JP3292619B2 - Manufacturing method of hot rolled steel sheet with excellent stretch flangeability - Google Patents

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 stretch flangeability, which can be suitably used as an original sheet for producing a complicated pressed part.

[0002]

2. Description of the Related Art In order to ensure the stretch flangeability of a hot-rolled steel sheet, it is necessary to control the formation of inclusions and hard layers to suppress the occurrence of cracks and voids in a punched cross section. For example, 5
Winding at a temperature of 00 ° C. or higher causes embrittlement to proceed due to grain boundary segregation of P and the like, and induces cracks and voids in the punched cross section of the steel sheet, while leaving untransformed austenite and causing
If it is wound at a temperature of 00 ° C. or less, the formation of a hard phase such as martensite is unavoidable. Therefore, in order to stably obtain a hot-rolled steel sheet having excellent stretch flangeability, it is necessary to substantially complete the transformation by cooling until winding, and to wind at 500 ° C. or lower.

However, ordinary hot rolling equipment cannot provide a sufficient cooling time between rolling and winding from the viewpoint of maintenance of the finishing temperature and productivity, so that in actual operation, it is necessary to reduce the cooling time to winding. It is difficult to complete the transformation. In particular, in a high-strength hot-rolled steel sheet, since an alloying element such as Cr that strongly suppresses ferrite transformation and pearlite transformation is added, austenite tends to remain until after winding. Therefore, conventionally, there has been a strong demand for a method capable of producing a steel sheet having excellent stretch flangeability even if some austenite remains during winding.

In such a situation, Japanese Patent Laid-Open Publication No.
No. 6127 discloses a method for producing a hot-rolled steel sheet for processing having excellent fatigue properties. According to this method, after hot rolling, a cooling rate of 760 ° C. to 600 ° C. is 30 ° C.
Since cooling is performed at a rate of at least ° C / sec, in the case of a high-strength hot-rolled steel sheet containing a large amount of alloying elements, pearlite does not sufficiently precipitate during this cooling, and a large amount of austenite remains until winding. The remaining austenite is transformed after winding, and becomes harder as the winding temperature is lower. This hard phase is likely to be a starting point of a void during the press working, and thus deteriorates the stretch flangeability of the steel sheet, which is not preferable.

[0005] If the winding is performed at a high temperature, the retained austenite does not harden significantly after the transformation even if it is transformed after the winding, but there is a problem. For example, Japanese Patent Application Laid-Open No. 4-88125 describes a method for producing a high-strength hot-rolled steel sheet for automobile undercarriage having excellent stretch flangeability. According to this method, a winding temperature of 500 ° C. or higher is used. Then, since the grain boundary segregation such as P is gradually cooled from 600 ° C. to 500 ° C., cracks are apt to be generated in the punched section before pressing, and cracks are generated from the cracks, and the stretch flangeability is deteriorated. The reason for the slow cooling after the winding is that the steel strip becomes a cylindrical coil.

Japanese Unexamined Patent Publication (Kokai) No. 4-276024 discloses the production of a high-strength hot-rolled steel sheet having excellent stretch flangeability, but presupposes a steel sheet containing retained austenite. However, since retained austenite is unstable, in order to obtain a steel plate of a stable material, it is preferable to completely transform during the process of cooling to room temperature.
As described above, when austenite remains at the time of winding, if winding at a low temperature, the remaining austenite is transformed and hardened after transformation, while if winding at a high temperature, embrittlement due to segregation of P and the like occurs. It was difficult to produce a high-strength hot-rolled steel sheet having excellent stretch flangeability.

[0007]

SUMMARY OF THE INVENTION The present invention has been made in order to solve the above-mentioned problems in the production of a conventional high-strength hot-rolled steel sheet. To provide a method for producing a hot-rolled steel sheet having a structure in which the retained austenite is made as harmless as possible, the segregation of P is suppressed by low-temperature winding, and thus the stretch flangeability is excellent. With the goal.

