JPH1036917A - Production of high strength hot-rolled steel plate excellent in stretch-flanging property - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the unevenness of strength and stretch-flanging property by regulating chemical components and hot-rolling condition. SOLUTION: Silicon is the most important element and acts to restrain the precipitation of cementite harmful to the stretch-flanging property. When 0.04-0.07wt.% of C concn. range, and 1.0-1.4wt.% of So concn. range, the above restraining effect is remarkable. Finish-rolling is executed at a temp. above the Ar3 transformation point or higher. The stretch-flanging property is improved by the above operation because the recrystallization and granulation of γ till starting the cooling are progressed and Ar3 point at the time of cooling after rolling is lowered to make fine the structure of a product plate. Therein, the coiling temp. is particularly important, and when it is in the range of 400-600 deg.C, the accuracy to hit a coiling temp., is increased and the unevenness of the strength and the stretch-flanging property are reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a method for producing a coil, which has a small variation in the material inside the coil and between the coils, has an excellent stretch flangeability (generally referred to as a hole expanding property), and
The present invention relates to a method for manufacturing a hot-rolled steel sheet having a tensile strength of 0 MPa or more at low cost. Its use is in automobiles, home appliances, building materials, etc., but it is particularly suitable for use in undercarriage of automobiles.

[0002]

2. Description of the Related Art With increasing interest in global environmental protection, there is an urgent need to improve fuel efficiency of automobiles. As a means for that, there is a reduction in the weight of the vehicle body. For this reason, there is a strong demand for a thinner steel sheet by increasing its strength. Above all, hot-rolled steel sheets used as automobile undercarriage parts are subjected to severe processing such as stretch flange forming, and therefore are required to have high strength and excellent workability at the same time. From such a background, a hot-rolled steel sheet having high workability and enhanced strength has been conventionally developed by using various methods.

[0003] A dual phase steel composed of ferrite and martensite has a low yield ratio and excellent ductility, but a relatively coarse and hard martensite phase exists at the ferrite grain boundary, so that the stretch flangeability is low. Not enough. A steel sheet composed of ferrite and bainite is slightly inferior in ductility to the above dual structure (Dual Phase) steel, but is excellent in stretch flangeability, and various inventions have been disclosed. JP-A-57-1
The inventions disclosed in Japanese Patent Publication No. 01649, Japanese Patent Publication No. 1-46583, Japanese Patent Publication No. Sho 62-37089, and Japanese Patent Publication No. 7-62178 are typical known examples thereof.
However, the formation of bainite is extremely sensitive to the cooling and winding conditions after hot rolling, so that the amount of bainite and the hardness of bainite change due to slight changes in the conditions, and the strength within the coil or between each coil is reduced. There is a drawback in that the material such as heat resistance, stretch flangeability, and ductility varies and the yield is poor.

[0004] A relatively advantageous structure against such material variations is a structure composed of ferrite and pearlite. JP-B-61-40015, JP-A-58-0015
The inventions disclosed in Japanese Patent Publication No. 11734 and Japanese Patent Publication No. Sho 64-10563 are typical known examples thereof. However, even these techniques for making ferrite and pearlite fine do not satisfy the recent demand for stretch flangeability.

As a means for suppressing the formation of pearlite, addition of Si can be mentioned. Japanese Patent Publication No. 7-62178 discloses a technique for suppressing the generation of pearlite in a structure mainly composed of bainite by actively utilizing Si. However, since this is a bainite structure, as described above, there is a problem of variations in the material and, consequently, a decrease in yield.

[0006]

SUMMARY OF THE INVENTION The present invention is to improve the stretch flangeability of steel composed of ferrite and cementite metal structures, thereby improving the strength of the material inside and between the coils, particularly strength and stretch flangeability. It is an object of the present invention to provide a method for producing a high-strength hot-rolled steel sheet having small variations.

[0007]

Means for Solving the Problems The present inventors have studied the chemical composition and hot rolling conditions based on the structure composed of ferrite and cementite to significantly reduce the volume fraction of cementite harmful to stretch flangeability. It has been newly found that the above problems can be solved by exercising.

