JP6052078B2 - Manufacturing method of cold rolled steel sheet with high strength and low yield ratio - Google Patents

Description

  The present invention relates to a method for producing a high-strength, low-yield ratio cold-rolled steel sheet having a tensile strength TS of 980 MPa or more, which is used for automobile frame parts and reinforcing parts.

  In recent years, the need for thin steel sheets with higher strength than ever has been increasing for automobile bodies in order to improve fuel economy by reducing weight and to ensure safety in the event of a collision. There are more opportunities to be used for reinforced parts. Since these parts are manufactured by press forming, TS is a cold-rolled steel sheet with TS of 980 MPa class (thickness 1.0-1.2 mm) as a low yield ratio steel sheet according to the Japan Iron and Steel Federation standard, yield strength YS of 590-930 MPa, 10 An elongation El of more than% is specified. Further, in recent years, a steel sheet having a strength TS of 980 MPa class, a yield strength YS of 800 MPa or less, and an elongation El of 15% or more is required from the viewpoint of securing better shape freezing property and workability.

  As such a high-strength, low-yield-ratio cold-rolled steel sheet, a DP (Dual Phase) steel sheet that has a hard martensite phase dispersed in a soft ferrite phase and has been strengthened by strengthening the structure is well known. Yes. However, when trying to achieve TS, El, and YS as described above, there is a problem that the content of C and Mn must be increased, resulting in a decrease in weldability.

In view of this, a high strength low yield ratio cold-rolled steel sheet has been proposed in which a high strength is achieved by using a combination of structure strengthening and precipitation strengthening by fine precipitates such as Ti, Nb, and V. For example, Patent Document 1 includes mass%, C: 0.04 to 0.14%, Si: 0.4 to 2.2%, Mn: 1.2 to 2.4%, P: 0.02% or less, S: 0.01% or less, Al: 0.002 to 0.5. %, Ti: 0.005 to 0.1%, N: 0.006% or less, and (% Ti) / (% S) ≧ 5 [(% M) represents the content of element M. The same applies hereinafter. The slab having the composition consisting of the balance Fe and inevitable impurities is hot-rolled at a finishing temperature of (900 + 50 × (% Si)) ° C. or lower, and cold-rolled at a reduction rate of 50 to 85%. After that, annealing is performed for 10 s to 5 min in the two-phase coexistence temperature range of 700 to 900 ° C. ferrite phase and austenite phase, and the 700 to 500 ° C. temperature range is cooled to 250 to 500 ° C. at an average cooling rate of 1 to 120 / s, After reheating as necessary, hold for 30 s to 10 min in the temperature range of 250 to 600 ° C and then cool to room temperature. The volume ratio of martensite phase and residual austenite phase is 6% or more in total, and martensite Low yield ratio high strength with excellent hole expansibility, characterized by α ≦ 50000 × (% Ti) / 48, where α% is the volume fraction of the hard phase structure of the phase, residual austenite phase and bainite phase A method for manufacturing a cold-rolled steel sheet is disclosed. Patent Document 2 includes, in mass%, C: 0.02 to 0.3%, Mn: 0.05 to 3%, Si and Al in total of 0.05 to 3%, and further, one or two of Ti and Nb in total. A hot-rolled steel sheet containing 0.01 to 0.40% and the balance consisting of Fe and inevitable impurities is cold-rolled at a reduction rate corresponding to the texture of the hot-rolled steel sheet, and then heated at 3 to 100 ° C./s, Ac Workability and shape freezing characterized by cooling from ₁ transformation temperature to (Ac ₃ transformation temperature +150) ° C annealing temperature and then cooling from annealing temperature to 500 ° C or less at a cooling rate of 1 to 250 ° C / s A method for producing a low-yield ratio type high-strength cold-rolled steel sheet having excellent properties is disclosed. Patent Document 3 includes, in mass%, C: 0.02 to 0.3%, Mn: 0.05 to 3%, Ni: 3% or less, Cr: 3% or less, Cu: 3% or less, Mo: 1% or less, Co: 3% or less, Sn: 0.2% or less, and one or more of these, including 0.1 to 3.5% in total, Si: 3% or less, Al: 3% or less, and one of these Or, both include 0.02 to 3% in total, further Ti: 0.4% or less, Nb: 0.4% or less, and one or both of these include 0.01 to 0.4% in total, the balance being Fe and inevitable The slab with the impurity composition is hot-rolled by controlling the reduction rate and finishing start / end temperature from Ar ₃ transformation temperature to (Ar ₃ transformation temperature +150) ° C, and then the temperature range of 600 to 700 ° C on the runout table. After cooling to 0.2 to 15 seconds at a temperature below the critical temperature determined by the chemical composition and at 550 ° C. or less, pickling, cold rolling at a rolling reduction of 20 to 70%, heating rate 3 to Heat at 100 ° C / s, Ac ₁ transformation temperature to Ac ₃ transformation temperature Of low yield ratio type high strength cold-rolled steel sheet with excellent shape freezing property, characterized by cooling at an annealing temperature of 1 degree to 250 ° C / s from annealing temperature to 500 ° C or less A manufacturing method is disclosed.

