JP2011144409A - High-strength steel sheet superior in workability and method for manufacturing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To attain a high-strength steel sheet superior in workability on an industrial scale, and to provide a method for manufacturing the same on the industrial scale. <P>SOLUTION: The high-strength steel sheet includes, by mass%, 0.07-0.20% C, 0.005-1.5% Si, 1.0-3.1% Mn, 0.001-0.06% P, 0.001-0.01% S, 0.0005-0.01% N, 0.005-1.2% Al and the balance Fe with unavoidable impurities; further has such mass% of C and Mn and a target strength value (TS) as to satisfy expression (A), wherein [TS target value] is a designed strength value of the steel sheet and the unit is MPa; [Mn] is mass% of Mn; and [C] is mass% of C; and has such a hardness distribution as to satisfy expression (B). The manufacturing method includes an annealing step in which in the first heating zone, such a temperature T1 as to satisfy expression (C) is kept for 30 seconds or longer, wherein T1 is temperature (°C); [Mn] is mass% of Mn; [Si] is mass% of Si; [Ti] is mass% of Ti; [Nb] is mass% of Nb; and [Mo] is mass% of Mo; and in a cooling zone after soaking treatment in the annealing step, a cooling rate V1 in such a period that the temperature of the steel sheet is in a range of 800-400°C and a cooling rate V2 is such a period that the temperature is lower than 400°C satisfy expression (D), wherein V1 is a cooling rate m/s in a former stage; and V2 is a cooling rate m/s in a latter stage. The high-strength steel sheet also has a metallographic structure containing ferrite and martensite. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a high-strength steel sheet excellent in workability and a method for producing the same.

  In recent years, it has become necessary to improve automobile fuel efficiency as a measure against global warming. For this reason, the weight reduction of a vehicle body is requested | required further. In order to reduce weight, it is necessary to reduce the thickness of the steel sheet used, but on the other hand, legislation on regulations on collision safety and standards have been raised. Absent. In order to achieve both weight reduction and collision safety standards, it is essential to use high-strength steel sheets.

  In general, as the strength of a steel plate increases, press forming becomes difficult. On the other hand, in order to reduce the weight of the components as much as possible, the shape tends to be complicated, and the steel sheet is required to further improve the workability. In response to these conditions, it has been said that improvement in workability and improvement in elongation and hole expansibility.

A steel plate (hereinafter referred to as TRIP steel) using work-induced transformation of retained austenite has been invented for improving elongation, and disclosed in, for example, Patent Document 1 and Patent Document 2.
However, a normal TRIP steel sheet requires a large amount of C addition, and has a welding problem such as nugget cracking.

  Further, as an index representing the workability of a steel sheet, there is hole expansibility as in Patent Document 3, but it is understood that this alone is not sufficient to actually represent the workability of stretch flange forming of automotive parts. ing. In addition, Patent Document 4 and Patent Document 5 focus on the hardness of crystal grains to express workability, but Patent Document 4 is limited to the hardness ratio of ferrite and martensite, while Patent Document 5 It is limited to the hardness ratio depending on the position in the martensite grains, and both are insufficient.

JP-A 61-157625 JP-A-10-130776 JP 2001-355043 A JP 2007-302918 A JP 2008-063804 A

  An object of the present invention is to solve the problems of the prior art as described above, and to realize a high-strength steel sheet excellent in workability and a manufacturing method thereof on an industrial scale.

First, the technical idea of the present invention will be described.
As a result of intensive studies on high-strength steel sheets excellent in workability, the present inventors have found that the hardness distribution of the steel sheets is specified, and high-strength steel sheets that can ensure workability higher than before can be industrially produced. Moreover, the manufacturing conditions for manufacturing the steel plate excellent in workability were also discovered.

  The steel sheet of the present invention was evaluated by the side bend test method, not the elongation and hole expansibility, which are conventional indexes indicating workability. This evaluation method can more accurately represent the workability of automobile parts. Furthermore, in order not to cause the problem of delayed fracture and secondary work brittleness, DP steel that allows unavoidable 5% or less of retained austenite and does not substantially contain retained austenite was obtained.

