JP2002327241A - High tension hot-rolled steel sheet superion in surface property, and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high tension hot-rolled steel sheet, which has low surface roughness along with high strength of 550 MPa or higher, elongation of 30% or higher, and adequate fatigue strength. SOLUTION: The hot-rolled steel sheet is characterized by including 0.05-0.15 wt.% C, 0.5-2.0 wt.% Mn, 0.02-0.2 wt.% Si, 0.02-0.07 wt.% P, 0.01 wt.% or less S, and 0.1 wt.% or less Al, satisfying 40×[Si wt.%]+200×[P wt.%]<=15, and having ferrite grains with an average particle diameter of 6 μm or less, of 60% or more at an area rate in a cross sectional structure, martensite with an average cross sectional area of 25 μm<2> or less, of 10% or more, and surface roughness of 2 μm or less by Ra (an arithmetic mean roughness specified in JIS). The manufacturing method is characterized by finish rolling the steel material at 800-900 deg.C, cooling it to 650-730 deg.C at a cooling rate of 10 deg.C/s or lower, then immediately cooling it to 200 deg.C or lower at a cooling rate of 50 deg.C/s or higher, and winding it up.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION An object of the present invention is to provide a steel sheet for automobiles having both strength and surface properties, which has been increasingly demanded due to environmental and fuel consumption problems.

[0002]

2. Description of the Related Art At present, automobiles are frequently used as daily necessities, and are equipped with various functions according to the demands of users. In order to pursue convenience, the weight of the vehicle body has increased, and accordingly, fuel efficiency has been reduced and environmental degradation has been caused. Therefore, reducing the weight has become a major issue.
On the other hand, what is called collision safety, which protects human lives even when a vehicle collides, is required, and materials are required to be lighter and more secure. In particular,
There is a strong demand for steel materials that account for more than 70% of the body weight of automobiles.

In order to solve this problem, a method of increasing the strength of a steel material to reduce the required thickness has been considered, and much research has been conducted. However, since steel materials such as undercarriage parts are molded into a complicated shape, it is not possible to withstand use only by increasing the strength, and it is necessary to provide not only strength but also workability such as elongation. Furthermore, fatigue strength is required for long-term utilization even after production.

[0004] As a high tensile strength steel sheet having workability, martensite (including retained austenite) is added to the ferrite phase.
Is dispersed in dual face steel (DP steel). This steel is a high strength steel having a low yield point and relatively high elongation.
JP-A-57-137452, JP-A-59-742
No. 17 discloses a high-strength steel sheet that can secure strength together with elongation using inexpensive Mn, Si, and P.

[0005] Further, when Si is used as the high-tensile steel, there is a problem that a pattern called Si scale is formed on the surface after rolling, making it difficult to carry out a chemical conversion treatment. As a countermeasure, Japanese Patent Application Laid-Open No. Hei 8-206723 proposes a method of applying a high-pressure deske to a material in which P and Si are regulated.

[0006]

SUMMARY OF THE INVENTION The above-mentioned Japanese Patent Application Laid-Open No.
When a large amount of Si and P are used as in JP-A-137452 and JP-A-59-74217, the following problem occurs even if the material is good.

[0007] When a large amount of Si is used, a red streak pattern called a Si scale is formed on a steel plate, and the pickling efficiency is reduced. The pattern also remains on the steel plate after pickling, and the roughness of this portion becomes rough.

[0008] Excessive use of P causes embrittlement due to segregation and lowers weldability.

In order to solve this problem, when the Si scale is eliminated by a high-pressure desk, as in Japanese Patent Application Laid-Open No. 8-206723, the effect differs depending on the installation position. In other words, when installed immediately after heating or before and after rough rolling, the Si scale is once peeled off, but the Si scale is generated again after deske and the pattern remains after rolling. The degree of roughness increases. In order to eliminate this, it is effective to carry out before the finish rolling, but in this case, the high-pressure water removes the Si scale and causes the temperature of the steel material to drop, and it is necessary to secure the final finishing rolling temperature of the steel material. It becomes difficult, and the surface texture often becomes a processed texture or coarse grains to deteriorate the material properties.

