JP5167865B2 - High-strength hot-dip galvanized steel sheet excellent in workability and weldability and method for producing the same - Google Patents

Description

The present invention is a high-strength hot-dip galvanized steel sheet having a tensile strength (TS) of 980 MPa or more, excellent in workability and weldability, and suitable for use in automobile parts that are required to be press-formed into a strict shape It is about the method.
The hot dip galvanized steel sheet in the present invention includes a so-called galvannealed steel sheet that has been subjected to an alloying heat treatment after galvanization.

High-strength hot-dip galvanized steel sheets used for automobile parts and the like are required to have excellent workability in addition to high strength due to the characteristics of their applications.
Recently, high-strength steel sheets have been demanded for automobile bodies from the viewpoint of improving fuel efficiency and ensuring collision safety by reducing the weight of the vehicle body, and its application is expanding. Conventionally, light processing has been mainly used, but application to complex shapes is also being studied.

However, since the workability generally tends to decrease with the increase in strength of the steel sheet, cracking during press forming can be cited as the primary problem when applying a high-strength steel sheet. Therefore, it is required to improve workability such as stretch flangeability according to the part shape.
Moreover, since resistance spot welding is performed in an assembly process after molding, in addition to workability, excellent weldability is also required.

In order to meet the above requirements, for example, in Patent Documents 1 to 8, there is a method for obtaining a hot-dip galvanized steel sheet having high workability by limiting the steel components and the structure, optimizing hot rolling conditions and annealing conditions, and the like. Proposed.
JP 2004-232011 Japanese Patent Laid-Open No. 2002-256386 JP 2002-317245 A JP 2005-105367 A Japanese Patent No. 3263143 Japanese Patent No. 3596316 Japanese Patent Laid-Open No. 2001-11538 JP 2006-342373 A

Among the above-mentioned patent documents, Patent Document 1 discloses a TS: 980 MPa grade steel material having a large C and Si content, but no consideration is given to stretch flangeability.
Patent Documents 2 to 4 disclose steel materials using Cr, but no consideration is given to stretch flangeability.
Patent Documents 5 to 7 describe a hole expansion ratio λ that is one of the indexes for evaluating stretch flangeability, but the tensile strength (TS) does not reach 980 MPa.
Patent Document 8 discloses a hot-dip galvanized steel sheet that is excellent in balance between strength and ductility, bendability, spot weldability, and the like. The technique disclosed in Patent Document 8 focuses on precipitation strengthening and balances the above characteristics, and it is not possible to achieve a high strength of 980 MPa or more in tensile strength (TS). However, when high strength of 980 MPa or more is aimed, there is a problem that sufficient stretch flangeability cannot be secured.

  Moreover, in the high-strength steel sheet having a tensile strength (TS) of 980 MPa or more, there remains a problem in that the material is likely to vary due to variations in manufacturing conditions. Therefore, in addition to good workability and spot weldability, there has been a demand for a steel plate that has a small variation in material, particularly a small variation in material in the longitudinal direction of the steel plate.

  The present invention was developed in view of the above situation, and a high strength hot-dip galvanized steel sheet having high tensile strength of TS ≧ 980 MPa, excellent workability and weldability, and a manufacturing method with reduced material variations It aims to be proposed together.

Now, the inventors have intensively studied to solve the above problems.
as a result,
(1) From the viewpoint of workability and weldability, it is necessary to reduce the amount of C, P and S.
(2) In order to achieve good surface properties, it is necessary to keep the amount of Si low,
(3) About the strength reduction accompanying the reduction of C and P, by using Cr, it is possible to increase the strength even when there are few alloy elements.
(4) In the above component system, the steel structure is, in volume fraction, ferrite phase: 20-60%, martensite phase: 40-80%, and the average crystal grain size of the ferrite phase is 5 μm or less. This makes it possible to obtain excellent workability and weldability even under high strength of TS: 980 MPa or higher.
I got that knowledge.
The present invention is based on the above findings.

That is, the gist configuration of the present invention is as follows.
1. By mass%, C: 0.05% or more and less than 0.10%, Si: 0.01% or more and less than 0.35%, Mn: 2.0 to 3.5%, P: 0.020% or less, S: 0.0020% or less, Al: 0.005 to 0.1%, N: Contains 0.0050% or less, Cr: more than 1.0% and 2.0% or less, Ti: 0.010-0.080%, Nb: 0.010-0.080% and B: 0.0001-0.0030%, the balance is Fe and inevitable impurities, steel The structure consists of 20-60% ferrite phase, 40-80% martensite phase, and 5% or less (including 0%) balance structure in volume fraction, and the average grain size of the ferrite phase Is a high-strength hot-dip galvanized steel sheet excellent in workability and weldability, characterized by having a tensile strength of 980 MPa or more and a hot-dip galvanized layer on the steel sheet surface.

2. 2. The high-strength hot-dip galvanized steel sheet according to 1 above, wherein the steel sheet further contains Ca: 0.0001 to 0.0050% by mass%.

