JPH10273755A - High strength hot rolled steel plate, excellent in press formability and impact characteristic, and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength hot rolled steel plate, improved in press formability and impact characteristic, and its production. SOLUTION: This high strength hot rolled steel plate has a composition which consists of, by weight, 0.10-0.20% C, 1.0-1.4% Si, 1.8-2.2% Mn, <=0.35% Cu, <=0.35% Ni, 0.005-0.040% sol.Al. 0.0020-0.0040% N, and the balance Fe with inevitable impurities and to which <=2%, in total, of one or more elements among Mo, W, V, and Co and <=1%, in total, of one or more elements among Cr, Nb, and Ti and further added, if necessary, and also has a metallic structure composed of three phase of polygonal ferrite, lower bainite, and 5 to 10%, by volume ratio, of retained austenite. In the course between the hot rolling of a steel slab and the coiling of a steel plate, cooling velocity is controlled to three steps and coiling is done at 350 to 500 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-rolled steel sheet having excellent press formability and impact properties, and a method for producing the same. More specifically, the present invention relates to a hot-rolled steel sheet having a tensile strength of 590 N / mm ² or more.
The tensile strength (TS) × total elongation (EL) is 20000N / mm ²
% Or more and a 2 mmV notched Charpy impact value of 25 J or more.

[0002]

2. Description of the Related Art In the automobile industry, in addition to pursuing and improving safety, there is a demand for a lighter vehicle body as part of measures to improve fuel economy. Strengthening is required. But,
The higher the strength, the lower the ductility and the more difficult it is to form by pressing or the like.

Further, in the recent automobile industry, improvement of safety has been taken up as a very important issue. Above all, in order to ensure the safety inside the car, it is important to determine how much the steel material absorbs energy at the time of the collision, and high-strength hot-rolled steel constituting automobile parts is also required to have improved impact characteristics. I have.

Hitherto, as a high-strength steel sheet having high ductility, a two-phase steel sheet having a mixed structure of ferrite and martensite or a mixed structure of ferrite and bainite has been manufactured. The mechanical properties of the duplex steel sheet have a low yield ratio (yield strength / tensile strength) and have a high strength level among steel sheets having the same ductility. However, the tensile strength is 780
At present, it is difficult to obtain a sufficient strength-ductility balance because the ductility of a steel sheet of N / mm ² or more is remarkably reduced.

[0005] As a method for improving the strength-ductility balance expressed by tensile strength (TS) x total elongation (EL), a metal structure of a steel sheet is mixed with ferrite, bainite and retained austenite, or a mixed structure containing a part of martensite. A high-strength composite microstructure steel sheet has been proposed. This steel sheet utilizes so-called "transformation-induced plasticity" in which retained austenite remaining in the metal structure is transformed into martensite during forming to obtain high strength and high ductility.
Since the amount of retained austenite affects the mechanical properties of the steel sheet, ductility, particularly uniform elongation, is improved even at the same strength.

Under these circumstances, Japanese Patent Application Laid-Open No. H4-32512 discloses a method for producing a high-strength composite structural steel sheet utilizing "transformation-induced plasticity". 0.8 to 2.5% of one or more of Si, Mn and Cr in total of 0.8 to 4.0% and 0.001 to 0.050% of sol. After reheating a steel sheet having a steel composition composed of 0.008 to 0.025% Al and N at an Ac ₃ point or more and an austenite average particle diameter of 50 μm or less, the final pass rolling reduction is 40% or more. , finishing temperature of Ar ₃ ~Ar ₃ +50
After hot rolling at 60 ° C./second and cooling to 550 to 700 ° C. at a cooling rate of 60 ° C./second or less, 0 to 30
Second stage cooling is performed at a cooling rate of 0 to 5 ° C./sec, and 300 to 45 ° C. or more at a cooling rate of 30 ° C./sec or more.
Winding after cooling to the third stage of cooling to 0 ° C, polygonal ferrite 60% or more by volume ratio and residual austenite 10%
A high-strength composite structure hot-rolled steel sheet for processing having a super composite structure is disclosed. By controlling the hot-rolling heating temperature, heating time, heating rate, or adding Nb and Ti to reduce the average austenite grain size during heating to less than 50 μm, to promote ferrite formation in the subsequent cooling process, Stabilizing austenite by enriching C in untransformed austenite and further using low Al high N steel,
It has the feature of obtaining sufficient retained austenite.

