JP4333379B2 - Method for producing high-strength thin steel sheet with excellent workability, surface texture and flatness - Google Patents

Description

The present invention, automobile bodies, reinforcements, wheels, undercarriage components, a method of manufacturing other optimal high strength thin steel sheet as any mechanical structural parts.

  In order to protect the global environment and improve the safety of passengers, high strength and thinning of steel sheets for automobiles are being studied. However, generally, when the strength of the material is increased, the press formability is lowered. Therefore, improvement of formability is cited as one of the important issues in expanding the application of high-strength steel sheets.

  As a response to the above, there is conventionally a dual-phase steel sheet (otherly called dual phase steel, DP steel, composite structure steel, etc.) having ferrite and martensite as the main phase, and the duplex steel sheet has a yield ratio ( (Hereinafter referred to as YR) and high elongation, it has excellent press formability (drawing formability, shape freezing property), and has been attracting attention and developed as an automotive material.

  For example, the formation of a two-phase structure in a hot-rolled steel sheet increases the hardenability by concentrating solute elements in the remaining austenite by precipitating a large amount of equiaxed ferrite in the cooling process after hot rolling. It is done by making it.

  However, when winding is performed at a high temperature of approximately 350 ° C. or more, austenite undergoes bainite transformation after winding, and the target two-phase structure of ferrite and martensite cannot be obtained. On the other hand, when the coiling temperature is low, there is a problem that the plate shape deteriorates due to cooling heat distortion. In order to solve these problems, Cr-added steel was developed. In addition, in order to promote the formation of ferrite in the cooling process after hot rolling, steels with a large amount of ferrite stabilizing elements such as Si and P were developed. As a cooling method after hot rolling, a so-called two-stage cooling method is proposed in which the cooling is temporarily stopped in the vicinity of the point A1 where the precipitation of ferrite is promoted and held for about 10 seconds. Patent Documents 1 to 3 disclose methods combining these techniques.

Furthermore, Patent Documents 4 to 6 disclose a method of performing rapid cooling immediately after hot rolling. In particular, in Patent Document 4, the above method is used for low Si-containing steel.
JP-A-4-289126 JP 9-67641 A Japanese Patent Laid-Open No. 10-195588 JP 2002-69534 JP Japanese Patent Laid-Open No. 2001-192736 JP 2001-355023

  However, although Patent Documents 1 to 3 all have good mechanical properties, it is necessary to add a large amount of Si, P, and Al. Therefore, surface properties deteriorate due to red scale generation, paintability deterioration and weldability. There is a problem of deterioration, and its application range is limited.

  In Patent Documents 4 and 5, YR is not considered.

  Further, since Patent Document 6 is a technique for producing high-concentration Si-added steel, the surface properties of the steel sheet are inferior. Although it is conceivable to lower Si in order to improve the surface properties, when the Si is lowered, the YR characteristics are inferior this time. Thus, both YR and surface properties cannot be satisfied.

The present invention has been made in view of the above circumstances, and a method for improving the mechanical properties of a steel sheet without adding a large amount of ferrite stabilizing elements (Si, P, Al) that adversely affect surface properties, weldability, and the like. by developing, processability (YR: 0.6 or less), and an object thereof is to provide a method for producing a high strength thin steel sheet excellent in surface properties and strip flatness. It should be noted that YR needs to be 0.6 or less in order to increase the strain resolution and to improve the shape freezing property.

  The present inventors have intensively studied to solve the above problems. As a result, after hot rolling, ultra-rapid cooling at 150 ° C / second or more within 2 seconds and holding at 750 to 650 ° C for a certain period of time, even when a large amount of ferrite stabilizing element is not added, As compared with the stage cooling method, a phenomenon in which the formation of fine ferrite was remarkably promoted was found, and this was applied to the production of a two-phase hot-rolled high-strength steel sheet to complete the present invention.

  The present invention has been made based on the above findings, and the gist thereof is as follows.

[ 1 ] In mass%, C: 0.05 to 0.15%, Si: 0.5% or less, Mn: 1.0 to 1.8%, Cr: 0.5 to 1.5%, P: 0.06% or less, S: 0.01% or less, N: 0.005% Hereinafter, SolAl: 0.01 to 0.1% contained, the balance is Fe or unavoidable impurities after casting, directly or heated, hot-rolled at an Ar3 point temperature or higher, then hot-rolled 2 Cool to 750-600 ° C at a cooling rate of 150 ° C / second or more within 2 seconds, then hold at a temperature range of 750-600 ° C for 2-15 seconds, then cool at a cooling rate of 20 ° C / second or more, A method for producing a high-strength thin steel sheet excellent in workability, surface properties and plate flatness, characterized by winding at a temperature of 350 to 550 ° C.

