JP5320681B2 - High strength cold rolled steel sheet and method for producing high strength cold rolled steel sheet - Google Patents

Description

  The present invention relates to a high-strength cold-rolled steel sheet having excellent impact resistance suitable for use as a steel sheet for automobiles.

In recent years, improving the fuel efficiency of automobiles has become an important issue from the viewpoint of conservation of the global environment. For this reason, a movement to reduce the thickness of the vehicle body by increasing the strength of the vehicle body material and to reduce the weight of the vehicle body has become active. Since the strain rate which each part receives at the time of a car collision reaches about 10 ³ / s, the impact resistance characteristic in such a high speed region is particularly important.
A typical example of a high-strength steel plate having excellent shock absorption characteristics is a dual-phase steel plate (DP steel plate) composed of a composite structure of ferrite and martensite as disclosed in Patent Document 1. Further, Patent Document 2 discloses a technique for improving impact resistance characteristics in TRIP steel using a plasticity-induced transformation of residual γ.
JP-A-9-111396 JP 2001-11565 A

However, the DP steel sheet, which originally has a low yield strength, shows a high impact absorption capacity because of its high work hardening by press working, and when processing strain enters, strain aging occurs in the subsequent paint baking process, resulting in a significant increase in yield strength. This is the reason for this, and there is a problem that a part with a small amount of processing such as bending does not necessarily exhibit a sufficient shock absorbing ability. Furthermore, DP steel is characterized by high shock absorption energy in a high strain range of about 10 to 30%, and is suitable for a part that deforms to some extent during a collision, such as a frontal collision part, and absorbs the collision energy. It could not be said that the required characteristics were sufficiently satisfied for parts that require high absorbed energy in a small strain region from the viewpoint of securing passenger space, such as a collision part. The TRIP steel shown in Patent Document 2 also has the same problem as the DP steel.
The present invention relates to cold rolled steel sheets used for automobiles, home appliances, machine structures, etc., even if no strain is introduced by press working, the absorbed energy is large in the low strain region and the high strength is excellent in the impact resistance characteristics. It aims at providing a cold-rolled steel plate and its manufacturing method.

In order to achieve the above-described problems and to produce a cold-rolled steel sheet having excellent impact resistance, the present inventors have made extensive studies from the viewpoints of the composition and microstructure of the steel sheet. As a result, the alloy elements are appropriately adjusted, and the temperature range from 500 ° C. to the Ac ₁ transformation point, which is the recrystallization temperature range, is heated at an average heating rate (average heating rate) of 10 ° C./s or more during annealing. To suppress recrystallization at the time of temperature rise, and then hold for 10 seconds or more in the temperature range from A ₁ transformation point to (A ₃ transformation point + 30 ° C.) to produce fine γ by reverse transformation and then cool. As a result, it was found that the ferrite phase is the main phase and the low-temperature transformation phase (one or more of martensite, bainite and residual γ) is finely dispersed, and the impact resistance is remarkably improved.

At this time, the reason why γ generated by the reverse transformation becomes fine is not necessarily clear, but is considered as follows. That is, at least one of Ti, Nb, and V is added, and recrystallization during heating is suppressed by the pinning force of these fine carbonitrides. In particular, when the heating rate is 10 ° C./s or more, almost no recrystallization occurs and the non-recrystallized α remains in the α + γ2 phase region temperature (the two-phase region of ferrite and austenite) above the A ₁ transformation point, and the high dislocation density part, At the preferential nucleation site such as a non-uniformly deformed portion, a competition occurs between recrystallization α nucleation from the processed α and α → γ transformation nucleation. At this time, since the driving force of the α → γ transformation is larger than the driving force of recrystallization, γ nuclei are generated one after another in preference to the recrystallization α nucleation and occupy the preferential nucleation site. As a result, it is considered that γ generated by reverse transformation becomes very fine.

