JPH06179922A - Production of high tensile strength steel sheet for deep drawing - Google Patents

Abstract

PURPOSE:To inexpensively produce a high tensile strength steel sheet excellent in deep drawability and minimal in partial dispersion of strength with superior productivity. CONSTITUTION:A slab of a steel which has a composition consisting of 0.0005-0.012% C, <=1.5% Si, 0.05-3.0% Mn, <=0.15% P, <=0.01% S, <=0.1% sol.Al, <=0.005% N, 0.5-2% Cu, 1/3Cu to 2/3Cu% Ni, 0.01-0.2% Ti, and the balance Fe with inevitable impurities or further containing 0.003-0.1% Nb and/or 0.0003-0.003% B is hot-rolled and coiled at <=550 deg.C. After cold rolling at 65% draft, the resulting steel sheet is subjected to primary annealing by means of a continuous annealing line, to temper rolling at 0.5-5% elongation percentage, and then to secondary annealing consisting of heating and holding at 500-750 deg.C for 5sec-2min in a continuous annealing line again or a continuous hot dip galvanizing line.

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 high-strength thin steel sheet which is formed into various shapes by press working or the like and is used as a structural member, which is excellent in deep drawability and has small variations in strength. It is a thing.

[0002]

[Prior art and its problems] Based on an "ultra-low carbon Ti-added steel" in which Ti was added after sufficient decarburization at the steelmaking stage,
Many proposals have been made so far regarding a high-strength cold-rolled steel sheet in which Si, Mn, Cr, or P is added to increase the strength. For example, referring to Japanese Patent Publication No. 57-57945, there is disclosed a cold-rolled steel sheet obtained by adding a large amount of P to the above ultra-low carbon Ti-added steel, and Japanese Patent Laid-Open No. 57-47832 discloses ultra-low carbon. A cold-rolled steel sheet in which a large amount of Mn is added alone to Ti-added steel is disclosed.

However, none of such high-tensile cold-rolled steel sheets has sufficient formability and cannot be applied to applications requiring deep drawing. In other words, in the above high-tensile cold-rolled steel sheet, when the tensile strength is about 50 kgf / mm ² , the r value (rankfode value) shows only a low value of about 1.6, and it is difficult to perform deep drawing without trouble. It is due to the fact.

Therefore, the inventors of the present invention previously proposed a high-tensile cold-rolled steel sheet for deep drawing, in which a predetermined amount of P and Mn are added to ultra-low carbon Ti-added steel to improve the deep drawability. (See JP-A-63-190141). However, the above-mentioned steel sheet proposed by the present inventors was able to increase the tensile strength to 500 N / mm ² or more and was able to secure a relatively good deep drawability. However, there is a problem in that the cost becomes high because of the addition of. Moreover, when manufacturing a cold-rolled steel sheet, the strength of the hot-rolled sheet, which is an intermediate material, is at the same high level as that of the product, so a considerable burden is placed on the cold rolling mill, making it difficult to control the flatness of the steel sheet. It was also pointed out that there was an inconvenience.

Separately from this, the inventors of the present invention "cold-rolled an ultra-low carbon Ti-added steel in which a predetermined amount of Cu was added and the composition of Mn, S, and P was adjusted, and then at high temperature. By continuous annealing, it shows relatively good deep drawability and
It has been proposed as a useful "method for producing a high-formability high-strength steel sheet" by finding that "a cold-rolled steel sheet satisfying product strength can be obtained by performing heat treatment after forming processing" (JP-A-2- (See Japanese Patent No. 156025). However, the steel sheet obtained by this method has a problem in practical use that "the heat treatment cost for improving strength after forming is high".

Similarly, as an ultra low carbon steel cold-rolled steel sheet to which Cu is added, Japanese Patent Laid-Open No. 64-4429 discloses, "Cold-added ultra low carbon steel is cold-rolled and then recrystallized.
A method for producing a high-strength steel sheet for deep drawing, which comprises continuously annealing to form a solid solution of Cu and further performing secondary annealing to precipitate ε-Cu in the steel sheet before forming ”is disclosed. .