As described above, in the conventional high-strength steel sheet, Si
However, it is difficult to manufacture a steel material having both good strength and good surface condition.

The present invention solves such a problem, reduces the surface roughness at the same time as the increase in the strength of the steel material, and further improves the fatigue strength together with the strength of 550 MPa or more and the elongation of 30% or more by making the structure finer. It is an object of the present invention to provide a high-tensile steel material that does not impair the appearance.

[0012]

Means for Solving the Problems In order to solve the above-mentioned problems, the present inventors have reviewed the material components, extracted the influencing factors on the required strength, elongation and fatigue characteristics by experiments, and obtained the present invention. Was able to reach.

The features of the hot-rolled steel sheet disclosed in the present invention are as follows: C: 0.05 to 0.15%, Mn: 0.5 to 100% by mass.
2.0%, Si: 0.02-0.2%, P: 0.02-
0.07%, S: 0.01% or less, Al: 0.1% or less, and the mass% of Si and P is 40 · [Si%] + 20
In a steel material which satisfies 0 · [P%] ≦ 15, the balance being Fe and unavoidable impurities, the ferrite grains having an average grain size of 6 μm or less are 60% or more and the average cross-sectional area is 25 μm ^{2 in} terms of a sectional structure area ratio. A high-tensile hot-rolled steel sheet having excellent surface properties, characterized in that the following martensite has a total cross-sectional area ratio of 10% or more and a steel sheet surface roughness of Ra ≦ 2 μm.

Further, as a component, B: 0.01% or less;
Ni: 1.0% or less, Cr: 1.0% or less, Cu: 2.
0% or less, Mo: 1.0% or less, Ti: 0.1% or less,
Nb: 0.1% or less, Zr: 0.1% or less, V: 0.1
%, The characteristics of the present invention can be obtained in a steel material containing one or more of the steel materials of not more than%.

[0015] The manufacturing method is such that after finish rolling at 800 to 900 ° C, 65 ° C at a cooling rate of 10 ° C / s or less.
Immediately after cooling to the range of 0 ° C to 730 ° C, 50 ° C /
It is characterized by cooling to 200 ° C. or less at a cooling rate of at least s and winding.

Further, after the rough rolling is completed, the steel is rolled up and then unwound, and the rear end portion of the steel material and the front end portion of the following material are joined and the finish rolling is performed successively. Can be obtained.

Hereinafter, the reasons for limiting the steel sheet and the manufacturing method of the present invention will be described in detail.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail.

At present, a number of high-strength composite structure steel sheets are manufactured, and the structure that determines the material properties is determined by the cooling pattern after rolling is completed. The structures formed in this step are ferrite, pearlite, bainite, martensite, and retained austenite. Regarding the strength of the steel material, martensite, bainite, and pearlite have a large effect, and ferrite and retained austenite have a large effect on elongation.

[0020] DP steel is composed of ferrite and martensite (including retained austenite). According to the investigation by the present inventors, when pearlite and bainite are mixed in this, both the strength and the elongation tend to deteriorate. To maintain the strength, hard martensite, which is the second phase, is required. For that purpose, it is necessary to concentrate C in the austenite phase before transformation into martensite.

Therefore, a certain amount of base C and austenite stabilizing elements Si, Mn, P, etc., which easily discharge C into the austenite phase, are utilized.

However, as described above, heavy use of Si and P has an adverse effect. Therefore, as a result of detailed studies on the effects of these two elements, the use of a combination of Si and P in a hot-rolled steel sheet effectively works to refine the structure and improve the surface, and to improve the workability and fatigue characteristics. It was found to improve.

