JP4272541B2 - Super high strength hot-rolled steel sheet with good workability - Google Patents

Description

  The present invention relates to an ultra-high strength hot-rolled steel sheet having good workability, and particularly to an ultra-high-strength hot-rolled steel sheet having good stretch flangeability and ductility and having a tensile strength of 900 MPa or more.

  In fields such as automobiles and industrial machines, there is a strong demand for reducing the weight of members, and accordingly, the importance of ultra-high strength hot-rolled steel sheets with significantly increased strength of 900 MPa or more, and further 1000 MPa or more is increasing. However, even for such an ultra-high strength steel sheet, there is a strong demand for workability, and it is desired that the steel sheet can be satisfactorily formed according to the part shape. In particular, in the manufacture of automobile parts, it is often pressed into a complicated shape, and it is desired to be excellent in various processing characteristics.

  As a technique for simultaneously improving the strength of the hot-rolled steel sheet and stretch flangeability, which is one of the workability, for example, Patent Document 1 proposes that the component composition be specified and that the metal structure be mainly composed of fine bainite. Yes. However, if the structure is mainly composed of bainite, it is difficult to ensure sufficient elongation, and it is difficult to simultaneously increase the elongation and stretch flangeability required when processing automobile parts.

  Patent Document 2 shows that the shape freezing property and stretch flangeability can be improved at the same time by controlling the amount of iron carbide and grain orientation at the grain boundary with ferrite or bainite as the metal structure. . However, this technique is intended for a steel plate having a strength of 750 MPa, and cannot meet the demand for further strengthening.

  The present inventors have been further researching for the realization of a hot-rolled steel sheet having enhanced stretch flangeability and strength at the same time, and so far, the metallographic structure is bainitic ferrite and / or granular bainitic.・ A steel sheet that is mainly composed of ferrite and has improved stretch flangeability and improved press formability (Patent Document 3).

  In addition, the inventors of the present invention have a technique in which stretch flangeability is further improved by making the metal structure mainly composed of granular bainitic ferrite and / or bainitic ferrite and controlling the contents of S and N ( For example, Patent Document 4), and the metal structure is also bainitic ferrite or granular bainitic ferrite, and the relationship between the amount of C and the addition amount of Ti and Nb and the ratio between the amount of Ni and the amount of Cu are controlled. Therefore, a technique (for example, Patent Document 5) in which bainitic ferrite is efficiently generated to enhance stretch flangeability is also proposed.

However, although the steel sheets disclosed in these documents are excellent in workability such as stretch flangeability and press formability, the tensile strength is at a level of 700 to 800 MPa, and further improvements can be made to meet the demand for higher strength. Desired.
JP 2000-109951 A JP 2002-363893 A JP 2002-180191 A Japanese Patent Laid-Open No. 2001-20030 JP-A-8-157957

  The present invention has been made in view of the above circumstances, and the object thereof is an ultra-high-strength hot-rolled steel sheet having good workability, particularly stretch flangeability and ductility (stretch flangeability is stretch flangeability in Examples). An object of the present invention is to provide a hot-rolled steel sheet having a λ value of 50% or more, an elongation of El of 10% or more, and a tensile strength of 900 MPa or more (particularly 1000 MPa or more).

  The ultra-high-strength hot-rolled steel sheet with good workability according to the present invention is in mass% (the same applies to the components below), C: 0.03 to less than 0.10%, Si: 1.5% or less (0% Not including), Mn: 0.5 to 2.5%, Cr: 0.1 to 0.50%, Mo: 0.1 to 0.50%, Ti: 0.1 to 0.3% included, The following formulas (1) and (2) are satisfied, the balance is iron and inevitable impurities, the metal structure is an area ratio (the same applies to the metal structure below), bainitic ferrite: 80% or more, martensite: 10% This is characterized by the following (including 0%) and the tensile strength is 900 MPa or more.