The present invention is configured based on the above knowledge, and the high-strength cold-rolled steel sheet according to claim 1 of the present invention is
C: 0.05-0.25 mass% or less, Si: 1.0-2.0 mass% , Mn: 1.0-3.0 mass%, P: 0.10 mass% or less, S: 0.0. 02% by mass or less, Al: 0.01 to 2% by mass, and one or two of Nb and V in total, 0.01 to 0.2% by mass, with the balance being iron and inevitable impurities Having a steel composition,
As a steel structure, it has ferrite and a low temperature transformation phase composed of one or more of martensite, bainite and residual γ,
The volume ratio of the low temperature transformation phase is 10 to 50 vol. %, The average crystal grain size of the low temperature transformation phase is 2 μm or less, and the average distance between the adjacent low temperature transformation phases is 2 μm or less.

Furthermore, the high-strength cold-rolled steel sheet according to claim 2 of the present invention is as follows.
The steel composition is further characterized by containing a component defined in one group or two or more of the following groups A, B and C within the specified range.
(Group A) Cr and Mo: 0.05 to 2.0 mass% in total of 1 type or 2 types
(Group B) B: 0.005 mass% or less (Group C) Ca and rare earth elements: 0.01 mass% or less in total of 1 type or 2 types or more

Furthermore, the high-strength cold-rolled steel sheet according to claim 3 of the present invention is as described in claim 1 or 2,
It has a hot dip galvanized layer or an alloyed hot dip galvanized layer on its surface.
Furthermore, the manufacturing method of the high-strength cold-rolled steel sheet according to claim 4 of the present invention includes
After subjecting the steel material having the steel composition according to claim 1 or 2 to hot rolling and cold rolling, continuous annealing is performed,
And in the continuous annealing, the steel sheet produced by rolling the cold, 500 ° C. to A the average heating rate was raised to A ₁ transformation point as 10 ° C. / s or more at ₁ transformation point, then A _l transformation point ~ (A ₍₃ transformation point + 30 ° C.) is maintained for 10 seconds or more, and the average cooling rate at 550 ° C. or more is set to 5 ° C./s or more and cooled to 550 ° C. or less.

Furthermore, the manufacturing method of the high-strength cold-rolled steel sheet according to claim 5 of the present invention,
A hot dip galvanizing process is performed on the steel sheet surface during or after the cooling.
Furthermore, the manufacturing method of the high-strength cold-rolled steel sheet according to claim 6 of the present invention is
After the hot dip galvanizing treatment, the galvanized layer formed by the hot dip galvanizing treatment is alloyed by holding at 480 to 550 ° C. for 5 to 60 seconds.
In addition, about description of this specification and a claim, the description of "A-B" is the meaning which includes A and B used as a boundary value in a range (A and B are arbitrary numbers or a character).

  The steel sheet of the present invention has a large absorbed energy in a low strain region and is excellent in impact resistance.

Hereinafter, embodiments of the present invention will be specifically described. In addition, in the following description, the "%" display regarding a component shall mean the mass% unless there is particular notice.
[About high-strength cold-rolled steel sheets]
The high-strength cold-rolled steel sheet according to the present embodiment includes C: 0.05 to 0.25% or less, Si: 2.0% or less, Mn: 1.0 to 3.0%, P: 0.10% or less. , S: 0.02% or less, Al: 0.01-2% included, and one or more of Ti, Nb and V are included in a total of 0.01-0.2%, the balance being iron and It has a composition consisting of inevitable impurities. Furthermore, the steel structure has ferrite and a low-temperature transformation phase composed of one or more of martensite, bainite and residual γ (hereinafter sometimes simply referred to as “low-temperature transformation phase”). The volume fraction of the low temperature transformation phase is 10 to 50 vol. %, And the average crystal grain size of the low temperature transformation phase is 2 μm or less, and the average distance between the adjacent low temperature transformation phases is 2 μm or less.

Next, each component composition will be specifically described.
(C: 0.05-0.25%)
C is an element useful for strengthening steel, and is also an important element in generating a low-temperature transformation phase such as martensite. However, if the content is less than 0.05%, the effect of addition is poor. On the other hand, if the content exceeds 0.25%, ductility and weldability deteriorate, so C is 0.05 to 0.25%. It was limited to the range.
(Si: 2.0% or less)
Si, as a solid solution strengthening component, effectively contributes to improving strength while improving the strength-elongation balance, but excessive addition deteriorates ductility, surface properties, and weldability. The content was 0% or less. In addition, when performing hot dip galvanization, it is preferable to make Si amount into 1.0% or less from a viewpoint of plating property.