The high-strength steel sheet of the present invention can realize a tensile strength of 590 Mpa to 1500 Mpa, but has a remarkable effect with a high-strength steel sheet of 980 Mpa or higher.
The present invention is based on the technical idea as described above, and includes the following contents described in the claims.

(1) By mass%, C: 0.07-0.20%, Si: 0.005-1.5%, Mn: 1.0-3.1%, P: 0.001-0.06%, S: 0.001-0.01%, N: 0.0005-0.01%, Al : Containing 0.005-1.2%, consisting of the balance Fe and inevitable impurities, and further, the mass% of C and Mn and the target strength value (TS) satisfy the following formula (A),
Furthermore, the hardness is measured at 100 or more points with a nanoindenter, the hardness distribution satisfies the following formula (B) shown in Formula 1, and the metal structure contains ferrite and martensite. High-strength steel sheet with excellent workability.
0.0004 × [TS target value] -0.11-0.12 [Mn] <[C] <0.0005 × [TS target value] -0.07-0.12 × [Mn] (A)
Here, [TS target value] is the strength design value of the steel sheet, the unit is MPa, [Mn] is the mass% of Mn, and [C] is the mass% of C.

(2) Further, B: 0.0005 to 0.002%, Mo: 0.01 to 0.5%, Cr: 0.01 to 0.5%, V: 0.01 to 0.1%, Ti: 0.01 to 0.1%, Nb: 0.005 to 0.1%, Ca: 0.0005 The high-strength steel sheet having excellent workability as set forth in (1), comprising one or two of ˜0.005% and REM: 0.0005 to 0.005%.

(3) The slab having the components described in (1) or (2) is subjected to hot rolling and then subjected to cold pickling after normal pickling, and then annealed in a continuous annealing step. The temperature T1 satisfying the following (C) in the first heating zone in the annealing step is set to 30 seconds or more, and the steel plate temperature in the cooling zone after soaking in the annealing step. A method for producing a high-strength steel sheet excellent in workability, characterized in that a cooling rate V1 in the range of 800 to 400 ° C and a cooling rate V2 of less than 400 ° C satisfy the following (D).
0.9 ≦ T1 / (750 + 10 [Mn] +10 [Si] +100 [Ti] +500 [Nb] +40 [Mo]) ≦ 1.1 (C)
Here, T1 is temperature ° C, [Mn] is Mn mass%, [Si] is Si mass%, [Ti] is Ti mass%, [Nb] is Nb mass%, and [Mo] is Mo mass%.
0.5 <V1 / (3 × V2) <3.5 (D)
Here, V1 is the former cooling rate m / s, and V2 is the latter cooling rate m / s.

(4) A high-strength steel sheet excellent in workability, wherein the high-strength steel sheet according to (1) or (2) is a cold-rolled steel sheet.
(5) The high-strength steel sheet having excellent formability as described in (1) or (2), wherein the steel sheet is subjected to a hot dip galvanized surface treatment.

  According to the present invention, an excellent high-strength steel sheet capable of ensuring workability more than ever and a manufacturing method thereof on an industrial scale by controlling the hardness distribution of the steel sheet by setting the balance of C and Mn to a specific range. It can be realized and has an industrially useful and remarkable effect.

It is a figure which shows the amount of C and Mn which can ensure workability, and the target TS range. It is a figure which shows a side bend test piece. It is a figure which shows the range of the parameter | index showing the hardness distribution which can ensure workability. It is a figure which shows the range of the parameter | index showing the component which can ensure workability, and heating temperature. It is a figure which shows the range of the parameter | index showing the cooling rate which can ensure workability.

Hereinafter, embodiments of the present invention will be described in detail.
First, the reasons for limiting the components and metal structure of the high-strength steel sheet of the present invention will be described.
C is an essential component from the viewpoint of securing strength and as a basic element for stabilizing martensite. If C is less than 0.07%, the strength is not satisfactory, and a martensite phase is not formed. On the other hand, if it exceeds 0.2%, the strength is excessively increased, the ductility is insufficient, and the weldability is deteriorated, so that it cannot be used as an industrial material.
Therefore, the range of C in the present invention is 0.07 to 0.2%, preferably 0.10 to 0.16%.

