JP4501716B2 - High-strength steel sheet with excellent workability and method for producing the same - Google Patents

Description

  The present invention relates to a high-strength steel sheet excellent in workability used in industrial fields such as automobiles and electricity, and a method for producing the same.

  In recent years, improving the fuel efficiency of automobiles has become an important issue from the viewpoint of global environmental conservation. For this reason, efforts are being made to reduce the thickness of the vehicle body by increasing the strength of the vehicle body material and to reduce the weight of the vehicle body itself. However, the development of a material having both high strength and high workability is desired since the increase in strength of the steel sheet causes a decrease in forming workability.

In response to such demands, various composite steel sheets have been developed, such as ferrite-martensite duplex steel (Dual-Phase steel) and TRIP steel using transformation-induced plasticity of retained austenite. For example, in Patent Document 1, a steel sheet excellent in press formability by controlling the chemical composition and the amount of retained austenite in the steel sheet, and in Patent Document 2, press formability by controlling the chemical composition and the steel sheet structure is good. In a high-strength steel sheet, Patent Document 3, a manufacturing method thereof is proposed. Patent Document 4 proposes a steel sheet having excellent workability, particularly local ductility, containing 5% or more of retained austenite.
Japanese Patent Publication No.6-145892 Japanese Patent No. 2660644 Japanese Patent No. 2704350 Japanese Patent No. 3317303

  However, most of these inventions have been developed to improve ductility, and are sufficient for securing formability that is balanced with stretch flangeability, which is an important workability during molding. There is no particular consideration. Moreover, even when considered, the effect was not sufficient. For example, in Patent Documents 1, 2 and 3, although the ductility can be sufficiently obtained by utilizing the TRIP effect, the stretch flangeability is inferior to that of the ferrite-martensite duplex steel. Further, in Patent Document 4, it is proposed to improve local elongation and secure stretch flangeability by making it difficult to cause strain-induced transformation up to a high strain region. Then, strain-induced transformation was caused, and the effect of improving the stretch flangeability after that was limited.

  In actual press molding or the like, in order to ensure excellent formability, it is very important to balance not only excellent ductility but also stretch flangeability. However, as described above, the present invention has not been sufficiently compatible with the present invention.

  The objective of this invention is providing the high strength steel plate which solves the subject mentioned above, and has the outstanding ductility and stretch flangeability, and its manufacturing method.

In order to achieve the above object, the gist of the present invention is as follows.
(I) By mass%, C: 0.05 to 0.30%, Si: 2.0% or less, Mn: 0.8 to 3.0%, P: 0.1% or less, S: 0.07% or less, Al: 0.1 to 2.5% and N: 0.007% Containing the following, the balance is made of Fe and inevitable impurities, the second phase grains are present in isolation in the ferrite matrix, and the second phase grains include a tempered martensite phase and a bainite phase. A high-strength steel sheet with excellent workability, characterized in that the abundance ratio of second phase grains consisting of a mixed structure is 20% or more.

(II) As described in (I) above, further comprising one or more elements selected from Cr: 2.0% or less, V: 2.0% or less, and Mo: 2.0% or less in terms of mass% High-strength steel sheet with excellent workability.

(III) By mass%, Ti: 0.1% or less, Nb: 0.1% or less, B: 0.0050% or less, Ni: 2.0% or less, and Cu: 2.0% or less, further containing one or more elements A high-strength steel sheet excellent in workability as described in (I) or (II) above.

