JP2002226943A - High-yield-ratio and high-tensile hot-rolled steel plate having excellent workability, and its manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-yield-ratio and high-tensile hot-rolled steel plate having excellent workability, particularly elongation and stretch-flange formability, in an as-hot-rolled state, having excellent weldability and chemical convertibility. SOLUTION: A chemical composition consisting of, by mass, 0.02-0.15% C, 0.2-1.5% Si, 1.0-3.5% Mn, 0.1-1.0% Mo, <=0.03% P, <=0.001% S and the balance Fe with inevitable impurities is provided, and a steel structure which contains ferrite as a principal phase and 10-40% by volume fraction of a secondary phase composed of bainite or bainite partially containing martensite is also provided. Further, the average grain size (d2) of the secondary phase is <=8 μm, and the ratio of the (d2) to the average grain size (d1) of the principal phase, d2/d1, is controlled to 0.7-1.3.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION Structural members of automobiles, undercarriage members such as wheels, rims and chassis, and strength members such as bumpers and door guard bars require not only tensile strength but also rigidity. In order to secure a high yield ratio, a material having a high yield ratio type is required. The present invention relates to a high-strength hot-rolled steel sheet having such a high yield ratio and a method for producing the same, having a tensile strength of 590 MPa class to 980 MPa class, and having excellent workability, particularly excellent stretchability and stretch flangeability. It is intended to provide a tension hot-rolled steel sheet.

[0002]

2. Description of the Related Art Conventionally, as a method of strengthening the quality of steel sheets for automobiles, ferrite single-phase steel is mainly made of solid solution strengthening by adding a substitution element such as Si, Mn, or P, or martensite, bainite or bainite in a ferrite phase. A method of strengthening by precipitating retained austenite or the like was generally used.

For example, as described in JP-A-56-139654 and the like, in order to improve processability and aging,
Many proposals have been made to increase the strength by adding Ti and Nb to ultra-low carbon steel. In addition, for example, Japanese Patent Application Laid-Open No. Sho 60-52528 proposes a method of producing a high-strength steel sheet having excellent ductility by annealing low-carbon steel at a high temperature and precipitating a martensite phase after cooling. I have. In Japanese Patent Publication No. 61-12008, by adding Nb and B in addition to Si, Mn and P to ultra-low carbon steel,
There has been proposed a method for producing a steel sheet which achieves high strength and has both high bake hardenability at a low yield ratio and high r-value and ductility.

[0004] By increasing the strength by the above-described method, the thickness of automobile steel sheets has certainly been reduced to some extent. However, in the above proposal,
Although high strength, furthermore ductility, improvement of r value is taken into consideration, stretch flangeability, which is particularly important in various structural members such as wheel discs, is hardly described or described. However, it has not been sufficiently improved to satisfy the required characteristics. Therefore, the above-mentioned prior art cannot be said to provide a truly effective means for reducing the weight of an automobile body.

[0005]

SUMMARY OF THE INVENTION The present invention has been developed in view of the above-mentioned circumstances, and optimizes the steel composition and hot rolling conditions and the steel structure so that the workability, particularly the elongation, can be improved while hot rolling. It is an object of the present invention to propose a high-yield-ratio high-strength hot-rolled steel sheet having excellent stretch flangeability, and also excellent weldability and chemical conversion treatment properties, together with its advantageous production method.

In the present invention, the reason for aiming for a high yield ratio is that formability deteriorates with increasing strength, but by reducing the crystal grain size to some extent and increasing the yield ratio, the same strength is achieved. This is because the elongation is improved, and the strength-elongation balance is improved as compared with those having a low yield ratio. The purpose of the high yield ratio type is to further improve the strength-elongation characteristics, including the elongation and the stretch flange characteristics, as compared with the related art. In the present invention, the high yield ratio means that the yield ratio (yield strength / tensile strength × 100) is 70%.
It means above.