3. By mass%, C: 0.05% or more and less than 0.10%, Si: 0.01% or more and less than 0.35%, Mn: 2.0 to 3.5%, P: 0.020% or less, S: 0.0020% or less, Al: 0.005 to 0.1%, N: Steel slab containing 0.0050% or less, Cr: more than 1.0% and 2.0% or less, Ti: 0.010-0.080%, Nb: 0.010-0.080% and B: 0.0001-0.0030%, with the balance being Fe and inevitable impurities , After hot rolling, after winding into a coil, pickling, then cold rolling, hot-dip galvanized to produce a hot-dip galvanized steel sheet,
In the above hot rolling, after hot rolling with a slab heating temperature of 1150 to 1300 ° C and a hot finish rolling temperature of 850 to 950 ° C, a temperature of hot finish rolling temperature to (hot finish rolling temperature-100 ° C) The zone is cooled at an average cooling rate of 5 to 200 ° C./second, wound at a coiling temperature of 400 to 600 ° C., then pickled, cold-rolled, and then primary from 200 ° C. to an intermediate temperature. Heating to an intermediate temperature of 500 to 800 ° C with an average temperature increase rate of 10 to 50 ° C / second, and 750 to 900 ° C with a secondary average temperature increase rate from the intermediate temperature to the annealing temperature of 0.1 to 10 ° C / second After heating to the annealing temperature of the steel and holding it for 10 to 500 seconds, it is cooled to a cooling stop temperature of 450 to 550 ° C. at an average cooling rate of 1 to 30 ° C./second, and then hot dip galvanized or further alloyed. that is,
The steel structure has a volume fraction of 20-60% ferrite phase, 40-80% martensite phase, and a remaining structure of 5% or less (including 0%), and the average grain size of the ferrite phase A method for producing a high-strength hot-dip galvanized steel sheet excellent in workability and weldability, characterized in that a steel sheet having a tensile strength of 980 MPa or more is obtained .

4). 4. The method for producing a high-strength hot-dip galvanized steel sheet according to 3 above, wherein an elapsed time from the end of the hot finish rolling to a temperature of (winding temperature) × 1.1 is 5 seconds or more.

5. 5. The method for producing a high-strength hot-dip galvanized steel sheet according to 3 or 4 above, wherein the steel slab further contains, by mass%, Ca: 0.0001 to 0.0050%.
In the present invention, excellent workability means satisfying both TS × El ≧ 15000 MPa ·% and TS × λ ≧ 35000 MPa ·%, and excellent weldability means nugget diameter: The base material breaks at 4t ^1/2 (mm) (t: plate thickness of the steel plate) or more, and high strength means that the tensile strength (TS) is 980 MPa or more.

According to the present invention, a high-strength hot-dip galvanized steel sheet excellent in workability and weldability can be obtained.
Moreover, according to the manufacturing method of the present invention, it is possible to obtain a high-strength hot-dip galvanized steel sheet having a small variation in material in the coil longitudinal direction of the steel sheet, that is, a small material variation in the coil longitudinal direction.
The high-strength hot-dip galvanized steel sheet obtained by the present invention satisfies both strength and workability required for automobile parts, and is suitable as an automobile part that is press-formed into a strict shape.

Hereinafter, the present invention will be specifically described.
First, in the present invention, the reason why the component composition of the steel sheet is limited to the above range will be described. Note that the “%” indication regarding the component means mass% unless otherwise specified.
C: 0.05% or more and less than 0.10% Since the strength of the martensite phase tends to be proportional to the amount of C, C is an indispensable element for strengthening steel using the martensite phase. In order to obtain TS of 980 MPa or more, 0.05% or more of C is necessary, and TS increases as the amount of C increases. However, when the C content exceeds 0.10%, the spot weldability is remarkably deteriorated, the martensite phase is hardened, and further, a retained austenite phase harder than the martensite phase is generated, so that workability such as stretch flangeability is also achieved. It tends to decrease significantly. Therefore, the C content is limited to a range of 0.05% or more and less than 0.10%. From the viewpoint of stably securing TS of 980 MPa or more, a preferable C amount is 0.08% or more and less than 0.10%.

Si: 0.01% or more and less than 0.35%
Si is an element that contributes to strength improvement by solid solution strengthening. However, if the content is less than 0.01%, the effect is poor. On the other hand, if the content is 0.35% or more, the effect is not only saturated but also the ductility of the ferrite phase is lowered. Moreover, by containing excessively, the scale of difficulty peelability is produced | generated at the time of hot rolling, the surface property of a steel plate is deteriorated, and workability is reduced. Moreover, it concentrates as an oxide on the steel plate surface and causes non-plating. From the above, the Si content was limited to 0.01% or more and less than 0.35%. Preferably it is 0.01 to 0.20% of range.

Mn: 2.0-3.5%
Mn contributes effectively to strength improvement, and this effect is recognized by containing 2.0% or more. On the other hand, if it exceeds 3.5% and excessively contained, it becomes a structure in which the transformation point is partially different due to segregation of Mn and the like, and as a result, it becomes a non-uniform structure in which the ferrite phase and the martensite phase exist in a band shape, Workability is reduced. Moreover, it concentrates as an oxide on the steel plate surface and causes non-plating. From the above, the amount of Mn was limited to the range of 2.0 to 3.5%. Preferably it is 2.2 to 2.8% of range.

P: 0.020% or less P is an element that contributes to strength improvement, but on the other hand, it is also an element that deteriorates weldability. When the amount of P exceeds 0.020%, the effect becomes remarkable. On the other hand, since excessive P reduction is accompanied by an increase in manufacturing cost in the steel making process, the lower limit of the P amount is preferably about 0.001%. From the above, the P content is limited to 0.020% or less. Preferably it is 0.015% or less, More preferably, it is 0.010% or less.