However, this Japanese Patent Laid-Open No. 64-4429
In the method disclosed in the publication, when "box annealing" is applied to the secondary annealing, (a) temperature unevenness occurs in the longitudinal direction of the steel strip, and the precipitation state of ε-Cu is the position of the steel strip. Change, resulting in variations in strength. It has been pointed out that this tendency occurs especially when the box annealing temperature is low, and that (b) the productivity is extremely poor because heating and cooling take a long time, which increases the manufacturing cost.

When "continuous annealing" is applied to the secondary annealing, it is necessary to add a large amount of Cu because it must be age-hardened in a short time. Therefore, a large amount of Ni is added together with this. Therefore, it is inevitable that not only the deep drawability after annealing deteriorates, but also the manufacturing cost increases.

In view of the above, an object of the present invention is to establish means for manufacturing a high-strength thin steel sheet which is excellent in deep drawability and has a small partial strength variation with good productivity. .

[0010]

Means for Solving the Problems The inventors of the present invention have conducted the earnest research to achieve the above object, and have obtained the following findings.

That is, the inventors of the present invention have described the above-mentioned Japanese Patent Laid-Open No. 2-1.
In the process of studying the invention relating to No. 56025, it was found that "in an ultra-low carbon Ti-added steel containing a predetermined amount of Cu, the strain added prior to the heat treatment significantly promotes the precipitation of ε-Cu". , And proceeded with the research, thinking that this phenomenon might be the key to solve the problems pointed out in the "method of manufacturing high-strength steel sheet for deep drawing by double annealing method".

That is, when cold-rolled steel sheet containing ultra-low carbon Ti containing Cu is subjected to primary and secondary annealing for modification, 2
If a pre-strain is added to the steel sheet prior to the next annealing, ε-Cu can be obtained by simply performing heat treatment in a rapid heating / cooling facility with good productivity such as a “continuous annealing line” or “continuous hot dip galvanizing line”. Based on the assumption that the uniform precipitation of P should be performed, the effect of the amount of pre-strain and the secondary annealing temperature on the strength of the steel sheet was investigated. Here, the pre-strain was applied by temper rolling.

As a result, the following has been clarified. a) The higher the elongation of temper rolling, the higher the tensile strength after secondary annealing. However, as the elongation percentage of temper rolling increases, the total elongation decreases. b) When the annealing temperature in the secondary annealing is around 680 ° C, 2
The tensile strength after the next annealing becomes maximum. c) When the same strength increase occurs, "when the elongation of temper rolling is reduced and the secondary annealing is performed at high temperature" is "when the elongation of temper rolling is increased and the secondary annealing is performed at low temperature. The degree of deterioration of total elongation is smaller than that of ".

The present invention has been completed through further studies based on the above-mentioned findings and the like. "C: 0.0005 to 0.012% (hereinafter,% representing the component ratio shall be% by weight), Si : 1.5% or less, Mn: 0.05 to 3.0%,
P: 0.15% or less, S: 0.01% or less, acid-soluble Al: 0.1%
Below, N: 0.005% or less, Cu: 0.5 to 2%, Ni: ¹ /
_{^{3 Cu ~ 2/3 Cu%}} , Ti: or containing 0.01 to 0.2 percent, or even Nb: 0.003 ~0.1%, B: also comprise from 0.0003 to 0.003% of one or the balance Fe and incidental Steel slab consisting of mechanical impurities is hot-rolled, wound in a temperature range of 550 ° C or lower, then cold-rolled at a rolling reduction of 65% or more, and then primary-annealed in a continuous annealing line, followed by an elongation of 0.5 to 5 %, And then subjected to secondary annealing by heating and holding again in a continuous annealing line or a continuous hot dip galvanizing line in a temperature range of 500 to 750 ° C. for 5 seconds to 2 minutes to obtain excellent strength and deep drawability. Another feature is that high-strength thin steel sheets can be manufactured with high productivity and at low cost. "

In the following, the reason why the composition of the raw material steel and the manufacturing conditions are limited as described above in the present invention will be explained.