First, as for the addition of Si at 0.02% or more, the effect of rejecting C is exhibited.
When it is 5% or more, Si scale starts to be formed on the steel sheet surface, and its area increases as the amount of Si increases, and the surface roughness becomes rough even after pickling. However, the effect of suppressing the formation of Si scale is seen by the addition of P. The upper limit of this effect was 0.2% of Si. It is considered that a low melting point ternary eutectic FeO—Fe ₂ SiO ₄ —P ₂ O ₅ is formed by simultaneous addition of Si and P, so that the Si scale is easily peeled off by a normal deske.

Next, P added together with Si is 0.0%.
If less than 2%, neither C-exclusion effect nor Si scale exclusion effect was observed. This effect is seen when the content exceeds 0.02%, and when the content of Si and P is 40 · [S
i%] + 200 · [P%] ≦ 15 was found to be effective for uniform micronization of the structure.

In DP steel, Si or P is used to secure the material.
Was often used. Each of these elements is A
there is work to increase the r ₃ transformation point. According to the experimental results of the present inventors, the transformation point is increased at a rate of Si: 40 ° C./% and P: 200 ° C./%. However, all of these tend to segregate on the surface at high temperatures to form oxides. For this reason, at a high temperature, the transformation point temperature at the center of the thickness of the steel material increases, so that the surface layer is more likely to form ferrite first. In DP steel, ferrite is formed, the amount of C in the austenite region is enriched, and the steel is cooled to form a two-phase steel. Was.

Therefore, as shown in FIG. 1, the investigation was carried out by changing the combination of Si and P. As a result, if the temperature difference between the calculated transformation points was within 15 ° C., a uniform fine grain structure was obtained.
It has been found that a regulation condition of [Si%] + 200 · [P%] ≦ 15 is required.

By maintaining the above-mentioned appropriate amounts of Si and P, the average ferrite grain size is 6 μm or less, the average cross-sectional area of martensite is 25 μm ² or less, the ferrite space factor is 60% or more, and the martensite breakage. A steel material having an area of 10% or more and having high fatigue strength as well as uniform material properties can be obtained. Also, despite being DP steel, Si
Since the scale disappears, the surface roughness of the steel sheet after pickling is Ra
≦ 2 μm, the steel sheet is excellent in appearance and has excellent surface properties, and also has improved fatigue strength characteristics.

The reasons for limiting the range of the components of the present invention will be described below.

C: 0.05 to 0.15% 0.05% is necessary for securing the strength and forming martensite, but if it exceeds 0.15%, the ductility is significantly deteriorated. Was set to 0.05% to 0.15%.

Si: 0.02 to 0.2% Si is made of firelite (2FeO.SiO.
₂ ) is an element that easily causes red scale by leaving fine Fe ₂ O ₃ on the outermost surface, but is an element effective for excluding C into austenite. If less than 0.02%, this effect is lost. Further, the occurrence of red scale can be prevented up to 0.2% when used in combination with P, and this is set as the upper limit.

P: 0.02 to 0.07% P is a strengthening element and has an effect of rejecting C. 0.02
%, The effect is not seen. If it exceeds 0.07%, the workability is degraded, and weld cracks are liable to occur. Therefore, the upper limit is made 0.07%.

Further, as described above, in combination with Si,
By satisfying the condition of 40 · [Si%] + 200 · [P%] ≦ 15, an effect of uniformly miniaturizing the structure can be obtained.

Mn: 0.5 to 2.0% Mn fixes M in steel to form MnS, and has the effect of improving the workability of the steel sheet. It is used as a strengthening element in the production of high-strength steel sheets. Mn is a strengthening element for steel, and when producing DP steel, 0.5% or more is required. However, when the content exceeds 2.0%, the workability is deteriorated as well as the strength is improved. Therefore, the Mn content is 0.5% to 2.
0%.