Further, according to the present invention, the tensile strength is 590 M
Although the target is a steel sheet of Pa class to 980 MPa class, more specifically, a steel sheet having a tensile strength of 590 to 880 MPa obtains excellent elongation and stretch flangeability together with tensile strength. On the other hand, the tensile strength exceeds 880 MPa and 1050 M
A steel sheet less than Pa is a case where higher tensile strength is required at the expense of some elongation properties. Here, the target characteristics in each case are as follows.・ Tensile strength (TS): 590 to 880 MPa TS × El ≧ 20000 MPa ・%, TS × λ ≧ 50000 MPa ・% ・ Tensile strength (TS): Over 880 MPa, less than 1050 MPa TS × El ≧ 14000 MPa ・%, TS × λ ≧ 30,000 MPa ・% where λ: hole expansion rate (%)

[0008]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies in order to achieve the above-mentioned object, and as a result, have properly adjusted the steel composition, and have properly adjusted the hot rolling conditions and the subsequent cooling conditions. It has been found that the intended purpose can be advantageously achieved by controlling the steel structure appropriately and controlling the steel structure appropriately. The details are as follows. (1) By adding Mo, the initial austenite grains are refined, the crystal grains in the final product are refined, and the strength −
Not only is the elongation balance property improved, but also the high yield ratio results in improved stretch flangeability. (2) By increasing the rolling temperature, recrystallization of austenite grains is promoted, and the main phase after ferrite transformation and the second phase
The difference in average particle size of the phases is reduced, resulting in improved stretch flangeability. (3) When Cr and B are added in addition to Mo, ferrite and pearlite transformation are suppressed, and not only the average particle size difference between the main phase and the second phase becomes small, but also the second phase becomes Since the structure is mainly composed of inite, stretch flangeability is improved. (4) In addition, Cr and B can reduce the rolling load during hot rolling, and at the same time, reduce the rolling in the unrecrystallized austenite region, thereby reducing the deformation zone and reducing the ferrite formation site. However, since the ferrite transformation is suppressed, the difference in the average grain size between the main phase and the second phase is reduced, and the second phase has a structure mainly composed of bainite, so that the stretch flangeability is improved. (5) By setting upper limits for the contents of P and S, stretch flangeability and weldability are improved.

The present invention is based on the above findings. That is, the gist configuration of the present invention is as follows. 1. C: 0.02 to 0.15% by mass percentage, Si: 0.2 to 1.5
%, Mn: 1.0 to 3.5%, Mo: 0.1 to 1.0%, Al: 0.01 to
The second phase contains 0.1%, P: 0.03% or less and S: 0.001% or less, with the balance being a composition of Fe and unavoidable impurities, the main phase being ferrite and bainite or bainite partially containing martensite. 10 to 40% by volume fraction
And the average particle size of the second phase is 8 μm or less, and the ratio of the average particle size to the main phase is as follows: 1.3 ≧ d ₂ / d ₁ ≧ 0.7 where d _{1 is the} main phase A high-yield-ratio high-tensile-strength hot-rolled steel sheet excellent in workability, characterized by satisfying a range of an average particle diameter, d ₂ : an average particle diameter of the second phase.

[0010] 2. In the above item 1, the steel further contains, by mass percentage, Cr: 0.3% or less, B: 0.0005 to 0.01%, C:
a: high yield ratio type high tensile strength hot-rolled steel sheet excellent in workability, characterized by having a composition containing one or more selected from a: 0.001 to 0.005% and REM: 0.001 to 0.005%. .

3. C: 0.02 to 0.15% by mass percentage, Si:
0.2-1.5%, Mn: 1.0-3.5%, Mo: 0.1-1.0%,
Al: 0.01 to 0.1%, P: 0.03% or less and S: 0.001%
A steel slab with a composition containing: finishing temperature: 82
Hot rolling at 0 to 970 ° C, cooling to 750 to 700 ° C at a rate of 5 to 50 ° C / s within 1 second after completion of hot rolling, and then reducing the temperature of ferrite phase precipitation to 750 to 650 ° C After staying for 2 to 30 seconds, cool at a rate of 5 to 50 ° C / s, 350 to 650
A method for producing a high-yield-ratio high-strength hot-rolled steel sheet excellent in workability, characterized by winding at a temperature of ° C.