S: 0.0020% or less When the amount of S increases, it may cause hot red hot brittleness, which may cause problems in the manufacturing process, and inclusion MnS is formed, and is present as a plate-like inclusion after cold rolling. Especially, the ultimate deformability of the material is lowered, and the moldability such as stretch flangeability is lowered. There is no problem until the S content is 0.0020%. In addition, since excessive reduction accompanies the increase in the desulfurization cost in a steelmaking process, it is preferable that the lower limit of S is about 0.0001%. From the above, the amount of S was limited to 0.0020% or less. Preferably it is 0.0015% or less.

Al: 0.005-0.1%
Al is an effective element as a deoxidizer in the steelmaking process, and is also a useful element in separating non-metallic inclusions that reduce local ductility into slag. Furthermore, Al has the effect of suppressing the formation of Mn and Si-based oxides on the surface layer that hinders plating properties during annealing and improving the plating surface appearance. In order to obtain such an effect, 0.005% or more of Al is necessary. On the other hand, if added over 0.1%, not only the steel component cost increases, but also the weldability decreases. Therefore, the Al content is limited to the range of 0.005 to 0.1%. Preferably it is 0.01 to 0.06% of range.

N: 0.0050% or less The influence of N on material properties in the structure-reinforced steel is not so great, but if it is 0.0050% or less, the effect of the present invention is not impaired. On the other hand, it is preferable that the amount of N is small from the viewpoint of improving ductility by cleaning ferrite. In addition, since the cost on steelmaking increases, the lower limit is preferably about 0.0001%. From the above, the N content is limited to 0.0050% or less.

Cr: more than 1.0% and less than 2.0%
Cr is an element that contributes to the hardenability of steel. In the present invention, the addition of Cr secures the hardenability and stabilizes the formation of martensite. In order to obtain this effect, a Cr content exceeding 1.0% is required. Cr also strengthens the ferrite phase by solid solution, reduces the hardness difference between the martensite phase and the ferrite phase, and contributes to the improvement of stretch flangeability. However, if the Cr content exceeds 2.0%, Cr is unevenly distributed locally on the surface of the steel sheet, resulting in non-uniform surface properties, resulting in non-uniform deformation during forming and reduced workability. From the above, the Cr content was limited to the range of more than 1.0% and less than 2.0%. Preferably it is 1.1 to 1.6% of range.

Ti: 0.010-0.080%
Ti effectively acts to impart refinement and precipitation strengthening of the hot-rolled sheet structure and the steel sheet structure after annealing by forming fine carbides and nitrides with C or N in the steel. In order to obtain this effect, 0.010% or more of Ti is necessary. However, when the Ti amount exceeds 0.080%, not only this effect is saturated, but also precipitates are generated excessively in the ferrite, which lowers the ductility of the ferrite. Therefore, the Ti content is limited to the range of 0.010 to 0.080%. More preferably 0.020
It is in the range of ~ 0.060%.

Nb: 0.010-0.080%
Nb is an element that contributes to improvement in strength by solid solution strengthening or precipitation strengthening. It also contributes to the improvement of stretch flangeability through the effect of reducing the hardness difference from the martensite phase by strengthening ferrite. Such an effect is obtained when the Nb content is 0.010% or more. However, if it is contained excessively exceeding 0.080%, the hot-rolled sheet becomes hard and causes an increase in rolling load during hot rolling and cold rolling. In addition, the ductility of the ferrite is lowered and workability is deteriorated. Therefore, the Nb content is limited to a range of 0.010 to 0.080%. From the viewpoint of strength and workability, the Nb content is preferably in the range of 0.030 to 0.070%.

B: 0.0001-0.0030%
B improves hardenability, suppresses the formation of ferrite that occurs during the annealing cooling process, and contributes to obtaining a desired amount of martensite. In order to acquire this effect, it is necessary to contain B amount 0.0001% or more, but if it exceeds 0.0030%, the above effect is saturated. Therefore, the amount of B is limited to the range of 0.0001 to 0.0030%. Preferably it is 0.0005 to 0.0020% of range.

Although the essential components have been described above, in the present invention, Ca can be appropriately contained as necessary.
Ca: 0.0001 to 0.0050%
Ca has the effect of improving ductility and stretch flangeability by controlling the shape of sulfides such as MnS, but the effect tends to be saturated even if contained in a large amount. Therefore, when Ca is contained, the content of Ca is 0.0001 to 0.0050%, more preferably 0.0001 to 0.0020%.

  The steel sheet of the present invention has the above as basic components for obtaining desired workability and weldability, and the balance is composed of Fe and inevitable impurities. Among inevitable impurities, V precipitates carbides and lowers the ductility of the ferrite phase. Therefore, the V amount is preferably less than 0.05%. More preferably, it is less than 0.005%.