[ 2 ] In mass%, C: 0.05 to 0.15%, Si: 0.5% or less, Mn: 1.0 to 1.8%, Cr: 0.5 to 1.5%, P: 0.06% or less, S: 0.01% or less, N: 0.005% Hereinafter, including SolAl: 0.01-0.1%, further containing Mo: 0.3% or less, Nb: 0.05% or less, Ti: 0.1% or less, B: 0.002% one or more, the balance is Fe and After casting a slab composed of inevitable impurities , directly or by heating, perform hot rolling at an Ar3 point temperature or higher, then 750-600 ° C at a cooling rate of 150 ° C / second or more within 2 seconds after the end of hot rolling Next, after maintaining at a temperature range of 750 to 600 ° C for 2 to 15 seconds, cooling at a cooling rate of 20 ° C / second or more and winding at a temperature of 350 to 550 ° C A method for producing a high-strength thin steel sheet having excellent surface properties and flatness.

Contact name herein,% indicating the components of the steel are all mass%.

  In the present invention, the high-strength thin steel sheet is a thin steel sheet having a tensile strength suitable for machine structural parts exceeding 590 MPa.

  According to the present invention, a high-strength thin steel sheet excellent in workability, surface properties, and plate flatness can be obtained. Thus, the thin steel sheet obtained by the present invention has high strength and low YR (0.6 or less), high ductility, excellent press formability, and excellent surface properties and spot weldability, so it is easy to use. It can be used for automobile parts and machine structural parts. Moreover, since it can be manufactured in the same process as a conventional soft steel plate and good performance can be obtained without adding a special element, the manufacturing cost can be reduced. Therefore, it is expected to be widely put into practical use in the future, and it is thought that it contributes to the development of society through the preservation of the global environment and the improvement of safety by reducing the weight of automobiles.

  The high-strength thin steel sheet of the present invention is characterized in that the components are defined as shown below, the equiaxed ferrite volume fraction is 60% or more, and the martensite volume fraction is 5 to 30%. It is the most important requirement. By defining the components and the structure in this way, a high-strength thin steel plate excellent in workability, surface properties, and plate flatness can be obtained. The high-strength thin steel sheet is hot-rolled at an Ar3 point temperature or higher, then cooled to 750-600 ° C at a cooling rate of 150 ° C / second or more within 2 seconds after the hot rolling is completed, It is possible to manufacture by holding in a temperature range of 750 to 600 ° C. for 2 to 15 seconds, cooling at a cooling rate of 20 ° C./second or more, and winding at a temperature of 350 to 550 ° C. Thus, in the manufacturing method, after hot rolling, it is also an important requirement in the present invention to perform ultra-rapid cooling at 150 ° C./second or more within 2 seconds and hold at 750 to 650 ° C. for a certain time.

  Hereinafter, the present invention will be described in detail.

  First, the reasons for limiting the chemical components of the steel sheet of the present invention will be described.

C: 0.05-0.15%
C is an important element for strengthening the martensite phase, and it is necessary to add 0.05% or more in order to achieve a sufficient effect. On the other hand, when the addition amount exceeds 0.15%, austenite is stabilized, it becomes difficult to form a two-phase, and ductility is lowered. From the above, C is made 0.05% to 0.15%. In consideration of spot weldability, if the addition amount is less than 0.07%, the tensile shear strength may decrease, so 0.07% or more is preferable. Further, if it exceeds 0.10%, the cross tensile strength may decrease, so 0.10% or less is preferable.

Si: 0.5% or less
Si not only deteriorates the surface properties due to the red scale, but also deteriorates the paintability and weldability. If it exceeds 0.5%, the adverse effect of Si becomes remarkable. From the above, Si is 0.5% or less. In applications where surface properties are particularly important, Si is preferably less than 0.25%.

Mn: 1.0-1.8%
Mn plays an important role in the formation of a two-phase structure in order to suppress the formation of pearlite during cooling after hot rolling. If it is less than 1.0%, the effect is not sufficient, pearlite is generated, YR increases, and press formability deteriorates. On the other hand, if it exceeds 1.8%, austenite is excessively stabilized, and the formation of equiaxed ferrite is prevented. From the above, Mn is 1.0% or more and 1.8% or less. Note that Mn is preferably set to 1.3% or more in order to suppress variation in strength in the coil.

