JPH04333524A - Production of high strength dual-phase steel sheet having superior ductility - Google Patents

Abstract

PURPOSE:To produce a high strength steel sheet excellent in ductility by regulating the components in a steel and the annealing method after cold rolling, respectively, and controlling the amount of retained austenite in a steel sheet. CONSTITUTION:A steel having a composition containing, by weight, 0.05-0.12% C, 0.5-3.0% Si, and 0.5-2.5% Mn is cold-rolled, heated up to a temp. in the two- phase region, and subjected, as subsequent cooling, to slow cooling down to 550-700 deg.C at a rate of 1-10 deg.C/sec, to rapid cooling down to 200-450 deg.C at a rate of 10-200 deg.C/sec, and then to holding at 300-450 deg.C for a proper time, by which the microstructure of the resulting steel sheet can be controlled. By this method, the steel sheet containing fetrite and bainite as main phases and further containing 3-10% austenite stable at room temp. can be produced, and the steel sheet combining high strength with high ductility can be produced. Moreover, the thickness of an automobile steel sheet for working can be reduced, and this steel sheet can contribute to the lightening in weight of the automobile body.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing high-strength steel sheets having excellent formability.

0002

BACKGROUND OF THE INVENTION In recent years, there has been an increasing demand for greater comfort and safety of automobiles, as well as for lighter vehicle bodies. This is a demand for energy conservation and environmental issues considered on a global scale, and aims to improve vehicle fuel efficiency through weight reduction and reduce harmful exhaust gases such as CO2. In order to achieve this goal, it is necessary to improve the strength of the materials used in the car body structure and reduce the thickness of the materials.
It is necessary to use new materials with low specific gravity. When using new low-density materials (e.g. Al, Mg, etc.), from the viewpoint of price and stable supply, it is considered that they must be used in coexistence with steel sheets, which have traditionally been used as the main material for car bodies. . In this case, the most important problem is the recycling of scrap, and steel plate scrap mixed with other materials needs to be reused after spending a lot of energy and cost. Therefore, in order to minimize energy and preserve the environment for the entire planet, it is extremely important to use a single material (i.e., steel) to reduce weight, except for special parts, and it is expected that steel materials will have even higher strength. has been done. In addition to the above requirements, integral molding of vehicle body components is considered to be an important technical requirement for simplifying and continuous manufacturing processes. Among the steel materials used in such modernized forming processes, especially considering thin steel sheets, the criterion for selecting the steel sheet is that it has good formability. The formability of a thin steel sheet is judged by its elongation, Lankford plastic strain ratio (r value), work hardening index (n value), and yield strength.
One necessary condition is that the value is high. Dual Phase (DP) steel, which has a two-phase structure of ferrite and martensite, is conventionally known as an example of a steel plate with a large elongation or a large n value. As shown in Japanese Patent Publications No. 56-18051 and Japanese Patent Publication No. 59-45735, DP steel can obtain a maximum total elongation of about 30-35% at 50-80 kgf/mm2. However, it cannot be said that there is a sufficient strength-ductility balance when applied to parts that require complicated machining, where thin steel sheets with relatively low strength (35 to 45 kgf/mm2) have conventionally been used. In addition, the existing continuous annealing line with an overaging zone produces 50 to 60 kgf/mm.
There is also the drawback that process conditions suitable for manufacturing DP steel of about 2 cannot be achieved.

[0003] As a method for further improving the quality of this material, a high-strength composite steel sheet having a microstructure of a mixed structure of ferrite, bainite, and austenite (or partially containing martensite) has recently been proposed. This steel sheet utilizes "transformation-induced plasticity," which exhibits high ductility due to the austenite remaining at room temperature transforming into martensite during forming. Steels that utilize transformation-induced plasticity are known as TRIP steels, such as those proposed by Zackay et al. (V.F. Zackay et al.: T
rans. ASM vol. 60 (1967) 252
), high ductility of up to about 90% is achieved at 70 kgf/mm2 or more. However, such T
RIP steel does not necessarily meet the requirements here, as it requires the addition of large amounts of expensive alloying elements. As a solution to these problems, a method for producing thin steel sheets suitable for inexpensive uses that require mass production, such as steel sheets for automobiles, has been proposed, as described in Japanese Patent Application Laid-Open No. 61-157625. ing. However, since the aim is to have a large amount of austenite present, it is assumed that a large amount of carbon is contained, and conventionally the strength is relatively low (35 to 45 kgf/mm).
The strength is too high to be applied to the parts where thin steel plates in 2) were used, and there is very little possibility that it will be put to practical use. The technology described in this patent publication suppresses the precipitation of carbides by adding Si and promotes ferrite transformation, thereby effectively enriching carbon in untransformed austenite and stabilizing the austenite. It is. JP-A-1-168819 is an invention that applies this technique to a lower intensity region. This invention shows that even if the lower limit of carbon concentration is expanded to 0.05 wt%, a high-strength steel plate with excellent ductility can be obtained.