Hereinafter, the present invention will be described specifically. First, the reason why the composition of steel in the present invention is limited to the above range will be described. The percentages of the component compositions shown below are “% by mass”. C: 0.02 to 0.15% C effectively contributes to the improvement of the strength, and at least 0.02% is required to obtain the expected tensile strength in the present invention, but if it exceeds 0.15%, the weldability sharply increases. To deteriorate,
C was limited to the range of 0.02 to 0.15%.

Si: 0.2-1.5% Si is a useful element having a large solid solution strengthening ability and capable of increasing the strength without impairing the yield ratio and the strength-elongation balance. Further, it is an element indispensable for the formation of a mixed structure, for example, by activating the γ → α transformation and promoting the C concentration in the γ phase. Further, as a deoxidizing element at the time of steel making, it effectively contributes to cleaning of steel. However, if the content is less than 0.2%, the effect of the addition is poor. On the other hand, if the content exceeds 1.5%, the effect not only reaches saturation, but also deteriorates the surface properties,
Since disadvantages such as deterioration of chemical conversion property occur,
Limited to 1.5% range.

Mn: 1.0 to 3.5% Mn not only contributes to improvement in strength but also has an effect of improving hardenability, and is particularly useful for forming the second phase into a bainite structure partially containing martensite. However, if the content is less than 1.0%, the above effects cannot be expected. On the other hand, if the content exceeds 3.5%, it becomes easy to form a band-like rolled structure, and deterioration of stretch flangeability and weldability is caused. Therefore, Mn is limited to the range of 1.0 to 3.5%.

Mo: 0.1-1.0% Mo is a particularly important element in the present invention. That is, Mo contributes not only to the strength but also to the improvement of hardenability, and not only to refine the crystal grains to improve the strength-elongation balance, but also to suppress the pearlite transformation to improve the pearlite transformation. The formation of bainite in two phases is facilitated, and the high yield ratio contributes to the improvement of stretch flangeability. To achieve the above effects, it is necessary to add at least 0.1%. However, if the content exceeds 1.0%, the effect reaches saturation, and adverse effects such as an increase in cost and deterioration in weldability occur. 0.1 to 1.0%. Particularly preferred
It is in the range of 0.1-0.5%.

Al: 0.01 to 0.1% Al is an element useful as a deoxidizing agent, but has a content of 0.01 to 0.1%.
If it is less than 0.1%, there is no effect as a deoxidizing agent, while if it exceeds 0.1%, this effect not only saturates but also raises the cost. Therefore, Al was limited to the range of 0.01 to 0.1%.

P: 0.03% or less Since P is a harmful element in the present invention, setting the upper limit value is important. That is, when P is contained in a large amount, the weldability deteriorates, and the formation of a ferrite band due to the center segregation causes a significant deterioration in stretch flangeability. These phenomena become remarkable when the P content exceeds 0.03%, so that P is suppressed to 0.03% or less.

S: 0.001% or less S, like P, is a harmful component in the present invention. That is, when S is contained in a large amount, not only the weldability is deteriorated, but also the stretch flangeability is deteriorated due to the formation of MnS. In the present invention, in order to increase the tensile strength, Mn
It is particularly important to limit the upper limit of the amount of S because it tends to be added in a large amount. The deterioration of the characteristics as described above becomes remarkable when the amount of S exceeds 0.001%.
Suppressed to 0.001% or less.

Although the essential components and the inhibiting components have been described above, the present invention may further include the following elements as appropriate. Cr: 0.3% or less Cr is an element that is useful not only for obtaining a two-phase structure but also for suppressing the pearlite transformation to make the second phase a bainite-based structure. However, the content
If the content exceeds 0.3%, not only is the chemical conversion property significantly reduced, but also the weldability is adversely affected and the addition cost is increased. Therefore, the content of Cr is set to 0.3% or less.

B: 0.0005 to 0.01% B is an element useful for producing a second phase mainly composed of bainite. This effect is obtained when the content is 0.0005% or more, but exceeds 0.01%. And this effect is saturated, and rather, the steel sheet tends to crack during hot rolling.
It was made to contain in the range of -0.01%.

Ca: 0.001 to 0.005% Ca has the effect of reducing the size of the sulfide, and effectively contributes to the improvement of elongation and stretch flangeability. However, if the content is less than 0.001%, the effect is poor, while if it exceeds 0.005%, not only the effect reaches saturation, but also the cleanliness of the steel is deteriorated and it is not economical. The content was set in the range of 0.005%.