JP 2002-69574 A JP 2004-124123 A JP 2005-256020 JP

  However, the low yield ratio high-strength cold-rolled steel sheet manufactured by the method described in Patent Document 1 cannot provide a steel sheet that simultaneously satisfies TS of 980 MPa or more, El of 15% or more, and YS of 590 to 800 MPa. In addition, TS of 980 MPa or more cannot be obtained with the low yield ratio type high strength cold-rolled steel sheets manufactured by the methods described in Patent Document 2 and Patent Document 3.

  An object of the present invention is to provide a method capable of producing a high-strength, low-yield ratio cold-rolled steel sheet having excellent weldability and having TS of 980 MPa or more, El of 15% or more, and YS of 590 to 800 MPa.

When the present inventors examined the manufacturing method of the said high strength low yield ratio cold-rolled steel plate made into the said objective, the following knowledge was acquired.
i) If the C content is 0.12% by mass or less and the Mn content is 3.3% by mass or less, it is possible to prevent deterioration of weldability.
ii) Set 0.36 ≧ (% C) × (% Mn) ≧ 0.13, add 0.025 mass% or more of Ti, and control the heating rate and cooling rate during continuous annealing according to the amount of Ti. Achieve TS, El, and YS.

The present invention has been made based on such findings, and in mass%, C: 0.05 to 0.12%, Si: 0.2 to 2.5%, Mn: 2.0 to 3.3%, Al: 0.1% or less, and Ti: A steel slab containing 0.025 to 0.100%, the content of C and Mn satisfies the following formula (1), and the balance is composed of Fe and inevitable impurities, is heated and then hot-rolled, and then acid After washing and cold rolling into a cold-rolled sheet, the cold-rolled sheet is heated to a temperature range of 300 to 700 ° C. at an average temperature increase rate that satisfies the following formula (2), and 750 to 900 ° C. A high-strength, low-yield specific cooling, characterized in that after annealing for 5 to 600 s in a temperature range of 750 to 500 ° C at an average cooling rate satisfying the following formula (3), continuous annealing is performed. A method for producing a rolled steel sheet is provided.
0.36 ≧ (% C) × (% Mn) ≧ 0.13 (1)
Average heating rate (℃ / s) ≧ 0.24 / (% Ti) (2)
75 ≧ Average cooling rate (℃ / s) ≧ 100 × (% Ti) +13 ... (3)
However, (% M) represents the content (mass%) of the element M as defined above.

  In the production method of the present invention, it is further preferable to use a steel slab having a composition containing B: 0.0005 to 0.0050% by mass.

  Further, at least one selected from the group consisting of Nb: 0.008 to 0.5%, Cr: 0.05 to 2%, Mo: 0.05 to 2%, P: 0.05 to 0.5%, and V: 0.05 to 0.5% in mass%. It is preferable to use a steel slab having a composition containing the above.

Moreover, it is preferable to heat up the temperature range of 300-700 degreeC with the average temperature increase rate which satisfy | fills following formula (4).
Average heating rate (℃ / s) ≧ 0.5 / (% Ti) (4)
Furthermore, when the maximum temperature during continuous annealing is T _M (° C.), it is preferable to cool the temperature range of T _{M to} 0.98 × T _M ° C. at an average cooling rate of 0.2 to 5 ° C./s.

