JP2011202244A - High-strength thin steel sheet having excellent material quality stability and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a precipitation strengthening type high-strength thin steel sheet which has highly stable material quality, particularly strength, and to provide a method for producing the same.SOLUTION: The high-strength thin steel sheet having excellent material quality stability has a composition containing, by mass, 0.05 to 0.15% C, 0.06 to 0.7% Si, 1.0 to 2.5% Mn, 0.01 to 0.05% P, ≤0.0050% S, 0.01 to 0.10% Al, ≤0.0050% N, 0.01 to 0.10% Nb and 0.001 to 0.010% Ta, and the balance Fe with inevitable impurities.

Description

  The present invention relates to a high-strength thin steel sheet having excellent material stability suitable as a member for structural parts such as automobiles and home appliances, and a method for producing the same.

In recent years, CO ₂ emission regulations have become stricter due to increasing environmental problems, and in the automobile field, improvement of fuel consumption by reducing the weight of the vehicle body has become a major issue. For this reason, thinning is being promoted by applying high-strength steel sheets to automobile parts, and steel parts of TS590 MPa class or higher are applied to parts that have been used with steel sheets of 270 to 440 MPa class of tensile strength (TS). Is underway.

  Among high-strength steel sheets of TS590MPa class or higher, so-called precipitation-strengthened high-strength steel sheets to which carbide-generating elements such as Ti and Nb are added are applied to structure-strengthened steel sheets that generate hard second phases such as martensite. In comparison, since a small amount of alloy element is required to ensure a predetermined strength, it can be manufactured at low cost and is widely used industrially.

  For example, Patent Document 1 discloses a method for producing a hot-dip galvanized steel sheet having excellent secondary work brittleness resistance after press forming at TS 590 MPa or more, which is precipitation strengthened by addition of Nb, and Patent Document 2 discloses Nb and A high-strength cold-rolled steel sheet, which is precipitation-strengthened by addition of Ti and has excellent stretch flange formability and impact absorption energy characteristics with a TS of 490 MPa or more and less than 720 MPa, and a method for producing the same are disclosed.

  However, high-strength steel plates of TS590MPa class or higher have material variations such as strength and elongation that are larger than those of low-strength steel plates such as mild steel that have been used in the past. There is a problem that productivity decreases.

  As a technique for improving such material variation, Patent Literature 3 discloses a method for improving the balance between strength and ductility of an alloyed hot-dip galvanized steel sheet, and Patent Literature 4 provides spot weldability and material stability. An excellent high strength hot dip galvanized steel sheet is disclosed.

Japanese Patent No. 3873638 JP 2008-174776 JP 2007-327123 A JP 2005-320561 A

  However, the technique disclosed in Patent Document 3 ensures the stability of elongation by controlling the amount of carbon in retained austenite, and does not give any knowledge about the reduction in strength variation. In addition, the technique disclosed in Patent Document 4 is required to add a large amount of Mn, resulting in high cost and stable generation of the martensite phase. Since it is considered that the effect is small even if it is applied to a precipitation strengthening type high strength steel sheet that contains only or not at all, it is not a technique that can be applied to a precipitation strengthening type high strength steel sheet.

  As described above, there is no technology for sufficiently suppressing fluctuations in strength for precipitation strengthened high-strength steel sheets, and variations in materials remain a problem when applying precipitation-strengthened high strength steel sheets. Therefore, there is a problem that productivity when using a steel sheet is hindered due to such variation in material.

  Accordingly, an object of the present invention is to provide a precipitation-strengthened high-strength steel sheet excellent in material, particularly strength stability, and a method for producing the same.

  As a result of careful examination of the factors of material variation, the present inventors have found that precipitates are the main factor of material variation. And further examination was continued in order to improve the stability of the precipitate, and the present invention was achieved.

