JPH11189842A - High-strength and high-workability hot rolled steel plate excellent in impact resistance, balance between strength and elongation, fatigue resistance, and bore-expandability, and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hot rolled steel plate with high strength and high workability, excellent in fatigue resistance and fore-expandability as well as in impact resistance and a balance between strength and elongation. SOLUTION: This hot rolled steel plate has a composition consisting of, by mass, 0.05-0.40% C, 1.0-3.0% Si, 0.6-3.0% Mn, 0.2-2.0% Cr, at least one kind selected from 0.005-0.25% Ti, 0.003-0.1% Nb, and 0.003-0.1% V, and the balance essentially Fe and also has a steel structure where primary phase is composed of pro-eutectoid ferrite and secondary phase is composed of martensite, acicular ferrite, and retained austenite. Further, the hardness Hv of the pro-eutectoid ferrite phase as primary phase is regulated to >=180, and the difference ΔHv between the hardness of the primary phase and that of the secondary phase is regulated to <=200.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength, high-workability hot-rolled steel sheet having excellent impact resistance, strength-elongation balance, fatigue resistance, and hole expandability suitable for use as a steel sheet for automobiles. And a method of manufacturing the same.

[0002]

2. Description of the Related Art With the aim of reducing the weight of automobiles, the demand for high-strength thin steel sheets having excellent formability has become particularly strong. In addition, recently, importance has been placed on the safety of automobiles, and for that purpose, an improvement in impact resistance, which is a measure of safety in a collision, is required. Furthermore, consideration for economic efficiency is also required, and in consideration of such economic efficiency, a hot-rolled steel sheet is more advantageous than a cold-rolled steel sheet.

[0003] Against the background of the above situation, various high-strength hot-rolled steel sheets have been developed. For example, Japanese Patent Publication No. 6-41617, Japanese Patent Publication No. 5-65566 and Japanese Patent Publication No. 5-67682 each disclose a so-called high-workability, high-strength hot-rolled steel sheet containing ferrite, bainite and 5% or more retained austenite. A method for producing Transformation Induced Plasticity steel (hereinafter referred to as TRIP steel) is disclosed. However, although this TRIP steel has high elongation and good formability (TS × El ≧ 24000 MPa ·%), it still has a problem in that it cannot meet the current severe impact resistance. There is also a problem that the work hardening amount (WH) at the time of press molding and the baking hardening amount (BH) at the time of subsequent baking of paint are as low as about 70 MPa. If the amount of work and bake hardening (WH + BH) is low, there is a great disadvantage in terms of guaranteeing the strength after work-paint baking.

On the other hand, as a high-strength hot-rolled steel sheet having excellent impact resistance, as disclosed in Japanese Patent Application Laid-Open No. 9-111396, a so-called Dual Phase steel (hereinafter, referred to as a two-phase structure of ferrite and martensite) is disclosed. DP steel) has been developed. However, although this DP steel is excellent in impact resistance, it cannot be said that elongation is sufficient, and there is a problem in formability.

As described above, no hot rolled steel sheet satisfying both sufficient formability and strict safety has been found, and its development has been desired. In this regard, the present inventors have previously disclosed in Japanese Patent Application Nos. 9-139794 and 9-139802 that pro-eutectoid ferrite is the main phase and needle-like ferrite is disclosed in Japanese Patent Application Nos. 9-139794 and 9-139802. ＋ Martensite +
A hot-rolled steel sheet having a mixed structure having retained austenite as a second phase was proposed. The above-mentioned hot-rolled steel sheet is a conventional steel T
As a result of investigating the relationship between the structure and properties of RIP steel, the bainite phase, which is indispensable for obtaining a sufficient amount of retained austenite, which is advantageous for improving formability, deteriorates impact resistance of TRIP steel. It has been developed based on new knowledge of what is causing this.

[0006] Fig. 1 shows a typical continuous cooling transformation curve (CCT diagram) of a conventional TRIP steel. As shown in the figure, the conventional TRIP steel has a proeutectoid ferrite region after hot rolling. To slightly precipitate eutectoid ferrite (also referred to as polygonal ferrite), and at the same time promote the concentration of solid solution carbon in the untransformed austenite phase, increase the stability of austenite, and then lead it to the bainite region, By slowly cooling this region, bainite transformation occurs,
A predetermined amount of austenite was left. However, although the TRIP steel manufactured in this manner is excellent in strength and workability, sufficient impact resistance cannot be obtained due to the formation of the bainite phase.

