JPH01184251A - High strength cold rolled steel plate for deep drawing and its manufacture - Google Patents

Abstract

PURPOSE:To improve the baking hardenability of paint and secondary working britleness resistance of the title steel plate by subjecting a steel in which C, Si, P, Mn, N and Al are specified to hot rolling, cold rolling and temp. treat ment under the prescribed conditions. CONSTITUTION:The steel consisting of, by weight, <=0.004% C, 0.1-0.8% Si, 0.02-0.1% P, 0.05-0.5% Mn, <=0.005% N and 0.001-0.1% acid soluble Al and the balance constituted of Fe is refined. The steel is hot-rolled at >=Ar3 transformation point, is then reeled at <=720 deg.C, is descaled and is thereafter cold-rolled at 60-95% draft rate. The steel is furthermore heated to the recrystallization temp.-Ac3 transformation point and is annealed.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a high-strength cold-rolled steel sheet for deep drawing that has excellent bake-hardening properties and secondary work brittleness resistance, and an inexpensive and stable manufacturing method thereof.

(Prior art) Recently, in order to improve the safety, fuel efficiency, and durability of automobiles, for example, there has been a sudden need for high-strength cold-rolled steel sheets of 32 kgf/mm2 class or higher, especially steel sheets suitable for deep drawing. It's increasing. Moreover, since it is used as a coated steel sheet, it is required to improve the baking hardenability as well as the resistance to secondary work brittleness.

Conventionally, high-strength cold-rolled steel sheets used for such applications include ultra-low carbon Ti-added steel, Sis Mn, and Cr.
Many materials have been proposed that have increased strength by adding P.

For example, in Japanese Patent Publication No. 57-57945, 0.050
A cold-rolled steel sheet to which a large amount of P is added, 0°120%, is disclosed. However, in this case, the Si content is rather limited to less than 0.10%, making it an expensive material.

In addition, in Japanese Patent Publication No. 50-31089, the above-mentioned ultra-low carbon Ti
Added steel (TiS 0.01-0.4%) with 0.5-0.5% Si
An example of adding 2.5% is disclosed. However, the r-value level is not necessarily high, and oxidation of the surface of the steel sheet is a problem, making it difficult to put it into practical use.

On the other hand, in such ultra-low carbon Ti-added steel, it is difficult for solid solution carbon and nitrogen to remain in the steel sheet. Therefore, it is known that baking not only makes it difficult to impart hardenability, but also tends to cause secondary processing brittleness. Therefore, in boat fishing, B is added in combination to prevent secondary processing brittleness. However, adding a large amount of B can cause cracking of the slab, and it is difficult to stably add such a large amount of B, which may lead to operational instability. It is not a definitive means of preventing brittleness.

(Problem to be Solved by the Invention) From the above, the present inventors, as well as the industry, believe that tensile strength is 32 kgf/mm2 or more, yield strength is tensile strength -10 kgf/mm2 or less, r Value 1.
A high tensile strength cold-rolled steel sheet with a hardness of 8 or more, which is hardenable and less susceptible to secondary work brittleness, and a method for manufacturing it using low-cost alloy additions have been researched and sought for many years.

Therefore, the object of the present invention is to obtain a tensile strength of 32 kgf/m.
m2 or more, yield strength is tensile strength -10 kgf/mm2 or less, r value is 1.8 or more, and if necessary, baking is hardenable. High tensile strength cold rolled for deep drawing processing that does not easily cause secondary processing brittleness. It is an object of the present invention to provide a steel plate and a method for manufacturing the same at low cost and stably.

(Means for Solving the Problems) By the way, the present inventors have developed an ultra-low carbon steel as a steel type that can be produced more cheaply and stably in order to ensure hardenability and prevent secondary work embrittlement. Focusing on this, we conducted a detailed investigation on the steel composition and manufacturing conditions that affect the mechanical properties of ultra-low carbon steel after continuous annealing, and obtained the following results.

That is, (1) in ultra-low carbon aluminum killed steel, when rolled under normal hot rolling conditions, the structure of the hot-rolled sheet becomes coarse grained, and the deep drawability of the annealed sheet is improved.

This is thought to be because the purity of the steel is so high that there is nothing to suppress recrystallization and grain growth during the hot rolling and cold rolling steps. However, this deep drawability has a large in-plane anisotropy.