  According to the present invention, it is possible to produce a high-strength, low-yield ratio cold-rolled steel sheet having excellent weldability, TS of 980 MPa or more, El of 15% or more, and YS of 590 to 800 MPa. For example, when TS is 980 MPa, it has become possible to produce a cold rolled steel sheet having a high strength and a low yield ratio that can achieve a yield ratio (YR) of 0.59 to 0.82. The high-strength, low-yield ratio cold-rolled steel sheet produced according to the present invention is suitable for automobile frame parts and reinforcing parts.

It is a figure which shows the relationship between Ti amount, tensile strength TS, and yield strength YS (conventional manufacturing method). It is a figure which shows the relationship between Ti amount, tensile strength TS, and yield strength YS (this invention manufacturing method).

  Details of the present invention will be described below.

1) Composition In the following,%, which is a unit of content of component elements, means mass%.

C: 0.05-0.12%
C is an essential element for strengthening the structure by forming a martensite phase and a retained austenite phase in steel. It is possible to increase the strength by increasing the amount of C and promote the formation of retained austenite to improve the elongation. However, if the amount is less than 0.05%, such an effect does not appear. On the other hand, when the C content exceeds 0.12%, the strength of the welded portion is lowered, resulting in a decrease in weldability. Therefore, the C content is 0.05 to 0.12%.

Si: 0.2-2.5%
By adding Si, it is possible to increase the strength while suppressing a decrease in ductility. Si also increases the C concentration in the austenite phase to increase the strength of the martensite phase and promote a lower yield ratio. In order to secure strength while limiting the amount of C as in the present invention, the amount of Si needs to be 0.2% or more. On the other hand, when the Si content exceeds 2.5%, YS exceeds 800 MPa and chemical conversion processability deteriorates. Therefore, the Si content is 0.2 to 2.5%.

Mn: 2.0-3.3%
Mn is an element that improves the hardenability of steel together with C, and can increase the strength by increasing the amount of martensite phase according to the content. From the viewpoint of securing TS of 980 MPa or more and YS of 590 to 800 MPa, at least 2.0% of Mn content is necessary. On the other hand, if the amount of Mn exceeds 3.3%, weldability is deteriorated and YS is excessively increased. Therefore, the Mn content is 2.0 to 3.3%, preferably 2.3 to 3.3%.

0.36 ≧ (% C) × (% Mn) ≧ 0.13
In the present invention, the amount of C and Mn is limited from the viewpoint of ensuring weldability. Therefore, even if the amount of C and Mn is controlled within the above range, if (% C) × (% Mn) is less than 0.13, 980 MPa The above TS cannot be obtained. If (% C) × (% Mn) exceeds 0.36, YS cannot be controlled to 590 to 800 MPa. Therefore, 0.36 ≧ (% C) × (% Mn) ≧ 0.13.

Al: 0.1% or less
Al is necessary for deoxidation of steel and strengthens the ferrite phase. In order to utilize this effect, the Al content is preferably 0.01% or more. However, if the amount exceeds 0.1%, coarse inclusions increase and formability deteriorates. Therefore, the Al amount needs to be 0.1% or less.

Ti: 0.025-0.100%
Since Ti has a small effect on reducing weldability, Ti is an effective element to reinforce strength deficiencies caused by limiting the amount of C and Mn from the viewpoint of securing weldability. Although details will be described later, in the present invention, the target TS, El, and YS are achieved by controlling the heating rate and cooling rate during continuous annealing according to the Ti amount. In order to obtain such an effect, the Ti amount needs to be 0.025% or more. On the other hand, if the Ti content exceeds 0.100%, YS increases excessively, making it difficult to achieve a low yield ratio. Therefore, the Ti amount is 0.025 to 0.100%.

  The balance is Fe and inevitable impurities, but it is preferable to contain B: 0.0005 to 0.0050% for the following reasons, and Nb: 0.008 to 0.5%, Cr: 0.05 to 2%, Mo: 0.05 to 2% , P: 0.05 to 0.5%, V: 0.05 to 0.5%, preferably at least one selected from the group consisting of 0.05 to 0.5%.