(Mn: 1.0-3.0%)
Mn is an element effective for strengthening steel and promotes the generation of low-temperature transformation phases such as martensite. Such an effect is recognized when the Mn content is 1.0% or more. However, if Mn is added excessively exceeding 3.0%, the strength rises significantly due to an excessive increase in the second phase fraction (volume ratio of martensite, bainite, residual austenite) and an increase in the amount of solid solution strengthening. , Leading to a decrease in ductility. Therefore, the amount of Mn is set to 1.0 to 3.0%.
(P: 0.10% or less)
P is an element effective for strengthening steel, but if added in excess of 0.10%, it causes embrittlement due to grain boundary segregation and deteriorates impact properties. Therefore, the P content is 0.10% or less.

(S: 0.02% or less)
S is an inclusion such as MnS, which causes deterioration of impact resistance and cracks along the metal flow of the weld. Therefore, it is better to be as low as possible, but 0.02% or less from the viewpoint of manufacturing cost. .
(Al: 0.01-2.0%)
Al acts as a deoxidizer and is an element effective for the cleanliness of steel, and is preferably added in the deoxidation step. If the amount of Al is less than 0.01%, the effect of addition becomes poor, so the lower limit was made 0.01%. Moreover, Al contributes effectively to improve the strength while improving the strength-elongation balance as a solid solution strengthening component like Si, but if it exceeds 2%, the effect is not only saturated. The amount of inclusions in the steel increases and the ductility decreases. Therefore, the addition amount of Al is limited to 2.0% or less.

(Totally 0.01% to 0.2% of one or more of Ti, Nb and V)
Ti, Nb, and V are important elements in the present invention. By adding these elements, TiC, NbC, VC, and the like are precipitated, and effectively work to refine the structure by suppressing recrystallization of the steel sheet. When the total amount of Ti, Nb, and V is less than 0.01%, the above effect is poor, and adding more than 0.2% not only saturates the effect, but also causes excessive precipitates and The ductility is reduced. Therefore, the total addition amount of Ti, Nb, and V is set to a range of 0.01 to 0.2%. A preferable range is 0.02 to 0.1%.

The basic components have been described above. However, in the present invention, other elements described below can be appropriately contained.
(Cr and Mo: 0.05 to 2.0% in total of 1 type or 2 types)
Both Cr and Mo can be contained as a reinforcing component as required, but a large amount of addition deteriorates the strength-ductility balance. For this reason, it is desirable that one or two of Cr and Mo be 2.0% or less in total. Moreover, in order to exhibit said effect | action, it is preferable to contain 0.05% or more of 1 type or 2 types of Cr and Mo in total.

(B: 0.005% or less)
B has an effect of improving workability through an increase in grain boundary strength, and can be contained as required. However, excessive content deteriorates workability, so the upper limit is made 0.005%.
(Ca and rare earth elements (REM): 0.01% or less in total of 1 type or 2 types or more)
Both Ca and REM have the effect of improving workability by controlling the form of sulfides, and can contain one or more as required. However, excessive addition may adversely affect cleanliness, so the Ca and REM contents are made 0.01% or less in total.

Next, the steel structure will be described.
The low temperature transformation phase works effectively to increase strength and improve impact resistance. If the volume ratio of the low temperature transformation phase is less than 10%, the effect is poor, and if it exceeds 50%, the workability deteriorates, so the volume ratio of the low temperature transformation phase is 10 to 50%. Further, the impact resistance is improved by finely dispersing the low temperature transformation phase, and the effect becomes remarkable when the average crystal grain size of the low temperature transformation phase is 2 μm or less and the average distance between adjacent low temperature transformation phases is 2 μm or less. Therefore, the average crystal grain size of the low temperature transformation phase is 2 μm or less, and the average distance between adjacent low temperature transformation phases is 2 μm or less. In order to obtain even higher impact resistance, it is preferable that the average crystal grain size of the low temperature transformation phase is 1.5 μm or less. The ferrite particle size is not particularly limited in the present invention, but usually 3 μm or less when the configuration of the present invention is adopted. Further, the phase other than ferrite, martensite, bainite, and residual γ may contain pearlite, but if it satisfies the above phase configuration, it does not matter if pearlite is contained.