REM: 0.001 to 0.005% REM (rare earth element) also has an effect of controlling the size of sulfide to improve elongation and stretch flangeability, like Ca, but the content is less than 0.001%. And its effect is poor. On the other hand, if it exceeds 0.005%, not only does the effect reach saturation, but also the cleanliness of the steel deteriorates and it is not economical, so the REM should be contained in the range of 0.001 to 0.005%. .

The preferred component composition range of the present invention has been described above. However, in the present invention, it is not sufficient to limit the component composition to the above range, and it is important to adjust the steel structure as well. That is, it is important that the main phase be ferrite and the second phase be bainite or a bainite structure partially containing martensite. Here, the second
The reason why the phase was composed mainly of bainite (including some martensite) was to reduce the hardness difference from ferrite by obtaining a high yield ratio and by using a second phase softer than martensite and pearlite. This is to reduce the occurrence of initial cracks during deformation of the stretch flange. Further, if the fraction of the second phase is less than 10%, a sufficient strength level cannot be obtained, while if it exceeds 40%, the elongation is remarkably reduced, and the press formability is deteriorated. Therefore, in the present invention, the fraction of the second phase is in the range of 10 to 40%. Preferably, 20-30%
Range. In the present invention, the fraction of the main phase ferrite is preferably 60 to 90%, more preferably 70 to 80%. Further, the fraction of bainite in the second phase is preferably 70% or more.

The reason why the average particle size of the second phase is 8 μm or less is that if it exceeds 8 μm, good strength-elongation characteristics cannot be obtained at a high yield ratio. Further, the difference between the average particle diameter of the main phase and the average particle diameter of the second phase is expressed by the following equation: 1.3 ≧ d ₂ / d ₁ ≧ 0.7, where d _{1 is} the average particle diameter of the main phase, and d _{2 is} the average particle diameter of the second phase. The reason for limiting the range is that if the ratio is out of this range, the homogeneity of the structure is degraded, that is, the particle size difference between the main phase and the second phase is increased, and the number of initial cracks during stretch flange deformation is increased. It is.

Next, a manufacturing method according to the present invention will be described. In the present invention, the hot rolling finishing temperature is set to 820.
The reason why it was set to ~ 970 ° C is that when the finishing temperature exceeds 970 ° C,
Since the final average particle size of the second phase is coarse, it is not possible to achieve a finer particle size. On the other hand, if the average particle size is lower than 820 ° C., the accumulation of strain increases, and the slow cooling process (the ferrite phase This is because the transformation into the ferrite phase proceeds excessively in the precipitation process), and not only the particle size difference between the main phase and the second phase becomes large, but also the formation of the second phase mainly composed of bainite becomes difficult. . If the temperature is further lowered, rolling occurs in the two-phase region, and the ferrite phase becomes wrought and extended, which adversely affects the stretch flangeability. In this regard, when rolling is completed in the temperature range of 820 to 970 ° C., moderate austenite grain growth and transformation to ferrite phase and grain growth occur in the slow cooling process after rapid cooling, and the main phase and the second phase The particle size difference becomes smaller. Note that a more preferable finishing temperature is 880 to 930 ° C.
It is.

After the completion of the above-mentioned hot rolling, the cooling is started at a rate of 5 to 50 ° C./s within 1 second because the rapid cooling prevents the austenite grains from being excessively coarsened and the austenite. This is for accumulating distortion. Here, if the quenching start time exceeds 1 second or the cooling rate is less than 5 ° C./s, rolling distortion is released, and the transformation to the ferrite phase in the subsequent slow cooling process is delayed. Is 50 ℃
If the value exceeds / s, excessive strain accumulation promotes ferrite transformation, not only increases the particle size difference between the main phase and the second phase, but also makes it difficult to form a second phase mainly composed of bainite. A more preferred quenching start time is 0.5 seconds or less, and a more preferred cooling rate is 10 to 30 ° C / s.