[Cr] / [Mo] = 0.50-1.20 (1)
([Ti] / 48) / ([C] / 12) ≧ 0.8 (2)
{[Cr], [Mo], [Ti], [C] in the above formulas indicate the contents (mass%) of Cr, Mo, Ti, and C, respectively}
The steel plate of the present invention may further contain Ca: 20 ppm or less (not including 0%) and / or REM: 20 ppm or less (not including 0%) as other elements.

  The steel sheet of the present invention has a high strength of 900 MPa or more, and has stretch flangeability and elongation. Therefore, when manufacturing automobile parts and other industrial machine parts that require high strength, the forming process is excellent. It can be carried out.

  As described above, the present inventors have intensively studied to make workability such as elongation and stretch flangeability compatible in a steel sheet having an ultrahigh strength of 900 MPa or more. As a result, the inventors have found that it is only necessary to contain Cr, Mo and Ti in the following concentration ranges and to control the ratio of the content of Cr and Mo.

  FIG. 1 shows an experiment No. 2, 13-15, 18-30, 49-51 based on data, only Mo added to low C (0.05%) steel plate and high C (0.13%) steel plate, It is the graph which arranged and showed the tensile strength of each steel plate at the time of adding only Cr, adding Mo and Cr, or adding Mo, Cr, and Ti.

  From FIG. 1, it can be seen that in the case of a high C steel plate, a high strength of 1000 MPa or more can be achieved regardless of the type of additive element. However, those containing 0.1% or more of C have poor workability and cannot satisfy stretch flangeability. Therefore, in order to improve stretch flangeability, it is necessary to suppress the C content to less than 0.1%. 1 indicates the tensile strength of a steel sheet that can be expected to have excellent stretch flangeability with a low C content of 0.05%. In such a steel material with a low C content, only Mo is added, and only Cr is added. Alternatively, even when Mo and Cr are added simultaneously, the strength is limited to the 800 MPa level and does not reach the strength level of the high C steel plate. However, it was confirmed that when Ti was added in addition to these Mo and Cr, the tensile strength dramatically increased to the high C steel plate level. Therefore, in the present invention, Ti is an essential element in addition to Cr and Mo.

  FIG. 2 shows the relationship between the Cr / Mo content ratio ([Cr] / [Mo]) and the λ value, which is an index of stretch flangeability, based on the data obtained in the examples described later. . It can be seen from FIG. 2 that the stretch flangeability can be improved by controlling the content ratio of Cr and Mo, and that the effect becomes more remarkable particularly when an appropriate amount of Ti coexists. To obtain a more reliable stretch flangeability improvement effect, 0.1 to 0.3% of Ti is contained as will be described later, and the ratio of [Cr] / [Mo] is within the range of 0.50 to 1.20. It is better to control. In order to ensure higher stretch flangeability, the ratio of [Cr] / [Mo] is preferably controlled to 0.8 to 1.10.

  In order to effectively exhibit the effects shown in FIG. 1 and FIG. 2, each content of Cr, Mo, and Ti needs to satisfy the following range.

Cr: 0.1 to 0.50%
In order to exhibit the above effect effectively, it is necessary to contain Cr by 0.1% or more (preferably 0.2% or more). However, when the amount of Cr is excessive, a low temperature transformation product such as martensite structure is produced in a large amount and the elongation is lowered, so it is suppressed to 0.50% or less. Preferably it is 0.4% or less.

Mo: 0.1 to 0.50%
Mo needs to be contained in an amount of 0.1% or more (preferably 0.2% or more). However, if it is excessive, the elongation deteriorates.

Ti: 0.1 to 0.3%
In order to exert the above effect, it is necessary to contain 0.1% or more (preferably 0.15% or more) of Ti. However, when the amount of Ti is excessive, the hot-worked structure tends to remain, and excessive precipitates may be deposited, which may cause deterioration of stretch flangeability. Preferably it is 0.25% or less.

