JPH0665645A - Production of high ductility hot rolled high tensile strength steel sheet - Google Patents

Abstract

PURPOSE:To establish means stably manufacturing a hot rolled high tensile strength steel sheet combining sufficiently excellent strength, ductility, weldability and bore expandability and good in surface properties as well. CONSTITUTION:A hot rolled steel sheet contg. 0.05 to 0.25% C, >=0.05% Si, 0.8 to 2.5% Mn and 1.0 to 2.5% sol.Al or furthermore contg. one or more kinds among 0.0002 to 0.0100% Ca, 0.01 to 0.10% Zr, 0.002 to 0.10% rare earth elements, 0.005 to 0.10% Nb, 0.005 to 0.10% Ti and 0.005 to 0.20% V, and the balance Fe with inevitable impurities is heated to>=850 deg.C. In the subsequent cooling, it is held to 650 to 800 deg.C for 15sec to 15min, is furthermore held to 300 to 500 deg.C for 30sec to 40min and is then cooled to a room temp. at the cooling rate of air cooling or above, by which the high ductility hot rolled high tensile strength steel sheet having a structure essentially consisting of ferrite and bainite contg. retained austenite by >=5% volume rate and excellent in ductility, bore expandability and surface properties can be obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high-strength hot-rolled steel sheet for processing, which is suitable as a steel sheet for high-strength members in automobiles, industrial machines, etc.

[0002]

2. Description of the Related Art Today, so-called "hot rolled steel sheet" manufactured by continuous hot rolling has been widely applied to various industrial equipment such as automobiles as a relatively inexpensive structural material. However, there are many members that are molded by press processing for their applications, and therefore there is a strong demand for "combination of high strength and high ductility".

As a steel material excellent in both strength and ductility, for example, DP steel (Dual Phase steel: ferrite + martensite dual phase steel) as described in JP-A-55-44551 is known. . This feature of the DP steel is that it "ductile low yield ratio", nevertheless the tensile strength: material that growth in the 60 kgf / mm ² is the current situation is of about 30%, ductility In view of the above, there has been a demand for a further improved material.

By the way, a method utilizing TRIP (transformation induced plasticity) of retained austenite has been developed as a means for improving the ductility of high strength steel sheets (for example, Japanese Patent Laid-Open No. 55-1).
45121). And according to this method
It is possible to manufacture a high-ductile high-strength steel sheet having a tensile strength of 110 kgf / mm ² or more and an elongation of 22% or more, and having a “value of [tensile strength] × [elongation]” exceeding 2400. However, with this method, it is necessary to adjust the C content to a high value of 0.35 to 0.85% (hereinafter,% representing the component ratio is taken as% by weight), so that the steel sheet obtained is inferior in weldability and is not suitable for automobiles. The range of application as a steel sheet was narrow.

As a means for securing retained austenite under a low C content to obtain high ductility in the steel, a method has been proposed in which a high Si content steel is largely reduced in the low temperature austenite region ( JP-A-63-4017). As described above, the addition of Si is effective for obtaining residual austenite, and therefore it is possible to secure high ductility in the steel material by adding an appropriate amount of Si, but on the other hand, coarse bainite is easily generated, and
If the amount of Si added is high, hard martensite is also likely to be formed, so hole expansibility, which is one of the important workability for steel sheets for automobiles, was low. In particular, when a large reduction is applied in the low temperature austenite region as described above, coarse bainite is likely to be formed into a band shape, which significantly deteriorates the hole expandability. Further, in the case of high Si-added steel, poor surface quality of hot-rolled steel sheet called island scale has also become a big problem.

Therefore, the inventors of the present invention first hot-rolled a low-C, low-Si steel to which an appropriate amount of Al was added to perform 780-940 ° C.
It was newly found that a hot-rolled high-strength steel sheet having excellent hole expansibility and surface properties can be obtained by accelerating cooling under a specific condition and winding after finishing.
Proposed as No. 8. However, as a result of further study after that, in this method, the rolling load becomes large because the finish rolling is performed in the low temperature austenite region, and depending on the rolling mill, the load such as thin material and wide width becomes larger due to the load restriction. The problem is that it is not possible to manufacture such steel sheets.