[Action]

 A) Composition of raw steel

The C content C is a component that is unavoidably accompanied in steel, and it is not practical to reduce it to less than 0.0005% in the current steelmaking technology because the cost becomes too high. But 0.012%
If the content of C exceeds the above range, the required addition amounts of Ti and Nb increase, resulting in high cost, and at the same time, deterioration of steel sheet properties. Therefore, the C content is set to 0.0005 to 0.012%.

Si Content Si is a component added as a solid solution strengthening element for adjusting the strength of the steel sheet. However, if Si is contained in excess of 1.5%, austenite does not appear in an ultra low carbon steel such as the steel of the present invention, and it becomes difficult to control the grain size. Therefore, the upper limit of Si content was set at 1.5%.

The Mn content Mn is a component that is added to prevent hot embrittlement of steel by fixing S as MnS, and at the same time is added as a solid solution strengthening element to adjust the strength of the steel sheet. Is.
In particular, the combined addition of P makes it possible to increase the strength without impairing the deep drawability. However, if the content is less than 0.05%, the above effect is not sufficient, while 3.0%
%, The deep drawability is adversely affected, so the Mn content was set to 0.05 to 3.0%.

The P content P is a component added as a solid solution strengthening element for adjusting the strength of the steel sheet. However, if the content exceeds 0.15%, the steel sheet becomes too hard and the ductility decreases significantly, so the upper limit of the P content was made 0.15%.

The S content S is an impurity element inevitably accompanied in the steel, and if the content exceeds 0.01%, the hot workability of the steel and the deep drawability of the steel sheet are adversely affected. Therefore, the upper limit of S content is
It was set at 0.01%.

The N content N is also an impurity element necessarily contained in steel, and if the content exceeds 0.005%, the amount of Al added increases and the manufacturing cost increases. Therefore, the N content is limited to 0.005% or less, preferably 0.0030% or less.

Acid-soluble Al content Acid-soluble Al (sol. Al) is Ti, Nb after vacuum degassing of molten steel.
It is added for deoxidation prior to addition, and it may be considered that deoxidation is completed if traces remain.
Generally, it is judged to be sufficient if the final product contains 0.005% or more. However, if the content exceeds 0.1%, the steel becomes hard and the ductility deteriorates.
l.Al content was set to 0.1% or less.

Cu Content Cu is one of the main components in the steel of the present invention,
Precipitated as ε-Cu during secondary annealing and added to increase the strength of the steel sheet. However, its content is 0.5
If it is less than%, the increase in strength during secondary annealing is small and the desired strength cannot be secured. On the other hand, as the Cu content increases, the strength increase under the same manufacturing conditions increases, but the effect is almost saturated at 2%, and when Cu is contained in excess of 2%, the steel sheet is formed at low temperature after hot rolling. Since ε-Cu precipitates in the hot-rolled sheet even after winding, it cannot be expected to develop a recrystallization texture suitable for deep drawability in the primary annealing. Therefore, the Cu content is set to 0.5 to 2%.

Ni-containing Cu-added steel is liable to cause a unique “surface glaze cracking” during hot working, but the Ni component is added to prevent this glaze cracking. Ni addition amount is determined in correspondence to the Cu content, but is less than _^1/3 of the content Cu content of Ni can not be completely prevented tortoise shell cracking, whereas, the Cu content of _^2/3 If the content of Ni exceeds Ni, the steel plate cost will be too high. Therefore, Ni content is specified to ¹ / _{3-1 ^2/3} of the Cu content.

Ti content Ti is such that by fixing C and N in the steel as TiC and TiN, the steel sheet is recrystallized by primary annealing in a state where the amount of solid solution C and N is small, and good deep drawability is obtained. It is added to give.
However, if the content is less than 0.01%, sufficient deep drawability cannot be ensured, and if it exceeds 0.2%, the amount of Ti oxide in the steel sheet increases and the ductility deteriorates.
The content was set to 0.01 to 0.2%. However, the Ti content is preferably in the range where the formula Ti / 48- (N / 14 + S / 32) + Nb / 93 ≥ C / 12 is satisfied when "Nb / 93> C / 12", and "Nb When "/ 93≤C / 12", it is better to adjust the range to satisfy the expression Ti / 48- (N / 14 + S / 32) + Nb / 93≥0.

