JPS63190141A - High-tensile cold-rolled steel sheet having superior formability and its production - Google Patents

Abstract

PURPOSE:To manufacture a high-tensile cold-rolled steel sheet hardly causing secondary working brittleness and having superior formability, by successively applying hot rolling, winding at low temp., cold rolling, and the recrystallization annealing to a Ti-added steel with very low carbon content in which proper amounts of Mn and P coexist together. CONSTITUTION:A steel having a composition which consists of, by weight, 0.001-0.012% C, 0.001-0.008% Ni, <=0.08% solAl, <=0.010% S, 0.01-0.15% Ti, and the balance Fe with inevitable impurities and satisfies Ti>=4(C+12/14N) and in which >0.90-3.0% Mn and 0.05-0.15% P are further added in combination is hot-rolled. The hot-rolled plate is wound up at a temp. between ordinary temp. and 450 deg.C, cold-rolled, and then subjected to recrystallization annealing. If necessary, 0.0001-0.0004% B is further added to the steel of this composition. In this way, the high-tensile cold-rolled steel sheet excellent in formability can be obtained by means of a simplified line.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a high-strength cold-rolled steel sheet that has high strength and is excellent in press forming, and a method for manufacturing the same.

The cold-rolled steel sheet according to the present invention is used, for example, in automobiles, home appliances, steel structures, etc. after being subjected to appropriate surface treatment and press working, and in particular, it is important to simultaneously impart formability and strength required for these products. is possible. As a result, these products can be made thinner or lighter.

(Conventional technology) After thorough decarburization treatment at the steel manufacturing stage to achieve extremely low carbon
Based on ultra-low carbon 7II processed steel with addition of Si, M
There are already many proposals for high-strength cold-rolled steel sheets that have increased strength by adding n, Cr, or P.

For example, Japanese Patent Publication No. 57-57945 discloses a cold-rolled steel sheet in which a large amount of P is added to the ultra-low carbon Ti-added steel. In this case, the Mn content may be less than 0.90%, and the resulting r value is 1.6.
~1.9 is the limit. Furthermore, there is no mention of the amounts of N and S, or of secondary processing embrittlement.

In addition, Japanese Patent Publication No. 58-29129 discloses an example in which a large amount of 10 is added alone to the ultra-low carbon Ti-added steel, but in this case too, a high r (ll!! As a result, it is necessary to use water quenching for cooling after continuous annealing, and the practicality is poor.

Other methods have been disclosed, such as those in which Si is added to the ultra-low carbon Ti-added steel, and those in which Cr is added, but these have not been put into practical use because of the problem of oxidation on the surface of the steel sheet. This is the reality.

On the other hand, it is well known that adding alloying elements to such ultra-low carbon Ti-added steel tends to cause secondary work embrittlement, and therefore B is generally added in combination to reduce secondary work embrittlement. Measures are being taken to prevent this. However, adding a large amount of B can cause cracks in the slab, and adding such a large amount of B is difficult to do reliably and can lead to operational instability. It is not a definitive means of preventing processing embrittlement.

(Problems to be Solved by the Invention) From the above, the present inventors, as well as the industry, believe that the tensile strength is 38 kg4/+wm" or more, the yield stress is the tensile strength -12 kgf/ms" or less, r value 1.
High-strength cold-rolled steel sheets with a tensile strength of 8 or higher and less susceptible to secondary work brittleness, and a method for manufacturing the same by conventional continuous annealing and low-cost alloy addition, have been studied and sought for many years.

Therefore, the object of the present invention is to have a tensile strength of 38 kgf/am or more and a yield stress of (tensile strength 12
An object of the present invention is to provide a high-tensile strength cold-rolled steel sheet having an r value of 1.8 or more and less than kgf/w+IIJ and less likely to cause secondary work brittleness, and a method for manufacturing the same.

(Means for Solving the Problems) In order to achieve the above object, the present inventors have continued to conduct intensive research focusing on the above-mentioned ultra-low carbon Ti-added steel.

