JP2011219809A - High strength steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength steel sheet having low yield stress and high tensile strength and breaking elongation.SOLUTION: An Ni equivalent amount indicated by the following equation (A) is 21.5 to 25.5%, Md30 indicated by the following equation (B) is 98 to 260°C, the ratio of an austenitic phase occupying a metal structure is ≥85%, the crystal grain size of the austenitic phase is ≥20 μm, 0.2% proof stress is ≤400 MPa, tensile stress is ≥1,000 MPa, and stretch is ≥60%: wherein (A) Ni equivalent amount=Ni+12.93C+1.11Mn+0.72Cr+0.88Mo-0.27Si+0.53Cu-0.69Al+7.55N, and (B) Md30(°C)=551-462(C+N)-9.2Si-8.1Mn-13.7Cr-29(Ni+Cu)-18.2Mo.

Description

  The present invention relates to a high strength steel sheet having a low yield ratio, high ductility, excellent press formability, and high strength.

  In recent years, there has been an increasing demand for collision safety of automobiles, and the development of vehicle body technology for protecting the occupant's living space and protecting the occupant in the event of a collision has been promoted. On the other hand, demands for improving fuel efficiency are increasing day by day from the viewpoint of global warming, and there is a strong demand for weight reduction of automobile bodies. In order to satisfy such conflicting requirements, it is important to increase the strength of the steel sheet that occupies most of the vehicle body components.

  However, in general, if the strength of the steel sheet is increased, the ductility decreases, and the tensile strength exceeds 1 GPa, and the elongation at break decreases to several percent. This causes problems such as difficulty in productivity and a significant reduction in productivity. In the past, the strength of the steel sheet has been improved, and the thickness of the steel sheet has been reduced to achieve both collision safety and weight reduction. However, the reduction of the plate thickness may lead to a decrease in the rigidity of the vehicle body. . In order to compensate for this drawback, it is necessary to improve the rigidity of the vehicle body by the shape of the parts, for example, by providing ribs or the like, and as a result, the shape of the parts becomes complicated.

  In this regard, when an ultra-high strength steel plate is used, the shape of the part is restricted due to a decrease in ductility, so that it is difficult to maintain vehicle body rigidity. For these reasons, steel plates with a tensile strength of 270 MPa are currently used for automobile bodies that require complex shapes, and steel plates with a tensile strength of 590 MPa are used for members that require strength. Has been widely applied. Therefore, a steel sheet having a yield stress (400 MPa or less) equivalent to or less than that of the 590 MPa class, a forming freedom degree (elongation> 50%) comparable to that of the 270 MPa class, and a tensile strength (1000 MPa or more) comparable to that of an ultra-high strength steel sheet. If this can be realized, the industrial utility value is very high.

  The reason why it is difficult to achieve both high strength and high ductility with the current technology for cold-working automotive steel sheets is because hardening structure strengthening is used as a technique for increasing the strength of steel. On the other hand, it is difficult to ensure a breaking elongation of 60% or more while increasing the tensile strength to 1000 MPa or more with the ferrite structure. This is because martensite, which is representative of a hardened structure, has high strength but lacks ductility, so that the elongation of the steel sheet is also greatly deteriorated. With the extension of the prior art, both high strength exceeding 1 GPa and high ductility are achieved. Is not feasible.

  The need for a steel sheet that has both high strength and formability has been very high, but there is a limit as described above in the structure mainly composed of ferrite. On the other hand, an austenitic structure typified by austenitic stainless steel has a relatively high strength, and its elongation is better than that of ferritic structure steel. For example, Patent Document 1 discloses austenitic stainless steel having excellent collision absorption performance.

  In addition, a technique for achieving both high strength and high ductility in a TWIP steel using twinning induced plasticity (TWIP) has been studied. For example, Patent Document 2 discloses an austenitic steel containing about 25% by weight of Mn, Si and Al in a total range of 12% by weight or less, and includes TWIP (twinning induced plasticity) and TRIP (transformation induced plasticity). ), A steel sheet with a yield stress of 400 MPa or more, a tensile strength of 1100 MPa, a uniform elongation of 70%, and a maximum elongation of 90% is disclosed.

JP 2002-20843 A Special table 2002-507251 gazette

  However, although the austenitic stainless steel described in Patent Document 1 has a low yield stress and a high elongation at break, the tensile strength is about 800 MPa, and the strength is insufficient. Moreover, since the steel plate described in Patent Document 2 contains a large amount of elements such as Si and Al, there is a problem that the yield stress is high. In addition, it contains 25% of Mn and does not contain Cr at all, so the corrosion resistance is remarkably deteriorated. In order to apply it to automobiles, development of chemical conversion treatment and plating technology is required. It is.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a high-strength steel sheet having low yield stress and high tensile strength and elongation at break.