[0007] The inventors of the present invention have conducted numerous experiments and studies in order to avoid the formation of bainite. As a result, (1) when a small amount of Cr is contained as a steel component, the nose of the bainite transformation region in the CCT diagram becomes larger. It recedes and suppresses the precipitation of bainite (especially the precipitation of carbides) and instead precipitates acicular ferrite (also called acicular ferrite). (2) Acicular ferrite, residual austenite thus formed It has been newly found that the second phase composed of and martensite significantly improves the impact resistance without impairing the formability.

FIG. 2 shows a typical CCT diagram in the above Cr-containing component system. As shown in the figure, by adding a small amount of Cr, the nose of the bainite transformation region recedes, and instead, a needle-like ferrite region appears remarkably. By rapidly quenching, the second phase can have a mixed structure consisting of acicular ferrite, retained austenite and martensite, and thus has both excellent formability and impact resistance (specifically, dynamic n Value is 0.35 or more, strength-
It is now possible to obtain high-strength, high-workability hot-rolled steel sheets with an elongation balance (TS × El) of 24000 MPa ·% or more and a work / bake hardening amount (WH + BH) of 100 MPa or more.

Here, the dynamic n value is newly found by the present inventors as an index of impact resistance characteristics, and by using this dynamic n value, the impact resistance characteristics can be more accurately determined than in the prior art. Can be evaluated. In other words, in the past, collision safety was considered in relation to strength, and it was considered that the higher the strength, the higher the crash safety. However, the relationship between strength and collision safety is not necessarily unique. It turned out not to be the case. Therefore, as a result of diligent research on this point, it has been found that the collision safety is improved, that is, when the vehicle is deformed at high speed (the strain rate is

[Outside 1] The energy in but increased to 2 × 10 ³ / s), in order to absorb more with the steel sheet, the steel sheet

[Outside 2] It has been clarified that it is effective to increase the n value (hereinafter referred to as dynamic n value) when tensile deformation is performed under the following conditions. Here, an instantaneous n value at an elongation of 10% is defined as a dynamic n value. In addition, it was also found that increasing the dynamic n value is effective for improving strength during high-speed deformation.

[0010]

However, the above-mentioned hot-rolled steel sheet has excellent impact resistance and strength-elongation balance, but the hardness of the proeutectoid ferrite phase as the main phase is H.
_V is as low as about 140, and the difference in hardness between the second phase (about 380 in H _V ) and the main phase is large, ΔH _V is large (ΔH _V > 200). However, there remains a problem in that it is difficult to obtain excellent characteristics with respect to fatigue resistance (fatigue crack propagation resistance) and hole expandability.

The present invention advantageously solves the above-mentioned problems, and is not only excellent in impact resistance and strength-elongation balance, but also in high-strength and high-working with excellent fatigue resistance and hole-expandability. It is an object to propose a hot rolled steel sheet together with its advantageous production method.

[0012]

The details of the invention will be described below. As one of methods for increasing the strength and hardness of a steel structure, a precipitation strengthening method, that is, a method of finely dispersing carbides and nitrides in crystal grains is known. Then, the inventors tried to improve the hardness of the steel structure using this precipitation strengthening method.

FIG. 3 shows a comparison between the nose in the pro-eutectoid ferrite transformation region and the nose of carbonitride precipitation in the CCT diagram of the Cr-containing component system described above. As shown by the symbol a in the figure, under general manufacturing conditions, the precipitation nose of carbonitride has receded to a considerably long time side. An increase in the hardness of proeutectoid ferrite cannot be expected. However, if the precipitation nose of the carbonitride can be advanced to the left as shown by the symbol b to make the precipitation nose of the carbonitride coincide with the nose in the pro-eutectoid ferrite transformation zone, the formation of pro-eutectoid ferrite At the same time, carbonitride precipitates, so that the hardness of proeutectoid ferrite should increase.

Then, the present inventors next examined means for advancing the precipitation nose of carbonitride to the left. As a result, if the slab heating temperature is set lower and the rolling reduction in the final pass of rough rolling and the rolling reduction in the final pass of finish rolling are each set higher than usual, the precipitation nose of carbonitride will be left. It has been found that the intended purpose can be advantageously achieved. The present invention is based on the above findings.

