JP3362739B2 - Manufacturing method of hot rolled steel sheet with excellent deep drawability - Google Patents

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 hot-rolled steel sheet having excellent deep drawability, specifically, a hot-rolled steel sheet having a high rank Ford value (r value).

[0002]

2. Description of the Related Art Cold rolled steel sheets have hitherto been used for applications requiring excellent workability, particularly deep drawability. As is well known, cold-rolled steel sheet is produced by a hot-rolled steel sheet obtained by hot rolling through a process of pickling-cold rolling-annealing-tempered rolling. The manufacturing cost is significantly higher than that of hot-rolled steel sheet. On the other hand, hot-rolled steel sheets are often manufactured as raw materials for the cold-rolled steel sheets, welded steel pipes, etc., and when used as hot-rolled steel sheets themselves, they are used where relatively high workability is not required.

In recent years, there has been a demand for reducing the cost of steel materials from various directions, and in combination with this and the improvement of the manufacturing technology for hot-rolled steel sheets, hot-rolled steel sheets have been used in fields where cold-rolled steel sheets have been conventionally used. It has started to be done. However, in order to further expand the field of use of hot rolled steel sheet, its workability, especially deep drawability, must be greatly improved.

As an attempt to improve the workability of a hot rolled steel sheet, there is, for example, the invention disclosed in Japanese Patent Application Laid-Open No. 61-3844. A feature of the invention is that steel of a specific composition is hot-rolled under high pressure in a high temperature range, and then lubricated in a relatively low temperature range, whereby a hot-rolled steel sheet with an r value of 1.0 or more is obtained. It will be done. However, this invention does
Only lubrication rolling with a total reduction of 50% or more is performed in a temperature range of Ar ₃ points or less, and a high r as high as that of a cold rolled steel sheet is obtained.
No consideration has been given to the optimum rolling reduction or temperature range for obtaining the value.

Therefore, as a result of studying the optimum reduction ratio and temperature range for obtaining a high r value comparable to that of a cold-rolled steel sheet, the present inventors have found that 1) it has a deep drawability comparable to that of a cold-rolled steel sheet. The optimum rolling reduction exists to obtain hot-rolled steel sheet, 2) Ar
_Even after hot rolling in the temperature range of ₃ points to (Ar ₃ points −150) ° C., ferrite (hereinafter referred to as “α”) is recrystallized during rolling and the rolling texture does not develop. Even if crystallized, deep drawability comparable to that of cold-rolled steel sheet cannot be obtained. 3) To ensure deep drawability comparable to that of cold-rolled steel sheet, (Ar ₃ points -150) ° C below α recrystallization temperature range We found that it is necessary to roll at. Then, the aluminum-killed steel slab of a specific composition is subjected to primary rolling including large reduction rolling in which the final pass has a reduction point of ₃ points or more and the reduction rate of the final pass is 30% or more, and then (Ar ₃
(Point -150) ℃ ~ 450 ℃ temperature range of 80 ~ 97% reduction ratio, the friction coefficient of 0.18 or less while performing the secondary rolling while performing the lubrication process Patent Applied
(Japanese Patent Application No. 2-289795, filed on October 26, 1990, hereinafter referred to as "prior invention").

[0006]

The hot-rolled steel sheet is generally produced by a continuous hot rolling mill composed of a rough rolling stand and a finishing stand. In the production method of the prior invention, the primary rolling including large reduction rolling is used. Rolling should be carried out in a rough rolling stand, and secondary rolling in lubrication rolling should be carried out in a finishing rolling stand, but it is not easy to carry out lubricating rolling with the above-mentioned reduction ratio in secondary rolling, and rolling is difficult. There is a problem that is. That is, since the amount of biting in lubricated rolling is lower than that in non-lubricated rolling, it is not easy to secure a reduction rate of 80% or more while performing lubrication at a finish rolling stand suitable for performing secondary rolling. Absent.

An object of the present invention is to provide a method for manufacturing a hot-rolled steel sheet having a deep drawability comparable to that of a cold-rolled steel sheet, which solves such a problem.

