JP6048040B2 - Manufacturing method of high-tensile hot-rolled steel sheet with excellent workability - Google Patents

Description

  The present invention relates to a hot-rolled steel sheet used for press forming, and more particularly to a high-tensile hot-rolled steel sheet excellent in workability, suitable for applications such as automobile structural members and undercarriage parts, and a method for producing the same. .

  In order to reduce the environmental burden, improving the fuel efficiency of automobiles has become an important issue. One of the most effective methods is to use a high-tensile steel plate. In particular, for structural members, suspension members, etc., the thinning of hot-rolled steel sheets using high-tensile steel sheets has been studied, and the use of high-strength steel sheets is expanding.

  In general, when the tensile strength of a steel sheet is increased, the ductility becomes smaller and the workability becomes worse. However, various research and development of hot-rolled steel sheets having excellent workability have been conventionally performed. For example, when ductility is required for a high-tensile steel plate having a tensile strength of 590 to 690 MPa, a dual phase steel composed of a ferrite phase and a martensite phase is selected. Further, when stretch flange formability is required, TRIP steel that contains residual austenite and uses the transformation induced plasticity phenomenon during deformation of this phase is selected.

  For example, in Patent Document 1, by weight%, C: 0.05 to 0.12%, Si: 0.6 to 1.3%, Mn: 1.0 to 1.8%, P: 0.01 to 0.04%, balance Fe and inevitable impurities composition, and ferrite, bainite and microstructure composed of 7% or less residual austenite, and the hardness ratio of ferrite and bainite is 2.5 or less Disclosed is a technology related to a high-tensile hot-rolled steel sheet characterized by being satisfied, having a tensile strength (TS) of 590 to 690 MPa, a TS × El value of 18000 MPa ·% or more, and a hole expansion ratio (λ) of 80% or more. Yes.

  Moreover, in patent document 2, C: 0.06-0.15%, Si: 0.50-1.50%, Mn: 1.0-2.0%, S: 0.002% by weight% 1 or 2 or more selected from Ti, Nb, V, and Zr in total, 0.005 to 0.020% in total, and further 1 or 2 selected from B and Ca in total. 0010-0.0060% is contained, the balance is composed of Fe and inevitable impurities, and is composed of an acicular ferrite single phase structure, and has a tensile strength of 540 MPa or more, an elongation of 30%, and a hole expansion ratio of 140%. The technique regarding the above high-tensile steel plate is disclosed.

Japanese Patent Laid-Open No. 11-172372 JP 2002-161339 A

  However, the high-tensile hot-rolled steel sheet described in Patent Document 1 has a problem that a high hole expansion rate cannot always be obtained and stretch flangeability is not satisfied. In particular, when the coiling temperature in hot rolling is 500 ° C. or less, it is difficult to achieve a stable coiling temperature because it is a transition boiling region. However, in Patent Document 1, the coiling temperature is 350 to 500 ° C. It is difficult to obtain uniform characteristics over the entire length, and the yield is considered to be relatively low. In the high-tensile hot-rolled steel sheet described in Patent Document 2, although the hole expansion rate is excellent, there is a problem that a tensile strength of 590 MPa or more cannot be stably achieved. Further, in Patent Document 2, the winding temperature is 400 to 600 ° C., and as described above, it is difficult to obtain uniform characteristics over the entire length because the stable winding temperature cannot be achieved. It is considered to be relatively low.

  The present invention has been made in view of such circumstances, and is a high-tensile hot-rolled steel sheet having a tensile strength of 590 MPa or more, particularly high-strength hot-rolled steel with excellent stretch flangeability and ductility and excellent workability. It aims at providing a rolled steel plate and its manufacturing method.

  In order to solve the above-mentioned problems, in the present invention, a high-tensile hot-rolled steel sheet having a specific component composition, a ferrite as a main phase, and a metal structure composed of retained austenite in a second phase is stretched flange formability. It was also found that the ductility is good and the workability is excellent. Further, it has been found that the high-tensile hot-rolled steel sheet of the present invention can be obtained by controlling the hot rolling method and the heat treatment conditions after hot rolling.