0004

[Problem to be solved by the invention] The above-mentioned Japanese Patent Application Laid-Open No. 1-168
The invention of Publication No. 819 requires that 10% or more of austenite remain at room temperature even in a steel sheet with a very low carbon concentration. Satisfying this condition is not necessarily easy.

[0005]

[Means for Solving the Problems] The present inventors have set manufacturing conditions that increase the addition of Si and the amount of ferrite produced even in steel sheets with a relatively low carbon content, thereby achieving a relatively low austenite content. We have discovered a method for producing a high-strength composite steel sheet with excellent ductility of 50 to 70 kgf/mm2, which can utilize residual transformation-induced plasticity. That is, (1) C: 0.05-0.12%, Si: 0.5-3.
00%, Mn: 0.5 to 2.50%, and the remainder Fe and unavoidable impurities, after cold rolling, Ac1 to Ae
3 Heating to the transformation temperature range, then 1 to 10℃/sec
Cooled to a temperature range of 550 to 700°C at a cooling rate of
A method for producing a high-strength steel sheet with excellent ductility, characterized by having ferrite and bainite as main phases and further containing retained austenite in a volume fraction of 3 to 10%, by holding for 0 minutes and cooling to room temperature. . (2) C: 0.05-0.12%, Si: 0.5-3.
00%, Mn: 1.5 to 2.50%, and the balance is Fe and unavoidable impurities after cold rolling.
3 Heating to the transformation temperature range, then 1 to 10℃/sec
Cooled to a temperature range of 550 to 700°C at a cooling rate of
A method for producing a high-strength steel sheet with excellent ductility, characterized by having ferrite and bainite as main phases and further containing retained austenite in a volume fraction of 3 to 10%, by holding for 0 minutes and cooling to room temperature. . (3) The highly ductile steel according to claim 1 or 2, characterized in that one or more of Ni, Cr, Cu, Mo, Nb, and Ti are added in a total amount of 2% or less. This is a method for manufacturing a high-strength steel plate.

[0006]

[Function] Conventionally reported high-strength steel sheets containing retained austenite contain a relatively large amount of carbon in order to maximize the amount of residual austenite for the purpose of significantly improving ductility through the TRIP effect. This is N
In this type of steel that does not contain a large amount of austenite-stabilizing elements such as This is because it is thought that the average carbon concentration contained almost determines the amount of residual austenite finally obtained. However, increasing the carbon concentration in the steel plate is equivalent to increasing the hardenability of the steel plate, and therefore the strength of the steel plate inevitably increases. 50 to 70 kgf/mm targeted by this invention
Therefore, in order to produce a relatively low-strength, high-strength steel plate with a tensile strength of did. On the premise that austenite remains, it can be said that the tensile strength of a cold-rolled and annealed steel sheet to which Si and Mn are added is determined by the alloy addition amount and annealing conditions for each C content. A summary of the experimental results that have formed the basis of the present invention yields the relationship shown in FIG. 1. That is, it has been found that in order to industrially stably obtain a strength of less than 70 kgf/mm2, it is necessary to limit the carbon content to 0.12% or less. Further, the relationship between the ductility (elongation at break El) and the amount of retained austenite obtained under these conditions is shown in FIG. It has been found that in order to stably obtain an elongation at break of 35% or more, the amount of retained austenite is in the range of 3 to 10%.