Similarly, it is preferable to reduce the content of Zr, Mg, and the like that generate precipitates as much as possible, and it is preferable that each content be less than 0.0200%. More preferably, each is less than 0.002%, and more preferably less than 0.0002%.
As other elements, for example, Cu adversely affects weldability and Ni adversely affects the surface appearance after plating. Therefore, the Cu content and the Ni content are each preferably less than 0.4%. More preferably, it is less than 0.04%.
In addition, as other elements, for example, REM having an action of controlling the form of sulfide inclusions without greatly changing the plating property and Sb having an action of adjusting the grain size of the steel sheet surface layer are 0.0001 to 0.1, respectively. Even if contained in the range of%, the effect of the present invention is not affected.

Next, the limited range and reason for limiting the steel structure, which is one of the important requirements for the present invention, will be described.
Average crystal grain size of ferrite phase: 5 μm or less Refinement of crystal grains contributes to improvement of stretch flangeability of a steel sheet. Therefore, in the present invention, the average crystal grain size of the ferrite phase in the composite structure is limited to 5 μm or less. If the average crystal grain size of the ferrite phase becomes excessively large, the steel plate surface may be roughened after press working. In addition, if the soft region and the hard region are present roughly, the processing becomes non-uniform and the formability deteriorates. In this respect, if the ferrite phase and the martensite phase are present uniformly and finely, the deformation of the steel sheet becomes uniform during processing, and therefore it is desirable that the average crystal grain size of the ferrite phase is small. In order to suppress the deterioration of workability, the preferred range is 1 to 3.5 μm.

Volume fraction of ferrite phase: 20-60%
Since the ferrite phase is a soft phase and contributes to the ductility of the steel sheet, the steel sheet of the present invention needs to contain the ferrite phase in a volume fraction of 20% or more. On the other hand, if the ferrite phase exceeds 60%, it becomes too soft and it is difficult to ensure the strength. Therefore, the ferrite phase has a volume fraction of 20 to 60%, preferably 30 to 50%.

Volume fraction of martensite phase: 40-80%
In addition to the ferrite phase, an excellent material can be obtained by forming a martensite phase, which is a low-temperature transformation phase from austenite, in a volume fraction of 40 to 80%. The martensite phase is a hard phase and has an effect of increasing the strength of the steel sheet by strengthening the transformation structure. In addition, since the generation of movable dislocation is accompanied at the time of transformation generation, it also has the effect of reducing the yield ratio of the steel sheet. However, when the martensite phase is less than 40% in volume fraction, there is a problem that it becomes soft and it is difficult to ensure strength. On the other hand, when it exceeds 80%, it becomes excessively hard and workability such as stretch flangeability There is a problem that the remarkably decreases. A preferred martensite phase is 50 to 70% by volume.

  As the remaining structure other than the ferrite phase and the martensite phase described above, a bainite phase, a retained austenite phase, cementite, and the like can be considered, but the total of one or more of these is 5% or less in volume fraction. If present, the effect of the present invention is not impaired. The volume fraction of the remaining tissue may be 0%.

Next, the manufacturing method of the high intensity | strength hot-dip galvanized steel plate of this invention is demonstrated.
First, a slab is manufactured from the molten steel prepared to the above-mentioned preferred component composition by a continuous casting method or an ingot-bundling method. Subsequently, the obtained slab is cooled and then reheated, or hot rolling is performed as it is without undergoing a heat treatment after casting. The slab heating temperature is 1150-1300 ° C, the hot-rolled sheet is uniformly structured, and the finish rolling temperature is 850-950 ° C to improve workability such as stretch flangeability, and consists of two phases: ferrite phase and pearlite phase. Average between [Hot Finishing Rolling Temperature ~ (Hot Finishing Rolling Temperature – 100 ° C)] in order to suppress the formation of band-like structure to make hot rolled sheet uniform and further improve workability such as stretch flangeability. The cooling rate is 5 to 200 ° C./second, the rolling temperature is 400 to 600 ° C. in order to improve the surface properties and the cold rolling property, the hot rolling is finished, and after pickling, the desired plate is obtained by cold rolling. Thickness. The cold rolling rate (cold rolling reduction rate) is desirably 30% or more in order to improve ductility by promoting recrystallization of the ferrite phase. In addition, since productivity will fall when rolling load increases too much, it is preferable that the upper limit of a cold rolling rate shall be about 70%.

Next, in the hot dip galvanizing process, in order to control the microstructure during annealing before the start of cooling and optimize the ferrite fraction finally obtained, the primary average rate of temperature increase from 200 ° C to the intermediate temperature is set to 10 -50 ° C / second, intermediate temperature 500-800 ° C, secondary average heating rate from intermediate temperature to annealing temperature 0.1-10 ° C / second, annealing temperature 750-900 ° C, this temperature range After 10 to 500 seconds, cooling is stopped at an average cooling rate of 1 to 30 ° C./second from 450 to 550 ° C. After cooling, the steel sheet is subsequently immersed in a molten zinc bath, and the amount of galvanized coating is controlled by gas wiping or the like, or further heated and alloyed, and then cooled to room temperature.
Thus, the intended high-strength hot-dip galvanized steel sheet can be obtained in the present invention, but skin-pass rolling may be applied to the steel sheet after plating.

