JP3400351B2 - Manufacturing method of high strength and high workability hot rolled steel sheet with excellent impact resistance - 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 high-strength and high-workability hot-rolled steel sheet which is suitable for use as a steel sheet for automobiles and has excellent impact resistance.

[0002]

2. Description of the Related Art With the trend toward weight reduction of automobiles, the demand for high-strength thin steel sheets having excellent formability has become particularly strong. Further, recently, safety of automobiles has been emphasized, and for that purpose, improvement of impact resistance, which is a measure of safety at the time of collision, is required. Further, it is necessary to consider the economical efficiency, and in consideration of such economic efficiency, the hot rolled steel sheet is more advantageous than the cold rolled steel sheet.

Against the background of the above situation, various high-strength hot-rolled steel sheets have been developed so far. For example, Japanese Examined Patent Publication No. 6-41617, Japanese Examined Patent Publication No. 5-65566 and Japanese Examined Patent Publication No. 5-67682 disclose high workability high strength hot rolled steel sheets containing ferrite, bainite and 5% or more of retained austenite. A method for manufacturing Transformation Induced Plasticity steel (hereinafter referred to as TRIP steel) is disclosed. However, although this TRIP steel has high elongation and good formability (TS × El ≧ 24000 MPa ·%), it still has a problem in that it cannot reach the current severe impact resistance property. Further, there is a problem that the work hardening amount (WH) at the time of press molding and the baking hardening amount (BH) at the time of coating baking thereafter are as low as about 70 MPa. If this working / baking hardening amount (WH + BH) is low, there is a large disadvantage in terms of guaranteeing strength after working-baking.

On the other hand, as a high-strength hot-rolled steel sheet excellent in impact resistance, so-called Dual Phase steel (hereinafter referred to as Dual Phase steel having a two-phase structure of ferrite and martensite, as disclosed in JP-A-9-111396) , DP steel) has been developed. However, although this DP steel is excellent in impact resistance, it cannot be said that the elongation is sufficient, and there is a problem in formability.

[0005]

As described above, up to now, no hot rolled steel sheet satisfying both sufficient formability and severe safety has been found, and its development has been desired. The present invention advantageously responds to the above demands, and has excellent formability and impact resistance. Specifically, the strength-elongation balance (TS × El) is 24000 MPa ·% or more and the dynamic n If the value is 0.35 or more, and the amount of processing and baking hardening (WH
(BH) is as high as 100 MPa or more, and an object is to propose an advantageous manufacturing method of a high-strength and high-workability hot-rolled steel sheet excellent in impact resistance.

Here, the dynamic n value is newly found by the inventors as an index of the impact resistance characteristic. By using this dynamic n value, the impact resistance characteristic can be more accurately compared with the conventional one. Can be evaluated. That is, hitherto, the collision resistance has been considered in relation to the strength, and it has been considered that if the strength is large, the collision resistance is also high, but the strength and the collision resistance are not always unique. It turned out not to be. Therefore, as a result of repeated studies on this point, the collision safety is improved, that is, when the vehicle is deformed at a high speed (when the vehicle crashes, the strain rate is

[Outer 1] The energy in but increased to 2 × 10 ³ / s), in order to absorb more with the steel sheet, the steel sheet

[Outside 2] It has been clarified that it is effective to increase the n value (hereinafter, referred to as a dynamic n value) when the tensile deformation is performed under the condition. Here, the instantaneous n value at 10% elongation is defined as the dynamic n value. It was also found that increasing the dynamic n value is also effective for improving strength during high-speed deformation.

[0007]

Means for Solving the Problems The clarification process of the present invention will be described below. Now, in order to achieve the above-mentioned object, the inventors first investigated the relationship between the structure and properties of the conventional TRIP steel. As a result, T
In the RIP steel, it has been indispensable to generate a bainite phase in order to obtain a sufficient amount of retained austenite, which is advantageous for improving the formability. However, this bainite phase causes deterioration of impact resistance. It turned out.

[0008] Therefore, the inventors of the present invention have suppressed the formation of such bainite phase, particularly carbide, that is, the second phase other than the primary phase proeutectoid ferrite from the conventional bainite + retained austenite in the form of needles. When we changed to a mixed structure of ferrite + martensite + retained austenite, we obtained unexpected results in achieving the intended purpose. The present invention is based on the above findings.