[0008]

The method for producing a hot-rolled steel sheet having excellent stretch flangeability according to the present invention comprises: 0.05 to 0.20% by weight of C; 0.50 to 2.00% of Mn; Steel containing 0.01-1.50% of Si, 0.10% or less of P, 0.005% of S or less, and 0.20-2.00% of Cr in the austenitic region with the balance being iron and unavoidable impurities Rolling is completed, and maintained at 650 ° C. and a temperature T ° C. at which the pearlite transformation stops for 5 seconds or more.
To 500 ° C. at an average cooling rate of 30 ° C./sec or more, and winding at a temperature in the range of 500 to 300 ° C.

First, the experimental facts underlying the method of the present invention will be described. C 0.1%, Mn 1.0%, Si 0.5
% And Cr 0.5% were heated at 950 ° C. (austenitic range) for 5 minutes as shown in FIG.
It was kept at a constant temperature of 00 ° C. for 1 minute, further kept at a temperature in the range of 680 to 490 ° C. for 10 seconds, and then water-quenched.
The constant temperature holding at 700 ° C. is for precipitating the ferrite sufficiently, and when rolling is performed by using a hot rolling equipment, the rolling reduction of the rolling is very high, and the precipitation of the ferrite is fast. This takes into account the point that ferrite precipitates to almost equilibrium even when quenched.

The constant temperature holding at 680 ° C. to 490 ° C. is for precipitating pearlite. FIG. 2 shows the effect of the holding temperature on the pearlite transformation rate. Perlite precipitates only between 650 ° C and 580 ° C. However, in FIG. 2, pearlite of less than 5% is treated as 0%. At 580 ° C or lower, the experimental steel
Since pearlite transformation does not proceed, the temperature is 580 to 580 ° C.
Unless the temperature is gradually cooled down to ℃, a large amount of austenite remains after winding. Since this austenite becomes a hard phase after winding, as shown in FIG.
The shorter the holding time to ° C, the lower the critical hole expansion ratio λ value.
Here, the λ value refers to the expansion rate of the hole diameter when a steel plate punched out of a hole having a diameter of 10 mm is pushed up with a conical punch at 60 ° C. and a crack occurs.

Further, even when the amount of retained austenite is small, when the temperature is gradually cooled from 580 ° C. to 500 ° C., the stretch flangeability deteriorates. That is, FIG.
% (Holding time from 650 ° C to 580 ° C for 2 seconds) and 5%
(8 seconds) show stretch flangeability when the same steel was cooled at various cooling rates. In any case, when the temperature is gradually cooled from 580 ° C. to 500 ° C., the λ value deteriorates. Although the reason for this is not clear, it is considered that a structure harmful to stretch flangeability is precipitated from the residual austenite between 580 ° C and 500 ° C.

When quenched from 580 ° C. to 500 ° C., the remaining austenite is transformed into bainite, but when cooled slowly, an abnormal structure in which ferrite and martensite are mixed is actually observed. However, 500-5
Even when the temperature is rapidly cooled to 50 ° C., the residual austenite remains 13%.
% Have significantly lower λ values than 5%.

As is apparent from the above, in order to obtain a hot-rolled steel sheet having excellent stretch flangeability using the above-mentioned experimental steel, the temperature is gradually cooled between 650 ° C. and 580 ° C. where pearlite is precipitated, and after the ferrite transformation, The remaining austenite is made as pearlite as possible and at 580 ° C to 50
It is understood that the temperature may be rapidly cooled to 0 ° C. to suppress the generation of abnormal tissue, and the film may be wound at a temperature of 500 ° C. or less. As described above, when the winding temperature is 500 ° C. or less, the occurrence of cracks in the punched section due to segregation of P and the like is small.

Next, other steels than the above-mentioned experimental steels were studied. FIG. 5 shows the effect of the amount of Cr on the temperature T (° C.) at which the pearlite transformation stops, as the Cr amount increases, the lower limit temperature at which the pearlite transformation proceeds increases. This is presumably because the precipitation of cementite is suppressed as the amount of Cr increases. Perlite is
Austenite is separated into ferrite and cementite. At this time, the distribution of Cr occurs between the ferrite and cementite, so that pearlite precipitation is suppressed.
Since the upper limit temperature of pearlite precipitation hardly changes regardless of the amount of Cr, it is important to gradually cool from 650 ° C. to the lower limit temperature of pearlite precipitation. Perlite transformation is C
Since the same is suppressed by a substitutional alloy element other than r, the same occurs with other alloy elements.