■On the other hand, when 0.1% or more of Si and 0.02% or more of P are added to such ultra-low carbon steel, (1) a fine grain structure is obtained in the hot rolled sheet, and (2) recrystallization is prevented. This suppresses the generation of recrystallized oriented grains that occur at the initial stage of recrystallization. The occurrence of such recrystallized grains is unfavorable for deep drawability.

Moreover, since both Si and P are elements that strengthen steel, a high-strength steel plate with good deep drawability can be obtained as a result.

■Even though it is called ultra-low carbon steel, the carbon content in the steel is, for example, 0.0.
If it exceeds 0.040%, the recrystallization suppressing effect is too strong and the structure of the hot rolled sheet becomes fine grained, but the recrystallization texture becomes random and the deep drawability of the annealed sheet is not good.

In other words, the present inventors made the carbon extremely low, 0.0040% or less, and added Si 0.1% or more, 20
The present invention was completed based on the knowledge that the above object can be achieved by adding 6025 or more.

Here, the gist of the present invention is as follows: C: 0.0040% or less, Si: 0.10 to 0.0% by weight.
80%, P: 0.02-0.10%, Mn:
Baking with a steel composition consisting of 0.05-0.5%, N: 0.0050% or less, acid-soluble Q: O, 001-0.1%, the balance Fe and unavoidable impurities has good hardenability and resistance. This is a high-strength cold-rolled steel sheet for deep drawing with excellent secondary processing brittleness.

In addition, from another aspect, the present invention involves forming steel having the above-mentioned composition into a steel billet, subjecting it to hot rolling at a temperature higher than the transformation point, then coiling at a temperature of 720°C or lower, and then pickling, etc. Baking is characterized by descaling, cold rolling at a reduction rate of 60 to 95%, and further heating to a temperature range above the recrystallization temperature and below the Ac3 transformation point for annealing. This is a method for producing high-strength cold-rolled steel sheets for deep drawing that have excellent resistance to secondary work brittleness.

The above-mentioned steel billet is generally a slab slab produced by continuous casting, but it may also be a steel billet that is bloomed by an ingot-forming method, and there is no particular restriction. Furthermore, as the annealing method, any of box annealing, continuous annealing, and continuous hot-dip Zn plating can be applied.

As described above, the steel sheet according to the present invention differs from conventional ultra-low carbon steel sheets for deep drawing by adding carbonitride-forming elements such as Ti and Nb in order to obtain a recrystallized texture favorable for deep drawability. I haven't. Therefore, the carbon in the steel remains as a solid solution in the steel plate, which not only makes the steel plate less susceptible to secondary work brittleness, but also makes it easier to bake and impart hardenability to the steel plate.

(Function) Next, the reason why the components of the steel plate in the present invention are limited as described above will be explained. In this specification, unless otherwise specified, "%J" means "%J by weight."

C: This is one of the important constituent elements of the present invention.

It is naturally contained in steel, and if it exceeds 0.0040%, a recrystallized texture favorable for deep drawability will not develop. In addition, the strain aging property of the steel sheet at room temperature increases significantly.

Preferably it is 0.0020% or less.

Sj: One of the important constituent elements of the present invention. Sj is added as a reinforcing element for the steel, and when added in combination with P, contributes to the formation of a recrystallized texture that is favorable for deep drawability. If it is less than 0.10%, the effect of strengthening the steel and controlling the recrystallization texture cannot be sufficiently obtained. Moreover, if it exceeds 0.80%, the rope becomes brittle, which is not preferable.

P: One of the important constituent elements of the present invention, which is added as a reinforcing element to the steel, and is added in combination with 0.10 to 0.80% Si to improve recrystallization, which is favorable for deep drawability. Contributes to the formation of collective tissue. If it is less than 0.02%, the effect of strengthening the steel and controlling the recrystallization texture cannot be sufficiently obtained.

Moreover, if it exceeds 0.10%, the steel becomes brittle, which is not preferable.

Mn: 0.05% is inevitably added to combine with S to form MnS and prevent hot embrittlement of steel.
or more is required. However, if it exceeds 0.5%, strengthening by solid solution Mn will occur and the steel will become hard, which is not preferable.