  Specifically, it has been found that a stable precipitate can be precipitated by simultaneously adding Nb and Ta. And after adding Nb and Ta simultaneously in this way, the material after annealing is actually stabilized by controlling the cooling conditions and winding temperature after finish rolling in hot rolling, and annealing conditions. I found out. Although details are not necessarily clear, the precipitate becomes a composite carbonitride such as (Nb, Ta) (C, N), so that the stability of the precipitate is improved and the coarsening rate is significantly reduced. It is considered that the strength contribution due to precipitation strengthening is stabilized.

That is, the present invention provides the following (1) to (6).
(1) By mass%, C: 0.05 to 0.15%, Si: 0.06 to 0.7%, Mn: 1.0 to 2.5%, P: 0.01 to 0.05% S: 0.0050% or less, Al: 0.01 to 0.10%, N: 0.0050% or less, Nb: 0.01 to 0.10%, Ta: 0.001 to 0.010% A high-strength thin steel sheet excellent in material stability, characterized in that the balance is comprised of Fe and inevitable impurities.
(2) In addition to the component of (1) above, further contains at least one of mass%, V: 0.10% or less, Ti: 0.100% or less, with the balance being Fe and inevitable impurities. High strength thin steel sheet with excellent material stability.
(3) In addition to the component (1) or (2) above, further in terms of mass, Cr: 0.5% or less, Mo: 0.5% or less, Cu: 0.50% or less, Ni: 0.00. A high-strength thin steel sheet excellent in material stability, comprising one or more of 50% or less and B: 0.0030% or less, with the balance being Fe and inevitable impurities.
(4) The high strength thin steel sheet excellent in material stability according to any one of (1) to (3), wherein the tensile strength TS is 590 MPa or more.
(5) A steel slab having the composition described in any one of (1) to (3) above is hot-rolled under conditions of a heating temperature of 1100 to 1270 ° C. and a finish rolling finishing temperature of 830 to 950 ° C. Cooling is started within 1 s after the completion, rapidly cooled to 650-750 ° C. at an average cooling rate of 20-200 ° C./s, air-cooled at this temperature for 2 s or longer, wound up at 500-650 ° C., pickled, and cooled Then, it is heated to 720 to 860 ° C. at an average heating rate of 3 to 30 ° C./s, the soaking time at 720 to 860 ° C. is set to 30 to 300 s, and the range from the soaking temperature to 510 ° C. The material was excellent in material stability, characterized in that annealing was performed at an average cooling rate of 3 to 30 ° C./s, and then temper rolling was performed at an elongation rate of 0.3 to 2.0%. Manufacturing method of high strength thin steel sheet.
(6) The method for producing a high-strength thin steel sheet having excellent material stability according to (5), wherein the obtained thin steel sheet has a tensile strength TS of 590 MPa or more.

  According to the present invention, it is possible to provide a high-strength thin steel sheet having excellent material stability, which greatly contributes to the improvement of productivity.

Hereinafter, the present invention will be specifically described.
First, the reasons for limiting the composition of the high strength thin steel sheet of the present invention will be described. In the following, “%” notation of components means mass%.

C: 0.05 to 0.15%
C is an element effective for increasing the strength of a steel sheet, and in particular, forms carbide forming elements such as Nb and Ta and fine alloy carbides or alloy carbonitrides, thereby contributing to strengthening of the steel sheet. In addition, forming a second phase such as pearlite or martensite also contributes to an increase in strength. In order to acquire this effect, it is necessary to contain 0.05% or more. On the other hand, if contained excessively, the steel sheet is hardened and not only the formability is lowered but also the spot weldability is lowered, so the content is made 0.15% or less. From the viewpoint of ensuring good weldability, 0.12% or less is preferable.

Si: 0.06 to 0.7%
Si is an element that contributes to high strength mainly by solid solution strengthening, is a relatively small decrease in ductility with respect to strength increase, and is an element that contributes not only to strength but also to improvement of strength-ductility balance. In order to acquire this effect, it is necessary to contain 0.06% or more. From the viewpoint of improving the strength-ductility balance, the content is preferably 0.2% or more. On the other hand, if it exceeds 0.7%, the chemical conversion processability decreases, so the content is made 0.7% or less.