S: 0.01% or less S forms a sulfide such as MnS, and when it becomes an elongation inclusion, it causes deterioration of workability. Since this effect exceeding 0.01% appears remarkably, the upper limit was made 0.01%.

Al: 0.1% or less Al is added as a deoxidizing element if necessary.
If the content exceeds 1%, the ductility is degraded with an increase in oxides, so the upper limit was made 0.1%.

Cr: 1.0% or less Cr is added for strength adjustment. It is an element effective in forming martensite by stabilizing austenite. This effect is obtained at 0.2% or more, but 1.0%
If the ratio exceeds 1, the effect stagnates, so the upper limit is set to 1.0%.

Cu: 2.0% or less Cu is effective for improving solid solution strength and improving fatigue characteristics. Also, it has a role of improving corrosion resistance. However, 2.
When added in a large amount exceeding 0%, the generation of Cu scab becomes remarkable and the surface properties deteriorate, so the upper limit was made 2.0%.

Ni: 1.0% or less Ni is effective in improving corrosion resistance and strength, and is used simultaneously with the addition of Cu as a measure against Cu baldness. It is expensive, and if it exceeds 1.0%, the effect is diminished.
%.

Mo: 1.0% or less Mo is used for improving heat resistance and strength. 1.
If it exceeds 0%, the workability is significantly deteriorated.
0%.

Ti, Nb, Zr, V Ti, Nb, Zr, V are carbonitride forming elements, and are used for strengthening and grain refinement by precipitates. In any case, when the content exceeds 0.1%, this effect is saturated. Therefore, the upper limit is set to 0.1%.

B: 0.01% or less B is an element which enhances the hardenability of austenite and is effective for forming martensite. The effect is manifested in a trace amount, but if it exceeds 0.01%, the effect is saturated and the workability deteriorates, so the upper limit was made 0.01%.

Next, a method for producing a high-tensile hot-rolled steel sheet having excellent surface properties in the hot-rolling step according to the present invention will be described in detail.

The present inventors conducted investigations on the above-described method for producing a high-tensile hot-rolled steel sheet while changing various production conditions. As a result, it was found that the production of the target hot-rolled steel sheet was possible by restricting to the following method. The details are described below.

When the DP steel is manufactured in the hot rolling step, the surface properties are formed before the finish rolling, and the ferrite and martensite which form the DP structure are separately formed by controlling the cooling pattern after the rolling.

The slab is heated and subjected to finish rolling via rough rolling. The winding and rewinding after rough rolling is performed by removing the scale on the surface while securing the temperature and making it uniform. It is for improving. Thereafter, the rear end of the preceding material and the front end of the succeeding material are joined and finish rolling is sequentially performed.Continuous rolling is performed by rolling a coil having a uniform temperature as described above and uniformly cooling the entire length during cooling. This is to take a cooling pattern. In continuous rolling, except for the leading portion of the first steel material and the trailing portion of the final steel material, the portion sandwiched between the rolling and rolling is always performed at the same time,
Since moderate tension is maintained between rolling and winding, a stable cooling pattern can be obtained. Therefore, it is easy to obtain a uniform material / surface property state.

The reason why the finish rolling is performed at 800 to 900 ° C. is to ensure that the rolling in the austenite region is performed so that the subsequent ferrite transformation is likely to occur.
The reason why the upper limit is set to 900 ° C. is to prevent scale flaws generated when the finishing temperature is too high.

650 to 7 at 10 ° C./s or less after the end of rolling
Cooling to 30 ° C. is for keeping ferrite as high as possible because ferrite is easily formed in a high-temperature ferrite region below the Ar ₃ transformation point. When the cooling rate is 10 ° C./s or more, or when the cooling stop temperature is 730 ° C. or more, the amount of ferrite generated becomes insufficient, and the ferrite area ratio of 60% or more cannot be secured. Therefore, even if the subsequent cooling is accelerated, a structure mainly composed of pearlite and bainite is formed, and the martensite area ratio is 5
% Or more cannot be obtained, resulting in reduced strength and ductility. Further, when the cooling stop temperature is 650 ° C. or less, since the temperature is in the pearlite or bainite generation temperature range even if C is concentrated in austenite, martensite is reduced and a DP structure cannot be obtained.