That is, the gist of the present invention is as follows. 1. C: 0.05 to 0.40 mass%, Si: 1.0 to 3.0 mass%, Mn: 0.6 to 3.0 mass%, Cr: 0.2 to 2.0 mass%, Ti: 0.005 to 0.25 mass%, Nb: 0.003 to 0.1 mass% V: at least one selected from the group consisting of 0.003 to 0.1 mass%, the balance being substantially Fe, and consisting of martensite, acicular ferrite and retained austenite with proeutectoid ferrite as the main phase. impact of a second phase, yet the hardness of pro-eutectoid ferrite phase is equal to or over 180 in H _V, and the main phase and hardness of the difference [Delta] H _V of the second phase is 200 or less Characteristic,
High strength and high workability hot rolled steel sheet with excellent strength-elongation balance, fatigue resistance and hole expandability.

2. In the above item 1, the steel composition has a composition containing at least one selected from the group consisting of P: 0.01 to 0.2 mass% and Al: 0.01 to 0.3 mass%. High strength, high workability hot rolled steel sheet with excellent fatigue resistance and hole expandability.

[0017] 3. C: 0.05 to 0.40 mass%, Si: 1.0 to 3.0 mass%, Mn: 0.6 to 3.0 mass%, Cr: 0.2 to 2.0 mass%, Ti: 0.005 to 0.25 mass%, Nb: 0.003 to 0.1 mass% , V: A steel slab having a composition containing one or more selected from 0.003 to 0.1 mass% is heated to 1050 to 1150 ° C, and the rolling reduction of the final pass of the rough rolling is 30% or more. After rough rolling at 780, finish rolling at final pass rolling reduction: 15% or more
Finish at a temperature of 980 ° C, then cool to 620-780 ° C, then keep isothermally for 1-10 seconds or cooling rate: 20
It is characterized in that it is subjected to slow cooling at a temperature of less than 300 ° C / s, then cooled to 350-500 ° C, wound around a coil, and then cooled to 300 ° C or less at a cooling rate of 10-100 ° C / h. A method for producing a high-strength, high-workability hot-rolled steel sheet having excellent impact resistance, strength-elongation balance, fatigue resistance, and hole expandability.

4. In the above item 3, after the coil is wound, a gradual cooling process of cooling to 300 ° C. or less at a cooling rate of 10 to 100 ° C./h is performed. After slow cooling of less than / h
Isothermal holding (cooling slowly) cooling at a cooling rate of 50 ° C / h or more to 300 ° C or less by forced cooling-Impact resistance and strength characterized by changing to forced cooling treatment-High strength with excellent elongation balance Manufacturing method (manufacturing method) of workable hot-rolled steel sheet.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be specifically described below. As a means for increasing the hardness of pro-eutectoid ferrite, the inventors first focused on the slab heating temperature and studied a suitable temperature range. As a result, the hardness of the proeutectoid ferrite could be increased by limiting the slab heating temperature to the range of 1050 to 1150 ° C. However, the hardness obtained by controlling the slab heating temperature is 150 at most in H _V, it did not reach more than 180 as a target.

Then, a number of experiments and examinations were carried out by paying attention to the reduction rate of the final pass of the finish rolling. However, even if the reduction rate of the final pass of the finish rolling was set to 15% or more, it was obtained. The hardness of proeutectoid ferrite is around 180,
The target hardness of 180 or more could not be obtained stably. The fact that the target hardness cannot be obtained stably
This means that the desired characteristics cannot be stably obtained, and cannot be used industrially as it is.

Therefore, as a result of various studies on means for increasing the hardness of the proeutectoid ferrite, the reduction ratio in the final pass of the rough rolling was set to 30% or more, which was higher than the normal value. The deposition nose of the object can be effectively advanced to the left, thus H _V ≧
The hardness of 180 can be obtained stably.

[0022] Here, the hardness of pro-eutectoid ferrite in H _V 1
80 The reason for limiting the above, the hardness of pro-eutectoid ferrite is H _V ≧
If it is 180, good fatigue resistance (fatigue crack propagation resistance) can be obtained as shown in FIG. In the figure, the fatigue crack propagation characteristic is evaluated by da / dN when a crack gauge is put on the fatigue tester and the propagation behavior of the fatigue crack is quantitatively measured under the condition of ΔK = 20 MPa · m ^1/2. did. If da / dN is 1 × 10 ⁻⁵ or less, it can be said that the material has excellent resistance to fatigue fracture.

