JP5728856B2 - Manufacturing method of member - Google Patents

Description

  The present invention relates to a method for producing a member mainly used for automobiles, particularly a member excellent in impact absorbing performance, and is formed by press-forming a cold-rolled steel sheet and then performing a heat treatment.

In recent years, in order to regulate CO ₂ emissions from the viewpoint of global environmental conservation, there has been a demand for improved fuel efficiency of automobiles. In addition, in order to ensure the safety of passengers in the event of a collision, it is also required to improve safety centered on the collision characteristics of the automobile body.
In order to satisfy the weight reduction and strengthening of automobile bodies at the same time, it is said that weight reduction by reducing the plate thickness is effective in increasing the strength of component materials within a range where rigidity does not matter. Tensile steel plates are actively used in automotive parts.

  On the other hand, since many automotive parts made of steel plates are formed by press working, the steel plates for automobiles are required to have excellent press formability. However, conventionally, attention has not been paid to ductility and work hardening ability after press forming into a part. Aside from the parts that do not deform, such as the door impact beam, there is a problem that if the ductility after being incorporated in the vehicle body is not ensured, cracking occurs at the time of collision, and sufficient collision energy cannot be secured. Furthermore, even if it is said to be ductile, it is conceivable that local constriction occurs immediately due to deformation at the time of collision only by local ductility, and impact performance is also affected.

In Non-Patent Document 1, as a means for improving the forming ability of low carbon aluminum killed steel sheet using continuous annealing with excellent productivity, in Non-Patent Document 1, after annealing, sufficient overaging treatment is performed at around 300 to 350 ° C., and solid solution C in ferrite is cementite. As a result, it is possible to obtain a steel sheet having excellent aging resistance and high ductility.
However, in this case, there is a problem that the uniform elongation sharply decreases when processed.

  Moreover, in patent document 1, as a steel plate for motor vehicles which obtains high bake hardenability while having workability, C: 0.003-0.01% N: 0.003% or less, and solid solution C is 0.002. %, A steel sheet having a ferrite single phase with a grain size number of 9.5 or more is disclosed, and BH exceeding 100 MPa is achieved. Similarly, Patent Document 2 includes C: 0.008 to 0.010%, Si: 0.01 to 1%, Mn: 0.05 to 2%, P: 0.005 to 0.1%, A steel plate for an automobile body inner plate is disclosed that exhibits a structure mainly composed of ferrite and is excellent in bake hardenability that is not easily affected by variations in workability and bake treatment conditions. However, paying attention to the formability at the time of pressing, no mention is made of the ductility after becoming a part.

  Further, in Patent Document 3, strain aging is achieved by setting C: 0.01 to 0.02% to a total solid solution C and solid solution N of 0.0015% and a ferrite phase having an average crystal grain size of 10 μm or less. A high-tensile steel plate of TS440 MPa or more that is excellent in heat resistance, impact resistance, secondary work brittleness resistance and workability is disclosed. In addition, as a manufacturing method, it is shown that the product plate is subjected to 1-15% skin pass or leveler processing, and then subjected to baking coating equivalent processing, thereby increasing TS and absorbing energy in high-speed tension. . However, this is insufficient from the viewpoint of the balance between the strength of the material and the energy absorbed by the member.

JP 2006-199997 A JP 2005-126793 A JP 2001-335889 A

Nishiyama Memorial Technology Course "Progress in Continuous Annealing Technology of the 88th and 89th Strips" Japan Iron and Steel Institute (1983)

Thus, conventionally, the formability after annealing, press workability, bake hardenability, etc. have been pursued, but the impact absorption ability in the state where the baking coating treatment after press molding has been performed has been sufficient in the past. It has not been examined.
An object of the present invention is to provide a method for producing a member having excellent impact absorbing ability in a state in which a high-strength steel plate having an alloy-saving component is press-formed and subsequently subjected to a heat treatment (baking coating treatment).

  The present invention has been intensively studied to solve the above-mentioned problems.The microstructure of the steel sheet after annealing, the control of the solid solution C in the matrix, and the amount of strain after annealing, It has been found that by optimizing the heat treatment conditions after forming the member, it is possible to manufacture a low-cost member having excellent shock absorption capability.