The Nb content Nb has an effect of fixing C in the steel as NbC and further improving the deep drawability of the steel sheet by making the crystal grain size of the hot rolled sheet finer, so that it is added if necessary. To be done. The optimum addition amount of Nb is determined in relation to the addition amount of Ti, but its content is
If it is less than 0.003%, the deep drawability improving effect is not sufficient, while
If the content exceeds 0.1%, the recrystallization temperature of the steel sheet becomes too high and sufficient ductility cannot be obtained. Therefore, although the Nb content was set to 0.003 to 0.1%, preferably when Ti / 48- (N / 14 + S / 32)> 0, the formula Ti / 48- (N / 14 + S / 32) + Nb Within the range of / 93 ≥ C / 12, and "Ti / 48- (N / 14 + S / 32
) ≦ 0 ”, it is better to adjust to the range that satisfies the expression Nb / 93 ≧ C / 12.

B content The present invention is also directed to the production of so-called "extremely low C-IF steel sheet", but when this extremely low C-IF steel sheet is applied to deep drawing applications, it is possible to obtain low temperature after deep drawing. A phenomenon called "secondary working brittleness" occurs in which almost no toughness is observed.
This is because the solid solution C and N disappear in the steel sheet and the grain boundary strength decreases. B is an element having a strong segregation property at the grain boundaries and has the effect of segregating at the grain boundaries of the steel to strengthen the grain boundaries, so it is added as necessary to suppress the above-mentioned secondary work embrittlement. . However, its content is 0.0003
If it is less than 0.00%, the secondary working brittleness cannot be sufficiently suppressed, while if it exceeds 0.0030%, not only the effect is saturated but also the deep drawability of the steel sheet is deteriorated. The B content was set to 0.0003 to 0.0030%.

B) Manufacturing conditions Hot rolling coiling temperature When the steel strip is coiled at a high temperature after completion of hot rolling, ε-Cu precipitates in the slow cooling process after coiling, and during the subsequent primary recrystallization annealing. The growth of crystal grains having a “recrystallization orientation that enhances deep drawability” is suppressed, and the deep drawability of the product deteriorates. Therefore, it is better that the hot rolling temperature is lower, but it is actually 550.
Since no problem will occur if the temperature is ℃ or less, the upper limit of the hot rolling coiling temperature is set to 550 ° C in consideration of productivity. However, it is preferable to wind it at 500 ° C. or lower.

Cold Rolling Reduction Ratio If the cold rolling reduction ratio is less than 65%, the crystals having preferred crystal orientations are not sufficiently aligned, and the r value of the product steel sheet deteriorates to ensure good deep drawability. I can't. Therefore, the rolling reduction of cold rolling is set to 65% or more.

Elongation of temper rolling As described above, temper rolling is carried out to promote precipitation of ε-Cu during secondary annealing. The elongation of temper rolling is selected according to the amount of Cu added and the required amount of strength increase, but if it is less than 0.5%, the effect of promoting the precipitation of ε-Cu is small and a sufficient strength increase cannot be obtained. On the other hand, as the elongation percentage of temper rolling increases, the amount of increase in strength increases, but the promoting effect saturates in the vicinity of 5%. Therefore, the elongation of temper rolling is set to 0.5 to 5%, but it is preferable to set it to the range of 1 to 2%.

Secondary Annealing Condition The secondary annealing is a continuous annealing line or continuous hot dip galvanizing in order to precipitate ε-Cu by utilizing the acceleration effect by the strain introduced in the temper rolling to increase the strength of the steel sheet. It is carried out in line. However, if the maximum heating temperature in this secondary annealing is less than 500 ° C, the diffusion of Cu atoms does not occur sufficiently so that the precipitation of ε-Cu does not proceed. On the other hand, if the maximum heating temperature exceeds 750 ° C, the Since the solid solution limit of Cu in the inside becomes large and Cu that is in solid solution in a supersaturated state disappears, precipitation of ε-Cu does not proceed, and a sufficient strength increase cannot be obtained in any case. Therefore, the secondary annealing temperature is 500 to 750.
Although the temperature is set to 0 ° C, it is preferably adjusted to 650 to 720 ° C.