[0007] The details of the operation of the important elements of the present invention will be described below. C: As mentioned above, the target of the present invention is 50 to 70 kgf/m
Since we are manufacturing steel plates in the relatively low strength range of m
If it exceeds 0.12%, it is difficult to achieve the objective, so this is set as the upper limit. At this time, another way to reduce the strength of the steel sheet is to reduce the amounts of Mn and Si, which are other additive elements, but as will be shown later, reducing these alloying elements at the same time will reduce the austenite. This is disadvantageous for remaining at room temperature. Also,
In the present invention, it is also an important technical element to avoid cost disadvantages by avoiding the addition of expensive alloying elements as much as possible. Therefore, in order to stabilize austenite and make it remain at room temperature, it is essential to enrich the austenite with carbon. In order for austenite stabilized by carbon enrichment to improve ductility through the TRIP effect and have a clear advantage over other conventional steels, it is necessary to have a carbon content of at least 0.05%. Set as the lower limit of carbon.

Si: Si is the most important additive element for making austenite so carbon-enriched that it is stable even at room temperature. The core of the technology of the present invention is to heat a steel sheet to a ferrite/austenite two-phase region and to advance ferrite transformation during cooling, thereby enriching carbon in austenite. As the carbon concentration increases, carbide formation becomes more likely to occur, pearlite is formed at high temperatures and upper bainite is formed at low temperatures, reducing the total carbon content in austenite and, as a result, the amount of retained austenite. becomes. As is well known, Si does not form a solid solution in carbides (cementite here), and therefore has the function of significantly delaying the formation of carbides. This enables efficient carbon enrichment into austenite without wasting carbon atoms in the form of carbides. 0 for this work
．． It is essential to add 5% or more of Si. At this time, Si dissolves in the ferrite and strengthens the ferrite, so
Addition of an unnecessarily large amount leads to a decrease in workability of the steel sheet. Therefore, the amount added was limited to 3% or less.

Mn: Like Si, Mn is also an additive element that contributes to the retention of austenite because it has the function of delaying the formation of carbides. In addition to this, Mn addition lowers the martensitic transformation start temperature of austenite. In order to make austenite stable at room temperature, it is necessary to suppress the precipitation of carbides and increase the carbon concentration in austenite as described above, but at the same time, it is also important to lower the martensite transformation start temperature of the austenite. If the martensitic transformation temperature is higher than room temperature, a portion of austenite will inevitably transform into martensite, increasing the strength of the steel sheet and deteriorating its ductility. For this purpose, a Mn concentration of 0.5% or more is required, but addition of 1.5% or more is particularly effective in significantly lowering the martensitic transformation temperature and allowing austenite to remain at room temperature. be. However, addition of a large amount of Mn unnecessarily increases the hardenability of the steel sheet and increases the possibility of deterioration of ductility as well as increase in strength. Therefore, the upper limit of Mn is set to 2.5%.
shall be.

[0010] Other additive elements: Ni and Cr can stabilize austenite by adding them alone or in combination, and are considered to be advantageous for retaining austenite. It is also effective to add elements such as Mo that increase the hardenability of steel. Nb and Ti are carbides (Nb(C,N)
, Ti (C, N), etc.) is added to adjust the strength by precipitation strengthening. Further, since Cu, like Si, is difficult to form a solid solution in cementite, it is used to delay the start of carbide precipitation, assist the progression of carbon concentration in austenite, and at the same time adjust the strength. However, adding more of these alloying elements than necessary not only increases the manufacturing cost of the steel sheet, but also causes deterioration of ductility as the strength increases, so the total addition is limited to 2% or less.