Therefore, the object of the present invention is to produce a hot-rolled high-strength steel sheet which has sufficiently excellent strength, ductility, weldability and hole expandability, and has a good surface property. ， It was to establish a method that can easily and stably manufacture a wide product.

[0008]

Therefore, the present inventors have
Satisfaction in weldability is achieved to achieve the above object C
The TRIP shows a high strength sufficient for automobiles etc. due to the content range, and "TRIP leading to excellent ductility and hole expandability".
Various investigations were conducted in search of a simple method capable of realizing a high ductility hot rolled high tensile strength steel sheet containing a sufficient amount of austenite to utilize the effect.

[0009] Then, the findings obtained in the proposal of Japanese Patent Application No. 3-240618, namely, a) island shape by reducing "Si" effective for securing retained austenite in the low C range. Even in the low Si content steel that suppresses the deterioration of surface quality due to scale, the Al content should be 1.0% or more.
It is possible to allow austenite to remain, so that excellent tensile strength / elongation balance can be obtained. B) In the conventional Si-added retained austenitic high-strength steel,
Although a high elongation is obtained in the tensile test, only a hole expansion ratio similar to DP steel is obtained in the hole expansion test.
In the case of Al-added retained austenite type high-strength steel, unexpectedly excellent elongation and high hole expansion ratio can be satisfied at the same time, c) In addition, since Al is a ferrite stabilizing element, austenite is added. Although it was considered that the addition of Al was disadvantageous for remaining it, the addition of an appropriate amount of Al improves the elongation of the product and is effective in improving the hole expansibility because the second phase is uniformly dispersed. , D) Therefore, when 1% or more of Al is contained in the low Si steel having a C content range in which weldability is not significantly deteriorated, a structure containing 5% or more of retained austenite by volume is obtained, and Based on the technical matter of obtaining a hot-rolled high-strength steel sheet that sufficiently satisfies various properties such as strength, ductility, and hole expandability required for steel sheets, etc.
As a result of further research in search of a method that makes it easier to manufacture hot-rolled high-strength steel sheets having ductility, hole expandability, etc., the following new findings have been obtained.

As described above, according to the knowledge obtained so far, finish rolling in a low temperature austenite region is required to realize the above-mentioned "hot rolled high tensile strength steel sheet having excellent strength, ductility, hole expandability, etc." It was necessary, and thin steel sheets and wide steel sheets could not be manufactured due to the high rolling load. However, since the deformation resistance of steel decreases as the deformation temperature increases, the finishing temperature during rolling is increased to produce thin and wide steel sheets with the above composition under low deformation resistance, and then this "high temperature finish rolling" is performed. When the heat-treated steel sheet obtained by performing the heat treatment is subjected to heat treatment under specific conditions, an appropriate amount of residual
A structure containing austenite appears, and a retained austenite type high strength hot rolled steel sheet having the above-mentioned excellent properties can be obtained.

That is, not only the retained austenite cannot be sufficiently obtained only by hot rolling with high-temperature finishing, but also a structure containing a large amount of martensite cannot be obtained and high ductility cannot be obtained. It has been found that when a specific heat treatment is further applied, a hot-rolled high-strength steel sheet containing an appropriate amount of retained austenite and having excellent ductility and hole expandability can be stably produced.

The present invention has been completed by further research based on the above findings and the like, and "C: 0.05 to 0.25"
%, Si: 0.05% or less, Mn: 0.8 to 2.5%, sol.
Al: 1.0-2.5%, or even Ca: 0.0002
~ 0.0100%, Zr: 0.01 ~ 0.10%, rare earth element: 0.002 ~ 0.
10%, Nb: 0.005 to 0.10%, Ti: 0.005 to 0.10%, V:
A hot-rolled steel sheet containing 0.005 to 0.20% of 1 type or more and the balance consisting of Fe and unavoidable impurities is heated to 850 ° C or higher, and is kept at a temperature of 650 to 800 ° C for 15 seconds to 15 minutes during the subsequent cooling. After that, the temperature is further maintained at 300 to 500 ° C. for 30 seconds to 40 minutes, and then cooled to room temperature by air cooling or at a cooling rate higher than “5% by volume or more of retained austenite is contained. The main feature is that "a high-ductility hot-rolled high-strength steel sheet having a structure mainly composed of ferrite and bainite and having excellent ductility, hole expandability, and surface properties can be stably manufactured."