Cr: 0.5-1.5%
Cr is an important element for suppressing the bainite transformation of austenite during the cooling process of the coil wound and achieving the target ferrite and martensite two-phase structure. If it is less than 0.5%, this effect is not sufficient. On the other hand, if it exceeds 1.5%, the ferrite transformation during the primary cooling after hot rolling is delayed and the steel sheet properties are deteriorated. From the above, Cr is 0.5% to 1.5%. Further, if it exceeds 1.2%, chemical conversion processability may be deteriorated, so Cr is preferably 1.2% or less. Further, in order to suppress the intensity variation in the coil, Cr is preferably 0.8% or more.

P: 0.06% or less
Since P deteriorates the toughness of the welded portion, it lowers the joint strength of the welded portion. If it exceeds 0.06%, this adverse effect becomes significant. Therefore, P is set to 0.06% or less.

S: 0.01% or less
S is an impurity contained in the crude steel, and deteriorates the formability and weldability of the raw steel plate. Therefore, it is desirable to remove and reduce it as much as possible in the steelmaking process. However, if S is reduced more than necessary, the refining cost increases, so S is made 0.01% or less, which is substantially harmless.

N: 0.005% or less
N is an impurity contained in the crude steel and deteriorates the formability of the raw steel plate. Therefore, it is desirable to remove and reduce it as much as possible in the steel making process. However, since refining costs increase if N is reduced more than necessary, N is made 0.005% or less, which is substantially harmless.

SolAl: 0.01-0.1%
Al is added to deoxidize and precipitate N as AlN. If it is less than 0.01%, the effect of deoxidation / denitrification is not sufficient. On the other hand, if it exceeds 0.1%, the effect of Al addition is saturated and uneconomical. From the above, SolAl should be 0.01% or more and 0.1% or less.

  In addition, the steel of the present invention can achieve the desired characteristics with the above-mentioned essential additive elements, but in addition to the above-mentioned essential additive elements, Mo, Nb, Ti, B may be used alone or as needed to increase the strength. Two or more kinds may be added. In that case, if each added amount exceeds 0.3%, 0.05%, 0.1%, 0.002%, the formation of the two-phase structure is hindered, and the mechanical properties deteriorate (YR increases or elongation decreases). In this case, Mo is 0.3% or less, Nb is 0.05% or less, Ti is 0.1% or less, and B is 0.002% or less.

As non avoidable impurities, eg, O because an adverse effect on the quality to form a nonmetallic inclusion, O is desirably reduced to 0.003% or less.

  Next, the reason for limiting the metal structure of the present invention will be described.

  First, the equiaxed ferrite volume fraction is set to 60% or more. The equiaxed ferrite volume fraction is extremely important for the expression of the low YR characteristic that is a feature of the present invention. In order to make YR 0.6 or less, the volume ratio of equiaxed ferrite needs to be 60% or more. For the above reason, it is preferably 95% or less.

  Next, the martensite volume ratio is set to 5 to 30%. Since the martensite volume fraction affects the strength, ductility, and low YR characteristics, it is an important requirement in the present invention, as with the equiaxed ferrite volume fraction. When the martensite volume fraction is less than 5%, the strength is low and low YR characteristics cannot be obtained. On the other hand, if it exceeds 30%, the ductility decreases. Therefore, the martensite volume ratio is set to 5% or more and 30% or less. In order to obtain better low YR characteristics, the martensite volume ratio is preferably 10% or more and 20% or less. The remaining structure is acicular ferrite, bainite, pearlite, etc., but if the volume ratio of equiaxed ferrite and martensite is in the above range, the effect of the present invention is exhibited, so the volume ratio of the remaining structure is particularly limited. do not do.

  Next, the manufacturing method of the high-strength thin steel plate excellent in workability, surface properties and plate flatness of the present invention will be described.

  The high-strength thin steel sheet of the present invention, after casting a slab adjusted to the above chemical composition range, directly or by heating, performs hot rolling at an Ar3 point temperature or higher, and then within 2 seconds after the end of hot rolling. Cool to 750-600 ° C. at a cooling rate of 150 ° C./second or higher, then hold at a temperature range of 750-600 ° C. for 2-15 seconds, then cool at a cooling rate of 20 ° C./second or higher, 350-550 It is obtained by winding at a temperature of ° C.

  In the above, the casting method of a slab is not limited. In the case of continuous casting, it may be directly hot-rolled as it is, or may be reheated after cooling and hot-rolled.

  Hot rolling is performed at an Ar3 point temperature or higher. At temperatures below the Ar3 point, hot rolling is performed in the two-phase region of ferrite and austenite, which prevents the formation of equiaxed ferrite, increases YR, and decreases ductility.

