JPH1161273A - Manufacture of cold-rolled steel sheet small in in-plane anisotropy and excellent in secondary working brittleness resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stably provide the steel sheet which is small in in-plane anisotropy and excellent in secondary working brittleness resistance by hot rolling and coiling the Ti-added ultra low carbon steel, cold-rolled under the specified condition, and achieving the recrystallization annealing. SOLUTION: The steel has the composition consisting of, by weight, 0.0005-0.005% C, <=0.3% Si, 0.01-0.4% Mn, 0.01-0.05% sol Al, 0.01-0.10% Ti so as to satisfy the inequality of Ti> (48/12)C+(48/14)N+(48/32)S}, 0.0003-0.003% B, and the balance Fe with inevitable impurities. The steel is hot rolled, and coiled at <=560 deg.C. Then, the cold rolling is achieved at the draft in the range to satisfy the condition indicated by the formula at >=40 deg.C. Then, recrystallization annealing is achieved in the temperature range of >=650 deg.C to < the Ac3 transformation point. Excellent characteristics can be obtained without adding Nb or Al by optimizing the B content, the coiling temperature and the cold rolling draft.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a cold-rolled steel sheet having a small in-plane anisotropy and excellent secondary work brittleness resistance.

[0002]

2. Description of the Related Art Since excellent press formability is required for cold-rolled steel sheets used for automobiles and home electric appliances, it is desired to have a high Rankford value (r value) and ductility.

[0003] JP-B-44-18066 discloses Ti
A method for producing a cold-rolled steel sheet for added deep drawing is disclosed. This means that C is 0.001 to 0.020% by weight, Ti is 0.2
0.5% by weight, and 4 × C (% by weight) or more, and carbon and nitrogen in the steel were all fixed as carbonitrides.
It relates to so-called IF steel. IF steel has such features that a cold-rolled steel sheet for deep drawing can be manufactured stably and that normal-temperature strain aging does not occur. However, on the other hand, the strength of crystal grain boundaries is reduced due to the absence of solute carbon, and secondary working embrittlement occurs. There is a problem that defects such as are likely to occur. The in-plane anisotropy of the r value (△ r) is an index indicating the non-uniformity of the r value depending on the tensile direction, and Δr = (r ₀ +
r ₉₀ −2 × r ₄₅ ) / 2. Here, r ₀ denotes a rolling direction, r ₄₅ denotes a direction at 45 degrees to the rolling direction, and r ₉₀ denotes an r value measured by a tensile test in the width direction. It is considered that the closer the absolute value of Δr (hereinafter referred to as | Δr |) to 0, the smaller the in-plane anisotropy and the more preferable.

Japanese Patent Application Laid-Open No. 2-77558 discloses a cold-rolled steel sheet having a small in-plane anisotropy and an excellent secondary work brittleness resistance, and a method for producing the same. However, here, the B content is limited to a low value in order not to increase the in-plane anisotropy.
The effects of improving the secondary working brittleness and the in-plane anisotropy are not sufficient.
Japanese Patent Application Laid-Open No. H5-195079 discloses a method for producing a cold-rolled steel sheet having excellent deep drawability and secondary work brittleness resistance. However, in the method disclosed herein, according to the study of the present inventors, the winding temperature after hot rolling is high, and
Because care has not been taken to control the cold rolling rate,
The effect of improving in-plane anisotropy is not sufficient.

[0005] JP-A-63-310924 discloses that T
Japanese Patent Laid-Open No. 5-117 discloses a method for producing an ultra-thin steel sheet having a small in-plane anisotropy using an ultra-low carbon steel to which i-Nb-B is added in combination.
No. 758 discloses a method for producing a cold-rolled thin steel sheet having excellent secondary work embrittlement resistance and low in-plane anisotropy using an ultra-low carbon steel to which Ti-Nb-B-Al is added in a complex manner. . In either method, the hot-rolled sheet is made into fine grains by adding Nb, Al, or the like, and the in-plane anisotropy is reduced. However, the addition of Nb or Al increases the material cost, and the addition of Nb raises the recrystallization temperature. Therefore, in order to obtain good formability, the annealing temperature must be increased or the annealing time must be increased. Or detract from thermal energy costs and productivity.