[0013]

Next, the reason why the component composition of the steel sheet and the production conditions thereof are limited as described above in the present invention will be described in detail together with its action. A) The chemical composition C C of the steel sheet contributes to the strengthening of the steel sheet while stabilizing the austenite by concentrating in untransformed austenite with the progress of ferrite transformation in the heat treatment process after hot rolling and winding. However, if its content is less than 0.05%, the strength is not secured and the austenite stabilizing effect is not sufficient. On the other hand, if the content of C exceeds 0.25%, the weldability becomes remarkable. In addition to deterioration, the amount of ferrite decreases and the amount of bainite increases too much, so the hole expandability also deteriorates. Therefore, C
The content was set to 0.05 to 0.25%.

Si In general, Si has the effect of promoting the formation of ferrite to assist the concentration of C in untransformed austenite, and delaying the precipitation of cementite. It is considered to be an effective element, and is usually contained in this type of steel sheet at about 1%. However, this Si causes the "deterioration of appearance" of the steel plate surface called island scale, and is also an element that deteriorates the GA processability (hot dip galvanizing processability), and also coarse bainite and hard bainite. And promotes the formation of martensite, and easily deteriorates the hole expandability. Since these disadvantages become remarkable when the Si content exceeds 0.05%, the Si content is particularly limited to 0.05% or less in the present invention.

Mn Mn is an austenite stabilizing element, and has an action of lowering the Ms point of untransformed austenite, improving hardenability, and suppressing pearlite transformation of untransformed austenite. Exert. However, if the Mn content is less than 0.8%, the desired effect due to the above-mentioned action cannot be obtained.
%, It becomes difficult to generate a sufficient amount of ferrite by heat treatment, so that the concentration of C in untransformed austenite becomes insufficient and the austenite cannot be stabilized. . Therefore, the Mn content is set to 0.8 to 2.5%.

Sol.Al Al is a particularly important component in the present invention. That is, Al
Like Si, it is a ferrite stabilizing element, promotes the formation of ferrite to promote the concentration of C in untransformed austenite, and promotes the retention of austenite through the action of delaying the precipitation of cementite. Moreover, the effect is more remarkable than when Si is added at the same weight ratio. In addition, it has the effect of promoting the uniform and fine formation of ferrite and suppressing the formation of coarse bainite that deteriorates the hole expansibility, and the addition of Al does not cause the formation of island scales. Good steel plate surface properties can be obtained. But Al
If the content is less than 1.0% as sol.Al, the desired effect due to the above action cannot be obtained, while if it is added in excess of 2.5%, the effect is saturated and the formation of inclusions promotes ductility. Since the hole expandability deteriorates, the sol.Al content is set to 1.0 to 2.5% in the present invention.

Ca, Zr, and rare earth elements All of these components have the effect of adjusting the shape of inclusions to improve cold workability, and therefore, if necessary, one or more of them may be added. However, if the content of Ca is less than 0.0002%, the content of Zr is less than 0.01%, and the content of rare earth element is less than 0.002%, the desired effect due to the above action is obtained. Not obtained, while Ca
0.0100%, Zr 0.10%, and rare earth elements 0.10%
%, The inclusions in the steel become too large and the workability deteriorates. Therefore, the Ca content is 0.
0002 to 0.0100%, Zr content was 0.01 to 0.10%, and rare earth element content was 0.002 to 0.10%.

Nb, Ti, and V Nb, Ti, and V are all elements that precipitate as carbonitrides on the ferrite ground and are effective for further strengthening the steel sheet. Therefore, if necessary, one or more of them may be used. However, in any case, if the content is less than 0.005%, the desired effect cannot be obtained. On the other hand, in the case of Nb and Ti, the content exceeds 0.10%, and in the case of V, the content exceeds 0.20%. However, the effect is saturated and it is not economical. Therefore, the Nb content is 0.0
05-0.10%, Ti content was 0.005-0.10%, and V content was 0.005-0.20%.