([Ti] / 48) / ([C] / 12) ≧ 0.8 (2)
(In the above formula, [Ti] and [C] indicate the contents (mass%) of Ti and C, respectively)
In order to efficiently produce bainitic ferrite, the atomic ratio of Ti and C; ([Ti] / 48) / ([C] / 12) should satisfy the above formula (2). If the atomic ratio of Ti and C satisfies the above formula (2), generation of cementite and martensite is suppressed, and a fine bainitic ferrite structure can be obtained. In order to exhibit these characteristics more effectively, the atomic ratio is preferably set to 1.0 or more. On the other hand, if the atomic ratio is too high, ductility deteriorates, so it is preferable to keep it to 1.5 or less.

  The reason why other components are specified will be described below.

C: 0.03 to less than 0.10% C is indispensable for increasing the amount of dissolved C in steel by heating the slab and generating a bainitic ferrite structure during cooling after hot rolling. It is an element. In order to effectively exhibit such an effect, the content should be 0.03% or more, preferably 0.04% or more. However, when the amount of C is excessive, a structure that inhibits stretch flangeability such as a martensite structure is excessively generated in the cooling process after hot rolling, so it is suppressed to less than 0.10%, preferably 0.08% or less.

Si: 1.5% or less (excluding 0%)
Si is an element effective for increasing the strength without deteriorating stretch flangeability, and is preferably contained in an amount of 0.5% or more in order to exert the effect. However, if the amount of Si is too large, polygonal ferrite is likely to be generated, and the intended level of strength cannot be secured. Moreover, since it also has an adverse effect on the surface properties of the steel sheet, the Si content is limited to 1.5% or less, preferably 1.3% or less.

Mn: 0.5 to 2.5%
In addition to effectively acting as a solid solution strengthening element, Mn also exerts an effect of promoting transformation and promoting the formation of bainitic ferrite structure. In order to exhibit such an effect effectively, Mn is contained by 0.5% or more, preferably 0.8% or more. However, if the amount is too large, the hardenability becomes high and a large amount of transformation product is formed, which impairs the stretch flangeability.

  The contained elements defined in the present invention are as described above, and the remaining component is substantially Fe, but P (phosphorus) is an inevitable impurity brought into the steel depending on the situation of raw materials, materials, manufacturing equipment, etc. Of course, it is allowed to be contained within a range of 0.02% or less, S (sulfur) 0.01% or less, Al 0.05% or less, N (nitrogen) 0.008% or less. In order not to adversely affect the operation of the present invention, other elements may be positively contained as described below in order to give further effects.

Ca: 20 ppm or less (not including 0%) and / or REM: 20 ppm or less (not including 0%)
Ca exhibits the effect | action which suppresses the production | generation of MnS which has a bad influence on hole expansibility by couple | bonding with S in steel, and producing | generating spherical sulfide CaS harmless to stretch flangeability. REM (rare earth element) is also an effective element for improving the stretch flangeability by spheroidizing the sulfide. In order to exert such an effect, it is preferable to contain 5 ppm or more (more preferably 10 ppm or more) of Ca and 5 ppm or more (more preferably 10 ppm or more) of REM. However, since the effect is saturated even if these elements are contained excessively, the upper limit of the Ca amount is set to 20 ppm (more preferably 15 ppm or less), and the upper limit of the REM amount is set to 20 ppm (more preferably 15 ppm or less).

  Other elements may be contained in a range that does not adversely affect the action of the present invention (for example, does not inhibit the formation of bainitic ferrite), Nb: 0.05% or less, Cu: 0.1% or less Ni: 0.1% or less, B: 20 ppm or less, and the like may be included.

  In the present invention, by controlling the metal structure as described below together with the component composition in this way, the elongation and stretch flangeability of the ultra-high-strength steel sheet can be reliably improved.