[0007] Japanese Patent Application Laid-Open No. H4-228538 discloses that a steel strip prepared by hot rolling a steel slab adjusted to a predetermined composition and wound is immersed in water or sprayed with mist at a cooling rate of 30 ° C./hour or more. Cooling to below ℃, volume ratio is about 1-4
It is disclosed to obtain a% retained austenite.
When the winding temperature is lowered, martensite is generated and becomes a factor inhibiting mechanical properties, and further, post-treatment after winding is performed for the purpose of suppressing excessive bainite transformation.

As described above, the strength-ductility balance of tensile strength (TS) × total elongation (EL) is 20,000 N / mm 2
% In order to obtain a high-strength, high-ductility steel sheet of not less than%, after hot rolling, heat treatment is again performed to generate retained austenite, or after hot rolling, post-treatment such as mist spray cooling, Alternatively, by adding a large amount of C or N as an austenite stabilizing element, the volume ratio of retained austenite was secured to 10% or more. Further, at present, it is not clear about the improvement of the impact characteristics of the steel sheet.

[0009]

Accordingly, an object of the present invention is to provide a high-strength hot-rolled steel sheet having excellent press formability and impact properties. Specifically, the tensile strength is 590
N / mm ² or more, a tensile strength (TS) × total elongation (EL) is 20000N / mm ² ·% or more, a high-strength hot-rolled steel sheet is -40～20 Charpy impact value-out 2mmV notch at ℃ or more 25J And a method for producing the same.

[0010]

The above object is achieved by the invention of claim 1, that is, C: 0.10 to 0.20%, Si:
1.0-1.4%, Mn: 1.8-2.2%, Cu:
0.35% or less, Ni: 0.35% or less, Sol. A
l: 0.005 to 0.040%, N: 0.0020 to
This can be achieved by a high-strength hot-rolled steel sheet comprising 0.0040%, the balance being Fe and inevitable impurities, and a metal structure including polygonal ferrite, lower bainite, and 5 to 10% by volume retained austenite.

According to a second aspect of the present invention, the steel to which the present invention is applied further comprises one or more of Mo, W, V, and Co elements in a total amount of 2% or less.

According to a third aspect of the present invention, there is provided the steel according to the first or second aspect, wherein Cr, N
b, one or more of Ti elements are 1% in total
The following are added.

A fourth aspect of the present invention is a method for producing a high-strength hot-rolled steel sheet according to any one of the first to third aspects,
The hot rolling is completed with the steel slab having the components described in claims 1 to 3, and the finishing temperature Ar3 point is −20 ° C. to Ar3.
When hot rolling at a temperature of + 50 ° C. and a winding temperature of 350 to 500 ° C., the temperature from the finishing temperature to the ferrite formation starting temperature is cooled at 50 to 100 ° C./sec. After cooling for 2 seconds, 50-100 ° C up to the winding temperature
Per second.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have studied in detail the steel composition, metal structure, manufacturing conditions such as hot rolling, and the tensile properties and impact properties of a steel sheet. As a result, it was found that a high-strength hot-rolled steel sheet excellent in press formability and impact properties was obtained. Hereinafter, matters for specifying the present invention will be described.

C: 0.10 to 0.20% C is an extremely important element in the present invention, and is indispensable for containing retained austenite in a cooling process after hot rolling. The effect is to concentrate C in the untransformed austenite to stabilize the austenite and to effectively secure sufficient retained austenite in the steel sheet. If the content of C is less than 0.10%,
The strength is insufficient, and the concentration of C in the austenite becomes insufficient. As a result, the retained austenite is not sufficiently satisfied, and it is difficult to obtain high ductility. On the other hand, if the C content exceeds 0.20%, there is no problem with the generation of retained austenite, but a problem arises because workability and weldability are significantly deteriorated. Also, by adding C,
One of the problems is that the transition temperature rises and the notch brittleness in the V-notch Charpy impact test is deteriorated. Therefore, the content of C is set to 0.10 to 0.20%.