Hereinafter, the limitation range and limitation reason of the manufacturing conditions will be specifically described.
Slab heating temperature: 1150 ~ 1300 ℃
Precipitates present in the heating stage of steel slabs exist as coarse precipitates in the finally obtained steel sheet and do not contribute to strength, so the Ti and Nb-based precipitates precipitated during casting are redissolved. There is a need. Here, contribution to strength is recognized by heating at 1150 ° C. or higher. In addition, it is advantageous to heat to 1150 ° C. or higher from the viewpoint of scaling off defects such as bubbles and segregation on the surface of the slab, reducing cracks and irregularities on the steel sheet surface, and achieving a smooth steel sheet surface. However, when the heating temperature exceeds 1300 ° C., the austenite grains become coarse, the final structure becomes coarse, and stretch flangeability deteriorates. Therefore, the slab heating temperature was limited to the range of 1150 to 1300 ° C.

Hot finish rolling temperature: 850-950 ° C
By setting the hot finish rolling temperature to 850 ° C. or higher, workability (ductility, stretch flangeability) can be remarkably improved. When the finish rolling temperature is less than 850 ° C, it becomes a processed structure in which crystals are stretched after hot rolling, and tends to be a non-uniform structure after cold rolling and annealing, which prevents the uniform deformation of the material during processing and is excellent. It is difficult to obtain high workability.
On the other hand, when the hot finish rolling temperature exceeds 950 ° C., the amount of oxide (scale) generated increases rapidly, the iron-oxide interface becomes rough, and the surface quality after pickling and cold rolling tends to deteriorate. It is in. In addition, after the pickling, unsettled portions of the hot-rolled scale are likely to be present in part, which adversely affects resistance spot weldability. Furthermore, when the hot finish rolling temperature exceeds 950 ° C., the crystal grain size becomes excessively large, and the surface of the pressed product may be roughened during processing. Therefore, the hot finish rolling temperature is set to 850 to 950 ° C, preferably 900 to 950 ° C.

Average cooling rate between [Hot finish rolling temperature-(Hot finish rolling temperature-100 ° C)]:
5 to 200 ° C./second If the cooling rate is less than 5 ° C./second in the high temperature region [Hot finishing temperature to (Hot finishing temperature−100 ° C.)] immediately after hot finish rolling, Crystals and grains grow, the hot-rolled sheet structure becomes coarse, and a band-like structure is formed in which ferrite and pearlite are formed in layers. If a band-like structure is formed before annealing, heat treatment is performed in a state in which the concentration of the components is uneven. Therefore, it becomes difficult to make the structure fine and uniform by the heat treatment in the plating process, resulting in a non-uniform structure and elongation. Flangeability decreases. For this reason, the average cooling rate in [hot finishing temperature to (hot finishing temperature−100 ° C.)] is set to 5 ° C./second or more. On the other hand, since the effect tends to be saturated even if the average cooling rate in the temperature range exceeds 200 ° C./second, the average cooling rate in the temperature range is set to a range of 5 to 200 ° C./second. Note that the cooling rate in the temperature region after the above-described control cooling and before winding is not particularly limited.

Winding temperature: 400 ~ 600 ℃
When the coiling temperature exceeds 600 ° C., the hot-rolled scale thickness increases, the surface after pickling and cold rolling becomes rough, and irregularities are formed on the surface, leading to a decrease in workability, and pickling. Later, the hot-rolled scale tends to remain and adversely affects resistance spot weldability. On the other hand, when the coiling temperature is less than 400 ° C., the hot rolled sheet strength increases, the rolling load in cold rolling increases, and the productivity tends to decrease. Therefore, the coiling temperature is in the range of 400 to 600 ° C.

Primary average rate of temperature rise (from 200 ° C. to intermediate temperature): 10 to 50 ° C./second, Intermediate temperature: 500 to 800 ° C. Secondary average rate of temperature rise (from intermediate temperature to annealing temperature): 0.1 to 10 ° C. / Second When the primary heating rate is slower than 10 ° C./second, the crystal grains become coarse and stretch flangeability deteriorates. The primary heating rate may be high, but tends to saturate when it exceeds 50 ° C./second. Therefore, the primary average heating rate was set in the range of 10 to 50 ° C./second.
When the intermediate temperature exceeds 800 ° C., the crystal grain size becomes coarse and stretch flangeability deteriorates. The intermediate temperature may be low, but if it is less than 500 ° C., the effect is saturated, and the difference in the final structure is reduced. Therefore, the intermediate temperature is set in the range of 500 to 800 ° C.
When the secondary average temperature rising rate is faster than 10 ° C./second, austenite formation is slow, and the finally obtained ferrite phase fraction increases, making it difficult to ensure strength. On the other hand, when the secondary average temperature rising rate is slower than 0.1 / second, the crystal grain size becomes coarse and stretch flangeability deteriorates. Therefore, the secondary average temperature rising rate was set in the range of 0.1 to 10 ° C./second.

