JPS58141334A - Production of hot rolled steel plate having >=60kgf/mm2 tensile strength and excellent workability and weldability - Google Patents

Abstract

PURPOSE:To provide a hot rolled steel plate having excellent tensile strength, workability and weldability as a steel plate for automobiles, by combining control of components consisting mainly of C, Si and Mn and specific conditions for hot rolling thereby forming fine composite structures of ferrite and pearlite. CONSTITUTION:In the stage of producing a steel plate having 1.6-4.0mm. thickness, >=60kgf/mm.<2> tensile strength and excellent workability and weldability as a steel plate for automobiles, the steel slab contg. 0.15-0.25% C, <1.5% Si, 0.7- 1.5% Mn, <0.01% P, <0.005% S and 0.01-0.10% Al, or further contg. >=1 or >=2 kinds among 0.0005-0.0050% Ca, 0.005-0.015% REM and 0.0005-0.010% Mg as granulating agents of sulfide inclusions is hot-rolled at <=1,200 deg.C and after the finish rolling is finished at the Ar3 transformation point -930 deg.C, the steel plate is cooled at >=15 deg.C/sec average cooling rate and is coiled at 350-560 deg.C, whereby the fine composite structures of ferrite and pearlite are obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is mainly aimed at automobile applications, and has a plate thickness of 1.6 to
The present invention relates to a method for producing a high-strength hot-rolled steel sheet having a tensile strength of about 4.0-60 to 9 f/■3 or more, and excellent workability and weldability.

Conventionally, high-strength hot rolled steel with a tensile strength of 60kIif/■1 or more
Steel sheets were manufactured using a precipitation strengthening system in which fil-Mn, KNb, V, and TI were added to provide strength through precipitation strengthening of carbohydrates. However, this method is not economical due to the use of expensive Nb, V, TI, etc., and the low elongation characteristic of precipitation strengthening has hindered its use.

A material expected to overcome this situation is so-called composite structure steel (Dual Phase W4) consisting of ferrite and martensite. It has particularly favorable properties from the viewpoint of workability, such as a low yield point, high work hardening, and high elongation. The original manufacturing method for this steel was (t
/r Continuous annealing equipment is suitable for this by heating to a two-phase region and then rapidly cooling. However, since there is generally no continuous annealing equipment for hot-rolled steel sheets, research and development has been conducted on manufacturing in hot-rolled steel sheets, that is, non-temperature treatment, and several methods have been proposed, but the process conditions Problems arise such as excessive load being applied to the metal and the need for many alloying elements, so it has not been put into practical use from the viewpoint of economy and properties.

Against the background of the above, the present invention has been developed and put into practical use. The gist of the present invention is as follows.

(1) C: over 0.15 to 0.25 whisper, 81: 1.5
('Mn: 0.7-1.5%, P: 0.01- or less, 10.005- or less, Mut: 0.01-0.1
0 * t - Contains remainder F ↓ and steel consisting of unavoidable impurity elements is made into a slab, heated to 1200℃ or less, hot rolled and finish rolled at Ar transformation point ~930℃, and then Average cooling series 1i 15℃/S or more cooling 35
60 to 9f/- characterized by winding at 0 to 560℃
It has a tensile strength of 1141! A method for producing nine high-strength hot-rolled steel sheets with excellent properties.

(z) C: over 0.15 to 0.25 arrogant, 81:1.
5- or less, Mn: 0.7-1.5 sSP: 0
．． 01 whisper or less, 8: 0.005 whisper or less, muz:o
, ox~0. 10, then C1: o, ooos~o,
ooso-1RIM: 0.005~0.0151G
, Mg: 0.0005 to 0.010% (1 of D
Danma-ku is made by forming a slab of steel consisting of two or more Dan and the balance F and unavoidable impurity elements, then heating it to 1000℃ or less, hot rolling it, and finishing rolling at a transformation point of ~930℃. After that, it is cooled at an average cooling rate of 15'C/8 or more and wound up at 350-560°C.It has a tensile strength of 60yf/- or more as a percentage of 1, and has workability and #a bondability. A method for producing hot-rolled steel sheets with excellent arc.

In other words, the combination of component regulation centered on C, 81, and Mni and specific hot rolling conditions results in fine ferrite and dense).
Strength and elongation are ensured by the set fII&'ft@ consisting of 萐-light. Although the elongation at this time is inferior to that of composite structure steel, it is better than precipitation strengthened steel, and it can withstand forming sufficiently for applications such as automobile parts. The workability of hot-rolled steel sheets for automobiles is not just about elongation, but also stretchability. Therefore, sulfide-based inclusions are thoroughly reduced, and in some cases, Ca
, RIM.

Add one or more layers of Mg. Spot weldability is the next most important property for hot rolled steel sheets for automobiles.

There is a simple way to judge spot weldability based on whether the welded part is peeled off vertically or peeled off, but more precisely, it is determined that the weld does not peel off from the joint surface with cross tensile strength (JI8.Z3137) and that fatigue strength is It becomes a problem. It has been found that limiting the amount of C in the steel and reducing P by 80 degrees are effective in improving such spot weldability.