  Si is an element that is usually added to ensure ductility in addition to ensuring strength. However, if it exceeds 1.5%, the scale removal in hot rolling is costly and economical. Due to disadvantages, the upper limit is 1.5%. Si is added to improve the deoxidizer and hardenability, but if it is less than 0.005%, the deoxidation effect is not sufficient, so the lower limit is made 0.005%.

  Mn is an element that delays the formation of carbides and is effective for the formation of ferrite, in addition to the need for addition from the viewpoint of securing strength. If Mn is less than 1.0%, the strength is not satisfied, and the formation of ferrite is insufficient and the ductility deteriorates. In addition, if the Mn content exceeds 3.1%, the hardenability is increased more than necessary, so a lot of martensite is generated, resulting in an increase in strength, resulting in increased product variation and insufficient ductility. Cannot be used as a material. Therefore, the range of Mn in the present invention is set to 1.0 to 3.1%.

  P is added according to the strength level required as an element for increasing the strength of the steel sheet. However, if the addition amount is large, segregation to the grain boundary causes deterioration of local ductility. In addition, the weldability is deteriorated. Therefore, the P upper limit value is set to 0.06. The reason why the lower limit is set to 0.001% is that a further reduction leads to a cost increase during refining in the steelmaking stage.

  S is an element that degrades local ductility and weldability by generating MnS, and is preferably an element that does not exist in steel. Therefore, the upper limit is made 0.01%. The reason why the lower limit is set to 0.001% is that, as in the case of P, a further reduction leads to an increase in cost during refining at the steelmaking stage.

  N is an element inevitably included, but if it is contained in a large amount, not only deteriorates the aging property but also increases the amount of precipitated AlN and decreases the effect of Al addition, so 0.01% The following contents are preferred. Further, unnecessarily reducing N increases the cost in the steelmaking process, so it is usually preferable to control it to about 0.0005% or more.

  Al is added in an amount of 0.005% or more for deoxidation, but if the addition amount increases, inclusions such as alumina increase and workability deteriorates, so the upper limit is 1.2%.

  Furthermore, one or more of B, Mo, Cr, V, Ti, Nb, Ca, and REM can be contained as strength, hardenability, and other material improvements.

  B may be added in a range of B: 0.0005 to 0.002% for the purpose of ensuring hardenability. If the addition of B is less than 0.0005%, the solid solution B becomes insufficient, and a sufficient effect of improving hardenability cannot be seen. Moreover, the effect of hardenability does not change even if B exceeds 0.002%. Therefore, the B addition range is set to 0.0005 to 0.002%.

  Mo and Cr are elements that are effective in securing strength and hardenability. Excessive addition of Mo suppresses ferrite formation in DP, causes ductility deterioration, and may deteriorate chemical conversion property and hot dip galvanizing property, so the upper limit was made 0.5%.

  V, Ti and Nb are added in the range of V: 0.01 to 0.1%, Ti: 0.01 to 0.1%, Nb: 0.005 to 0.1% for the purpose of securing strength. Also good.

  Ca and REM may be added within the range of Ca: 0.0005 to 0.005% and REM: 0.0005 to 0.005% for the purpose of inclusion control and improvement of hole expansion.

  Inevitable impurities include, for example, Sn, but the effects of the present invention are not impaired even if these elements are contained in the range of 0.01% by mass or less.

  In order to obtain a high-strength steel plate, it is generally necessary to add a large amount of elements. Moreover, the structure strengthening is common for high strength steel sheets of TS 980 MPa class. Thus, in order to produce a hard structure such as bainite or martensite, it is necessary to utilize a highly hardenable element and to rapidly cool from a two-phase structure of austenite or austenite / ferrite.

Here, the inventors focused on C and Mn, which are elements that have high hardenability and are economical. If the amount of C and Mn is small, the target TS will not be reached. Moreover, when there are many amounts of C and Mn, workability will worsen. Furthermore, if the balance between the amount of C and the amount of Mn is poor, the workability will be inferior due to the hardness difference between the soft phase and the hard phase becoming too high. Thus, it has been found that a high-strength steel sheet with good workability can be produced industrially when it has a C and Mn balance that satisfies the relationship of formula (A).
0.0004 × [TS target value] -0.11-0.12 [Mn] <[C] <0.0005 × [TS target value] -0.07-0.12 × [Mn] (A)
Here, [TS target value] is the strength design value of the steel sheet, the unit is MPa, [Mn] is the mass% of Mn, and [C] is the mass% of C.