B: 0.0005-0.0050%
B segregates at the grain boundaries to increase the strength, but has a small effect on weldability and is therefore an effective element for increasing the strength of steel. However, if the amount of B is less than 0.0005%, the effect of increasing the strength is poor, and if it exceeds 0.0050%, the effect is not only saturated but also the moldability is deteriorated. Therefore, the B amount is preferably 0.0005 to 0.0050%.

At least one selected from Nb: 0.008 to 0.5%, Cr: 0.05 to 2%, Mo: 0.05 to 2%, P: 0.05 to 0.5%, V: 0.05 to 0.5%
Nb, Cr, Mo, P and V are effective elements for increasing the strength of steel. In obtaining such actions, the Nb content is preferably 0.008% or more, the Cr content is 0.05% or more, the Mo content is 0.05% or more, the P content is 0.05% or more, and the V content is preferably 0.05% or more. . However, when the Nb content exceeds 0.5%, the Cr content exceeds 2%, the Mo content exceeds 2%, the P content exceeds 0.5%, and the V content exceeds 0.5%, coarse precipitates are formed in the steel and the elongation is increased. Decreases. Therefore, the Nb amount is 0.008 to 0.5%, the Cr amount is 0.05 to 2%, the Mo amount is 0.05 to 2%, the P amount is 0.05 to 0.5%, and the V: amount is 0.05 to 0.5%. Is preferred.

  Inevitable impurities include S: 0.008% or less, N: 0.008% or less, or P: 0.04% or less and Cr: 0.04% or less when P and Cr are not actively contained.

2) Continuous annealing conditions In the present invention, the steel slab having the above composition is heated, hot-rolled under normal conditions, then pickled, then cold-rolled into a cold-rolled sheet, and the following Continuous annealing is performed under conditions.

2-1) Temperature increase condition: Average temperature increase rate (° C / s) ≧ 0.24 / (% Ti) in the temperature range of 300 to 700 ° C [Formula (2)]
In order to obtain the target TS and YS, it is necessary to control the average heating rate of 300 to 700 ° C. according to the amount of Ti as shown in Equation (2).

  The reason is considered as follows. In general, in the temperature rising process, an unrecrystallized structure containing a lot of dislocations has an action of promoting austenite transformation, but when the residence time in the temperature range below the ferrite / austenite transformation temperature is increased, dislocation release proceeds, The austenite transformation amount decreases, the martensite phase generated during cooling decreases, and the final strength decreases. On the other hand, when Ti is added as in the present invention, Ti forms fine precipitates and has the action of suppressing recrystallization, so that the unrecrystallized structure can easily remain up to a high temperature. Can be increased, and the final strength can be stably increased. Therefore, the temperature increase rate in the temperature range between 300 to 700 ° C. where dislocation release proceeds below the transformation temperature is increased when the amount of Ti decreases and the amount of precipitates decreases, that is, the formula (2) By controlling in this way, the amount of austenite transformation can be increased, and target TS and YS can be obtained. On the other hand, when the average heating rate (° C / s) <0.24 / (% Ti), the residence time in the temperature range below the ferrite / austenite transformation temperature becomes longer, the austenite transformation amount decreases, and the target TS and YS are reduced. I can't get it.

  In order to achieve low YS with a higher TS, it is preferable to satisfy the average rate of temperature increase (° C./s)≧0.5/(% Ti) [Formula (4)].

2-2) Heating conditions: 5 to 600 s in the temperature range of 750 to 900 ° C
In the present invention, it is necessary to form a two-phase structure in which the martensite phase is dispersed in the ferrite phase in order to increase the strength and reduce the yield ratio. Therefore, the heating temperature needs to be heated to a temperature range of 750 to 900 ° C. where two phases of the ferrite phase and the austenite phase coexist. However, if the heating time is less than 5 s, the austenite transformation does not proceed sufficiently and the martensite phase is not generated, and if it exceeds 600 s, the austenite structure becomes coarse and the two-phase structure of the coarse ferrite phase and the martensite phase Since El decreases, the heating time is 5 to 600 s.