Here, the inventors newly discovered that when an appropriate amount of Mn and P coexist based on ultra-low carbon T+ added steel, the tensile strength after cold rolling and annealing increases. Not only that, but at the same time the r value was significantly improved, and even a small amount of solid? I
JC will remain.

The residual solid solution C effectively prevents secondary work brittleness.

This is due to the interaction between Ti, Mn, P, S and C, for example, Mn,! :In steel where P does not coexist, TiC and MnS are each formed as stable precipitates, so Ti≧4 (C+12/1
If 4N) Ti is added, no solid TiC will remain, but if a large amount of Mn and P coexist as in the present invention, part of the TiC will be decomposed, and TiC, Mn
S.

It is thought that precipitates such as TiPSTiS and MnP are formed, and C in a solid solution state exists. When recrystallization annealing is performed under such conditions, a recrystallization texture favorable to the r value develops due to this small amount of solid solute C, and the r value is significantly improved. remains, strengthens grain boundaries and prevents secondary processing brittleness,
It also becomes possible to exhibit a small amount of bake hardenability.

Furthermore, the present inventors have found that r(i) can be further improved by significantly lowering the coiling temperature after hot rolling from the usual 550 to 700°C.

Here, the gist of the present invention is expressed in weight%: C: 0.001 to 0.012%, N: 0. OO1~0
, OO, 8%, 3o1. AQ: 0.08% or less,
S≦0.010%, Ti 40.0, 90%, and T
Contains i≧4 (C+12/14 N), and further includes Mn: more than 0.90% and 3.0% or less and P: 0
High tensile strength with good formability, having a composition in which 0.05 to 0.15% of B is added in combination, and if desired, 1 to 0.0004% of B is added, with the remainder being Fe and unavoidable impurities. It is a cold rolled steel plate.

In addition, from another aspect, the present invention hot-rolls the steel having the above composition, and maintains the coiling temperature after hot rolling at a constant temperature. ! A to 450℃,
This is a method for producing a high-strength cold-rolled steel sheet with good formability, which is then subjected to cold rolling and recrystallization annealing.

(Function) Here, the reason why the steel composition and manufacturing conditions are limited as described above in the present invention will be further explained.

C: C is naturally contained in steel. The C required for grain boundary strengthening as mentioned above is about 0.0005%, but since lowering C leads to cost increase, the lower limit is set to 0.001%.
%. An increase in C contributes to strengthening, but the amount of Ti required increases, leading to an increase in cost. therefore,
In the present invention, the upper limit of C is set to 0.012%.

N: It is desirable that N be less, but reducing it is costly, so the lower limit was set at 0.001%.

On the other hand, if it is too large, a large amount of Ti needs to be added, so the upper limit was set at 0.008%.

s o l , AQ: Added to deacidification adjustment. It is not necessary to add Ti, but in that case, the addition yield of Ti will decrease.* If there is too much 5olJ9, the cost will increase, so the upper limit was set at 0.08%.

S: In the present invention, it is particularly desirable to reduce the amount.

When the amount of S exceeds 0.010%, MnS is formed, which deteriorates workability and makes it difficult to form the above-mentioned MnP.

T1; 11% is determined by Ti≧4 (C+ 12/14N). This is the conventional formula, where C and N are Tic,
This indicates that enough Tifll should be added to fix as TiN. Less than 0.01% is not realistic based on the above formula, and addition of more than 0.15% not only increases cost but also makes it difficult for TiC to decompose, so it was limited to 0.0.90%.

? Ins This is necessary to form Mis, MnP. If it is less than 0.90%, its formation is insufficient and high r value and grain boundary strengthening cannot be obtained.On the other hand, if it exceeds 3.0%, Mn
Too much P is formed, and the r value actually decreases. Therefore, it was limited to more than 0.90% and 3.0% or less. Preferably it is 1.2 to 2.0%.

P: P is also required to form MnP-TiP.

In particular, it has the effect of capturing TI and dissolving C more than TiC. If it is less than 0.05%, such effects are insufficient and a high r value and grain boundary strengthening cannot be achieved. On the other hand, if it exceeds 0°15%, P segregation in the steel increases, making slab cracking more likely. Therefore, it was limited to 0.05 to 0.15%.