[0008]

Means for Solving the Problems As a result of further studies based on the invention of the prior application, the present inventors have found that (Ar ₃ points −150) ° C. to 450 ° C.
The inventors have found that the above-mentioned problems can be achieved by performing the secondary rolling in the temperature range of 1) in combination with the lubrication rolling and the non-lubrication rolling in the range of an appropriate reduction ratio, and have reached the present invention.

Here, the gist of the present invention is the following method for producing a hot-rolled steel sheet. (I)% by weight, C: 0.08% or less, Si: 0.3% or less, M
n: 0.01 to 0.4% or less, Sol.Al: 0.01 to 0.08%, N: 0.0
A method for producing a hot-rolled steel sheet excellent in deep drawability, which comprises subjecting steel slabs of a material containing 1% or less and the balance being Fe and unavoidable impurities to thermomechanical treatment in order in the following steps.

A step of performing the primary rolling in which the rolling of the final pass is in the temperature range of Ar ₃ points or more and the rolling reduction of the final pass is 30% or more, (Ar ₃ points −150) ° C. to 450 ° C. In total rolling reduction (R ₁ ) of 70 to 97%, the rolling reduction (R ₂ ) of lubrication rolling with a friction coefficient of 0.2 or less satisfies the following formula (a) , and Distortion over 100s ^-1 over the path
Secondary rolling under the condition that the rolling reduction is 30% or more at speed , 4500 ≦ R ₁ (%) × R ₂ (%) ≦ 7760 ··· (a) where R ₁ (% ) ≧ R ₂ (%), R ₂ ≦ 80 (%) Recrystallization after secondary rolling.

(II) In addition to the above-mentioned components in the material, a total of one or more selected from Ti, Nb, Zr and V is 0.001 to 0.350% by weight and / or B is 0.0001.
The method for producing a hot-rolled steel sheet having excellent deep drawability according to (I) above, which comprises using a steel slab containing 0.0050% by weight.

[0012]

The function and effect of each constituent of the present invention and the reason for limiting the numerical value will be described below. First, the reason for limiting the composition of the raw steel billet used in the present invention as described above will be described. In addition, "%" regarding a steel composition is "weight%."

C: Since C is an element that adversely affects deep drawability, it is desirable that its content be small. When the C content exceeds 0.08%, the deep drawability is remarkably deteriorated.
It was set to 08% or less.

Si: Si is preferably as small as possible in order to improve the deep drawability. If the Si content exceeds 0.3%, not only the deep drawability deteriorates, but also the scale properties deteriorate and the product quality is impaired, so the content was made 0.3% or less.

Mn: Mn is an element effective for fixing S mixed as an impurity in steel as MnS and improving hot workability.
However, if the content exceeds 0.4%, the deep drawability deteriorates significantly, so the upper limit was made 0.4%.
On the other hand, due to the recent progress of desulfurization technology, it has become possible to manufacture extremely low S steel, and if the Mn content is 0.01%, S can be sufficiently fixed as MnS and hot work cracking can be prevented. Therefore, the lower limit was made 0.01%.

Sol.Al: Al is an important element in the present invention. Al is used as a deoxidizer, and also has the effect of fixing N in the steel as AlN and improving the deep drawability. However, if the content is less than 0.01% in sol.Al, the above-mentioned action and effect are not sufficiently obtained, and if it exceeds 0.08%, the effect is saturated and only the cost rises. 0.01 to 0 in quantity.
It was set to 08%.

N: N is an element that forms a nitride with Al and Ti, Nb, and V described later, and contributes to miniaturization of ferrite. However, on the other hand, it is also an element that adversely affects the deep drawability. When the N content exceeds 0.01%, the solid solution N remains, and the adverse effect of the deep drawability deterioration becomes more remarkable than the effect of refining the ferrite, so the content was made 0.01% or less.

In addition to the above components, the balance of the raw steel billet used in the present invention is Fe and inevitable impurities. Or
In addition to these components, a steel slab containing 0.001 to 0.350% by weight in total of one or more selected from Ti, Nb, Zr and V and / or 0.0001 to 0.0050% by weight of B is used as a raw material. can do. The action and effect of these components which may be added additionally are as follows.