Therefore, the present invention employs the following means in order to solve the above problems.
[1] By mass%
C: 0.005 to 0.03%,
Mn: 0.05 to 1.5% or less,
Si: 1.5% or less,
P: 0.10% or less,
S: 0.0100% or less,
Al: 0.02 to 0.80% or less,
N: 0.0100% or less, and a steel slab having a composition composed of the balance Fe and inevitable impurities is heated to 1050 ° C. or more and subjected to finish rolling and finish rolling, and the finish rolling exit temperature is the Ar ₃ transformation point. ˜950 ° C., wound at 600 ° C. or lower, pickled and cold-rolled, then annealed in a temperature range of 700 ° C. to Ac ₁ transformation point, and subsequently from 700 to 250 ° C. After cooling at an average cooling rate of 15 ° C./s or higher, a steel sheet made of a ferrite phase having a structure with an area ratio of 95% or more obtained by temper rolling with a pressure regulation ratio of 15 to 30% is pressed. A method for producing a member having an absorbed energy ratio of 1.4 or more, wherein heat treatment is performed at 140 to 300 ° C. after molding.
[2] The steel slab is further mass%,
Ti: 0.05% or less,
Nb: 0.05% or less,
Cr: 0.2% or less,
Mo: One or more types of 0.2% or less are included, The manufacturing method of the member whose absorption energy ratio as described in [1] is 1.4 or more .
[3] The steel slab is further mass%,
B: The manufacturing method of the member whose absorption energy ratio as described in [1] or [2] is 1.4 or more characterized by including 0.0030% or less.

  By controlling the microstructure of the cold-rolled steel sheet after annealing and the solid solution C in the matrix, optimizing the amount of strain after annealing, the heat treatment conditions after forming the member, etc. An excellent member can be manufactured. ADVANTAGE OF THE INVENTION According to this invention, the impact absorption capability of the member for motor vehicles which takes charge of impact absorption, such as a front side member of a motor vehicle, a rear side member, for example can be improved greatly, and the collision characteristic of a motor vehicle body can be improved.

  The inventors set the steel plate structure after annealing as a ferrite main phase and adjusted the skin pass amount (pressure regulation rate) after annealing higher than before in a state where sufficient solid solution C was secured in the matrix phase ferrite. We have found that heat-treated cold-rolled steel sheets are heat-treated after forming members at an appropriate temperature after press forming, so that when the material is impacted, the material can exhibit sufficient work-hardening properties to ensure absorbed energy. It was.

  This is because, when sufficient aging treatment is performed to ensure the ductility of the material while being continuously annealed, carbon precipitates as cementite, and under high skin pass conditions, it becomes a material that cannot be work hardened. If there is enough, strong processing is performed, and even after a large amount of dislocations are introduced, the solid solution C that exists in large amounts is fixed by the heat treatment, and the dislocations are propagated again by heat treatment, so that the absorbed energy at the time of impact is increased. It is thought to increase.

The present invention is described in detail below.
Unless otherwise specified below, “%” of the element content indicates “mass%”.
First, the reason for limiting the component composition (chemical component) of the steel slab in the present invention will be described. In addition, the component composition of the steel plate in this invention is the same as that of this steel slab.

・ C: 0.005 to 0.03%
C is important for securing solid solution C after annealing, and it is necessary to contain 0.005% or more. However, if the content is excessively large, undissolved cementite exists at the time of annealing, and the solid solution C after annealing decreases, so the upper limit is made 0.03%. Preferably it is 0.02% or less.

Mn: 0.05 to 1.5%
Mn is also an effective element for preventing hot cracking due to S. From such a viewpoint, it is necessary to contain Mn 0.05% or more. However, excessive addition lowers the transformation point, reduces the effect of increasing the ductility after press baking by forming a composite structure and reducing solid solution C in ferrite. Therefore, the Mn content is 1.5% or less. Preferably it is 0.8% or less.

Si: 1.5% or less Si is an element effective for strengthening solid solution without impairing moldability, and is contained in a range of 1.5% or less. In order to acquire this effect, it is preferable to contain Si 0.01% or more, More preferably, it is 0.05% or more. On the other hand, if the content exceeds 1.5%, the transformation point temperature becomes high, and hot rolling cannot be finished in the austenite single-phase region, resulting in a decrease in ductility. Further, the surface appearance due to the scale is also deteriorated and the chemical conversion treatment is deteriorated, so the Si content is set to 1.5% or less. Preferably it is 1.0% or less.