That is, the present invention contains C: 0.05 to 0.40 mass%, Si: 1.0 to 3.0 mass%, Mn: 0.6 to 3.0 mass%, Cr: 0.2 to 2.0 mass%, and the balance substantially Fe. The steel slab with the composition
After heating to 1000 to 1300 ℃, rough rolling and finish rolling from 780 to
Finish at a temperature of 980 ℃, then cool to 620 ~ 780 ℃, then keep isothermal for 1-10 seconds or cooling rate: 20
After annealing at ℃ / s or less and then cooling to 350-500 ℃, coil it, and cool it to 300 ℃ or less at a cooling rate of 10-100 ℃ / h. Manufacturing method of high-strength and high-workability hot-rolled steel sheet excellent in impact resistance, characterized in that a steel structure having a second phase composed of martensite, acicular ferrite and retained austenite in a ferrite main phase (No. 1) 1 invention).

Further, the present invention contains C: 0.05 to 0.40 mass%, Si: 1.0 to 3.0 mass%, Mn: 0.6 to 3.0 mass%, Cr: 0.2 to 2.0 mass%, and the balance substantially Fe. The steel slab with the composition
After heating to 1000 to 1300 ℃, rough rolling and finish rolling from 780 to
Finish at a temperature of 980 ℃, then cool to 620 ~ 780 ℃, then keep isothermal for 1-10 seconds or cooling rate: 20
Slowly cool down to ℃ / s or less, then cool to 350-500 ℃, wind around coil, hold isothermal for 2-60 minutes or slow cooling speed: less than 50 ℃ / h And then forced cooling at a cooling rate of 50 ° C / h or more 3
By cooling to a temperature below 00 ° C, a steel structure having a second phase consisting of martensite, acicular ferrite and retained austenite in the main phase of proeutectoid ferrite, has high impact resistance and high strength. It is a manufacturing method (2nd invention) of a high workability hot rolled steel plate.

According to the present invention, in addition to the basic composition described above, the composition of the steel slab includes at least one selected from P: 0.01 to 0.2 mass% and Al: 0.01 to 0.3 mass% as an austenite forming element. Further, at least one selected from Ti: 0.005 to 0.25 mass% and Nb: 0.003 to 0.1 mass% can be contained as a strength improving component.

In the present invention, the proportion of the second phase in the steel structure is preferably 3-40%, and the proportion of each phase in the second phase is martensite: 10-80%. , Retained austenite: 8 to 30%, acicular ferrite: preferably 5 to 60%.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be specifically described below. FIG. 1 shows a typical continuous cooling transformation curve diagram (CCT diagram) of a conventional TRIP steel. As shown in the figure, the conventional TRIP steel, after hot rolling, holds a little in the pro-eutectoid ferrite region to precipitate pro-eutectoid ferrite (also referred to as polygonal ferrite), and at the same time, forms a solid solution in the untransformed austenite phase. After promoting the concentration of carbon and increasing the stability of austenite, the carbon was introduced into the bainite region, and by gradually cooling this region, a predetermined amount of austenite was retained while causing the bainite transformation. However, as described above, the TRIP steel manufactured in this manner is excellent in strength and workability, but cannot obtain sufficient impact resistance.

Therefore, as a result of repeated experiments and studies for avoiding the formation of bainite, the inventors have found that (1) when a small amount of Cr is contained as a steel component, the nose of the bainite transformation region in the above CCT diagram is reduced. Retreating suppresses bainite precipitation (especially carbide precipitation) and acicular ferrite (also called acicular ferrite) precipitates instead. (2) Acicular ferrite and residual austenite formed in this way. It was clarified that the second phase composed of and martensite markedly improves the impact resistance without impairing the moldability.

FIG. 2 shows a typical CCT diagram in the component system of the present invention. As shown in the figure, by adding a small amount of Cr, the nose of the bainite transformation region recedes, and instead the acicular ferrite region appears remarkably, so hold in this acicular ferrite region for a short time, preferably after that The second phase can be made to have a mixed structure of acicular ferrite, retained austenite and martensite by quenching to a hot rolled steel sheet having excellent formability and impact resistance. It was.