Bainitic ferrite: 80% or more In the present invention, “bainitic ferrite” means that the bainite structure has a substructure having a lath-like structure with a high dislocation density, and has a carbide in the structure. This is clearly different from the bainite structure. It is also different from a polygonal ferrite structure with a substructure with little or no dislocation density, or a quasi-polygonal ferrite structure with a substructure such as fine subgrains. See Bainite Photobook-1 ”). This structure exhibits an acicular shape when observed with an optical microscope or SEM, and is difficult to distinguish. Therefore, in order to determine a clear difference from a bainite structure or a polygonal / ferrite structure, the structure of the lower structure by TEM observation is determined. Identification is necessary.

  In the matrix structure of the present invention, bainitic ferrite occupies 80% or more in area ratio, and since the soft polygonal ferrite structure is suppressed, the crack susceptibility is reduced, resulting in improved stretch flangeability. To do. Bainitic ferrite is preferably 85% or more, and when the following martensite is present, the upper limit may be determined in consideration of the area ratio of the martensite.

Martensite: 10% or less (including 0%)
The steel sheet of the present invention may be a bainitic ferrite single phase, but in order to increase strength and lower YR and have excellent elongation (El), 2% or more of martensite may be present. . However, if the martensite becomes excessive, the stretch flangeability deteriorates, so it is preferable to keep it at 10% or less (preferably 6% or less).

Other: Polygonal ferrite, cementite, bainite, etc. (including 0%)
In principle, the steel sheet of the present invention has only the above-described structure (only bainitic ferrite and martensite or bainitic ferrite). Such heterogeneous structures may include polygonal ferrite, cementite, bainite and the like. Although these structures can inevitably remain in the production process of the present invention, the smaller the amount, the better. It is recommended to do.

  The high-strength hot-rolled steel sheet of the present invention is characterized in that the component composition and the metal structure are defined in this way, and the manufacturing method is not particularly limited. Therefore, the steel sheet of the present invention can be obtained by melting steel by a conventional method, for example, hot rolling under the following conditions. In order to efficiently obtain a steel sheet that satisfies both the intended high strength and stretch flangeability and ductility (total elongation), it is particularly recommended to manufacture under the following conditions.

  -Heating during hot rolling should be over 1200 ° C. This is because when the temperature is 1200 ° C. or lower, Ti is not sufficiently dissolved, and the effects of Ti as described above are not sufficiently exhibited. More preferably, it heats to 1230 degreeC or more. However, if the heating temperature is too high, the economical efficiency is deteriorated, so that the temperature is preferably 1280 ° C. or lower.

  -Finish rolling in hot rolling is preferably performed at 880 ° C or higher (preferably 900 ° C or higher). When finish rolling is performed at a temperature lower than this, since hot rolling is performed in a two-phase region, a structure in which a processed ferrite structure is mixed is obtained, and it is difficult to obtain an intended level of stretch flangeability.

  -The cooling rate after hot rolling is preferably 50 ° C / second or more (more preferably 60 ° C / second or more). By cooling at this speed, the production of polygonal ferrite can be suppressed and bainitic ferrite can be efficiently produced.

  -It is preferable to perform winding in the temperature range of 300-500 degreeC. This is because if the coiling temperature is too high, the structure tends to be polygonal ferrite, and the tensile strength is reduced. On the other hand, when the coiling temperature is too low, a large amount of martensite structure is generated and stretch flangeability is deteriorated, which is not preferable.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. It is also possible to implement, and they are all included in the technical scope of the present invention.

  Using the slab obtained by melting and casting each steel material having the chemical composition shown in Table 1, conditions shown in Tables 2 to 4 (SRT; heating temperature, FDT; finish rolling temperature, CR; hot rolling) The steel sheet was hot-rolled at a subsequent cooling rate (CT; coiling temperature) and wound to obtain a hot-rolled steel sheet having a thickness of about 2.0 mm.

  And the metal structure, tensile strength (TS), elongation [total elongation (EI)], yield strength (YP), and stretch flangeability (hole expandability: evaluated by λ value) of the obtained hot-rolled steel sheet are as follows: I examined each in a manner.