(IV) By mass%, C: 0.05 to 0.30%, Si: 2.0% or less, Mn: 0.8 to 3.0%, P: 0.1% or less, S: 0.07% or less, Al: 0.1 to 2.5% and N: 0.007% After holding a steel plate containing the following, the balance being Fe and inevitable impurities in a first temperature range of 700 to 900 ° C. for 15 to 600 seconds, at a cooling rate of 5 ° C./s or more, the following formula (1) After cooling to the temperature range of MS to MS-50 ° C obtained in step 1, hold at the second temperature range of 350 to 600 ° C for 15 to 600 seconds, then cool the temperature range to at least 200 ° C at 3 ° C / s or more A method for producing a high-strength steel sheet excellent in workability, characterized by cooling at a speed.
Record
MS (° C.) = 540−350 × {[C%] / (1− [α%] / 100)} − 40 × [Mn%] + 30 × [Al%]
−20 × [Cr%] − 35 × [V%] − 10 × [Mo%] − 17 × [Ni%]
−10 × [Cu%] (1)
However, [X%] means mass% of the alloy element X, and [α%] means volume fraction (%) of polygonal ferrite.

(V) The steel sheet described above, wherein the steel sheet further contains one or more elements selected from Cr: 2.0% or less, V: 2.0% or less, and Mo: 2.0% or less in mass% ( A method for producing a high-strength steel sheet having excellent workability as described in IV).

(VI) The steel sheet is, by mass%, Ti: 0.1% or less, Nb: 0.1% or less, B: 0.0050% or less, Ni: 2.0% or less, and Cu: 2.0% or less. The method for producing a high-strength steel sheet having excellent workability as described in (IV) or (V) above, further comprising an element.

  According to the present invention, a high-strength steel sheet having excellent ductility and stretch flangeability and a method for producing the same can be provided, and the industrial utility value is very large, which is an invention with a large industrial effect.

  The inventors investigated factors affecting the ductility and stretch flangeability of high-strength steel sheets, particularly TRIP steel. In TRIP steel, retained austenite undergoes martensite transformation when strain is applied, and the strain concentrated portion is work-hardened to disperse the strain, thereby obtaining high ductility. For this reason, the uniform elongation is excellent, but when processing is performed after the retained austenite is transformed into martensite, voids are generated from the grain boundaries because the hardness difference between martensite containing high C and hardened is significantly large. Progresses and breaks. For this reason, at the time of stretch flange processing, in particular, when punching is performed, the end face is subjected to remarkably strong processing, so the characteristics deteriorate.

  The inventors examined a method for suppressing the generation of voids resulting from such a hardness difference from ferrite. As a result, it has been clarified that stretch flangeability is improved by making bainite containing retained austenite adjacent to tempered martensite. Furthermore, the method of realizing such an organization was examined. As a result, when a part of the untransformed austenite remaining after austemper undergoes martensite transformation, heat treatment is performed for martensite tempering after the austempering treatment, so that carbide precipitates from the retained austenite and ductility deteriorates. For this reason, a part of untransformed austenite is martensitic transformed before austemper, and at the same time the bainite transformation of untransformed austenite proceeds during austempering, and at the same time, the martensite generated before austemper is softened by tempering, It was clarified that it is effective to reduce the hardness difference with ferrite.

  The present invention is characterized by the fact that the tempered martensite phase and the bainite phase containing residual austenite between the laths are adjacent to each other, thereby reducing the hardness difference of ferrite and improving stretch flangeability and the method thereof. There is.

Next, the reasons for limiting the chemical components of the present invention will be described. In addition,% represented with the following chemical components means the mass%.
C: 0.05-0.30%
C is an element that stabilizes austenite, and is an element necessary for ensuring the amount of martensite and for retaining austenite at room temperature. If the amount of C is less than 0.05%, even if the production conditions are optimized, it is difficult to secure the amount of retained austenite at the same time as securing the strength of the steel sheet and satisfy the predetermined characteristics. On the other hand, if the amount of C exceeds 0.30%, the welded part and the heat-affected zone are markedly cured, and the weldability deteriorates. From such a viewpoint, the C content is in the range of 0.05 to 0.30%, preferably 0.05 to 0.2%.