[0006]

The problem to be solved by the present invention is that Ti which is advantageous for improving the deep drawing formability.
Cold rolled steel sheet using an added ultra-low carbon steel and having low in-plane anisotropy and excellent secondary work brittleness resistance. More specifically, the average r value is 1.5 or more, and | △ r | is 0.25. Hereinafter, it is an object of the present invention to provide a method for stably manufacturing a cold-rolled steel sheet having a brittle transition temperature of −90 ° C. or less after drawing.

[0007]

The gist of the present invention resides in the following method for producing a cold-rolled steel sheet having small in-plane anisotropy and excellent secondary work brittleness resistance.

C in weight%: 0.0005 to 0.005
%, Si: 0.3% or less, Mn: 0.01 to 0.4%,
sol. Al: 0.01 to 0.05%, Ti: 0.01
~ 0.10% and Ti> ｛(48/12) C + (4
8/14) N + (48/32) S｝, B: 0.0003
Hot rolled steel containing up to 0.003%, the balance consisting of Fe and unavoidable impurities, wound around a coil at 560 ° C or less, and cold rolled at a rolling rate of 40% or more and satisfying the following formula And a method for producing a cold-rolled steel sheet having small in-plane anisotropy and excellent secondary work brittleness resistance, wherein recrystallization annealing is performed in a temperature range of 650 ° C. or more and less than the A _c3 transformation point.

[0009]

(Equation 1)

[0010] Here, CT represents the winding temperature (° C), CR represents the cold rolling reduction (%), and B represents the B content (% by weight) in the steel.

The present inventors investigated the effects of the B content in steel and the manufacturing conditions in order to improve the in-plane anisotropy of the Ti-added ultra-low carbon steel sheet. The steel used was in weight%, C:
0.0025%, Si: 0.01%, Mn: 0.15
%, P: 0.01%, S: 0.0050%, sol. A
l: 0.04%, N: 0.002%, Ti: 0.05
%, B: 0.0005 to 0.0030%. A steel slab of this chemical composition is hot-rolled at 900 ° C. or more, wound at various temperatures of 650 ° C. or less, the obtained hot-rolled sheet is pickled, and cold-rolled at a rolling rate of 60 to 95%. Then, the sample was annealed continuously, and the r value was measured by a tensile test.

FIG. 1 and FIG. 2 show a part of the results of the above investigation, in which B is 0.0005% by weight or 0.003% by weight.
The relationship between the winding temperature and the cold rolling reduction with respect to Δr of a steel sheet containing 0% by weight is shown. In the figure, Δ indicates that Δr exceeds 0.25, Δ indicates less than -0.25, and ○ indicates | △ r |
Means 0.25 or less. | Δr | is 0.25
If it is below, it is judged that the in-plane anisotropy is good. As shown in FIG. 1, there is a correlation between Δr, the winding temperature during hot rolling, and the cold rolling reduction. To reduce | △ r | It can be seen that when the winding temperature at the time is high, it is better to increase the cold rolling reduction. Also, B
It can be seen that it is better to lower the cold rolling reduction as the content increases. 3 and 4 show the relationship between the winding temperature and the cold rolling reduction with respect to Δr. It can be seen that to reduce | Δr |, it is better to lower the cold rolling reduction as the B content increases.

[0013] These phenomena are caused by the fact that the r value in the plate surface changes as follows in accordance with the fluctuation of the manufacturing factor.

As the B content increases, r ₀ and r ₉₀ decrease but r ₄₅ does not change much. Therefore, Δr decreases as the B content increases. This is presumably because B affects the orientation selectivity of recrystallization nucleation and nucleation during annealing.

B When the winding temperature increases, Δr increases because r ₄₅ decreases. This is presumably because the crystal grains of the hot-rolled steel sheet become coarser as the winding temperature increases.

C When the cold rolling reduction is increased, r ₄₅ increases but r ₀ and r ₉₀ do not change much, so that Δr decreases.