Ti, Nb, Zr and V: These elements are expensive, but N is firmly fixed as a nitride rather than Al, and the ferrite grain size before secondary rolling is refined to deep draw. It is a very preferable element that has an improving effect. However, if the content of one or more of them is less than 0.001% in total, the above effect cannot be obtained sufficiently, and if it exceeds 0.350%, the effect saturates and only the cost increases, so these elements In the case of adding, the total content of one or more kinds is preferably 0.001 to 0.350%.

B: B is an element effective in preventing "vertical cracking" which is a problem in drawn parts. However, if the content is less than 0.0001%, the effect is small, and if the content exceeds 0.0050%, the effect is saturated and only the cost is increased. Therefore, B
When adding, the content should be 0.0001 to 0.0050%.

Next, the thermo-mechanical treatment of the raw steel billet having the above composition will be described.

The material steel slab (slab) having the above-mentioned composition to be subjected to hot rolling may be one produced by slab rolling from an ingot, or one produced by continuous casting. Further, the raw steel billet may be subjected to hot rolling after heating the cold steel billet after slab rolling or continuous casting to a predetermined temperature, and it is called high temperature from a line of slab rolling or continuous casting called direct feed rolling. It is also possible to employ a method in which the steel bill sent as it is or as it is, or after being subjected to some auxiliary heating, is subjected to hot rolling. In either case, the hot rolling is preferably carried out at a temperature of Ar ₃ points or higher.

The hot-rolled steel sheet is produced by subjecting a steel slab having the above composition to primary rolling, secondary rolling and recrystallization as described below.

[Primary Rolling] The purpose of primary rolling is to reduce the size of α grains in the cooling process until the next secondary rolling.
(Hereinafter referred to as “γ”) grains are refined by recrystallization, and further processing strain is introduced into γ grains to increase the precipitation sites of α grains. For that purpose, the primary rolling is performed by Ar ₃
It is necessary to perform it in the temperature range above the point and at a reduction rate of 30% or more. In particular, it is preferable to roll at a large reduction of 45% or more.

When the rolling reduction in the final pass is less than 30%, not only does γ not recrystallize into fine grains, but also the processing strain is small, so α grains do not become fine in the next cooling process. Further, when the temperature of the rolling in the final pass is lower than the Ar ₃ point, the α phase becomes mixed and the working strain is concentrated in the soft α phase, and the working strain is not accumulated in the γ phase. The α-grains generated by the γ / α transformation in the process are not refined.

It is preferable that this primary rolling is carried out for one or more passes, and the final pass is conducted under the above conditions. The rolling before the final pass may be normal rolling without particular limitation. The Ar ₃ point (° C.) in the present invention is 910-3
It is calculated by 10 × C (%) − 80 × Mn (%).

[Secondary Rolling] Secondary rolling is performed in a temperature range of (Ar ₃ points −150) ° C. to 450 ° C., so at least a high-temperature rolled material that has been subjected to primary rolling in a temperature range of Ar ₃ points or higher is used. It is necessary to cool to a temperature of (Ar ₃ points −150) ° C. This cooling condition is not particularly limited, but it is desirable to cool at a cooling rate of 5 ° C./s or more.
The reason for this is that the {111} recrystallized texture, which is preferable for improving the r value, originates from the α grain boundaries, so it is desirable to refine the α grains and increase the area of the α grain boundaries before secondary rolling.
If cooled at a cooling rate of 5 ° C./s or more, a large number of α can be obtained from γ that is recrystallized and refined in the primary rolling process and processing strain is accumulated.
In addition to generating nuclei, the effect of accelerated cooling suppresses the growth of α-grains after the transformation from γ to α, and the α-grains become finer, resulting in fine α-grains with an ASTM grain size number of 8 or more. Because you can.