Si: 2.0% or less
Si is an element effective for strengthening steel. In addition, it is a ferrite-forming element, and since it acts to promote the formation of retained austenite because it promotes the concentration of C in austenite and suppresses the formation of carbides, it must be added to composite structure steel and TRIP steel. There are many. However, excessive addition of Si exceeding 2.0% causes deterioration of workability and toughness due to an increase in the amount of solid solution in ferrite, and deterioration of surface properties due to the occurrence of red scale, etc. Causes deterioration of plating adhesion. Therefore, the Si addition amount is set to 2.0% or less, preferably 0.01 to 2.0%.

Mn: 0.8-3.0%
Mm is an element effective for strengthening steel. In addition, it is an element that stabilizes austenite, and is an element that is necessary for increasing the volume of martensite and retained austenite. This effect is obtained when Mn is 0.8% or more. On the other hand, when Mn is added excessively exceeding 3.0%, strength increase due to excessive second phase fraction or solid solution strengthening becomes remarkable. Therefore, the Mn content is set to 0.8 to 3.0%, preferably 1.0 to 3.0%.

P: 0.1% or less P is an element effective for strengthening steel. However, when P is added in excess of 0.1%, it causes embrittlement due to segregation at the grain boundaries and deteriorates impact resistance. Therefore, the P content is 0.1% or less.

S: 0.07% or less S is an inclusion such as MnS, which causes deterioration in impact resistance and cracks along the metal flow of the weld. Thus, the S content is set to 0.07% or less.

Al: 0.1-2.5%
Al is a ferrite-forming element, and has the effect of promoting the concentration of C in austenite and suppressing the formation of carbides and promoting the formation of retained austenite. In order to exert this effect, it is necessary to add 0.1% or more of Al. In particular, in the case of composite structure steel and TRIP steel, a large amount of Al may be added in order to exert such an effect. However, excessive addition of Al exceeding 2.5% leads to embrittlement of ferrite, which deteriorates the strength-ductility balance of the material, and increases the inclusions in the steel sheet to deteriorate the ductility. Therefore, the Al addition amount is 0.1 to 2.5%, preferably 0.1 to 2.0%.

N: 0.007% or less Since N is an element that most deteriorates the aging resistance of steel, the smaller the amount, the better. In particular, when the N content exceeds 0.007%, the deterioration of aging resistance becomes significant. Therefore, the N content is 0.007% or less.

  The steel sheet of the present invention is mainly composed of the above basic components and iron. Here, the main component does not impede the inclusion of elements that inevitably improve the action of these elements or improve the mechanical and chemical characteristics without impairing the action of the inevitable impurities and the basic ingredients. For example, one or more elements selected from Cr, V and Mo shown below can be contained.

Cr: 2.0% or less
Cr has an effect of suppressing the formation of a pearlite phase during cooling from the annealing temperature. However, if the Cr content exceeds 2.0%, the ferrite content becomes too small and there is a concern about workability deterioration, so the upper limit of the Cr content is preferably 2.0%. The lower limit of the Cr content is not particularly limited, but is preferably 0.01% in order to obtain the effect of suppressing the formation of pearlite phase.

V: 2.0% or less V has an effect of suppressing the formation of a pearlite phase during cooling from the annealing temperature. However, if the V content exceeds 2.0%, the amount of ferrite becomes too small and there is a concern about workability reduction, so the upper limit of the V content is preferably 2.0%. The lower limit of the V content is not particularly limited, but is preferably 0.005% in order to obtain the effect of suppressing the formation of pearlite phase.

Mo: 2.0% or less
Mo is an element effective for delayed fracture resistance and the like, but if the Mo content exceeds 2.0%, the workability tends to decrease. Therefore, the Mo content is preferably 2.0%. The lower limit of the Mo content is not particularly limited, but is preferably 0.005% in order to sufficiently obtain an effect on delayed fracture resistance.