Here, the acicular ferrite refers to one having a major axis of crystal grains of 10 μm or less, an aspect ratio of 1: 1.5 or more, and a cementite precipitation amount of 5% or less. In the bainite of the conventional TRIP steel, a large amount of precipitation of cementite is recognized (10% or more), so that the acicular ferrite of the present invention and bainite of the TRIP steel are clearly distinguished.

FIG. 3 (a) shows the second embodiment obtained according to the present invention.
The characteristic phase composition of the phase is shown in Fig. 3 (b).
The phase constitution of the second phase of the IP steel is schematically shown. The second phase of the conventional TRIP steel has a phase structure in which retained austenite is interspersed in bainite, whereas the second phase of the present invention has acicular ferrite and martensite in a layered form, and its interface The retained austenite is scattered on the (martensite side). Thus, the precipitation of acicular ferrite in the second phase is one of the features of the present invention. This acicular ferrite phase increases TS × El and improves the dynamic n value. It is considered to be a thing. According to the knowledge of the inventors, it has been confirmed that the dynamic n value tends to increase as the interfacial area ratio between the acicular ferrite and martensite increases.

In the present invention, the ratio of the second phase in the steel structure is preferably 3-40%. This is because if the phase ratio is less than 3%, sufficient impact resistance cannot be obtained, whereas if it exceeds 40%, the elongation and thus the strength-elongation balance deteriorate. A more preferable ratio is 10 to 30%. In the present invention, the phase ratio was calculated by polishing a steel sample, etching it with a 2% nitric acid + ethyl alcohol solution, and performing image analysis on a micrograph.

Regarding the ratio of each phase in the second phase, martensite: 10-80% (preferably 30-60)
%), Retained austenite: 8-30% (preferably 10-)
20%), acicular ferrite: 5-60% (preferably 20-50)
%) Is desirable. This is because if the martensite ratio is less than 10%, sufficient impact resistance cannot be obtained, while if it exceeds 80%, the elongation and thus the strength-elongation balance deteriorate. Also, if the ratio of retained austenite is less than 8%, sufficient elongation cannot be obtained.
This is because the impact resistance deteriorates when the content exceeds%. Further, if the proportion of acicular ferrite is less than 5%, good impact resistance cannot be obtained, while if it exceeds 60%, the elongation is lowered.

The proportion of each phase in the entire steel structure is preferably 5 to 15% for martensite and acicular ferrite, and about 2 to 10% for retained austenite. Further, in the present invention, the steel structure is not always composed of a mixed phase of proeutectoid ferrite which is a main phase and martensite which is a second phase, acicular ferrite and retained austenite, and a bainite phase and the like are not necessarily formed. There may be some precipitation, but even if such a third phase is mixed, there is no problem in terms of characteristics as long as the ratio is 10% or less of the entire second phase.

Next, the reason why the composition of the steel sheet is limited to the above range in the present invention will be explained. C: 0.05 to 0.40 mass% C is an element that not only effectively contributes to the strengthening of steel, but is also useful in obtaining retained austenite. However, if the content is less than 0.05 mass%, the effect is poor,
On the other hand, if it exceeds 0.40 mass%, the ductility decreases, so the C content was limited to the range of 0.05 to 0.40 mass%.

Si: 1.0 to 3.0 mass% Si is an element indispensable for the formation of retained austenite, and for that purpose, addition of at least 1.0 mass% is necessary, but addition of more than 3.0 mass% reduces ductility. Not only is it caused, but also the scale properties deteriorate and there is a problem in surface quality, so the Si content was limited to the range of 1.0 to 3.0 mass%.

Mn: 0.6 to 3.0 mass% Mn is an element not only useful as a strengthening element for steel, but also useful for obtaining retained austenite. However, if the content is less than 0.6 mass%, the effect is poor, while if it exceeds 3.0 mass%, the ductility decreases, so the Mn content is
It was limited to the range of 0.6 to 3.0 mass%.