Mn: 1.0 to 2.5%
Mn is an element that contributes to strengthening by forming a solid solution strengthening and a second phase. In order to obtain this effect, it is necessary to contain 1.0% or more. On the other hand, when the content is excessive, the moldability is remarkably lowered, so the content is made 2.5% or less.

P: 0.01-0.05%
P is an element contributing to an increase in strength by solid solution strengthening, and in order to obtain this effect, it is necessary to contain 0.01% or more. When the content is excessive, segregation to the grain boundary becomes remarkable and the grain boundary becomes brittle or easily segregates at the center. Therefore, the content is made 0.05% or less.

S: 0.0050% or less When the content of S is large, a large amount of sulfide such as MnS is generated, and the local ductility represented by stretch flangeability is lowered, so the upper limit is made 0.0050%. Preferably, it is 0.0030% or less. Although there is no particular lower limit, it is preferable to make it 0.0005% or more because extremely low S increases the steelmaking cost.

Al: 0.01-0.10%
Al is an element necessary for deoxidation, and in order to obtain this effect, it is necessary to contain 0.01% or more, but even if added over 0.10%, the effect is saturated. 10% or less.

N: 0.0050% or less N, like C, forms a compound with Nb or Ta to become an alloy nitride or an alloy carbonitride, contributing to an increase in strength. However, since nitrides are likely to be formed at a relatively high temperature, they tend to be coarse, and their contribution to strength is relatively small compared to carbides. In other words, to increase the strength, it is advantageous to reduce the amount of N and produce more alloy carbide. From such a viewpoint, the N content is set to 0.0050% or less. Preferably it is 0.0030% or less.

Nb: 0.01 to 0.10%
Nb forms a compound with C or N to become a carbide or carbonitride, contributing to an increase in strength. In order to acquire this effect, it is necessary to contain 0.01% or more. However, when excessively added, the moldability is remarkably lowered, so the content is made 0.10% or less.

Ta: 0.001 to 0.010%
Ta, like Nb, forms alloy carbides and alloy carbonitrides and contributes to high strength, but also partially dissolves in Nb carbides and Nb carbonitrides, and (Nb, Ta) (C, By forming the composite precipitate as in N), it is considered that the coarsening of the precipitate is remarkably suppressed and the contribution to the strength by precipitation strengthening is stabilized. In order to acquire such an effect, it is necessary to contain 0.001% or more. However, when added excessively, not only the above-mentioned precipitate stabilization effect is saturated but also the alloy cost increases, so the content is made 0.010% or less.

  In the present invention, in addition to the above components, the following components may be added within a predetermined range.

V: 0.10% or less V, like Nb, can contribute to an increase in strength by forming fine carbonitrides, and can be added as necessary. In order to exhibit such an effect, it is preferable to contain 0.01% or more. On the other hand, even if a large amount of V is contained, the effect of increasing the strength exceeding 0.10% is small, and the alloy cost is also increased. Therefore, the V content is 0.10% or less.

Ti: 0.100% or less Ti, like Nb, can contribute to an increase in strength by forming fine carbonitride, and can be added as necessary. In order to exert such an effect, the Ti content is preferably 0.005% or more. On the other hand, when Ti is added in a large amount, the moldability is remarkably lowered, so the content is made 0.100% or less.

Cr: 0.5% or less Cr is an element that contributes to increasing the strength by improving the hardenability and generating the second phase, and can be added as necessary. In order to exhibit such an effect, it is preferable to contain 0.1% or more. On the other hand, even if the content exceeds 0.5%, the effect is saturated, so the content is made 0.5% or less.