Moreover, the steel plate of this invention can further contain the 1 type (s) or 2 or more types of element chosen from Ti, Nb, B, Ni, and Cu.
Ti and Nb: 0.1% or less each
Ti and Nb are effective elements for precipitation strengthening of steel, and may be used for strengthening steel as long as the structure satisfies the structure defined in the present invention. However, when the content of Ti and Nb exceeds 0.1%, the workability tends to decrease. Accordingly, the Ti and Nb contents are each preferably 0.1% or less. In addition, although the minimum of content of Ti and Nb is not specifically limited, In order to fully acquire effects, such as precipitation strengthening, it is preferable to set it as 0.003%.

B: 0.0050% or less B has an action of suppressing the formation of a ferrite phase from the austenite grain boundary and increasing the second phase. However, if the B content exceeds 0.0050%, the amount of ferrite becomes too small, and the workability tends to decrease. Therefore, the B content is 0.0050% or less. The lower limit of the B content is not particularly limited, but is preferably 0.0003% in order to sufficiently obtain effects such as an increase in the second phase.

Ni and Cu: 2.0% or less each
Ni and Cu are austenite stabilizing elements, and are effective in increasing the strength while retaining austenite. However, if Ni and Cu are added in excess of 2.0%, the ductility of the steel sheet tends to be reduced. Accordingly, the Ni and Cu contents are each preferably 2.0% or less. In addition, although the minimum of content of Ni and Cu is not specifically limited, In order to acquire an austenite stabilization effect, it is preferable to set it as 0.005%.

Next, the reason which limited the steel structure of the steel plate of this invention is demonstrated.
The high-strength steel sheet of the present invention needs to have a so-called composite structure in which second phase grains are present in isolation in the ferrite matrix. This is because, by using a composite structure of a ferrite phase and a second phase, both high strength and high workability can be achieved.

  Furthermore, in the present invention, it is necessary that the abundance ratio of the second phase grains composed of the mixed structure including the tempered martensite phase and the bainite phase among the second phase grains is 20% or more. By adjoining the tempered martensite phase and the bainite phase containing retained austenite between the laths in this way, it is possible to suppress the rapid change in hardness between the phases and improve the stretch flangeability. Because it can. Such an effect can be exhibited effectively when the abundance ratio of the second phase grains composed of the mixed structure including the tempered martensite phase and the bainite phase is 20% or more. The abundance ratio of the second phase grains is preferably 30 to 90% in order to improve the balance between ductility and stretch flangeability. The remaining second phase grains are martensite (including tempered martensite) single phase or bainite (including residual austenite between laths) single phase.

  Further, from the viewpoint of workability, it is preferable that the abundance ratio of the ferrite phase and the second phase is 30 to 80% in terms of the volume ratio of the ferrite phase in the entire steel structure.

Next, an example of the manufacturing method of the high strength steel plate of the present invention will be described below.
First, the cold-rolled steel sheet having the above chemical composition is annealed for 15 to 600 seconds in a first temperature range of 700 to 900 ° C., specifically, in an austenite single phase region or a two-phase region of an austenite phase and a ferrite phase. If the annealing temperature is less than 700 ° C or if the annealing time is less than 15 seconds, the carbides in the cold-rolled steel sheet will not dissolve sufficiently or the recrystallization of ferrite will not be completed, and the target characteristics will be obtained. There may not be. On the other hand, when the annealing temperature exceeds 900 ° C., the austenite grains grow remarkably, which may cause a decrease in ferrite nucleation sites from the second phase caused by subsequent cooling. In addition, annealing with an annealing time exceeding 600 seconds causes an increase in cost due to a large energy consumption. For this reason, annealing temperature shall be 700-900 degreeC, and annealing time shall be 15-600 seconds.

After annealing, it is cooled to a temperature range of MS to MS-50 ° C. obtained by the following formula (1) at a cooling rate of 5 ° C./s or more.
Record
MS (° C.) = 540−350 × {[C%] / (1− [α%] / 100)} − 40 × [Mn%] + 30 × [Al%]
−20 × [Cr%] − 35 × [V%] − 10 × [Mo%] − 17 × [Ni%]
−10 × [Cu%] (1)
However, [X%] means mass% of the alloy element X, and [α%] means volume fraction (%) of polygonal ferrite.