B: B segregates at grain boundaries and has the effect of strengthening the grain boundaries.

The present invention is characterized in that C segregates at grain boundaries, strengthens the grain boundaries, and prevents secondary work embrittlement, but even if a small amount of B is added in combination as necessary, the effect of the present invention will be reduced. It may be added as necessary to ensure the strengthening of grain boundaries.

In this case, it is meaningless if it is less than o, oooi%, and 0
If it exceeds 0.0004%, it increases the addition cost and causes cracking of the slab, so it was set to 0.0001 to 0.0004%.

In the present invention, compared to the conventional case, this small amount of B
One of its characteristics is that it is good.

Next, the reason for limiting the conditions in the manufacturing method will be described.

Hot rolling, cold rolling, annealing: The coiling temperature after hot rolling is usually 550 to 700°C, and even if variations due to coil position are included, the coiling temperature is 500 to 750°C.
'C.

In the present invention, even under such normal winding conditions, the r value is high and the effect can be exhibited, but the inventors have found that the r value can be significantly improved by lowering the winding temperature from 450°C to room temperature. I found out. This is presumed to be because the low-temperature winding causes the precipitates such as TiC mentioned above to reach a desirable size to increase the r value.

At temperatures above 450°C, the effect is small and is no different from that under normal winding conditions, but below 450°C, the r value becomes significantly improved. On the other hand, since winding cannot be performed at temperatures below room temperature, the lower limit in that case was set at room temperature.

After hot rolling, cold rolling and annealing are performed, and even in this case, normal manufacturing methods for cold rolled steel sheets and surface-treated steel sheets are applied.

Note that continuous annealing is preferable for annealing. In that case, the annealing temperature is preferably 700 to 920°C. The same applies to continuous annealing in a continuous hot-dip galvanizing line.

In the case of batch annealing, it is preferable to carry out the annealing at 700 to 750° C. After that, an appropriate amount of temper rolling is performed to produce the product. Next, the present invention will be explained in detail with reference to Examples.

Example 1 Based on the composition shown in Table 1, steel with varying amounts of Mn and P was melted, and the slab heating temperature was 1150°C, the finishing temperature was 900°C, and the temperature was 600°C. A steel plate with a thickness of 3.2 as was finished by hot rolling at the bottom.

After pickling, these were cold rolled to O,hs thickness at a reduction rate of 75%, and then soaked at 850°C x 4 at a heating rate of 20°C.
Continuous annealing was performed for 0 seconds at a cooling rate of 20° C. for 8 ec.

JIS No. 5 test pieces were taken from these samples and subjected to a tensile test to measure the r value (average value in 3 directions), etc.

FIG. 1 is a graph showing the relationship between Mn%, P%, r value, and tensile strength, and it can be seen that within the scope of the present invention, both a high r value and high tensile strength can be obtained.

The data points of the yield stress, r value, and tensile strength according to the present invention are shown in the relationship diagram of the r value, yield stress, and tensile strength of conventionally manufactured high-tensile cold rolled steel sheets as shown in Figure 2. It can be seen that according to the present invention, a steel plate with a higher r value, lower yield stress, and better press formability than a conventional steel plate with the same strength level can be obtained.

Example 2 Steel having the composition shown in Table 2 was melted and made into a slab, heated at 1100°C for 1 hour, and hot rolling was immediately started.
A hot-rolled steel plate with a time of 3.2+ms 11 was completed at a finishing temperature of 880°C. After pickling, these were cold rolled to a thickness of 0.8 mm, and then soaked at 820°C at a heating rate of 40°C for 8 ec.
Continuous annealing was performed for 60 seconds at a cooling rate of 40° C./sec.

After that, it is temper rolled with an elongation rate of 0.3% and then JI
A No. S5 tensile test piece was taken and subjected to a tensile test.

Here, the aging index is 100℃, 1 after adding 8% prestrain.
The specimen was aged for hr, then re-stretched, and the yield stress was determined from the increase in yield stress at this time. It is known that this aging index exhibits a high value when the amount of solid solute carbon in the steel sheet is large.