FIG. 5 shows the result of examining the relationship between the hardness difference ΔH _V between the second phase and the main phase and the hole expansion ratio. As shown in FIG. 5, ΔH _V is 200 7 if
Excellent hole expansion ratio of 0% or more is obtained. The hole expansion rate was calculated based on the Japan Iron and Steel Federation Standard JFST1001-1996. Therefore, in this invention, it was limited to a range of 200 or less for the difference [Delta] H _V in hardness between the second phase and the main phase. Incidentally, the hardness of the second phase is at H _V as described above 380
Therefore, if the hardness of the proeutectoid ferrite is H _V ≧ 180, the difference ΔH _{V in} hardness between the second phase and the main phase is 200 or less.

In the present invention, the acicular ferrite is
It means a crystal grain having a major axis of about 5 μm or less, an aspect ratio of 1: 1.5 or more, and a cementite precipitation of 5% or less. In the bainite of the conventional TRIP steel, precipitation of cementite is often observed (10% or more).
Therefore, the acicular ferrite of the present invention and the bainite of TRIP steel are clearly distinguished.

In the present invention, the ratio of the second phase in the steel structure is preferably 3 to 40%. The reason is that if the phase ratio is less than 3%, sufficient impact resistance cannot be obtained, while if it exceeds 40%, the elongation and hence the strength-elongation balance are reduced. A more desirable ratio is 10 to 30%. In the present invention, the phase ratio was calculated by polishing a steel sample, etching with a 2% nitric acid + ethyl alcohol solution, and performing image analysis on a micrograph.

Further, the ratio of each phase in the second phase is as follows: martensite: 10 to 80% (preferably 30 to 60%).
%), Retained austenite: 8 to 30% (preferably 10 to 10%)
20%), acicular ferrite: 5-60% (preferably 20-50)
%). The reason is that if the martensite ratio is less than 10%, sufficient impact resistance cannot be obtained, while if it exceeds 80%, the elongation and hence the strength-elongation balance are reduced. If the ratio of retained austenite is less than 8%, sufficient elongation cannot be obtained, while 30%
%, The impact resistance deteriorates. Further, if the proportion of acicular ferrite is less than 5%, good impact resistance cannot be obtained, while if it exceeds 60%, elongation is reduced.

The proportion of each phase in the entire steel structure is preferably about 5 to 15% for martensite and acicular ferrite, respectively, and about 2 to 10% for retained austenite. Further, in the present invention, the steel structure does not always consist of a mixed phase of proeutectoid ferrite as a main phase and martensite, acicular ferrite and retained austenite as a second phase. Although some precipitation may occur, even if such a third phase is mixed, there is no problem in characteristics as long as the ratio is 10% or less of the entire second phase.

Next, the reason why the composition of the steel sheet is limited to the above range in the present invention will be described. C: 0.05 to 0.40 mass% C is an element that not only effectively contributes to the strengthening of the steel by the second phase and the precipitation strengthening of the proeutectoid ferrite phase, but is also a useful element in obtaining retained austenite. However, when the content is less than 0.05 mass%, the effect is poor, while on the other hand, the content is 0.1%.
If the amount exceeds 40 mass%, the ductility is reduced.
Limited to the range of ~ 0.40 mass%.

Si: 1.0 to 3.0 mass% Si is an element indispensable for the formation of retained austenite. For this purpose, at least 1.0 mass% must be added. In addition, Si content is limited to the range of 1.0 to 3.0 mass%, because not only does this cause deterioration of the scale properties, but also poses a problem in surface quality.

Mn: 0.6 to 3.0 mass% Mn is not only useful as a steel strengthening element, but also a useful element in obtaining retained austenite. However, if the content is less than 0.6 mass%, the effect is poor, while if it exceeds 3.0 mass%, the ductility is reduced.
Limited to the range of 0.6 to 3.0 mass%.