-P: 0.10% or less P is segregated at the grain boundary if it is excessively contained in excess of 0.10%, and the secondary work brittleness resistance and weldability are deteriorated. Therefore, the upper limit was made 0.10%.
S: 0.0100% or less S is an impurity, which causes hot cracking and is present as an inclusion in steel and deteriorates various properties of the steel sheet. Since up to 0100% is acceptable, it is set to 0.0100% or less.

-Al: 0.02-0.80%
Since Al is useful as a deoxidizing element for steel, it is contained in an amount of 0.02% or more. On the other hand, if the content exceeds 0.80%, the transformation point temperature becomes high, hot rolling cannot be finished in the austenite single phase region, and ductility is lowered, which is not preferable. Moreover, since a high alloy cost is caused and weldability is further reduced, it is made 0.80% or less.

-N: 0.0100% or less In the present invention, since the solute C is used, the N content is not particularly specified. However, if it exceeds 0.0100%, it penetrates when deformed after pressing. Since the balance between the total amount of the solid solution element and the amount of strain introduced after annealing is lost, nonuniform deformation is likely to occur, and evaluation of uniform elongation becomes difficult, so the upper limit is made 0.0100%.

Ti: 0.05% or less, Nb: 0.05% or less, Cr: 0.2% or less, Mo: 0.2 or less, one or more types Ti, Nb, Cr, Mo are carbide forming elements, Addition of a large amount is not preferable in order to reduce the solid solution C and N, but the strength ductility balance is improved by fine graining, so that Ti: 0.05% or less, Nb: 0.05% or less, Cr: 0.00. If it is 2% or less and Mo: 0.2% or less, there is no problem even if one or more of these elements are contained. In order to obtain the above effects, the contents of Ti, Nb, Cr, and Mo are Ti: 0.005% or more, Nb: 0.005% or more, Cr: 0.1% or more, Mo: 0.05 % Or more is preferable.

B: 0.0030% or less B is an element having an effect of improving the refinement and bake hardenability of steel, and can be contained as required. However, if the content exceeds 0.0030%, the effect is saturated, so the content is made 0.003% or less. In order to obtain the above effect, B is preferably 0.0003% or more.

The above are the basic components of the steel of the present invention, but there is no problem even if Ca, REM, etc. are contained within the normal steel composition range.
Ca and REM have an action of controlling the form of sulfide inclusions, thereby preventing deterioration of various properties of the steel sheet. Since such an effect is saturated when the content of one or two selected from Ca and REM exceeds 0.01% in total, it is preferable to make it less than this.

  Other inevitable impurities include, for example, Sb, Sn, Zn, Co, etc. The allowable ranges of these contents are Sb: 0.01% or less, Sn: 0.1% or less, Zn: The range is 0.01% or less and Co: 0.1% or less.

Next, the reason why the structure of the steel sheet in the present invention is limited will be described.
-Structure: ferrite phase with an area ratio of 95% or more In order to improve uniform elongation by increasing the temper rolling ratio (pressure regulation ratio) as in the present invention, the ferrite single phase must be preferable. In the case of a composite structure containing martensite, bainite, etc., the effect is reduced, so at least the second phase fraction of martensite, bainite, etc. is 5% or less in terms of area ratio. That is, the area ratio of the ferrite phase is 95% or more.

Next, the manufacturing method of the steel plate used for this invention which has the said structure | tissue is demonstrated.
The steel slab used in the steel sheet manufacturing method used in the present invention has the above composition. The steel slab is desirably manufactured by a continuous casting method in order to prevent macro segregation of components, but may be manufactured by an ingot-making method or a thin slab casting method. In addition to the conventional method in which the steel slab is manufactured and then cooled to room temperature and then heated again, direct feed rolling in which the steel slab is charged without being cooled and charged in a heating furnace and hot-rolled, or a little heat retention Energy-saving processes such as direct-rolling and direct rolling, which are hot-rolled immediately after carrying out, can be applied without any problem.

