JPH09176745A - Production of hot rolled steel sheet excellent in workability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a steel sheet having high r value and elongation and excellent workability equal to those of the conventional cold rolled steel sheet by a hot rolling method. SOLUTION: A slab of a steel having a compsn. contg., by weight, <=0.02% C, <=2.0% Si, 0.05 to 0.20% Mn, <=0.03% S, <=0.010% N, <=0.10% P, 0.002 to 0.10% sol.Al, 0.01 to 0.30% Ti, <=0.1% Nb, <=0.5% V and <=0.003% B and satisfying the relationship of Ti/48>=(C/12)+(N+14)+(S/32) and the relationship of 0.7<=(S/32)/(C/12)<=2.0, and the balance Fe with inevitable impurities is held in the temp. range of 700 to 1150 deg.C for 1 to 60min before the finish rolling of hot rolling, is thereafter subjected to rolling at >=50% draft in the temp. range of 550 deg.C to less than the Ar3 point and is subsequently recrystallized.

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 workability, particularly deep drawability and ductility, which is used for automobiles, home appliances, steel structures and the like.

[0002]

2. Description of the Related Art With the demand for reduction of steel cost and the improvement of manufacturing technology of hot rolled steel sheet, the use of hot rolled steel sheet has come to be tried even in the field where cold rolled steel sheet was conventionally used.

The cold-rolled steel sheet has a beautiful surface, good plate thickness accuracy, and excellent deep drawability. However, compared with the hot-rolled steel sheet, the manufacturing process is long and the cost is high. On the other hand, even if the surface condition and the plate thickness accuracy are slightly inferior, if the workability, especially the deep drawability is sufficiently good, the hot rolled steel plate is also low in cost, and its application can be expanded.

The reason why the deep drawability of general hot-rolled steel sheet is inferior to that of the cold-rolled steel sheet is the r value which is an index of the deep-drawability (hereinafter, 0 ° to the rolling direction unless otherwise specified, The average of r values measured in three directions of 45 ° and 90 °)
This is because it rarely exceeds 1.0. This r value is an index showing the plastic anisotropy that the plate thickness direction is less likely to be deformed with respect to the plate surface direction during tensile deformation, and cold rolled steel plate is usually 1.0 or more, and for deep drawing. Then, the value becomes 1.5 or more. On the other hand, hot-rolled steel sheets are usually rolled in the austenite region, so the texture obtained by rolling is different from that of cold rolling, and on top of that, they are transformed and randomized. It is easy to become.

On the other hand, even in the hot rolling process,
A method for improving the r value of a steel sheet has been proposed. For example, in JP-A-61-1 3844, after rolling in Ar ₃ point or more austenite temperature region, performs a lubrication rolling over 50% Ar ₃ point or less of the ferrite zone, by subsequent recrystallization
A manufacturing method for obtaining an r value of 1.0 or more has been proposed. Also, by adding a trace amount of Ti or Nb to ultra low carbon steel, solid solution C
It has also been reported that a steel slab with solute N fixed as carbonitride is roughly rolled in a temperature range of Ar ₃ points or higher and then finish rolled with a total reduction of 73% in a ferrite range of 800 ° C or lower. (Iron and Steel: 74 (1988), 1617-1624.).

[0006] In all of these methods, rolling in the ferrite region, which is usually performed by cold rolling, is performed in a hot rolling process such as a hot strip mill. Therefore, as compared with cold rolling using a cold strip mill or the like, it is difficult to obtain a sufficient workability in the ferrite region, so that the r value is often less than that of cold rolled steel sheets. However, there is a possibility that the cold rolling step can be omitted and the construction period can be shortened to provide a steel sheet with good deep drawability at low cost, and since it can be sufficiently used depending on the application,
Further improvement and stability of steel sheet properties by only this hot rolling process are desired.

[0007]

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for producing a hot-rolled steel sheet which is excellent in deep drawability and ductility and which is used for rolling the ferrite region in the hot rolling step.