This invention is made | formed based on the said knowledge, The summary is as follows.
[1] By mass%, C: 0.08% to 0.16%, Si: 0.70% to 1.50%, Mn: 0.50% to 1.00% or less, P: 0.050% Hereinafter, S: 0.005% or less, Al: 0.070% or less, Cr: 0.10 to 0.50%, the balance has a component composition of Fe and inevitable impurities, and the metal structure is A high-tensile hot-rolled steel sheet characterized in that the main phase is ferrite and the second phase is residual austenite of 15% or less by volume.
[2] After the steel was heated, it was hot-rolled at a finishing temperature of Ar3 transformation point or higher to obtain a rolled hot-rolled steel sheet at a winding temperature of 550 to 700 ° C, and then reheated at Ac1 transformation point or higher for 30 seconds or more. The high tension according to claim 1, wherein after the cooling, cooling is performed at a cooling rate of 10 ° C./s or more until the cooling stop temperature becomes 350 to 450 ° C., and heat treatment is performed for 400 seconds or less at the cooling stop temperature. A method for producing a hot-rolled steel sheet.
The high-tensile hot-rolled steel sheet excellent in workability of the present invention refers to a high-tensile hot-rolled steel sheet having a tensile strength of 590 MPa or more, a total elongation of 30% or more, and a hole expansion rate of 100% or more.

  According to the present invention, it is possible to provide a high-tensile hot-rolled steel sheet having a tensile strength of 590 MPa or more, excellent stretch flangeability and ductility, and excellent workability, and a method for producing the same.

  Hereinafter, the present invention will be specifically described. In the following description, the unit of the content of each element of the component composition is “mass%”, and hereinafter, simply indicated by “%” unless otherwise specified.

(1) Component composition C: 0.08% to 0.16%
C is an element that increases the strength, and in the heat treatment after hot rolling, it is concentrated in austenite to increase the stability of austenite. In the present invention, it is necessary to contain 0.08% or more in order to obtain the desired retained austenite. However, if the C content exceeds 0.16%, the fatigue characteristics deteriorate due to the increase in the hardness of the weld heat affected zone. Therefore, the C content is 0.08% to 0.16%. More preferably, the content is 0.11 to 0.14%.

Si: 0.70% to 1.50%
Si is an element effective for improving the strength by solid solution strengthening. In order to obtain a tensile strength of 590 MPa or more, it is necessary to contain 0.70% or more. On the other hand, if it exceeds 1.50%, the weldability is deteriorated, and a firelite is formed on the surface of the steel sheet to deteriorate the descalability. Therefore, the Si amount is set to 0.70% to 1.50%. More preferably, the content is 1.20 to 1.40%.

Mn: 0.50% to 1.00%
Mn is an element effective for improving the strength by solid solution strengthening and refines the ferrite grains to improve the ductility. In order to acquire such an effect, it is necessary to contain 0.50% or more of Mn amount. On the other hand, if it exceeds 1.00%, the strength increases, and the improvement of the hole expansion rate is hindered. For this reason, the amount of Mn shall be 0.50%-1.00%. More preferably, the content is 0.70 to 0.90%.

P: 0.050% or less P is an element that contributes to strength improvement by solid solution strengthening, but if it exceeds 0.050%, it causes work cracking due to segregation. For this reason, the amount of P is made into 0.050% or less. More preferably, it is 0.030% or less.

S: 0.005% or less S forms sulfides with Mn and reduces stretch flange formability. Therefore, the S amount is 0.005% or less, but it is preferable to reduce it as much as possible. More preferably, it is 0.002% or less.

Al: 0.070% or less Al is added as a deoxidizing material for steel and is an effective element for improving the cleanliness thereof. However, if it exceeds 0.070%, inclusions in the steel increase, so that surface defects are likely to occur and ductility decreases. For this reason, the Al content is 0.070% or less. More preferably, it is made into 0.050% or less.