By optimizing the B content, the winding temperature and the cold rolling reduction, a cold-rolled steel sheet having a small in-plane anisotropy can be produced without adding elements such as Nb and Al. Also,
Since this cold-rolled steel sheet can contain a necessary amount of B, it can also take measures against secondary working brittleness.

The present invention has been completed based on the above findings and the like.

[0019]

Embodiments of the present invention will be described below in detail. The percentages of the chemical compositions described below mean% by weight.

Chemical composition of steel C: If the C content exceeds 0.005%, TiC increases, the deep drawability of the steel sheet is impaired, and the recrystallization temperature becomes excessively high, so that the difficulty in production is reduced. Increase. On the other hand, 0.00
If the content is less than 0.05%, the precipitation of TiC becomes insufficient and solid solution C remains. For this reason, the range of the C content is set to 0.0005 to 0.005%. Preferably, 0.
0015 to 0.004%.

Since Si: Si has an effect of strengthening a steel sheet, it can be contained for the purpose of strengthening steel.
When steel strength is not required, Si need not be contained. If the Si content exceeds 0.3%, the possibility of occurrence of scale-like surface defects increases, so the upper limit of the content is 0.3%.

Mn: S contained as an unavoidable impurity
0.01%
It is contained above. If it exceeds 0.4%, the steel is hardened, the ductility is deteriorated, and the deep drawability is impaired. For this reason, Mn
Is 0.01 to 0.4%.

Sol. Al: Used for deoxidizing molten steel. sol. If the Al content is less than 0.01%, a sufficient deoxidizing effect cannot be obtained, and even if the Al content exceeds 0.05%, the effect is saturated and uneconomical. Therefore, sol. The content of Al is set to 0.01 to 0.05%.

Ti: C, N, S in steel are precipitated and fixed,
It is added to obtain excellent deep drawability, ductility and non-aging property. Further, by precipitating and fixing N, the added B can be present in a solid solution state, and the effect of improving the secondary working brittleness of B can be exhibited. In order to obtain these effects, 0.01% or more and (48/12) C%
Contains more than + (48/14) N% + (48/32) S%. On the other hand, if it exceeds 0.1%, the effect of containing Ti is saturated, and the economy is impaired.
The upper limit of the content is 0.1%.

B: B does not affect r ₄₅ of the steel sheet but has the effect of lowering r ₀ and r ₉₀ , and when contained in an appropriate amount, can improve the in-plane anisotropy of the steel sheet. Also, B
Has the effect of strengthening the crystal grain boundaries and improving the resistance to secondary working brittleness. B is contained in an amount of 0.0003% or more to improve the resistance to secondary working brittleness. On the other hand, when the B content is 0.
If it exceeds 0030%, the deep drawability of the steel sheet is significantly impaired. For this reason, the range of the B content is from 0.0003 to 0.1.
0030%. Desirably 0.0006 to 0.00
25%.

Other than the above are Fe and inevitable impurities. Among the unavoidable impurities, P segregates at the crystal grain boundaries to make the grain boundaries brittle, and significantly impairs the secondary work embrittlement resistance of the steel sheet. Therefore, the P content is desirably 0.03% or less. Since S impairs the moldability, it is preferable that S is small.
When the content increases, the amount of Mn and Ti required for detoxification increases, and the cost increases. Therefore, the content is preferably set to 0.02% or less. When the N content increases, the Ti content required for fixing N increases, and the economic efficiency is impaired.
Precipitates increase and impair ductility. Therefore, the N content is preferably set to 0.01% or less.

Processing conditions Hot rolling: A slab having a chemical composition within the above range is produced by continuous casting of molten steel, or a method of forming a steel ingot followed by slab rolling. The slab is re-heated, hot-rolled as it is after continuous casting or slab rolling, or hot-rolled with auxiliary heating. After hot rolling, the steel sheet is cooled to a winding temperature and wound into a coil.