[0008]

[Means for Solving the Problems] A method of improving the r-value of a hot-rolled steel sheet is basically to introduce a rolling texture in the ferrite region as in the case of the cold-rolled steel sheet and recrystallize it by annealing or the like. This is to develop a recrystallized grain having a preferred orientation for the value. Therefore, in the second half of the hot rolling process, in the manufacturing method in which the rolling is performed in the temperature range of Ar ₃ point or less, the influence of the components in the case of recrystallizing by annealing after winding and the conditions up to the Ar ₃ point I examined it in detail. As a result, in order to obtain an r-value as high as 1 or higher as much as possible, first of all, as a steel, solute C and solute N are substantially absent at the time of the final annealing, so-called IF (Inter
stitial Free) Steel was essential. On top of that, rolling in the austenite region above the Ar ₃ point is required a certain degree or more working ratio at a temperature range close to the _3-point Ar, further rolling at a temperature of Ar less than ₃ points, possible reduction Increasing the rate was effective in improving the r value of the obtained steel sheet.

In order to further improve the r-value in these manufacturing processes, a study was conducted focusing on the behavior of the precipitates that inhibit the crystal growth. As a result, Ti ₄ C ₂ S ₂ was formed and precipitated positively. Is used to coarsen precipitates and
It was effective to suppress the precipitation of iC. For that purpose, it was particularly effective to perform a Ti ₄ C ₂ S ₂ precipitation treatment in which the temperature was maintained for a certain period of time in a rolling process in a temperature range above 700 ° C.

Further, 0.15% Mn-0.01% P-0.03% A
1-0.003% N-0.05% Ti composition, C is 0.001-
Steels with various S contents varying in the range of 0.005% were smelted on a laboratory scale, and after rough rolling, a precipitation treatment was carried out by holding at 950 ° C for 5 minutes before finish rolling. All steels were rolled at a total rolling reduction of 80% using lubricating oil within a temperature range of Ar ₃ points of 870 to 880 ℃)
Finished thick. After descaling, recrystallization was performed by annealing equivalent to a continuous annealing method at 820 ° C for 1 min soaking. A JIS No. 5 tensile test piece was sampled from the obtained steel sheet, and the elongation and r value (average value of r values measured in three directions of 0 °, 45 ° and 90 ° with respect to the rolling direction) were measured. As a result of examining the relationship between the atomic concentration ratio of C and S (according to the formula) and the elongation and the r value of these steels, FIG. 1 was obtained. Thus, it was found that there is an optimum range of the atomic concentration ratio of C and S for increasing the elongation and the r value. When the atomic concentration ratio of C and S is 1, the elongation and the r value are maximum.
_It was considered that the precipitation of ₄ C ₂ S ₂ was effectively performed. Based on these findings, clarify the limit conditions,
This has led to the present invention.

The gist of the present invention is that C: 0.02%
Below, Si: 2.0% or less, Mn: 0.05 to 0.20%, S: 0.
03% or less, N: 0.010% or less, P: 0.10% or less, sol.A
1: 0.002 to 0.10%, Ti: 0.01 to 0.30%, Nb: 0.1
% Or less, V: 0.5% or less, B: 0.0030% or less, and satisfy the following formula and formula (where the element symbol represents the content% by weight of the element): Ti / 48 ≧ (C / 12 ) + (N / 14) + (S / 32) 0.7 ≦ (S / 32) / (C / 12) ≦ 2.0 A steel slab with the balance Fe and unavoidable impurities is 700 ~ 1 to 60min in the temperature range of 1150 ℃
The method for producing a hot-rolled steel sheet having excellent workability is characterized in that, after holding, rolling is performed at a rolling reduction of 50% or more in a temperature range of 550 ° C. or more and less than Ar ₃ point, and then recrystallization is performed.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION The chemical composition of the present invention and the reasons for limiting the production conditions will be described in detail below.

(1) C C is an element that adversely affects the deep drawability of the steel sheet, so the smaller the content, the better. Particularly, in the process of annealing for recrystallization, it is considered that the less solid solution C is, the better the deep drawability is. Therefore, Ti is added to bond with C, and substantially no solid solution C is present. To do so. In order to reduce the addition amount of Ti necessary for fixing C, it is preferable that the amount of C is small. Therefore, the C content is 0.02%
The following is assumed.

(2) Si Si hardens the steel and deteriorates the surface properties of the steel sheet.
The less, the better. 0.05% or less is desirable to have substantially no effect. However, it may be added as a means for improving the strength without significantly lowering the workability in the case where the deterioration of the surface properties is not a serious problem. However, if the amount is too large, the workability decreases, so the content limit is 2.0%.

(3) Mn Mn is generally added for the purpose of fixing S in order to prevent hot embrittlement due to S in steel. However, in the method of the present invention, since the production and precipitation of Ti ₄ C ₂ S ₂ is positively utilized, it is necessary to keep the content of Mn, which tends to inhibit the production, low. Therefore, the upper limit is 0.2%. On the other hand, if the amount is too small, the quality of the steel is deteriorated due to an increase in oxides, so at least 0.05% or more is necessary.
Therefore, the Mn content range is 0.05 to 0.20%.