Cr: 0.10% to 0.50%
Cr is an element that contributes to strength improvement by forming carbides. In order to acquire this effect, it is necessary to contain 0.10% or more. If it exceeds 0.50%, stretch flange formability and ductility are lowered. Therefore, the Cr content is 0.10% to 0.50%. More preferably, it is 0.20 to 0.40%.

  As other components, you may contain Ti and Mo suitably as needed. Ti forms carbides, and Mo is an element that contributes to strength improvement by precipitation strengthening. However, in the hot-rolled steel sheet of the present invention, sufficient performance is obtained by the addition of Cr, so it is not specifically added to avoid an increase in cost.

(2) Metal structure The metal structure of the steel sheet of the present invention is characterized by containing 15% or less of ferrite in the main phase and 15% or less of retained austenite in the second phase. In the present invention, by setting the volume ratio of retained austenite to 15% or less, it is possible to obtain a high-tensile hot-rolled steel sheet that achieves both stretch flangeability and ductility. On the other hand, if the retained austenite exceeds 15%, stretched flange formability is lowered during severe processing. For this reason, the volume fraction of retained austenite in the second phase is set to 15% or less. Preferably, it is 5 to 15%. The volume fraction of retained austenite is obtained by measuring a plate surface having a thickness of 1/4 by X-ray diffraction.

  In the present invention, the volume ratio of ferrite as the main phase is preferably 60% or more. Due to the main phase of ferrite, excellent stretch flange formability can be obtained. In the present invention, the main phase means a phase or a structure having the highest proportion in the metal structure. The remainder of the metal structure is mainly composed of martensite. Here, the volume ratio of the ferrite structure is obtained by etching the cross section after polishing, photographing with an optical microscope, and image processing.

(3) Manufacturing method As a manufacturing method of the high-tensile hot-rolled steel sheet of the present invention, after heating the steel having the above-mentioned composition, it is hot-rolled at a finishing temperature Ar3 transformation point or higher, and a winding temperature is 550 to 700 ° C. After obtaining the rolled hot-rolled steel sheet, after reheating for 30 seconds or more above the Ac1 transformation point, cooling is performed at a cooling rate of 10 ° C./s or more until the cooling stop temperature becomes 350 to 450 ° C., and the cooling stop temperature The heat treatment is performed for 400 seconds or less. The reason for limitation will be described below.

  Although the heating conditions of steel are not particularly defined, it is preferable to produce the steel in a temperature range of 1100 ° C to 1280 ° C. Heating above 1280 ° C. is not preferable because it causes carbonitride dissolution and austenite grain coarsening.

  In the hot rolling after heating, the finishing temperature is set to the Ar3 transformation point or higher, and the coil is wound around a coil at a winding temperature of 550 ° C to 700 ° C. If the finishing temperature is lower than the Ar3 transformation point, ferrite may be generated during rolling, and the hole expansion rate may be reduced. Therefore, the finishing temperature is not less than the Ar3 transformation point. In the present invention, when the coiling temperature is less than 550 ° C., it becomes a transition boiling region, so that stable temperature control is difficult, and uniformity of characteristics may not be ensured. On the other hand, when the coiling temperature exceeds 700 ° C., the generated crystal grains are coarsened and desired characteristics cannot be obtained. Therefore, the coiling temperature is set to 550 to 700 ° C.

Next, heat treatment is performed on the hot-rolled steel sheet obtained under the above conditions. In the present invention, the heat treatment condition is a particularly important condition. First, it reheats 30 seconds or more above the Ac1 transformation point. Thereby, a part of steel plate is transformed into austenite. If the reheating time is less than 30 seconds, the cementite remains undissolved, so the reheating time is 30 seconds or more. Preferably, it is 35 seconds or more. Thereafter, cooling is performed at a cooling rate of 10 ° C./s or more until the cooling stop temperature becomes 350 ° C. to 450 ° C. When the cooling rate is less than 10 ° C./s, C diffusion occurs and it becomes difficult to make the second phase into retained austenite. Therefore, the cooling rate is 10 ° C./s or more. Preferably, it is set to 13 ° C./s or more. Further, when the cooling stop temperature is less than 350 ° C., the martensite fraction becomes high. On the other hand, when the cooling stop temperature exceeds 450 ° C., pearlite is generated, so that it is difficult to secure necessary residual austenite. . For this reason, cooling stop temperature shall be 350 to 450 degreeC. Next, the cooling stop temperature is maintained for 400 seconds or less. When the holding time at the cooling stop temperature is long, the volume ratio of retained austenite becomes excessive, and the stretch flangeability is deteriorated. Accordingly, the holding time is 400 seconds or less.
In addition, you may perform the pickling process for removing the scale produced | generated on the steel plate surface before heat processing.