  When the cooling rate is less than 5 ° C./s, pearlite is precipitated and the solid solution C in the untransformed austenite is significantly reduced, so that the target structure cannot be obtained. When the cooling stop temperature is in the range of MS to MS-50 ° C., part of untransformed austenite can be martensitic transformed. At a temperature at which the cooling stop temperature exceeds MS ° C., the alloy components contained in the second phase vary, so part of the second phase undergoes martensitic transformation, but the transformation amount is not sufficient, and MS-50 If it is less than 0 ° C., on the contrary, the martensite transformation amount becomes excessive, and the target characteristics may not be obtained. For this reason, cooling stop temperature shall be the range of MS-MS-50 degreeC.

  Then, in this invention, it hold | maintains for 15 to 600 second in the 2nd temperature range of 350-600 degreeC. This is performed to promote the bainite transformation of untransformed austenite and increase the amount of dissolved C to obtain stable retained austenite at room temperature, and at the same time, temper martensite. If the temperature range exceeds 600 ° C., carbide precipitates from the untransformed austenite. Conversely, if the temperature is less than 350 ° C., carbide precipitates due to the lower bainite transformation, and as a result, stable retained austenite cannot be obtained sufficiently. If the holding time is less than 15 seconds, the amount of bainite transformation is not sufficient. Conversely, if it exceeds 600 seconds, carbides precipitate from untransformed austenite, and as a result, stable retained austenite cannot be obtained sufficiently.

  In the present invention, after holding in the second temperature range, it is necessary to cool the third temperature range up to at least 200 ° C. at a cooling rate of 3 ° C./s or more. By cooling the temperature range up to at least 200 ° C. at a cooling rate of 3 ° C./s or more, it is possible to suppress deterioration of characteristics due to precipitation of carbides. If the cooling end temperature at a cooling rate of 3 ° C./s or higher is higher than 200 ° C., carbide may be precipitated, and if it is less than 3 ° C./s, characteristic deterioration may occur due to carbide precipitation or coarsening. There is sex.

  In the series of heat treatments in the production method of the present invention, the holding temperature does not have to be constant as long as it is within the specified temperature range, and even if the cooling rate changes during cooling, it may be within the specified range. Thus, the gist of the present invention is not impaired. Further, as long as the thermal history is satisfied, the steel sheet may be heat-treated in any equipment including a continuous annealing equipment and a hot dip galvanizing equipment. In addition, it is also included in the scope of the present invention that the steel sheet of the present invention is subjected to temper rolling or the surface layer is coated with electroplating or the like for shape correction after heat treatment. In the present invention, it is assumed that the steel material is manufactured through normal steelmaking, casting, and hot rolling processes, but the manufacturing process is performed by omitting part or all of the hot rolling process by thin casting, for example. You may do it.

  Hereinafter, the present invention will be described in more detail by way of examples. However, the following examples are not intended to limit the present invention, and any design changes in accordance with the gist of the preceding and following descriptions are technical aspects of the present invention. It is included in the range.

  A slab obtained by melting steel having chemical components shown in Table 1 was hot-rolled, pickled, and then cold-rolled to obtain a cold-rolled steel sheet having a thickness of 1.2 mm. Then, after heat treatment under the conditions shown in Table 2 and Table 3, 0.5% temper rolling was performed. The performance of each steel plate thus obtained was evaluated. Further, the steel sheet structure was observed with a scanning electron microscope (SEM), and the volume fraction of each phase in the structure was determined by a line segment method.

1. Tensile test Tensile test is performed using JIS No. 5 test piece punched from each steel plate obtained, TS (tensile strength) and El (total elongation) are measured, and product of strength and elongation (TS x El) The value of strength-elongation balance represented by In the present invention, the case of TS × El ≧ 19800 (MPa ·%) was determined to be good.