  The inventors of the present invention have intensively studied to improve the balance between strength and ductility while suppressing an increase in yield stress of austenitic steel. As a result, we established a design method for alloy components that optimizes the balance between strength and ductility. Furthermore, the present inventors have found a technique for suppressing an increase in yield stress associated with high alloying. Specifically, by optimizing the Ni equivalent and Md30, the composition range of the austenitic steel where the processing-induced transformation is most likely to occur while the metal structure before deformation is mainly composed of austenite is found. Low yield stress was achieved despite its strength and high ductility. As a result, a high-strength steel sheet capable of obtaining high press formability while having a tensile strength exceeding 1000 MPa was found. Since austenite does not easily exhibit a clear yield phenomenon, the present invention specifies 0.2% yield strength, which is the same kind of characteristic. In the following description, “0.2% yield strength” is used.

The high-strength steel sheet of the present invention has been made based on the above findings, and in mass%, C: 0.005 to 0.25%, N: 0.005 to 0.2%, Si: 0.01 to 3%, Mn: 8.9 to 17.5%, Cr: 2 to 14.5%, the balance being Fe and unavoidable impurities, the total content of C and N being 0.01 to 0.00. It is 25%, Ni equivalent represented by the following formula (A) is 21.5 to 25.5%, Md30 represented by the following formula (B) is 98 to 260 ° C., and occupies the metal structure The area ratio of the austenite phase is 85% or more, the crystal grain size of the austenite phase is 20 μm or more, the 0.2% proof stress is 400 MPa or less, the tensile stress is 1000 MPa or more, and the elongation is 60% or more.
Ni equivalent = Ni + 12.93C + 1.11Mn + 0.72Cr + 0.88Mo-0
. 27Si + 0.53Cu-0.69Al + 7.55N (A)
Md30 (° C) = 551-462 (C + N) -9.2Si-8.1 Mn-13.7
Cr-29 (Ni + Cu) -18.2Mo (B)
However, each element symbol indicates a mass percentage of the element.

  In the present invention, since the Ni equivalent is 21.5 to 25.5 within the range of the alloy components described above, the structure includes an austenite phase of 85% or more in area ratio. Further, since Md30 is set to 98 to 260 ° C., deformation-induced martensitic transformation occurs at a higher level than that of the conventional material at the time of deformation, so that very high strength and elongation can be obtained. Md30 is the temperature at which half of the material transforms when 30% strain is applied to the steel, and the higher this value, the easier it is to transform into martensite. On the other hand, since the crystal grain size of the austenite phase is 20 μm or more, the 0.2% proof stress can be suppressed to 400 MPa or less while being a high alloy component.

  According to the present invention, it is possible to provide a high-strength steel sheet having a low 0.2% yield strength and a high tensile strength and elongation at break. Therefore, the structural member can be thinned to reduce the weight, and the rigidity can be maintained by forming the structural member into a complicated shape having ribs and the like.

It is a graph which shows the relationship between the annealing temperature and the crystal grain diameter in the Example of this invention. It is a diagram which shows the relationship between the crystal grain diameter and 0.2% yield strength in the Example of this invention. It is a top view which shows the sample used in the Example of this invention. It is a graph which shows the relationship between elongation and stress in the Example of this invention.

  The austenitic steel of the present invention has a metal structure mainly composed of austenite before deformation, and realizes high strength and elongation by expressing work-induced martensite with deformation. Therefore, it is necessary to adjust the Ni equivalent and Md30, which are indices indicating the stability of austenite, to alloy components that satisfy an appropriate range. Here, the appropriate range is a range that simultaneously satisfies Ni equivalent: 21.5 to 25.5 and Md30: 98 to 260 ° C. Below, the effect of an alloy component and the reason for limitation will be described in detail. In the following description, “%” means “mass%”.

C: 0.005-0.25%
C is an austenite forming element and is an element effective for increasing the work hardening rate. If the C content is less than 0.005%, such an effect cannot be expected. On the other hand, if the content of C exceeds 0.25%, the hardness difference between the work-induced martensite phase and the untransformed austenite phase increases, showing a brittle fracture form, resulting in a decrease in strength and ductility. May end up. Therefore, the C content is 0.005 to 0.25%. However, when the C content is less than 0.03%, ductility increases, but the work hardening rate decreases. For this reason, when it is less than 0.03%, it is necessary to make an alloy component design that keeps the Ni equivalent low to about 22%. The content of C is preferably 0.05 to 0.15% in consideration of corrosion resistance and refinability.

N: 0.005 to 0.2%
N, like C, is an element that forms austenite, and since it also has the effect of improving corrosion resistance, it is positively added. The N content is an amount inevitably mixed in the steel material. If the N content exceeds 0.2%, the steel becomes brittle and the hot formability is remarkably reduced, so the upper limit is made 0.2%.