Mo: 0.5% or less Mo is an element that improves hardenability and contributes to high strength by generating a second phase, or partially generates carbides and contributes to high strength. Can be added accordingly. In order to exhibit these effects, it is preferable to make it contain 0.05% or more. On the other hand, even if the content exceeds 0.5%, the effect is saturated, so the content is made 0.5% or less.

Cu: 0.50% or less Cu is an element that contributes to strengthening by solid solution strengthening, improves hardenability, and contributes to strengthening by generating a second phase. can do. In order to exhibit these effects, it is preferable to make it contain 0.05% or more. On the other hand, even if the content exceeds 0.50%, the effect is saturated, and surface defects due to Cu are likely to occur, so the content is made 0.50% or less.

Ni: 0.50% or less Ni, like Cu, is an element that contributes to strengthening by solid solution strengthening, improves hardenability, and contributes to strengthening by generating a second phase. Necessary It can be added depending on. In order to exhibit these effects, it is preferable to make it contain 0.05% or more. Moreover, since it has the effect of suppressing the surface defect resulting from Cu when it contains with Cu, it is effective at the time of Cu addition. On the other hand, since the effect is saturated even if the content exceeds 0.50%, the content is made 0.50% or less.

B: 0.0030% or less B is an element that improves the hardenability and contributes to increasing the strength by generating the second phase, and can be added as necessary. In order to exhibit this effect, it is preferable to contain 0.0005% or more. On the other hand, even if the content exceeds 0.0030%, the effect is saturated, so the content is made 0.0030% or less.

Stability and Strength of Material The high strength thin steel sheet of the present invention is excellent in material stability. In evaluating the stability of the material, a tensile test was carried out at a total of 9 points in the center in the width direction and at both 1/4 width positions at the longitudinal tip, center and tail ends in the coil to determine the yield stress. Evaluate the difference between the maximum and minimum values of YS and tensile strength TS, ΔYS, ΔTS, and if both ΔYS and ΔTS are 30 MPa or less, the material variation is within the allowable range, and the material stability It is excellent. The high strength thin steel sheet of the present invention preferably has a tensile strength TS of 590 MPa or more.

Next, the manufacturing method of the high intensity | strength thin steel plate of this invention is demonstrated.
The high-strength thin steel sheet of the present invention is a steel slab having the above composition, heated in a hot rolling process, subjected to rough rolling and finish rolling, wound, and then removed from the scale of the hot rolled sheet surface layer in a pickling process. Then, it is manufactured by performing a cold rolling process, and subsequently performing an annealing process and a temper rolling process.

[Hot rolling conditions]
Heating temperature: 1100-1270 ° C
When the heating temperature of the slab is less than 1100 ° C., the rolling load increases and the productivity decreases, and when it exceeds 1270 ° C., the heating cost increases, so the heating temperature is set to 1100 to 1270 ° C.

Finishing rolling finish temperature: 830-950 ° C
Since it is necessary to finish the hot rolling in the austenite single phase region, it is necessary to finish the finish rolling at 830 ° C. or higher. On the other hand, if it exceeds 950 ° C., the hot-rolled structure becomes coarse and the characteristics after annealing deteriorate. For this reason, finish rolling end temperature shall be 830-950 degreeC.

Rapid cooling conditions after finish rolling: Cooling started within 1 s and rapidly cooled to 650-750 ° C. at an average cooling rate of 20-200 ° C./s Intermediate air cooling: Air cooling at 650-750 ° C. for 2 s or more The ferrite transformation is promoted by rapid cooling, and fine and stable alloy carbide ((Nb, Ta) (C, N)) or the like is precipitated, so that stabilization of the material can be achieved. If it stays at a high temperature after completion of hot rolling, the precipitate becomes coarse. Therefore, after completion of rolling, it is necessary to start cooling within 1 s and rapidly cool to 650 to 750 ° C. at an average cooling rate of 20 ° C./s or more. However, when the cooling rate is too high, uneven cooling is likely to occur, which causes an unstable factor of the material. Therefore, the average cooling rate needs to be 200 ° C./s or less. Further, even in the ferrite region, precipitates are likely to be coarsened at high temperatures, and precipitation is suppressed at low temperatures. Therefore, after quenching, air cooling at 650 to 750 ° C. for 2 seconds or more is required.