Conventionally, the change in the stop temperature of the pearlite transformation due to the difference in the alloying elements has not been taken into consideration, and when the alloying elements are different, it has not been possible to find the conditions for sufficiently proceeding the pearlite transformation. According to the present invention, by measuring this temperature for each component system, it is possible to adapt to the case where the alloy elements are different. The present invention has been completed based on the above findings.

Next, the chemical composition of steel in the present invention will be described. C is an element necessary to secure the strength of the steel sheet, and needs to be added at least 0.05%, and preferably added at 0.10% or more. However, the amount added
If it exceeds 0.20%, it becomes difficult to reduce the amount of residual austenite during winding, and the stretch flangeability deteriorates. Preferably, the amount added is in the range of 0.15% or less.

For the same reason as for C, Mn is added in an amount of 0.50 to 2.00%, preferably 0.80 to 2.00%.
It is in the range of 1.50%. Si is added at 1.50%
When the value exceeds, it becomes difficult to reduce the amount of residual austenite during winding, and the stretch flangeability deteriorates. However, if the amount is less than 0.01%, not only does the production cost increase unnecessarily, but also no improvement in material quality can be expected. Therefore, the addition amount is set to 0.01% or more.

When P is added in a large amount, cracking due to embrittlement occurs, so P is set to 0.10% or less. However, if the amount is less than 0.01%, not only does the production cost increase unnecessarily, but also no improvement in material quality can be expected. Therefore, the addition amount is set to 0.01% or more. Since S generates inclusions such as MnS and deteriorates stretch flangeability, S is set to 0.005% or less, and preferably 0.002% or less.

Cr is effective in improving the strength of the steel sheet.
To obtain this effect effectively, it is necessary to add at least 0.20%. However, if it is added excessively, it becomes difficult to reduce the amount of residual austenite at the time of winding, and the stretch flangeability is deteriorated. Therefore, the addition amount is 2.00% or less.

According to the present invention, in addition to the above elements, Cu
1.0% or less, Ti 0.05% or less, and Nb 0.05
% Or less can be added.

Since Cu enhances the ferrite strength by precipitation strengthening, Cu is added for the purpose of increasing the strength of the steel sheet and improving the fatigue strength. However, the addition of a large amount requires the addition of a large amount of expensive Ni at the same time in order to prevent hot embrittlement and increases the manufacturing cost of the steel sheet. Therefore, the addition amount is set to 1.0% or less. According to the present invention, the amount added is 1.0.
% Or less, it is preferable to add at least about half of Cu to prevent surface flaws.

Ti and Nb, like Cr, are useful for increasing the strength of a steel sheet. However, when excessively added, the yield ratio becomes high, and the shape freezing property at the time of press working deteriorates. Therefore, when adding for high strength, the upper limit of any element is 0.05. %.

Further, according to the present invention, in order to suppress the precipitation of MnS which is harmful to the stretch flangeability, Ca 0.000 is added to the steel.
At least one element selected from the group consisting of 20% or less and REM 0.0020% or less can be added. However, if these elements are added excessively, not only does the production cost of the steel sheet increase, but also the above effects are saturated. Therefore, the addition amount of each element is 0.002.
0% or less.

According to the present invention, in addition to the above, the following alloy elements can be added as required. In particular, in steel to which Cu has been added, as described above, it is preferable to add Ni in order to prevent surface cracks and the like.
However, the amount of addition is in the range of 1.0% or less, and the addition of more than that is not preferable because the production cost is unnecessarily increased.

Although V and Zr are effective in improving the strength of the steel sheet, the effect is saturated even if added in an excessive amount. Therefore, the upper limit is 0.2% by weight for V and 0% for Zr. .1
%. Mo is also effective in improving the strength of the steel sheet, like V, but even if it is added excessively, the effect is saturated.
The upper limit is set to 1.0%. B is useful for improving longitudinal cracking resistance and strength, and becomes brittle when added in excess, so the upper limit is 20 ppm.