Next, the temperature at which the cooling is stopped is set to 750 to
The reason for setting the temperature at 700 ° C. is that if the temperature is less than 700 ° C. or exceeds 750 ° C., the ferrite phase comes out of the precipitation nose and the ferrite transformation in the slow cooling process is delayed.

Next, the temperature is kept in the range of 750 to 650 ° C. for 2 to 30 seconds, because this temperature range is an optimum ferrite phase precipitation temperature range. If the residence time in this temperature range is less than 2 seconds, transformation to ferrite does not sufficiently occur, and γ
→ The two-phase separation of α does not proceed, the C concentration in γ is insufficient, and the bainite transformation of the second phase in the subsequent winding step is unlikely to occur, and the desired structure cannot be obtained. on the other hand,
If the residence time exceeds 30 seconds, the ferrite transformation proceeds excessively, the two-phase separation of γ → α is promoted, and not only the particle size difference between the main phase and the second phase becomes large, but also bainite is mainly used. This is because the formation of the second phase becomes difficult, the pearlite transformation starts, and the formation of the bainite phase decreases. A more preferred residence time is between 4 and 10 seconds.

After the two-phase separation process of γ → α in the above-mentioned slow cooling process (ferrite phase precipitation process), it is further cooled at a speed of 5 to 50 ° C./s and wound around a coil at a temperature of 350 to 650 ° C. . This is because if the above cooling rate is less than 5 ° C./s, pearlite is generated, so that a second phase mainly composed of bainite cannot be obtained after winding, while cooling at a rate exceeding 50 ° C./s This is because a large amount of martensite is formed, a second phase mainly composed of bainite cannot be obtained, the yield ratio decreases, and the stretch flangeability decreases. If the winding temperature is lower than 350 ° C, a large amount of martensite is generated, and the yield ratio is lowered. Also. On the other hand, if the winding temperature is higher than 650 ° C, pearlite precipitates, the amount of bainite phase decreases, the elongation-strength balance is lowered, a high yield ratio is not obtained, and the stretch flangeability is markedly reduced. to degrade. A more preferred winding temperature is in the range of 550-400 ° C.

[0030]

EXAMPLES Example 1 A steel slab having the composition shown in Table 1 was prepared under the conditions shown in Table 2.
Then, after hot rolling, cool and then wind it up into a coil
Was. In Table 2, FDT is hot rolling finishing temperature, t₁ Is the end of hot rolling
Cooling start time after completion, v₁ Is the cooling rate, T₁ Is that
Cooling stop temperature, T _Two Is the slow cooling temperature (the ferrite phase precipitation temperature
Degree), t_Two Is the residence time in that temperature range, v_TwoIs its temperature
Cooling rate from the area, and CT is the winding temperature. Or
Structure, mechanical properties and chemical treatment of the hot rolled sheet thus obtained
Table 3 shows the results of the examination for reason.

The tensile properties were determined by using a JIS No. 5 tensile test piece with a thickness of 3.2 mm, which was sampled from the rolling width direction (C direction), and subjected to a tensile test to determine the yield strength (YS) and tensile strength.
(TS) and elongation (El) were measured, and the yield ratio (YR) was determined. The stretch flangeability was examined by a hole expansion test. Hole expansion test is based on JFS-T1001-199
6 According to the hole expansion test method, the hole diameter = 10 mmφ
Clearance: punch out at 12.5% to make an initial hole (d ₀ ), then insert the conical punch with a vertex angle: 60 ° into the initial hole with the burr side of the initial hole as the die side (opposite to the punch) The hole diameter (d) at the time when the crack penetrated the plate thickness was determined, and the hole expansion ratio λ (%) was calculated by the following equation. λ = {(d−d ₀ ) / d ₀ } × 100 where d: diameter at the time of occurrence of crack d ₀ : punching diameter

Further, the particle diameters of the ferrite and the second phase were measured by taking a photograph with an electron microscope and then determining the cutting diameter in the method for testing the ferrite crystal grain size of steel shown in JIS G 0552. The fraction of the second phase was determined by image analysis of an electron micrograph.

Regarding the chemical conversion property, a steel sheet of a test material having a mass W ₀ was washed and degreased, immersed in a solution containing a chemical conversion agent (zinc phosphate solution) for a certain period of time, and after washing, the mass was measured ( W), and the mass increase per unit area (W-W ₀ ) due to the adhesion of zinc phosphate crystals was evaluated. The goal
2.0 g / m ² or more.