Cr: 0.2 to 2.0 mass% This addition of Cr is one of the features of the present invention. As described above, the addition of Cr causes the second phase to become acicular ferrite. For this purpose, it is necessary to add 0.2 mass% or more. However, if it exceeds 2.0 mass%, coarse Cr carbides are formed, ductility is inhibited, and the strength-elongation balance and the dynamic n value deteriorate. , Cr content is 0.2-2.0 mass
%. Preferably it is 0.3 to 1.8 mass%.

FIGS. 6 and 7 show the results of examining the relationship between the Cr content and the strength-elongation balance and dynamic n value, respectively. As is apparent from FIGS.
TS × El ≧ 240 in the range of 0.2 mass% or more and 2.0 mass% or less
Excellent workability and impact resistance of 00 (MPa ·%) and dynamic n value ≧ 0.35 are obtained.

Ti: 0.005 to 0.25 mass%, Nb: 0.003 to 0.2%
1 mass%, V: 0.003 to 0.1 mass% Ti, Nb and V are all effective as carbonitride forming elements in the proeutectoid ferrite phase to improve the strength and hardness of the proeutectoid ferrite phase. I do. However,
If the content is too small, the effect of the addition is poor. On the other hand, excessive addition will rather lead to a decrease in ductility. Therefore, each content is set in the above range.

Although the basic components have been described above, in the present invention, P or Al can be appropriately contained as an austenite-forming element in the following range. P: 0.01 to 0.2 mass% P is useful as a retained austenite-forming element, but if the content is less than 0.01 mass%, the effect of its addition is poor, while if it exceeds 0.2 mass%, the secondary workability deteriorates. Therefore, when it is added, it is desirable to set it in the range of 0.01 to 0.2 mass%.

Al: 0.01 to 0.3 mass% Al is also useful as a retained austenite forming element, like P, but if the content is less than 0.01 mass%, the effect of its addition is poor. If the amount exceeds the above range, the ductility is reduced. Therefore, when added, the content is preferably in the range of 0.01 to 0.3 mass%. In addition, about other elements, in order to maintain formability, S is 0.01 mass% or less,
N is preferably set to 0.01 mass% or less.

Next, the production method of the present invention will be specifically described. In the present invention, basically, a mixed structure consisting of martensite, acicular ferrite, and retained austenite may be formed as the second phase.
The cooling may be performed along the cooling curve shown in FIG. Then, in the above manufacturing process, the hardness of the proeutectoid ferrite due to the precipitation of the carbonitride may be increased so that the precipitation nose of the carbonitride coincides with the nose of the pro-eutectoid ferrite transformation region.

First, slab heating is performed prior to hot rolling. The heating temperature must be 1050 to 1150 ° C. That is, if the slab heating temperature is less than 1050 ° C, the effect of improving the strength due to sufficient fine precipitates in the subsequent process is not recognized, while if it exceeds 1150 ° C, the crystal grains are coarsened and sufficient strength-elongation balance Is not obtained. The heating time is not particularly limited, but if it is too long, the crystal grains become coarse, so that it is preferable to set the heating time to about 60 minutes or less.

Next, hot rolling is performed. In the present invention, it is important that the rolling reduction of the final pass in hot rough rolling is set to 30% or more (preferably 40% or more). That is, if the final pass rolling reduction is less than 30%, no matter how the above-mentioned slab heating temperature and the final pass rolling reduction in the hot finish rolling described later are adjusted, the precipitation nose of carbonitride and The nose of the pro-eutectoid ferrite transformation region could not be matched well, and as a result, the hardness H _V ≧ 180 of the pro-eutectoid ferrite phase targeted by the present invention, and the difference ΔH in hardness between the main phase and the second phase. _{This is because V} ≦ 200 cannot be achieved. The upper limit of the rolling reduction in the final pass in hot rough rolling is not particularly limited. However, if the rolling reduction is too high, not only the load on the rolling equipment is increased but also the cost and production disadvantages such as shortening of the roll life are reduced. Therefore, the upper limit is preferably set to about 50%.

Similarly, in the hot finish rolling, it is important that the rolling reduction in the final pass is 15% or more (preferably 20% or more). That is, similarly to the final pass in the hot rough rolling, when the rolling reduction in the final pass in the hot finish rolling is less than 15%, the hardness of the proeutectoid ferrite phase targeted in the present invention H _V ≧ 180, because a hardness difference ΔH _V ≦ 200 between the main phase and the second phase cannot be obtained. The upper limit of the reduction ratio of the final pass in the hot finishing rolling is not particularly limited. However, if the reduction ratio is too high, the load on the rolling equipment increases as in the case of the rough rolling. Preferably, it is about 50%.