By identifying the above-mentioned ingredients and hot rolling conditions, and creating an exquisite combination, we have been able to manufacture a high-strength, thermodynamic steel sheet that balances workability, point **a, and economic efficiency.9.

Next, the reason for limiting the number of features of each configuration of the present invention will be described.

C is 60kliiZ as ferritic/4-lite steel
-6 is necessary to ensure strength of 4 or more, and the minimum @0.15% is required for the 9th point. However, when it exceeds 0.25 inches, the 9-light part increases too much and becomes 60 and 41f/
Even if a tensile strength of wi' or more is considered, the ductility deteriorates significantly and the spot weldability also deteriorates. The ninth C is 0.15fI
4 to 0.25-.

Sl is substituted into the ferrite phase and is effective in increasing the strength. Furthermore, it has the effect of increasing the degree of work hardening of ferrite and increasing its ductility. However, if the amount exceeds 1.5, these effects tend to become saturated, and because of the 8-ton scale, pickling properties are poor, and economic efficiency is also impaired, so the amount of 81 added is less than 1.5. Tojiku.

Mn creates a dense ferrite Δ-lite structure, and -
Since it improves the ductility t- as well as the strength, it costs 0.71 seconds. However, if too much Kn is added, a layered structure will appear, which will deteriorate ductility and increase costs, so the upper limit should be set at 1.5%.

Next KP is point melting W! It is necessary to thoroughly lower the level from a gender perspective to 0. Ol- and friends below. Furthermore, since it is necessary to lower S to the lowest point in terms of spot weldability and stretch flangeability, it is set to 0.005 or less.

In order to improve elongation 7 run properties, it is necessary to reduce the amount of sulfide inclusions, and for this purpose it is necessary to reduce the amount of 8 sulfide inclusions as described above. In addition to reducing this, it is preferable to make it spherical. For that purpose, 1 of Cm, REM, Mg
It is preferable to add more than one species to form a sulfide with less plasticity. 0.0005%, 0.0051G, 0 respectively
．． Below 000511, the effect of spheroidization is small, while o and o o s respectively.

If the temperature exceeds 0.015- or 0.010-, the effect of spheroidization is saturated, and on the contrary, oxide inclusions increase and the ductility deteriorates.
When adding 11 or more, o, ooos ~ o, o, respectively.
os.

-,0,005~0.015!1.0.0005~0.
It needs to be 010-.

MuL is necessary as a deoxidizing agent. 0. If it is less than 0.10, there is no effect, and if it exceeds 0.10, alumina inclusions will increase and the ductility of the steel will deteriorate.

Next, regarding the KM rolling conditions, the hot rolling conditions are very important constituent requirements in combination with the components in the present invention.

First, the heating temperature needs to be 1200°C or less. In the steel of the present invention, TI and Nb are used from the viewpoint of ductility.

■Not added. Therefore, austenite is difficult to break down into fine grains during hot rolling, and the temperature range in which it is not recrystallized is also small. Therefore, unless the austenite grains in the state before rolling are made small, a fine final structure cannot be obtained. Therefore, it is necessary to heat it at a low temperature. This also results in the advantage of energy saving. Furthermore, it is preferable that the heating temperature of K is set to 1150° C. or lower to obtain a more thorough microstructure. Lower limit of heating temperature □ value is low i within the range where hot rolling is possible.
Although it is better, it is usually 1050°C.

Next, the temperature at which finish rolling is finished must be between the inflection point and 930°C. When the temperature exceeds 930°C, the austenite before transformation becomes coarse and exhibits a coarse bainite-like structure, resulting in deterioration of ductility. If rolling is carried out below the 9mr3 transformation point, ferrite transformation occurs and the 7e2ite undergoes processing, deteriorating ductility.

Next, cooling from the end of finish rolling to coiling is important in that the structure is formed by transformation during this period, and for this purpose it is necessary to maintain an average speed of tsc/s or more. 15℃
If it is less than /8, it tends to become a coarse ayulite/armorite structure, which does not provide sufficient strength, and tends to result in a band-like structure, which also deteriorates stretch flangeability. In order to ensure more stable strength, it is preferable to use 25C/8 or more.

The winding temperature is an important component in obtaining the properties unique to the present invention in relation to the components and the heating temperature. If the coiling temperature exceeds 600°C, a fine ferrite structure cannot be obtained and strength cannot be ensured. On the other hand, if the coiling temperature is lower than 350° C., the rolling speed will be limited, resulting in reduced productivity and deformation, which will require a straightening process, which will impair economic efficiency. Therefore, the winding temperature is 350~
The temperature was 560°C. However, if the winding temperature is below 400℃, the productivity will be slightly lower than the normal winding temperature.
It is preferable that the temperature exceeds 10°C. On the other hand, in order to stably obtain a fine structure, it is preferable that the winding temperature be 480°C or less. Light tissue is 5~10 μml! The structure is composed of 4-ripnal ferrite with a diameter of 1.5° and fine morpholites present at the grain boundaries, and gnorite does not show a complete layered structure. In some cases, island-like martinsite and bainite are also present.