Annealing temperature: 750-900 ° C, holding time at this temperature: 10-500 seconds When annealing temperature is lower than 750 ° C, strain introduced by cold working is present in unrecovered unrecrystallized ferrite, elongation, Workability such as hole expansion rate tends to deteriorate. On the other hand, when the annealing temperature is higher than 900 ° C., austenite becomes coarse during heating, and the amount of ferrite phase generated in the subsequent cooling process decreases, and the elongation tends to decrease. Moreover, the crystal grain diameter finally obtained becomes excessively coarse, and the hole expansion rate tends to decrease. Therefore, the annealing temperature was set in the range of 750 to 900 ° C.
In addition, if the holding time at the annealing temperature is less than 10 seconds, there is a high possibility that undissolved carbides are present during annealing, and the austenite phase may be less present during annealing or at the cooling start temperature. In particular, it tends to be difficult to ensure the strength of the steel sheet. On the other hand, crystal grains tend to grow and become coarse due to long-term annealing, and when the holding time at the annealing temperature exceeds 500 seconds, the grain size of the austenite phase during heating annealing becomes coarse and finally obtained after heat treatment. The structure of the obtained steel sheet is coarsened, and the hole expansion rate tends to decrease. In addition, it is not preferable because it is caused by coarse grains and causes rough skin after press molding. Furthermore, the amount of ferrite phase produced during the cooling process to the cooling stop temperature also decreases, so that the elongation tends to decrease. Therefore, in order to achieve both the achievement of a finer structure and the reduction of the influence of the structure before annealing to obtain a uniform and fine structure, the holding time is set in the range of 10 to 500 seconds. A preferred holding time is in the range of 20 to 200 seconds.

Average cooling rate up to the cooling stop temperature: 1-30 ° C / sec This cooling rate controls the ratio of soft ferrite phase and hard martensite phase to ensure TS: 980 MPa class strength and workability It plays an important role. That is, when the average cooling rate exceeds 30 ° C./second, the generation of ferrite phase during cooling is suppressed, and the martensite phase is excessively generated. Therefore, it is easy to secure TS: 980 MPa class, but the formability deteriorates. Invite. On the other hand, if it is slower than 1 ° C./second, the amount of ferrite phase generated during the cooling process becomes too large, and the TS tends to decrease. A preferable range of the average cooling rate is 5 to 20 ° C./second. The cooling in this case is preferably gas cooling, but may be performed in combination using furnace cooling, mist cooling, roll cooling, water cooling, or the like.

Cooling stop temperature: 450-550 ° C
When the cooling stop temperature is higher than 550 ° C., a pearlite transformation or a bainite transformation that forms a softer structure than a martensite phase proceeds from austenite, and it becomes difficult to secure TS: 980 MPa class. Further, when a hard retained austenite phase is generated, stretch flangeability is deteriorated. On the other hand, when the cooling stop temperature is less than 450 ° C., the generation of ferrite during cooling becomes excessive, and it becomes difficult to secure TS: 980 MPa class.

After the cooling is stopped, a general hot dip galvanizing process is performed to obtain hot dip galvanizing. Or, further, after the above hot dip galvanizing treatment, an alloying treatment is performed in which reheating is performed using an induction heating device or the like to obtain an alloyed hot dip galvanized steel sheet.
Here, the adhesion amount of hot dip galvanizing is preferably about ²⁰ to 150 g / m ² per side.
This is because it is difficult to ensure corrosion resistance if the plating adhesion amount is less than 20 g / m ² , while if it exceeds 150 g / m ² , the corrosion resistance effect is saturated, and the cost is rather increased.

  In addition, after continuous annealing, the galvannealed steel sheet finally obtained may be subjected to temper rolling (also called skin pass rolling) for the purpose of shape correction or surface roughness adjustment. When rolling is performed, excessive strain is introduced and the crystal grains are stretched to form a rolled structure and the ductility is lowered. Therefore, the rolling rate (elongation rate) of skin pass rolling is preferably about 0.1 to 1.5%.

  According to the production method of the present invention described above, a high-strength hot-dip galvanized steel sheet having desired workability and weldability can be obtained. After hot finish rolling, hot finish rolling temperature to (hot finish rolling temperature- 100 ° C) after cooling at an average cooling rate of 5 to 200 ° C / second, and if necessary, after hot finish rolling, the elapsed time to (winding temperature) x 1.1 should be 5 seconds or more. Then, the material variation in the coil longitudinal direction can be effectively suppressed by performing winding.

  When the steel sheet is rapidly cooled to a temperature close to the coiling temperature after completion of the hot finish rolling, specifically to a temperature below (coiling temperature) × 1.1, austenite having crystal grains partially expanded by hot rolling, Recrystallized sized austenite may be mixed. Such a mixed structure is noticeably generated immediately after hot rolling, particularly at an edge portion in the width direction of the steel sheet. In this case, in the finally obtained hot-rolled sheet, a layered low-temperature transformation phase structure was present in the region where partially expanded crystal grains were present, and sized recrystallized austenite was present. In the region, there is a low-temperature transformation structure of sized particles produced by transformation from austenite. Also, in the case of the rapid cooling as described above, it is difficult to perform uniform rapid cooling over the entire length of the coil, and during cooling, temperature unevenness occurs at the tip, tail, and coil longitudinal center of the coil, Since the water used in this process remains on the steel sheet after cooling, it tends to be a non-uniform hot rolled sheet structure.

  The unevenness or uneven distribution of the components and structure in the steel sheet due to the non-uniform structure generated in the hot rolling stage cannot be completely eliminated even in the annealing process of the hot dip galvanizing process. As a result, the structure of the final product is not uniform. When the second phase is hardened due to the non-uniformity of the structure, a difference in hardness from soft ferrite occurs, which inhibits uniform deformation of the material during press molding and causes deterioration of stretch flangeability.