In order to reduce | △ r | of the steel sheet after cold rolling and annealing, as shown in FIGS.
As the winding temperature after hot rolling increases, it is necessary to increase the cold rolling reduction. However, if the winding temperature exceeds 560 ° C., even in steel having a B content of 0.0030% by weight, a cold rolling reduction of 95% or more is required, and cold rolling becomes difficult in ordinary rolling. Therefore, the winding temperature is 560
It is good to be below ° C. Preferably, the temperature is 500 ° C. or lower. The lower limit is not particularly defined, but if the winding temperature is too low, the precipitates become finer and the ductility is impaired.
The temperature is preferably set to 0 ° C. or higher.

The hot rolling conditions other than the winding temperature are not particularly limited. However, in order to refine the crystal grains of the hot rolled sheet and improve the deep drawability, the finishing temperature is set to (Ar3 transformation point + 30 ° C.) or lower, and the rolling is performed. Then, the cooling rate to the γ / α transformation completion temperature is increased by 10
C./s or more is desirable.

Cold rolling: In order to reduce in-plane anisotropy, it is necessary to perform cold rolling at an appropriate rolling reduction according to the B content of steel and the winding temperature. The appropriate range of the cold rolling reduction is required to be 40% or more in order to obtain a recrystallized texture necessary for securing formability. Further, in order to make | △ r | 0.25 or less, it is made to fall within the range defined by the following formula, which is experimentally obtained from FIGS.

[0031]

(Equation 1)

Here, CT represents the winding temperature (° C.), and CR represents the cold rolling reduction (%).

Annealing: The cold-rolled steel sheet is subjected to a treatment such as degreasing according to a known method, if necessary, and then annealed for recrystallization. The annealing temperature at this time is in a temperature range from 650 ° C. to less than the A _c3 transformation point. If the annealing temperature is lower than 650 ° C., it takes too much time to complete the recrystallization. If the annealing temperature is _{equal to} or higher than A _{c3, the} recrystallization texture preferable for deep drawability is undesirably reduced by transformation.

In order to maintain good formability of the steel sheet, (r ₀ °
It is also necessary to increase the average r value defined by (+ 2r ₄₅ ° + r ₉₀ °) / 4. B in steel has a value of
1) Has the effect of suppressing the development of texture. For this reason,
Usually, the average r-value of steels with a high B content will be low. Further, when the cold rolling reduction is reduced, the formation of the rolling texture becomes weak, so that the development of the texture after annealing becomes insufficient, and the average r value of the steel sheet does not improve. However, by performing annealing after cold rolling in a temperature range exceeding {8 × B (% by weight) × 10 ⁴ + 1550-10 × cold rolling rate (%)} ° C., the B and the cold rolling rate can be reduced. A decrease in the average r value due to the influence can be suppressed. Therefore, when increasing the average r value, it is preferable that the annealing temperature be in a range exceeding the temperature determined by the above equation.

The annealing means is optional, and any method such as a continuous annealing method or a box annealing method may be used. However, since the productivity is high, it is desirable to carry out by the continuous annealing method.

After annealing, it is desirable to perform temper rolling according to a conventional method, but temper rolling may be omitted. The cold-rolled steel sheet manufactured according to the manufacturing method of the present invention can be used as a base material for electroplating or as a coated steel sheet. The steel sheet after cold rolling is annealed in a heating furnace equipped with a known hot-dip coating apparatus, and hot-dip,
It may be a plated steel sheet. Needless to say, after annealing in a continuous annealing furnace, hot-dip plating may be performed to form a plated steel sheet.

[0037]

EXAMPLE A steel having the chemical composition shown in Table 1 was melted and cast in a vacuum melting furnace for experiments.

[0038]

[Table 1]

These ingots were hot forged into steel slabs having a thickness of 25 mm, and heated to 1250 ° C. using an electric heating furnace.
The sample was held for a time, and was rolled into a hot-rolled sheet having a thickness of 5 mm in three passes in a temperature range of 1150 ° C. to 930 ° C. using an experimental hot rolling mill. Immediately after hot rolling, it is cooled to various temperatures within a temperature range of 450 to 600 ° C. by forced air cooling or water spray cooling, set to a winding temperature, and charged into an electric heating furnace maintained at that temperature to obtain 1. After holding for 20 hours,
The furnace was cooled at the cooling rate at the time to perform a slow cooling process after winding. Both surfaces of the obtained steel sheet were ground to form a cold-rolled base material having a thickness of 4 mm, and were cold-rolled at a reduction ratio of 70 to 90%.
Recrystallization annealing equivalent to continuous annealing held for 2 seconds or 750
Recrystallization annealing equivalent to box annealing maintained at 5 ° C. for 5 hours was performed.
Thereafter, these annealed sheets were subjected to temper rolling at an elongation of 0.8% to evaluate the performance.