  The area ratio of the metal structure was obtained as follows. That is, observation was performed at a magnification of 10,000 using a TEM (transmission electron microscope), micrographs of 10 visual fields were randomly taken, and an average value of the area ratio of each tissue in these 10 visual fields was obtained.

  The tensile test used a JIS No. 5 test piece and measured tensile strength (TS), elongation (EI) and yield strength (YP). The strain rate in the tensile test was 1 mm / sec.

  The stretch flangeability test was performed using a disk-shaped test piece having a diameter of 100 mm and a plate thickness of 2.0 mm. Specifically, after punching out a hole of φ10 mm, a hole expansion rate (λ) at the time of crack penetration was measured by performing hole expansion processing on a burr using a 60 ° conical punch (Iron and Steel Federation Standard JFST 1001). ). These results are shown in Tables 2-4.

  It can consider as follows from Tables 1-4 (in addition, the following No. shows experiment No. of Tables 2-4).

  Class A is the result of investigating the effects of C content and Ti content. Nos. 2 and 3 satisfy the requirements defined in the present invention, so that those exhibiting excellent characteristics were obtained. No. 1 has too little C and Ti content. No. 4 had too much C and Ti, so that a bainitic ferrite structure was not sufficiently obtained, and the stretch flangeability was not preferable.

  Classification B is the result of examining the effect of Si content. Nos. 6 to 9 satisfy all the requirements to be specified, so that hot-rolled steel sheets having a stretch flangeability and a good tensile strength of 900 MPa or more are obtained. In No. 5, since the amount of Si was too large, the amount of polygonal ferrite increased and the bainitic ferrite structure was not sufficiently secured, so that the stretch flangeability was not preferable.

  Classification C is the result of examining the effect of the amount of Mn. Nos. 11 to 14 exhibit excellent characteristics because the amount of Mn and other conditions satisfy the requirements of the present invention. In contrast, no. No. 10 cannot secure sufficient strength because the amount of Mn is too small.

  Classification D is the result of investigating the effects of the amounts of C, Cr and Ti. In Nos. 15 to 17, since the mass ratio of Cr and Mo or the atomic ratio of Ti and C is out of the specified range, bainitic ferrite cannot be sufficiently secured and the stretch flangeability is poor.

  Class E is the result of examining the effect of Cr and Ti content. No. 20 is excellent in characteristics because the Cr amount and the Ti amount are within the specified ranges. 18 and no. Since 19 does not contain Cr or Ti, the strength could not be sufficiently increased.

  The classification F is an experimental result in which the mass ratio of Cr and Mo is changed without including Ti. No. From the results shown in 21 to 25, it can be seen that when Ti is not included, a bainite structure is easily formed, and sufficient strength cannot be secured or stretch flangeability is poor.

  Classification G is the result of an experiment in which the mass ratio of Cr and Mo is changed (the amount of Mo is substantially constant and the amount of Cr is changed) for those containing a prescribed amount of Ti. Nos. 27 to 29 are excellent in characteristics because the mass ratio of Cr and Mo is within the specified range. No. 26 has a mass ratio of Cr and Mo that is outside the lower limit. No. 30 is inferior in stretch flangeability because the mass ratio of Cr and Mo is outside the upper limit.

  Class H is the result of an experiment in which the mass ratio of Cr and Mo is changed (the Cr amount is almost constant and the Mo amount is changed) for those containing a prescribed amount of Ti. Nos. 32, 33 and 34 are steel plates having excellent characteristics because the mass ratio of Cr and Mo is within the specified range. No. 31, the mass ratio of Cr and Mo is outside the upper limit. 35 is inferior in stretch flangeability because the mass ratio of Cr and Mo is outside the lower limit.