Immediately after stopping at 650-730 ° C.
The reason for cooling at or more than / s is to form martensite by quenching the C-enriched austenite so that pearlite bainite does not appear. For the same reason, the stop temperature was set to 200 ° C. or lower.

[0049]

EXAMPLE After steel types 1 to 13 having the components shown in Table 1 were tapped, as shown in Table 2, they were heated to 1000 ° C. to 1200 ° C. to perform rough rolling—rolling after roughing—bar joining—finish rolling— In the cooling-winding step, rolling was performed under different manufacturing conditions, and then the material properties were examined.

In Table 2, No. Regarding 1 to 13, the conditions in the hot rolling step were performed within the scope of the present invention, and the influence of the components was evaluated. No. 14 to 22 are steel types No. 1 in Table 1. 3
A test was carried out by changing the hot-rolling conditions using.

After completion of the test, a microscopic examination was conducted to determine an average grain size (average from the surface layer to the center) area ratio of ferrite, an average area per martensite, and a microstructure area ratio. Further, a JIS No. 5 tensile test piece was processed and a tensile test was performed to determine a yield strength (YP), a tensile strength (TS), and a total elongation due to abutting (El). Further, a swing-type bending fatigue test piece was prepared and subjected to a fatigue test.
The fatigue limit strength was determined for 10 million times, the ratio of the fatigue limit strength to the base metal strength was determined, and the ratio was defined as the fatigue limit ratio.

Regarding the surface of the steel material, as an evaluation of the appearance of the as-rolled material, the presence or absence of a red scale (Si scale) was evaluated by ○ ×, and then the surface roughness of the steel plate after pickling (R
a) was measured.

Test No. 2 in Table 2 1-9, since the steel of the present invention conditions component created in manufacturing conditions of the present invention, the average particle size of 6μm or less and a microstructure area ratio of 60% ferrite, and average cross-sectional area 25 [mu] m ² or less and The DP steel was fine and composed of martensite (including retained austenite) having a structure area ratio of 10% or more. In this material, since the balance between Si and P was controlled to prevent the occurrence of Si scale, to avoid the problem of welding cracks due to P, and to minimize the variation in transformation point temperature, the structure was fine from the surface layer to the center. It became a grain. For this reason, the elongation is 30 while securing the strength of 540 MPa or more.
% Was obtained. Furthermore, a high fatigue limit ratio of 0.5 or more could be obtained due to the effect of the fine structure.

As a comparative example, steel type No. 10 is
Both Si and P were low and the concentration of C in austenite was insufficient, the martensite amount was small, and sufficient strength and elongation could not be obtained. Steel type No. In Nos. 11 and 12, since the addition balance of Si and P is not good, the size of the structure of the surface layer and the central portion varies, and the ferrite grains become large.
Due to the formation of bainite, the strength and elongation were insufficient. Steel type No. Comparative Example No. 13 has a small amount of P with respect to Si. Sample No. 14 did not take up after rough rolling, the scale peeling was insufficient, the generation of Si scale could not be avoided, a red scale was generated, and the roughness was deteriorated.

Comparative Example No. In Comparative Example No. 15, the structure was not sufficiently refined because continuity by bar joining was not performed. In No. 16, the finish rolling temperature was as high as 950 ° C., so that the ferrite grain size was large.
o. In No. 17, rolling was performed at a temperature lower than the transformation point, and coarse grains were formed on the surface layer.

Comparative Example No. In Comparative Example No. 18, the primary cooling rate after finish rolling was high, and the concentration of C in austenite was insufficient. In No. 19, the stop temperature of the primary cooling was too low, and the pearlite structure was mixed. In each case, the martensite area ratio was low, and the strength and elongation were low.