The finishing temperature in the above-mentioned finish rolling needs to be 780 to 980 ° C. If the finish rolling end temperature is less than 780 ° C, the work structure remains in the steel and the ductility is deteriorated.On the other hand, if the temperature exceeds 980 ° C, the structure becomes coarse and the ferrite transformation delays. This is because the property is lowered.

Then, after cooling to the vicinity of the nose of the pro-eutectoid ferrite region at 620 to 780 ° C., the temperature is maintained at this temperature for 1 to 10 seconds or gradually cooled at a rate of 20 ° C./s or less to obtain the main phase. A certain pro-eutectoid ferrite is deposited. Above
The temperature range of 620 to 780 ° C. is a temperature range in which the ferrite transformation proceeds most smoothly, so that a desired amount of pro-eutectoid ferrite can be obtained by a short holding treatment or slow cooling treatment for about 1 to 10 seconds. In the case of the slow cooling treatment, if the cooling stop temperature is lower than 600 ° C., pearlite transformation may occur. Therefore, the cooling stop temperature is preferably set to 600 ° C. or higher. Here, the cooling rate up to the temperature range of 620 to 780 ° C is important for promoting ferrite transformation.
The temperature is preferably 30 ° C./s or more. However, 300
If the temperature exceeds ℃ / s, the shape of the steel sheet is damaged, which is not preferable.

Then, the mixture is cooled to a region of 350 to 500 ° C. acicular ferrite, and the region is gradually cooled at a cooling rate of 10 to 100 ° C./h to precipitate a desired amount of acicular ferrite. In the above slow cooling treatment, if the cooling rate is less than 10 ° C./h, there is a high possibility that bainite transformation occurs.
If the temperature exceeds 00 ° C./h, it is difficult to obtain a desired amount of acicular ferrite. Therefore, the cooling rate is limited to the range of 10 to 100 ° C./h. The cooling rate to the temperature range of 350 to 500 ° C is preferably 30 ° C / s or more from the viewpoint of accelerating the precipitation of acicular ferrite. However, if the temperature exceeds 300 ° C./s, the shape of the steel sheet is still damaged, which is not preferable.

Then, by the above-mentioned slow cooling treatment, 300 ° C.
During cooling to below, a part of untransformed austenite is transformed into martensite, and a part remains as austenite as it is. The reason why the cooling stop temperature in the slow cooling treatment is set to 300 ° C. or lower is also to avoid the possibility that bainite transformation occurs.

By the above-described series of treatments, a hot-rolled steel sheet having a desired steel structure and a desired hardness in which a second phase composed of acicular ferrite, martensite, and retained austenite is present in the proeutectoid ferrite main phase is obtained. You can do it.

As described above, as the manufacturing method, the case where the coil is wound and then gradually cooled at a cooling rate of 10 to 100 ° C./h to 300 ° C. or less (a in FIG. 3) has been described.
In the present invention, in place of the above-described slow cooling treatment, after winding on a coil, isothermal holding for 2 to 60 minutes or cooling rate: 50 ° C./h
After gentle cooling to less than 300 ° C by forced cooling, the cooling rate should be 50 ° C / h or more to 300 ° C or less (b in Fig. 3).
May be adopted.

In the isothermal holding (slow cooling) -forced cooling treatment, the holding or slow cooling time is limited to 2 to 60 minutes because the holding or slow cooling time is less than 2 minutes and a sufficient amount of needle-shaped This is because ferrite cannot be obtained, and if it exceeds 60 minutes, bainite transformation may be caused. The reason why the cooling rate in slow cooling was set to less than 50 ° C./h is that if this rate was too high, a sufficient amount of needle-like ferrite could not be obtained. The reason why the temperature was set to 50 ° C./h or more is that if this rate is too low, bainite transformation may occur.