  As shown in FIG. 1, when the TS target value is 980 MPa, workability is affected by the C and Mn balance. The workability is judged by ε × TS, which is the product of ε obtained in the side bend test described later and the actual strength value, and if this value is 40,000% MPa or more, the workability is good, and 0000 Less than% MPa was judged as x. The TS target value is a TS presented when placing an order from the user, and examples thereof include 440 MPa, 590 MPa, 780 MPa, 980 MPa, 1180 MPa, and 1470 MPa.

  The reason why the metal structure of the present invention is characterized by containing ferrite and martensite is that when such a structure is taken, the steel sheet has an excellent balance of strength and ductility. The ferrite here refers to polygonal ferrite and bainetic ferrite, and martensite is effective not only in martensite obtained by ordinary quenching but also in martensite tempered at a temperature of 600 ° C. or lower. does not change.

  The ferrite fraction and martensite fraction vary depending on the steel sheet strength. When TS is 500 to 800 MPa, the preferred ferrite fraction is 50 to 90% and the martensite fraction is 10 to 40%. When TS is 800 to 1100 MPa, the preferred ferrite fraction is 20 to 60%, and the martensite fraction is 30 to 60%. When TS is over 1100 MPa, the preferred ferrite fraction is 30% or less and the martensite fraction is 40% or more.

  Moreover, in all the steel plates, the bainite fraction is preferably 10 to 40%. Further, if austenite remains in the structure, the secondary work brittleness and delayed fracture characteristics deteriorate, so in the present invention, 3% or less of retained austenite that is unavoidably present is allowed, and substantially no retained austenite is contained.

  In addition, to show the workability of automobile parts, it was found that the strain amount measured by the side bend test was superior to the elongation and hole expansion values that are conventional evaluation methods, so this method is used for evaluation. It was decided.

  The side bend test method is a method in which in-plane bending is applied to the shear end face and the value of the elongation strain when a through crack occurs is measured. Fig. 2 shows the shape of the test piece. In order to evaluate stretch flangeability, the test piece is provided with a notch sheared with a large R. In addition, in order to measure the elongation strain after the test, a marking line is inserted. When the test is started, the material is bent and fractured while being pulled in the circumferential direction. The determination of breakage is made when a through crack in the thickness direction occurs. Unlike the hole expansion test, the elongation strain after the through crack is not affected by the size of the crack. Therefore, there is no variation in crack determination.

When this strain amount is large, workability is good. As shown in FIG. 3, it is found that controlling the hardness distribution of the steel sheet is effective for increasing this value, and the following formula (B ) Was found to satisfy sufficient processability. There was no problem with moldability when the value of ε × TS was 40000% MPa or more. Although the upper limit was not obtained, the maximum value of Yave was 250 as a result of the test.

  In addition, there is no problem in formability if the product of strain and TS obtained in the side bend test is 40,000% MPa or more, but more preferably, the product of elongation and TS is 16000% MPa or more. It was found that the moldability was excellent.

The reasons for limiting the manufacturing process of the present invention are as follows.
A manufacturing process may be a manufacturing process of a hot-rolled steel sheet, a cold-rolled steel sheet, and a plated steel sheet that are generally performed. Hot rolling is performed under general conditions. Preferably, however, hot rolling is performed at Ar ₃ or higher in order to prevent the ferrite grains from being excessively strained and the workability is deteriorated. The recrystallized grain size of 940 ° C. or lower is desirable because the recrystallized grain size becomes larger than necessary. With regard to the coiling temperature, recrystallization and grain growth are promoted at high temperatures, and improvement in workability is desired. However, scale formation that occurs during hot rolling is also promoted and pickling properties are reduced. Since pearlite is generated in a layered manner and C diffuses unevenly, the temperature is set to 550 ° C. or lower. On the other hand, since it hardens | cures when it becomes low temperature, the load at the time of cold rolling becomes high. For this reason, it shall be 400 degreeC or more.