Winding temperature: 500-650 ° C
When the coiling temperature exceeds 650 ° C., precipitates such as alloy carbides generated in the cooling process after hot rolling become extremely coarse, so the upper limit of the coiling temperature is set to 650 ° C. On the other hand, when the coiling temperature becomes too low, hard bainite and martensite are generated, the cold rolling load increases, and the productivity is hindered. Therefore, the lower limit of the coiling temperature is set to 500 ° C.

[Pickling process]
After the hot rolling step, a pickling step is performed to remove the scale of the hot rolled sheet surface layer. The pickling step is not particularly limited, and may be performed according to a conventional method.

[Cold rolling process]
A cold rolling process is implemented to the predetermined plate | board thickness with respect to the hot-rolled sheet after pickling. What is necessary is just to implement a cold rolling process according to a conventional method.

[Annealing process]
In the annealing process, it is important for the stabilization of the material that the recrystallization of the ferrite structure proceeds and that the dissolution and coarsening of the precipitates are suppressed. In order to form such a structure, recrystallization proceeds sufficiently during the temperature rise, soaking in the two-phase region of ferrite and austenite and partly transformed into austenite, and pearlite, bainite, martensite during cooling. What is necessary is just to produce | generate a small amount of 2nd phases containing a site, and it anneals on the following conditions for that purpose.

Average heating rate: 3-30 ° C./s
The material can be stabilized by sufficiently allowing recrystallization to proceed in the ferrite region before heating to the two-phase region. Since recrystallization hardly proceeds when heated rapidly, the upper limit of the average heating rate is set to 30 ° C./s. On the other hand, if the heating rate is too low, the ferrite grains become coarse and the strength decreases, so an average heating rate of 3 ° C./s or more is required.

Soaking temperature: 720-860 ° C
If the soaking temperature is low, a large amount of unrecrystallized structure remains even at the heating rate described above, and the moldability is lowered. Therefore, the lower limit of the soaking temperature is set to 720 ° C. Further, when the soaking temperature is higher than 860 ° C., the precipitates become coarse and the strength decreases, so the upper limit of the soaking temperature is set to 860 ° C. Preferably it is 820 degrees C or less.

Soaking time: 30-300s
At the soaking temperature described above, it is necessary to hold for 30 s or longer in order to progress recrystallization and partially austenite transform. On the other hand, if the holding time is too long, the ferrite becomes coarse and the strength decreases, so it is necessary to set it to 300 s or less.

Average cooling rate to 510 ° C: 3-30 ° C / s
When the cooling rate is small, the strength decreases due to the coarsening of the ferrite, so an average cooling rate of 3 ° C./s or more is necessary. On the other hand, if the cooling rate is too high, the martensite fraction in the second phase increases and YS decreases, so the upper limit of the average cooling rate is set to 30 ° C./s.

[Temper rolling process]
When the yield point or yield elongation occurs, the temper rolling is carried out because the strength, particularly the variation in YS, increases.

Elongation rate of temper rolling: 0.3-2.0%
In order to eliminate the yield point and yield elongation, an elongation rate of 0.3% or more is required. However, even if the elongation rate is increased beyond 2.0%, not only the above effects are saturated but also the elongation is lowered, so the upper limit of the elongation rate is set to 2.0%.

  Within the scope of the present invention, in the annealing step, hot dip galvanization may be performed to obtain a hot dip galvanized steel sheet, or after hot dip galvanization, an alloying treatment may be performed to obtain an alloyed hot dip galvanized steel sheet. .