  Within 2 seconds after the end of hot rolling, it is cooled to a holding temperature of 750 to 600 ° C. at a cooling rate of 150 ° C./second or more. The primary cooling immediately after the hot rolling is the most important requirement for the effect of the present invention (low YR effect by promoting the formation of equiaxed ferrite). By defining primary cooling in this way and performing immediate and rapid cooling, it is possible to dramatically accelerate fine precipitation of equiaxed ferrite in the holding at 750 to 600 ° C, which is performed next to primary cooling. It becomes. If the time from the end of hot rolling to the start of cooling exceeds 2 seconds, ferrite is generated nonuniformly at the austenite grain boundaries, preventing precipitation of equiaxed ferrite during holding after cooling. If the cooling rate is less than 150 ° C./second, the heterogeneous precipitation of ferrite on the austenite grain boundaries during cooling cannot be suppressed, and the precipitation of equiaxed ferrite during holding after cooling is prevented.

  After primary cooling, hold in the temperature range of 750-600 ° C for 2-15 seconds. If the holding temperature range exceeds 750 ° C., the driving force for ferrite formation is small, and the precipitation promoting effect cannot be obtained. On the other hand, if the temperature is lower than 600 ° C., ferrite precipitation limited by the diffusion of Fe atoms is delayed and sufficient equiaxed ferrite formation cannot be obtained. Also, if the holding time is less than 2 seconds, the ferrite precipitation time is not sufficient, and low YR characteristics cannot be obtained. On the other hand, if the holding time is longer than 15 seconds, the generation of pearlite starts and the mechanical properties deteriorate.

  After holding, secondary cooling is performed at a cooling rate of 20 ° C./second or more, and winding is performed at a temperature of 350 to 550 ° C. or less. The cooling rate in the secondary cooling needs to be 20 ° C./second or more in order to suppress the formation of pearlite and bainite during cooling. When the coiling temperature exceeds 550 ° C, the pearlite transformation cannot be suppressed. On the other hand, when the coiling temperature is less than 350 ° C., the flatness of the plate deteriorates due to cooling thermal distortion. From the above, the winding temperature is 350 to 550 ° C. Furthermore, the coiling temperature is preferably 450 ° C. or higher in order to improve the temperature control of the secondary cooling end temperature and suppress the intensity variation in the coil.

  The high-strength thin steel sheet of the present invention obtained as described above may be subjected to skin pass rolling for further shape correction. Various surface treatments such as hot dip galvanization or electrogalvanization may be performed using the developed steel plate as a base.

  Slabs having chemical components shown in Table 1 were continuously cast and then cooled, then heated to 1100 to 1300 ° C., and finally rolled at an Ar 3 point temperature of 850 ° C. to a thickness of 1.6 to 3.0 mm. Next, within 1 second after the end of the final rolling, it is cooled to 680-720 ° C at a cooling rate of 300-500 ° C / second, held for 7-12 seconds within the same temperature range, and then cooled at 25-30 ° C / second And it wound up at 370-420 degrees C or less, and obtained the hot-rolled steel plate. However, in Steel No. 4, the primary cooling rate stop temperature was set to 550 ° C., and in Steel No. 5, the coiling temperature was set to 570 ° C. to adjust the structure shown in Table 1.

  Mechanical properties, surface properties, spot weldability, and plate flatness were evaluated for the hot-rolled steel plates obtained as described above. The results obtained are shown in Table 2. Each evaluation method is as follows. For mechanical properties, JIS No. 5 tensile test specimens were sampled at right angles to the rolling direction and tested according to JISZ2241. : The surface property was visually determined for the presence or absence of a red scale. Spot weldability is ◯ when the base material breaks in the form of fracture in the chisel test after spot welding under the condition that a 5 × √thickness (mm) nugget is formed, and × when the weld breaks. Judged. The flatness of the steel sheet was evaluated based on the wave height.

  Table 2 shows that the steels of the present invention are all excellent in mechanical properties (YR: 0.6 or less), and have good surface properties, weldability and plate flatness. Steel Nos. 12 and 20 were judged to have practically no problem although the surface properties were slightly deteriorated because the Si concentration was slightly high.