  After the hot finish rolling, if the elapsed time to the temperature of (winding temperature) × 1.1 is less than 5 seconds, after the hot rolling is finished, the steel is rapidly cooled to the vicinity of the winding temperature, causing the above-mentioned problems. This is thought to be due to the fact that material fluctuations in the coil longitudinal direction tend to be large. For this reason, after completion of hot finish rolling, the elapsed time up to the temperature of (winding temperature) × 1.1 is set to 5 seconds or more. The upper limit of the elapsed time depends on the production equipment, but if it is too long, it only reduces the productivity, so the upper limit is preferably about 30 seconds, and the more preferable upper limit of elapsed time is 20 seconds. Degree.

After melting the steel having the composition shown in Table 1 into a slab, hot rolling, pickling, cold rolling with a reduction ratio of 50%, continuous annealing and plating treatment under various conditions shown in Table 2 Thus, hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets having a plate thickness of 1.4 mm and a coating adhesion amount of 45 g / m ² per side were produced. About the obtained hot dip galvanized steel sheet and alloyed hot dip galvanized steel sheet, the material test shown below was done and the material characteristic was investigated.
The obtained results are shown in Table 3.

The material test and the evaluation method of material properties are as follows. The sampling position was 5 m from the tail end of the final product coil.
(1) Structure of steel plate The cross-section in the rolling direction, the thickness: 1/4 plane position was investigated by observing with an optical microscope or a scanning electron microscope (SEM). The crystal grain size of the ferrite phase was measured according to the method specified in JIS G 0551, and converted to an average crystal grain size. In addition, the volume fraction of the ferrite phase is obtained by obtaining the area occupied by the ferrite phase existing in a square area of 100 mm x 100 mm square arbitrarily set by image analysis using a cross-sectional structure photograph at a magnification of 1000 times. Is the volume fraction of the ferrite phase. The volume fraction of the martensite phase was determined in the same manner.

(2) Tensile properties Using the No. 5 test piece described in JIS Z 2201 with the rolling direction and 90 ° direction as the longitudinal direction (tensile direction), a tensile test based on JIS Z 2241 was performed and evaluated. The evaluation standard for tensile properties was a TS × EI value of 15000 MPa ·% or higher.

(3) Hole expansion rate This was carried out based on the Japan Iron and Steel Federation standard JFST1001. When a hole with an initial diameter of d ₀ = 10 mm is punched and the hole is widened by raising a 60 ° conical punch, the punch stops rising when the crack penetrates the plate thickness, and the punched hole diameter d is measured after crack penetration. The following formula hole expansion rate (%) = ((d−d ₀ ) / d ₀ ) × 100
Was used to calculate the hole expansion rate.
This test was performed three times for the same number of steel plates, and the average value (λ) of the hole expansion rate was obtained. The evaluation standard for the hole expansion rate was good when the TS × λ value was 35000 MPa ·% or more.

(4) Resistance spot weldability Electrode: DR6mm-40R, pressure: 4.8kN, initial pressurization time: 30cycles / 60Hz, energization time: 17cycles / 60Hz, holding time: 1cycle / 60Hz Spot welding of steel plates with the same number under welding conditions (changes at a 0.2 kA pitch from 4.6 to 10.0 kA, and at a 0.5 kA pitch from 10.5 kA to welding), cross tension test, and the nugget diameter of the weld It used for the measurement. The cross tension test of the resistance spot welded joint was performed in accordance with JIS Z 3137. The nugget diameter was carried out according to the description of JIS Z 3139. For the cross section perpendicular to the plate surface of the symmetrical circular plug after resistance spot welding, the cross section passing through the approximate center of the welding point is half-cut by an appropriate method, and after polishing and corrosion, by cross-sectional structure observation by optical microscope observation It was measured. The maximum diameter of the molten region excluding the corona bond was defined as the nugget diameter. When a cross tensile test was performed on a welded material having a nugget diameter of 4 t ^1/2 (mm) (t: plate thickness of the steel plate) or more, the weldability was improved when the nugget fractured at the base material.

As shown in Table 3, in the invention example, a high strength hot dip galvanized steel sheet with excellent workability satisfying TS × EI ≧ 15000 MPa ·%, TS × λ ≧ 35000 MPa ·% and simultaneously satisfying good resistance spot weldability is obtained. It was confirmed that it was obtained.
On the other hand, Nos. 11, 12 and 13 whose steel components were outside the proper range of the present invention could not achieve both workability and weldability. Nos. 14, 15 and 17 in which any one of the primary heating rate, intermediate temperature and annealing temperature is out of the proper range of the present invention has poor stretch flangeability due to coarse ferrite grains. It was.
Nos. 16, 18 and 20 whose secondary heating rate, holding time or cooling stop temperature were outside the proper range of the present invention had a high fraction of ferrite phase and TS was lower than 980 MPa. No. 19 having an average cooling rate outside the appropriate range of the present invention had a low El fraction, a low hole expansion rate λ, and a poor workability because the ferrite phase fraction was small.