(4) sol.Al Al is used for deoxidation of molten steel and Ti for obtaining sound cast pieces.
It is added to secure the yield of additional elements. If the content is 0.002% or more of acid solid solution Al (sol. Al) in steel, the action and effect are not sufficient. On the other hand, if the content exceeds 0.10%, the effect is saturated, so the Al content is sol.
It is set as 0.002 to 0.10% as l.

(5) Ti Since Ti is strongly bonded to C and N, it is added to eliminate solid solution C and solid solution N in steel. Further, the method of the present invention forms Ti ₄ C ₂ S ₂ together with S, and reduces fine TiC precipitates that may be formed as a result of bonding with C. For this purpose, 0.01% or more is necessary, but it is important that the content is sufficient, corresponding to the contents of C, N and S, and the above formula must be satisfied.

On the other hand, the content of more than 0.30% is not preferable because it deteriorates the surface properties such as chemical conversion treatment property. Therefore, the Ti content is in the range of 0.01 to 0.30% and satisfying the above formula.

(6) BB does not need to be added because it tends to deteriorate deep drawability, but it is effective in preventing "secondary work cracking" which is a problem when drawing an ultra-low carbon steel sheet. Therefore, it is added if necessary. If you add it and expect its effect, add 0.
The content of 0002% or more is desirable, but even if the content exceeds 0.0030%, the effect may be saturated and the steel may become brittle, so the B content is defined as 0.0030% or less.

(7) Nb and V Nb and V may not be added, but they are added if necessary because they have the effect of refining the crystal grains of the steel and increasing their strength. However, if too much is added, Ti ₄ C ₂ S
_Since it inhibits the formation of ₂ , the content of Nb should be 0.1% or less and V should be 0.5% or less.

(8) S S is an unavoidable impurity element, and the smaller the content, the better for ensuring the quality of steel such as workability. The content is 0.03%.
The following is assumed. However, in the method of the present invention, Ti and C
It is necessary to react them with each other to form a Ti ₄ C ₂ S ₂ precipitate, and it is considered desirable that the atomic concentration ratio of S and C is close to 1. Therefore, from Fig. 1, the r value is
The content range in which the result exceeds 1.4, that is, the range satisfying the following formula 0.7 ≦ (S / 32) / (C / 12) ≦ 2.0 is regulated according to the C content. If the r value exceeds 1.4, it can be said that the improvement is obvious. As described above, the S content is 0.03% or less, and is in the range represented by the above formula according to the C content.

(9) P P tends to make the steel hard and brittle, so the smaller the strength, the better. However, the addition of a small amount can increase the strength without significantly impairing the workability, so that it is added if necessary.
When it is added, if it is too much, embrittlement becomes conspicuous, so the content is made 0.10%.

(10) N N is an unavoidable impurity element like S, and when Ti is added as in the present invention, TiN is formed to form an extra T.
Since i will be consumed, and TiN formed on the i-side may cause a ground defect, the smaller the better, the better. Therefore, N is 0.010% or less.

(11) Hot rolling conditions (a) Rough rolling process Rolling by a normal hot strip mill is divided into two: rough rolling and finishing tandem mill rolling.
In the rough rolling in which the heated slab is rolled down to finish tandem mill rolling, it is desirable that the total rolling reduction in the temperature range from Ar ₃ point to 1200 ° C. is 40% or more. Rolling in this temperature range has an effect of promoting precipitation and growth of precipitates by breaking coarse cast grains and introducing strain. If the total reduction is less than 40% or the rolling temperature is higher than 1200 ° C, the desired effect cannot be obtained sufficiently. However, 200 by normal continuous casting
This condition is almost satisfied when the slab with a thickness of about mm is finished to a final thickness of about 3 mm or less.

(B) Precipitation treatment In order to positively form Ti ₄ C ₂ S ₂ precipitates, in the temperature range of 700 to 1150 ° C. before finish rolling, 1 min or more
Perform a precipitation treatment that keeps the time within 60 min. It is desirable that this precipitation treatment be performed in the subsequent stage of rough rolling in which the slab is rolled to some extent. This is because the addition of work strain promotes precipitation. This treatment is carried out by suspending the rolling during the rough rolling or after the rough rolling and allowing to cool or keep the temperature, but it is also possible to temporarily hold the rough pressure material by using a coil box developed in recent years. Further, the precipitation treatment may be performed during the rough pressure and the rough rolling may be further continued.