[0008] The present invention has been made based on the above findings, and the gist thereof is as follows. (1) C: 0.04% or more and less than 0.07%, Si: more than 1.0%, less than 1.4%, Mn: 1.0% by weight%
Super, 1.5% or less, P: 0.02% or less, S: 0.00
8% or less, Al: 0.005% or more, 0.1% or less,
N: contained in a range satisfying 0.01% or less, and the remainder is a slab composed of iron and unavoidable impurities of 1000 to 13
After heating at 00 ° C. and rough rolling, finish rolling is performed at a temperature not lower than the Ar ₃ transformation point, and after finishing rolling, cooling is performed at a cooling rate of 40 ° C./s or more, and 400 ° C. or more and less than 600 ° C. A method for producing a high-strength hot-rolled steel sheet having excellent stretch flangeability characterized by winding at a temperature of

(2) As a steel component, further,
The method for producing a high-strength hot-rolled steel sheet having excellent stretch flangeability according to the above (1), wherein the content of Ca is 0.0005% or more and 0.0050% or less. (3) Further, as a steel component, Ti: 0.0
01% or more, 0.015% or less, Nb: 0.001% or more, 0.015% or less, B: 0.0001% or more, 0.1% or less.
The high-strength heat excellent in stretch flangeability according to the above (1) or (2), wherein at least one of 0040% or less is contained so as to satisfy 0.001% ≦ Ti + Nb + 5B ≦ 0.020%. Manufacturing method of rolled steel sheet.

(4) After the finish rolling, the cooling is started at a cooling rate of 40 ° C./s or more, and then the cooling is performed in a temperature range of (CT + 30) to (CT + 130) ° C. with respect to the winding temperature CT.
Any of (1) to (3) above, wherein cooling at a cooling rate of 0 ° C./s or more is completed, and cooling from the temperature range to CT is performed at a cooling rate of 3 to 20 ° C./s. 4. The method for producing a high-strength hot-rolled steel sheet excellent in stretch flangeability according to item 1.

Hereinafter, the present invention will be described in detail, and first, the reasons for limiting the chemical components will be described. C is necessary for securing the strength. For this reason, C is added in an amount of 0.04% or more. However, if it is too much, the amount of cementite increases or a second phase other than cementite is generated, resulting in a large variation in the material. And the stretch flangeability deteriorates.
%. From the balance between strength and stretch flangeability,
C is preferably in the range of 0.05 to 0.065%.

[0012] Si is the most important element in the present invention. It is well known that the positive addition of Si suppresses the precipitation of cementite. This is because Si is an element that hardly forms a solid solution in cementite, and therefore, in order to generate and grow cementite, Si must be diffused and discharged from the place. According to this concept, the greater the amount of Si, the more the precipitation of cementite is suppressed. However, the study leading to the present invention has revealed a very important and novel fact that there is an optimum value of the amount of Si for suppressing the precipitation of cementite. That is, in the range of the C content in the present invention, when Si is more than 1.0% and less than 1.4%, a remarkable inhibitory effect on the precipitation of cementite is exhibited.

The details of the study which led to the discovery of the existence of an optimum value of the amount of Si for suppressing the precipitation of cementite will be described below. We have 0.055% C-1.2%
Mn-0.01% P-0.001% S-0.002% C
In steel having a chemical composition of a-0.02% Al-0.004% N, the influence of the Si content on the precipitation of cementite was studied by changing the Si content from 0.5 to 2.0%. . Various conditions in the hot rolling are as follows. That is, the heating temperature is 1160 ° C., the finishing temperature is (Ar ₃ +50) to (Ar ₃ +70) ° C., and A
then cooled r ₃ or more temperature at a rate of about 60 ℃ / s, 40
A heat treatment equivalent to winding was performed at a temperature of 0 to 550 ° C. The results obtained are shown in FIG.

From FIG. 1, it has been found that there is an appropriate range of the amount of Si for suppressing the precipitation of cementite.
The reason for this is not necessarily clear, but is considered as follows. That is, when the amount of Si increases, γ after hot rolling
→ The α transformation is delayed, and as a result, the two-phase region of α + γ is expanded. Therefore, when winding is performed at a constant temperature, if the amount of Si is too large, the winding temperature is maintained during the γ → α transformation, and it is estimated that the amount of pearlite increases. Further, Si is important for ensuring strength.