C + N: 0.01 to 0.25%
As described above, since C and N have the same effect on the stabilization and embrittlement of austenite, the total content of C and N is set to 0.25% or less.

Si: 0.01-3.0%
Si is an element effective for improving corrosion resistance, time crack resistance, and stress corrosion crack resistance. If the Si content is less than 0.01%, such an effect is not exhibited. If the Si content exceeds 3.0%, the hot formability deteriorates. Therefore, the Si content is set to 0.01 to 3.0%.

Mn: 8.9 to 17.5%
Mn is an austenite stabilizing element and has a high effect of increasing Md30. In order to ensure the target value of Md30> 98, Mn needs to be contained in an amount of 8.9% or more. On the other hand, if the content of Mn exceeds 17.5%, Mn has a high vapor pressure, so that the refinability is significantly lowered. Therefore, the Mn content is 8.9 to 17.5%. In view of corrosion resistance and manufacturability, the Mn content is desirably 8.5 to 13.0%.

Cr: 2.0-14.5%
Cr stabilizes austenite, reduces stacking fault energy, and increases the work hardening rate, so is an element useful for increasing the strength. If the Cr content is less than 2.0%, such an effect is not exhibited, and if it exceeds 14.5%, a large amount of expensive Ni must be added to suppress δ ferrite, This causes deterioration of inter-workability and an increase in yield stress. In consideration of corrosion resistance, 7 to 14.5% is desirable.

The above is an essential component of the present invention. The components added and desired and their contents will be described below.
Cu: 0.5 to 1.6%
Cu is an austenite stabilizing element and is an element effective for improving corrosion resistance and ductility. In order to acquire such an effect, it is necessary to contain Cu 0.5% or more. On the other hand, if the Cu content exceeds 1.5%, the effect of increasing the stacking fault energy is very high, and the formation of work-induced martensite is remarkably reduced, leading to a reduction in strength. Further, excessive addition of Cu may cause hot brittleness. Therefore, the Cu content is set to 0.5 to 1.6%.

Ni: 0.5 to 7.2%
Ni is an austenite stabilizing element, and is an element effective for improving corrosion resistance and ductility. In order to acquire such an effect, it is necessary to contain 0.5% or more. On the other hand, if the Ni content exceeds 7.2%, the Md30 is lowered, the work hardening rate is lowered, and sufficient strength cannot be obtained. Ni is not usually added because it is very expensive. Since the effect of improving the ductility of steel is higher than that of Mn, when ductility is particularly required, Mn is added instead of Mn. The Ni content is preferably 0.5 to 4% in consideration of strength and cost.

Mo: 0.01 to 2.5%
Mo is combined with Si in the steel to form precipitates such as Mo3Si and MoSi2, which may increase the yield point, and is an extremely expensive element, so it is not usually added. However, since it is an element effective for improving corrosion resistance, it can be added in an amount of 0.1% or more depending on the use environment. However, addition of 2.5% or more causes an increase in yield stress and a decrease in ductility, so the upper limit is made 2.5%.

Al: 0.02 to 0.05%
Al is not usually added because it significantly increases the stacking fault energy and significantly reduces the formation of work-induced martensitic transformation. However, since Al is highly effective as a deoxidizer, it can be added up to 0.05%.

B: 0.0004 to 0.002%
B has an effect of strengthening the grain boundary of austenitic steel, and can be added as necessary. However, if the amount added is too high, brittleness will develop, so the upper limit is made 0.002%.

Metallic structure In the present invention, by setting the Ni equivalent to 21.5 to 25.5% within the above-described alloy component range, a structure containing an austenite phase of 85% or more in area ratio is obtained. Further, by setting Md30 to 98 to 260 ° C., deformation-induced martensite transformation occurs at a higher level than that of the conventional material at the time of deformation, so that the steel sheet has very high strength and elongation. In addition to this, it is possible to suppress the 0.2% proof stress to 400 MPa or less, even though it is a high alloy component, by adjusting the crystal grain size to 20 μm or more by heat treatment for the requirement of low yield stress. It becomes.

Hereinafter, the present invention will be described in more detail with reference to specific examples.
Steels of alloy components shown in Table 1 were melted by vacuum melting, and hot rolled steel sheets having a thickness of 7 mm were produced by hot rolling. This hot-rolled steel sheet was annealed at 1100 ° C. for 30 minutes, and the oxidized scale on the steel sheet surface was ground to produce a hot-rolled steel sheet having a thickness of 4.5 mm. Further, these hot-rolled steel sheets were heated to 300 ° C., removed from the furnace, rolled to a thickness of 1.4 mm, annealed at 1050 ° C. for 10 minutes, air-cooled to 575 ° C., and immediately annealed at 575 ° C. for 10 minutes. Then, pickling was performed to produce a cold-rolled / annealed steel sheet.