N: The less the better. Like C, it is naturally contained in steel, and is fixed by AQ. But N
If the content is large, the amount of M added increases accordingly, so 0
．． 0050% or less. Preferably it is 0.0020% or less.

Acid-soluble AQz Added to fix N in steel, generally 0.001% or more, (acid-soluble t8
It is sufficient if the atomic ratio (AQ/N) is 1 or more. Shigashi, 0.
If it is added in an amount exceeding 1%, the steel becomes hard and the ductility deteriorates, which is not preferable.

Further, the lower the amount of unavoidable impurities such as S, the better. Next, the manufacturing conditions of the manufacturing method according to the present invention are limited for the following reasons.

Slab heating conditions: In the present invention, such conditions are not particularly limited. However, in the continuous casting method, after casting the slab,
3. When hot rolling is performed by reheating after cooling to below the transformation point, if the temperature is less than 1000"C, temperature unevenness will occur in the slab, and hot rolling may not be completed above the Ar + transformation point. It becomes difficult.

Therefore, it is preferable to heat to 1050 to 1150°C. Moreover, if hot rolling is performed without cooling the slab to below the Ar3 transformation point after casting, there is no particular problem.

Natural rolling finish peel: When the hot rolling finishing temperature falls below the Ar3 transformation point, the steel will be hot rolled in the α+γ region or the α region, and normally disappears with the γ-α transformation. The hot-rolled texture remains in the hot-rolled sheet, hindering the development of a recrystallized texture favorable for drawability.

3-W degrees: When the winding temperature exceeds 720°C, the scale becomes thick during winding, descaling performance deteriorates, and abnormal grain growth occurs to generate coarse grains. Further, unless the amount of N in the steel is sufficiently low, it is necessary to coil the steel at a high temperature in order to precipitate the N in the steel as AQN. Preferably 550
~700°C.

Flat plate rolling ■Reduction ratio: If this is less than 60%, a recrystallized texture favorable for deep drawability will not develop. A higher rolling reduction ratio is preferable. However, if it exceeds 95%, the deep drawability deteriorates.

It goes without saying that since it is recrystallization annealing, it is necessary to heat it above the recrystallization temperature, but when it is heated above the Ac3 transformation point, it transforms from α-γ→α, forming during the recrystallization process. Since this will erase the recrystallized texture that is favorable for deep drawability, it is necessary to keep the temperature below the transformation point. In addition, if the annealing temperature exceeds Ac, the transformation point,
Read after soaking.

It is preferable to slowly cool the material to a temperature below the transformation point at a cooling rate of 5° C./S or less.

The steel sheet manufactured in this way is annealed and temper rolled before being shipped as a product. Furthermore, if necessary, the surface is electroplated or painted before being shipped.

Next, examples of the present invention will be shown, but these are merely illustrative of the present invention and are not intended to unduly limit the present invention.

Example 1 In an experimental vacuum melting furnace, steels with different amounts of carbon, nitrogen, and amounts of Si and P added were melted as shown in Table 1. These were made into experimental slabs with a thickness of 25 mm by hot forging. Next, it was heated in an electric furnace at 1200°C for 1 hour, and then rolled in a temperature range of 1150°C to 930°C for 3 passes using an experimental hot rolling mill to obtain a hot rolled plate with a thickness of 4.0 mm. As a simulation of winding, the steel plate was immediately cooled to a temperature of 700 to 500'C by forced air cooling or water spray cooling after hot rolling, then inserted into an electric furnace maintained at that temperature, and further heated at that temperature. After holding for 1 hour, it was cooled in the furnace at 20° C./hr. Next, after pickling,
It was cold rolled to a thickness of 0.8 mm. The cold-rolled sheet was heated in an infrared heating furnace at 10°C/s to 850°C, which is a temperature above the recrystallization temperature and below the AC 3 point, held at that temperature for 40 s, and then slowly heated to 750°C at 3°C/S. Cool, then add 10”Cl
3 and cooled to room temperature. After annealing and temper rolling at an elongation rate of 1.2%, a tensile test was conducted using a JISS No. test piece.

The results are shown in Table 1 and graphically shown in FIGS. 1 to 4.