-Slab heating temperature: 1050 ° C or higher The steel sheet used in the present invention is a slab having a composition within the above-mentioned range, and a hot rolling process in which the raw material is hot rolled to form a hot rolled sheet, and the hot rolling It can be manufactured by subjecting a plate to pickling and cold rolling to form a cold rolled steel sheet, and sequentially subjecting the cold rolled steel sheet to an annealing process for achieving recrystallization and complexing. In this case, the slab heating temperature is preferably 1050 ° C. or higher in order to secure the finishing temperature described later. However, when the temperature exceeds 1300 ° C., precipitation during heating is not sufficient, and it is preferable to heat at 1300 ° C. or less from the viewpoint of γ grain growth, cost increase due to an increase in heating temperature, and scale loss.

  The slab heated under the above conditions is formed into a sheet bar by rough rolling. The conditions for rough rolling need not be specified, and may be performed according to a conventional method. Further, from the viewpoint of lowering the slab heating temperature and preventing troubles during hot rolling, a so-called sheet bar heater for heating the sheet bar can be used.

· Finish rolling delivery temperature: Ar ₃ transformation point ~950 ℃
Subsequently, the sheet bar is finish-rolled with a finish rolling outlet temperature (FT) of Ar ₃ or more and the transformation point or more to obtain a hot-rolled sheet. If the FT is less than Ar ₃ , the hot rolled structure becomes coarse due to rolling in the high temperature region of the ferrite region, and the formability decreases. On the other hand, if the FT is too high, surface property problems such as scale defects occur. The Ar ₃ transformation point can be obtained by measuring a thermal expansion curve using a processing formaster.

  In addition, in order to reduce the rolling load at the time of hot rolling, it is good also as lubrication rolling between some or all passes of finishing rolling. Lubrication rolling is effective from the viewpoint of uniform steel plate shape and uniform material. The coefficient of friction during lubrication rolling is preferably in the range of 0.10 to 0.25. Furthermore, it is also preferable to use a continuous rolling process in which the adjacent sheet bars are joined and finish-rolled continuously. It is desirable to apply the continuous rolling process from the viewpoint of the operational stability of hot rolling.

-Winding temperature: 600 degrees C or less The hot-rolled sheet after finish rolling is wound up by a coil. The coil winding temperature (CT) is 600 ° C. or lower. In particular, when CT exceeds 600 ° C., the crystal grains are coarsened and the strength is lowered, and cementite is coarsened, so that it is difficult to ensure solid solution C during annealing. Preferably it is set to 500-580 degreeC.

-Cold rolling Next, after pickling the hot-rolled sheet, it is cold-rolled to obtain a cold-rolled steel sheet. Pickling may be performed under normal conditions. The cold rolling condition is not particularly limited as long as it can be a cold rolled steel sheet having a desired size and shape, but the rolling reduction during cold rolling is preferably at least 50% or more, more preferably 60% or more. To do. On the other hand, in the present invention, the r value increases as the cold rolling reduction is increased up to 90%, but if it exceeds 90%, not only the effect is saturated, but also the load on the roll during rolling increases. The upper limit is preferably 90%.

-Annealing temperature: 700 degreeC-Ac ₁ transformation point The cold-rolled steel plate after cold rolling is annealed. If recrystallization is not completed by annealing, the ductility is greatly reduced and the anisotropy is also increased, which is not preferable. In that sense, the annealing temperature needs to be 700 ° C. or higher. However, if the temperature exceeds the Ac ₁ transformation point, the structure of the steel sheet becomes a composite structure, and it becomes impossible to obtain a steel sheet composed of a ferrite phase with an area ratio of 95% or more. Since it cannot be secured, Ac is set to ₁ point or less. The Ac ₁ transformation point can be obtained by measuring a thermal expansion curve using a Formaster.

-Average cooling rate from 700 degreeC to 250 degreeC in the cooling process following annealing: 15 degree-C / s or more The cold-rolled steel plate after annealing is cooled in order to ensure the solid solution C after annealing. When the average cooling rate from 700 ° C. to 250 ° C. is less than 15 ° C./s, cementite is precipitated and sufficient solute C cannot be secured. Therefore, the average cooling rate from 700 ° C. to 250 ° C. is 15 ° C./s or more. To do.