In this case, the Ti ₄ C ₂ S ₂ does not sufficiently precipitate even if it is held at a temperature higher than 1150 ° C. because of its large solubility. Precipitation is also sufficient because the cooling rate is too fast and the diffusion rate is low. If the precipitation is insufficient, fine TiC and TiS will be left, and the crystal grain growth during recrystallization after rolling will be insufficient and r
The value is low, and the fine precipitates impair the ductility of the steel sheet. If the holding time is less than 1 min, precipitation of Ti ₄ C ₂ S ₂ is insufficient, and even if held for 60 min or more, the progress of precipitation is saturated and does not proceed any further. In the precipitation treatment temporarily held during rolling, if the residence time at the Ar ₃ point or higher is 1 min or longer, the temperature of the steel may be lower than the Ar ₃ point at the end.

The Ar ₃ point in the actual rolling process is affected not only by the composition but also by the heat history of the steel reaching it, the rolling speed, the reduction ratio, etc., and in the hot rolling process by the tandem mill, the deformation resistance changes. I know its existence. Therefore,
Strictly speaking, the rolling temperature range should be regulated on the basis of the Ar ₃ point detected in the hot rolling process, but an estimated value from generally applied components may be used.

(C) Finish rolling The steel after rough rolling is finished to a final plate thickness, and the total rolling reduction in the temperature range of 550 ° C or higher and lower than Ar ₃ point is 50% or higher. . Finish rolling may be started from a temperature lower than Ar ₃ point, be continuous so that it becomes rolling first temperature range to less than ₃ points 550 ° C. or higher Ar transformation from the middle of the rolling from Ar ₃ point or more temperature May be.
The purpose of rolling in this temperature range is to roll the α grain (crystal grain of the ferrite phase) to form a rolling texture and to introduce strain due to the working, and to recrystallize it in the next step. To improve the crystallinity and the reduction of in-plane anisotropy to develop a 111 recrystallized texture and improve the workability (r value, elongation) of the final product. Even if the rolling reduction is sufficiently large, the processing at a temperature of Ar ₃ point or higher does not sufficiently result in the formation of the texture and the accumulation of strain due to the rolling.
The value of steel plate cannot be obtained. If the temperature is too low, the deformation resistance increases and rolling becomes difficult, so the lower limit is set to 550 ° C.

When rolling is carried out in the temperature range of 550 ° C. or higher and lower than Ar ₃ point, it is desirable to carry out some or all of the passes by using rolling lubricant. By using the rolling lubricant, the work deformation in the plate thickness direction is made uniform, and the r value of the recrystallized steel plate is improved.

When this rolling is carried out using the rolling lubricant, it is preferable that the friction coefficient μ between the rolling roll and the steel sheet be 0.2 or less.

(12) Recrystallization It is necessary to sufficiently recrystallize the rolling texture introduced in the rolling process in the α region of less than Ar ₃ point in order to develop a texture preferable for deep drawability. As a method,
It is recrystallized by self-held heat during cooling after completion of hot rolling or in the state of being wound into a coil, or by heating and annealing after winding.

In the case of self-held heat after winding, high temperature winding of 650 ° C. or higher is desirable for sufficient recrystallization.
However, if rolling with a total reduction of 50% or more is performed in the α region, the temperature decrease will be large, and it will be difficult to wind at a sufficient temperature. In that case, in order to obtain a good surface condition, the product is pickled and the scale is removed, and then heated to a temperature range of 600 to 900 ° C. and annealed. At a temperature lower than 600 ° C, recrystallization does not proceed sufficiently even if annealed for a long time, while at a temperature higher than 900 ° C, the r value decreases. This annealing method may be a box annealing method in the state of a coil, a continuous annealing method, or an annealing during hot dip galvanizing treatment.

[0033]

【Example】

[Example 1] Using a steel slab having a chemical composition of 200 mm and having a thickness of 200 mm as shown in Table 1, precipitation treatment conditions in hot rolling as shown in Table 2 and a rolling reduction in an α region of 550 ° C or higher and less than Ar ₃ points Was controlled to finish a 1.0 mm thick steel plate and wind it. Table 1 shows the temperature at the Ar ₃ point, the Ti equivalents of C, S and N corresponding to the formula, and the C and S content equivalent ratios corresponding to the formula. In the method of the present invention, the Ti content must be above this Ti equivalent, and the C / S content equivalent ratio must be within the regulation range of 0.7 to 2.0.