  FIG. 1 shows the relationship between the annealing temperature and crystal grain size of the steel type of Invention Example 14, and FIG. 2 shows the relationship between the crystal grain size and 0.2% proof stress of the steel type of Example 14 of the invention. As shown in FIG. 1, when the annealing temperature is 1025 ° C. or higher, a crystal grain size of 20 μm or more is obtained. As a result, as shown in FIG. 2, the 0.2% proof stress can be reduced to 400 MPa or less. Is possible. However, a temperature of about 1050 to 1100 ° C. is desirable in consideration of variations in manufacturing conditions and temperature management costs. In addition, although the heat processing for 10 minutes was implemented at 575 degreeC in the annealing performed after cold rolling, this heat processing is unnecessary if the cooling rate after annealing at 1050 degreeC is 3 degrees C / min or less, but more cooling than this Needed for speed. After completing the above steps, a micro tensile test piece shown in FIG. 3 was produced from the cold-rolled / annealed steel sheet, and a static tensile test was performed. The tensile test was carried out at a tensile speed of 0.05 mm / min using a high-speed tensile tester TS-2000 manufactured by Kinomiya Seisakusho.

  In the structure observation, the steel sheet was cut so that a cross section parallel to the rolling direction could be observed, wet polishing was performed up to # 1000, and finally surface finishing was performed by electrolytic polishing. The crystal grain size and the phase distribution state were analyzed by an EBSP apparatus manufactured by TSL. For the measurement of the martensite content, for example, a method of measuring and estimating the magnetic permeability of a steel sheet can be used. However, as a result of detailed examination in the present invention, the magnetic permeability is affected by the alloying elements and residual strain. Therefore, it became clear that it was difficult to accurately measure the amount of ferrite. Therefore, in the present invention, the measurement result by the EBSP apparatus is adopted.

  Hereinafter, the above results in Examples and Comparative Examples will be described in detail. In Table 1, the slabs 1 to 14 of the present invention all satisfy the formulas (A) and (B), the alloy component range, and the crystal grain size, and the comparative slabs have the formulas (A), (B) and One or more conditions of the alloy component range and the crystal grain size are not satisfied. Inventive slabs 1 to 6 are slabs supplied at low cost without adding any expensive Ni or Mo.

  FIG. 2 shows the relationship between the crystal grain size and the 0.2% proof stress. The Hall Petch rule holds for crystal grain sizes from 1 to 20 μm, but the 0.2% proof stress is stable at 400 MPa or less at 20 μm or more. I understand that. Therefore, the crystal grain size needs to be 20 μm or more.

  FIG. 4 shows stress-strain curves of the example and the comparative example. It can be seen that the present invention material achieves high strength and elongation while keeping the yield stress low compared to the comparative material.

  As described above, according to the present invention, a tensile strength of over 1 GPa and an elongation of 60% or more can be imparted with a yield stress of a steel plate level of 590 MPa, thereby reducing the weight of the automobile and improving the collision safety. It is possible to make various social contributions such as environmental contributions and human life protection.

  The high-strength steel sheet of the present invention has a low 0.2% proof stress, and has an excellent deep drawability as well as a high tensile strength and strength having elongation at break. Therefore, it should be applied as a structural steel sheet in automobiles and other technical fields. Can do.

Claims (2)

In mass%, C: 0.005 to 0.25%, N: 0.005 to 0.2%, Si: 0.01 to 3%, Mn: 8.9 to 17.5%, Cr: 2 to 14.5% is included, the balance is Fe and inevitable impurities, the total content of C and N is 0.01 to 0.25%, and the Ni equivalent represented by the following formula (A) is 21. 0.5 to 25.5%, Md30 represented by the following formula (B) is 98 to 260 ° C., the proportion of the austenite phase in the metal structure is 85% or more, and the crystal grain size of the austenite phase is 20 μm or more. A high-strength steel sheet having a 0.2% proof stress of 400 MPa or less, a tensile stress of 1000 MPa or more, and an elongation of 60% or more.
Ni equivalent = Ni + 12.93C + 1.11Mn + 0.72Cr + 0.88Mo-0
. 27Si + 0.53Cu-0.69Al + 7.55N (A)
Md30 (° C) = 551-462 (C + N) -9.2Si-8.1 Mn-13.7
Cr-29 (Ni + Cu) -18.2Mo (B)
However, each element symbol indicates a mass percentage of the element. In mass%, Cu: 0.5-1.6%, Ni: 0.5-7.2%, Mo: 0.01-2.5%, Al: 0.02-0.05%, B: The high-strength steel sheet according to claim 1, further comprising one or more of 0.0004 to 0.002%.

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