Heat treatment conditions after cold rolling: The annealing heating temperature of the cold rolled steel sheet is in the ferrite/austenite two-phase region (Ac1 to A
e3), it is necessary to control the recrystallization of processed ferrite, the amount of transformed austenite, and component concentration. The final microstructure of a steel plate with good ductility of 50 to 70 kgf/mm2, which is the object of the present invention, is a composite structure in which the main phase is ferrite, ferrite, bainite, and retained austenite (and may include some martensite). . It has been found that good ductility depends not only on the presence of retained austenite but also on the presence of a soft ferrite phase, so it is important to control the cooling that follows the heating in the two-phase region to allow the ferrite transformation to proceed sufficiently. It is. For this purpose, in order to increase the amount of ferrite transformation as much as possible after heating in the two-phase region, a cooling rate of 1 to 10 °C/s was used to
Cool to 0°C to 700°C. The lower limit of the cooling rate is 1° C./second, which is the lowest cooling rate that can be achieved in actual operation. Further, since cooling in this temperature range at a cooling rate of 10° C./second or more inhibits sufficient progress of ferrite transformation, this is set as the upper limit. Cooling at 1-10℃/sec to 550℃
When the following steps are carried out, not only ferrite but also pearlite is formed, consuming carbon in the steel in the form of carbide (cementite), greatly inhibiting the concentration of carbide into the final remaining austenite, and When this cooling is completed at 700° C. or higher, the amount of ferrite transformation obtained is not sufficient. After obtaining a sufficient amount of ferrite transformation in this way, the steel plate is cooled at a cooling rate of 10 to 200°C/sec.
Cooled to 00-450°C. At this time, since pearlite formation was observed at a cooling rate of 10° C./second or less, this was set as the lower limit. In addition, the upper limit cooling rate was determined from the cooling rate that could be achieved in the actual operating line. Furthermore, setting the cooling stop temperature to 200°C or lower may cause martensitic transformation and must be avoided; if it exceeds 450°C, carbide (cementite) will precipitate at the same time as bainite transformation, leaving austenite to remain at room temperature. These are set as upper and lower limits. It is clear from the equilibrium phase diagram that the carbon concentration in untransformed austenite at this stage is not high enough to be stable at room temperature. Therefore, it is necessary to further increase the carbon concentration in austenite by the subsequent bainite transformation treatment (austenburization treatment). The temperature for the bainite transformation treatment at this time should be avoided at 450° C. or higher, at which the formation of carbides is observed, and should be set at 300° C. or higher, at which the formation of lower bainite containing carbides in ferrite is not observed. It has been confirmed that at least 15 seconds or more is required at these appropriate bainite transformation treatment temperatures to obtain a bainite transformation amount that is of substantial significance for carbon enrichment in untransformed austenite. However, holding it for an unnecessarily long time leads to a decrease in the carbon concentration in austenite due to the transformation of untransformed austenite to pearlite and the precipitation of carbides from untransformed austenite, and as a result, the bainite transformation continues to progress and the remaining austenite is reduced. Not expected. In order to avoid this, the maximum holding time at the bainite transformation treatment temperature is limited to 20 minutes or less.

Austenite content in the final microstructure: The retained austenite that is finally contained in the steel sheet transforms into martensite during the tensile test of the steel sheet, thereby increasing the ductility (particularly the (elongation). This is called TRIP (transformation-induced plasticity), and it is generally said that the ductility obtained can be determined by the amount of retained austenite. However, the deformation-induced transformation of retained austenite is greatly influenced by the stability of austenite, and a steel plate containing a large amount of retained austenite does not necessarily exhibit high ductility. At this time, the biggest factor governing the stability of retained austenite is the martensitic transformation temperature (Ms
). If the Ms of retained austenite is near room temperature, most of the martensitic transformation of austenite is completed with slight deformation, so a TRIP effect cannot be expected in a high strain region, and as a result, only small ductility is exhibited. On the other hand, when the Ms of retained austenite is very low temperature, local deformation of the steel sheet progresses before deformation-induced transformation of austenite occurs, and no TRIP effect can be expected. The Ms temperature of such retained austenite is determined by the chemical components in the austenite, and for substitutional additive elements, concentration occurs due to diffusion accompanying ferrite transformation during annealing in the two-layer region or during slow cooling from the annealing temperature. (or thinning), its concentration changes. However, because the annealing time is short and the distribution of substitutional elements is small during ferrite transformation except in the extremely high temperature range, it is assumed that the retained austenite contains substitutional elements that are close to the average concentration of the steel sheet. Conceivable. Therefore, it can be said that Ms in retained austenite is determined by its carbon concentration. The importance of carbon concentration in retained austenite is also understood from the fact that the influence of carbon concentration on Ms of austenite is much greater than that of other elements. Although it is technically possible to contain 10% or more retained austenite in the 0.05 to 0.12 wt% C steel sheet targeted by the present invention, it is possible to contain carbon in the retained austenite obtained at that time. It was found that the concentration was about 1 wt % or less, and that it transformed into martensite with slight deformation and did not exhibit the TRIP effect in the high strain region. Further, when the amount of retained austenite is 3% or less, the superiority over conventionally known steel plates such as DP steel is very small. Therefore, it is important to control the amount of retained austenite in the final steel plate to 3 to 10%.