The "unavoidable impurities" that are inevitably mixed in the steel include P, S, Cu, Ni, Cr, Mo, etc. For example, the contents of P and S are as follows. It is desirable to regulate it as follows. Since P P is an impurity element that adversely affects weldability, it is desirable to keep its content to 0.05% or less, but 0.010% from the viewpoint of more evenly dispersing ferrite.
The following restrictions are recommended. Since S S is an impurity element that forms sulfide-based inclusions and deteriorates the workability, it is desirable to keep its content to 0.05% or less, but from the viewpoint of securing more excellent workability, It is more preferably 0.003% or less.

By the way, the steel having the above-described composition is melted by, for example, a converter, an electric furnace, an open furnace or the like. The steel type may be any of rimmed steel, capped steel, semi-killed steel or killed steel. Further, the production of the steel slab to be subjected to hot rolling may be performed by any means of "ingot-segmentation rolling" or "continuous casting".

B) Manufacturing Conditions for Hot Rolled Steel Sheet Now, a hot rolled high tensile steel sheet having the composition as described above and having a structure mainly composed of ferrite and bainite containing 5% or more by volume of retained austenite. First, a steel sheet obtained by hot rolling a steel slab having the above-mentioned composition is heated to 850 ° C. or higher, then kept at a temperature of 650 to 800 ° C. for 15 seconds to 15 minutes, and then at a temperature of 300 to 500 ° C. It can be produced by holding for 30 seconds to 40 minutes, and then cooling to room temperature at a cooling rate of air cooling or higher.

The hot-rolled steel sheet is first heated to a temperature of 850 ° C. or higher in order to prepare for the next treatment by eliminating hard martensite phase and the like to generate austenite. If the temperature is lower than 850 ° C, austenitization is not sufficiently performed.

In the cooling process following the above heating, holding is carried out at a temperature of 650 to 850 ° C. for 15 seconds to 15 minutes. The holding in the first step is for producing ferrite, and with it The concentration of C in untransformed austenite is promoted. The holding temperature of the first stage is 80
If the temperature exceeds 0 ° C, ferrite will not be sufficiently generated, and 65
When the temperature is lower than 0 ° C, pearlite is formed, and in each case, a sufficient amount of retained austenite cannot be finally obtained. Further, if the holding time is less than 15 seconds, the formation of ferrite is insufficient, and finally a sufficient amount of residual alloy is formed.
Stenite cannot be obtained, on the other hand, even if it is held for more than 15 minutes, no particular effect can be expected, and only energy is wasted.

After holding the first stage, 300 to 500
The holding at the temperature of 30 ° C. for 30 seconds to 40 minutes is performed. In the holding in the second step, the austenite that has not been transformed in the holding in the first step is transformed into bainite, thereby forming retained austenite. It is carried out to When the holding temperature in the second stage exceeds 500 ° C, pearlite is formed and residual austenite does not remain, while at temperatures below 300 ° C, martensite formation is promoted and ductility and hole expandability are promoted. Deteriorates. Further, when the holding time is less than 30 seconds, the bainite transformation is insufficiently progressed and the concentration of C in the untransformed austenite is insufficient, and most of the untransformed austenite does not reach the room temperature where it is continuously performed. During the cooling, it transforms to martensite and high ductility cannot be obtained. On the contrary,
If the holding temperature exceeds 40 minutes, cementite is formed and a sufficient amount of retained austenite cannot be obtained.

Cooling from the second stage holding temperature to room temperature is performed by air cooling or at a cooling rate higher than that. If the cooling rate at this time is extremely slower than air cooling, cementite is generated during cooling and sufficient cooling occurs. No residual austenite can be obtained. The upper limit of the cooling rate is not particularly limited.

The progress of the above-mentioned ferrite formation or bainite transformation is governed by the components, temperature and time,
It goes without saying that the optimum time varies depending on the components and the holding temperature.