[0047]

EXAMPLES Example 1 A steel slab having the composition shown in Table 1 was heated to 1100 ° C., then rough-rolled at a final pass reduction rate of 45%, then a final pass reduction rate of 25%, and a rolling end temperature. : After finish rolling at 820 ° C, cool to near the nose of the pro-eutectoid ferrite region at 700 ° C, and then gradually cool at 15 ° C / s.
After applying to 650 ° C, it is cooled to the needle-like ferrite region of 400 ° C, wound up in a coil, and then gradually cooled to room temperature at a rate of 70 ° C / h (processing a) or at a rate of 40 ° C / h. , And then subjected to a process of forced cooling to room temperature at a rate of 150 ° C./h (process b). In addition, S is 0.0010-0 in each steel.
0030 mass%, N was in the range of 0.0020 to 0.0030 mass%.

From the hot-rolled sheet thus obtained, tensile test pieces were cut out, and the test pieces were subjected to a strain rate of 2
A tensile test was performed under the conditions of × 10 ^-2 / s, and the yield strength (YS), tensile strength (TS) and elongation (El) were determined. In addition, Hopkinson pressure bar test materials (Materials and Process vol.9 (19
96) Using P.1108-1111), strain rate: 2 × 10 ³ / s
The tensile test was carried out under the conditions described above, and the instantaneous n value (dynamic n value) when the elongation was 10% was determined. Furthermore, the work hardening amount (WH) during press molding and the subsequent baking (170
C)), the bake hardening amount (BH) was also measured.
In addition, WH and BH were determined from FIG. 8 using a tensile tester having a strain rate of 2 × 10 ⁻² / s. Further, a fatigue crack propagation test and a hole expansion test were performed under the conditions described above. Steel structure of each hot-rolled steel sheet, TS × El balance, dynamic n value,
Table 2 summarizes the results of a study on WH + BH, hole expansion characteristics, and fatigue resistance characteristics.

[0049]

[Table 1]

[0050]

[Table 2]

As shown in Table 2, according to the present invention, any of the alloys in which a mixed structure of martensite, acicular ferrite, and retained austenite was formed as the second phase, and the hardness of the main phase was increased was excellent. -Elongation balance (TS x El ≥ 24000 MPa ·%), impact resistance (dynamic n value ≥ 0.35) and workability and bake hardening (WH + BH ≥ 100 MPa)
) As well as excellent fatigue resistance (da / dN at ΔK = 2)
0 MPa · m ^1/2 ≦ 1 × 10 ⁻⁵ ) and hole expanding characteristics (hole expanding ratio ≧ 70%). On the other hand, G steel which does not contain any of Ti, Nb and V is inferior in both fatigue resistance characteristics and hole expanding characteristics. Further, the H-K steels whose component compositions and structures deviate from the appropriate ranges have good fatigue resistance properties and hole expanding properties due to precipitation strengthening of the pro-eutectoid ferrite phase, but have strength-
Poor elongation balance, impact resistance and WH + BH.

Example 2 C: 0.15 mass%, Si: 1.4 mass%, Mn: 0.8 mass%, C
A steel slab containing r: 0.7 mass%, Ti: 0.01 mass%, Nb: 0.14 mass% and V: 0.07 mass%, with the balance being substantially Fe, was hot-rolled under the conditions shown in Table 3. After that, it was treated in the same manner as in Example 1 to obtain a hot-rolled steel sheet (Note that, for the final cooling treatment, the treatment B was adopted). The steel structure, TS × El balance, dynamic n value, WH of the hot rolled steel sheet thus obtained
Table 4 summarizes the results of examining the + BH, hole expanding characteristics, and fatigue resistance characteristics.

[0053]

[Table 3]

[0054]

[Table 4]

In each of the hot-rolled steel sheets obtained according to the present invention, not only a mixed structure of martensite, acicular ferrite and residual austenite is formed as the second phase, but also the hardness of the main phase is increased. As a result, TS ×
El ≧ 24000 MPa ・%, dynamic n value ≧ 0.35, WH + BH ≧ 10
Excellent strength-elongation balance of 0 MPa, da / dN (atΔK = 20 MPa · m ^1/2 ) ≦ 1 × 10 ^-5 and hole expansion ratio ≧ 70%,
Impact resistance, processing and bake hardenability, fatigue resistance and hole expansion characteristics are obtained.