Cr: 0.2-2.0 mass% This Cr addition is one of the features of the present invention. By adding Cr, the second phase becomes acicular ferrite, as described above. For that purpose, it is necessary to add 0.2 mass% or more, but if added in excess of 2.0 mass%, coarse Cr carbide is generated and ductility is hindered, and strength-elongation balance and dynamic n value are deteriorated. , Cr amount is 0.2 to 2.0 mass
It was limited to the range of%. It is preferably 0.3 to 1.8 mass%.

FIG. 4 and FIG. 5 show the results of examining the relationship between the Cr content and the strength-elongation balance and the dynamic n value, respectively. As is clear from FIGS. 4 and 5, the Cr content is
Within the range of 0.2 mass% or more and 2.0 mass% or less, TS × El ≧ 240
Excellent workability and impact resistance of 00 (MPa ·%) and dynamic n value ≧ 0.35 are obtained.

Although the basic components have been described above, in the present invention, P and Al as austenite forming elements and Ti and Nb as strength improving components can be appropriately contained within the following ranges. P: 0.01 to 0.2 mass% P is useful as a retained austenite forming element, but if the content is less than 0.01 mass%, its effect of addition is poor, while if it exceeds 0.2 mass%, the secondary workability deteriorates. Therefore, when it is added, it is desirable to set it in the range of 0.01 to 0.2 mass%.

Al: 0.01 to 0.3 mass% Al, like P, is also useful as a retained austenite forming element, but if the content is less than 0.01 mass%, the effect of addition is poor, while 0.3 mass% If it exceeds the above range, the ductility is deteriorated. Therefore, when it is added, the range is preferably 0.01 to 0.3 mass%.

Ti: 0.005 to 0.25 mass%, Nb: 0.003 to 0.1 mass% Ti and Nb all contribute to the improvement of strength by refining the ferrite, which is the main phase, and are therefore necessary. It can be added accordingly. In particular, when Ti is contained, the nose of the acicular ferrite moves to a short time side, and the acicular ferrite is sufficiently precipitated even at the coil end where the cooling rate becomes faster compared to the coil middle part,
It also has the effect of improving the yield. However, if the content is too small, the effect of addition is poor, and on the other hand, excessive addition leads to a decrease in ductility, so it is preferable to contain each in the above range.

Next, the manufacturing method of the present invention will be specifically described. In the present invention, it is sufficient to form a mixed structure of martensite, acicular ferrite and retained austenite as the second phase.
Cooling may be performed along the cooling curve shown in FIG. First, slab heating is performed prior to hot rolling.
This heating temperature must be 1000 to 1300 ° C. This is because if the slab heating temperature is less than 1000 ° C, the surface quality of the steel sheet deteriorates significantly, while if it exceeds 1300 ° C, the crystal grains of the steel become coarse and the homogeneity and ductility of the material deteriorate. is there. The heating time is not particularly limited, but if it is too long, the crystal grains become coarse. Therefore, it is preferable to set the heating time to about 60 minutes or less.

Next, finish rolling is performed after hot rough rolling, and the finish rolling temperature at this time must be 780 to 980 ° C. If the finish rolling finish temperature is less than 780 ° C, the work structure remains in the steel and the ductility deteriorates.
On the other hand, if the temperature exceeds 980 ° C., the structure becomes coarse and the formability is lowered due to the delay of ferrite transformation.

Then, after cooling to near the nose of the pro-eutectoid ferrite region at 620 to 780 ° C., the temperature is maintained at this temperature for 1 to 10 seconds or gradually cooled at a rate of 20 ° C./s or less to form a main phase. It precipitates a certain proeutectoid ferrite and promotes the concentration of solute carbon in the austenite phase. Since the above-mentioned temperature range of 620 to 780 ° C is the temperature range in which the ferrite transformation proceeds most smoothly, it is possible to obtain a desired amount of proeutectoid ferrite by holding treatment or gradual cooling treatment for a short time of about 1 to 10 seconds. it can. In the case of the slow cooling treatment, if the cooling stop temperature is lower than 600 ° C, pearlite transformation may occur, so the cooling stop temperature is preferably 600 ° C or higher.