[0027]

The hot-rolled steel sheet according to the present invention has a C content of 0.05 to 0.25% and thus has a weldability of a level required for a steel sheet for high-strength members, and further utilizes TRIP. A sufficient amount of austenite. Moreover, since the Si content is 0.05% or less, it has an excellent surface property without island scale. Therefore, according to the present invention, despite the good weldability, residual austenite is present and the tensile strength / elongation balance is good,
Moreover, it is possible to realize a hot-rolled steel sheet having excellent surface properties and hole expandability. When the steel sheet of the present invention is subjected to a surface treatment such as hot dip galvanizing, galvannealing, electroplating, etc., a surface-treated steel sheet having further excellent surface properties, ductility and hole expandability can be obtained.

Next, the effects of the present invention will be described more specifically by way of examples.

EXAMPLES First, steels A to Y having the chemical composition shown in Table 1
Was hot-rolled to obtain a hot-rolled steel sheet having a plate thickness of 2.3 mm. Next, each of these hot-rolled steel sheets was subjected to heat treatment under various conditions shown in Tables 2 and 3.

[0029]

[Table 1]

[0030]

[Table 2]

[0031]

[Table 3]

Then, JIS No. 5 tensile test pieces were sampled from each of the steel sheets thus obtained, and their mechanical properties were investigated. Also, the dimensions are 2.3mm thickness x 120mm width x 120mm.
Clearance of 14mmφ hole in long steel plate specimen: 15%
Then, the hole expandability was investigated using a conical punch. Further, a test piece for X-ray test was sampled from the central portion of the steel sheet, and the amount of retained austenite was also investigated. The results are also shown in Tables 2 and 3.

As is clear from the results shown in Tables 2 and 3, the hot-rolled steel sheet produced according to the present invention has a high tensile strength of 60 kgf / mm ² or more and an excellent ductility of 30% or more. At the same time, it has excellent punching workability with a hole expansion ratio of 100% or more, and it is understood that both high strength and high workability are satisfied at the same time. Furthermore, it can be confirmed that the hot-rolled steel sheet of the present invention does not form an island scale and exhibits a good surface condition.

On the other hand, in the test number 28 using the steel type U having the C content exceeding the specified range of the present invention, the elongation is relatively high but the hole expansibility is poor. In the test number 29 using the steel type V whose sol.Al content is less than the specified range of the present invention, residual austenite is not formed and the elongation is poor. In the test number 30 using the steel type W whose C content is below the specified range of the present invention, the strength is significantly inferior. In the test number 31 using the steel type X whose Si content exceeds the specified range of the present invention, the strength, elongation, and hole expansibility are all relatively good, but the surface properties of the steel sheet are inferior. Further, in the test number 32 using the steel type Y in which the Si content and the sol.Al content are out of the specified ranges of the present invention, the hole expandability and the steel sheet surface property are inferior.

Further, in the test number 21 manufactured under the condition that the first stage holding temperature is out of the lower limit of the range specified by the present invention,
When the first stage is held, pearlite is generated and residual
Stenite is not formed and elongation is poor. Test No. 22 manufactured under the condition that the first stage holding temperature is outside the upper limit of the range specified by the present invention, or test produced under the condition that the first stage holding temperature is outside the lower limit of the range specified by the present invention In No. 23, since the formation of ferrite by holding the first stage is insufficient, martensite is formed when holding the second stage, and retained austenite is not sufficiently formed. For this reason, the strength is high, but the elongation is low, and the hole expandability is not good.

In the test No. 24 manufactured under the condition that the second stage holding temperature was outside the upper limit of the range specified by the present invention, residual austenite was produced because pearlite was produced during the second stage holding. No, the growth is inferior. In the test number 25 manufactured under the condition where the second stage holding time was out of the lower limit of the range specified in the present invention, the retained austenite was not sufficiently produced because bainite was not sufficiently produced by the second stage holding. The elongation is poor and the hole expandability is not good.

In Test No. 26, which was manufactured under the condition that the second stage holding time was out of the upper limit of the range defined by the present invention, cementite was generated due to the long second stage holding time and a sufficient amount of residual o Stenite cannot be obtained and elongation is poor. Two
In Test No. 27 manufactured under the condition that the stage holding temperature was out of the lower limit of the range specified in the present invention, the austenite which was not transformed in the first stage holding due to the low second stage holding temperature was Martensite transformation and insufficient generation of retained austenite are insufficient. Therefore, although the strength is high, the elongation is low and the hole expandability is not good.