The purpose of the secondary rolling is to develop a {111} texture that is preferable for improving the r value and minimizing the in-plane anisotropy in the next recrystallization treatment step, and use an unfavorable {100} texture for them. RD (rolling direction) // <11
0> To develop a rolling texture mainly composed of fiber texture. Therefore, this secondary rolling is (Ar ₃ points -150) ℃ ~ 450
It is desirable to carry out in the temperature range of ℃. In the temperature range higher than (Ar ₃ points −150) ° C., the above object is not achieved, and in the temperature range lower than 450 ° C., the deformation resistance increases, the energy required for rolling increases, and the economic merit decreases. In the secondary rolling in this temperature range, in order to further improve the r value, it is important to uniformly distribute the strain in the plate thickness direction and reduce the difference in texture between the center part of the plate thickness and the surface layer part. In order to do so, it is preferable to employ lubrication rolling, in which the friction coefficient (μ) between the steel sheet and the rolling roll is 0.2 or less, preferably 0.15 or less, while performing lubrication for the secondary rolling. In that case, the reduction ratio (R ₂ ) of the lubrication rolling is preferably determined so as to satisfy the following (a) from the relationship with the total reduction ratio (R ₁ ) of the secondary rolling. 4500 ≦ R ₁ (%) × R ₂ (%) ≦ 7760 ··· (a) However, when R ₁ (%) ≧ R ₂ (%), R ₂ ≦ 80 (%) 70 ≦ R ₁ (%) ≤97 Even if lubrication rolling with a friction coefficient of 0.2 or less is adopted for secondary rolling,
When the value obtained by multiplying the rolling reduction by the lubrication rolling and the total reduction is less than 4500, even if recrystallization treatment is performed after the secondary rolling, {11
1} for texture sufficiently not develop {100} texture Ru is formed, r value is low. On the other hand, when the value obtained by multiplying the rolling reduction by the lubrication rolling and the total reduction exceeds 7760, that is, when rolling with a reduction of more than 97%, {111} texture does not develop and {001} <110> Texture near the orientation and {211} 〈0
11> The r value is low due to the development of texture near the orientation. (Ar ₃ points -150) ℃ ~ 450 ℃ temperature range, 4500 ≤ R
{111} texture develops when recrystallization treatment is performed after only secondary rolling by lubrication rolling or combination of lubrication rolling and non-lubrication rolling so that ₁ (%) × R ₂ (%) ≤ 7760. However, the r value is comparable to that of cold rolled steel sheet.

In addition, this secondary rolling has a final pass of 100s.
^It is preferable to carry out at a strain rate of ^-1 or more and a reduction rate of 30% or more. The r value of the steel sheet recrystallized after the secondary rolling, in other words, the formation of texture also has a great influence on the strain rate during the secondary rolling. From the experimental results on hot rolling, it is possible to control the {111} texture and suppress the {100} texture by setting the strain rate in the final pass to 100 s ^-1 or more and the rolling reduction to 30% or more. It has been found.

It should be noted that the number of passes of the secondary rolling may be one or more, and the number of passes may be determined in consideration of the thickness of the raw material and the thickness of the product.
Although there is no restriction on where in the hot rolling line the primary rolling and the secondary rolling are performed, it is advantageous in terms of equipment to perform the former in the rough rolling step and the latter in the finish rolling step.

[Recrystallization Treatment] Recrystallization treatment is an indispensable step for imparting excellent workability to the steel sheet after rolling. It may be recrystallized by cooling after completion of hot rolling or by self-annealing in a state of being wound on a coil, or may be heated and recrystallized after winding. The treatment method is not particularly limited. When recrystallizing by heating after winding, it can be performed by, for example, a normal continuous annealing process or a batch annealing process, and when a final product is galvanized, a continuous annealing process in a hot dip galvanizing line is performed. Can be done at.

The hot-rolled steel sheet produced in this manner has extremely excellent workability comparable to that of the conventional cold-rolled steel sheet, despite being a hot-rolled steel sheet. The specific characteristics are shown by examples. This embodiment is an example of the present invention and does not unduly limit the present invention.