From the above, the Si content exceeds 1.0%,
It shall be less than 1.4%. As is clear from FIG.
~ 1.3% is the most preferred range. Mn has an effect of refining the ferrite structure by suppressing the ferrite transformation and also finely dispersing the cementite in order to secure the strength, and is added in an amount of more than 1.0%.
However, if added in excess of 1.5%, the structure becomes non-uniform due to segregation, and the formation of a hard second phase other than cementite causes the material to vary, further deteriorates the weldability, and reduces the cost of the alloy. Because it will increase
The upper limit is 1.5%.

P is a highly segregated element, which causes a problem of brittle fracture and lowers the weldability. Therefore, the upper limit of P is 0.02%, but for applications requiring higher workability, 0.01% is required.
% Is preferred. Since S forms an elongated compound such as MnS and remarkably deteriorates stretch flangeability, the lower the S, the better. Therefore, the upper limit of S is set to 0.008.
%. In addition, it is preferably 0.004% or less for applications requiring higher stretch flangeability, and more preferably 0.002% or less for more difficult applications.

Al is at least 0.005 as a deoxidizing agent.
%Added. On the other hand, the addition of more than 0.1% saturates the deoxidizing effect, and rather increases the inclusions to deteriorate the stretch flangeability, so the upper limit is 0.1%. To properly balance the deoxidizing effect and processability, 0.01 to 0.0
It is better to be in the range of 5%. Since N deteriorates stretch flangeability, the upper limit is made 0.01%. For applications requiring higher stretch flangeability, the content is preferably 0.005% or less.

Ca may be positively added for controlling the form of sulfide. If less than 0.0005%, this effect is not sufficient, so the lower limit of Ca is set to 0.0005%. Further, even if added in excess of 0.0050%, there is no remarkable effect, but rather, inclusions increase to deteriorate the stretch flangeability, so the upper limit is 0.0050%. Ti, Nb, B
Is effective in rendering the harmful effects of N harmless and in making the structure finer, so one or more kinds may be added in a small amount. That is, Ti and Nb are in the range of 0.001 to 0.015%, and B is in the range of 0.0001 to 0.0040%.
001% ≦ Ti + Nb + 5B ≦ 0.020%. Below each range, N is fixed,
The effect of refining the structure is not sufficient, and when added beyond the respective ranges, compounds with elements other than N are formed,
It may cause material variation.

The raw materials for obtaining the above-mentioned components are not particularly limited. However, in addition to the method of preparing the components by using a blast furnace and a converter using iron ore, scrap may be used as a raw material, and the scrap may be melted in an electric furnace. May be. When scrap is used as all or a part of the raw material, Cu, Cr, N
Elements such as i, Sn, Sb, Zn, Pb, Mo, etc.
% Or less.

Next, the reasons for limitation on the manufacturing process will be described. The slab to be subjected to hot rolling is not particularly limited. That is, it may be any as long as it is manufactured using a continuous cast slab or a thin slab caster. It is also suitable for processes such as continuous casting-direct rolling (CC-DR) in which hot rolling is performed immediately after casting.

The heating temperature in hot rolling is 1000 to 1
The range is 300 ° C. If it exceeds 1300 ° C., the pickling property after winding may be deteriorated, or the surface properties may be deteriorated. On the other hand, if the temperature is lower than 1000 ° C., a load is applied to the rolling, and it is difficult to secure a finishing temperature. In order to obtain good pickling properties, the temperature is preferably 1200 ° C. or lower. After roughly rolling the slab hot, a bending process with a curvature of 1.5 m or less may be performed. That is, after winding in a coil shape at a temperature of 950 to 1150 ° C. and holding for 0.5 seconds or more and 30 minutes or less, rewinding is performed from the end of winding at the time of winding, and finish hot rolling is performed. By winding the rough rolling bar into a coil as described above, a heat retention effect is generated, so that temperature variation in the coil is reduced, and a uniform material is obtained. Furthermore, since scale is removed by bending, pickling properties and surface properties are improved. In the present invention, Si
, And a red scale or a cloud pattern is likely to be generated, and thus the bending after the rough rolling is particularly important.

Further, the rough bar once wound into a coil shape may be joined to the preceding rough bar, and finish rolling may be performed continuously. It goes without saying that by performing finish rolling continuously, the temperature distribution in the longitudinal direction of the coil becomes uniform, and excellent material is secured over the entire length. Furthermore, the rough rolling bar wound in a coil shape may be subjected to finish rolling after being transferred into a facility having a heat retaining or heating ability.

