if the content is less than 0.0001%, the effect is insufficient, and if it exceeds 0.0030%, the cost of addition increases and slab cracks may be caused.

Although the present invention relates to a high-strength hot-rolled steel sheet having the above-mentioned composition, it is preferable to produce the steel sheet by the following method. That is, when hot-rolling a slab manufactured by a normal method, the finishing temperature is set to Ar ₃ -100 ° C. or higher from the viewpoint of ensuring the workability of the product sheet, and the winding temperature is set to 750 ° C. from room temperature. Good to do. If the hot-rolling finishing temperature is lower than Ar ₃ -100 ° C., a processed structure is likely to remain, which impairs the good workability characteristic of the steel of the present invention. On the other hand, the present invention is characterized in that the product material is not significantly affected by the hot rolling winding temperature. This is considered to be because, in addition to the fact that N is a very low carbon steel fixed as TiN, Mn, Cr and the like are considerably added, and the structure of the hot rolled sheet is uniform.
The reason why the upper limit of the winding temperature is set to 750 ° C. is from the viewpoint of preventing the yield from decreasing due to the deterioration of the material at both ends of the coil.

Further, in the present steel, the strength level can be easily changed by changing the cooling rate after finish rolling. In the present invention, Mn,
It is considered that Cr was positively added, and a low-temperature transformation product was formed by increasing the cooling rate after hot rolling.

As described above, according to the present invention, 30 to 60
kgf / mm ² tensile strength (TS) and yield strength (Y
P) is 13 to 36 kgf / mm ² and the yield ratio (YR = YP / T
S) is set to a low YR of 0.60 or less, the elongation (T.El) is 33% or more, and the secondary workability is excellent without cracking due to impact during or after processing, and if necessary. 2
Can impart paint bake hardenability of ~8kgf / mm ^2,
WH amount (2% deformation stress-yield stress), which is an index of work hardening ability in the low strain range, is 4 kgf / mm ² or more, and has very good adhesion to plating applied for rust prevention. Hot rolled steel sheet can be obtained.

[0029]

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 sheet having a thickness of 1.4 mm.
A JIS No. 5 tensile test piece was collected from this hot-rolled steel sheet and subjected to a tensile test. Table 2 shows the results of various mechanical tests.

Here, the amount of BH was 170 ° C. × 2% pre-strained material.
This is the amount of increase in stress (value obtained by subtracting 2% deformation stress from falling yield stress during re-tensile test) when a tensile test is performed again after heat treatment equivalent to 20 minutes of paint baking. Also,
The secondary working brittle transition temperature is the ductile-brittle transition temperature when a blank having a diameter of 50 mm is punched from a temper-rolled steel sheet, cup-formed with a punch having a diameter of 33 mm, and subjected to a drop test at various temperatures. It is.

[0031]

[Table 1]

[0032]

[Table 2]

As is clear from Table 2, the steel of the present invention has a lower yield strength (YP) and a lower yield ratio (YR) as compared with a high-strength steel sheet having the same level of tensile strength (TS) as a comparative steel. . Further, it has high elongation (El) and is excellent in BH property and secondary workability.

[0034]

Example 2 A hot-rolled steel sheet having a thickness of 1.4 mm was manufactured from various steels shown in Table 1 in the same manner as in Example 1. The heating rate is 10
About 10 ℃ after heating to 600 ℃
After cooling at 460 ° C., conventional hot-dip galvanizing is performed at 460 ° C. (Al concentration in the bath: 0.10%), and further heated to 52 ° C.
After alloying at 0 ° C. for 20 s, it was cooled to room temperature at about 10 ° C./s. Plating adhesion (powdering property) of the obtained alloyed galvanized steel sheet and Fe in the plating film
The concentration was measured. Table 3 shows the results. Here, the plating adhesion was determined by 180 ° adhesion bending, and the peeling state of the zinc film was determined by adhering a cellophane tape to the bent portion, peeling the cellophane tape, and peeling the cellophane tape from the amount of peeling plating adhered to the tape. The evaluation was based on the following five levels. 1: large peeling, 2: peeling, 3: small peeling, 4: small amount of peeling,
5: No peeling The Fe concentration in the plating phase was determined by X-ray diffraction.

[Table 3]

As is clear from Table 3, the steel of the present invention has better plating adhesion than the conventional comparative steel, and the Fe concentration in the alloy layer is equivalent to that of the δ ₁ phase, which is considered to be a desirable phase. Amount to be.

[0036]

Example 3 Samples E and F in Table 1 were heated at a slab heating temperature of 1.
Hot rolling was performed at 150 ° C. and a finishing temperature of 910 ° C., and 1.4
mm, and the cooling rate after finish rolling is 10 to 60 ° C /
s and wound at 650 ° C. A JIS No. 5 test piece was sampled from this hot-rolled steel sheet and subjected to a tensile test. Table 4 shows various mechanical properties. The evaluation of each characteristic was performed in the same manner as in Example 1.

[0037]

[Table 4]

As is clear from Table 4, in the comparative steel, there was almost no change in strength due to the cooling rate after the hot rolling, but in the steel of the present invention, the strength could be largely changed and the yield was high. It has low strength and yield ratio, high BH and WH, and is excellent in secondary workability.

[0039]

As is apparent from the above description, according to the present invention, a high-strength hot-rolled steel sheet excellent in workability, which has not been achieved conventionally,
Obtained by low cost manufacturing methods. Further, the steel sheet of the present invention is excellent in BH property and secondary workability, and can be widely used for automobile members, frames, container covers and the like.

Claims (4)

(57) [Claims] 1. C: 0.0003 to 0.01%, Si: 0.005 to 1.0%, Mn: 0.2 to 3.0%, Cr: 0.01 to 3.0% by weight And Mn and Cr are combined in a total amount of 0.5% or more.
Was added, P: 0.001~0.12%, S: 0.001~0.015%, Al: 0.005~0.10%, further, Ti: 0.003-0.10% and Nb : 0.003 to 0.10% or one kind, and Ti ^* = T. When Ti−3.42N, 2 ≦ (Ti ^* + 0.
52Nb) / C ≦ 20, a high-strength hot-rolled steel sheet excellent in workability, characterized by being composed of the balance of Fe and unavoidable impurities. 2. The high-strength hot-rolled steel sheet according to claim 1, further comprising 0.0001 to 0.0030% by weight of B. 3. C: 0.0003-0.01%, Si: 0.005-1.0%, Mn: 0.2-3.0%, Cr: 0.01-3.0% by weight And Mn and Cr are combined in a total amount of 0.5% or more.
Was added, P: 0.001~0.12%, S: 0.001~0.015%, Al: 0.005~0.10%, further, Ti: 0.003-0.10% and Nb : 0.003 to 0.10% or one kind, and Ti ^* = T. When Ti−3.42N, 2 ≦ (Ti ^* + 0.
A slab containing 52Nb) / C ≦ 20, the balance being Fe and unavoidable impurities was Ar ₃ -100 ° C.
A method for producing a high-strength hot-rolled steel sheet excellent in workability, comprising hot rolling at the above finishing temperature and then winding at a temperature of 750 ° C. or lower. 4. The slab for producing a hot-rolled steel sheet according to claim 3, further comprising 0.0001 to 0.0030% by weight.
A method for producing a high-strength hot-rolled steel sheet excellent in workability, characterized by containing B.