Then, the acicular ferrite is cooled to a temperature range of 350 to 500 ° C., and this area is gradually cooled, or isothermally held or slowly cooled and then forcedly cooled to deposit a desired amount of acicular ferrite. In the case of slow cooling (a in Fig. 2), if the cooling rate is less than 10 ° C / h, bainite transformation is likely to occur, while if it exceeds 100 ° C / h, it becomes difficult to obtain the desired amount of acicular ferrite. The cooling rate during slow cooling was limited to the range of 10 to 100 ° C / h. Also,
When forced cooling is performed after isothermal holding or slow cooling (B in FIG. 2), the holding or slow cooling time is set to 2 to 60 minutes. This is because if the holding or slow cooling time is less than 2 minutes, a sufficient amount of acicular ferrite cannot be obtained, while if it exceeds 60 minutes, the bainite transformation may be induced.
Furthermore, the reason for setting the cooling rate in slow cooling to less than 50 ° C / h is that if this rate is too high, sufficient acicular ferrite cannot be obtained, and the cooling rate after isothermal holding or slow cooling is 50 ° C. The reason for setting / h or more is to prevent bainite transformation.

Then, by the above-mentioned slow cooling treatment or isothermal holding (slow cooling) -forced cooling treatment, the temperature is lowered to 300 ° C. or lower. The cooling stop temperature in such treatment is 300 ° C.
The reason for setting below is to avoid the risk of bainite transformation.

By the series of treatments described above, it is possible to obtain a desired steel structure in which the second phase consisting of acicular ferrite, martensite and retained austenite is present in the proeutectoid ferrite main phase.

[0035]

【Example】

Example 1 A steel slab containing C: 0.15 mass%, Si: 1.50 mass%, Mn: 1.0 mass% and Cr: 0.20 mass% and the balance being substantially Fe composition was heated to 1200 ° C., After rough rolling, according to the pattern X and the pattern Y shown in FIGS. 6 (a) and 6 (b),
After finish rolling at 880 ° C, after cooling near the nose of the pro-eutectoid ferrite region at 750 ° C, it is subjected to isothermal holding treatment or slow cooling treatment under the conditions shown in Table 1 and then cooled to the acicular ferrite region at 500 ° C. In this region, slow cooling treatment or isothermal holding (slow cooling) -forced cooling treatment was performed under the conditions shown in Table 1, and then cooled to room temperature. Tensile test pieces were cut out from the obtained hot-rolled sheet, and strain rate:
Tensile test was conducted under the condition of 2 × 10 ^-2 / s, yield strength (YS),
Tensile strength (TS) and elongation (El) were obtained. Also, Hopkinson Pressure Bar test materials (Materials and Process vol.9
(1996) P.1108-1111), strain rate: 2 × 10 ³ /
The tensile test was conducted under the condition of s, and the moment when the elongation was 10% n
The value (dynamic n value) was determined. Furthermore, the work hardening amount (WH) during press molding and the subsequent coating baking (170
The bake hardening amount (BH) at (° C.) was also measured.
In addition, WH and BH were obtained from FIG. 7 using a tensile tester with a strain rate of 2 × 10 ⁻² / s. Table 1 summarizes the results of the examination of the steel structure, TS × El balance, dynamic n value, WH + BH and YR of each hot rolled steel sheet.

[0036]

[Table 1]

As shown in Table 1, according to the present invention, any of those having a mixed structure of martensite, acicular ferrite and retained austenite as the second phase is excellent in TS × El ≧ 24000 MPa ·%. Strength-elongation balance, excellent impact resistance with dynamic n value ≥ 0.35 and WH + B
Excellent processing and bake hardenability of H ≧ 100 MPa are obtained.

Example 2 A steel slab having the chemical composition shown in Table 2 was processed according to pattern X and pattern Y shown in FIG. 6 to obtain a hot rolled steel sheet. Table 3 shows the results obtained by examining the steel structure, TS × El balance, dynamic n value, WH + BH and YR of the obtained hot rolled steel sheet.
Shown in.