-Temper rolling: Temper rolling with a pressure regulation ratio of 15-30% Cold-rolled steel sheet cooled after annealing is temper-rolled and applied to press work. If the pressure regulation ratio is less than 15%, normal temperature aging is achieved. Is concerned. Further, when the amount of strain after press molding is small, work hardening after heat treatment after press molding cannot be expected. On the other hand, if it exceeds 30%, even if solid solution C is sufficiently present, the uniform elongation after the heat treatment (baking treatment) is lowered. Preferably it is 25% or less.
Next, the steel plate (cold rolled steel plate) obtained as described above is press-formed into a member shape, and then subjected to heat treatment.

-Heat treatment temperature: 140-300 ° C
Cold-rolled steel sheet has low ductility because it only introduces processing strain unless heat treatment such as paint baking is performed after it is pressed to form a molded part. In order to sufficiently fix the solid solution C to the dislocation, the heat treatment temperature needs to be 140 ° C. or higher. When the temperature exceeds 300 ° C., dislocations introduced by temper rolling or press work are recovered, resulting in a decrease in strength, and a decrease in strength due to cementite precipitation is increased, resulting in a decrease in impact absorption capability. Preferably it is 170-250 degreeC.

  Moreover, although this invention is related to a cold-rolled steel plate, you may perform plating processes, such as galvanization suitably. However, from the viewpoint of setting the average cooling rate from 700 ° C. to 250 ° C. in the cooling process to 15 ° C./s or more, for example, in the case of zinc plating, electrogalvanizing and hot dip galvanizing (no alloying) are preferable.

Next, examples of the present invention will be described.
Molten steel having the composition (steel A to K) shown in Table 1 was melted in a converter and made into a slab by a continuous casting method. These steel slabs were heated to 1250 ° C. and roughly rolled into sheet bars, then hot rolled into finish bars under the conditions shown in Table 2, and wound around coils. Subsequently, these hot-rolled sheets were made into cold-rolled steel sheets by pickling and a cold rolling process with a rolling reduction of 65%. Subsequently, these cold-rolled steel sheets were continuously annealed in the continuous annealing line under the conditions shown in Table 2, and subsequently cooled. The steel sheet thus obtained was further subjected to temper rolling at a pressure regulation rate (elongation rate) shown in Table 2. The plate thickness after temper rolling was 1.2 mm.

(1) Tensile properties JIS5 is applied to a steel plate that has been subjected to a heat treatment equivalent to a paint baking process under the heat treatment conditions shown in Table 2 by applying a rolling strain (elongation rate 5%) corresponding to press forming to the steel sheet after the temper rolling. Tensile test specimens were subjected to a tensile test at a crosshead speed of 10 mm / min in accordance with JIS Z 2241, yield stress (YS), tensile strength (TS), uniform elongation (Uel), total elongation ( Tel).

(2) Ferrite fraction For the same material as the test piece in (1) above, the phase that was corroded black in the optical microscope photograph of 400 times was judged as the second phase, and the area ratio of the remaining ferrite phase was measured. .

(3) Impact absorption capacity The steel sheet specimen after temper rolling was press-formed into a hat-shaped member (hat member) having a cross section of 40 × 40 mm, 5R and a length of 300 mm, and the same material was spot welded to the back plate. . The nugget diameter was 4√t when the plate thickness was t, and the spot spacing was 30 mm. This was heat treated for 20 minutes at the temperature of the heat treatment conditions shown in Table 2.
And after fixing the both ends of the produced hat member with the hot rolled sheet of thickness 5mm, the impact test was done from the top at a constant speed of 20km / hr, and the load-displacement curve was measured. The absorbed energy was evaluated by the ratio of the energy up to the time of displacement of 15 cm to the value of the annealed material (No. 13 in Table 2) of a general low carbon steel plate that is steel H having the same thickness.

Based on Table 2, Example No. 1 to 16 will be described.
Comparative Example No. 1 has a composition of steel A, which is within the scope of the present invention, but is not subjected to heat treatment after press molding, so that the processing strain is still introduced, and the tensile strength (TS) is high. However, the elongation [uniform elongation (Uel), total elongation (Tel)] is remarkably low, the absorption energy ratio is less than 1, and the shock absorbing ability is inferior.

  On the other hand, No. of the example of this invention currently heat-processed at 170 degreeC. 2 and no. No. 3 has a composition of steel B and 1 and within the scope of the present invention, heat treatment was performed at 170 ° C. and 200 ° C., and the production conditions other than the composition were No. in Comparative Example. Although it is substantially the same as 1, the tensile strength is high, the elongation is high, the absorption energy ratio is much higher than 1, and the shock absorbing ability is excellent.