As shown in Figure 1, if the carbon content in the steel exceeds the range of the present invention, not only will the F value decrease to less than 1.8, but the BH content will become higher than necessary. This increases the risk of room temperature strain aging.

Furthermore, as shown in FIGS. 2 and 3, if Si and P are contained in the steel in amounts exceeding the range of the present invention, the value of Si and P decreases, and the steel sheet becomes hard and elongation deteriorates.

As shown in Figure 4, a decrease in the coiling temperature after hot rolling not only causes a decrease in f value, but also increases the amount of BH due to an increase in solid solution N, leading to the risk of cold strain aging. increases.

In addition, the baking hardenability in the present invention is evaluated by the BH value, that is, the increase in yield point when 2% prestrain is applied, baking is performed at 170°C for 20 minutes, and the yield point is tensed again. Values of .5 kgf/mm2 or more and 6 kgf/mm2 or less are acceptable. On the other hand, secondary processing brittleness is determined by punching an annealed plate into a disk, drawing it into a cup at a drawing ratio of 1.6, and placing it face down on a truncated cone-shaped tool at various temperatures.
The evaluation is based on the ductile-brittle transition temperature when a weight of 1.5 g is dropped and an impact is applied, and a value of -20°C or higher is acceptable.

Example 2 Steel having the composition shown in Table 2 was melted in a converter,
A part of the slab was subjected to vacuum degassing treatment and a slab was manufactured by continuous casting. These were heated to 1200"c and hot rolled 6
It was coiled at 50°C, pickled, and then cold-rolled at a rolling rate of 75% to produce a cold-rolled sheet with a thickness of 0.8 mm.
(consisting of holding at 0°C for 40s and overaging treatment at 400°C for 3°5 minutes) or box annealing (heating/cooling rate 20°C/h)
r, holding at 720° C. for 6 hours), followed by temper rolling with an elongation rate of 1.0% to produce a cold rolled steel sheet.

The mechanical properties of the obtained cold-rolled steel sheets were tested.

The results are also summarized in Table 2. As can be seen from these results, Steel B1 is a steel plate according to the present invention, and has an appropriate f value and high<BH amount.

However, steel B2 contains a large amount of C beyond the scope of the present invention, and has a low BH value and an excessively high BH content.

”5 συ *: Indicates that it is outside the scope of the present invention.

(Effects of the Invention) As detailed above, according to the present invention, baking is performed using Ti, Nb, Since it does not require the addition of any alloying elements or the addition of B, it simply has an extremely low carbon content, making its production cheaper and more stable than conventional methods. The significance of the present invention is significant from a practical standpoint, such as the resulting high-tensile cold-rolled steel sheet being inexpensive.

[Brief explanation of the drawing]

Figure 1 is a graph showing the influence of the amount of C in steel on the mechanical properties of annealed plates; Figure 2 is a graph showing the influence of the amount of St in steel on the mechanical properties of annealed plates; Figure 3 is , a graph showing the effect of the amount of P in steel on the mechanical properties of an annealed plate; and FIG. 4 is a graph showing the effect of hot rolling winding temperature on the mechanical properties of an annealed plate.

Claims (2)

[Claims] (1) In weight%, C: 0.0040% or less, Si: 0.10 to 0.80%
, P: 0.02-0.10%, Mn: 0.05-0.5
%, N: 0.0050% or less, acid-soluble Al: 0.001
A high-strength cold-rolled steel sheet for deep drawing that has a steel composition of ~0.1%, the balance being Fe and unavoidable impurities, and has excellent bake hardenability and secondary work brittleness resistance. (2) In weight%, C: 0.0040% or less, Si: 0.10 to 0.80%
, P: 0.02-0.10%, Mn: 0.05-0.5
%, N: 0.0050% or less, acid-soluble Al: 0.001
~0.1%, the balance Fe and unavoidable impurities are formed into a steel billet, hot-rolled above the Ar_3 transformation point, then coiled at a temperature below 720°C, and then descaled. After doing this, reduce the rolling reduction rate to 60
~95% cold rolled and further recrystallized temperature or higher Ac_3
A method for producing a high-strength cold-rolled steel sheet for deep drawing that has excellent bake hardenability and secondary work brittleness resistance, the method comprising heating to a temperature range below the transformation point and annealing.