  For pickling, normal pickling is performed, and for cold rolling after pickling, normal cold rolling is also performed. Preferably, in cold rolling, if the rolling reduction is low, it becomes difficult to correct the shape of the steel sheet, so the lower limit is set to 30%. Further, if rolling is performed at a rolling reduction exceeding 70%, the upper limit is preferably set to 70% because of the occurrence of cracks in the edge portion of the steel sheet and disorder of the shape.

In the first heating zone in the annealing process, it was found that the workability is improved as shown in FIG. 4 when the range of the temperature T1 calculated backward from the following formula (C) is 30 seconds or more. Formability was judged by the value of ε × TS, and when this value was 40000% MPa or more as described above, it was judged that the moldability was good, and less than 40000% MPa was judged as x. If the lower limit of the formula (C) is exceeded, a large amount of unrecrystallized structure remains, resulting in poor workability. On the other hand, if the upper limit of the formula (C) is exceeded, toughness and low-temperature brittleness are deteriorated and workability is not good.
0.9 ≦ T1 / (750 + 10 [Mn] +10 [Si] +100 [Ti] +500 [Nb] +40 [Mo]) ≦ 1.1 (C)
Here, T1 is temperature ° C., [Mn] is Mn mass%, [Si] is Si mass%,
[Ti] is Ti mass%, [Nb] is Nb mass%, and [Mo] is Mo mass%.

  To check whether the temperature range of T1 is 30 seconds or more, a simulation is performed from the result of the actual machine test, and it is indirectly determined from the result whether the temperature and time are within the range. For example, if you install multiple thermometers in a furnace, perform actual machine tests using steel sheets of various steel types and sizes, and perform simulations using a computer using the data, the temperature range of T1 can be measured in seconds. You can check if there was.

  Further, in the annealing step, in the cooling zone after soaking, the cooling rate V1 in the range of 800 to 400 ° C. and the cooling rate V2 of less than 400 ° C. satisfy the following (D) as shown in FIG. Found out that it would be better. Formability was judged by the value of ε × TS, and when this value was 40000% MPa or more as described above, it was judged that the moldability was good, and less than 40000% MPa was judged as x.

When V1 is low, a sufficient hard phase is not generated and the strength is insufficient, and when V1 is high, the C solid capacity in the ferrite becomes excessive and workability deteriorates. Further, if V2 is low, productivity is hindered, and if V2 is high, the end point temperature of cooling greatly varies, leading to variations in product materials.
0.5 <V1 / (3 × V2) <3.5 (D)
Here, V1 is the former cooling rate m / s, and V2 is the latter cooling rate m / s.
In the annealing process, annealing is performed at a temperature of Ac1 or higher and Ac3 + 100 ° C or lower. Below this, the organization becomes uneven. On the other hand, at a temperature higher than this, since the formation of ferrite is suppressed by the coarsening of austenite, the elongation is deteriorated. Also, the annealing temperature is desirably 900 ° C. or less from an economical point. At this time, in order to eliminate the layered organization, it is necessary to hold for 30 seconds or more. However, even if it exceeds 30 minutes, the effect is saturated and the productivity is also lowered. Therefore, it is 30 seconds or more and 30 minutes or less.

Subsequently, the cooling end temperature is set to a temperature of 600 ° C. or lower. If it exceeds 600 ° C., austenite tends to remain, and problems of secondary workability and delayed fracture tend to occur.
The effect of the present invention does not change even if a tempering treatment at 600 ° C. or lower for the purpose of improving brittleness is performed after the heat treatment.

  Examples of the present invention and comparative examples will be described below with reference to Tables 1 to 3. Table 2 is a continuation of Table 1, and Table 3 is a continuation of Table 2.

  By manufacturing a steel having the composition shown in Table 1 in a vacuum melting furnace, reheating to 1200 ° C after cooling and solidification, performing finish rolling at 880 ° C, and holding at 500 ° C for 1 hour after cooling, The hot rolling coil heat treatment was reproduced. The obtained hot-rolled sheet was removed from the scale by grinding and cold-rolled 60%. Thereafter, annealing was performed using a continuous annealing simulator.

  Workability is evaluated by a side bend test method and a tensile test, and the product of breaking elongation strain ε (%) × TS (MPa) is preferably 40000% · MPa or more, and more preferably, elongation (%) × It was also evaluated whether the product of TS (MPa) was 16000% · MPa or more.