No. of the comparative example whose composition is steel B. No. 4 in which the finish rolling exit temperature (FT) is lower than the Ar ₃ point (see “Ar ₃ ” in Table 1) and the composition is No. 4 steel A. No. 5 has a high coiling temperature (CT), the structure and crystal grains become coarse, both TS and elongation are slightly low, the absorption energy ratio is 1.2, and the shock absorption capacity is slightly inferior. In addition, No. whose composition is steel A. No. 6 is a cooling rate after annealing of 3 ° C./s outside the range of the present invention, solid solution C cannot be sufficiently secured, uniform elongation (Uel) is extremely low, absorption energy ratio is less than 1, and shock absorption Inferior to Noh.

  No. of the example of the present invention. 7-No. No. 11 is a steel C to G having a composition within the scope of the present invention, and the production conditions all satisfy the conditions of the present invention. 2, no. Similar to 3, TS is high, the absorbed energy ratio is much higher than 1, and the shock absorbing ability is excellent.

Comparative Example No. No. 12, the composition is G and the composition is within the range of the present invention, but the pressure regulation rate is 2%, which is outside the conditions of the present invention, the work hardening amount is small, the absorbed energy ratio is 1.1, and the impact Slightly inferior in absorption capacity.
Comparative Example No. No. 13 has a composition of steel H and the amount of C is outside the scope of the present invention, and even under the production conditions, the cooling rate after annealing is 5 ° C./s, heat treatment is not performed, and is outside the conditions of the present invention. As already described, no. Reference numeral 13 is a conventional annealing material for a general low-carbon steel sheet, and the absorption energy ratio is 1 because it is used as a reference for evaluating the absorption energy.

  Comparative Example No. No. 14 has a composition of Steel I, the amount of Cr exceeds the range of the present invention, the ferrite fraction is 94% lower than the range of the present invention, and has a composite structure including martensite and bainite. The elongation is low, the absorbed energy ratio is 1.1, and the shock absorbing ability is slightly inferior.

Comparative Example No. 15, no. No. 16 also has a steel composition of J and K, respectively, which is outside the scope of the present invention, both of which have a low ferrite fraction and are outside the scope of the present invention. Similar to 14, the uniform elongation is low, the absorption energy ratio is around 1, and the shock absorbing ability is slightly inferior or inferior.
As described above, in the example of the present invention, TS is sufficiently high, and the absorbed energy of the member is significantly higher than that of the comparative example, and the shock absorbing ability is excellent.

According to the present invention, it is possible to stably and inexpensively manufacture a member having excellent shock absorption capacity after press molding, and there is a remarkable industrial effect.

Claims (3)

% By mass
C: 0.005 to 0.03%,
Mn: 0.05 to 1.5% or less,
Si: 1.5% or less,
P: 0.10% or less,
S: 0.0100% or less,
Al: 0.02 to 0.80% or less,
N: 0.0100% or less, and a steel slab having a composition composed of the balance Fe and inevitable impurities is heated to 1050 ° C. or more and subjected to finish rolling and finish rolling, and the finish rolling exit temperature is the Ar ₃ transformation point. ˜950 ° C., wound at 600 ° C. or lower, pickled and cold-rolled, then annealed in a temperature range of 700 ° C. to Ac ₁ transformation point, and subsequently from 700 to 250 ° C. After cooling at an average cooling rate of 15 ° C./s or higher, a steel sheet made of a ferrite phase having a structure with an area ratio of 95% or more obtained by temper rolling with a pressure regulation ratio of 15 to 30% is pressed. A method for producing a member having an absorbed energy ratio of 1.4 or more, wherein heat treatment is performed at 140 to 300 ° C. after molding. The steel slab is further mass%,
Ti: 0.05% or less,
Nb: 0.05% or less,
Cr: 0.2% or less,
Mo: One or more types of 0.2% or less are included, The manufacturing method of the member whose absorption energy ratio of Claim 1 is 1.4 or more characterized by the above-mentioned. The steel slab is further mass%,
B: 0.0030% or less is included, The manufacturing method of the member whose absorption energy ratio of Claim 1 or 2 is 1.4 or more characterized by the above-mentioned.