[0038]

[Summary of Effects] As described above, according to the present invention, it is possible to stably provide a high-ductility high-strength hot-rolled steel sheet having excellent strength, ductility, hole expandability, weldability, and surface properties. It can be applied to industrial equipment members such as automobile undercarriage parts to further improve the performance and life of those products, which brings extremely useful effects in industry.

Claims (4)

[Claims] 1. A weight ratio of C: 0.05 to 0.25%, S
i: 0.05% or less, Mn: 0.8 to 2.5%, sol.Al: 1.0
A hot rolled steel sheet containing ˜2.5% and the balance being Fe and unavoidable impurities is heated to 850 ° C. or higher, and when being cooled thereafter, the temperature is kept at 650 to 800 ° C. for 15 seconds to 15 minutes, and further 300 to 30 seconds to 40 at a temperature of 500 ° C
A surface texture having a structure mainly composed of ferrite and bainite containing residual austenite at a volume ratio of 5% or more, characterized by holding for a minute and then cooling to room temperature at a cooling rate of air cooling or higher. Manufacturing method of high ductility hot rolled high strength steel sheet with excellent ductility and hole expansion. 2. A weight ratio of C: 0.05 to 0.25%, S
i: 0.05% or less, Mn: 0.8 to 2.5%, sol.Al: 1.0
~ 2.5%, Ca: 0.0002 to 0.0100%, Zr: 0.
01-0.10%, rare-earth element: 0.002-0.10% hot-rolled steel sheet containing at least one kind and the balance consisting of Fe and unavoidable impurities,
In addition to heating to 850 ° C or higher, in the subsequent cooling,
It is characterized by holding at a temperature of 650 to 800 ° C. for 15 seconds to 15 minutes, further holding at a temperature of 300 to 500 ° C. for 30 seconds to 40 minutes, and then cooling to room temperature at a cooling rate of air cooling or higher. , Residual volume of 5% or more by volume
A method for producing a high-ductility hot-rolled high-strength steel sheet having a surface texture, a ductility, and hole expansion, which has a structure mainly composed of ferrite and bainite containing stenite. 3. A weight ratio of C: 0.05 to 0.25%, S
i: 0.05% or less, Mn: 0.8 to 2.5%, sol.Al: 1.0
~ 2.5%, Nb: 0.005 ~ 0.10%, Ti: 0.0
05-0.10%, V: 0.005-0.20% of hot-rolled steel sheet containing at least 1 type and Fe and inevitable impurities at 850 ° C
In addition to heating above, when cooling thereafter, 650-
After keeping the temperature at 800 ° C for 15 seconds to 15 minutes, further 3
A ferrite containing retained austenite in a volume ratio of 5% or more, characterized by holding at a temperature of 00 to 500 ° C. for 30 seconds to 40 minutes, and then cooling to room temperature at a cooling rate of air cooling or higher. A method for producing a high-ductility hot-rolled high-strength steel sheet having a bainite-based structure and having excellent surface properties, ductility, and hole expansion. 4. A weight ratio of C: 0.05 to 0.25%, S
i: 0.05% or less, Mn: 0.8 to 2.5%, sol.Al: 1.0
~ 2.5%, Ca: 0.0002 to 0.0100%, Zr: 0.
01 to 0.10%, rare earth element: 0.002 to 0.10%, 1 or more,
And Nb: 0.005 to 0.10%, Ti: 0.005 to 0.10%, V:
A hot-rolled steel sheet containing 0.005 to 0.20% of 1 type or more and the balance consisting of Fe and unavoidable impurities is heated to 850 ° C or higher, and is kept at a temperature of 650 to 800 ° C for 15 seconds to 15 minutes during subsequent cooling. After that, the temperature is kept at 300 to 500 ° C. for 30 seconds to 40 minutes, and then cooled to room temperature by air cooling or a cooling rate higher than 5% by volume of residual austenite. Surface texture and ductility with a structure mainly containing ferrite and bainite,
A method for producing a high-ductility hot-rolled high-strength steel sheet with excellent hole expansion.