[0034]

[Table 1]

[0035]

[Table 2]

When the coiling temperature is sufficiently high, the subsequent annealing can be omitted due to the annealing effect of self-held heat after coiling, but when the reduction ratio is increased or the rolling start temperature is low, the coiling temperature is changed. Since it cannot be raised, it was annealed after descaling. Annealing for recrystallization was a continuous annealing method at a temperature of 820 ° C for 1 minute or a box annealing method at a temperature of 700 ° C for 5 hours.

Table 2 shows the results of a JIS 5 tensile test piece taken from the obtained steel sheet, and the yield strength, tensile strength, elongation and r value were investigated. The r values are r ₀ , r _45, and r ₉₀ measured with test pieces taken from the directions of 0 °, 45 °, and 90 ° with respect to the rolling direction, respectively (r ₀ + 2r ₄₅ + r ₉₀ ) / 4 is the average value obtained.

As is clear from the results shown in Table 2, the components specified in the present invention are conditions for precipitation treatment, and the reduction ratio in the α region at 550 ° C. or higher and lower than Ar ₃ point within the range defined by the present invention. The steel sheet manufactured in accordance with Table 1 shows excellent elongation and r-value as compared with the steel sheet under other conditions. For example, Test No. 11 has the same steel composition as Test No. 1, but the rolling conditions are out of the range specified in the present invention, and the elongation and r value are inferior to those of Test No. 1. Test Nos. 12 and 13 are also the same steel as Test No. 1, but the precipitation treatment conditions are outside the range defined by the present invention, and the elongation and r value are not improved. Further, even when the rolling process is the method of the present invention, even when the chemical composition of the steel does not satisfy the range of the present invention, sufficient improvement in steel plate properties is obtained as shown in Test Nos. 14 to 21. Absent.

Example 2 Using a slab of steel A shown in Table 1, a precipitation treatment of 930 ° C. for 5 minutes was performed immediately before finish rolling, and the total reduction ratio in the temperature range of 660 to 790 ° C. was 20. It was changed within the range of up to 92%, and the target winding temperature was 600 ° C, and the finish was 1.0 mm. Lubricating oil was used for rolling at a temperature below the Ar ₃ point. After descaling, it was annealed by continuous annealing at 820 ° C for 1 min. A JIS 5 tensile test piece was sampled from the obtained steel sheet in the same manner as in Example 1, and the r value was measured.

The relationship between the rolling reduction and the r value of the obtained steel sheet is shown in FIG. 2. The larger the rolling reduction at a temperature lower than the Ar ₃ point, the higher the r value. It can be seen that the rolling reduction is required to be 50% or more to obtain the above results.

[0041]

EFFECTS OF THE INVENTION According to the method of the present invention, a steel sheet having an r value comparable to that of a conventional cold-rolled steel sheet and excellent workability with high elongation is rationalized and cost-effective by the hot rolling step and subsequent annealing. Can be manufactured.

[Brief description of the drawings]

[Fig. 1] Atomic concentration ratio of S and C {(S / 32) / (C / 12
)} And the r value and elongation of the obtained steel sheet.

FIG. 2 is a diagram showing the relationship between the total rolling reduction in the temperature range of 550 ° C. or higher and lower than the Ar ₃ point and the r value of the obtained steel sheet.

Claims (1)

[Claims] 1. A weight ratio of C: 0.02% or less, Si: 2.0
% Or less, Mn: 0.05 to 0.20%, S: 0.03% or less, N:
0.010% or less, P: 0.10% or less, sol.Al: 0.002-0.1
0%, Ti: 0.01 to 0.30%, Nb: 0.1% or less, V: 0.
5% or less, B: 0.0030% or less, and the following formulas and formulas are satisfied: Ti / 48 ≧ (C / 12) + (N / 14) + (S / 32) 0.7 ≦ (S / 32) / (C / 12) ≤ 2.0 A steel slab with the balance Fe and unavoidable impurities is held for 1 to 60 minutes in the temperature range of 700 to 1150 ° C before finish rolling of hot rolling, and then 550 ° C or more Ar _A method for producing a hot-rolled steel sheet with excellent workability, which comprises rolling at a rolling reduction of 50% or more in a temperature range of less than ₃ points and then recrystallization.