The r value was measured by performing a tensile test on a JIS No. 5 tensile test piece taken from the rolling direction, the 45 ° direction, and the width direction.

The secondary work brittleness was evaluated by the following method.
A circular base plate having a diameter of 59.4 mm was sampled from each cold-rolled steel plate, and subjected to deep drawing at a drawing ratio of 1.8 using a cylindrical deep drawing tester to form a cylindrical cup having a diameter of 33 mm. The ears of these cylindrical cups were cut and removed to form cylindrical cups having a depth of 17 mm, which were used as samples for measuring the secondary working brittleness of steel plates. The above cylindrical cup cooled to various temperatures is covered with a bottom face up on a truncated cone-shaped mold having a tip angle of 60 degrees, and a mass of 5 cm from a height of 80 cm above the cup.
kg of weight is dropped on the bottom of the cylindrical cup, and the critical temperature at which brittle cracking occurs on the side wall of the cylindrical cup is determined.
This was used as an index of secondary work brittleness resistance.

Table 2 shows the rolling conditions and performance evaluation results of the prototype cold rolled steel sheet.

[0043]

[Table 2]

As shown in Table 2, the cold rolled steel sheets manufactured under the conditions specified in the present invention were all:
| △ r | was 0.25 or less, and the in-plane anisotropy was small and good. Ears of the cylindrical cup hardly occurred during deep drawing. Further, the brittle transition temperature of these cold-rolled steel sheets was −90 ° C. or lower, indicating good secondary work brittleness resistance. On the other hand, steel D has an excessively low B content, so that the brittle transition temperature is as high as −50 ° C., and the secondary working embrittlement resistance is not preferable. Steel E had an excessive B content, an average r value of less than 1.5, and poor deep drawability.
Steels A, B and C whose chemical compositions are within the range specified by the present invention
However, when the processing conditions were out of the range defined by the present invention, | Δr | exceeded 0.25, and the in-plane anisotropy was not good.

[0045]

The cold-rolled steel sheet manufactured according to the method specified by the present invention has a small in-plane anisotropy, has few forming defects during deep drawing, and has excellent secondary work brittleness resistance. The production method of the present invention is an economical production method because excellent properties can be obtained by specifying production conditions without using expensive alloy elements.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between Δr of steel containing 0.0005% of B, winding temperature, and cold rolling reduction.

FIG. 2 is a diagram showing the relationship between Δr, winding temperature and cold rolling reduction of steel containing 0.0030% of B.

FIG. 3 is a diagram showing the relationship between Δr, B content and cold rolling reduction when the winding temperature is 450 ° C.

FIG. 4 is a diagram showing the relationship between Δr, B content and cold rolling reduction when the winding temperature is 500 ° C.

Claims (1)

[Claims] (1) C: 0.0005 to 0.005 by weight%
%, Si: 0.3% or less, Mn: 0.01 to 0.4%,
sol. Al: 0.01 to 0.05%, Ti: 0.01
0.1% and Ti%> ｛(48/12) C% +
(48/14) N% + (48/32) S%｝, B: 0.
A steel containing 0003 to 0.003%, the balance consisting of Fe and unavoidable impurities is hot-rolled and wound into a coil at 560 ° C or less, and a rolling rate of 40% or more and in a range satisfying the following formula: A cold-rolled steel sheet having low in-plane anisotropy and excellent secondary work brittleness resistance, characterized by cold rolling at 650 ° C. or higher and _lower than the A _c3 transformation point. (Equation 1) Here, CT represents the winding temperature (° C.), CR represents the cold rolling reduction (%), and B represents the B content (% by weight) in the steel.