The finish rolling is performed at the Ar ₃ transformation point or higher. Ar
_When hot rolling is performed at a temperature lower than ₃ , the stretch flangeability deteriorates and the thickness accuracy also decreases. It is more preferable to finish at a temperature of Ar ₃ + 50 ° C. or higher. This is γ by the start of cooling.
Recrystallization and grain growth progress, and Ar during cooling after rolling
₃ is reduced, and the structure of the product plate becomes finer, so that the stretch flangeability is improved.

After the finish rolling, rapid cooling is performed at a cooling rate of 40 ° C./s or more. This cooling is preferably performed from a temperature of Ar ₃ or more. By cooling from a temperature below Ar ₃ ,
The γ → α transformation proceeds by cooling down to the start of cooling,
Since the degree of supercooling is still low, the initial ferrite structure becomes large. Furthermore, since the ferrite structure transformed at a lower temperature by the subsequent quenching becomes fine, the ferrite structure becomes uneven as a result, and the stretch flangeability deteriorates. The cooling rate of 40 ° C./s or more
It is essential for making the structure finer and more uniform.

The end point temperature of the rapid cooling after hot rolling may be set with respect to a target winding temperature (CT), and in particular, a temperature range of (CT + 30) to (CT + 130) ° C. with respect to the winding temperature. Further, by performing gentle cooling at 3 to 20 ° C./s until winding, accuracy of the target of the winding temperature can be improved, and variation in the material inside and between the coils can be further reduced. . When the end point temperature of the rapid cooling is lower than (CT + 30) ° C. or the cooling rate of the slow cooling exceeds 20 ° C./s, the effect of improving the accuracy of CT accuracy is reduced. In addition, the end point temperature of rapid cooling is (CT +
If the temperature exceeds 130) ° C. or the cooling rate of the slow cooling is lower than 3 ° C./s, not only does the productivity decrease, but also the cementite precipitation increases and the structure becomes coarse.

The winding temperature is of particular importance in the present invention. If the winding temperature is lower than 400 ° C., the accuracy of the winding temperature is significantly reduced, and the second phase is variously changed to cementite, martensite, bainite, etc. due to the variation of the winding temperature, and the strength and stretch flangeability are large. Variations and the desired steel material cannot be obtained. On the other hand, when the winding temperature is 600
If the temperature exceeds ℃, coarse pearlite is precipitated, and the stretch flangeability becomes poor. Therefore, the upper limit is set to 600 ℃.
In order to further improve the accuracy of the target temperature of the winding temperature, reduce the variation in the material, and ensure excellent stretch flangeability,
480-550 ° C is a preferred range.

The structure obtained by the present invention has an area ratio of 95%
It is composed of extra ferrite and less than 5% cementite. The ferrite includes both polygonal ferrite and acicular ferrite. Further, it may contain less than 2% of a bainite phase. In order to obtain excellent stretch flangeability, the ferrite fraction is preferably 98% or more.

Temper rolling is performed according to the purpose. That is,
From the viewpoint of shape correction and surface roughness adjustment, and further ensuring non-aging property, it is preferable to perform temper rolling at a rolling reduction of 0.5% or more. Note that the temper rolling may be performed in-line after finish hot rolling, or may be performed off-line after winding or pickling. The hot-rolled steel sheet according to the present invention may be used as it is after winding, after pickling, or after temper rolling, or may be subjected to various surface treatments. Further, the hot-rolled steel sheet may be used as a cold-rolled material.

The hot-rolled steel sheet obtained by the present invention has a strength of 440 MPa or more and is excellent not only in stretch flangeability but also in weldability and fatigue properties.