[0034]

[Table 1]

[0035]

[Table 2]

[0036]

[Table 3]

As shown in Table 3, the invention examples obtained according to the present invention all have tensile strength (TS) and elongation (El).
Excellent in hole expansion ratio (λ), TS × El is 20000 M
Excellent strength-elongation balance and TS × over Pa ·%
Excellent strength-hole expansion balance with λ of 50,000 MPa ·% or more could be obtained.

Example 2 A steel slab having the composition shown in Table 4 was prepared under the conditions shown in Table 5.
Then, after hot rolling, cool and then wind it up into a coil
Was. In Table 5, FDT is hot rolling finishing temperature, t₁ Is the end of hot rolling
Cooling start time after completion, v₁ Is the cooling rate, T₁ Is that
Cooling stop temperature, T _Two Is the slow cooling temperature (the ferrite phase precipitation temperature
Degree), t_Two Is the residence time in that temperature range, v_TwoIs its temperature
Cooling rate from the area, and CT is the winding temperature. Or
Structure, mechanical properties and chemical treatment of the hot rolled sheet thus obtained
Table 6 shows the results of the examination for reason.

[0039]

[Table 4]

[0040]

[Table 5]

[0041]

[Table 6]

This example is a case where the tensile strength exceeds 880 MPa.
≧ 14000 MPa ・% Excellent strength-elongation balance and TS
An excellent strength-hole expansion balance of × λ ≧ 30000 MPa ·% is obtained.

[0043]

Thus, according to the present invention, a high-yield-ratio high-strength hot-rolled steel sheet excellent in workability, particularly in elongation and stretch flangeability, and also excellent in weldability and chemical conversion treatment, as hot-rolled. Can be obtained stably.

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 4K037 EA01 EA02 EA05 EA06 EA09 EA11 EA15 EA16 EA17 EA23 EA25 EA27 EA28 EA36 EB05 EB08 EB09 EB11 FB00 FC03 FC04 FD02 FD03 FD04 FD08 FE01 FE07 JA

Claims (3)

[Claims] 1. A mass percentage of C: 0.02 to 0.15%, Si: 0.2 to 1.5%, Mn: 1.0 to 3.5%, Mo: 0.1 to 1.0%, Al: 0.01 to 0.1%, P: 0.03% or less and S : 0.001% or less, the balance is composed of Fe and unavoidable impurities, the main phase is ferrite, and the second phase consisting of bainite or bainite partially containing martensite is expressed in volume fraction.
It has a steel structure containing 10 to 40%, the average grain size of the second phase is 8 μm or less, and the ratio of the average grain size to the main phase is as follows: 1.3 ≧ d ₂ / d ₁ ≧ 0.7 where d ₁ A high yield ratio type high tensile strength hot rolled steel sheet excellent in workability, characterized by satisfying a range of: average particle size of main phase, and d ₂ : average particle size of second phase. 2. The steel according to claim 1, wherein the steel is selected from a mass percentage of Cr: 0.3% or less, B: 0.0005 to 0.01%, Ca: 0.001 to 0.005%, and REM: 0.001 to 0.005%. A high-yield-ratio high-tensile-strength hot-rolled steel sheet having excellent workability, characterized by having a composition containing one or more kinds. 3. Mass percentage: C: 0.02 to 0.15%, Si: 0.2 to 1.5%, Mn: 1.0 to 3.5%, Mo: 0.1 to 1.0%, Al: 0.01 to 0.1%, P: 0.03% or less and S : A steel slab having a composition containing 0.001% or less, finishing temperature: 820 ~
Hot-rolled at 970 ° C, cooled to 750-700 ° C at a rate of 5-50 ° C / s within 1 second after completion of hot rolling.
After staying in the ferrite phase precipitation temperature range of 50 to 650 ° C for 2 to 30 seconds, cooling at a rate of 5 to 50 ° C / s and winding at a temperature of 350 to 650 ° C. Manufacturing method of excellent high yield ratio type high tension hot rolled steel sheet.