After melting the steel having the composition shown in Table 4 into a slab, hot rolling, pickling, rolling reduction: 50% cold rolling, continuous annealing, and plating treatment under various conditions shown in Table 5 Thus, a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet having a sheet thickness of 1.4 mm and a coating adhesion amount of 45 g / m ² per side were produced.
The obtained hot-dip galvanized steel sheet and alloyed hot-dip galvanized steel sheet were investigated by conducting the material tests (1) to (4) described above and the material fluctuation evaluation shown in the following (5).
(5) Coil longitudinal direction material fluctuation investigation In the center in the width direction of the final product coil, the position in the longitudinal direction shows the hole expansion ratio λ (%) in the three points of the tip, tail and center. They were obtained by the same method as in 3), and were set to λ1, λ2, and λ3, respectively. Note that the tip portion is a position 5 m from the tip of the final product coil, the center portion is a center position with respect to the entire length of the final product coil, and the tail end portion is a position 5 m from the tail end of the final coil.
From the λ1, λ2 and λ3 obtained in this way, the hole expansion rate fluctuation amount Δλ (%) is calculated from the following equation, and when Δλ is in the range of −10 ≦ Δλ (%) ≦ 10, the material variation over the entire coil length Small and good material properties were assumed.
Δλ (%) = λ3− (λ1 + λ2) / 2

  The results obtained are shown in Table 6.

As shown in Table 6, in the invention example, TS × EI ≧ 15000 MPa ·%, TS × λ ≧ 35000 MPa ·%, the material variation is small over the entire length of the coil, and satisfactory resistance spot weldability is satisfied at the same time. It was confirmed that a high-strength hot-dip galvanized steel sheet having excellent properties was obtained.
On the other hand, No.32, 35, 36 and 37 whose elapsed time up to the temperature of (winding temperature) x 1.1 after hot finish rolling is outside the proper range are satisfactory in workability and weldability. Fluctuation is large. In addition, even if the residence temperature and residence time of the steel sheet after hot finish rolling were within the proper range, No. 34 whose steel plate components were outside the proper range was inferior in workability, although the material variation was small. .

The high-strength hot-dip galvanized steel sheet of the present invention not only has high tensile strength, but also has excellent workability and weldability. Therefore, strict dimensional accuracy and workability are required in fields such as automobile parts, architecture, and home appliances. It can be used suitably for a certain application.

Claims (5)

  By mass%, C: 0.05% or more and less than 0.10%, Si: 0.01% or more and less than 0.35%, Mn: 2.0 to 3.5%, P: 0.020% or less, S: 0.0020% or less, Al: 0.005 to 0.1%, N: Contains 0.0050% or less, Cr: more than 1.0% and 2.0% or less, Ti: 0.010-0.080%, Nb: 0.010-0.080% and B: 0.0001-0.0030%, the balance is Fe and inevitable impurities, steel The structure consists of 20-60% ferrite phase, 40-80% martensite phase, and 5% or less (including 0%) balance structure in volume fraction, and the average grain size of the ferrite phase Is a high-strength hot-dip galvanized steel sheet excellent in workability and weldability, characterized by having a tensile strength of 980 MPa or more and a hot-dip galvanized layer on the steel sheet surface.   The high-strength hot-dip galvanized steel sheet according to claim 1, wherein the steel sheet further contains Ca: 0.0001 to 0.0050% by mass. By mass%, C: 0.05% or more and less than 0.10%, Si: 0.01% or more and less than 0.35%, Mn: 2.0 to 3.5%, P: 0.020% or less, S: 0.0020% or less, Al: 0.005 to 0.1%, N: Steel slab containing 0.0050% or less, Cr: more than 1.0% and 2.0% or less, Ti: 0.010-0.080%, Nb: 0.010-0.080% and B: 0.0001-0.0030%, with the balance being Fe and inevitable impurities , After hot rolling, after winding into a coil, pickling, then cold rolling, hot-dip galvanized to produce a hot-dip galvanized steel sheet,
In the above hot rolling, after hot rolling with a slab heating temperature of 1150 to 1300 ° C and a hot finish rolling temperature of 850 to 950 ° C, a temperature of hot finish rolling temperature to (hot finish rolling temperature-100 ° C) The zone is cooled at an average cooling rate of 5 to 200 ° C./second, wound at a coiling temperature of 400 to 600 ° C., then pickled, cold-rolled, and then primary from 200 ° C. to an intermediate temperature. Heating to an intermediate temperature of 500 to 800 ° C with an average temperature increase rate of 10 to 50 ° C / second, and 750 to 900 ° C with a secondary average temperature increase rate from the intermediate temperature to the annealing temperature of 0.1 to 10 ° C / second After heating to the annealing temperature of the steel and holding it for 10 to 500 seconds, it is cooled to a cooling stop temperature of 450 to 550 ° C. at an average cooling rate of 1 to 30 ° C./second, and then hot dip galvanized or further alloyed. that is,
The steel structure has a volume fraction of 20-60% ferrite phase, 40-80% martensite phase, and a remaining structure of 5% or less (including 0%), and the average grain size of the ferrite phase A method for producing a high-strength hot-dip galvanized steel sheet excellent in workability and weldability, characterized in that a steel sheet having a tensile strength of 980 MPa or more is obtained .   4. The method for producing a high-strength hot-dip galvanized steel sheet according to claim 3, wherein after the hot finish rolling is finished, an elapsed time up to a temperature of (winding temperature) × 1.1 is 5 seconds or more.   The method for producing a high-strength hot-dip galvanized steel sheet according to claim 3 or 4, wherein the steel slab further contains, by mass%, Ca: 0.0001 to 0.0050%.