[0030]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The embodiments of the present invention will be specifically described below by way of examples. A slab having the chemical components shown in Tables 1 and 2 (continued from Table 1) was subjected to rough rolling at a heating temperature of about 1170 ° C. in an actual machine, and the final finish rolling temperature was (Ar ₃ +60) ° C. or higher, and after the finish rolling. At a cooling rate of 50 to 70 ° C./s from a temperature of Ar ₃ or more (CT + 60)
~ (CT + 80) ° C, then about 10 ° C / s
And cooled at about 500 ° C. The board thickness is 2.6mm
And After subjecting to 1.2% temper rolling, it was subjected to a tensile test and a hole expanding test. In the tensile test, the test material was JIS
No. 5 test piece described in Z 2201
The test was performed according to the test method described in H.241 In the hole expanding test, a hole having a diameter of 10 mm was formed in the center of a 90 × 90 mm plate by punching with a punching clearance of 12%.
The test was performed using a conical punch having a vertical angle of 60 °. The hole expansion ratio as an index of stretch flangeability was evaluated by the value of d / do when the original hole diameter was do and the hole diameter after the test was d. Table 3 shows the results. As is clear from this, it is understood that the steel having the chemical composition within the range of the present invention has excellent stretch flangeability (high hole expansion ratio) and ductility.

[0031]

[Table 1]

[0032]

[Table 2]

[0033]

[Table 3]

Next, steels B, D, E, G, and
Using I and J, the heating temperature was set to about 1200 in the laboratory.
C. and rough rolling was performed. The finish rolling temperature is Ar ₃ or more, and after cooling at about 60 ° C./s from the temperature of Ar ₃ or more,
The change of the material when the winding temperature was changed variously was examined. The plate thickness was 2.9 mm. Table 4 shows the results. ADVANTAGE OF THE INVENTION According to this invention, the hot-rolled steel plate of an excellent material is obtained, while the dependence of the hole expansion ratio and the strength on the winding temperature is extremely small. Also, the winding temperature is a stable region in the temperature range of 48.
When the temperature is 0 ° C. or higher, not only excellent characteristics can be obtained, but also
Yield also improves. In addition, in the sample I of the comparative example, the winding temperature was 430 ° C., and the hole expansion ratio showed a good value of 2.3, but as the winding temperature was increased, the amount of cementite increased, and the hole expansion ratio decreased. . That is, there is a problem that the range of the winding temperature for obtaining good stretch flangeability is narrow, which causes a decrease in yield.

[0035]

[Table 4]

[0036]

As described above, according to the present invention,
It is possible to provide a high-strength hot-rolled steel sheet that has excellent stretch flangeability and ductility, and also has excellent material uniformity within the coil or between each coil. Therefore, the present invention can be said to be an industrially excellent invention.

[Brief description of the drawings]

FIG. 1 is a diagram showing the influence of the Si content and the winding temperature on the cementite fraction obtained in a basic experiment leading to the present invention.

Continued on the front page (72) Inventor Jun Itami 1 Kimitsu, Kimitsu City Nippon Steel Corporation Kimitsu Works

Claims (4)

[Claims] 1. In weight%, C: 0.04% or more and less than 0.07%, Si: more than 1.0%, less than 1.4%, Mn: more than 1.0%, 1.5% or less , P: 0.02% or less, S: 0.008% or less, Al: 0.005% or more, 0.1% or less, N: 0.01% or less, the balance being iron and unavoidable The slab composed of the target impurities is heated at 1000 to 1300 ° C., coarsely rolled, finish-rolled at a temperature not lower than the Ar ₃ transformation point, and cooled at a cooling rate of 40 ° C./s or more after finish rolling. And a method for producing a high-strength hot-rolled steel sheet having excellent stretch flangeability, wherein the hot-rolled steel sheet is wound at a temperature of 400 ° C. or more and less than 600 ° C. 2. The high-strength heat excellent in stretch flangeability according to claim 1, wherein the steel component further contains, by weight%, Ca: 0.0005% or more and 0.0050% or less. Manufacturing method of rolled steel sheet. 3. As a steel component, further, by weight%, Ti: 0.001% or more, 0.015% or less, Nb: 0.001% or more, 0.015% or less, B: 0.0001% or more The high-strength heat excellent in stretch flangeability according to claim 1 or 2, wherein at least one of the following components is contained so as to satisfy 0.001% ≦ Ti + Nb + 5B ≦ 0.020%. Manufacturing method of rolled steel sheet. 4. After the finish rolling, cooling is started at a cooling rate of 40 ° C./s or more, and then (CT
+30) to (CT + 130) ° C.
The cooling at a cooling rate of s or more is terminated, and cooling from the temperature range to CT is performed at a cooling rate of 3 to 20 ° C./s. Manufacturing method of high strength hot rolled steel sheet with excellent stretch flangeability.