[About manufacturing method of high strength cold-rolled steel sheet]
Next, a method for producing a high-strength cold-rolled steel sheet having the above configuration will be described.
The high-strength cold-rolled steel sheet is obtained by melting the steel adjusted to the above-mentioned suitable component composition in a converter or the like, forming a slab by a continuous casting method or the like, and subjecting the slab to hot rolling and cold rolling, Manufactured by annealing. The continuous annealing, steel sheet, an average heating rate at 500 ° C. to A ₁ transformation point temperature was raised to A ₁ transformation point as 10 ° C. / s or higher, then A _l temperature of transformation point ~ (A ₃ transformation point + 30 ° C.) After holding in the region for 10 seconds or more, the steel structure as described above is formed by cooling to 550 ° C. or less by setting the average cooling rate at 550 ° C. or more to 5 ° C./s or more.

Hereinafter, each step will be specifically described. In addition, about casting, hot rolling, and cold rolling, it does not specifically limit in this invention, The conditions described below are suitable conditions.
[Casting conditions]
The slab to be used is preferably produced by a continuous casting method in order to prevent macro segregation of components, but may be produced by an ingot-making method or a thin slab casting method. After manufacturing the steel slab, in addition to the conventional method of cooling to room temperature and then heating again, without cooling to room temperature, insert it into a heating furnace as it is, or carry out slight heat retention Energy saving processes such as direct feed rolling and direct rolling, which are rolled immediately, can be applied without any problem.

[Hot rolling conditions]
(Slab heating temperature: 1100 ° C or higher)
As for the slab heating temperature, low-temperature heating is preferable in terms of energy, but if the heating temperature is less than 1100 ° C., the carbide cannot be sufficiently dissolved, or the risk of trouble during hot rolling due to an increase in rolling load increases. Problems arise. Note that the slab heating temperature is desirably 1300 ° C. or less because of an increase in scale loss accompanying an increase in oxidized weight.
From the viewpoint of preventing troubles during hot rolling even if the slab heating temperature is lowered, a so-called sheet bar heater that heats the sheet bar may be used.

(Finish rolling end temperature: A ₃ transformation point or higher)
The finish rolling completion temperature is A less than ₃ transformation point, and generates the α and γ during rolling, the band-like structure is liable to generate in the steel sheet, such band-like tissue also remain after or annealing after cold rolling, It may cause anisotropy in the material properties or cause a decrease in workability. Therefore, the finish rolling temperature is preferably set to A ₃ transformation point or more.
(Winding temperature: 450 ° C to 700 ° C)
When the coiling temperature is less than 450 ° C. or exceeds 700 ° C., there are problems such as insufficient carbonitride generation and difficulty in controlling the coiling temperature. For this reason, it is desirable that the coiling temperature be in the range of 450 to 700 ° C.

  In the hot rolling process of the present invention, part or all of finish rolling may be lubricated rolling in order to reduce the rolling load during hot rolling. Performing lubrication rolling is also effective from the viewpoint of uniform steel plate shape and uniform material. In addition, it is preferable to make the friction coefficient in the case of lubrication rolling into the range of 0.25-0.10. Moreover, it is preferable to set it as the continuous rolling process which joins the front and back sheet | seat bars and finish-rolls continuously. The application of the continuous rolling process is also desirable from the viewpoint of the operational stability of hot rolling.

Next, preferably after removing the oxidized scale on the surface of the hot-rolled steel sheet by pickling, it is subjected to cold rolling to obtain a cold-rolled steel sheet having a predetermined thickness. Here, pickling conditions and cold rolling conditions are not particularly limited, and may be in accordance with conventional methods.
The rolling reduction during cold rolling is preferably 40% or more from the viewpoint of increasing the number of γ nucleation sites generated by reverse transformation during annealing and promoting the refinement of crystal grains.