The reason for limiting the numerical values of the constituent elements has been described above.The slab used here can be made by either the blooming method or the continuous casting method, but considering economic efficiency, it is preferable to use the continuous casting method. , It is also preferable to warmly charge the slab into a heating furnace to save energy.

Next, the present invention will be explained with examples.

#I steel having the components shown in the first column was produced in a converter, made into a solid material by continuous casting, and then hot rolled.

@ill @o Chemical components in Examples (vtljJ
The hot rolling conditions are shown in j12fi. Among the steels in Table #11, codes M to D are steels of the present invention. The sign wealth, FOsl, is the amount of C,
The steel with the symbol G is different from the present invention in the amount of P and the amount of -FiS with the symbol H. Under hot rolling conditions, Hiromu l ~ 3.8,
11. 13 to 17 are based on the original @ conditions, A4 is the finishing temperature, 45 is the heating temperature, A6, 10 is the winding temperature, and A7, 9, 12 is the finishing temperature between the two windings. The average cooling rate is different from that of the present invention.

After washing the steel strips manufactured in this way, they are cut into round plates on a cutting line. At that time, it was subjected to 1% temper rolling. Afterwards, it was subjected to material testing.

A J18Z2201.5 test piece was used for the tensile test.

The cross tension of spot welded joints shall be in accordance with JI8 Z3137.

Welding is done at a single point, and the conditions are: electrode 8.0 mm, energization time 28 cycles, holding time 28 cycles, and pressure 50.
Set the current to 0kl and keep it close to the optimum value in terms of cross tensile strength. In the case of 2.3 plate thicknesses, it was 130OA. ,
In addition, the hole expansion test uses a method of expanding a sheared hole with a diameter of 20 mm using a conical punch, and the hole diameter at the time when the crack penetrates the plate thickness is divided by the original hole diameter (20 wm). Therefore, the hole expansion is 9 in comparison.

The results of the material tests are shown in Table 3. In addition, the material test results of typical composite structure steel (D%xal Phase steel) and precipitation strengthened steel are also shown in the 3rd fl.
．． 3-).

#I3 Table p 41 in which both the ingredients and hot rolling conditions were in accordance with the present invention.
The steels No. 3, 8, 11 and 13 have a tensile strength of 60 to above and an elongation superior to precipitation strengthened steels. These OS steels also show good values in spot welding cross tensile strength. However, although it is slightly lower than 414, which is a low C material, it is a good value considering that the C content is set to exceed 0.1511 for the purpose of increasing strength. As for the fracture type, some fractures in the slit were also observed1.
Further, the hole expansion ratios are both good values of 1.4 or more.

In contrast, 44. S steel has slightly higher tensile strength, but elongation is extremely poor, and the hole expansion ratio is also small.

Tensile strength 60k under A6, 7.9, 10.12 conditions
lf/m'' class cannot be obtained, and even with the steel of Idoro 14, which has a small amount of C, a tensile strength of 60:f/■3 class cannot be obtained. However, there are defects such as extremely low cross tensile strength and low expansion ratio during elongation.

Further, as described above, there are few factors inhibiting productivity under hot rolling conditions according to the present invention, and the yield drop due to shape etc. is also low t%p*, so economical efficiency is also good.

Finally, the steel strip according to I14 of the present invention may be used as a black-skinned steel strip, or it may be pickled and cut using a 4-hole shear twin. 70＠、Repeat the shape by heat rolling beforehand)、You can also correct the sound-0

Claims (2)

[Claims] (1) C: more than 0.1 s to 0.25%, 81:1
．． 5- or less, M! l: 0.7-1.51s%P
:, 0.011i or less, 8: 0.005- or less, A
A: Contains 0.01-0.10% and the remainder consists of F and unavoidable impurity elements.
Heating to 200℃ or less and hot rolling to achieve unevenness, transformation point ~9
Finish rolling was completed at 30C, and then the average cooling rate was 15℃.
It has a tensile strength of 60 kJf/;- or more and is workable, characterized by being cooled at a temperature of /S or higher and wound at 350 to 560°C.
A method for producing hot-rolled steel sheets with excellent weldability. (2) C: 0.15m-0,251,81:
1.5- or less, Mn: 0.7-1.5 L P:
0.011 or less, 8:0.005- or less, At:0
．． 01-0.10-1 further KCa: 0.0005-0.
0050%, Rj:M: 0.005-0.0151
! , Mg: 0.0005 to 0.01010, after forming a slab of steel containing 1 or more m or more and the remainder F@ and unavoidable impurity elements, it is heated to 1200°C or less and hot rolled to form a slab. Finish rolling was completed at the transformation point ~930°C, and then cooled at an average cooling rate of 15 c7 s or more to 35° C.
4 at 0-560℃! 60kII with 4 images to take f
, f/- or higher tensile strength and excellent workability and weldability.