According to the present invention, the steel having the chemical composition as described above is rolled in the austenitic region, and is subjected to 650.
5 ° C. and the temperature at which the pearlite transformation stops, 5 ° C. or more, and the average cooling rate 3 between T ° C. and 500 ° C.
By cooling at 0 ° C./sec or more and winding at a temperature in the range of 500 to 300 ° C., a hot-rolled steel sheet excellent in stretch flangeability can be obtained.

When rolling is performed in the austenite / ferrite region, the resulting steel sheet becomes unstable in hot strength and the thickness accuracy of the product is significantly deteriorated. It is necessary to

In the present invention, the pearlite precipitation stop temperature T (° C.) is set to a value obtained when the ferrite is sufficiently precipitated in the vicinity of the eutectoid temperature and held at a temperature in the range from the eutectoid temperature to 500 ° C. for 10 seconds. The maximum temperature at which the amount of precipitation is less than 5%. In order to obtain a hot-rolled steel sheet having excellent stretch flangeability in a high-strength hot-rolled steel sheet, the residual austenite after the ferrite transformation is transformed into pearlite as much as possible, and further from 550 ° C to 500 ° C.
It is necessary to rapidly cool the temperature range in which a harmful structure is considered to be generated from the residual austenite in the range described above and wind it at a temperature of 500 ° C. or less.

The method of the present invention cannot be applied to steel in which progress of the pearlite transformation is not stopped. In addition, the method of the present invention cannot be applied to steel in which pearlite does not precipitate during cooling after ordinary hot rolling.

Next, according to the present invention, as shown in FIG. 3, if the temperature is maintained for about 5 seconds in a temperature range in which pearlite is precipitated, the λ value is good.
Is maintained for 5 seconds or more. However, in a high-strength hot-rolled steel sheet, since the amount of alloying elements added is large, it is preferably maintained for 7 seconds or more.

As shown in FIG. 4, even if the steel is maintained from 650.degree. C. to T (.degree. C.) for 5 seconds or more and the austenite remains small, the steel is gradually cooled at a temperature in the range of 580.degree. Is deteriorated, so according to the present invention,
This temperature range is cooled at an average cooling rate of 30 ° C./sec or more. In particular, it is preferable to cool at an average cooling rate of 40 ° C./sec or more.

According to the present invention, since the temperature is gradually cooled after winding, there is a possibility that the temperature may be gradually lowered from the temperature T (° C.) to 500 ° C., and in order to avoid embrittlement due to segregation of P and the like, The winding temperature is set to 500 ° C. or less. Embrittlement deteriorates stretch flangeability as described above. When the film is wound at a temperature of 300 ° C. or lower, the remaining austenite becomes an extremely hard phase called martensite, which deteriorates stretch flangeability. Preferably, the winding temperature is 400-500 ° C
Temperature.

[0033]

EXAMPLES The present invention will be described below with reference to examples.
The present invention is not limited by these examples. Tables 1 to 3 show examples. In Tables 1 to 3, T is the stop temperature (° C.) of the pearlite transformation, and t is 650.
The holding time from ° C. to the temperature T (° C.), the phase indicates the steel phase at the end of rolling, and TS indicates the tensile strength. The cooling rate indicates an average cooling rate from the temperature T (° C.) to the winding temperature when the winding temperature is 500 ° C. or higher, and the cooling temperature is 500 ° C.
When it is lower than or equal to ° C., it indicates an average cooling rate from the temperature T (° C.) to 500 ° C. In general, λ decreases as the strength of the steel sheet increases, and thus the product of the tensile strength (TS) and λ is also shown.

Of the comparative steels, A13 has a low λ value because the structure at the end of rolling is in the two-phase region. Also, A14 having a winding temperature of 400 ° C. or lower, and A16 and A17 having a winding temperature of 500 ° C. or higher also have low λ values.

In general, when the amount of alloying elements increases,
The temperature T (° C.) tends to increase, and it is difficult to increase the holding time in the range from 650 ° C. to the temperature T (° C.). Therefore, in the present invention, from such a viewpoint, the Cr content is preferably 1.3% or less. Ti and Nb
Is effective in increasing the strength of the steel sheet,
Like r, it increases the temperature T, so it is not preferable to add a large amount.