[Continuous annealing conditions]
(Average heating rate at 500 ° C. to A ₁ transformation point: 10 ° C./s or more)
The heating rate is the most important condition in the production method of the present invention. By adjusting the average heating rate at the A ₁ transformation point from 500 ° C., which is the recrystallization temperature range in the steel of the present invention, to 10 ° C./s or more, recrystallization at the heating temperature rise is suppressed, and at the A ₁ transformation point or more. It works effectively for the refinement of the γ produced, and hence the refinement of the structure after annealing and cooling. If the average heating rate is less than 10 ° C./s, recrystallization of α occurs at the time of heating and heating, the strain introduced in α is released, and sufficient miniaturization cannot be achieved.
Accordingly, the average heating rate at the 500 ° C. to A ₁ transformation point is set to 10 ° C./s or more. A preferable range of the average heating rate is 20 ° C./s or more.

(A ₁ transformation point to (A ₃ transformation point + 30 ° C) held for 10 seconds or more)
When the heating temperature is less than the A ₁ transformation point or the holding time in the heating temperature range from the A ₁ transformation point to (A ₃ transformation point + 30 ° C.) is less than 10 seconds, the generation of γ during annealing becomes insufficient, and after annealing cooling A sufficient amount of the second phase cannot be secured. On the other hand, if the heating temperature exceeds (A ₃ transformation point + 30 ° C.), the growth of γ grains during annealing is intense and sufficient miniaturization cannot be achieved. The upper limit of the holding time in this heating temperature region is preferably set to 300 seconds or less from the viewpoint of energy and suppression of γ coarsening.

(Average cooling rate from heating temperature to 550 ° C: 5 ° C / s or more)
When the average cooling rate is less than 5 ° C./s, the degree of supercooling of the γ → α transformation during cooling becomes small, and the crystal grains become coarse. Further, pearlite is generated during cooling, and a sufficient amount of low-temperature transformation phase cannot be obtained.
Moreover, you may perform an overaging process after cooling to 550 degrees C or less. At that time, the conditions for the overaging treatment are not particularly limited, but it is preferable that the holding temperature is 250 to 550 ° C. and the holding time is 10 to 1000 seconds.
By setting it as the above manufacturing methods, the high intensity | strength cold-rolled steel plate which has the said microstructure (low temperature transformation phase) and is excellent in the impact-resistant characteristic in a low strain region can be obtained.

It is also possible to produce the hot dip galvanized steel sheet and the alloyed hot dip galvanized steel sheet by performing the above annealing in a continuous hot dip galvanizing line. In the production of the alloyed hot-dip galvanized steel sheet, after the plating treatment, the hot-dip galvanized film is alloyed by holding at 480 to 550 ° C. for 5 to 60 seconds.
In addition, you may add temper rolling for adjustment of shape correction, surface roughness, etc. to the steel plate after said annealing or the steel plate after a hot dip galvanization process (a galvannealed steel plate is included). In addition, there is no inconvenience even if treatments such as resin or oil coating and various paintings are applied.

Next, since the test which confirms the effect of this invention was done, it demonstrates.
First, steel having the composition shown in Table 1 with the balance being Fe and inevitable impurities was melted in a converter and made into a slab by a continuous casting method. The obtained slab was hot-rolled to a plate thickness of 3.0 mm. The hot rolling conditions were a finishing temperature of 900 ° C., a cooling rate of 10 ° C./s, and a winding temperature of 600 ° C.

Next, after pickling the hot-rolled steel sheet, cold rolling was performed to obtain a sheet thickness of 1.2 mm.
Next, the processes shown in Table 2 were performed on these cold-rolled steel sheets in a continuous annealing line or a continuous hot dip galvanizing line.

In Table 2, “heating temperature” and “holding time” under the continuous annealing conditions are the heating temperature and holding time in the temperature range from the _Al transformation point to (A ₃ transformation point + 30 ° C.).
After the treatment shown in Table 2, the microstructure, tensile properties and impact resistance properties of the obtained steel sheet were investigated, and the results are shown in Table 3.