2-3) Cooling condition: 75 ≧ average cooling rate (℃ / s) ≧ 100 × (% Ti) +13 in the temperature range of 750 to 500 ℃ [Formula (3)]
Fig. 1 shows C: 0.090%, Si: 1.0%, Mn: 2.0%, Al: 0.04%, B: 0.0008%, steel ingots with various additions of Ti, after hot rolling, cold rolling (Annealing rate of 50%) After setting the sheet thickness to 1.2 mm, annealing was performed at 750 to 700 ° C. during annealing at a heating rate of 300 ° C. to 700 ° C. at 10 ° C./s and a holding time of 750 to 900 ° C. for 100 seconds. The relationship between Ti addition amount, YS, and TS when the cooling rate between ~ 500 ° C is 17 ° C / s is shown.

  As shown in Fig. 1, the yield strength and tensile strength increase as the Ti content increases, but the amount of Ti added for both TS and YS to be in the target range is very limited and practically stable. Production is impossible. On the other hand, FIG. 2 shows the results when the cooling rate between 750 and 500 ° C. in the cooling process is set to 19 ° C./s and 25 ° C./s according to the amount of added Ti so as to conform to the present invention. As shown in this figure, it is possible to obtain appropriate tensile strength and yield strength by appropriately controlling the cooling rate of 750 to 500 ° C. according to the amount of added Ti. Therefore, by controlling the cooling rate of 750-500 ° C to 100 × (% Ti) +13 (° C / s) or more according to the amount of Ti, proper tensile strength and yield strength can be achieved. Will be obtained.

  The reason for this is estimated as follows. In other words, when obtaining a martensite structure dispersed in steel after being heated to a ferrite / austenite two-phase region and then cooled, the diffusion of C from the austenite phase, where C is concentrated in the high temperature range, to the ferrite phase is minimized. Therefore, it is considered that the hardness difference between martensite and ferrite increases and the yield strength decreases. On the other hand, since the ferrite / austenite structure becomes finer in the high temperature region as the Ti content increases, it is necessary to suppress the diffusion of C from the austenite phase more strongly in the cooling process, and a high cooling rate is considered appropriate. . In order to obtain such an effect, the cooling rate in the temperature range of about 500 ° C where the diffusion rate of C decreases sufficiently from around 750 ° C where the austenite phase is generated is 100 × (% Ti) + 13 (° C / s It is better to control as above.

  The present invention has an advantage that the target TS, El, and YS can be obtained without increasing the cooling rate as seen in Examples of Patent Documents 1 to 3. If the cooling rate becomes too fast, the target YS cannot be obtained, and the shape of the steel sheet becomes poor, so the cooling rate needs to be 75 ° C./s or less.

  Therefore, the average cooling rate (° C./s) in the temperature range of 750 to 500 ° C. is set to 100 × (% Ti) +13 (° C./s) or more and 75 (° C./s) or less.

Moreover, in order to ensure higher El, it is preferable to cool the temperature range from the maximum temperature T _M ° C to 0.98 × T _M ° C during heating at an average cooling rate of 0.2 to 5 ° C / s. The reason for this is thought to be that when Ti re-dissolved during heating begins to precipitate at the time of cooling, the precipitate is uniformly dispersed and higher El is obtained.

  In addition, if the heat treatment (tempering treatment) is performed at 250 to 450 ° C. for 50 to 800 s after cooling during the above-described continuous annealing, the martensite phase is tempered, and further improvement of El and improvement of hole expandability can be achieved. it can.

  Moreover, in order to improve corrosion resistance, the steel plate after continuous annealing can be subjected to plating such as hot dip galvanizing, alloying galvanizing, and electrogalvanizing.

  Further, the steel plate after continuous annealing may be subjected to skin pass rolling according to a conventional method for shape correction or the like.

  Steel slabs A to AI having the composition shown in Table 1 were heated to 1250 ° C. and hot-rolled to give a 3.5 mm hot-rolled sheet, and after pickling, cold-rolled to a 1.4 mm cold-rolled sheet. Next, the cold-rolled sheet was continuously annealed under the annealing conditions shown in Table 2, and when it was cooled to 250 ° C., it was subjected to a heat treatment (tempering process) for 240 s at the same temperature, subjected to skin pass rolling, and cold-rolled steel sheet No. 1 ~ 72 were made. In addition, a part of the tempering treatment was not performed. Then, JIS No. 5 test pieces in the direction perpendicular to the rolling direction were taken from the cold-rolled steel sheet and subjected to a tensile test according to JIS Z 2241 to obtain YS, TS, and El.