[0036]

[Table 1]

[0037]

[Table 2]

[0038]

[Table 3]

[0039]

As described above, according to the method of the present invention,
Optimum amount of alloying element is added, maintained for more than 5 seconds between 650 ° C and temperature T (° C) at which pearlite transformation stops, and average cooling rate between temperature T (° C) and 500 ° C By cooling at a rate of 30 ° C./sec or more and winding at a temperature in the range of 500 to 300 ° C., the amount of retained austenite at the time of winding is suppressed as much as possible. By forming into a structure and winding at a low temperature, segregation of P is suppressed, and thus a hot-rolled steel sheet having excellent stretch flangeability can be obtained. According to the present invention, if the product (TS × λ) of the tensile strength (TS) and λ is taken as an index of the stretch flangeability in this way, this is 5500 or more, and according to a preferred embodiment, it is 57
It is possible to obtain a hot-rolled steel sheet of not less than 00.

According to such a hot-rolled steel sheet, it is possible to reduce the thickness of the original press sheet, which can contribute to, for example, reducing the weight of a vehicle body.

[Brief description of the drawings]

FIG. 1 shows the temperature of the heat treatment used in the experiment of the experimental steel.
It is a graph of time.

FIG. 2 is a graph showing the effect of the holding temperature between 680 ° C. and 490 ° C. on the pearlite transformation rate.

FIG. 3 is a graph showing the effect of holding time at a temperature between 650 ° C. and 580 ° C. on the critical hole expansion rate λ value.

FIG. 4 is a graph showing the effect of the cooling rate from 580 ° C. to 50 ° C. on the λ value.

FIG. 5 shows Cr content and temperature T at which pearlite transformation stops.
6 is a graph showing a relationship with (° C.).

Continuation of the front page (56) References JP-A-4-341523 (JP, A) JP-A-6-57330 (JP, A) JP-A-57-137426 (JP, A) JP-A-4-88125 (JP) JP-A-6-264139 (JP, A) JP-A-7-118740 (JP, A) JP-A-4-246127 (JP, A) JP-A-4-276024 (JP, A) Field surveyed (Int. Cl. ⁷ , DB name) C21D 8/02-8/04 C21D 9/46-9/48 C22C 38/00-38/60

Claims (7)