  Class I is the result of examining the effect of Ti content. No. 38 has a Ti amount and an atomic ratio of Ti and C within the specified ranges, so that a steel sheet having excellent characteristics has been obtained. Since 36 did not contain Ti, sufficient bainitic ferrite could not be secured, resulting in inferior strength and stretch flangeability. No. 37 contains an appropriate amount of Ti, but since the atomic ratio of Ti and C is outside the specified range, a fine bainitic ferrite structure cannot be obtained, resulting in poor stretch flangeability. . No. No. 39 was inferior in elongation and stretch flangeability because the Ti amount was outside the upper limit.

  The classification J is an experimental result when Cr is contained among Cr, Mo, and Ti, and Cr and Ti or Mo and Ti are contained. In 40 (containing only Cr), bainitic ferrite is not generated so much and sufficient strength cannot be secured. No. 41 (containing Cr and Ti), No. 41 42 (containing Mo and Ti) resulted in inferior stretch flangeability.

  The following classifications K to N are the results of examining the influence of manufacturing conditions.

  Classification K shows the result of changing the heating temperature during hot rolling. No. 45 is heated at a desired temperature, so that a specified structure is obtained and an excellent characteristic is obtained. 43 and no. No. 44 has a low heating temperature, so that the prescribed structure cannot be obtained, and sufficient strength and stretch flangeability cannot be secured.

  Class L is the result of examining the effect of finish rolling temperature. In Nos. 46 to 48, finish rolling was performed at a desirable temperature, so that steel sheets having excellent characteristics were obtained. Nos. 49 and 50 were inferior in strength and stretch flangeability because the finish rolling temperature was low and sufficient bainitic ferrite could not be obtained.

  Classification M is the result of examining the effect of the cooling rate after hot rolling. Since No. 51 is cooled at a desirable speed, a steel plate having excellent characteristics is obtained. Nos. 52 and 53 have a low cooling rate, so that sufficient bainitic ferrite cannot be obtained, and the strength and stretch flangeability are poor.

  Class N is the result of examining the influence of the coiling temperature. Since Nos. 55 to 57 are wound at a desired temperature, steel sheets having excellent characteristics are obtained. However, no. In No. 54, since the coiling temperature was too low, the prescribed structure was not obtained, and the characteristics were not preferable. No. In No. 58, since the coiling temperature was too high, a prescribed structure was not obtained, and the strength and stretch flangeability were inferior.

  Class O is the result of examining the influence of the metal structure. No. 59 includes bainitic ferrite, polygonal ferrite, martensite and other structures. No. 60 includes bainitic ferrite, martensite and other structures. Nos. 61 and 62 consist of bainitic ferrite only. Although 63 and 64 have the structure which consists of bainitic ferrite and martensite, it turns out that any steel plate is excellent in the characteristic.

It is the graph which compared the tensile strength at the time of adding only Mo, only Cr, Mo + Cr, Mo + Cr + Ti, respectively in a low C steel plate and a high C steel plate. It is the graph which showed the relationship between content ratio of Cr and Mo, and (lambda) value.

Claims (2)

In mass% (the same applies to the ingredients below)
C: 0.03 to less than 0.10%,
Si: 1.5% or less (excluding 0%),
Mn: 0.5 to 2.5%
Cr: 0.1 to 0.50%
Mo: 0.1 to 0.50%,
Ti: 0.1 to 0.3% included,
Satisfying the following formulas (1) and (2), the remaining iron and inevitable impurities,
The metal structure is the area ratio (the same applies to the metal structure below)
Bainitic ferrite is over 80%,
Martensite is 10% or less (including 0%),
An ultra-high strength hot-rolled steel sheet with good workability, characterized by a tensile strength of 900 MPa or more.
[Cr] / [Mo] = 0.50-1.20 (1)
([Ti] / 48) / ([C] / 12) ≧ 0.8 (2)
{[Cr], [Mo], [Ti], [C] in the above formulas indicate the contents (mass%) of Cr, Mo, Ti, and C, respectively} As other elements,
Ca: 20 ppm or less (not including 0%) and / or REM: 20 ppm or less (not including 0%)
The ultra-high-strength hot-rolled steel sheet according to claim 1 comprising:

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