[0033]

EXAMPLE Aluminum-killed steel having the composition shown in Table 1 was melted in a 50 kg vacuum melting furnace and forged into a slab having a thickness of 80 mm. These slabs are 2 mm under the conditions shown in Table 2 (1) and Table 2 (2).
It was processed into a thick hot-rolled steel sheet, wound into a coil, and then recrystallized under any of the following conditions (a) to (d).

"Recrystallization treatment condition" (a) After being wound around a coil, recrystallization treatment is carried out by the heat possessed by itself during slow cooling (treatment a).

(B) After being wound around the coil, it is cooled to room temperature and then subjected to a recrystallization treatment by imparting a heat history corresponding to continuous annealing of holding at 800 ° C. for 2 minutes (treatment b). (c) Similarly, after cooling to room temperature, recrystallization treatment is performed by applying a thermal history equivalent to continuous annealing of a hot dip galvanizing line kept at a temperature of 800 ° C for 10 seconds (processing c) (d) also to room temperature After cooling, a recrystallization treatment is carried out by applying a heat history corresponding to batch annealing in which the temperature is maintained at 700 ° C. for 5 hours (treatment d).

Test pieces were cut out from the hot-rolled steel sheet thus obtained, and the mechanical properties were measured. The results are shown in Table 3.

[0037]

[Table 1]

[Table 2 (1)]

[Table 2 (2)]

[Table 3] As is clear from Table 3, the hot-rolled steel sheet manufactured by satisfying all the conditions of the present invention has much higher r-value and elongation than that of the comparative example, and has a low yield point, and is extremely workable. To be superior to. Among the examples of the present invention, No. 9 using aluminum-killed steel containing B in the raw material billet
11, resistance to fresh cracking transition temperature is not significantly low. Also, No. 11
Is the final pass of secondary rolling with a strain rate of 150s ^-1 and a reduction of 40%
Although having been subjected at the rate, this compound is excellent in mechanical properties than No.8 is substantially the same other conditions except for the final pass of the secondary rolling.

[0038]

As described above, according to the present invention, a hot rolled steel sheet having workability comparable to that of a conventional cold rolled steel sheet can be economically produced by hot rolling. Therefore, the present invention greatly contributes to the expansion of the use of hot-rolled steel sheets in the fields of automobiles, home appliances and building materials.

─────────────────────────────────────────────────── ─── Continuation of front page (56) References JP-A-1-301822 (JP, A) JP-A-62-253733 (JP, A) JP-A-61-3845 (JP, A) (58) Field (Int.Cl. ⁷ , DB name) C21D 9 / 48,8 / 04 C22C 38/00-38/06

Claims (2)

(57) [Claims] 1. By weight%, C: 0.08% or less, Si: 0.3% or less, Mn: 0.01 to 0.4% or less, Sol.Al: 0.01 to 0.08%,
N: A method for producing a hot-rolled steel sheet having excellent deep drawability, which comprises subjecting steel slabs of a material containing 0.01% or less and the balance being Fe and unavoidable impurities to sequential heat treatment in the following steps. Rolling the final pass in the temperature range of Ar ₃ points or more and performing the primary rolling in which the rolling reduction of the final pass is 30% or more, (Ar ₃ points −150) ° C. to 450 ° C. The rolling reduction (R ₁ ) is 70 to 97%, and the rolling reduction (R ₂ ) of the lubrication rolling of which the friction coefficient is 0.2 or less satisfies the following formula (a) and the final pass is 100s. ^-1 or more distortion
Secondary rolling under the condition that the rolling reduction is 30% or more at speed , 4500 ≦ R ₁ (%) × R ₂ (%) ≦ 7760 ··· (a) where R ₁ (% ) ≧ R ₂ (%), R ₂ ≦ 80 (%) Recrystallization after secondary rolling. 2. A steel slab as a raw material, in addition to the components described in claim 1, further comprises at least one selected from Ti, Nb, Zr and V in a total amount of 0.001 to 0.350% by weight and / or B. 0.
0001 to 0.0050% by weight is contained.
The method for producing a hot-rolled steel sheet having excellent deep drawability according to 1.