  By the above method, a high-tensile hot-rolled steel sheet having a specific component composition and having a metal structure of ferrite in the main phase and residual austenite in the second phase of 15% or less by volume can be obtained. The high-tensile hot-rolled steel sheet of the present invention has a tensile strength of 590 MPa or more, has good ductility and stretch flange formability, and is excellent in workability.

  After melting the steel of the component composition shown in Table 1 into a continuous cast slab, heating, hot rolling and cooling under each production condition shown in Table 2, winding up, hot rolled steel plate with a thickness of 1.8 mm Manufactured. Next, the obtained hot-rolled steel sheet was subjected to a heat treatment under the conditions shown in Table 2 after the pickling treatment.

Metal structures were observed for the various steel sheets obtained. The volume ratio (residual γ amount) of retained austenite was calculated by measuring a 1/4 thickness plate surface by X-ray diffraction. The volume ratio (α amount) of the ferrite was measured by polishing a section in the plate thickness direction parallel to the rolling direction and then performing nital etching, photographing with an optical microscope, and image processing. It was confirmed that the remainder of the metal structure was martensite.
Further, the tensile strength, stretch flange formability and ductility of the obtained high-tensile hot-rolled steel sheet were evaluated. Evaluation methods and evaluation criteria are shown below.
<Tensile strength and ductility>
Tensile strength and ductility were evaluated by performing a tensile test. The tensile test was performed according to JIS Z 2241 using a JIS No. 5 test piece, and the tensile strength (TS), yield strength (YP), and total elongation (EL) were measured. The tensile strength was 590 MPa or more. For the ductility, a total elongation of 30% or more was regarded as acceptable.
<Stretch flange formability>
Stretch flange formability was evaluated by performing a hole expansion test. The hole expansion test was conducted in accordance with the Japan Railway Standard JFST1001-1996. For stretch flange formability, a hole expansion rate (λ) of 100% or more was considered acceptable.

  The above evaluation results are shown in Table 3.

  All of the hot-rolled steel sheets of the examples of the present invention have a tensile strength of 590 MPa or more, a total elongation of 30% or more, and a hole expansion ratio of 100% or more, and all satisfy the tensile strength, stretch flangeability, and ductility. You can see that On the other hand, in the comparative example which is outside the scope of the present invention, any one or more of tensile strength, total elongation, and hole expansion rate is not satisfied.

Claims (1)

C: 0.08% to 0.16%, Si: 0.70% to 1.50%, Mn: 0.50% to 1.00 %, P: 0.050% or less, S: 0.005% or less, Al: 0.070% or less, Cr: comprises from 0.10 to 0.50%, the balance after heating the steel to have a chemical composition of Fe and unavoidable impurities, finishing temperature Ar3 transformation After hot rolling at a temperature above the point and obtaining a rolled hot-rolled steel sheet at a coiling temperature of 550 to 700 ° C., after reheating for 30 seconds or more at the Ac1 transformation point or higher, cooling is stopped at a cooling rate of 10 ° C./s or higher. It is cooled until the temperature reaches 350 to 450 ° C., and heat treatment is performed for 400 seconds or less at the cooling stop temperature . The metal structure has a ferrite phase volume ratio of 60% or more and a second phase. production of high strength hot rolled steel sheet residual austenite is less than 15% by volume to Law.