(57) [Claims] C. 0.05 to 0.20%, Mn 0.50 to 2.00%, Si 0.01 to 1.50% , P 0.10% or less, and S 0.005% by weight. % Or less, and 0.20 to 2.00% of Cr, the steel consisting of the balance iron and unavoidable impurities is rolled in the austenitic range, from 650 ° C to the temperature T (° C) at which the pearlite transformation stops. Hold for more than 5 seconds,
Further, an average cooling rate of 3 between the temperature T (° C.) and 500 ° C.
A method for producing a hot-rolled steel sheet having excellent stretch flangeability, wherein the hot-rolled steel sheet is cooled at a temperature of 0 ° C / sec or more and wound at a temperature in the range of 500 to 300 ° C. Wherein at wt% (a) C 0.05~0.20%, Mn 0.50~2.00%, Si 0.01~1.50%, P 0.10% or less, S 0.005% or less, and Cr 0.20 to 2.00% , and (b) at least one element selected from the group consisting of 0.0020% or less Ca and 0.0020% or less REM Rolling of the steel consisting of iron and unavoidable impurities is stopped in the austenitic region, and the temperature is maintained at 650 ° C. and the temperature T ° C. at which the pearlite transformation stops for 5 seconds or more.
Cool between 0 ° C at an average cooling rate of 30 ° C / sec or more,
A method for producing a hot-rolled steel sheet having excellent stretch flangeability, comprising winding at a temperature in the range of 00 to 300 ° C. 3. At wt% (a) C 0.05~0.20%, Mn 0.50~2.00%, Si 0.01~1.50%, P 0.10% or less, S 0.005% or less, and Cr 0.20 to 2.00% , and further selected from the group consisting of: (b) Cu 1.0% or less, Ti 0.05% or less, and Nb 0.05% or less. The steel containing at least one element and the balance consisting of iron and unavoidable impurities is subjected to rolling in the austenitic region, and is maintained for 5 seconds or more between 650 ° C. and the temperature T ° C. at which the pearlite transformation stops. ℃ to 50
Cool between 0 ° C at an average cooling rate of 30 ° C / sec or more,
A method for producing a hot-rolled steel sheet having excellent stretch flangeability, comprising winding at a temperature in the range of 00 to 300 ° C. 4. At wt% (a) C 0.05~0.20%, Mn 0.50~2.00%, Si 0.01~1.50%, P 0.10% or less, S 0.005% or less, and includes a Cr from .20 to 2.00%, further at least one element selected from (b) Ca 0.0020% or less, and REM 0.0020% group consists of the following (C) steel containing at least one element selected from the group consisting of Cu 1.0% or less, Ti 0.05% or less and Nb 0.05% or less, with the balance being iron and unavoidable impurities. Rolling is completed in the austenite region, and the temperature is maintained at 650 ° C. and the temperature at which the pearlite transformation stops at T ° C. for 5 seconds or more.
Cool between 0 ° C at an average cooling rate of 30 ° C / sec or more,
A method for producing a hot-rolled steel sheet having excellent stretch flangeability, comprising winding at a temperature in the range of 00 to 300 ° C. 5. At wt% (a) C 0.05~0.20%, Mn 0.50~2.00%, Si 0.01~1.50%, P 0.10% or less, S 0.005% or less, and Cr 0.20 to 2.00% . (B) Ni 1.0% or less, V 0.2% or less, Zr 0.1% or less, Mo 1.0% Below, and B at least one element selected from the group consisting of 20 ppm or less, the steel consisting of the balance of iron and unavoidable impurities is rolled in the austenitic region, and the temperature of 650 ° C. and the temperature T ° C. at which the pearlite transformation stops. Hold for more than 5 seconds between T ° C and 50 ° C.
Cool between 0 ° C at an average cooling rate of 30 ° C / sec or more,
A method for producing a hot-rolled steel sheet having excellent stretch flangeability, comprising winding at a temperature in the range of 00 to 300 ° C. 6. At wt% (a) C 0.05~0.20%, Mn 0.50~2.00%, Si 0.01~1.50%, P 0.10% or less, S 0.005% or less, and Cr 0.20 to 2.00% . (B) Ni 1.0% or less, V 0.2% or less, Zr 0.1% or less, Mo 1.0% And at least one element selected from the group consisting of: B and 20 ppm or less; and (c) at least one element selected from the group consisting of: 0.0020% or less Ca and 0.0020% or less REM Rolling of the steel consisting of the balance of iron and unavoidable impurities is stopped in the austenitic region, and the temperature is kept at 650 ° C. and the temperature T ° C. at which the pearlite transformation stops for 5 seconds or more.
Cool between 0 ° C at an average cooling rate of 30 ° C / sec or more,
A method for producing a hot-rolled steel sheet having excellent stretch flangeability, comprising winding at a temperature in the range of 00 to 300 ° C. 7. At weight% (a) C 0.05~0.20%, Mn 0.50~2.00%, Si 0.01~1.50%, P 0.10% or less, S 0.005% or less, and Cr 0.20 to 2.00% . (B) Ni 1.0% or less, V 0.2% or less, Zr 0.1% or less, Mo 1.0% And at least one element selected from the group consisting of B and 20 ppm or less; and (c) at least one element selected from the group consisting of Ca and 0.0020% or less and REM and 0.0020% or less; d) a steel containing at least one element selected from the group consisting of Cu 1.0% or less, Ti 0.05% or less, and Nb 0.05% or less, with the balance being iron and unavoidable impurities, At 650 ° C. and at a temperature T ° C. at which the pearlite transformation stops, for 5 seconds or more.
Cool between 0 ° C at an average cooling rate of 30 ° C / sec or more,
A method for producing a hot-rolled steel sheet having excellent stretch flangeability, comprising winding at a temperature in the range of 00 to 300 ° C.