Si: 1.0-1.4% Si is an important element in the present invention.
It has the effect of promoting the formation of ferrite and further promoting the concentration of C in untransformed austenite. Also Si
Is a solid solution strengthening element and therefore plays an important role in securing strength, and also has the effect of improving mechanical properties, especially uniform elongation. When the content of Si is less than 1.0%, this effect is not sufficiently exhibited, and as a result, sufficient retained austenite cannot be obtained. If the content exceeds 1.4%, the above-mentioned actions and effects tend to be saturated, the impact value decreases, and the transition temperature tends to increase.

Mn: 1.8-2.2% Mn is an austenite-forming element and is greatly involved in suppressing the transformation of untransformed austenite into pearlite, which inhibits ductility. This effect is not sufficient if the content is less than 1.8%, and if it exceeds 2.2%, it becomes difficult to produce a satisfactory ferrite, resulting in deterioration of ductility. Since the solid solubility limit of Mn in ferrite is about 3%,
Most of them are dissolved in ferrite to increase the cleavage fracture stress of ferrite, which is effective against notch embrittlement. That is, the transition temperature in the impact test is lowered and the deterioration of the impact value is suppressed.

Cu: 0.35% or less Cu is concentrated at the boundary between the oxide scale generated during hot rolling and the surface of the steel sheet to improve the scale releasability. However, 0.35%
If it exceeds 300, the impact value tends to decrease, and fine cracks tend to occur on the steel sheet surface due to embrittlement of the molten metal. Ni: 0.35% or less In order to suppress the embrittlement of molten metal due to the addition of Cu, approximately the same amount as Cu is required. However, when Cu is 0.20% or less, it is not necessary to add. Further, similarly to the effect of Mn, the transition temperature by the impact test is lowered and the deterioration of the impact value is suppressed.

Sol. Al: 0.005 to 0.040% Al is added for the purpose of deoxidation, but promotes the formation of ferrite similarly to Si, and further promotes the concentration of C in untransformed austenite. Therefore, it is desirable to add Si together with Si. However, if it exceeds 0.040%, inclusions increase, which causes a reduction in local elongation, and tends to lower the impact value and increase the transition temperature.

N: 0.0020 to 0.0040% Like C, it is an interstitial solid solution element, stabilizes austenite and promotes enrichment of C in untransformed austenite. Furthermore, although the transition temperature rises with an increase in N, if the treatment of AlN or the like is not performed reliably, it may be a hindrance factor, so large addition is not suitable. Further, the addition of N has the effect of lowering the Ms point (martensite formation temperature) and increasing retained austenite. However, if the content is 0.0040% or more, workability and toughness are reduced.

Mo, W, V, Co: 1 or 2 or more of them in a total of 2% or less Since all elements have an effect of lowering the impact temperature transition temperature, they may be added in a total of 2% or less. desirable.
Even if it is added more than this, the effect corresponding thereto cannot be obtained and the cost becomes high. Cr, Nb, Ti: 1% or 2 or more types in total of 1% or less Any of these elements is an effective element for stabilizing austenite and securing the amount of retained austenite, and for securing strength and refining. It is preferable to add one or more elements in a total amount of 1% or less. Even if it is added more than this, the effect corresponding thereto cannot be obtained and the cost becomes high.

Metallic structure: Three-phase structure of polygonal ferrite, lower bainite, and retained austenite A mixed structure of soft polygonal ferrite, hard lower bainite, and residual austenite, from which solid solution C contained in the ferrite has been sufficiently discharged. This improves ductility, especially uniform elongation, and yield ratio (yield strength / tensile strength)
Lower.

Retained austenite amount: 5 to 10% High strength and high ductility are obtained by the "transformation-induced plasticity" effect of retained austenite. If the volume fraction of retained austenite is less than 5%, sufficient ductility cannot be obtained. On the other hand, if the volume fraction of retained austenite exceeds 10%, the concentration of C in the untransformed austenite becomes insufficient, and the austenite becomes unstable, so that the amount of retained austenite fluctuates in the steel sheet.