  The results are shown in Tables 2-4.

  It can be seen that all of the inventive examples are high strength and low yield ratio cold rolled steel sheets having TS of 980 MPa or more, El of 15% or more, and YS of 590 to 800 MPa. In particular, it can be seen that when the temperature is increased at an average temperature increase rate that satisfies the above formula (4), a higher TS is obtained and the YS is lower than the TS.

  A steel slab of W in Table 1 was heated to 1250 ° C. and hot-rolled to obtain a 3.0 mm hot-rolled sheet, and after pickling, a cold-rolled sheet having a thickness of 1.0 mm was obtained. Next, the cold-rolled sheet is continuously annealed under the annealing conditions shown in Table 5, and then subjected to a tempering treatment that is held at 450 ° C. for 60 seconds, immersed in a galvanizing bath, and then alloyed with a plating layer of 30 s at 550 ° C. The galvanized steel sheets No. w1 to w21 were prepared by performing the treatment and performing the skin pass rolling. For these galvanized steel sheets, YS, TS, and El were determined in the same manner as in Example 1.

  The results are shown in Table 5.

It can be seen that all the inventive examples are high strength and low yield ratio galvanized steel sheets having TS of 980 MPa or more, El of 15% or more, and YS of 590 to 800 MPa. In particular, when the temperature is increased at an average temperature increase rate that satisfies the above formula (4), a higher TS is obtained, which is YS lower than TS, and in particular, temperatures from T _M ° C to 0.98 × T _M ° C It can be seen that higher El is obtained when the zone is cooled at an average cooling rate of 0.2 to 5 ° C./s.

Claims (5)

In mass%, C: 0.05-0.090%, Si: 0.5-2.5%, Mn: 2.3-3.3%, Al: 0.1% or less and Ti: 0.025-0.100%, and the contents of C and Mn are as follows: A steel slab satisfying the formula (1) and having the balance consisting of Fe and inevitable impurities is heated, hot-rolled, then pickled, cold-rolled into a cold-rolled sheet, After heating the temperature range of 300-700 ° C at an average temperature increase rate satisfying the following formula (2) on the rolled plate and heating it to the temperature range of 750-900 ° C for 5-600 s, the following formula (3) A tensile strength is 980 MPa or more, a yield strength is 590 to 800 MPa, and an elongation is 15% or more , characterized in that continuous annealing is performed under the condition of cooling a temperature range of 750 to 500 ° C. at an average cooling rate to be satisfied. A method for producing a cold rolled steel sheet having a high strength and low yield ratio;
0.36 ≧ (% C) × (% Mn) ≧ 0.13 (1)
Average heating rate (℃ / s) ≧ 0.24 / (% Ti) (2)
75 ≧ Average cooling rate (℃ / s) ≧ 100 × (% Ti) +13 ... (3)
However, (% M) represents the content (mass%) of the element M.   2. The method for producing a high-strength, low-yield ratio cold-rolled steel sheet according to claim 1, wherein a steel slab having a composition containing B: 0.0005 to 0.0050% by mass% is used.   Furthermore, at least one selected from Nb: 0.008 to 0.5%, Cr: 0.05 to 2%, Mo: 0.05 to 2%, P: 0.05 to 0.5%, and V: 0.05 to 0.5% by mass% 3. The method for producing a high-strength, low-yield ratio cold-rolled steel sheet according to claim 1, wherein a steel slab having a composition is used. The high-strength, low-yield ratio cold-rolled steel sheet according to any one of claims 1 to 3, wherein the temperature range of 300 to 700 ° C is increased at an average temperature increase rate that satisfies the following formula (4): Manufacturing method of
Average heating rate (℃ / s) ≧ 0.5 / (% Ti) (4) 2. The temperature range of T _{M to} 0.98 × T _M ° C. is cooled at an average cooling rate of 0.2 to 5 ° C./s, where T _M (° C.) is the maximum temperature during continuous annealing heating. To 4. The method for producing a high-strength, low-yield ratio cold-rolled steel sheet according to any one of items 1 to 4.

Images