  The side bend test method is a test method in which in-plane bending is applied to the shear end face and the value of the elongation strain when a through crack occurs is measured. The tensile test used a JIS No. 5 piece. The metal structure was observed with an optical microscope. Ferrite was observed by nital etching and martensite by repeller etching. The hardness was measured with TRIBOINDENTER (commonly known as Nanoindenter) manufactured by HYSITRON. The measurement interval was 3 μm and the measurement points were 300 points. As one of the evaluations of moldability, ε × TS passes 40000% MPa or more as described above.

As can be seen from the results shown in Table 2, the steel sheet according to the present invention can produce a high-strength steel sheet excellent in workability.
On the other hand, in the comparative example in which the component range in Table 1 deviates from the range of the present invention, the value of elongation El × TS indicating workability is less than 16000% MPa, or the value of elongation at break strain ε × TS is less than 40000% MPa. .

Further, in the comparative example (AM, AN) not satisfying the formula (A), the value of ε × TS is less than 40000% MPa.
Further, in the comparative example (AO, AP) not satisfying the formula (B), the value of ε × TS is less than 40000% MPa.
Further, in the comparative examples (AQ, AR) not satisfying the expression (C), the value of El × TS is less than 16000% MPa or the value of ε × TS is less than 40000% MPa.
In the comparative example (AS, AT) not satisfying the expression (D), the value of El × TS is less than 16000% MPa or the value of ε × TS is less than 40000% MPa.

Claims (5)

% By mass
C: 0.07 to 0.20%,
Si: 0.005 to 1.5%,
Mn: 1.0 to 3.1%
P: 0.001 to 0.06%,
S: 0.001 to 0.01%,
N: 0.0005 to 0.01%,
Al: 0.005-1.2%
And the balance Fe and inevitable impurities, and the mass% of C and Mn and the target strength value (TS) satisfy the following formula (A). A high-strength steel sheet having excellent workability, wherein hardness measurement is performed at the above-described locations, the hardness distribution satisfies the following formula (B) shown in Formula 1, and the metal structure contains ferrite and martensite.
0.0004 × [TS target value] -0.11-0.12 [Mn] <[C] <0.0005 × [TS target value] -0.07-0.12 × [Mn] (A)
Here, [TS target value] is the strength design value of the steel sheet, the unit is MPa, [Mn] is the mass% of Mn, and [C] is the mass% of C.

further,
B: 0.0005 to 0.002%,
Mo: 0.01 to 0.5%,
Cr: 0.01 to 0.5%,
V: 0.01 to 0.1%
Ti: 0.01 to 0.1%,
Nb: 0.005 to 0.1%,
Ca: 0.0005 to 0.005%,
REM: 0.0005-0.005%
The high-strength steel sheet excellent in workability according to claim 1, wherein one or two of them are contained. The slab of the component according to claim 1 or 2 is subjected to hot rolling, and then subjected to normal pickling, followed by cold rolling, followed by annealing in a continuous annealing step, and then tempering. It consists of a step of rolling, and the temperature T1 that satisfies the following (C) in the first heating zone in the annealing step is 30 seconds or more, and the steel plate temperature is 800 to 400 ° C. in the cooling zone after soaking in the annealing step. A method for producing a high-strength steel sheet excellent in workability, characterized in that a cooling rate V1 in the range of 1 and a cooling rate V2 of less than 400 ° C. satisfy the following (D).
0.9 ≦ T1 / (750 + 10 [Mn] +10 [Si] +100 [Ti] +500 [Nb] +40 [Mo]) ≦ 1.1 (C)
Here, T1 is temperature ° C., [Mn] is Mn mass%, [Si] is Si mass%,
[Ti] is Ti mass%, [Nb] is Nb mass%, and [Mo] is Mo mass%.
0.5 <V1 / (3 × V2) <3.5 (D)
Here, V1 is the former cooling rate m / s, and V2 is the latter cooling rate m / s.   A high-strength steel sheet excellent in workability, wherein the high-strength steel sheet according to claim 1 or 2 is a cold-rolled steel sheet.   The high-strength steel sheet having excellent formability according to claim 1 or 2, wherein the steel sheet has been subjected to surface treatment by hot dip galvanization.

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