2. Stretch flangeability Each steel plate obtained was cut to 100 mm x 100 mm, punched out a hole with a diameter of 10 mm with a clearance of 12%, and then with a dies with a diameter of 75 mm and a wrinkle holding force of 9 tons, The punch was pushed into the hole, the hole diameter at the crack initiation limit was measured, the critical hole expansion rate (%) was obtained from the following formula, and the stretch flangeability was evaluated from the value of the critical hole expansion rate. In the present invention, the critical hole expansion ratio λ ≧ 50% was determined to be good.
Limit hole expansion rate λ (%) = {(D _f −D ₀ ) / D ₀ } × 100
However, _{D f} hole diameter at crack initiation (mm), _{D 0} is the initial hole diameter (mm).

  Tables 2 and 3 summarize the evaluation results. As is clear from these results, it can be seen that the steel sheet that satisfies the requirements defined in the present invention is excellent in the balance between the strength-elongation balance value and the stretch flangeability, and the target characteristics are obtained.

  According to the present invention, a high-strength steel sheet having excellent ductility and stretch flangeability and a method for producing the same can be provided, and the industrial utility value is very large, which is an invention with a large industrial effect.

Claims (6)

In mass%, C: 0.05-0.30%, Si: 2.0% or less, Mn: 0.8-3.0%, P: 0.1% or less, S: 0.07% or less, Al: 0.1-2.5% and N: 0.007% or less And the balance is composed of Fe and inevitable impurities, the second phase grains are present in isolation in the ferrite matrix, and among the second phase grains, from the mixed structure including the tempered martensite phase and the bainite phase. A high-strength steel sheet excellent in workability, characterized in that the abundance ratio of the second phase grains is 20% or more.   The processability according to claim 1, further comprising one or more elements selected from Cr: 2.0% or less, V: 2.0% or less, and Mo: 2.0% or less in terms of mass%. Excellent high strength steel plate.   It further contains one or more elements selected from Ti: 0.1% or less, Nb: 0.1% or less, B: 0.0050% or less, Ni: 2.0% or less, and Cu: 2.0% or less in terms of mass%. The high-strength steel sheet excellent in workability according to claim 1 or 2. In mass%, C: 0.05-0.30%, Si: 2.0% or less, Mn: 0.8-3.0%, P: 0.1% or less, S: 0.07% or less, Al: 0.1-2.5% and N: 0.007% or less Then, after the steel plate having the balance of Fe and inevitable impurities is held for 15 to 600 seconds in the first temperature range of 700 to 900 ° C., it is obtained by the following formula (1) at a cooling rate of 5 ° C./s or more. After cooling to the temperature range of MS to MS-50 ° C, hold in the second temperature range of 350 to 600 ° C for 15 to 600 seconds, then cool the temperature range of at least 200 ° C at a cooling rate of 3 ° C / s or more A method for producing a high-strength steel sheet excellent in workability, characterized in that:
Record
MS (° C.) = 540−350 × {[C%] / (1− [α%] / 100)} − 40 × [Mn%] + 30 × [Al%]
−20 × [Cr%] − 35 × [V%] − 10 × [Mo%] − 17 × [Ni%]
−10 × [Cu%] (1)
However, [X%] means mass% of the alloy element X, and [α%] means volume fraction (%) of polygonal ferrite.   5. The steel sheet according to claim 4, further comprising one or more elements selected from Cr: 2.0% or less, V: 2.0% or less, and Mo: 2.0% or less in terms of mass%. Of high-strength steel sheet with excellent workability.   The steel sheet further includes one or more elements selected from mass%, Ti: 0.1% or less, Nb: 0.1% or less, B: 0.0050% or less, Ni: 2.0% or less, and Cu: 2.0% or less. The method for producing a high-strength steel sheet excellent in workability according to claim 4 or 5, characterized in that it is contained.