  In addition, the structure was observed with a scanning electron microscope in a rolling direction cross section of ¼ part of the plate thickness of the steel sheet with a field of view of 5000 times, and the area ratio of ferrite and the low-temperature transformation phase was obtained to be the volume ratio. . The average particle size of the low-temperature transformation phase was observed at a magnification of 5000 using a scanning electron microscope, and the total area of the low-temperature transformation phases in the visual field was divided by the number of the low-temperature transformation phases to obtain the average area. The square was taken as the average particle size. Moreover, the average distance between adjacent low temperature transformation phases was determined as follows. First, for the arbitrarily selected low-temperature transformation phase, the distance from one arbitrarily selected point in the low-temperature transformation phase to the nearest grain boundary of each other low-temperature transformation phase is obtained, and the distance is the most The average of three short measured values was taken as the proximity distance of the low temperature transformation phase. Similarly, the proximity distance was obtained for a total of 15 low-temperature transformation phases, and the average value of these 15 was taken as the average distance between the low-temperature transformation phases adjacent to the steel.

Tensile properties were measured by performing a tensile test at a strain rate of 10 ⁻³ / s using a JIS No. 5 test piece taken from a direction perpendicular to the rolling direction of the unprocessed steel sheet. In Table 3, “YS” is the yield strength, “TS” is the tensile strength, “YR” is the yield ratio, and “El” is the total elongation (breaking elongation).
Similarly, the shock absorption characteristic is that when a tensile test is performed at a strain rate of 2000 / s using a test piece having a width of 5 mm and a length of 7 mm of a parallel portion taken from a direction perpendicular to the rolling direction of a non-processed steel plate, the strain amount is 5 % (Iron and steel, 83 (1997), p.748). Absorbed energy was determined by integrating the stress-true strain curve within a strain range of 0-5%.

As a result, as shown in Table 3, in the example of the present invention, the ratio (AE / TS) between the absorbed energy and the static TS with a strain rate of 2000 / s and a strain amount of up to 5% is higher than 0.05, which is high. It was confirmed to have impact resistance characteristics.
Therefore, according to the example of the present invention, a cold-rolled steel sheet and a hot-dip galvanized steel sheet having excellent impact resistance characteristics can be obtained. There is an excellent effect of greatly contributing.

Claims (6)

C: 0.05-0.25 mass% or less, Si: 1.0-2.0 mass% , Mn: 1.0-3.0 mass%, P: 0.10 mass% or less, S: 0.0. 02% by mass or less, Al: 0.01 to 2% by mass, and one or two of Nb and V in total, 0.01 to 0.2% by mass, with the balance being iron and inevitable impurities Having a steel composition,
As a steel structure, it has ferrite and a low temperature transformation phase composed of one or more of martensite, bainite and residual γ,
The volume ratio of the low temperature transformation phase is 10 to 50 vol. %, And the average crystal grain size of the low-temperature transformation phase is 2 μm or less, and the average distance between the adjacent low-temperature transformation phases is 2 μm or less. 2. The high-strength cold according to claim 1, wherein the steel composition further contains a component defined in one or more of the following groups A, B and C within the specified range. Rolled steel sheet. (Group A) Cr and Mo: 0.05 to 2.0 mass% in total of 1 type or 2 types
(Group B) B: 0.005 mass% or less (Group C) Ca and rare earth elements: 0.01 mass% or less in total of 1 type or 2 types or more   The high-strength cold-rolled steel sheet according to claim 1 or 2, which has a hot-dip galvanized layer or an alloyed hot-dip galvanized layer on the surface. After subjecting the steel material having the steel composition according to claim 1 or 2 to hot rolling and cold rolling, continuous annealing is performed,
And in the continuous annealing, the steel sheet produced by rolling the cold, 500 ° C. to A ₁ the average heating rate in the transformation temperature was elevated to the A ₁ transformation point as 10 ° C. / s or higher, followed by the A ₁ transformation point ~ (A ₍₃ transformation point + 30 ° C.) After maintaining for 10 seconds or more in the temperature range, the average cooling rate at 550 ° C. or more is set to 5 ° C./s or more and cooled to 550 ° C. or less. .   The method for producing a high-strength cold-rolled steel sheet according to claim 4, wherein hot-dip galvanizing treatment is performed on the steel sheet surface during or after the cooling.   The high galvanizing layer according to claim 5, wherein after the hot dip galvanizing treatment is performed, the galvanized layer formed by the hot dip galvanizing treatment is alloyed by holding at 480 to 550 ° C for 5 to 60 seconds. A method for producing a high strength cold-rolled steel sheet.