On the contrary, in Comparative Example No. In No. 20, since the primary cooling stop temperature was high, the generation of ferrite was small, the area ratio was low, and the martensite area ratio was high, so that the strength was too high and the elongation was low.

Comparative Example No. Sample No. 21 had a low secondary cooling rate and generated pearlite bainite during cooling.
o. Sample No. 22 had a high secondary cooling stop temperature and could not completely become martensite, and thus all had insufficient strength and elongation.

[0059]

[Table 1]

[0060]

[Table 2]

[0061]

According to the present invention, a DP steel sheet having good surface properties and a fine structure can be obtained. For this reason, since the workability is good and the fatigue strength is improved, it can be used as a material for automobiles, and the economic effect on the industrial world is extremely large.

[Brief description of the drawings]

FIG. 1 is a diagram showing the influence of the amounts of P and Si on the properties of steel materials.

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 4K037 EA01 EA02 EA05 EA06 EA11 EA13 EA15 EA17 EA19 EA20 EA23 EA25 EA27 EA31 EA32 EA35 EB05 EB07 EB08 EB11 FA02 FC03 FC04 FD01 FD02 FD04 FE01 FE01 JA06

Claims (5)

[Claims] C: 0.05 to 0.15 as mass%
%, Mn: 0.5 to 2.0%, Si: 0.02 to 0.2
%, P: 0.02 to 0.07%, S: 0.01% or less,
Al: 0.1% or less, and the mass% of Si and P satisfies 40 · [Si%] + 200 · [P%] ≦ 15, the balance has Fe and unavoidable impurities, and the cross section In terms of the structure area ratio, the ferrite grains having an average grain size of 6 μm or less have a total cross-sectional area ratio of 60% or more, martensite having an average cross-sectional area of 25 μm ² or less of 10% or more, and a steel sheet surface roughness of Ra ≦ 2 μm. High tensile strength hot rolled steel sheet with excellent surface properties. 2. As mass%, B: 0.01% or less, N
i: 1.0% or less, Cr: 1.0% or less, Cu: 2.0
%, Mo: 1.0% or less, Ti: 0.1% or less, N
b: 0.1% or less, Zr: 0.1% or less, V: 0.1%
The high-tensile hot-rolled steel sheet having excellent surface properties according to claim 1, comprising one or more of the following. 3. C: 0.05 to 0.15 as mass%
%, Mn: 0.5 to 2.0%, Si: 0.02 to 0.2
%, P: 0.02 to 0.07%, S: 0.01% or less,
Al: A steel containing 0.1% or less and satisfying the mass% of 40% [Si%] + 200% [P%] ≦ 15 of Si and P, and the balance consisting of Fe and unavoidable impurity elements is heated. After performing the rough rolling, finishing the finish rolling at 800 to 900 ° C. or higher, cooling to a range of 650 to 730 ° C. at a cooling rate of 10 ° C./s or lower, and immediately cooling at a rate of 50 ° C./s or higher. A method for producing a high-tensile hot-rolled steel sheet having excellent surface properties, comprising cooling to 200 ° C. or less and winding. 4. As mass%, B: 0.01% or less, N
i: 1.0% or less, Cr: 1.0% or less, Cu: 2.0
%, Mo: 1.0% or less, Ti: 0.1% or less, N
b: 0.1% or less, Zr: 0.1% or less, V: 0.1%
The method for producing a high-tensile hot-rolled steel sheet having excellent surface properties according to claim 3, characterized by containing one or more of the following. 5. A continuous rolling process in which the rolling is performed after the rough rolling is completed, and then the film is unwound, and the rear end of the steel material and the front end of the following material are joined to sequentially perform finish rolling. A method for producing a high-tensile hot-rolled steel sheet having excellent surface properties according to claim 3.