In addition, blanks with a diameter of 50 mm are punched out from temper-rolled steel plates, and then deep drawn into cup shapes using a punch with a diameter of 33 mm.The blanks are then subjected to lightning tests at various temperatures to determine the temperature at which brittle fracture occurs. Examined.

This is the method of secondary processing brittleness test.

Table 2 also summarizes these results.

The steel plate according to the present invention has a tensile strength of 35 kgf/ms" or more and a yield stress of less than the tensile strength -12 kgf/ms", and has good elongation compared to its strength and has a very high r value of 1.8 or more. I understand that.

On the other hand, comparative steel 10 has poor elongation due to a large amount of S, comparative steel 11 has a low r value due to a lack of Mn, comparative steel 12 has a low r value due to a lack of P, and comparative steel 1113 has a low r value due to a lack of P.
has low elongation due to too much P, and comparative steel 14 has T
Because the amount of i added was insufficient, the yield stress was high and r (i was low).

Regarding secondary work embrittlement, all of the examples of the present invention have a temperature of -40°C or lower, which poses no practical problem, whereas the comparative steels have problems at temperatures of 0°C or higher.

Examples 15 to 21 of the present invention are data when the winding temperature after hot rolling is low. 600'Ct! According to the interview, r (l! Considering the current situation where line simplification is strongly required, it cannot be said that the effect is remarkable.

In particular, when the steel plate according to the present invention is used for automobile frames and other major structural members, it can be used for automobile bodies! This greatly contributes to reducing the amount of i, and its industrial effects are significant.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the relationship between Mn%, P%, r value and tensile strength; and FIG. 2 is a graph showing the relationship between Mn%, P%, r value and tensile strength; and FIG. It is a graph shown on the relationship diagram of r value, yield stress, and tensile strength of the high tensile strength cold-rolled steel plate manufactured conventionally.

Claims (4)

[Claims] (1) In weight%, C: 0.001-0.012%, N: 0.001-0.
008%, sol. Al: 0.08% or less, S≦0.0
10%, Ti: 0.01-0.15%, and Ti≧4
Contains (C+12/14N), and further includes Mn: 0, over 90%, 3.0% or less, and P: 0
．． A high-strength cold-rolled steel sheet with good formability, which has a composition in which 05 to 0.15% of composite addition is made, and the balance consists of Fe and unavoidable impurities. (2) In weight%, C: 0.001-0.012%, N: 0.001-0.
008%, sol. Al: 0.08% or less, S≦0.0
10%, Ti: 0.01-0.15%, and Ti≧4
Contains (C+12/14N), and further includes Mn: more than 0.90% and 3.0% or less and P: 0
．． 05 to 0.15%, and B: 0.0001 to 0.0004%,
A high-strength cold-rolled steel sheet with good formability, the balance being Fe and unavoidable impurities. (3) In weight%, C: 0.001-0.012%, N: 0.001-0.
008%, sol. Al: 0.08% or less, S≦0.0
10%, Ti: 0.01-0.15%, and Ti≧4
Contains (C+12/14N), and further includes Mn: more than 0.90% and 3.0% or less and P: 0
．． A steel having a composition of 05 to 0.15% composite addition and the balance consisting of Fe and unavoidable impurities is hot rolled, the coiling temperature after hot rolling is room temperature to 450°C, then cold rolling and re-rolling. A method for manufacturing high-strength cold-rolled steel sheets with good formability, which is characterized by crystal annealing. (4) In weight%, C: 0.001-0.012%, N: 0.001-0.
008%, sol. Al: 0.08% or less, S≦0.0
10%, Ti: 0.01-0.15%, and Ti≧4
Contains (C+12/14N), and further includes Mn: more than 0.90% and 3.0% or less and P: 0
．． 05 to 0.15%, and B: 0.0001 to 0.0004%,
Formability characterized by hot rolling a steel having a composition consisting of the remainder Fe and unavoidable impurities, the coiling temperature after hot rolling being room temperature to 450°C, then cold rolling and recrystallization annealing. A method for manufacturing high-strength cold-rolled steel sheets.