Next, the manufacturing conditions of the hot-rolled steel sheet according to the present invention will be described. The steel slab having the above composition is subjected to hot rolling immediately after continuous casting or after reheating. The conditions up to the start of hot rolling are not particularly limited, but the finishing temperature is Ar ₃ point−20 ° C. to Ar ₃ point + 5.
Since the temperature is relatively low at 0 ° C., it is desirable to combine with low-temperature heating.

Finish temperature: Ar ₃ point-20 ° C. to Ar ₃ point + 50 ° C. If Ar ₃ point is lower than −20 ° C., ferrite on the surface layer of the steel sheet generated during hot rolling appears as processed ferrite and deteriorates the workability of the steel sheet. Let it. If the temperature exceeds the Ar3 point + 50 ° C., the metal structure becomes coarse and the ductility and toughness deteriorate. Also,
If the finishing temperature is too high, it is difficult to control the winding temperature and the material cannot be obtained.

Winding temperature: 350 to 500 ° C. In order to prevent the formation of pearlite, which is a factor inhibiting ductility, to suppress excessive promotion of bainite transformation, and to secure the amount of retained austenite, the winding temperature is increased. Is 500 ° C. If the winding temperature is lower than 350 ° C., martensite is formed, which is a factor that impairs ductility and moldability.

Cooling rate from finishing temperature to ferrite formation starting temperature: 50 to 100 ° C./sec. A slab which has been hot-rolled at _three points of Ar—20 ° C. to _three points of Ar + 50 ° C. is cooled to 50 to 100 ° C./second. Cool at a cooling rate to a temperature below the ferrite formation start temperature and above the bainite formation start temperature. When the cooling rate is 50 ° C / sec or less,
The formation of pearlite which inhibits ductility is promoted, and the enrichment of C in untransformed austenite is impaired. Further, when the cooling rate is increased, the grain refinement of the structure is promoted.
Saturation occurs at cooling rates above ° C / sec.

Cooling at a cooling rate of 50 ° C./sec or less from the ferrite formation initiation temperature for 3 to 15 seconds: A cooling rate of 50 ° C./sec for the steel sheet cooled to a temperature below the ferrite formation initiation temperature and above the bainite formation initiation temperature. Cool with. As a result, the ferrite generated finely is grown into polygonal ferrite. At the same time, C in solid solution in ferrite
To concentrate C into untransformed austenite.
At a cooling rate of 50 ° C./sec or more, solid solution C in ferrite
Is not sufficient to promote the growth of acicular ferrite. When the slow cooling time in the second stage is short, the growth of the polygonal ferrite is suppressed, and the acicular ferrite in which C is enriched is easily generated. If this time is long, the formation of pearlite or carbide-rich upper bainite which reduces ductility is promoted. Therefore, the slow cooling time of the second stage is set to 6 to 1 in consideration of facility restrictions.
Desirably, it is 4 seconds.

Cooling rate to winding temperature: 50 to 100 ° C.
The steel sheet that has been subjected to the second stage of slow cooling is cooled at a cooling rate of 50 to 100 ° C./second, and wound up within a range of 350 to 500 ° C. At a cooling rate of 50 ° C./second or less, lower bainite that satisfies the present invention cannot be obtained, and upper bainite containing a large amount of carbide is formed, and pearlite, which significantly inhibits ductility, is formed. 100 ° C
At a cooling rate of / sec or more, the obtained retained austenite is small, and the material is not stable. When the winding temperature at which the cooling is terminated is lower than 350 ° C., the formation of martensite containing carbides increases, and the volume fraction of retained austenite for obtaining high ductility decreases. On the other hand, when the temperature exceeds 500 ° C., the production of pearlite increases, so that the ductility is impaired.

[0030]

EXAMPLE Steel type No. having the composition shown in Table 1 was used. After melting 1 to 9, a hot-rolled steel sheet having a thickness of 2.9 mm was manufactured under the hot-rolling conditions and cooling conditions shown in Table 2. Steel type No. 8-9
As shown in Table 2, simple cooling conditions were used in which the cooling rate from the finishing temperature to the winding temperature was kept constant. Other steel types No. With respect to 1 to 7, from the finishing temperature to the winding temperature, three cooling rates were set according to the present invention. Table 3 shows the results of identification of the main phase and the structure other than the main phase of the metal structure of the obtained hot-rolled steel sheet, the measurement results of the retained austenite volume fraction (VγR) by X-rays, and the measurement results of the mechanical properties.