[0056]

Thus, according to the present invention, the main phase is proeutectoid ferrite, the second phase is a mixed structure of martensite, acicular ferrite and retained austenite, and the hardness of the main phase is H _V ≧ 180. As described above, by setting the difference in hardness between the main phase and the second phase to ΔH _V ≦ 200, not only excellent moldability and impact resistance are obtained, but also fatigue resistance and hole expansion properties are improved. An excellent hot-rolled steel sheet can be obtained.

[Brief description of the drawings]

FIG. 1 is a typical continuous cooling transformation diagram (CCT diagram) of a conventional TRIP steel.

FIG. 2 is a typical continuous cooling transformation curve (CCT diagram) in the component system of the present invention.

FIG. 3 is a continuous cooling transformation curve diagram (CCT diagram) showing a comparison between a nose in a proeutectoid ferrite transformation region of a Cr-containing component system and a precipitation nose of carbonitride.

4 is a graph showing the relationship between the pro-eutectoid ferrite hardness H _V and da / dN (an index of fatigue crack propagation characteristics).

FIG. 5 is a graph showing a relationship between a hardness difference ΔH _V between a second phase and a main phase and a hole expansion ratio.

FIG. 6 is a graph showing the relationship between Cr content and strength-elongation balance.

FIG. 7 is a graph showing a relationship between a Cr amount and a dynamic n value.

FIG. 8: Work hardening amount (WH) and bake hardening amount (BH)
FIG.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Shusaku Takagi 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Inside the Technical Research Institute of Kawasaki Steel Co., Ltd. (72) Inventor Kazuya Miura 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Kawasaki (72) Inventor Takashi Ohara 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Pref.

Claims (4)

[Claims] Claims 1. C: 0.05 to 0.40 mass%, Si: 1.0 to 3.0 mass%, Mn: 0.6 to 3.0 mass%, Cr: 0.2 to 2.0 mass%, Ti: 0.005 to 0.25 mass%, Nb: 0.003 to 0.1 mass%, V: at least one selected from the group consisting of 0.003 to 0.1 mass%, and the balance is substantially Fe, with martensite and needle-like ferrite having proeutectoid ferrite as a main phase. and a second phase consisting of residual austenite, yet the hardness of pro-eutectoid ferrite phase is 180 or more H _V, and that the main phase and the difference [Delta] H _V of the hardness of the second phase is 200 or less Characteristic impact resistance,
High strength and high workability hot rolled steel sheet with excellent strength-elongation balance, fatigue resistance and hole expandability. 2. The impact resistance according to claim 1, wherein the steel composition further comprises at least one selected from the group consisting of P: 0.01 to 0.2 mass% and Al: 0.01 to 0.3 mass%. High strength and high workability hot rolled steel sheet with excellent properties, strength-elongation balance, fatigue resistance and hole expandability. 3. C: 0.05 to 0.40 mass%, Si: 1.0 to 3.0 mass%, Mn: 0.6 to 3.0 mass%, Cr: 0.2 to 2.0 mass%, Ti: 0.005 to 0.25 mass%, Nb: 0.003 to 0.1 mass% V: A steel slab having a composition containing one or more selected from 0.003 to 0.1 mass% is heated to 1050 to 1150 ° C, and the rolling reduction of the final pass of the rough rolling: 30 % After the rough rolling under the condition of more than 15%, and finish rolling under the condition of the final pass rolling reduction of 15% or more.
Finish at a temperature of 980 ° C, then cool to 620-780 ° C, then keep isothermally for 1-10 seconds or cooling rate: 20
It is characterized in that it is subjected to slow cooling at a temperature of less than 300 ° C / s, then cooled to 350-500 ° C, wound around a coil, and then cooled to 300 ° C or less at a cooling rate of 10-100 ° C / h. A method for producing a high-strength, high-workability hot-rolled steel sheet having excellent impact resistance, strength-elongation balance, fatigue resistance, and hole expandability. 4. The method according to claim 3, wherein after the coil is wound up, a gradual cooling process of cooling to a temperature of 300 ° C. or less at a cooling rate of 10 to 100 ° C./h is performed. Or cooling rate: After slow cooling at less than 50 ° C / h,
Isothermal holding (slow cooling), cooling to 50 ° C / h or less at a cooling rate of 50 ° C / h or more by forced cooling-Impact resistance, strength-elongation balance, fatigue resistance, and holes characterized by changing to forced cooling A method for producing high-strength, high-workability hot-rolled steel sheets with excellent spreadability.