If the heating and holding time in the secondary annealing is less than 5 seconds, the precipitation of ε-Cu will be insufficient and the desired strength of the steel sheet cannot be secured. Therefore, the heating and holding time in the secondary annealing is set to 5 seconds to 2 minutes because it causes a disadvantage in productivity. Of course, the primary annealing is 2
Since the subsequent annealing is also carried out uniformly on the continuous annealing line or the continuous hot dip galvanizing line, there is no partial strength variation in the longitudinal direction of the steel strip.

Next, the present invention will be described more specifically by way of examples.

EXAMPLE First, steel having the chemical composition shown in Table 1 was melted into an ingot in a laboratory vacuum melting furnace, and then hot forging was performed to prepare a 25 mm-thick laboratory slab.

[0034]

[Table 1]

Subsequently, this slab is heated at 1250 ° C. in an electric furnace.
After 1 hour of heating and holding, it was rolled in a temperature range of 1150 to 930 ° C. for 3 passes by an experimental hot rolling mill to obtain a 6 mm-thick hot rolled sheet. And as a simulation of winding,
The steel sheet after hot rolling is immediately cooled to a temperature of 700 to 400 ° C by forced air cooling or water spray cooling, then inserted into an electric furnace maintained at that temperature, and further held at that temperature for 1 hour, and then at 20 ° C. The furnace was cooled at / hr.

Next, the surface of these hot-rolled sheets was polished to 4.2
A cold-rolled base metal having a thickness of mm was cold-rolled to a thickness of 1.4 mm. Then, the obtained cold-rolled sheet imitates continuous annealing, is heated to 820 ° C. at a heating rate of 10 ° C./sec in an infrared heating furnace, is held at that temperature for 40 seconds, and is then heated to 700 ° C. for 3 seconds.
Gradually cool at a cooling rate of ℃ / sec, then 50 ℃ up to 400 ℃
After cooling at a cooling rate of / sec and holding at that temperature for 3 minutes, it was finally cooled to room temperature at a cooling rate of 10 ° C / sec.

Next, the elongation percentage of the steel sheet that has undergone the above primary annealing is:
After the temper rolling of 1%, similarly, the heat treatment by continuous annealing or continuous hot dip galvanizing is imitated, and 1
After heating to 680 ° C. at a heating rate of 0 ° C./sec and holding at that temperature for 40 seconds, secondary annealing was performed to cool to room temperature at a cooling rate of 50 ° C./sec.

For some steel sheets, the elongation percentage in temper rolling was changed to 0 to 8%, and the maximum heating temperature in secondary annealing was set to 400 to 840 ° C.

A tensile test (using JIS No. 5 test piece) and a secondary work embrittlement test were carried out on each of the annealed plates thus manufactured. In the secondary working brittleness test, a method was adopted in which the annealed plate was drawn into a cup with a drawing ratio of 1.6, and an impact load was applied at -60 ° C to determine whether the cup wall portion was brittlely broken. The results of these tests are shown in Table 2 together with the results of observation of the cracking occurrence state during hot rolling.

[0040]

[Table 2]

As is clear from the results shown in Table 2, the steel sheets obtained by the test Nos. 2, 3, 6 and 7 according to the conditions of the present invention show sufficient deep drawability and high strength, and are subjected to secondary working. It can be seen that the problem of brittleness does not occur.

On the other hand, in the test number 1, Cu of the raw steel
Since the content is too large, a larger increase in strength can be obtained after secondary annealing, but even if wound at 500 ° C after hot rolling, ε
-Since Cu precipitates, a recrystallization texture suitable for deep drawability does not develop during the primary annealing, and the deep drawability is poor. Moreover,
It can be seen that when steel A, which has a large increase in strength and does not contain B, is used as a material, secondary work embrittlement is exhibited.