[0031]

[Table 1]

[0032]

[Table 2]

[0033]

[Table 3]

From Table 3, the metallographic structure of the hot-rolled steel sheet according to the comparative example is such that the main phase is acicular ferrite,
The metal structure of the hot-rolled steel sheet of the present invention example was polygonal ferrite whose main phase was soft. Due to such a difference in the structure of the structure, in the steel sheet of the present invention, the yield ratio (YR), which is an index of stretch formability, is 60 to 80% lower than that of the comparative example. It can be seen that it has effective characteristics.

Further, in the example of the present invention, 4.5 other than the main phase was used.
Since the material has retained austenite of up to 8.1%, a strength-ductility balance expressed by tensile strength (TS) × total elongation (EL) of 20000 N / mm ² ·% or more is obtained. It can be seen that this is a high-strength hot-rolled steel sheet with excellent properties.

Table 4 shows the results of a Charpy impact test with a 2 mm V notch in a temperature range of -40 to 20 ° C. As a method for evaluating the deformability of a material, a tensile test or the like is known as a static test.However, when a large stress wave propagates during a collision, it is meaningless in a static test because it is affected by the strain rate. Therefore, a V-notched Charpy impact test, which is a dynamic test having a large strain rate, was performed and evaluated. As the test piece, a JIS Z2202 No. 4 sub-size test piece was used. The test piece was sampled at a position where the impact direction was perpendicular to the hot rolling direction, and the thickness was 2.0 ± 0.0
5mm, V-shaped notch depth 2mm, notch angle 45 ± 2
The notch tip radius was adjusted to 0.25 ± 0.025 mm. In the example of the present invention, the impact value is 25 J or more even at a test temperature of −40 ° C., and has excellent impact characteristics in a range from room temperature to −40 ° C. even when an impact having a large strain rate is applied. You can see that.

[0037]

[Table 4]

[0038]

As described above, in the hot-rolled steel sheet of the present invention, the adjustment of the steel composition and the metallographic structure of polygonal ferrite, 5 to 1
By forming a mixed structure of 0% retained austenite and lower bainite, it was possible to improve the press formability and impact characteristics of a hot-rolled steel sheet. Therefore, the high-strength hot-rolled steel sheet according to the present invention is suitably used as a material for various mechanical parts such as automobile members having a complicated part shape.

Claims (4)

[Claims] (1) C: 0.10 to 0.20% by weight,
Si: 1.0 to 1.4%, Mn: 1.8 to 2.2%, C
u: 0.35% or less, Ni: 0.35% or less, Sol.
Al: 0.005 to 0.040%, N: 0.0020 to
Excellent in press formability and impact properties consisting of 0.0040%, the balance being Fe and unavoidable impurities, and a metal structure consisting of three phases of polygonal ferrite, lower bainite, and 5-10% by volume retained austenite. High strength hot rolled steel sheet. 2. The high-strength hot-rolled steel according to claim 1, wherein one or more of Mo, W, V and Co are added in a total amount of 2% or less. steel sheet. 3. High strength excellent in press formability and impact characteristics according to claim 1 or 2, wherein one or more of Cr, Nb and Ti are added in a total amount of 1% or less. Hot rolled steel sheet. 4. When hot rolling a steel slab having the components described in claims 1 to 3, the finishing temperature of the exit side of the hot rolling mill is Ar ₃ points-20 ° C. to Ar ₃ points + 50. ℃, the winding temperature when winding the steel strip into a coil is 350 to 500 ℃
After cooling from the finishing temperature to the ferrite formation starting temperature at 50 to 100 ° C./sec, the cooling is performed at 50 ° C./sec or less for 3 to 15 seconds, and then 50 to 1 to the winding temperature.
A method for producing a high-strength hot-rolled steel sheet having excellent press formability and impact properties, characterized by cooling at a rate of 00 ° C./sec.