Examples of the present invention will be described below.
Steel of the components shown in Table 1 is melted and cast to produce a 230 mm-thick slab, hot rolling, pickling, cold rolling and annealing are performed under the manufacturing conditions shown in Table 2, and then a skin pass Rolling (temper rolling) was performed. The heating temperature during the hot rolling is 1200 ° C., the rapid cooling start time after finishing rolling is 0.1 s, the air cooling time at the rapid cooling end temperature (MT) is 2.5 s, and the thickness of the hot rolled sheet is 3 .2 mm, the thickness of the cold-rolled sheet was 1.4 mm, and the elongation by skin pass rolling was constant at 0.7%. In the table, FDT is the finish rolling finish temperature, CT is the coiling temperature, cooling speed 1 is the average cooling rate during quenching after finish rolling during hot rolling, and cooling speed 2 is the average cooling after soaking during annealing. Indicates speed.

  JIS No. 5 tensile test specimens were sampled from the vertical direction of rolling at 9 points at the center in the width direction and at both 1/4 width positions at the longitudinal tip, center, and tail end of the manufactured steel sheet, and subjected to a tensile test. Then, YS and TS and elongation El were measured, and the differences ΔYS and ΔTS between the maximum and minimum values of YS and TS were evaluated. The average values of YS, TS, and El and the values of ΔYS and ΔTS are also shown in Table 2.

  As shown in Table 2, steels A to N, which are within the scope of the present invention, have good strength such as TS ≧ 590 MPa, and ΔYS and ΔTS are both 30 MPa or less, and there is sufficient variation in strength within the coil. It was confirmed to be small.

On the other hand, it was confirmed that in steels Q to V whose composition is outside the scope of the present invention, ΔYS and ΔTS are large and the material stability is poor. Moreover, sufficient TS is not obtained with steel Q with low C and steel T with low Mn.

Claims (6)

  In mass%, C: 0.05 to 0.15%, Si: 0.06 to 0.7%, Mn: 1.0 to 2.5%, P: 0.01 to 0.05%, S: 0.0050% or less, Al: 0.01 to 0.10%, N: 0.0050% or less, Nb: 0.01 to 0.10%, Ta: 0.001 to 0.010%, the balance A high-strength thin steel sheet excellent in material stability, characterized in that is composed of Fe and inevitable impurities.   In addition to the components of claim 1, the composition further comprises one or more of V: 0.10% or less, Ti: 0.100% or less, and the balance is composed of Fe and inevitable impurities. High-strength thin steel sheet with excellent material stability.   In addition to the components of claim 1 or claim 2, further, in mass%, Cr: 0.5% or less, Mo: 0.5% or less, Cu: 0.50% or less, Ni: 0.50% or less, B: A high-strength thin steel sheet excellent in material stability, characterized by containing one or more of 0.0030% or less and the balance being made of Fe and inevitable impurities.   The high strength thin steel sheet excellent in material stability according to any one of claims 1 to 3, wherein the tensile strength TS is 590 MPa or more.   The steel slab having the composition according to any one of claims 1 to 3 is hot-rolled under conditions of a heating temperature of 1100 to 1270 ° C and a finish rolling finish temperature of 830 to 950 ° C, and within 1 s after the end of rolling. After cooling to 650-750 ° C. at an average cooling rate of 20-200 ° C./s, air-cooled at this temperature for 2 s or longer, wound up at 500-650 ° C., pickled, and then cold-rolled. Then, it is heated to 720-860 ° C. at an average heating rate of 3-30 ° C./s, the soaking time at 720-860 ° C. is 30-300 s, and the average cooling rate is in the range from the soaking temperature to 510 ° C. High strength thin steel sheet with excellent material stability, characterized by annealing under conditions of cooling at 3-30 ° C./s, followed by temper rolling at an elongation of 0.3-2.0% Manufacturing method. The tensile strength TS of the obtained thin steel plate is 590 MPa or more, The manufacturing method of the high strength thin steel plate excellent in material stability of Claim 5 characterized by the above-mentioned.