  On the other hand, Steel No. 1, which is a comparative example, has a low C concentration outside the scope of the present invention, so the martensite hardness is insufficient and, as a result, the YR is high. In Steel Nos. 4 and 5, the equiaxed ferrite volume fraction or martensite volume fraction is outside the range of the present invention, so that a good two-phase structure cannot be obtained and YR is high. Steel No. 9 has a high C concentration outside the scope of the present invention, so that ferrite formation is delayed, a good two-phase structure cannot be obtained, and YR is high. Also, the weldability deteriorated. Steel No. 13 had a high Si concentration outside the range of the present invention, so a red scale was generated and the surface properties were poor. In Steel No. 14, the Mn concentration is low outside the range of the present invention, so that austenite becomes unstable and pearlite is generated in the cooling stage, so that YR is high. Since Steel No. 17 has a high Mn concentration outside the range of the present invention, the production amount of equiaxed ferrite is small and the YR is high. Steel No. 18 has a low Cr concentration outside the range of the present invention, so that austenite undergoes bainite transformation after winding, and therefore YR is high. Steel No. 19 has a high Cr concentration outside the range of the present invention, so that equiaxed ferrite is not sufficiently generated during primary cooling and holding. As a result, a two-phase structure cannot be obtained and YR is high. Since Steel No. 21 had a high P concentration outside the range of the present invention, spot weldability was significantly deteriorated.

  Using a part of the slab having the chemical components shown in Table 1, hot rolling, cooling and winding were performed under the production conditions shown in Table 3 to obtain a hot rolled steel sheet.

  Mechanical properties, surface properties, spot weldability, and plate flatness were evaluated for the hot-rolled steel plates obtained as described above. The results obtained are shown in Table 4. Each evaluation method is the same as in Example 1.

  Table 4 shows that all of the steels of the present invention are excellent in mechanical properties (YR: 0.6 or less). Further, the surface properties and spot weldability were both good within the range of Example 2.

  In contrast, Code D, which is a comparative example, has a long time from the end of rolling to the start of primary cooling outside the scope of the present invention, so that ferrite is generated unevenly before the start of cooling, and a good two-phase structure is obtained. The YR is high. In the code E, the primary cooling rate is low outside the range of the present invention, so that ferrite is generated non-uniformly during cooling, does not form a good two-phase structure, and YR is high. Since the primary cooling stop temperature of Code I is high outside the range of the present invention, ferrite formation during the subsequent holding is insufficient, and a good two-phase structure is not obtained, and YR is high. Since the primary cooling stop temperature of Code M is low outside the range of the present invention, ferrite formation during the subsequent holding is insufficient, the two-phase structure is not good, and YR is high. Since the holding time after the primary cooling is not sufficient outside the range of the present invention, the symbol N has insufficient ferrite formation, does not have a good two-phase structure, and has a high YR. Since the symbol Q has a long holding time after the primary cooling outside the range of the present invention, pearlite is generated during the holding, and a good two-phase structure is not obtained, and the YR is high. Since the secondary cooling rate of the symbol R is low outside the range of the present invention, bainite is generated during cooling and does not form a good two-phase structure, and the YR is high. Since the winding temperatures of codes S and T were low outside the range of the present invention, the flatness of the plate deteriorated due to thermal strain during cooling. Since the coiling temperature U is high outside the range of the present invention, bainite is generated after winding, and a good two-phase structure is not obtained, and the YR is high.

  Since the steel sheet of the present invention has excellent press formability and excellent surface properties, it can also be applied to uses such as formed parts in which appearance properties are important.

Claims (2)

In mass%, C: 0.05-0.15%, Si: 0.5% or less, Mn: 1.0-1.8%, Cr: 0.5-1.5%, P: 0.06% or less, S: 0.01% or less, N: 0.005% or less, SolAl : 0.01-0.1% contained, the balance is Fe or unavoidable impurities slab after casting, directly or heated, hot-rolled at Ar3 point temperature or higher, then within 2 seconds after hot-rolling is finished Cool to 750-600 ° C. at a cooling rate of 150 ° C./second or higher, then hold at a temperature range of 750-600 ° C. for 2-15 seconds, then cool at a cooling rate of 20 ° C./second or higher, 350-550 A method for producing a high-strength thin steel sheet excellent in workability, surface properties and plate flatness, characterized by winding at a temperature of ° C. In mass%, C: 0.05-0.15%, Si: 0.5% or less, Mn: 1.0-1.8%, Cr: 0.5-1.5%, P: 0.06% or less, S: 0.01% or less, N: 0.005% or less, SolAl : Including 0.01 to 0.1%, Mo: 0.3% or less, Nb: 0.05% or less, Ti: 0.1% or less, B: 0.002% or more, containing the balance from Fe and inevitable impurities After casting, the slab is cast directly or heated and hot rolled at an Ar3 point temperature or higher, and then cooled to 750 to 600 ° C at a cooling rate of 150 ° C / second or more within 2 seconds after the hot rolling is completed. Next, after maintaining in the temperature range of 750 to 600 ° C for 2 to 15 seconds, cooling at a cooling rate of 20 ° C / second or more and winding at a temperature of 350 to 550 ° C, surface properties And a method for producing a high-strength thin steel sheet excellent in sheet flatness.