[0039]

[Table 2]

[0040]

[Table 3]

All the steel sheets obtained according to the present invention are
As the second phase, a mixed structure of martensite, acicular ferrite and retained austenite is formed. As a result, TS × El ≧ 24000 MPa ·%, dynamic n value ≧ 0.35, WH +
Excellent strength-elongation balance of BH ≧ 100 MPa, impact resistance and work / bake hardenability are obtained.

[0042]

As described above, according to the present invention, the main phase is proeutectoid ferrite, and the second phase is a mixed structure of martensite, acicular ferrite and retained austenite. It is possible to obtain a hot-rolled steel sheet having both of the above.

[Brief description of drawings]

FIG. 1 is a typical continuous cooling transformation curve diagram (CCT diagram) of a conventional TRIP steel.

FIG. 2 is a typical continuous cooling transformation curve diagram (CCT diagram) in the component system of the present invention.

FIG. 3 is a schematic view showing (a) a characteristic phase constitution of a second phase obtained according to the present invention and (b) a second phase constitution of a conventional TRIP steel.

FIG. 4 is a graph showing the relationship between the Cr content and the strength-elongation balance.

FIG. 5 is a graph showing the relationship between the Cr amount and the dynamic n value.

FIG. 6 is a schematic diagram of a cooling pattern according to the present invention.

FIG. 7: Work hardening amount (WH) and bake hardening amount (BH)
FIG.

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Osamu Furukun 1 Kawasaki-cho, Chuo-ku, Chiba, Chiba Prefecture Technical Research Laboratory, Kawasaki Steel Co., Ltd. (72) Shusaku Takagi 1 Kawasaki-cho, Chuo-ku, Chiba, Chiba Prefecture Kawasaki (56) References JP-A-6-65677 (JP, A) JP-A-4-276025 (JP, A) JP-A-60-43430 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C21D 9/46-9/48 C21D 8/00-8/04 C22C 38/00-38/60

Claims (4)

(57) [Claims] 1. C: 0.05 to 0.40 mass%, Si: 1.0 to
A steel slab containing 3.0 mass%, Mn: 0.6 to 3.0 mass%, Cr: 0.2 to 2.0 mass% and the balance being substantially Fe.
After heating to 1000 to 1300 ℃, rough rolling and finish rolling from 780 to
Finish at a temperature of 980 ℃, then cool to 620 ~ 780 ℃, then keep isothermal for 1-10 seconds or cooling rate: 20
After annealing at ℃ / s or less and then cooling to 350-500 ℃, coil it, and cool it to 300 ℃ or less at a cooling rate of 10-100 ℃ / h. A method for producing a high-strength and high-workability hot-rolled steel sheet excellent in impact resistance, comprising a steel structure having a second phase composed of martensite, acicular ferrite and retained austenite in a ferrite main phase. 2. C: 0.05 to 0.40 mass%, Si: 1.0 to
A steel slab containing 3.0 mass%, Mn: 0.6 to 3.0 mass%, Cr: 0.2 to 2.0 mass% and the balance being substantially Fe.
After heating to 1000 to 1300 ℃, rough rolling and finish rolling from 780 to
Finish at a temperature of 980 ℃, then cool to 620 ~ 780 ℃, then keep isothermal for 1-10 seconds or cooling rate: 20
Slowly cool down to ℃ / s or less, then cool to 350-500 ℃, wind around coil, hold isothermal for 2-60 minutes or slow cooling speed: less than 50 ℃ / h And then forced cooling at a cooling rate of 50 ° C / h or more 3
By cooling to a temperature below 00 ° C, a steel structure having a second phase consisting of martensite, acicular ferrite and retained austenite in the main phase of proeutectoid ferrite, has high impact resistance and high strength. Manufacturing method of high workability hot rolled steel sheet. 3. The steel slab composition according to claim 1 or 2, wherein the steel slab composition further contains at least one selected from P: 0.01 to 0.2 mass% and Al: 0.01 to 0.3 mass%. Of high strength and high workability hot rolled steel sheet with excellent impact resistance. 4. The steel slab composition according to claim 1, wherein the composition further contains at least one selected from Ti: 0.005 to 0.25 mass% and Nb: 0.003 to 0.1 mass%. A method for producing a high-strength, high-workability hot-rolled steel sheet having excellent impact resistance characteristics.