On the contrary, when steel D having a low Cu content is used as a material, it is not possible to obtain a sufficient strength increase by the secondary annealing, as can be seen from the result of test number 4.

On the other hand, from the results of test Nos. 9 and 12, Nb
It can be confirmed that the deep drawability can be improved when the steels G and J added with are used as the materials. In addition, test number 10 and
From the results of No. 11, it can be seen that even if the steels H and I to which Si, Mn, and P are added as the strengthening elements are used as raw materials, it is possible to manufacture a high-strength thin steel sheet that has good deep drawability and does not cause a problem of secondary work brittleness.

However, when steel E having a lower Ni content relative to the Cu content was used as a raw material, as can be seen from Test No. 5, cracks were observed on the surface and edges of the steel sheet in the observation after hot rolling. Was given. Further, when the winding temperature of Steel F is higher than the range of the present invention, the deep drawability is inferior as in Test No. 8.

1 and 2 summarize the effects of "tempered rolling elongation" and "secondary annealing maximum heating temperature" on the tensile strength and total elongation of steel F after secondary annealing. Indicated.

From FIG. 1, it can be seen that when the temper rolling elongation is small, a sufficient increase in strength cannot be obtained due to the secondary annealing, and when the temper rolling elongation is extremely large, the deterioration of the elongation becomes large and the depth becomes deep. It is clear that it cannot withstand drawing.
Furthermore, it can be confirmed from FIG. 2 that a sufficient increase in strength cannot be obtained by the secondary annealing even if the maximum heating temperature of the secondary annealing is too low or too high.

[0048]

[Summary of Effects] As described above, according to the present invention, a high-strength thin steel sheet exhibiting excellent deep drawability and small strength variation can be produced at low cost with high productivity by only performing secondary annealing for a short time. Industrially useful effects such as the ability to manufacture are brought about.

[Brief description of drawings]

FIG. 1 is a graph showing the effect of temper rolling elongation on tensile strength and total elongation after secondary annealing.

FIG. 2 is a graph showing the influence of the secondary annealing maximum heating temperature on the tensile strength and total elongation after the secondary annealing.

Claims (2)

[Claims] 1. A weight ratio of C: 0.0005 to 0.012%, Si: 1.5% or less, Mn: 0.05.
~ 3.0%, P: 0.15% or less, S: 0.01% or less,
Acid-soluble Al: 0.1% or less, N: 0.005% or less, C
u: 0.5 ~2%, Ni: 1/3 Cu ~ 2/3 Cu%, Ti: 0.01~
A steel slab containing 0.2% and the balance consisting of Fe and unavoidable impurities is hot-rolled, wound in a temperature range of 550 ° C. or lower, and then cold-rolled at a rolling reduction of 65% or more, and then primary in a continuous annealing line. After annealing, temper rolling with an elongation of 0.5 to 5%, and then perform secondary annealing again by heating for 5 seconds to 2 minutes in a temperature range of 500 to 750 ° C in a continuous annealing line or a continuous hot dip galvanizing line. A method for producing a high-strength thin steel sheet for deep drawing, characterized by: 2. A weight ratio of C: 0.0005 to 0.012%, Si: 1.5% or less, Mn: 0.05.
~ 3.0%, P: 0.15% or less, S: 0.01% or less,
Acid-soluble Al: 0.1% or less, N: 0.005% or less, C
u: 0.5 ~2%, Ni: 1/3 Cu ~ 2/3 Cu%, Ti: 0.01~
A steel slab containing 0.2% and further containing one or two of Nb: 0.003 to 0.1% and B: 0.0003 to 0.003% and the balance being Fe and inevitable impurities is hot-rolled to 550 ° C or less. , Then cold-rolled at a rolling reduction of 65% or more, then primary-annealed in a continuous annealing line, and then temper-rolled with an elongation of 0.5 to 5%, and then again in a continuous annealing line or continuous molten zinc. A method for producing a high-strength thin steel sheet for deep drawing, comprising performing secondary annealing in which a temperature is kept in a temperature range of 500 to 750 ° C. for 5 seconds to 2 minutes in a plating line.