[0013]

[Example] For each steel type shown in Table 1, 3
．． After making the thickness 0mm, cooling and winding (400℃~650℃
The hot rolled steel sheets were cold rolled to a thickness of 1.0 mm and then annealed, and mechanical properties were investigated and retained austenite was quantified. The annealing conditions were the thermal cycle shown in Figure 1, and the holding time at the annealing temperature (ST/℃) was 90
Seconds. After annealing, the steel plate is cooled to T1°C at a rate of 5-8°C/sec, and then cooled to T2°C at a cooling rate of 80-140°C/sec.
The mixture was cooled to room temperature, maintained isothermally at that temperature for a predetermined period of time, and then cooled to room temperature. Table 2 shows the mechanical properties and annealing conditions of the steel plate obtained by annealing. In addition, Vg% in the same table represents the volume percentage of retained austenite in the steel sheet. The table shows that the steel plate that satisfies the conditions of the present invention (indicated as the steel of the present invention in the table) has a target strength of 50 to 70 kgf/mm2 and an excellent elongation at break of 35% or more.

[0014]

[Table 1]

[0015]

[Table 2A]

[0016]

[Table 2B]

[0017]

Effects of the Invention As described above, by using the present invention, the relatively low strength (35 to 45 kg) currently used can be improved.
f/mm2) can be used as a replacement for thin steel sheets.
It becomes possible to manufacture high-strength steel sheets with excellent ductility of 0 kgf/mm2), and by applying it to automobile parts, it can greatly contribute to reducing the weight of automobile bodies.

[Brief explanation of the drawing]

FIG. 1 shows a thermal cycle of annealing after cold rolling.

FIG. 2 shows the influence of steel sheet carbon (C) concentration on the strength of a cold rolled and annealed steel sheet containing retained austenite.

FIG. 3 shows the relationship between the elongation at break and the amount of retained austenite of steel sheets obtained in the carbon range shown in FIG.

Claims (3)

[Claims] Claim 1: C: 0.05 to 0.12% by weight;
Si: 0.5-3.00%, Mn: 0.5-2.50%
A steel material consisting of Fe and unavoidable impurities,
After cold rolling, it is heated to a transformation temperature range of Ac1 to Ae3, then cooled to a range of 550 to 700 °C at a cooling rate of 1 to 10 °C/sec, and then cooled to a range of 200 to 450 °C at a cooling rate of 10 to 200 °C/sec. After cooling to 300-45℃
By keeping it in the temperature range of 0℃ for 15 seconds to 20 minutes and cooling it to room temperature, the main phase is ferrite and bainite,
A method for producing a high-strength steel sheet having excellent ductility, characterized in that the steel sheet further contains retained austenite in a volume fraction of 3 to 10%. [Claim 2] C: 0.05 to 0.12% by weight;
Si: 0.5-3.00%, Mn: 1.5-2.50%
A steel material consisting of Fe and unavoidable impurities,
After cold rolling, it is heated to a transformation temperature range of Ac1 to Ae3, then cooled to a range of 550 to 700 °C at a cooling rate of 1 to 10 °C/sec, and then cooled to a range of 200 to 450 °C at a cooling rate of 10 to 200 °C/sec. After cooling to 300-45℃
By keeping it in the temperature range of 0℃ for 15 seconds to 20 minutes and cooling it to room temperature, the main phase is ferrite and bainite,
A method for producing a high-strength steel sheet having excellent ductility, characterized in that the steel sheet further contains retained austenite in a volume fraction of 3 to 10%. [Claim 3] Ni, Cr, Cu, Mo, Nb, Ti
3. The method for manufacturing a high-strength steel sheet with excellent ductility according to claim 1 or 2, characterized in that one or more of these additive elements are added in a total amount of 2% or less.